FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 25 : Processing and Preparation of Milk and Milk products

Effect of Acid

At pH 6.6, casein is present largely as calcium caseinate. When the acidity of milk is increased either by the addition of acid or by natural souring, the acid removes calcium and phosphate from calcium caseinate changing it into casein . Casein coagulates when the pH has been reduced to about 5.2 and is least soluble at its iso-electric point pH 4.6. When the pH reaches about 4.6, the colloidally dispersed casein particles become unstable. They adhere together and form a coagulum or curd. This probably due to the usual negative charge on the casein particles, which causes them to repel each other and remain apart and is neutralised by acidic hydrogen ion.

Higher temperature hastens the action of acid coagulation. If the heat or acid is excessive, the precipitated curd shrinks and becomes tough and rubbery.