ICAR AIEEA PG Dairy Science 09 July 2023 (Shift 1) Paper

Chemistry of milk salts and ash analysis:

• Statement (I) is false: Ashing milk at $550^\circ\text{C}$ completely volatilizes organic counter-ions (citrates, lactates), converts organic phosphorus of caseins and phospholipids into inorganic phosphates, and partially volatilizes chlorides. Therefore, ash represents only the oxidized mineral residue after combustion and does NOT represent the true native salt equilibrium or ionic composition in fluid milk.
• Statement (II) is true: According to Sommer and Hart's classical salt balance theory, the heat stability of milk depends critically on the ionic ratio between polyvalent cations and polyvalent anions:
  $$\text{Salt Balance Ratio} = \frac{\text{Ca}^{2+} + \text{Mg}^{2+}}{\text{Citrate}^{3-} + \text{Phosphate}^{3-}}$$

Principles of analytical titrations for butter salt:

• Mohr's method is a precipitation titration that employs standard silver nitrate ($AgNO_3$) solution to quantify chloride ($Cl^-$) ions using potassium chromate ($K_2CrO_4$) as indicator. Because silver ($Ag^+$) is the active titrant yielding insoluble silver chloride ($AgCl$), this titration is termed an Argentometric titration. The end-point is indicated by the first appearance of reddish-brown silver chromate ($Ag_2CrO_4$).
• Iodometric titrations involve iodine/thiosulfate.
• Acid-base titrations involve hydronium/hydroxide neutralizations.
• Potentiometric titrations measure potential differences across electrodes.

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.

Cryoscopic analysis and colligative properties in souring milk:

• Assertion (A) is true: Freezing point depression (FPD) of milk is a colligative property strictly proportional to the number of dissolved solute particles. In sour or aged milk, microbial lactic acid fermentation increases the solute particle concentration, depressing the freezing point and artificially masking any dilution caused by added extraneous water.
• Reason (R) is true and provides the correct explanation: One mole of lactose ($C_{12}H_{22}O_{11} + H_2O$) is fermented by homofermentative lactic acid bacteria into 4 moles of lactic acid ($4\text{ CH}_3\text{CH(OH)COOH}$). This 4-fold increase in osmotically active solute particles increases osmotic pressure and directly increases freezing point depression.