Question

What is the Whey Protein Nitrogen Index (mg/mL) of low-heat skim milk powder?

• A. $>$ 6.0
• B. 1.5 - 6.0
• C. $<$ 1.5
• D. 6.6

Hint:

Remember the WPNI classification thresholds:
- Low-Heat: $> 6.0$ mg/g (minimum heat, high functional whey proteins).
- Medium-Heat: $1.5 \text{ to } 6.0$ mg/g.
- High-Heat: $< 1.5$ mg/g (maximum heat, highly denatured proteins).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Whey Protein Nitrogen Index (WPNI) is an analytical measure used to classify skim milk powder (SMP) based on the cumulative heat treatment it received during processing.
Heat treatment denatures whey proteins, making them insoluble in saturated sodium chloride solution.
The WPNI measures the amount of remaining undenatured whey protein nitrogen in milligrams per gram of powder.

Step 2: Detailed Explanation:
Whey proteins (such as \(\beta\)-lactoglobulin and \(\alpha\)-lactalbumin) are heat-sensitive, whereas caseins are heat-stable.
During the manufacture of skim milk powder, heat treatments like pasteurization and preheating denature some of these whey proteins.
By measuring the level of undenatured whey protein nitrogen remaining in the final powder, manufacturers can determine the heat class of the powder.
The standard classification system defines three categories of skim milk powder based on WPNI value:
1. Low-Heat Skim Milk Powder:
Received mild heat treatment, preserving most of the native whey proteins.
The WPNI value is greater than 6.0 mg/g of powder (often expressed as \(\ge 6.0\) mg/g).
This powder is preferred for recombined milk, cheese making, and infant formulas because the proteins retain their functional properties.
2. Medium-Heat Skim Milk Powder:
Received moderate heat treatment.
The WPNI value ranges from 1.51 to 5.99 mg/g of powder.
3. High-Heat Skim Milk Powder:
Received intense heat treatment, denaturing most of the whey proteins.
The WPNI value is less than or equal to 1.50 mg/g of powder.
This powder is typically used in bakery products where denatured whey proteins help improve loaf volume.
Note: Although the question lists the units as mg/mL, the standard scientific unit for WPNI is milligrams of undenatured whey protein nitrogen per gram of powder (mg/g).

Step 3: Final Answer:
The Whey Protein Nitrogen Index of low-heat skim milk powder is $>$ 6.0.