Practice question

Lactic acid fermentation is dominated by Gram-positive, non-sporulating, aerotolerant anaerobes grouped as lactic acid bacteria, including Lactobacillus delbrueckii, Lactococcus lactis, Streptococcus thermophilus, Pediococcus and Leuconostoc. These organisms lack functional electron transport chains containing cytochromes and rely exclusively on fermentative metabolism for energy. In homolactic fermentation, nearly 90 percent of hexose is metabolized via Embden-Meyerhof-Parnas pathway to two pyruvate molecules, which are immediately reduced by stereospecific NAD-dependent lactate dehydrogenases encoded by ldhL and ldhD genes to L- or D-lactate plus NAD+. Heterofermentative species employ phosphoketolase pathway encoded by xpk gene yielding lactate, ethanol or acetate, and CO2 from pentose phosphates. Rapid accumulation of lactic acid lowers pH below 4.5, denatures milk proteins in yogurt formation, suppresses spoilage organisms, preserves silage, and generates characteristic tang. In human type II muscle fibers, LDH-A isoform performs identical reduction during intense exertion, linking microbial and mammalian biochemistry. Industrial starter cultures monitored for bacteriophage resistance, titratable acidity 0.8 percent, and proteolytic activity ensure consistent yogurt texture, while symbiotic Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus cross-feed formate and amino acids enhancing growth and flavor compound acetaldehyde production.