Biochemical pathways of starter citrate fermentation in LAB:
In aroma-producing lactic acid bacteria (Lactococcus lactis subsp. lactis biovar diacetylactis and Leuconostoc mesenteroides):

• Citrate is cleaved by citrate lyase into oxaloacetate and acetate, which is further decarboxylated to pyruvate.
• Pyruvate condensation yields $\alpha$-acetolactate, which oxidatively decarboxylates to diacetyl (the primary buttery aroma compound).
• When milk fermentations are over-incubated, bacterial diacetyl reductase (acetoin dehydrogenase) reduces diacetyl to Acetoin (3-hydroxy-2-butanone), which has a much higher flavor threshold (bland flavour). Acetoin may be further reduced to 2,3-butanediol.

Physiological differentiation and starter culture purity testing:

• Oxidase test detects cytochrome c oxidase (Pseudomonas vs Enterobacteriaceae).
• True Lactic Acid Bacteria (Lactococcus, Streptococcus, Lactobacillus, Leuconostoc) lack a functional heme-based cytochrome system and are strictly catalase-negative. Therefore, the simple Catalase test (flooding culture colonies with $3\%\text{ }H_2O_2$) is the standard rapid diagnostic tool to check starter culture purity: instantaneous vigorous gas bubbling indicates contamination by catalase-positive organisms like Staphylococci, Micrococci, Bacilli, or coliforms.
• Indole test checks tryptophanase production in coliforms.
• Creatine test is used in Voges-Proskauer reaction.

Enantiomeric stereospecificity of lactate dehydrogenases in yogurt starters:

• Statement (I) is true: Lactobacillus delbrueckii subsp. bulgaricus possesses a stereospecific D-lactate dehydrogenase (D-LDH) and exclusively synthesizes the $D(-)$ enantiomer of lactic acid from pyruvate.
• Statement (II) is true: Streptococcus thermophilus possesses an L-lactate dehydrogenase (L-LDH) and exclusively produces the physiologically preferred $L(+)$ enantiomer of lactic acid.
  Because both organisms are co-cultured in traditional yogurt fermentation, yogurt contains a racemic mixture of both $L(+)$ and $D(-)$ lactic acid.