Polenske value of milk is due to

The correct option/answer is Water-insoluble fatty acids.

The Polenske value (also known as the Polenske number) is a chemical index used in the analysis of fats and oils, particularly in dairy chemistry to test the purity of butter and milk fat.
It is defined as the number of milliliters of $0.1\times$ Normal ($\text{N}$) aqueous alkali (such as sodium hydroxide) solution required to neutralize the volatile, water-insoluble fatty acids distilled from $5$ grams of a given fat under specific standardized conditions.

Milk fat contains a unique distribution of fatty acids. When butterfat undergoes saponification and subsequent acidification, both volatile water-soluble and volatile water-insoluble fatty acids are liberated.
During the steam distillation process:
1. The Reichert-Meissl value measures the volatile water-soluble fatty acids (primarily butyric and caproic acids) that distill over and remain in the aqueous filtrate.
2. The Polenske value measures the volatile water-insoluble fatty acids (principally caprylic, capric, and lauric acids) that distill over but filter out because they do not dissolve in water.

Therefore, the Polenske value of milk is directly due to the presence of these volatile, water-insoluble fatty acids.