Dairy Chemistry & Milk Constituents

Mechanisms of antioxidant defense in lipid systems:

• Primary / chain-breaking antioxidants (BHA, BHT, tocopherols) scavenge and inactivate lipid peroxy radicals ($ROO^\bullet$) by donating hydrogen atoms ($AH + ROO^\bullet \rightarrow A^\bullet + ROOH$).
• Secondary / preventative antioxidants (citric acid, EDTA, phytates) function by chelating transition metal ions ($Fe^{2+}, Cu^{2+}$) to prevent Fenton radical initiation.
• Secondary antioxidants (phosphites, thiodipropionates) decompose lipid hydroperoxides into stable, non-radical, non-volatile alcohols without forming sensory off-flavors.
• Inactivation of lipolytic enzymes is accomplished by thermal processing (pasteurization/boiling), NOT by antioxidants.

Biochemical precursors of ruminant milk fat biosynthesis:
In ruminant mammary epithelial cells, approximately $50\%$ of fatty acids (all $C_4$ to $C_{14}$ short- and medium-chain fatty acids, and ~50% of palmitic acid $C_{16:0}$) are synthesized de novo in the cytosol:

• Unlike non-ruminants (which utilize glucose via ATP-citrate lyase), ruminants lack functional ATP-citrate lyase and NADP-malate dehydrogenase in the mammary gland.
• Instead, ruminants utilize blood-borne ruminal fermentation products: Acetate ($C_2$) provides roughly $75\%$ of the carbon for de novo synthesis via acetyl-CoA carboxylase and fatty acid synthase, while Beta-hydroxybutyrate (BHBA, $C_4$, produced by rumen epithelium from butyrate) provides the initial 4-carbon priming unit for approximately $25\%$ of fatty acid chains.

Lactulose (4-O-beta-D-galactopyranosyl-D-fructose) is an epimerization isomer of lactose that is completely absent in raw milk. When milk is heated, the Lobry de Bruyn-Alberda van Ekenstein transformation converts lactose into lactulose in a temperature- and time-dependent manner. Because lactulose levels directly reflect the cumulative thermal load, international standards (IDF/ISO) use lactulose content as an unambiguous chemical index to differentiate pasteurized ($\lt 50\text{ mg/L}$), UHT treated ($100\text{ to }600\text{ mg/L}$), and in-bottle sterilized milk ($\gt 600\text{ mg/L}$).

Option analysis:

• Urea content: Incorrect. Reflects cow nutrition and rumen nitrogen metabolism.
• Lactose content: Incorrect. High natural baseline varying little with normal heat treatments.
• Lactulose content: Correct. Synthetic heat-formed marker used internationally as an index of milk heat processing severity.
• Formic acid content: Incorrect.