Step 1: Understanding Rancidity:
Rancidity in dairy products arises from the breakdown of milk fat (triglycerides hydrolysis) into free fatty acids (like butyric acid). These fatty acids impart a sharp, unpleasant taste and odor. The enzyme lipase catalyzes this process.
Step 2: Cause Analysis:
Analyzing potential causes:
(A) Insufficient Pre-heating: Milk undergoes pre-heating before concentration and drying to inactivate enzymes, especially heat-resistant lipase. Inadequate pre-heating (low temperature or short duration) allows lipase to survive. This active lipase then causes hydrolytic rancidity during powder storage. This is a key factor.
(B) Pitted Drum Surface: Pitted drum surfaces in drum drying lead to scorched particles, resulting in burnt or cooked flavors, not rancidity.
(C) Elevated Storage Temperature: High storage temperature accelerates defects like oxidative rancidity and Maillard browning. However, active lipase is the fundamental cause of hydrolytic rancidity.
(D) High Moisture Content During Storage: High moisture accelerates chemical reactions and microbial growth, but lipase must be present to cause rancidity.
Step 3: Conclusion:
The primary cause of hydrolytic rancidity in milk powder is lipase enzyme survival due to inadequate milk pre-heating before drying.