Impact of preacidification of milk and fermentation time on the properties of yogurt

Influence of preacidification of milk on the turbidity of milk sample. Turbidity was measured as absorbance (arbitrary units) at 600nm as a function of preacidification pH. Results are the means of triplicates with error bars for standard deviation.

Acid-base buffering curves of preacidified skim milk samples that were preacidified to pH values of 6.55 (□), 6.42 (×), 6.10 (▵), 5.78 (○), and 5.65 (–). Results are the means of triplicates. Arrows indicate the direction of titration.

Calcium content in (a) ultrafiltration permeate of milks as a function of preacidification pH, (b) supernatant measured at pH 4.6 of yogurt gels made with different preacidification pH values and fermentation times measured at pH 4.6, (c) supernatant of yogurt gels measured at pH 5.0. Yogurts were made from milks with different preacidification pH values and fermentation times as a function of time. Results are the means of triplicates with error bars for standard deviations.

Storage modulus (solid symbols) and loss tangent (open symbols) of yogurt gels made with (a) various preacidification pH values and constant fermentation time (375min), (b) with various fermentation times with a constant preacidification pH (6.10). (a) Yogurt gels were prepared with preacidification pH of 6.55 (■, □), 6.10 (▴, ▵), and 5.65 (●, ○) and incubated for 375min. (b) Yogurt gels were prepared with the same preacidification pH of 6.10 and incubated for 250 (■, □), 375 (▴, ▵), and 500min (●, ○). Results are the means of triplicates with error bars for standard deviation.

Whey separation of yogurt gel made from milk with different preacidification pH and different fermentation time. Whey separation was measured when gels reached pH 4.6. Results are the means of duplicates with error bars for standard deviation. ^a–dColumns with different letters are significantly different (P<0.05).

Microstructure of yogurt gels prepared with the preacidification pH of (a) 6.55, (b) 6.10, and (c) 5.65 and incubated for 375min. Microstructure of yogurt gels prepared with the same preacidification pH of 6.10 and incubated for (d) 250min, (b) 375min, and (e) 500min. Scale bar=50 μm. The protein matrix is white, whereas pores are dark.

Response surface plots for the effect of preacidification pH value and fermentation time on (a) storage modulus values (G′) at pH 4.6 and (b) permeability coefficient (B).