Impact of Gelation Conditions and Structural Breakdown on the Physical and Sensory Properties of Stirred Yogurts

Gelation profiles as a function of pH for a) yogurt gels made from milk preheated at 75°C for 30min and incubated at 32°C (■, □), 38°C (●, ○), and 44°C (▴, ▵), and b) yogurt gels made from milk preheated at 85°C for 30min and incubated at 32°C (■, □), 38°C (●, ○), and 44°C (▴, ▵). Solid symbols and open symbols indicate storage modulus (G′) and loss tangent (LT), respectively. Results were the mean of 3 replicates with error bars for the standard deviations.

Apparent viscosity as a function of shear rate for a) yogurt gels made from milk preheated at 75°C for 30min and incubated at 32°C (■), 38°C (○), and 44°C (▴), and b) yogurt gels made from milk preheated at 85°C for 30min and incubated at 32°C (■), 38°C (○), and 44°C (▴). SB = structural breakdown. Results were the mean of 3 replicates.

The shear stress as a function of strain at a) constant shear of 0.01s⁻¹ (♦), 0.03s⁻¹ (■), 0.05s⁻¹ (●), and 0.07s⁻¹ (▴), and b) continuously increasing shear rates from 0.01 to 0.07s⁻¹ for yogurt gels made from milk preheated at 85°C for 30min and incubated at 38°C. Arrows indicate shear rates at some points in the process. Results were the mean of 3 replicates with error bars for the standard deviations.

The apparent viscosity as a function of shear rate for a) stirred yogurts made from milk preheated at 75°C for 30min and incubated at 32°C (■), 38°C (○), and 44°C (▴), and b) stirred yogurts made from milk preheated at 85°C for 30min and incubated at 32°C (■), 38°C (○), and 44°C (▴). Results were the mean of 3 replicates.

Comparison between experimental flow curves of actual yogurt gels and predicted flow curves using rheological models: a) experimental flow curve of actual yogurt gel (●), predicted flow curve using Herschel-Bulkley model (□), and predicted flow curve using Casson model (▵); and b) experimental flow curve of actual yogurt gel (●), predicted flow curves using Carreau model (□), and predicted flow curve using Cross model (▵).

Microstructure for yogurt gels made from milk preheated at 75°C for 30min and incubated at a) 32, b) 38, and c) 44°C; and for yogurt gels made from milk preheated at 85°C for 30min and incubated at d) 32, e) 38, and f) 44°C. Protein matrix is white whereas pores are dark; scale bar = 20μm.

Microstructure for stirred yogurts made from milk preheated at 75°C for 30min and incubated at a) 32, b) 38, and c) 44°C; and for stirred yogurts made from milk preheated at 85°C for 30min and incubated at d) 32, e) 38, and f) 44°C. Protein matrix is white whereas pores are dark; scale bar = 20μm.