Dairy Chemistry & Milk Constituents

Chemical principles of saponification value (SV) in fats:

• Assertion (A) is true: The saponification value of ghee / milk fat is high ($220\text{ to }236\text{ mg KOH/g}$), whereas most vegetable oils (groundnut, soybean, sunflower) have saponification values around $188\text{ to }196\text{ mg KOH/g}$.
• Reason (R) is false: Saponification value is INVERSELY proportional to the average molecular weight of fatty acids in the triglyceride ($SV = \frac{3 \times 56100}{MW}$). Milk fat contains substantial proportions ($8\text{ to }10\%$) of short- and medium-chain fatty acids (butyric $C_4$, caproic $C_6$, caprylic $C_8$, capric $C_{10}$), meaning its average molecular weight is LOWER, not higher, than vegetable oils that are dominated by long-chain $C_{18}$ fatty acids.

Flavor chemistry of lipid peroxidation in milk fat:

• Lipid autoxidation initiates with free radicals generating primary hydroperoxides (ROOH). However, hydroperoxides themselves are completely odorless and tasteless.
• Hydroperoxides are chemically unstable and undergo homolytic $\beta$-scission cleavage to produce short- and medium-chain volatile secondary carbonyl compounds, primarily Aldehydes (hexanal, nonanal, 2-nonenal) and Ketones (methyl ketones, vinyl ketones). These volatile carbonyls have extraordinarily low odor thresholds (parts per billion) and are directly responsible for the repulsive, metallic, cardboard, and paint-like oxidized off-flavors in ghee.

Lipid chemistry and flavor defects in clarified butterfat (ghee):

• Riboflavin acts as an active photosensitizer in the presence of light, transferring energy to ground-state triplet oxygen to generate reactive singlet oxygen ($^1\text{O}_2$), thus acting as a potent Pro-oxidant $\to$ (II).
• Natural Tocopherols (Vitamin E) donate hydrogen atoms to lipid peroxyl radicals, terminating free radical chain propagation and acting as an Antioxidant $\to$ (III).
• The characteristic rich nutty, cooked flavor of ghee is derived from volatile Lactones ($\delta$-decalactone) and Carbonyl compounds (methyl ketones) formed during high-temperature clarification $\to$ (IV).
• Hydrolytic rancidity is caused by moisture and residual microbial lipases cleaving ester bonds to liberate Free fatty acids, especially butyric acid $\to$ (I).