Yeast fact sheet

General
Yeasts are unicellular fungi that are mainly assigned to the taxonomic group of sac fungi (Ascomycota). They play an important role in food production, since on the one hand they are required for fermentation processes in many foods. Yeasts ferment sugars to alcohol. The best-known yeast species is baker's yeast (Saccharomyces cerevisiae). On the other hand, yeasts play an important role as potential spoilage organisms in many foods.

• Further information on another taxonomic group within the kingdom of fungi: see fact sheet on moulds

Characteristics

• Spore-forming (ascospores)

Origin / occurrence

• Yeasts are widespread in our environment (e.g. soil, plants, air, water, humans, ...)
• They are part of the "normal" microbial flora in many foods (e.g. fruit) or are intentionally used in food production and exert a positive technological effect (e.g. baked goods, alcoholic beverages)

Significance

• Cultured yeasts are deliberately used in some foods because of their beneficial effects. For example, kefir is produced by the fermentation of milk by lactic acid bacteria and yeasts. The soy product tempeh is also produced with the help of yeasts. → In these foods, in addition to the yeasts, the aerobic mesophilic count (total viable count) is correspondingly high and cannot be evaluated as a hygiene or spoilage parameter
• However, in many foods yeasts are feared spoilage organisms. Especially in foods with a reduced pH value such as delicatessen products, cheese and cheese products, and fruit juices. Furthermore, osmotolerant or osmophilic yeasts play an important role in spoilage processes of foods with a low water content such as fruit concentrates, sugar-rich confectionery, ...
• Depending on the food group, a certain number of yeasts is tolerated in most foods. However, if these values are exceeded, this indicates possible spoilage of the food (exceptions are foods in which cultured yeasts are deliberately used during production)
• Yeasts can also grow under exclusion of oxygen and produce ethanol. Unlike most bacteria, yeasts have the particular ability to grow at low water contents and/or low pH values

Growth conditions

• Temperature:
  • Optimum: +30 °C to +35 °C
  • Minimum: 0 °C
  • In general no growth above +50 °C
• pH value: growth at pH 1.5 to 8.5
• a_w (water activity): growth up to a water activity of at least 0.80
• Some yeasts are osmophilic or osmotolerant or acid-tolerant. This means these organisms can also grow at low water and pH values
• Oxygen requirement: aerobic respiration or anaerobic fermentation (facultatively anaerobic)

Inactivation by heating

• Most yeasts are killed at +72 °C with a minimum exposure time of 2 minutes (Attention: monitor core temperature)
• Yeast spores are killed at a core temperature of +80 °C

Errors in cleaning and disinfection as well as air hygiene:
This can result in contamination of foods by, for example, microbiologically contaminated utensils, equipment, surfaces or machines.

Note on air hygiene:
The number of yeasts in the air can be problematic towards the end of the harvest season in the vicinity of viticulture and fruit-growing areas

1. Check the implementation of cleaning and disinfection:
   1. Hygiene plan
   2. Execution of cleaning and disinfection, e.g. disinfectant dosing, contact time, use of suitable and clean materials and cleaning cloths
2. Avoid residual moisture and biofilms. Ensure good drying of equipment, utensils and surfaces
3. Train staff regarding potential errors and corrective measures
4. Repeat cleaning and disinfection
5. In case of elevated airborne microbial counts, implement appropriate measures to reduce the number of yeasts in the air
6. Verify effectiveness by visual on-site inspections, environmental sampling and follow-up testing

1. Check incoming products at goods receipt for e.g. arrival/transport temperature, shelf life, packaging, deviations or abnormalities
2. The same applies to goods before use in production. In case of doubt, do not use the food and consult a supervisor
3. Check microbiological specifications. If necessary, arrange laboratory testing of suspicious products or request results from the supplier
4. Verify effectiveness by follow-up testing

1. Request traceability of the cold chain during transport from the supplier
2. Check incoming and storage temperatures
3. Monitor cooling temperatures and cold chains during processing, transport and sale
4. If overage (or excessively long shelf life) is suspected, perform appropriate storage tests
5. Verify effectiveness by follow-up testing

1. Check processes, production records and documentation
2. Inactivation of yeasts at a core temperature of +72 °C for at least 2 minutes. Yeast spores are inactivated above +80 °C
3. Check the correctness of processes and temperatures
   1. In general, holding temperatures above +60 °C are required
   2. Rapid cooling: after 3 hours a core temperature of at least +7 °C must be reached (special caution is required for large quantities)
4. Verify effectiveness by follow-up testing

1. Check processes, environmental conditions, production records and documentation
2. Verify effectiveness by follow-up testing