Dairy Chemistry & Milk Constituents

Biochemical etiology of primary hypolactasia:

• Galactokinase deficiency causes type II galactosemia (cataract formation).
  (B), (C) are not digestive enzymes in the human small intestine.
• Beta-galactosidase (commonly termed lactase / lactase-phlorizin hydrolase) is an apical brush-border disaccharidase situated in the microvilli of the small intestinal enterocytes. It hydrolyzes dietary lactose ($\beta$-D-galactopyranosyl-(1$\rightarrow$4)-D-glucose) into absorbable monosaccharides: D-glucose and D-galactose. In individuals with lactase non-persistence (deficiency of Beta-galactosidase), undigested lactose travels into the large intestine where it is fermented by colonic anaerobes, producing gas ($H_2, CO_2, CH_4$) and watery diarrhea (lactose intolerance).

Physico-chemical changes during milk drying:

• Statement (I) is true: Removal of water and thermal concentration shift the mineral equilibrium of milk. Soluble calcium and phosphate ions precipitate and migrate into the colloidal phase, thereby decreasing the concentration of soluble serum calcium/phosphate and increasing colloidal calcium phosphate (CCP) within the casein micelles.
• Statement (II) is true: In the roller (drum) drying process, milk is spread as a thin boiling film on rotating steam-heated metal drums ($120\text{ to }140^\circ\text{C}$) and scraped off by sharp doctor blades. The intense heat and mechanical shear rupture the milk fat globule membranes (MFGM), releasing over $90\%$ of the fat as extractable 'free fat'. In contrast, spray drying atomizes milk gently into fine droplets in hot air, preserving MFGM integrity and keeping free fat levels very low ($3\%$ to $10\%$).
  
  Option analysis:
• Both Statement (I) and Statement (II) are true: Correct.
• Both Statement (I) and Statement (II) are false: Incorrect.
• Statement (I) is true but Statement (II) is false: Incorrect.
• Statement (I) is false but Statement (II) is true: Incorrect.

In the mammary secretory epithelial cell, lactose biosynthesis follows the pathway:
Blood Glucose -> Glucose-6-phosphate -> Glucose-1-phosphate -> UDP-Glucose -> UDP-Galactose (catalyzed by UDP-galactose 4-epimerase).
In the Golgi apparatus, Lactose Synthase (galactosyltransferase complexed with alpha-lactalbumin) transfers the galactose moiety from UDP-galactose directly to free unphosphorylated D-glucose, yielding free Lactose and UDP:
$\text{UDP-Galactose} + \text{D-Glucose} \rightarrow \text{Lactose} + \text{UDP}$.
Lactose-6-phosphate is never produced during mammalian lactose biosynthesis (it is an intermediate in bacterial lactose transport via the PEP-PTS system in lactic acid bacteria).

Option analysis:

• Lactose-6-phosphate: Correct (not an intermediate in mammary lactose biosynthesis).
• UDP-Galactose: Incorrect (key donor substrate intermediate).
• Glucose-1-phosphate: Incorrect (precursor in pathway).
• UDP-Glucose: Incorrect (precursor in pathway).

Classical Galactosemia (Type I) is an inherited autosomal recessive metabolic disorder caused by the congenital deficiency or absence of the enzyme Galactose-1-phosphate uridylyltransferase (GALT). In affected infants consuming milk lactose, galactose cannot be converted to glucose, leading to the toxic accumulation of galactose-1-phosphate and galactitol in tissues, causing liver failure, cataracts, and intellectual disability unless milk and dairy products are completely removed from the diet.

Option analysis:

• Galactose-1-phosphate uridylyltransferase: Correct. The missing enzyme in classical galactosemia.
• UDP-galactosyl transferase: Incorrect.
• UDP-glucose pyrophosphorylase: Incorrect.
• Beta-galactosidase: Incorrect. Deficiency of beta-galactosidase (lactase) causes lactose intolerance, not galactosemia.