Fermentation fundamentals — the microbiology underneath

Fermentation is, at its most fundamental, microorganisms eating things and excreting compounds that humans find useful. The compounds are flavor (lactic acid's tang, ethanol's burn, acetic acid's sharpness, aromatic esters, complex amino acid breakdown products), preservation (acidic environments inhibit pathogens), nutrition (B-vitamin synthesis, predigested proteins, probiotic bacterial communities), and structure (CO2 leavening, alcohol-induced extraction, enzymatic protein breakdown). The microorganisms are diverse, and their interactions are dynamic.

Lactic acid bacteria (LAB) dominate the encyclopedia's vegetable, dairy, and many grain ferments. The genera Lactobacillus (now reclassified into 25+ separate genera as of 2020 but still commonly called Lactobacillus collectively), Leuconostoc, Pediococcus, Lactococcus, and Streptococcus convert simple sugars into lactic acid. The acidity drops pH to 3.0-4.5 range, inhibiting pathogens and creating the characteristic sour tang of sauerkraut, kimchi, yogurt, sourdough, and dozens of other ferments. Leuconostoc mesenteroides typically dominates early in vegetable ferments (producing CO2 and the initial pH drop); L. plantarum takes over as acidity develops; L. brevis and others complete the succession. This succession is why vegetable ferments evolve in flavor over weeks and months.

Yeasts are eukaryotic single-celled fungi that convert sugars primarily into ethanol and CO2. Saccharomyces cerevisiae is the most-domesticated species — the brewing/bread/wine yeast available commercially in countless strains. S. boulardii, S. bayanus, and S. pastorianus serve specialized roles. Brettanomyces yeasts produce the characteristic 'funk' of wild beers, lambic, and aged sourdough. Wild kahm yeasts (a phenotype, not a species — surface yeasts in oxygen-exposed environments) form harmless white surface films on vegetable ferments. Yeast tolerance to alcohol limits most beverages to 12-15% ABV without specialized strains.

Molds are filamentous fungi essential to several encyclopedia categories. Aspergillus oryzae (koji) is the foundational mold of Japanese sake, miso, shoyu, and Korean jang traditions — it produces enzymes (proteases, amylases) that break down soybean and rice proteins/starches into smaller compounds that subsequent yeast and bacterial fermentations can use. Rhizopus oligosporus binds Indonesian tempeh into its characteristic cake structure. Penicillium roqueforti and related species cure blue cheeses and certain meat products. Mold identification matters significantly — most kitchen-encountered molds are harmless to dangerous on a spectrum; some (Aspergillus flavus) produce aflatoxins; some (Penicillium) produce antibiotics; some (Aspergillus oryzae) produce the foundations of East Asian cuisine.

Acetic acid bacteria (Acetobacter aceti, A. pasteurianus, Gluconobacter oxydans) consume ethanol and oxygen to produce acetic acid. This is the underlying mechanism for all vinegar production — first an ethanol fermentation (yeasts), then an acetic fermentation (Acetobacter). The acetic step requires oxygen, hence vinegar mother surface films and stirring/agitation in production. Kombucha (which is technically a complex symbiotic ferment, not a vinegar) uses the same acetic-acid bacteria alongside yeasts.

Symbiotic communities are the most complex actors. Kefir grains are polysaccharide-bound communities of Lactobacillus, Acetobacter, Saccharomyces, and other organisms that aggregate physically into cauliflower-shaped 'grains' and reproduce by gradual mass increase. Kombucha SCOBYs (Symbiotic Culture Of Bacteria and Yeasts) form cellulose mats hosting Acetobacter, Gluconacetobacter, Saccharomyces, Zygosaccharomyces, and Brettanomyces communities. Sourdough starters host wild-captured Lactobacillus (especially L. sanfranciscensis) plus Saccharomyces exiguus (also called Kazachstania or Candida humilis). These communities are genuinely communities — no single species defines them, and the species composition shifts based on environmental conditions, substrate, and propagation history.

Succession dynamics matter as much as the individual actors. In a vegetable ferment: Leuconostoc mesenteroides dominates days 1-5 (producing CO2 and the initial acidity that excludes pathogens); Lactobacillus plantarum dominates days 5-21 (driving pH below 4.0); L. brevis and others continue developing complexity beyond 3 weeks. In a sourdough: wild yeasts and homofermentative LAB first; heterofermentative LAB second (producing acetic acid for that characteristic vinegary sourdough tang); equilibrium states reached after weeks of regular feeding. In a vinegar: yeasts ferment ethanol to ~10% ABV; Acetobacter then converts ethanol to acetic acid over weeks or months. Understanding the timeline of each species' contribution helps interpret what's happening and when to intervene or let nature continue.

The encyclopedia treats every culture profile as a vector into this microbial world. The 15 culture profiles document the major actors; the ferment profiles document what those actors produce when fed specific substrates under specific conditions. Cross-reading both is how to develop intuitive understanding of why fermentation works as it does.