Foodborne Pathogens, Toxins and Fermented Foods

Fermented food products

In food science, fermentation is the microbial conversion of food components, mainly carbohydrates, by bacteria, yeasts or moulds. The microbes produce acids, alcohol, gases or enzymes that change the flavour, texture and shelf life of the food. A product counts as fermented only when a microorganism actively transforms the substrate during processing. Vinegar is made by acetic acid fermentation, in which acetic acid bacteria oxidise ethanol to acetic acid.

The main fermented products and their starters are listed below.

• Starter culture — a defined group of microorganisms added deliberately to start and control a fermentation. Example: a yogurt starter of two thermophilic lactic acid bacteria.
• Tofu (not fermented) — made by coagulating hot soy milk with a salt such as calcium sulphate or nigari (magnesium chloride). No microbial fermentation step is involved, so tofu is a common exam trap.
• Tempeh — a soybean cake fermented by Rhizopus species, mainly Rhizopus oligosporus, which binds the beans into a firm block.
• Sauerkraut — shredded cabbage fermented in brine by lactic acid bacteria. Leuconostoc mesenteroides, a heterofermentative lactic acid bacterium, starts the spontaneous fermentation. Its lactic acid, carbon dioxide and other acids lower the pH and preserve the vegetable.
• Miso and natto — soybean products. Miso uses koji (Aspergillus oryzae) together with lactic acid bacteria and yeasts. Natto is fermented by Bacillus subtilis var. natto.
• Yogurt — milk fermented by Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. These two thermophilic organisms grow together, and the lactic acid they produce coagulates the milk protein.
• Mesophilic dairy cultures — Lactococcus lactis and Leuconostoc species are used in cultured buttermilk, cheese and butter, not in yogurt.
• Blue-veined cheese — the blue-green marbling of Roquefort and Gorgonzola comes from Penicillium roqueforti. The curd is pierced so that air reaches the mould, which grows along the channels.
• Camembert — its white surface rind is formed by Penicillium camemberti, which does not produce internal blue veins.

Foodborne pathogenic bacteria

Foodborne disease can be caused by bacteria, viruses, protozoa, helminths and, less often, fungi. Identifying the causal agent therefore requires knowing its class as well as its transmission route.