Fermentation

Fermentation is the controlled souring of a meat mix. Lactic acid bacteria consume simple sugars, chiefly dextrose, and produce lactic acid, which lowers the pH. In the Curesmith standard a fermented sausage is taken to about pH 4.9, and the amount of sugar is what sets that: too little and the cure never sours properly, while beyond a point extra sugar makes no difference, because the bacteria cannot keep growing in their own acid.

The falling pH is one of the Safety Triangle's forces, and its speed matters as much as its depth. A pH that drops quickly holds back Staphylococcus aureus before it can grow and form toxin, and the accumulated warm time before that drop is accounted as degree-hours; this is why warmer, faster fermentations lean on a rapid early acidification, while the cooler, slower fermentations of northern Europe manage the risk differently. Fermentation never acts alone: it works alongside the salt in the mix, 2.2 to 2.5% in the Curesmith standard, and with nitrite at 156 ppm.

Traditionally the souring was left to the bacteria already in the meat, or coaxed along by backslopping a little of a previous successful batch. Modern practice adds a measured starter culture instead, usually lactic acid bacteria such as Lactobacillus sakei or Pediococcus, often alongside nitrate-reducing micrococci or staphylococci that protect colour and build flavour.

One subtlety catches people out. On long, mould-ripened sausages the final pH often climbs back above 5.5, because the surface moulds consume lactic acid and give off ammonia. When that happens, safety leans more heavily on a low water activity than on acidity.