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Production and storage stability of concentrated micellar casein

Open AccessPublished:December 23, 2021DOI:https://doi.org/10.3168/jds.2021-21200

      ABSTRACT

      Concentrated micellar casein (CMC) is a high-protein ingredient that can be used in process cheese product formulations. The objectives of this study were to develop a process to produce CMC and to evaluate the effect of sodium chloride and sodium citrate on its storage stability. Skim milk was pasteurized at 76°C for 16 s and cooled to ≤4°C. The skim milk was heated to 50°C using a plate heat exchanger and microfiltered with a graded permeability (GP) ceramic microfiltration (MF) membrane system (0.1 μm) in a continuous feed-and-bleed mode (flux of 71.43 L/m2 per hour) using a 3× concentration factor (CF) to produce a 3× MF retentate. Subsequently, the retentate of the first stage was diluted 2× with soft water (2 kg of water: 1 kg of retentate) and again MF at 50°C using a 3× CF. The retentate of the second stage was then cooled to 4°C and stored overnight. The following day, the retentate was heated to 63°C and MF in a recirculation mode until the total solids (TS) reached approximately 22% (wt/wt). Subsequently, the MF system temperature was increased to 74°C and MF until the permeate flux was <3 L/m2 per hour. The CMC was then divided into 3 aliquots (approximately 10 kg each) at 74°C. The first portion was a control, whereas 1% of sodium chloride was added to the second portion (T1), and 1% of sodium chloride plus 1% of sodium citrate were added to the third portion (T2). The CMC retentates were transferred hot to sterilized vials and stored at 4°C. This trial was repeated 3 times using separate lots of skim milk. The CMC at d 0 (immediately after manufacturing) contained 25.41% TS, 21.65% true protein (TP), 0.09% nonprotein nitrogen (NPN), and 0.55% noncasein nitrogen (NCN). Mean total aerobic bacterial counts (TBC) in control, T1, and T2 at d 0 were 2.6, 2.5, and 2.8 log cfu/mL, respectively. The level of proteolysis (NCN and NPN values) increased with increasing TBC during 60 d of storage at 4°C. This study determined that CMC with >25% TS and >95% casein as percentage of TP can be manufactured using GP MF ceramic membranes and could be stored up to 60 d at 4°C. The effects of the small increase in NCN and NPN, as well as the addition of sodium chloride or sodium citrate in CMC during 60 d of storage on process cheese characteristics, will be evaluated in subsequent studies.

      Key words

      INTRODUCTION

      Microfiltration (MF) is a membrane process used to separate CN micelles (0.1–0.40 µm) and serum whey proteins (SP; 0.003–0.010 µm) from skim milk, using a semipermeable membrane with a pore size of 0.1 μm. When MF is applied to skim milk, CN is concentrated in the retentate and called micellar casein concentrate (MCC), whereas SP, lactose, soluble minerals, and water pass through the membrane to the permeate. The CN in the MCC exists in a micellar form, which is a relatively stable colloidal dispersion (
      • Rollema H.S.
      • Muir D.D.
      Casein and Related Products.
      ).
      Micellar casein concentrate has been manufactured from skim milk using different MF membranes, such as polymeric spiral-wound (SW) membranes (
      • Govindasamy-Lucey S.
      • Jaeggi J.J.
      • Johnson M.E.
      • Wang T.
      • Lucey J.A.
      Use of cold microfiltration retentates produced with polymeric membranes for standardization of milks for manufacture of pizza cheese.
      ;
      • Lawrence N.D.
      • Kentish S.E.
      • O'Connor A.J.
      • Barber A.R.
      • Stevens G.W.
      Microfiltration of skim milk using polymeric membranes for casein concentrate manufacture.
      ;
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C.
      ;
      • Beckman S.L.
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Production efficiency of micellar casein concentrate using polymeric spiral-wound microfiltration membranes.
      ;
      • Beckman S.L.
      • Barbano D.M.
      Effect of microfiltration concentration factor on serum protein removal from skim milk using spiral-wound polymeric membranes.
      ;
      • Hammam A.R.A.
      • Martínez-Monteagudo S.I.
      • Metzger L.E.
      Progress in micellar casein concentrate: Production and applications.
      ;
      • Marella C.
      • Sunkesula V.
      • Hammam A.R.A.
      • Kommineni A.
      • Metzger L.E.
      Optimization of spiral-wound microfiltration process parameters for the production of micellar casein concentrate.
      ) or ceramic membranes, including uniform transmembrane pressure (UTP) and graded permeability (GP) membranes (
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C.
      ;
      • Hurt E.
      • Barbano D.M.
      Processing factors that influence casein and serum protein separation by microfiltration1.
      ;
      • Hurt E.
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Micellar casein concentrate production with a 3×, 3-stage, uniform transmembrane pressure ceramic membrane process at 50°C.
      ;
      • Adams M.C.
      • Barbano D.M.
      Serum protein removal from skim milk with a 3-stage, 3× ceramic Isoflux membrane process at 50°C.
      ,
      • Adams M.C.
      • Barbano D.M.
      Effect of ceramic membrane channel diameter on limiting retentate protein concentration during skim milk microfiltration.
      ;
      • Zulewska J.
      • Barbano D.M.
      The effect of linear velocity and flux on performance of ceramic graded permeability membranes when processing skim milk at 50°C.
      ;
      • Hammam A.R.A.
      • Metzger L.E.
      Production and storage stability of liquid micellar casein concentrate.
      ). The SW membranes are less expensive and have lower operating costs but have limited viscosity range, low chemical stability, and shorter life compared with ceramic membranes (
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C.
      ;
      • Hammam A.R.A.
      • Martínez-Monteagudo S.I.
      • Metzger L.E.
      Progress in micellar casein concentrate: Production and applications.
      ). In a study that compared different MF membranes, the flux was higher in UTP and GP ceramic membranes (54 and 72 L/m2 per hour, respectively) than in SW membranes (16 L/m2 per hour) when skim milk was MF at 50°C in a continuous feed-and-bleed mode using a 3× concentration factor (CF;
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C.
      ). That study also reported that the efficiency of SP removal was 39% for SW, 64% for UTP, and 61% for GP MF membranes. To achieve SP removal with SW membranes comparable to that of ceramic membranes, more membrane surface area and diafiltration (DF) stages would be required (
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C.
      ;
      • Hammam A.R.A.
      • Martínez-Monteagudo S.I.
      • Metzger L.E.
      Progress in micellar casein concentrate: Production and applications.
      ). As a result, UTP and GP ceramic membranes are widely used for MF of skim milk. Uniform transmembrane pressure membranes require a higher investment and operating costs relative to GP membranes, due to the need for a permeate recirculation pump to produce co-current permeate flow, and thereby decreasing the fouling and increasing the SP removal (
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C.
      ;
      • Hammam A.R.A.
      • Martínez-Monteagudo S.I.
      • Metzger L.E.
      Progress in micellar casein concentrate: Production and applications.
      ). By contrast, GP membranes eliminate the need for a permeate recirculation pump and the associated electrical costs, due to their ability to maintain a constant and uniform flux.
      Micellar casein concentrate is a high-protein ingredient that can be used in a range of commercial applications, including protein fortification of dairy foods and ingredients for beverages, bakery, and meat products, due to its unique physicochemical and functional properties (e.g., water-binding, emulsifying, whipping, and foaming properties;
      • Mulvihill D.M.
      • Ennis M.P.
      Functional milk proteins: Production and utilization.
      ). The typical composition of liquid MCC is >9% true protein (TP) and >13% TS, using 3 MF stages with a 3× CF and DF (
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C.
      ). It has been reported that the first 3× CF MF stage using GP and UTP membranes produced MCC with 14.16 and 15.04% TS, respectively, and TP of 8.03 and 8.86%, respectively (
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C.
      ;
      • Hurt E.
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Micellar casein concentrate production with a 3×, 3-stage, uniform transmembrane pressure ceramic membrane process at 50°C.
      ;
      • Zulewska J.
      • Barbano D.M.
      The effect of linear velocity and flux on performance of ceramic graded permeability membranes when processing skim milk at 50°C.
      ;
      • Tremblay-Marchand D.
      • Doyen A.
      • Britten M.
      • Pouliot Y.
      A process efficiency assessment of serum protein removal from milk using ceramic graded permeability microfiltration membrane.
      ). The MCC produced from the second 3× CF MF stage using GP and UTP MF system had 10.30 and 11.41% TS, respectively, with TP of 7.5 and 8.68%, respectively (
      • Hurt E.
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Micellar casein concentrate production with a 3×, 3-stage, uniform transmembrane pressure ceramic membrane process at 50°C.
      ;
      • Zulewska J.
      • Barbano D.M.
      The effect of linear velocity and flux on performance of ceramic graded permeability membranes when processing skim milk at 50°C.
      ;
      • Tremblay-Marchand D.
      • Doyen A.
      • Britten M.
      • Pouliot Y.
      A process efficiency assessment of serum protein removal from milk using ceramic graded permeability microfiltration membrane.
      ). This MCC can be used as-is in many applications, or it can be further concentrated to produce concentrated micellar casein (CMC).
      The MCC can be further concentrated using UF with a 2.2× CF, followed by 3 MF stages with a 3× CF with DF, followed by UF again for further concentration (
      • Amelia I.
      • Barbano D.M.
      Production of an 18% protein liquid micellar casein concentrate with a long refrigerated shelf life.
      ) to produce CMC with 18 and 22% TP and TS, respectively. Micellar casein concentrate can also be concentrated by using vacuum evaporation.
      • Lu Y.
      • McMahon D.J.
      • Metzger L.E.
      • Kommineni A.
      • Vollmer A.H.
      Solubilization of rehydrated frozen highly concentrated micellar casein for use in liquid food applications.
      reported that CMC containing >25% TS and >20 TP can be produced using MF and vacuum evaporation. It may be possible to use GP MF membranes to produce CMC instead of using a combination of different equipment. Micellar casein concentrate and CMC can also be dried to produce MCC powder with a long shelf-life that has 84% TP and 96% TS (
      • Nasser S.
      • De Sa Peixoto P.
      • Moreau A.
      • Croguennec T.
      • Bray F.
      • Rolando C.
      • Tessier F.J.
      • Hédoux A.
      • Delaplace G.
      Storage of micellar casein powders with and without lactose: Consequences on color, solubility, and chemical modifications.
      ). If CMC is left in a concentrated form, it eliminates the drying cost and has more solubility compared with dried MCC. However, CMC has a shorter shelf-life as well as increased transportation costs compared with dried MCC (
      • Amelia I.
      • Barbano D.M.
      Production of an 18% protein liquid micellar casein concentrate with a long refrigerated shelf life.
      ). As a result, the shelf-life of CMC has become an important factor for its use by the dairy industry.
      • Muir D.D.
      The shelf-life of dairy products: 3. Factors influencing intermediate and long life dairy products.
      reported that the shelf-life of a dairy product is the time in which the product remains safe (no pathogenic bacteria) and shows no organoleptic defects (e.g., bitterness, acidic flavor). It has been reported that the shelf-life of dairy products is limited by the growth of spoilage bacteria (
      • Muir D.D.
      The shelf-life of dairy products: 3. Factors influencing intermediate and long life dairy products.
      ), which produce enzymes that can degrade milk constituents and cause unacceptable quality. The end of the shelf-life of CMC (18% TP and 22% TS) has been suggested as the point at which the total aerobic bacterial count (TBC) is >4.3 log cfu/mL (
      • Amelia I.
      • Barbano D.M.
      Production of an 18% protein liquid micellar casein concentrate with a long refrigerated shelf life.
      ), because this is the legal limit for the shelf-life of pasteurized milk based on the Pasteurized Milk Ordinance (
      • FDA (Food and Drug Administration)
      Grade “A” Pasteurized Milk Ordinance.
      ). Some additives may have an influence on the functionality as well as the shelf-life of CMC. The addition of sodium chloride to CMC could increase the shelf-life and decrease proteolysis because it is a potential preservative. Also, the addition of emulsifying salts (such as sodium citrate) increases the dispersibility and solubility of CMC (
      • Lu Y.
      • McMahon D.J.
      • Metzger L.E.
      • Kommineni A.
      • Vollmer A.H.
      Solubilization of rehydrated frozen highly concentrated micellar casein for use in liquid food applications.
      ), which could improve the functionality of process cheese products when CMC is used as an ingredient.
      We hypothesize that it may be possible to use a GP MF system to further concentrate CMC as an alternative to vacuum evaporation (
      • Lu Y.
      • McMahon D.J.
      • Metzger L.E.
      • Kommineni A.
      • Vollmer A.H.
      Solubilization of rehydrated frozen highly concentrated micellar casein for use in liquid food applications.
      ) or using multiple filtration systems (
      • Amelia I.
      • Barbano D.M.
      Production of an 18% protein liquid micellar casein concentrate with a long refrigerated shelf life.
      ). Consequently, there is a possibility of producing CMC with >25% TS using only a GP MF system. The objectives of this research were to develop a process to manufacture CMC [>25% TS and 95% CN as a percentage of TP (CN%TP)] using a GP ceramic MF system and to evaluate the effect of sodium chloride and sodium citrate on its storage stability during 60 d of storage at 4°C.

      MATERIALS AND METHODS

      Experimental Design

      The manufacture of CMC was completed over a period of 2 d at the South Dakota State University Dairy Plant (Brookings, SD). The experiment was repeated 3 times with different lots of skim milk. Chemical and microbiological analyses were performed on CMC at 0, 30, and 60 d of storage at 4°C, to examine shelf-life stability.

      Preparation of Skim Milk

      Approximately 685 kg of whole bovine milk was separated (model MSE 140-48-177 AirTight centrifuge, GEA Co.) at 4°C at the South Dakota State University Dairy Plant. Subsequently, pasteurization (76°C/16 s) was applied to the skim milk in a plate heat exchanger (model PR02-SH, AGC Engineering). The pasteurized skim milk was then kept at ≤4°C until MF was conducted the following day. Tanks and milk cans were covered during processing to minimize airborne contamination from the plant environment.

      Microfiltration Operation and CMC Manufacturing

      To produce CMC, a pilot-scale ceramic GP MF system (TIA) was used. This GP MF system was equipped with 7 ceramic tubes (19 channels with a diameter of 3.3 mm each) mounted in the system vertically. The ceramic Membralox GP membranes had a 0.1-μm pore size, 1.68-m2 surface area, and 1.02-m membrane length. The GP MF system was also equipped with a feed pump and a retentate recirculation pump (TIA).

      First Stage: Day 1

      The GP MF system was started with soft water at 50°C; subsequently, the system was transitioned from water to milk. Approximately 670 kg of skim milk was MF with the GP MF system at a constant flux (71.42 L/m2 per hour) using a 3× CF (1 kg of retentate to 2 kg of permeate) in a feed-and-bleed mode at 50°C (Figure 1). The skim milk was heated to 50°C with a heat plate exchanger (SABCO PlatePro Sanitary Chiller, NP925-41) before processing. The water at the beginning of the process was flushed out with skim milk by collecting about 37 kg of permeate and 18 kg of retentate in cans that were discarded. The permeate flow rate was 120 L/h (flux of 71.42 L/m2 per hour), and the retentate flow rate was 60 L/h, to produce a 3× retentate. After this startup, retentate and permeate were collected and weighed continuously. The following conditions were applied during MF of skim milk: retentate pressure inlet (Rpi), retentate pressure outlet (Rpo), and permeate pressure outlet (Ppo) were 400, 198.5, and 186.5 kPa, respectively. The CF was measured every 15 min by collecting permeate and retentate samples. The composition of retentate and permeate during MF was monitored using an infrared spectrophotometer (MilkoScan FT1-Lactoscope FTIR, FOSS Instruments–FOSS Analytical A/S). During the MF process, the collected retentate was kept in tanks at 4°C. At the end of the run, the retentate and permeate were sampled for compositional analysis. The processing time of the first stage was approximately 4 h.
      Figure thumbnail gr1
      Figure 1Schematic process diagram for manufacturing of concentrated micellar casein (CMC) using microfiltration (MF): CF = concentration factor; DF = diafiltration.

      Second Stage: Day 1

      The retentate from the first stage was diluted with soft water (approximately 204 kg of retentate was mixed with 408 kg of water) to obtain a DF of 3× to reach the original volume of skim milk. After mixing, the diluted retentate was heated to 50°C and processed with the GP MF system using a 3× CF, as described previously. The water at the beginning of the process was flushed out of the system with the diluted retentate as described in the first stage. The Rpi, Rpo, and Ppo were 400, 197, and 205 kPa, respectively. Permeate and retentate were weighed and sampled, as described in the first stage. The retentate was collected in sanitized cans, cooled to 4°C, and stored overnight at ≤4°C. The processing time of the second stage was approximately 3.5 h.

      Third Stage: Day 2

      The following day, about 154 kg of the retentate (9.65% TS and approximately 92% CN%TP) was MF in a recirculation mode to produce CMC. As soon as MF was started, the retentate was further heated to 63°C to reduce the viscosity while minimizing denaturation of whey protein. The following conditions were applied: Rpi, Rpo, and Ppo were 398, 199, and 200 kPa, respectively, the permeate flow rate was 120 L/h (flux of 71.42 L/m2 per hour), and the retentate flow rate was 60 L/h. When the TS reached approximately 22% (as measured using CEM Smart System5 SL7199), the temperature was increased to 74°C to maximize the final TS content that could be obtained. Increasing the solids content of CMC during MF led to decreasing the Ppo until it reached 0 kPa. The decrease of Ppo is related to the concentration polarization and membrane fouling that accumulated on the membrane during recirculation of the retentate. The process was stopped when the permeate flux reached approximately 3 L/m2 per hour and Ppo reached 0 kPa. The retentate from the third stage (approximately 30 kg) was collected and sampled. A composite sample of the permeate was sampled for compositional analysis. The processing time for the third stage was around 2 h. The retentate of the third stage was divided into 3 aliquots of approximately 10 kg each. The first portion was used as a control; 1% of sodium chloride was added into the second portion (T1), and 1% sodium chloride + 1% sodium citrate was added to the third portion (T2). The CMC treatments were sampled at 74°C and transferred to 45-mL sterilized vials (model 3040-00, Thermo Scientific–Capitol Vial Inc.) and stored at 4°C to evaluate the storage stability of CMC. This trial was replicated 3 times using 3 separate lots of raw milk.

      Cleaning After Processing

      After processing, the GP MF system was flushed with soft water to remove all retentate residues from the system. The initial flux was measured with approximately 60 kg of soft water at 27°C. During the flux measurement the retentate valves were closed and the permeate valves were completely opened, with the feed pump running. Subsequently, 30 kg of soft water was added to the system and heated to 74°C; then 900 mL of Ultrasil 110 Alkaline cleaner (Ecolab Inc.) and 200 mL of XY 12 (Ecolab Inc.) were added to get a pH of 11. This solution was recirculated for 30 min at a 350 L/h permeate flow rate (flux of 208 L/m2 per hour). After cleaning the MF system with the alkaline solution, the membrane was cooled to 50°C (less than 10°C per min). The alkaline solution was flushed out of the MF system with soft water until the pH of outlet water ranged from 8.3 to 8.5. The flux was measured again, as described previously. The system was cleaned with an acid solution (Ultrasil 78 acid cleaner, Ecolab Inc.) by adding 30 kg of soft water and heated to 52°C; subsequently, 400 mL of Ultrasil 78 (Ecolab Inc.) was added to obtain a pH of 2. The recirculation of the acid solution was applied for 20 min at a flux of 208 L/m2 per hour. Subsequently, the machine was stopped and the acid was retained inside the system. Before using the system again, the acid solution was flushed out with soft water until the pH reached 8.3 to 8.5. The flux was measured again after flushing the acid solution.

      Chemical Analyses

      Skim milk, permeate, and retentate samples collected during the process were analyzed using an infrared spectrophotometer to check if the system was running normally. Ash (AOAC International, 2000, section 33.2.10, method 945.46), TS (AOAC International, 2000, section 33.2.44, method 990.20), total nitrogen (TN; AOAC International, 2000, section 33.2.11, method 991.20), NPN (
      • AOAC Internationa
      Official Methods of Analysis.
      , section 33.2.12, method 991.21), and noncasein nitrogen (NCN;
      • Zhang H.
      • Metzger L.E.
      Noncasein nitrogen analysis of ultrafiltration and microfiltration retentate.
      ) were determined in skim milk, retentate, and permeate samples for each stage and during the shelf-life of CMC. The NCN was subtracted from TN and multiplied by 6.38 to calculate CN content; NPN was subtracted from TN and multiplied by 6.38 to calculate TP content; and NPN was subtracted from NCN and multiplied by 6.38 to calculate SP content.

      SP Removal

      A mass balance was conducted to determine the efficiency of SP removal from skim milk. The mass of SP in permeate was divided by the mass of SP in the skim milk and multiplied by 100 to calculate the percentage of SP removal at a given stage in the MF process. The mass of SP in permeate was calculated by multiplying the weight of removed permeate by the percentage of SP in this permeate, and the total mass of SP present in skim milk was calculated by multiplying the weight of skim milk by the percentage of SP in the skim milk.

      Capillary Gel Electrophoresis

      The protein fractions in the skim milk, permeate, and CMC were determined using capillary gel electrophoresis (CGE). Samples were diluted to <1% protein with distilled water. Subsequently, 10 µL of each diluted sample, 85 µL of sample buffer (ProteomeLab SDS-MW Analyses Kit, Beckman-Coulter), and 5 µL of β-mercaptoethanol were pipetted into a PCR vial (Fisher Scientific) and heated in a water bath at 90°C for 15 min. The samples were analyzed via CGE (P/ACE MIDQ, Beckman-Coulter) equipped with a UV detector set at 214 nm. The test was performed using a 50-µm bare fused silica capillary (20.2-cm effective length from the inlet to the detection window). Solution and reagents were obtained as a part of the ProteomeLab SDS-Molecular Weight Analysis Kit (Beckman-Coulter), which is designed for separation of protein SDS complexes, using a replaceable gel matrix. The gel is formulated to provide an effective sieving range of approximately 10 to 225 kDa. An SDS-molecular weight (MW) size standard (recombinant proteins 10–225 kDa supplied with the ProteomeLab SDS-MW Analysis Kit) was used to estimate MW of the proteins in each sample. A capillary preconditioning method (basic rinse-0.1 N NaOH-5 min-50 psi, acidic rinse-0.1 N HCl-2 min-50 psi, distilled water rinse-2 min-50 psi, and SDS gel rinse-10 min-40 psi) was run every 3 samples, and then the sample was electrokinetically introduced at 5 kV for 20 s. Separation was performed using the following conditions: a constant voltage of 15 kV, temperature 25°C, and 20 bar pressure with reverse polarity in the SDS-MW gel buffer. The MW standards (ProteomeLab and Beckman-Coulter) and available pure milk protein fractions (Sigma) were also separated using the method described earlier to calculate migration times.
      The migration time of the peaks resulting from the capillary electropherogram was compared with MW standards and pure standard samples to identify the peaks. Also, the peaks were compared with results reported by other researchers (
      • Creamer L.K.
      • Richardson T.
      Anomolous behaviour of bovine αS1- and β-caseins on gel electrophoresis in sodium dodecyl sulfate buffers.
      ;
      • Miralles B.
      • Bartolome B.
      • Amigo L.
      • Ramos M.
      Comparison of three methods to determine the whey protein to total protein ratio in milk.
      ,
      • Miralles B.
      • Ramos M.
      • Amigo L.
      Influence of proteolysis of milk on the whey protein to total protein ratio as determined by capillary electrophoresis.
      ;
      • Anema S.G.
      The use of “lab-on-a-chip” microfluidic SDS electrophoresis technology for the separation and quantification of milk proteins.
      ;
      • Salunke P.
      Impact of transglutaminase on the functionality of milk protein concentrate and micellar casein concentrate.
      ). The area of each identified peak was calculated from the electropherogram using a valley-to-valley approach, as described by
      • Miralles B.
      • Ramos M.
      • Amigo L.
      Influence of proteolysis of milk on the whey protein to total protein ratio as determined by capillary electrophoresis.
      . The areas of each identified individual CN fraction (such as αS1, αS2, β, κ, and γ-CN), SP fractions (such as α-LA and β-LG), and peptides (peaks between 10 and 20 kDa) were calculated as percentage of total area (positive peaks).

      Microbiological Analyses

      All CMC treatments were analyzed for TBC, coliforms, yeast, and mold at 0, 30, and 60 d of storage at 4°C. The TBC (method 6.040,
      • Wehr H.M.
      • Frank J.F.
      Standard Methods for the Examination of Dairy Products.
      ), coliform count (method 7.074,
      • Wehr H.M.
      • Frank J.F.
      Standard Methods for the Examination of Dairy Products.
      ), and yeast and mold counts (method 8.115,
      • Wehr H.M.
      • Frank J.F.
      Standard Methods for the Examination of Dairy Products.
      ) were determined during the shelf-life. Petrifilms (3M) were used for the TBC, coliform count, and yeast and mold counts. A sterile phosphate buffer was used for dilutions (Weber Scientific). The TBC Petrifilms were incubated at 32 ± 1°C for 48 h ± 3 h; coliform Petrifilm plates were incubated at 32°C ± 1°C for 24 h ± 2 h; and yeast and mold Petrifilm plates were incubated at 25°C ± 1°C for 5 d. All Petrifilms were counted and rounded, as described by
      • Wehr H.M.
      • Frank J.F.
      Standard Methods for the Examination of Dairy Products.
      .

      Shelf-Life Study

      Samples were analyzed chemically (ash, TN, NCN, NPN, TS, and CGE) and microbiologically (TBC, coliform count, and yeast and mold counts) at 0, 30, and 60 d of storage. The end of shelf-life was defined as TBC >20,000 cfu/mL (>4.3 log cfu/mL), as described by
      • Amelia I.
      • Barbano D.M.
      Production of an 18% protein liquid micellar casein concentrate with a long refrigerated shelf life.
      .

      Statistical Analyses

      Processing data were analyzed using R software (R ×64-3.3.3, R Foundation for Statistical Computing) to determine the differences among means of the 3 MF stages. Statistical analysis was also performed to study the effects of treatment and storage time, and the interaction of these factors on the shelf-stability of CMC. An ANOVA was performed to obtain the mean squares and P-values, using the GLM procedure available in R software. When a significant treatment, time, or interaction effect (P < 0.05) was detected, differences among means were compared using the least significant difference test.

      RESULTS AND DISCUSSION

      Composition of Skim Milk

      The composition of pasteurized skim milk used to produce CMC is shown in Table 1. We found a small amount of variation (0.12, 0.19, 0.02, 0.01, 0.17, 0.03, and 0.04% SD for the TS, TN, NCN, NPN, CN, SP, and ash contents, respectively), among the 3 replicates. The CN as a percentage of TN (CN%TN) and the CN%TP were typical (
      • Beckman S.L.
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Production efficiency of micellar casein concentrate using polymeric spiral-wound microfiltration membranes.
      ;
      • Hurt E.
      • Barbano D.M.
      Processing factors that influence casein and serum protein separation by microfiltration1.
      ;
      • Adams M.C.
      • Barbano D.M.
      Serum protein removal from skim milk with a 3-stage, 3× ceramic Isoflux membrane process at 50°C.
      ) and ranged from 79.23 to 80.45% for CN%TN and from 84.15 to 85.06% for CN%TP. The skim milk was pasteurized at 76°C for 16 s. Temperatures over 70°C can lead to interactions between β-LG and κ-CN through disulfide bonding (
      • Singh H.
      Heat-induced changes in casein, including interactions with whey proteins.
      ;
      • Hammam A.R.A.
      • Martínez-Monteagudo S.I.
      • Metzger L.E.
      Progress in micellar casein concentrate: Production and applications.
      ). Consequently, an elevated pasteurization temperature can decrease the SP available for removal during MF (
      • Hurt E.
      • Barbano D.M.
      Processing factors that influence casein and serum protein separation by microfiltration1.
      ;
      • Hammam A.R.A.
      • Martínez-Monteagudo S.I.
      • Metzger L.E.
      Progress in micellar casein concentrate: Production and applications.
      ).
      Table 1Mean (n = 3) composition (% by weight) of the pasteurized skim milk
      ReplicateComposition
      TN = total nitrogen × 6.38; NCN = noncasein nitrogen × 6.38; NPN = nonprotein nitrogen × 6.38; CN = TN − NCN; SP = serum protein: NCN − NPN; CN%TN = CN as a percentage of TN; CN%TP = CN as a percentage of TP.
      TSTNNCNNPNCNSPAshCN%TNCN%TP
      18.793.100.640.202.460.440.7479.2384.78
      28.943.480.680.182.800.490.6680.4585.06
      39.043.260.660.172.600.490.6779.5784.15
      Mean8.923.280.660.192.620.470.6979.7584.66
      SD0.120.190.020.010.170.030.040.620.028
      1 TN = total nitrogen × 6.38; NCN = noncasein nitrogen × 6.38; NPN = nonprotein nitrogen × 6.38; CN = TN − NCN; SP = serum protein: NCN − NPN; CN%TN = CN as a percentage of TN; CN%TP = CN as a percentage of TP.

      Composition of Permeate

      The composition of permeate from each stage of MF is presented in Table 2. The TS, TN, NPN, and SP contents of the permeate significantly decreased (P < 0.05) with each subsequent stage of MF. The SP concentrations in the permeate were 0.33, 0.26, and 0.15% for the first, second, and third stages, respectively, using 3× CF in the first and second stages and approximately 5× in the third stage, with 1 DF stage applied to the retentate of the first stage. The SP content in the first-stage permeate (0.33%) was lower than SP in the permeate portion of the pasteurized skim milk (0.47%). This was primarily due to some rejection of SP by the membrane. Similar results have been reported by
      • Tremblay-Marchand D.
      • Doyen A.
      • Britten M.
      • Pouliot Y.
      A process efficiency assessment of serum protein removal from milk using ceramic graded permeability microfiltration membrane.
      using a 0.1-µm GP ceramic membrane and 3 stages of 3× CF with DF. That study found that permeate of the first, second, and third stages contained 0.35, 0.13, and 0.06% SP, respectively, from MF of skim milk that contained 0.49% SP. However, another study found that permeate of the first, second, and third stages produced from a 0.1-µm GP membrane using 3 stages of 3× CF with DF contained 0.51, 0.25, and 0.13% of SP, respectively, from MF of skim milk that had 0.54% SP (
      • Zulewska J.
      • Barbano D.M.
      The effect of linear velocity and flux on performance of ceramic graded permeability membranes when processing skim milk at 50°C.
      ). The processing conditions, such as pasteurization temperature could affect SP removal in the permeate of the first stage, due to the denaturation of SP on CN. However, the SP removal in the second and third stages was similar to our study. Although the third stage of our study was processed in a recirculation mode using 5× CF without DF, to increase the solids, the SP removed in the permeate of this stage was similar to the typical feed-and-bleed mode using 3× CF with DF (
      • Zulewska J.
      • Barbano D.M.
      The effect of linear velocity and flux on performance of ceramic graded permeability membranes when processing skim milk at 50°C.
      ), and this can be related to that the majority of SP being removed during the first and second stages of MF. The ash content decreased significantly (P < 0.05) between the first and second stages; however, the ash content in the third stage was not significantly different (P > 0.05) from the second stage. This is due to the majority of the remaining ash content after the second stage being bound within the CN micelle, which is rejected by the membrane.
      Table 2Mean (n = 3) composition (% by weight) of permeates produced from the 3 stages of microfiltration (MF) using 0.1-μm ceramic graded permeability membranes
      MF stageComposition
      TN = total nitrogen × 6.38; NPN = nonprotein nitrogen × 6.3; SP = serum protein: TN − NPN.
      TSTNNPNSPAsh
      16.00
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.52
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.19
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.33
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.47
      Means in the same column not sharing a common superscript are different (P < 0.05).
      22.02
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.35
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.08
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.26
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.18
      Means in the same column not sharing a common superscript are different (P < 0.05).
      31.60
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.20
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.05
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.15
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.12
      Means in the same column not sharing a common superscript are different (P < 0.05).
      SEM0.700.050.020.030.05
      R20.990.950.970.890.93
      a–c Means in the same column not sharing a common superscript are different (P < 0.05).
      1 TN = total nitrogen × 6.38; NPN = nonprotein nitrogen × 6.3; SP = serum protein: TN − NPN.

      Composition of Retentate

      The composition of the retentate from the 3 MF stages is shown in Table 3. The TS and TN content decreased between stage 1 and stage 2 due to the passage of soluble minerals, lactose, and SP through the membrane. The composition of the first and second stages of MF retentate in our study was similar to the typical composition of retentate produced from those 2 stages using a 3× CF with DF (
      • Beckman S.L.
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Production efficiency of micellar casein concentrate using polymeric spiral-wound microfiltration membranes.
      ;
      • Zulewska J.
      • Barbano D.M.
      The effect of linear velocity and flux on performance of ceramic graded permeability membranes when processing skim milk at 50°C.
      ;
      • Tremblay-Marchand D.
      • Doyen A.
      • Britten M.
      • Pouliot Y.
      A process efficiency assessment of serum protein removal from milk using ceramic graded permeability microfiltration membrane.
      ). The composition of retentate produced from MF of skim milk can be slightly different based on the composition of initial milk and processing conditions. The TS, TN, TP, CN, and ash values of the retentate were significantly higher (P < 0.05) in the third stage compared with the first and second stages, due to the rejection of CN by the membrane. Because two-thirds of the ash is bound to the CN (
      • Hurt E.
      • Barbano D.M.
      Processing factors that influence casein and serum protein separation by microfiltration1.
      ), the ash content in the third stage increased with increasing CN content. The SP content in the retentate decreased (P < 0.05) with each successive stage of MF due to passage through the membrane into the permeate. As expected, the CN%TP and TN as a percentage of TS (TN%TS) increased significantly (P < 0.05) with each subsequent stage of MF. The CN%TP increased from 84.66% in the skim milk to approximately 98% in the final CMC product.
      Table 3Mean (n = 3) composition (% by weight) of retentates produced from the 3 stages of microfiltration (MF) using 0.1-μm ceramic graded permeability membranes
      MF stageComposition
      TN = total nitrogen × 6.38; NCN = noncasein nitrogen × 6.38; NPN = nonprotein nitrogen × 6.38; TP = true protein: TN − NPN; CN = TN − NCN; SP = serum protein: NCN − NPN; CN%TP = CN as a percentage of TP; TN%TS = TN as a percentage of TS.
      TSTNNCNNPNTPCNSPAshCN%TPTN%TS
      114.11
      Means in the same column not sharing a common superscript are different (P < 0.05).
      8.14
      Means in the same column not sharing a common superscript are different (P < 0.05).
      1.06
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.21
      Means in the same column not sharing a common superscript are different (P < 0.05).
      7.92
      Means in the same column not sharing a common superscript are different (P < 0.05).
      7.07
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.85
      Means in the same column not sharing a common superscript are different (P < 0.05).
      1.11
      Means in the same column not sharing a common superscript are different (P < 0.05).
      89.23
      Means in the same column not sharing a common superscript are different (P < 0.05).
      57.75
      Means in the same column not sharing a common superscript are different (P < 0.05).
      29.64
      Means in the same column not sharing a common superscript are different (P < 0.05).
      7.05
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.66
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.09
      Means in the same column not sharing a common superscript are different (P < 0.05).
      6.96
      Means in the same column not sharing a common superscript are different (P < 0.05).
      6.38
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.58
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.78
      Means in the same column not sharing a common superscript are different (P < 0.05).
      91.65
      Means in the same column not sharing a common superscript are different (P < 0.05).
      73.21
      Means in the same column not sharing a common superscript are different (P < 0.05).
      325.41
      Means in the same column not sharing a common superscript are different (P < 0.05).
      21.75
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.55
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.09
      Means in the same column not sharing a common superscript are different (P < 0.05).
      21.65
      Means in the same column not sharing a common superscript are different (P < 0.05).
      21.20
      Means in the same column not sharing a common superscript are different (P < 0.05).
      0.45
      Means in the same column not sharing a common superscript are different (P < 0.05).
      2.00
      Means in the same column not sharing a common superscript are different (P < 0.05).
      97.92
      Means in the same column not sharing a common superscript are different (P < 0.05).
      85.59
      Means in the same column not sharing a common superscript are different (P < 0.05).
      SEM2.001.930.0600.0171.941.950.050.151.435.05
      R20.980.990.960.930.990.990.930.980.980.98
      a–c Means in the same column not sharing a common superscript are different (P < 0.05).
      1 TN = total nitrogen × 6.38; NCN = noncasein nitrogen × 6.38; NPN = nonprotein nitrogen × 6.38; TP = true protein: TN − NPN; CN = TN − NCN; SP = serum protein: NCN − NPN; CN%TP = CN as a percentage of TP; TN%TS = TN as a percentage of TS.

      SP Removal

      Theoretical SP removal, actual SP removal, and CF of 3 MF stages are presented in Table 4. Theoretically, the cumulative SP removal from skim milk should be 67.8, 89.8, and 97.8% in the first, second, and third stages, respectively. The actual SP removal in our study was 46.20, 77.20, and 83.10% in the first, second, and third stages, respectively. It was expected that the theoretical SP removal would be higher than the actual SP removal from skim milk, because the theory of SP removal assumes that the membrane does not reject SP, but in reality some SP is rejected by the membrane.
      • Hurt E.
      • Barbano D.M.
      Processing factors that influence casein and serum protein separation by microfiltration1.
      reported percentages of cumulative SP removal from MF of skim milk (CN%TP = 85.00%) as approximately 56, 74, and 80% in the first, second, and third stages, respectively, using 3× CF with DF water within stages, which was close to our values. Another study has reported that SP removal in the first, second, and third stages was 47.0, 64.7, and 73.2%, respectively, using GP MF system (
      • Tremblay-Marchand D.
      • Doyen A.
      • Britten M.
      • Pouliot Y.
      A process efficiency assessment of serum protein removal from milk using ceramic graded permeability microfiltration membrane.
      ). The slight difference in SP removal between our study and others can be related to the initial milk composition and processing conditions, which can reflect on SP removal in the first, second, and third stages of MF. However,
      • Tremblay-Marchand D.
      • Doyen A.
      • Britten M.
      • Pouliot Y.
      A process efficiency assessment of serum protein removal from milk using ceramic graded permeability microfiltration membrane.
      have reported that the calculation of SP removal based on the mass balance might not be accurate because it does not take into consideration the remaining SP in the retentate.
      Table 4Mean (n = 3) serum protein (SP) removal for the 3 stages of microfiltration (MF) using 0.1-μm ceramic graded permeability membranes, determined by Kjeldahl analysis and concentration factor (CF)
      MF stageTheoretical SP removal
      Data in this column represent theoretical SP removal, assuming no rejection of serum proteins and complete rejection of CN: &percnt;SPremovalofeachstage=(&percnt;SPinpermeateofeachstage×amountofpermeate&percnt;SPinfeed×amountoffeed)×100.
      (%)
      Actual SP removal (%)CF
      CF = concentration factor.
      167.7846.20
      Means in the same column not sharing a common superscript are different (P < 0.05).
      3.01
      Means in the same column not sharing a common superscript are different (P < 0.05).
      289.7877.20
      Means in the same column not sharing a common superscript are different (P < 0.05).
      3.01
      Means in the same column not sharing a common superscript are different (P < 0.05).
      397.7883.10
      Means in the same column not sharing a common superscript are different (P < 0.05).
      5.12
      Means in the same column not sharing a common superscript are different (P < 0.05).
      SEM5.750.41
      R20.980.71
      a–c Means in the same column not sharing a common superscript are different (P < 0.05).
      1 Data in this column represent theoretical SP removal, assuming no rejection of serum proteins and complete rejection of CN:&percnt;SPremovalofeachstage=(&percnt;SPinpermeateofeachstage×amountofpermeate&percnt;SPinfeed×amountoffeed)×100.
      2 CF = concentration factor.
      The CF is the ratio of feed mass for each stage to the retentate mass of the same stage. In practice, the ability of the system to control the CF precisely may vary and may be limited, but the CF was close to the targeted values of 3× in the first and second stages and 5× for the third stage.

      Composition of CMC

      The composition of CMC after manufacturing is exemplified in Table 5. The addition of sodium chloride or sodium citrate to CMC did not result in significant differences (P > 0.05) in the TS, TN, NCN, NPN, TP, CN, CN%TP, and TN%TS after processing (Table 5). However, we found a significant difference (P < 0.05) in the ash content between control and T2. This was expected due to the addition of sodium chloride and sodium citrate to T2. The addition of 1% sodium chloride in T1 and 1% sodium chloride + 1% sodium citrate in T2 was expected to increase the TS content by 1% and 2% in T1 and T2, respectively, relative to the control. The ash content should also be higher in T1 and T2 by 1% and 2%, respectively, compared with control. However, the TS and ash contents were not as high as expected in T1 and T2 compared with the control. We theorized that this could occur because those salts were added and mixed manually in the CMC, and this may not have evenly dispersed the salts when samples were collected.
      Table 5Mean (n = 3) composition (% by weight) of the concentrated micellar casein (CMC), measured after manufacturing (d = 0)
      Treatment
      Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      Composition
      TN = total nitrogen × 6.38; NCN = noncasein nitrogen × 6.38; NPN = nonprotein nitrogen × 6.38; TP = true protein: TN − NPN; CN = TN − NCN; CN%TP = CN as a percentage of TP; TN%TS = TN as a percentage of TS.
      TSTNNCNNPNTPCNAshCN%TPTN%TS
      Control25.4121.750.550.0921.6521.202.00
      Means in the same column not sharing a common superscript are different (P < 0.05).
      97.8985.60
      T125.6221.230.550.0821.1420.672.26
      Means in the same column not sharing a common superscript are different (P < 0.05).
      97.8082.86
      T226.1321.150.550.1321.0220.602.52
      Means in the same column not sharing a common superscript are different (P < 0.05).
      98.0280.95
      SEM0.480.380.000.010.380.460.090.050.85
      a,b Means in the same column not sharing a common superscript are different (P < 0.05).
      1 Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      2 TN = total nitrogen × 6.38; NCN = noncasein nitrogen × 6.38; NPN = nonprotein nitrogen × 6.38; TP = true protein: TN − NPN; CN = TN − NCN; CN%TP = CN as a percentage of TP; TN%TS = TN as a percentage of TS.
      The NCN and NPN contents of CMC at 0 and 60 d of storage are shown in Table 6, and the ANOVA with mean squares and P-values for NCN and NPN during shelf-life are shown in Table 7. The NCN and NPN of the CMC were monitored for 60 d to determine the level of proteolysis (Table 6). No significant difference (P > 0.05) was found between treatments in the NPN or NCN content at 0 d. However, a significant difference (P < 0.05) was observed in the NCN content during 60 d of storage at 4°C (Table 7). The increase in NCN could be a result of the degradation of β-CN by proteolytic enzymes, such as plasmin, which produces γ-CN and small peptides.
      Table 6Mean (n = 3) noncasein nitrogen (NCN) and NPN of the concentrated micellar casein (CMC) measured over 60 d of storage at 4°C
      Composition
      NCN = noncasein nitrogen × 6.38; NPN = nonprotein nitrogen × 6.38.
      TimeTreatment
      Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      Mean
      ControlT1T2
      NCN00.550.550.550.55
      Means in the same column not sharing a common superscript are different (P < 0.05).
      600.760.820.940.84
      Means in the same column not sharing a common superscript are different (P < 0.05).
      Mean0.660.680.74
      NPN00.090.080.130.10
      600.120.130.140.13
      Mean0.100.110.14
      a,b Means in the same column not sharing a common superscript are different (P < 0.05).
      1 NCN = noncasein nitrogen × 6.38; NPN = nonprotein nitrogen × 6.38.
      2 Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      Table 7Mean squares and P-values (in parentheses) of the noncasein nitrogen (NCN) and NPN for the concentrated micellar casein (CMC) measured over 60 d of storage at 4°C
      FactordfNCNNPN
      Replication20.021 (0.26)0.0099 (<0.05)
      Treatment
      Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      20.012 (0.22)0.0012 (0.51)
      Time
      Time = 0 and 60 d of storage.
      10.38 (<0.05)0.0035 (0.18)
      Treatment × time20.012 (0.23)0.0009 (0.61)
      Error100.0070.001
      1 Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      2 Time = 0 and 60 d of storage.

      Microbiological Analysis of CMC

      The TBC (log cfu/mL) of CMC treatments during storage is shown in Table 8, and the mean squares and P-values for the TBC are presented in Table 9. The mean TBC of the CMC at d 0 (immediately after manufacturing) were 2.6, 2.5, and 2.8 log cfu/mL for control, T1, and T2, respectively (Table 8). This was slightly different from the 2.1 log cfu/mL in pasteurized CMC (18% TP and 22% TS) at d 0 reported by
      • Amelia I.
      • Barbano D.M.
      Production of an 18% protein liquid micellar casein concentrate with a long refrigerated shelf life.
      . All tanks and cans were covered during the experiment to reduce environmental contamination. The addition of sodium chloride or sodium citrate to CMC did not result in significant differences (P > 0.05) in the TBC (Table 9). The mean TBC increased significantly (P < 0.05) in all treatments during 60 d of storage at 4°C.
      • Amelia I.
      • Barbano D.M.
      Production of an 18% protein liquid micellar casein concentrate with a long refrigerated shelf life.
      defined the end of the shelf-life when the TBC is >4.3 log cfu/mL, which is the legal limit for the shelf-life of pasteurized milk (
      • FDA (Food and Drug Administration)
      Grade “A” Pasteurized Milk Ordinance.
      ). As a result, the end of the shelf-life of CMC in our study is 60 d when it is stored at 4°C. Additionally, no coliforms, yeast, or mold were detected (<1 estimated count) at any time point during 60 d of storage. It has been reported that CMC (18.27% TP and 21.78% TS) could be stored for 16 wk at 4°C with minimal proteolysis and low bacterial count (
      • Amelia I.
      • Barbano D.M.
      Production of an 18% protein liquid micellar casein concentrate with a long refrigerated shelf life.
      ). That study had a longer shelf-life than our study because they used UF before starting 3 stages of MF and at the end of the process to remove as much lactose and NPN as possible, considered nutrient sources for the microbes, and this, in turn, can result in longer shelf-life.
      Table 8Mean (n = 3) log cfu/mL of total aerobic bacterial count (TBC) of the concentrated micellar casein (CMC) measured every 30 d over 60 d of storage at 4°C
      TBCTimeTreatment
      Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      ControlT1T2
      Log cfu/mL02.632.502.79
      303.534.044.33
      604.334.065.30
      SEM0.450.400.46
      1 Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      Table 9Mean squares and P-values (in parentheses) of log cfu/mL of total aerobic bacterial count (TBC) of the concentrated micellar casein (CMC) measured every 30 d over 60 d of storage at 4°C
      FactordfTBC (log cfu/mL)
      Replication23.06 (0.08)
      Treatment
      Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      21.17 (0.35)
      Time
      Time = 0, 30, and 60 d of storage.
      28.75 (<0.05)
      Treatment × time40.32 (0.87)
      Error161.05
      1 Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      2 Time = 0, 30, and 60 d of storage.

      Protein Fractions

      Skim Milk and CMC Treatments

      A representative CGE electrophoretogram for skim milk is shown in Figure 2. Also, representative electrophoretograms of control, T1, and T2 CMC are presented in Figures 3, 4, and 5, respectively. The protein fractions are migrated and separated based on their MW. Among the CN fractions, the β-CN peak migrated first, followed by αS1-CN although the MW of αS1-CN is lower. Other researchers have reported similar results (
      • Creamer L.K.
      • Richardson T.
      Anomolous behaviour of bovine αS1- and β-caseins on gel electrophoresis in sodium dodecyl sulfate buffers.
      ;
      • Anema S.G.
      The use of “lab-on-a-chip” microfluidic SDS electrophoresis technology for the separation and quantification of milk proteins.
      ;
      • Salunke P.
      Impact of transglutaminase on the functionality of milk protein concentrate and micellar casein concentrate.
      ).
      • Creamer L.K.
      • Richardson T.
      Anomolous behaviour of bovine αS1- and β-caseins on gel electrophoresis in sodium dodecyl sulfate buffers.
      reported that αS1-CN has a reduced electrophoretic velocity due to its negatively charged regions, which extend its conformation in the presence of SDS, thereby giving an increased apparent size and slower migration under SDS-PAGE conditions (
      • Creamer L.K.
      • Richardson T.
      Anomolous behaviour of bovine αS1- and β-caseins on gel electrophoresis in sodium dodecyl sulfate buffers.
      ;
      • Anema S.G.
      The use of “lab-on-a-chip” microfluidic SDS electrophoresis technology for the separation and quantification of milk proteins.
      ). Although κ-CN has a low MW compared with other caseins, it eluted last after all other CN fractions. The late migration of κ-CN is attributable to the glycosylation of κ-CN (
      • Walstra P.
      • Jenness R.
      Dairy Chemistry and Physics.
      ;
      • Anema S.G.
      The use of “lab-on-a-chip” microfluidic SDS electrophoresis technology for the separation and quantification of milk proteins.
      ;
      • Salunke P.
      • Marella C.
      • Metzger L.E.
      Use of capillary gel electrophoresis for quantification of individual milk proteins in ultra- and microfiltration retentate.
      ). Changes in any protein fraction, such as hydrolysis or crosslinking, that change the MW lead to changes in the peaks' heights and migration times.
      Figure thumbnail gr2
      Figure 2Capillary gel electrophoretogram of skim milk. AU = absorbance units.
      Figure thumbnail gr3
      Figure 3Capillary gel electrophoretogram of concentrated micellar casein, control. AU = absorbance units.
      Figure thumbnail gr4
      Figure 4Capillary gel electrophoretogram of concentrated micellar casein (CMC) T1 (CMC + 1% sodium chloride). AU = absorbance units.
      Figure thumbnail gr5
      Figure 5Capillary gel electrophoretogram of concentrated micellar casein (CMC) T2 (CMC+ 1% sodium chloride and 1% sodium citrate). AU = absorbance units.
      The percent of protein fractions in pasteurized skim milk, control, T1, and T2 CMC measured by CGE are shown in Table 10. The percentage of peak areas in skim milk for β-CN, αS1-CN, αS2-CN, κ-CN, and γ-CN (CN fractions) were 33.8, 35.2, 8.5, 4.3, and 1.1%, respectively, whereas the percentages of β-LG and α-LA (SP fractions) obtained were 9.9 and 5.2%, respectively. It has been reported that the mean CN fractions in normal milk, including β-CN, αS1-CN, αS2-CN, κ-CN, and γ-CN were approximately 33.8, 34.4, 8.5, 8.5, and 3.0%, respectively, and means of SP fractions of β-LG and α-LA were 9.5 and 5.1%, respectively (
      • Walstra P.
      • Jenness R.
      Dairy Chemistry and Physics.
      ;
      • Fox P.F.
      • McSweeney P.L.H.
      Dairy Chemistry and Biochemistry.
      ;
      • Farrell Jr., H.M.
      • Jimenez-Flores R.
      • Bleck G.T.
      • Brown E.M.
      • Butler J.E.
      • Creamer L.K.
      • Hicks C.L.
      • Hollar C.M.
      • Ng-Kwai-Hang K.F.
      • Swaisgood H.E.
      Nomenclature of the proteins of cows' milk—Sixth revision.
      ), which are similar to our results. However, we noticed that the κ-CN determined by CGE is lower than we expected, and this is due to the carbohydrate moiety of κ-CN, which is difficult to detect using UV detection (214 nm) in CGE. The percentages of peak areas in control CMC for β-CN, αS1-CN, αS2-CN, κ-CN, and γ-CN determined by CGE were approximately 37.0, 39.1, 8.6, 4.4, and 1.6%, respectively, and the percentage of SP fractions for β-LG and α-LA were approximately 3.7 and 3.3%, respectively. The CN fractions of β-CN, αS1-CN, and γ-CN were higher (P < 0.05) in CMC than in skim milk, due to the concentration of CN during MF. As presented in Table 10 and the electrophoretogram figures of skim milk and CMC, the β-LG and α-LA are significantly higher (P < 0.05) in skim milk relative to CMC. This is expected because MF permeates SP through the membrane, which results in a lower β-LG and α-LA in the CMC. No differences in αS2-CN and κ-CN (P > 0.05) were detected between skim milk and CMC. The higher γ-CN in CMC compared with skim milk could be a result of proteolysis by plasmin during the 2 d of manufacturing. The CN and SP ratios measured using electrophoresis have been reported to range from 74 to 86% for CN and from 14 to 26% for SP (
      • Walstra P.
      • Jenness R.
      Dairy Chemistry and Physics.
      ;
      • Fox P.F.
      • McSweeney P.L.H.
      Dairy Chemistry and Biochemistry.
      ;
      • Farrell Jr., H.M.
      • Jimenez-Flores R.
      • Bleck G.T.
      • Brown E.M.
      • Butler J.E.
      • Creamer L.K.
      • Hicks C.L.
      • Hollar C.M.
      • Ng-Kwai-Hang K.F.
      • Swaisgood H.E.
      Nomenclature of the proteins of cows' milk—Sixth revision.
      ). The percentage of CN%TP in skim milk using CGE in our study was 84.61%, which is similar to the value of 84.66% determined using Kjeldahl analysis (Table 1).
      Table 10Mean (n = 3) relative protein fractions measured by using capillary gel electrophoresis of pasteurized skim milk and treatments of concentrated micellar casein (CMC)
      Treatment
      Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      β-CN
      Each fraction calculated as a percentage of total CN area.
      αS1-CN
      Each fraction calculated as a percentage of total CN area.
      αS2-CN
      Each fraction calculated as a percentage of total CN area.
      κ-CN
      Each fraction calculated as a percentage of total CN area.
      γ-CN
      Each fraction calculated as a percentage of total CN area.
      α-LA
      Each fraction calculated as a percentage of total serum protein (SP) area.
      β-LG
      Each fraction calculated as a percentage of total serum protein (SP) area.
      Peptides
      Peptides = peptide peaks (10–20 kDa) other than α-LA and β-LG.
      CNSP
      Skim milk33.76
      Means in the same column not sharing a common superscript are different (P < 0.05).
      35.24
      Means in the same column not sharing a common superscript are different (P < 0.05).
      8.504.311.13
      Means in the same column not sharing a common superscript are different (P < 0.05).
      5.17
      Means in the same column not sharing a common superscript are different (P < 0.05).
      9.91
      Means in the same column not sharing a common superscript are different (P < 0.05).
      1.96
      Means in the same column not sharing a common superscript are different (P < 0.05).
      82.95
      Means in the same column not sharing a common superscript are different (P < 0.05).
      15.08
      Means in the same column not sharing a common superscript are different (P < 0.05).
      Control36.97
      Means in the same column not sharing a common superscript are different (P < 0.05).
      39.15
      Means in the same column not sharing a common superscript are different (P < 0.05).
      8.594.401.63
      Means in the same column not sharing a common superscript are different (P < 0.05).
      3.30
      Means in the same column not sharing a common superscript are different (P < 0.05).
      3.75
      Means in the same column not sharing a common superscript are different (P < 0.05).
      2.19
      Means in the same column not sharing a common superscript are different (P < 0.05).
      90.74
      Means in the same column not sharing a common superscript are different (P < 0.05).
      7.06
      Means in the same column not sharing a common superscript are different (P < 0.05).
      T137.80
      Means in the same column not sharing a common superscript are different (P < 0.05).
      39.49
      Means in the same column not sharing a common superscript are different (P < 0.05).
      7.783.651.98
      Means in the same column not sharing a common superscript are different (P < 0.05).
      4.07
      Means in the same column not sharing a common superscript are different (P < 0.05).
      3.33
      Means in the same column not sharing a common superscript are different (P < 0.05).
      1.98
      Means in the same column not sharing a common superscript are different (P < 0.05).
      90.72
      Means in the same column not sharing a common superscript are different (P < 0.05).
      7.40
      Means in the same column not sharing a common superscript are different (P < 0.05).
      T237.25
      Means in the same column not sharing a common superscript are different (P < 0.05).
      38.51
      Means in the same column not sharing a common superscript are different (P < 0.05).
      7.524.422.82
      Means in the same column not sharing a common superscript are different (P < 0.05).
      3.73
      Means in the same column not sharing a common superscript are different (P < 0.05).
      3.75
      Means in the same column not sharing a common superscript are different (P < 0.05).
      1.88
      Means in the same column not sharing a common superscript are different (P < 0.05).
      90.53
      Means in the same column not sharing a common superscript are different (P < 0.05).
      7.48
      Means in the same column not sharing a common superscript are different (P < 0.05).
      SEM0.910.970.260.180.350.401.570.071.931.94
      SD1.821.940.530.360.710.803.150.133.863.88
      a,b Means in the same column not sharing a common superscript are different (P < 0.05).
      1 Control = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate.
      2 Each fraction calculated as a percentage of total CN area.
      3 Each fraction calculated as a percentage of total serum protein (SP) area.
      4 Peptides = peptide peaks (10–20 kDa) other than α-LA and β-LG.
      No significant difference (P > 0.05) was detected between control, T1, and T2 in the protein fraction contents at d 0. Similar results for MCC produced using SW membranes have been reported using CGE (
      • Salunke P.
      Impact of transglutaminase on the functionality of milk protein concentrate and micellar casein concentrate.
      ). In Salunke's study, β-CN, αS1-CN, αS2-CN, κ-CN, and γ-CN was 36.78, 35.76, 9.59, 6.40, and 3.72%, respectively, versus 37.0, 39.1, 8.6, 4.4, and 1.6%, respectively, in our study. The percentages of β-LG and α-LA were approximately 3.89 and 1.09%, respectively, in Salunke's study, versus 3.7 and 3.3%, respectively, in our study. The protein fractions of CMC treatments during the 60 d of storage are shown in Figure 6. No significant differences (P > 0.05) were observed between the control, T1, and T2 at 0, 30, and 60 d of storage at 4°C, although we observed a significant increase (P < 0.05) in NCN during 60 d of storage using Kjeldahl analysis (Table 6). This could be related to the fact that CGE is more accurate and determines each protein fraction individually.
      Figure thumbnail gr6
      Figure 6Mean (n = 3) relative protein fractions (%) observed in capillary gel electrophoresis of concentrated micellar casein (CMC) during 60 d of storage at 4°C. C = CMC; T1 = CMC + 1% sodium chloride; T2 = CMC + 1% sodium chloride and 1% sodium citrate. 0 = immediately after manufacturing; 30 = d 30 of storage; 60 = d 60 of storage.

      Permeate

      The protein fractions present in the permeate created during MF process are presented in Table 11. The percentages of β-LG and α-LA content were calculated in the permeate of each stage. The percentages of β-LG and α-LA in the first stage were approximately 70.0 and 26.9%, respectively.
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C.
      reported that the percentages of β-LG and α-LA in MF permeate were 76.3 and 23.7%, respectively, in the first stage using a 3× CF in GP ceramic membranes, which is close to our values. The percentages of β-LG and α-LA in the second stage were 75.04 and 20.8%, respectively, and 72.1 and 22.3% in the third stage. It has been reported that the relative percentages of β-LG to β-LG plus α-LA in permeate were 71.5, 73.0, and 74.6% in the first, second, and third stages, respectively, with a 3× CF using Isoflux membranes (
      • Adams M.C.
      • Barbano D.M.
      Serum protein removal from skim milk with a 3-stage, 3× ceramic Isoflux membrane process at 50°C.
      ). The relative percentages of β-LG to β-LG plus α-LA in the permeate of our study were 72.9, 78.2, and 76.4% in the first, second, and third stages, respectively, which are close to Adams and Barbano's values. These small differences may be related to the composition of skim milk, processing conditions, type of membranes, or experimental error (such as temperatures, flux, CF, and DF). Peptides increased, but not significantly (P > 0.05), from 3.09% in the first stage to 5.6% in the third stage. Interestingly, a small amount of β-CN (0.83%) was observed in the permeate of the second stage. It has been reported that the permeate of the first-stage MF had a 4.93% CN using GP ceramic membranes and 1.23% using UTP membranes (
      • Zulewska J.
      • Newbold M.
      • Barbano D.M.
      Efficiency of serum protein removal from skim milk with ceramic and polymeric membranes at 50°C.
      ). After each MF stage in our process, the retentate was cooled and kept at 4°C during cleaning the MF system. We hypothesized that β-CN is transferred from the CN micelles at 4°C with the addition of DF water. The amphiphilic nature (charged polar N-terminal region and less polar C-terminal region) of β-CN allows it to transit from the micelle into the serum phase depending on temperature (
      • O'Connell J.E.
      • Grinberg V.Y.
      • de Kruif C.G.
      Association behavior of β-casein.
      ) during heating the retentate to 50°C before MF. We may not have held the CMC long enough at 50°C for the β-CN to transit back into the CN micelles. As a result, solubilized β-CN (which has low MW) passed through the membrane to the permeate in stage 2.
      Table 11Mean (n = 3) relative protein fractions measured by using capillary gel electrophoresis of permeate during the 3 stages of microfiltration processing from skim milk
      Stageβ-CN
      Each fraction calculated as a percentage of total CN area.
      α-LA
      Each fraction calculated as a percentage of total serum protein (SP) area.
      β-LG
      Each fraction calculated as a percentage of total serum protein (SP) area.
      Peptides
      Peptides = peptide peaks (10–20 kD) other than α-LA and β-LG.
      CNSP
      126.87
      Means in the same column not sharing a common superscript are different (P < 0.05).
      70.04
      Means in the same column not sharing a common superscript are different (P < 0.05).
      3.0996.91
      20.8320.85
      Means in the same column not sharing a common superscript are different (P < 0.05).
      75.04
      Means in the same column not sharing a common superscript are different (P < 0.05).
      3.280.8395.89
      322.28
      Means in the same column not sharing a common superscript are different (P < 0.05).
      72.10
      Means in the same column not sharing a common superscript are different (P < 0.05).
      5.6294.38
      SEM1.000.950.550.52
      R20.820.580.540.48
      a,b Means in the same column not sharing a common superscript are different (P < 0.05).
      1 Each fraction calculated as a percentage of total CN area.
      2 Each fraction calculated as a percentage of total serum protein (SP) area.
      3 Peptides = peptide peaks (10–20 kD) other than α-LA and β-LG.

      CONCLUSIONS

      A process to produce CMC with a 60-d refrigerated shelf-life was developed. This study determined that CMC can be manufactured using a ceramic GP MF system with >25% TS and >95% CN%TP. The addition of sodium chloride or sodium citrate did not significantly (P > 0.05) affect the composition of CMC immediately after manufacturing (at d 0) and during the 60 d of storage at 4°C. However, the NCN content increased significantly (P < 0.05) during 60 d of storage in all treatments. The TBC also increased significantly (P < 0.05) in all treatments during 60 d of storage at 4°C. This indicates that the addition of those salts in T1 and T2 did not decrease the rate of proteolysis. The influence of the increase in NCN, as well as addition of sodium chloride or sodium citrate to CMC during storage on process cheese characteristics, will be evaluated in subsequent studies.

      ACKNOWLEDGMENTS

      The authors acknowledge and thank the Midwest Dairy Foods Research Center (St. Paul, MN) for their financial support. The authors have not stated any conflicts of interest.

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