Question

Which of the following are two indices used for heat classification of milk powder based on level of whey protein denaturation?

• A. Casein number and NPN
• B. WPNI and casein number
• C. Total nitrogen and WPNI
• D. Total casein and NPN

Hint:

The Whey Protein Nitrogen Index (WPNI) is the primary industry standard for heat classification, reflecting the concentration of undenatured, soluble whey proteins left in the powder.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
During the manufacture of skim milk powder or nonfat dry milk, the thermal treatments applied prior to spray drying denature the heat-sensitive whey proteins.
The level of whey protein denaturation determines the physical properties and industrial end-uses of the milk powder.

Step 2: Detailed Explanation:
The American Dairy Products Institute (ADPI) classifies milk powders into Low-Heat, Medium-Heat, and High-Heat classes using specific chemical indices:
Whey Protein Nitrogen Index (WPNI): This is the primary index, defined as the mass of undenatured whey protein nitrogen remaining in the powder, expressed in milligrams per gram of powder (\(\text{mg/g}\)).
Low-heat powder has a \(\text{WPNI} \ge 0\,\text{mg/g}\), while high-heat powder has a \(\text{WPNI} \le 5\,\text{mg/g}\).
Casein Number: The casein number represents the percentage of total milk nitrogen that is casein nitrogen.
When whey proteins are thermally denatured, they co-precipitate with casein during acidification.
This co-precipitation artificially increases the measured casein fraction, thereby altering the casein number.
Thus, both WPNI and casein number serve as indicators of the severity of heat treatment applied during milk powder manufacture.
Other parameters like total nitrogen or non-protein nitrogen (NPN) do not directly measure the denaturation of whey proteins.

Step 3: Final Answer:
Therefore, WPNI and casein number are the two indices used for the heat classification of milk powder.