Yeast and Mold Count (also written as Yeast and Mould Count) is a microbiological test that quantifies the number of viable yeast cells and mould colony-forming units (cfu) in food, feed, pharmaceutical, and environmental samples. Results are expressed in cfu/g or cfu/mL.
Yeasts and moulds are eukaryotic fungi with distinct roles in food safety and quality:
- Yeasts: Single-celled fungi that cause spoilage through fermentation (CO₂ production, off-flavours, gas swelling of packaging) in dairy products, juices, sauces, and soft drinks.
- Moulds (filamentous fungi): Multi-cellular fungi that cause visible surface growth (moulding) and — critically — may produce mycotoxins, hazardous secondary metabolites with carcinogenic and immunotoxic properties. Common mycotoxin-producing genera include Aspergillus, Penicillium, and Fusarium.
- Public health relevance: While most spoilage moulds and yeasts are not directly pathogenic, mould-contaminated food may contain mycotoxins even after the visible mould is removed.
- Yeast and mould testing is part of Ovalab’s accredited microbiological analysis service.
Testing Methods
The internationally harmonised methods for yeast and mould enumeration are defined in two complementary ISO standards:
- EN ISO 21527-1:2008 — Colony count technique for products with water activity >0.95: Applicable to high-moisture products (fresh meat, dairy, fruit, vegetables, fresh pasta). Sample dilutions are plated on Dichloran Rose Bengal Chloramphenicol (DRBC) agar or Oxytetracycline Glucose Yeast Extract (OGYE) agar, incubated at 25 °C ± 1 °C for 5 days. Antibiotics suppress bacterial growth, allowing selective enumeration of fungi.
- EN ISO 21527-2:2008 — Colony count technique for products with water activity ≤0.95: Targets osmophilic yeasts and xerophilic moulds in dried and low-moisture products (dried fruits, cereals, spices, confectionery, powders). Uses Dichloran 18% Glycerol (DG18) agar with incubation at 25 °C ± 1 °C for 5 days.
- Pharmaceutical applications: Total Yeast and Mould Count (TYMC) for pharmaceutical products and raw materials follows European Pharmacopoeia (Ph. Eur.) methods — separate from food analysis methods but using comparable principles.
Connection to Mycotoxin Risk
Elevated mould counts in food materials — particularly cereals, nuts, dried fruits, spices, and feeds — are a strong indicator of potential mycotoxin contamination. Key relationships include:
- Aspergillus flavus / A. parasiticus: Producers of aflatoxins (B1, B2, G1, G2) — potent carcinogens regulated under EU Regulation (EC) No 1881/2006.
- Penicillium verrucosum: Producer of Ochratoxin A (OTA) in cereal grains and dried vine fruits.
- Fusarium spp.: Producers of Deoxynivalenol (DON), zearalenone, and fumonisins in grain crops.
- Important note: A high mould count does not automatically indicate mycotoxin presence (conditions must be appropriate for toxin production), and mycotoxins can persist in food even after mould is no longer detectable. Both yeast/mould counts and targeted mycotoxin testing may be warranted for high-risk commodities.
Frequently Asked Questions
Why are yeasts and molds tested in food?
Spices and dried foods are low water-activity products that are particularly susceptible to contamination by xerophilic moulds capable of growing at reduced moisture levels. Many spice-contaminating moulds (particularly Aspergillus spp.) can produce aflatoxins and Ochratoxin A under appropriate conditions. Even if the contamination pre-dates arrival at the food facility, mycotoxins can be present in the incoming material. Routine yeast and mould testing of spice lots, combined with targeted mycotoxin screening, is essential for ingredient quality assurance.
What are acceptable yeast and mold levels?
Standard EN ISO 21527-1 and ISO 21527-2 methods require incubation at 25 °C for 5 days before colony counting, making the total test time approximately 5–7 working days including sample preparation and reporting. This relatively long incubation period (compared to bacteria) reflects the slower growth rate of fungi. Rapid methods (ATP bioluminescence, DNA-based) can provide earlier screening results but are typically used as presumptive tests requiring culture confirmation.
How long does a yeast and mold count take?
Commission Regulation (EC) No 2073/2005 does not set universal EU-wide yeast and mould count limits for all foods. However, specific limits exist in certain categories (e.g., for yeasts in specific fermented dairy products). Limits are more commonly set by: individual customer/retailer specifications, sector-specific codes of practice, and national guidelines. For pharmaceutical products and dietary supplements, the European Pharmacopoeia (Ph. Eur.) sets specific Total Yeast and Mould Count (TYMC) limits (e.g., ≤10² cfu/g for certain non-sterile oral preparations).
What is the connection between molds and mycotoxins?
The key difference lies in the target food matrix: ISO 21527-1:2008 is for products with water activity >0.95 (fresh, high-moisture products — dairy, meat, vegetables, juices) using DRBC or OGYE agar. ISO 21527-2:2008 is for products with water activity ≤0.95 (dried, low-moisture products — spices, cereals, confectionery, dried fruits) using DG18 agar, which contains glycerol to reduce the water activity of the medium and select for organisms adapted to low-moisture environments. Selecting the correct standard for the product type is essential for accurate results.
Which foods are most susceptible to yeast and mold?
Visible mould on food is generally a clear indicator that the product should not be consumed or used, particularly for soft, high-moisture foods where mycotoxins may have diffused throughout the product — not just at the visible mould surface. For hard cheeses, some regulatory authorities allow cutting away the mouldy portion plus a safety margin. However, for any food where mycotoxin-producing moulds are possible (cereals, nuts, spices, dried fruits), the presence of visible mould warrants laboratory analysis before any decision on product disposition, as toxins may be present even where mould growth appears superficial.