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Short communication: Evaluation of a sol-gel–based stainless steel surface modification to reduce fouling and biofilm formation during pasteurization of milk
Milk fouling and biofilms are common problems in the dairy industry across many types of processing equipment. One way to reduce milk fouling and biofilms is to modify the characteristics of milk contact surfaces. This study examines the viability of using Thermolon (Porcelain Industries Inc., Dickson, TN), a sol-gel-based surface modification of stainless steel, during thermal processing of milk. We used stainless steel 316L (control) and sol-gel-modified coupons in this study to evaluate fouling behavior and bacterial adhesion. The surface roughness as measured by an optical profiler indicated that the control coupons had a slightly smoother finish. Contact angle measurements showed that the modified surface led to a higher water contact angle, suggesting a more hydrophobic surface. The modified surface also had a lower surface energy (32.4 ± 1.4 mN/m) than the control surface (41.36 ± 2.7 mN/m). We evaluated the susceptibility of control and modified stainless steel coupons to fouling in a benchtop plate heat exchanger. We observed a significant reduction in the amount of fouled layer on modified surfaces. We found an average fouling weight of 19.21 mg/cm2 and 0.37 mg/cm2 on the control and modified stainless steel coupons, respectively. We also examined the adhesion of Bacillus and biofilm formation, and observed that the modified stainless steel surface offered greater resistance to biofilm formation. Overall, the Thermolon-modified surface showed potential in the thermal processing of milk, offering significantly lower fouling and bacterial attachment than the control surface.
). Fouling during pasteurization of milk on the stainless steel surfaces of plate heat exchangers can be classified as type A fouling, consisting of 50 to 60% proteins and 30 to 35% minerals. Fouling necessitates frequent clean-in-place, leading to increased down time and reduced production (
). The fouled layer and extracellular polymeric substances guard the microorganisms and help them survive most of the cleaning protocols used in the dairy industry. Biofilms formed by Bacillus spp. are resistant to high stress, are very hydrophobic, and can easily attach to processing equipment (
One way to control undesirable fouling and biofilms is to modify the surface properties of the processing equipment. Sol-gel surface modification converts inorganic liquid substances into a gel that can be applied on metal surfaces to improve the surface properties. Thermolon is a sol-gel-based surface modification developed from an inorganic ceramic polymer. According to its manufacturer (Porcelain Industries Inc., Dickson, TN), “It is environmentally friendly, durable, and most importantly, it has been approved by the FDA as a food contact surface (FDA 21CFR 175.300).” The objective of the present study was to evaluate the effectiveness of Thermolon surface modification on mitigating fouling and adhesion of microorganisms during the thermal processing of milk.
Stainless steel 316L coupons (25.4mm × 25.4mm × 0.5 mm) with a 2B finished surface were provided by Stainless Supply (Monroe, NC) and used to mimic the surface of a typical plate heat exchanger. Thermolon surface modification was done by Porcelain Industries (Dickson, TN).
We measured the contact angles of 3 liquids with known surface tension (
Determination of the acid-base characteristics of clay mineral surfaces by contact angle measurements—implications for the adsorption of organic solutes from aqueous media.
): water (72.8 mN/m), 1-bromonaphthalene (44.4 mN/m), and ethylene glycol (48 mN/m) on the stainless steel control and Thermolon-modified surfaces using a static method with a FTA 1000 B Drop Shape instrument (Portsmouth, VA) at room temperature.
Because
(where
and
are the dispersive and polar components of the solid surface energy, respectively), solid surface energy can be determined by combining Young's equation (Equation [1]) and the Owens-Wendt approach (Equation [2];
where
is the total interfacial surface tension between solid and liquid,
and
are the surface tension of the solid and the liquid, respectively, and the contact angle θ; and
[2]
where
and
are the dispersive and polar contributions of the liquid (Table 1).
Determination of the acid-base characteristics of clay mineral surfaces by contact angle measurements—implications for the adsorption of organic solutes from aqueous media.
Values are an average of 9 measurements (3 distinct regions on 3 independent samples) ± SE. γTOTl is the surface tension of the liquid; γdl and γpl are the dispersive and polar contributions of the liquid, respectively.
Liquid
γTOTl
γdl
γpl
Contact angle (°)
Stainless steel
Thermolon
Water
72.8
21.8
51
82.9 ± 1.2
105.5 ± 0.9
1-Bromonaphthalene
44.4
44.4
0
21.7 ± 1.0
44.9 ± 1.5
Ethylene glycol
48
29
19
65.9 ± 1.4
72.4 ± 0.8
1 Values are an average of 9 measurements (3 distinct regions on 3 independent samples) ± SE. γTOTl is the surface tension of the liquid; γdl and γpl are the dispersive and polar contributions of the liquid, respectively.
We determined the surface roughness of different substrates using a Wyko NT1100 Optical Profiler (VEECO, Tucson, AZ). The field of view was 450 × 592 μm, and the results were reported as an average of duplicate samples, with 5 scans of each sample.
We conducted fouling experiments in a laboratory-designed benchtop plate heat exchanger as shown in Figure 1, fitted with control and modified coupons, respectively. Different batches of raw milk were collected from the dairy plant at Kansas State University and kept at 4°C before use. Each batch of milk was divided into 2 for tests using control and Thermolon-modified stainless steel coupons. The milk inlet temperature was set at 40°C, and the hot water temperature in the second water bath was maintained at 88–90°C to maintain the milk outlet temperature at ∼85°C. Raw milk was pumped through the benchtop plate heat exchanger for 7.5 h, with a flow rate of 22 mL/min. After each test, the plate heat exchanger was dissembled, and the weight of the milk deposit on the coupons was measured after air drying for 15 min by recording the difference in weight of the clean plates versus the air-dried fouled substrates.
Figure 1Schematic of the benchtop plate heat exchanger (PHE) setup to simulate milk pasteurization and generate milk fouling. Color version available online.
We carried out scanning electron microscope analysis using the Hitachi S-3500N (Tokyo, Japan). Clean stainless steel coupons (control and modified) were analyzed directly at an accelerating voltage off 20 kV. Milk fouling on the coupons was air-dried at room temperature and then coated with a 10-nm layer of 99% gold. The fouling layer was then observed at an accelerating voltage of 10 kV.
We used aerobic spore-forming Bacillus licheniformis (ATCC 6643) to develop biofilms on control and modified stainless steel coupons at 50°C using the method described in our previous study (
). The biofilm embedded cells in 72 h, and matured biofilms formed on control and modified stainless steel coupons were enumerated by swabbing an area of 6.45 cm2 and plating on brain heart infusion (BHI) agar plates (
Milk fouling and biofilm formation tests were conducted in triplicate. We calculated milk deposit weight and bacterial counts as mean values and standard deviations. We compared fouling, bacteria attachment, and surface property results using SAS software (version 9.4; SAS Institute Inc., Cary, NC) and set the least significance difference at P < 0.05.
The properties of the milk contact surface play an important role in fouling and biofilm formation during dairy processing. We observed white, spongy-like deposit on control surfaces and less fouling on modified surfaces (Figure 2). The mean fouling weight on the control surfaces was 19.21 ± 2.25 mg/cm2, and on the modified surfaces, it was 0.37 ± 0.28 mg/cm2 (Table 2). Fouling weight was decreased up to 98% after sol-gel modification. We found more bacteria on control surfaces, at log 4.35 cfu/cm2; the modified surface showed a reduction to log 3.38 cfu/cm2 (Table 2).
Figure 2Representative images of fouling on (A) control and (B) Thermolon-modified stainless steel coupons after running raw milk for 7.5 h at 85°C in the benchtop plate heat exchanger.
). The water contact angle of control surfaces was slightly below 90°, and the modified surface was more hydrophobic, with a contact angle of 105° (Table 1). Early studies reported that less fouling and bacterial attachment were observed on hydrophobic surfaces (
). The relationship between fouling adhesion and material surface energy has been described by the Baier curve; substrates with surface energy 20–30 mN/m show the least atomic features of retention to protein deposition and bioadhesion (
). In the present study, the control surface had a surface energy of 41.36 ± 2.7 mN/m, and that of the modified surface was lower, with a value of 32.40 ± 1.4 mN/m (Table 2). This finding explains the easy-release property of the Thermolon-modified surface: it has a lower surface energy, close to the bio-fouling release zone (
Scanning electron microscopy images were acquired to investigate surface morphology. As shown in Figure 3A, images of the 2B finished stainless steel surfaces revealed cracks. This might have been due to the steps involved in producing the 2B surface: an annealing oxidizing atmosphere followed by picking to remove the oxide layer formed in the previous step (
). The sol-gel surface modification (Figure 3B) masked all surface characteristics of the stainless steel substrate, and the scanning electron microscope image showed a more heterogeneous topography, with no cracks on the top, leading to a more uniform surface. These surface differences could have led to the differences in the fouled microstructure. On the control surfaces, we observed thick and rugged milk deposits with sublayers and porous structures (Figure 3C), which might have been due to the fouling-induced heat transfer difference across the heat exchanger surface. On Thermolon-modified surfaces, the fouling layer was thinner and smooth, with small granules (Figure 3D).
Figure 3Scanning electron microscope images of clean and used coupons. (A) Clean control coupon; (B) clean Thermolon-modified coupon; (C) milk fouling on control coupon; (D) milk fouling on Thermolon-modified coupon.
Finally, the roughness of the modified surface was slightly higher (surface roughness = 199 ± 10.6 nm) than that of the control surface (surface roughness = 148.6 ± 15.0 nm; Table 2). There appears to be no direct correlation between surface roughness and bio-fouling formations (
Overall, the properties of the milk contact surface influenced the formation of milk deposit and biofilms. After Thermolon modification, the surface became more uniform and hydrophobic, with lower surface energy. Compared with the modified surface, we observed more milk fouling and bacterial attachment on the less hydrophobic control surfaces under the same conditions. These results provided evidence for the potential of using Thermolon-modified heat exchangers in dairy thermal processing to reduce milk fouling and biofilm formation, increasing process efficiency and enhancing the microbial quality of the final product.
The durability of surface modification is an important parameter for use in food-processing applications. In future studies, we will investigate the reusability of the Thermolon surface modification.
ACKNOWLEDGMENTS
This work was financially supported by the National Dairy Council. The authors also acknowledge Ravindra Thakkar from Kansas State University for scanning electron microscope analysis and Qiang Ye from Kansas University for surface roughness analysis. Contribution no. 17-113-J from the Kansas Agricultural Experiment Station.
REFERENCES
Baier R.E.
Surface behaviour of biomaterials: The theta surface for biocompatibility.
J. Mater. Sci. Mater. Med.2006; 17 (17122919): 1057-1062
Determination of the acid-base characteristics of clay mineral surfaces by contact angle measurements—implications for the adsorption of organic solutes from aqueous media.
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