Microstructural changes in casein supramolecules during acidification of skim milk

Transmission electron micrographs of skim milk, glutaraldehyde-fixed and agar-solidified at (A) 40°C, (B) 30°C, (C) 20°C, and (D) 10°C (short arrows=agarose fibers; d=dissociated protein; bar=500nm).

Reduction of pH in skim milk as a function of temperature based on (A) amount of glucono-δ-lactone (GDL) added and (B) addition of 1.2% freeze-dried starter culture.

Transmission electron micrographs during warm acidification at 40°C of skim milk by 1.5% glucono-δ-lactone, glutaraldehyde-fixed and agar-solidified at (A) pH 5.5, (B) pH 5.2, (C) pH 4.95, and (D) pH 4.8 (d=loosely entangled protein; short arrows=agarose fibers; bar=500nm).

Transmission electron micrographs during warm acidification at 40°C of skim milk by addition of 1.2% starter culture, glutaraldehyde-fixed and agar-solidified at (A) pH 5.4, (B) pH 5.0, (C) pH 4.75, and (D) pH 4.6 (d=loosely entangled protein; short arrows=agarose fibers; bar=500nm).

Transmission electron micrographs during cold acidification at 10°C of skim milk by 4.0% glucono-δ-lactone, glutaraldehyde-fixed and agar-solidified at (A) pH 5.9, (B) pH 5.3, (C) pH 5.0, and (D) pH 4.8 (d=loosely entangled protein; short arrows=agarose fibers; bar=500nm).

Transmission electron micrographs of acid milk gels at pH 4.8 formed after acidification of skim milk by glucono-δ-lactone at (A) 40°C, (B) 30°C, (C) 20°C, and (D) 10°C (bar=1 μm).

Influence of temperature on changes in turbidity upon acidification of skim milk by 4.0%, 2.5%, 2.0%, and 1.5% (wt/wt) glucono-δ-lactone (GDL) at 10, 20, 30, and 40°C, respectively.

Change in turbidity of skim milk as a function of slow, medium, and fast acidification rates using glucono-δ-lactone (GDL) at (A) 10°C and (B) 40°C, as well as by using 1.2% starter culture at 40°C (B).

Microstructural changes occurring during chemical acidification of milk as a function of temperature and pH. Images derived from transmission electron micrographs, with colloidal casein supramolecules depicted in black and loosely entangled protein aggregates depicted in gray, and all background digitally converted to white.