Dairy Chemistry- ICAR JRF/SRF 2024

• Antimicrobial Metabolites of Lactic Starter Cultures:
• Endogenous Antimicrobials Produced by LAB:
• Organic acids: Lactic and acetic acids lower pH and permeabilize bacterial cell membranes.
• Bacteriocins: Ribosomal antimicrobial peptides (nisin, pediocin) that create pores in bacterial cell membranes.
• Hydrogen peroxide ($H_2O_2$): Generates oxidative stress in catalase-negative pathogens under aerobic conditions.
• Phenols: Phenols are plant secondary metabolites or industrial chemicals, NOT metabolites synthesized by dairy starter cultures.
• Conclusion: Phenols (Option 2).

Correct option: (2) Phenols

• Statement (I) is True: The family Enterobacteriaceae is a large, diverse family of Gram-negative, facultatively anaerobic, non-spore-forming rod-shaped bacteria that ferment D-glucose with the production of acid and/or gas, reduce nitrates to nitrites, and are oxidase-negative.
• Statement (II) is True: The majority of medically and agriculturally important members of Enterobacteriaceae are pathogens or opportunistic pathogens in animals and humans, including Salmonella, Escherichia coli (ETEC, STEC/EHEC), Shigella, Yersinia pestis, Klebsiella pneumoniae, and Proteus.
• Conclusion: Both Statement (I) and Statement (II) are true (Option 1).

Correct option: (1) Both Statement (I) and Statement (II) are true.

• Microbial Load Comparison in Dairy Commodities:
• Raw Milk:
• Fresh, unprocessed bovine milk directly from farm milking contains endogenous udder microflora, teat canal contaminants, and environmental dust/manure bacteria, easily yielding total viable counts of $10^5 - 10^7\text{ CFU/mL}$.
• In contrast, Skim milk powder, Sweetened condensed milk, and Butter undergo pasteurization, thermal evaporation, and water-activity suppression, resulting in drastically lower microbial counts.
• Conclusion: Raw Milk (Option 4).

Correct option: (4) Raw Milk