Question

1. Can you make yogurt with lactose - free milk? Why?
2. Which milks are best suited to making yogurt? Which are not well - suited to making yogurt? Explain your recommendations.
3. Rennin is widely used in the food industry to make cheese. Can you explain why you can’t use the enzyme (such as rennin) to make cheese from hard boiled milk?
4. Can you still coagulate cheese protein when the pH is far lower than 4.6?  Explain your suggestions.

Answer 1

1) Yes, yogurt can be prepared from lactose free milk.

Reason- The individuals having lactose intolerance prefer making and eating lactose free food and drinks. In lactose intolerance, an individual is unable to metabolize lactose sugar present in milk or dairy products due to deficiency of enzyme lactase hence may suffer from diarrhea, constipation, stomach pain etc. on consuming lactose containing products.

2) Milks best suited for making yougurt are, goat milk, sheep milk, cow milk.

Recommendation- Cow milk and Goat milk could be used for making yogurt because these milk are rich in protiens, and the protein get easily coagulate on heating thereby providing thicker yogurt. these milk are highly recommended for culturing and preparing curd and yogurt.

Non suited milk for yogurt- Lactose free milk, lactose reduced milk, as these milk do not contain enough food for culturing bacteria (lactobacillus) hence cannot be used for making yogurt.

3) Renin cannot be used to make cheese from hard boiled milk

Renin also known as Chymosin is a proteolytic enzyme, secreted by chief cells of stomach. It gets activated from inactive prochymosin to chymosin on exposure to acidic environment. Renin is responsible for digesting and precipitating milk protein casein and curdling the milk. Milk majorly consists casein protein. 4 types of caseins, alpha-s1, alpha-s2 (calcium precipitable), beta and kappa (calcium in-precipitable) are present in the milk. alpha-s1, alpha-s2 are hydrophobic protein which are surrounded by beta and kappa (non-hydrophobic) protein forming micellar structure. Renin after activation proteolytically cleaves the kappa casein and the calcium precipitable alpha-s1, alpha-s2 are exposed to calcium concentration of milk, which precipitates these casein proteins and milk converts into curd.

In hard boiled milk, the excess heat and excess pasteurization causes delay in clotting process and weak gel formation (network between casein proteins). High temperature causes denaturation of whey proteins and absorption of insoluble whey protein onto casein proteins of milk due to which casein does not form strong gel.

4) at very low pH, the structure stability and quality of casein protein gel gets effected.

low pH, increase protonation of amino acids and also effects the hydrogen, hydrophobic, ionic interactions responsible for stabilizing the cheese structure. extreme lowering of pH, changes the ionic composition of protein which effects the final structure of cheese due to denaturing of proteins.