Starter Cultures & Fermented Products

Galactose fermentation enzymology in thermophilic starters:

• Streptococcus thermophilus metabolizes galactose via the Leloir pathway. Galactose is transported across the membrane and converted to glucose-1-phosphate via sequential reactions catalyzed by the gal operon enzymes: galactokinase (GalK), galactose-1-phosphate uridylyltransferase (GalT), and UDP-galactose 4-epimerase (GalE).
• Tagatose 6-phosphate pathway is used by Lactococcus lactis and Staphylococcus aureus for phosphorylated galactose from lactose-PTS.
• Phosphoketolase pathway is used by heterofermentative leuconostocs and lactobacilli.
• Glycolysis (Embden-Meyerhof pathway) metabolizes glucose, not direct galactose.

Proteolytic system architecture of Lactic Acid Bacteria (Lactococcus lactis):

• Statement (I) is false: Intact milk casein molecules are too large to pass into the cell. LAB initiate casein breakdown extracellularly using a single cell-wall-anchored extracellular PROTEINASE (specifically cell envelope proteinase, CEP / PrtP), NOT peptidases. Proteinases hydrolyze proteins into oligopeptides of 4 to 30 amino acids.
• Statement (II) is false: The generated oligopeptides are actively internalized into the bacterial cytoplasm intact via the oligopeptide ABC transport system (Opp). Once inside the cytoplasm, they are degraded into di/tripeptides and free amino acids by a cascade of INTRACELLULAR (cytoplasmic) peptidases (PepN, PepX, PepC, PepO, endopeptidases), NOT by membrane-bound peptidases.

Biochemical and protective agents in dairy microbiology:

• Starter distillate is steam-distillate of fermented starter culture concentrated in natural Diacetyl ($CH_3-CO-CO-CH_3$), used commercially to impart butter aroma to spreads and margarines $\to$ (III).
• Nisin is an approved Class I lantibiotic antimicrobial peptide naturally produced by Lactococcus lactis subsp. lactis $\to$ (IV).
• Calcium-deficient medium (Phage Inhibitory Medium, PIM / PRM) binds or chelates $Ca^{2+}$ ions required for phage adsorption and tail contraction, thereby protecting bulk starter cultures from lytic Bacteriophage destruction $\to$ (I).
• Cobalamin (Vitamin $B_{12}$) is industrially and biosynthetically produced by dairy Propionibacteria (Propionibacterium freudenreichii) during Swiss cheese ripening $\to$ (II).