Quality Assurance

• 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

• Acetaldehyde Biosynthesis in Streptococcus thermophilus:
• Metabolic Pathway:
• Acetaldehyde is the signature flavor compound of yogurt.
• In Streptococcus thermophilus, it is synthesized primarily from threonine catalyzed by the enzyme Threonine aldolase:

• Conclusion: Threonine (Option 3).

Correct option: (3) Threonine

• Total Solids Determination in Composite Ice Cream Cake:
• Product Composition & Standards:
• Ice cream mix formulated for an overrun of $90\%$ requires minimum $36\%$ Total Solids (fat, MSNF, sugars, stabilizers) to maintain structural air-cell stability.
• Chocolate cake layers contain $70 - 75\%$ Total Solids.
• For a composite dessert exhibiting $4\%\text{ w/w}$ protein and $90\%$ overrun, the combined weighted total solids content inherently exceeds $36\%\text{ w/w}$.
• Conclusion: More than 36% w/w (Option 3).

Correct option: (3) More than 36% wiw