Question

Rancid flavour in dried milk is due to -

• A. Low pre-heating temperature
• B. Drum surface has pits
• C. Storage at high temperature
• D. High moisture during storage

Hint:

Distinguish between the two types of rancidity: {Hydrolytic Rancidity:} Enzyme (lipase) + Water → Free Fatty Acids. Prevented by heat treatment. {Oxidative Rancidity (Tallowy flavor):} Oxygen + Unsaturated Fat → Aldehydes/Ketones. Prevented by removing oxygen (e.g., gas flushing) and avoiding light/metal catalysts.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Rancidity in dairy products is a specific off-flavor caused by the breakdown of milk fat (hydrolysis of triglycerides) into free fatty acids (like butyric acid), which have a sharp, unpleasant taste and odor. This breakdown is catalyzed by the enzyme lipase.
Step 2: Detailed Explanation:
Let's analyze the causes:
(A) Low pre-heating temperature: Before milk is concentrated and dried to make powder, it undergoes a high-temperature heat treatment (pre-heating). A primary purpose of this step is to inactivate enzymes present in the raw milk, especially the heat-resistant lipase. If the pre-heating is insufficient (too low a temperature or too short a time), the lipase enzyme survives the process. It can then act on the fat during storage of the powder, causing hydrolytic rancidity. This is a primary cause of this defect.
(B) Drum surface has pits: This is a problem in drum drying that can lead to scorched particles, causing a burnt or cooked flavor, not rancidity.
(C) Storage at high temperature: High storage temperature primarily accelerates other defects like oxidative rancidity (a different chemical process) and Maillard browning, but the root cause of hydrolytic rancidity is the presence of the active lipase enzyme.
(D) High moisture during storage: High moisture can accelerate many chemical reactions and allow microbial growth, but it doesn't cause rancidity on its own without the presence of the lipase enzyme.
Step 3: Final Answer:
The primary cause of rancid flavor (hydrolytic rancidity) in milk powder is the survival of the lipase enzyme due to inadequate pre-heating of the milk before drying.