Yeasts and molds are significant spoilage organisms in foods where bacterial competition is limited, typically due to low pH, low water activity (aw), low temperature, or the presence of preservatives. Zygosaccharomyces is the most important spoilage yeast genus due to its extreme osmotolerance and resistance to weak acid preservatives. Z. bailii and Z. rouxii can grow at aw as low as 0.62 and tolerate concentrations of sorbic acid, benzoic acid, and acetic acid that inhibit most other yeasts. They cause spoilage of fruit juice concentrates, sauces, salad dressings, wine, and dried fruit.
Debaryomyces hansenii is halotolerant and can grow at aw 0.83, frequently causing spoilage of salted and cured meats, cheeses, and brined vegetables. Candida species (C. parapsilosis, C. zeylanoides) are also common spoilage agents, particularly in dairy products. Yeast spoilage is manifested as surface growth (pellets, films, or turbidity in liquids), production of CO₂ (swollen packages), off-flavors (estery, yeasty, stale), and changes in pH. Detection involves plating on malt extract agar or oxytetracycline glucose yeast extract agar (OGYE) with incubation at 25°C.
Mold spoilage of foods is caused primarily by species of Aspergillus, Penicillium, and Fusarium. Aspergillus flavus and A. parasiticus produce aflatoxins (B₁, B₂, G₁, G₂), which are potent hepatocarcinogens. A. ochraceus produces ochratoxin A, a nephrotoxin found in cereals, coffee, and wine. Penicillium expansum is the primary cause of blue mold rot in apples and produces patulin, a mycotoxin regulated in apple juice products. Fusarium species (F. graminearum, F. verticillioides) produce deoxynivalenol (DON), fumonisins, and zearalenone in cereal grains.
Control of fungal spoilage relies on multiple hurdles including reduction of water activity (drying, addition of humectants), pH adjustment, use of weak acid preservatives (sorbic, benzoic, propionic acids), and modified atmosphere packaging (low O₂, elevated CO₂). The concept of hurdle technology emphasizes the synergistic effect of combining multiple sub-lethal stresses. For mycotoxin prevention, control of fungal growth in the field (good agricultural practices, fungicide application, resistant cultivars) and during storage (temperature, moisture control) is essential. Mycotoxin testing using HPLC, LC-MS/MS, or ELISA is employed by food processors for raw material and finished product verification. Fungal spoilage is distinct from bacterial microbial spoilage and can result in mycotoxin production. Some spoilage molds are related to those used in beneficial mold fermentations. Control strategies include reducing water activity through drying.
