How to know if a fermentation has gone bad?

Normal: cloudy brine, bubbles, white sediment on the bottom, a thin white film on the surface (kahm yeast), sour smell, tangy taste. Bad: fuzzy spots in blue, green, black, or pink on the surface (mold), foul or rotten smell, slimy texture that was not there before. If you see actual mold, discard…

Use 2-3% salt by weight of the total contents (vegetables plus water for brines). Too little salt allows harmful bacteria to compete with beneficial ones. Too much salt slows fermentation and makes the product overly salty. A kitchen scale is the most reliable way to measure.

The single most important safety rule: everything being fermented must stay below the liquid surface. Vegetables exposed to air above the brine are where mold grows. Use a glass weight, a small plate, a zip-lock bag filled with brine, or a cabbage leaf pressed on top.

Wash all jars, weights, and utensils with hot soapy water before each batch. Use fresh, unblemished vegetables — not wilted or damaged produce. Use non-chlorinated water (filtered or boiled and cooled). These basics prevent contamination before fermentation even begins.

For extra confidence, test the pH of your ferment with inexpensive pH test strips ($5-8 for 100 strips). Finished fermented vegetables should be at pH 4.6 or below. Most well-fermented products reach pH 3.0-3.5. Below 4.6, the acidity is too high for dangerous pathogens like botulism to survive.