Fermentation

Fermentation is the transformation of food by microorganisms - bacteria, yeasts, and molds - through enzymatic activity. In food preservation, the term most commonly refers to lactic acid fermentation (lacto-fermentation): the conversion of sugars to lactic acid by naturally occurring Lactobacillus bacteria, producing preserved vegetables with complex, sour flavors and significant probiotic content.

This is the oldest known food preservation method. Sauerkraut, kimchi, traditional dill pickles, miso, tempeh, sourdough bread, cheese, yogurt, beer, and wine are all fermented foods. The mechanisms differ (different organisms, different substrates, different products), but the common element is microbial transformation.

How Lactic Acid Fermentation Works

Lactobacillus bacteria and related species are present on the surface of virtually all fresh vegetables. When vegetables are submerged in brine (salt solution) without access to air, these bacteria are favored over spoilage organisms:

1. Salt draws moisture from vegetables through osmosis, creating brine
2. The anaerobic environment (no air) prevents mold and aerobic bacteria
3. Lactobacillus begins converting sugars to lactic acid
4. Accumulating lactic acid lowers pH (typically to 3.5-4.5 within 1-4 weeks)
5. The acidic environment inhibits all pathogenic and spoilage organisms
6. The result is a preserved, acidified product that’s stable without refrigeration (refrigeration slows the process and extends holding time after fermentation)

Salt Concentration

Salt concentration is the most critical variable. Salt suppresses spoilage while allowing Lactobacillus to dominate early in fermentation.

Brine fermentation (cucumbers, carrots, turnips): 2-3% salt by weight (roughly 1 tablespoon non-iodized salt per 2 cups water). Too little salt allows spoilage organisms; too much inhibits even Lactobacillus and slows fermentation.

Dry salt fermentation (sauerkraut, kimchi): 1.5-2% salt by weight of the shredded vegetable (roughly 1 tablespoon per 1.75 lbs cabbage). Salt draws moisture out of the vegetable to create its own brine; the vegetable ferments in its own juices.

Use non-iodized salt. Iodine in iodized table salt inhibits fermentation bacteria.

Anaerobic Environment

Keeping vegetables submerged below brine is essential. Surfaces exposed to air develop kahm yeast (a harmless but unappealing white film) or mold. Various approaches:

• Weighted plate or bag of brine pressing vegetables down
• Ziplock bag filled with brine placed on top of vegetables in a wide-mouth jar
• Commercial fermentation weights (glass or ceramic discs sized for Mason jars)
• Dedicated fermenting crocks with water-seal lids

Kahm yeast on the surface is harmless and can be scraped off. Green or black mold typically means the ferment was exposed to air and should be discarded.

Safety

Lactic acid fermentation is considered one of the safest home preservation methods when done correctly. C. botulinum cannot grow in acidic environments below pH 4.6, and properly fermenting brine reaches this pH within a few days to a week. Salmonella and E. coli are rapidly inhibited by the increasing acidity.

The risk with fermented vegetables is spoilage (off flavors, mold) rather than serious foodborne illness in correctly made ferments. Trust your senses: a properly fermented vegetable smells sour and clean; a spoiled one smells putrid or unpleasant in a way distinct from the expected sourness.

Temperature

Optimal fermentation temperature is 65-75°F. Higher temperatures speed fermentation but can favor undesirable bacteria and softer textures. Lower temperatures slow fermentation and produce different flavor profiles (cooler ferments are often more complex). Traditional sauerkraut fermentation in many European traditions occurred in cool cellars at 55-65°F over months.

See also: Lacto-Fermentation