What Fermentation Actually Is
Fermentation is microbial metabolism producing compounds (acids, alcohols, gases) that transform food. The "controlled" part is what separates fermentation from rot: you're creating conditions that favor useful organisms and suppress harmful ones, mainly through salt concentration, anaerobic environments, and pH.
Most kitchen fermentation falls into two categories: lacto-fermentation (vegetables, dairy, some condiments) and yeast fermentation (bread, beer, wine). They use different organisms and produce different outputs, but the underlying logic is the same: create conditions for one thing to thrive, starve out everything else.
Lacto-Fermentation: Salt, pH, and Lactobacillus
Lacto-fermentation relies on Lactobacillus species, bacteria that are naturally present on vegetables and in dairy. They consume sugars and produce lactic acid, which drops the pH of the environment. As pH drops, conditions become hostile to pathogens and spoilage bacteria (most of which can't survive below pH 4.5), and the Lactobacillus population dominates.
Salt is the initial gatekeeper. It creates an osmotic environment that draws water out of vegetables (forming brine) and suppresses a wide range of microorganisms while Lactobacillus, which is salt-tolerant, survives and begins working.
Salt ratios by weight:
- 2% brine (20g salt per liter of water): standard for lacto-fermented vegetables. Works for sauerkraut, kimchi, pickles, hot sauce.
- 3-5% brine: slower fermentation, firmer texture, longer shelf life. Use for whole vegetables or anything you want to ferment over weeks rather than days.
- Below 1.5%: ferment may proceed too fast, increasing risk of off-flavors or mold.
- Above 5%: fermentation slows significantly. Salt-preserved foods like salt cod use 10%+ and don't really ferment, they cure.
For direct-pack methods (sauerkraut, kimchi): use 2% salt by weight of the vegetable. Massage it in to draw out the liquid, then pack tightly so the vegetables are submerged in their own brine.
Kimchi vs. Sauerkraut vs. Pickles
These are often lumped together, but they're different processes:
Sauerkraut: direct-pack lacto-fermentation of shredded cabbage. Salt at 2% by weight, pack tightly, let sit at room temperature for 3-7 days minimum. Simple, reliable, and a good baseline for understanding the process.
Kimchi: functionally similar to sauerkraut (lacto-fermented cabbage), but with added aromatics (garlic, ginger, scallion, gochugaru). The extra ingredients introduce more sugars and compounds for Lactobacillus to work with, producing a more complex flavor profile. Traditional kimchi uses fish sauce or fermented shrimp as an additional nitrogen source, which feeds the bacteria and deepens flavor.
Pickles: here's where the categories split. Lacto-fermented pickles (traditional dill pickles) are a real ferment: cucumbers in 3% brine, 1-4 weeks, genuinely probiotic. Vinegar pickles (most commercial pickles) aren't fermented at all. They're preserved in acidic brine from the start. No microbial activity, no probiotics. They're preserved, not fermented. Both are fine, but they're not the same thing.
Wild Yeast vs. Commercial Yeast in Bread
Commercial yeast (Saccharomyces cerevisiae in a standardized, concentrated form) is reliable and fast. It produces CO2 quickly and predictably, gives you a consistent rise, and produces relatively neutral flavor.
Wild yeast fermentation (sourdough) uses a culture of naturally occurring yeasts along with lactic and acetic acid bacteria. The yeast does the leavening, the bacteria produce acids that lower pH and create flavor. The result is a more complex, tangy bread with better crust and longer shelf life (the acidity inhibits mold).
Sourdough takes longer because wild yeast strains are generally less aggressive than commercial yeast, and because acid bacteria activity needs time to develop flavor. A levain (active starter) builds over hours. Bulk fermentation can run 4-12 hours depending on temperature. Colder temperatures slow both yeast and bacterial activity, which is why retarding dough in the fridge overnight produces more flavor without over-proofing.
Starter Care
A sourdough starter is a live culture. The basics:
Keep a 1:1:1 ratio by weight: equal parts starter, flour, water. Feed once daily at room temperature, or once a week if refrigerated. The starter is ready to use when it's at peak activity: roughly doubled in size, domed on top, and passes the float test (a small spoonful floats in water because of the CO2 it's produced).
Signs of a healthy ferment:
- Consistent rise and fall cycle after feeding
- Sour, yeasty smell (tangy but not rancid)
- Bubbles throughout, not just on top
- For vegetables: active bubbling in the jar during the first few days, gradual shift from salty to sour over 1-2 weeks
Signs something's wrong:
- Pink, orange, or black mold (discard immediately, don't scrape it off)
- Strong acetone or nail polish remover smell in starter (too much acid, not enough feeding)
- Slimy texture on fermented vegetables (could be kahm yeast or early spoilage; slimy vegetables are usually a loss)
- No activity after 5+ days at room temperature (starter may be dead; start over with a new culture)
Kahm yeast (a white, flat film on the surface of vegetable ferments) is generally harmless and normal. Skim it off and continue. Fuzzy mold is not harmless. Toss it.