5 Targets to Test for in Your Production Environment: #4 Quality indicator organisms

#4 Quality indicator organisms (yeasts and molds)

What are they?

Unlike bacteria, yeasts and molds are eukaryotic organism in the fungus kingdom. Most of them belong to the phyla Ascomycota and Zygomycota. Yeasts and molds are commonly enumerated in foods as quality indicators. They have no predictive value for the occurrence of toxigenic fungi or other pathogens. As a group, the yeasts and molds are diverse and can grow on virtually any foodstuff.

Where do they come from?

As their spores are airborne, molds are present virtually everywhere. Yeasts are most commonly used for the fermentation of sugars to alcohol and in the baking industry; the most industrially important strain is Saccharomyces cerevisiae

Yeasts and molds can survive in a variety of environmental conditions: pH levels can range from 2 to 9, temperatures from 5° to 35°C and water activity (Aw) can be as low as 0.85 or lower. As quality indicators, they can be used to assess the stability and shelf life of a product. Although they have diverse growth habits, yeasts and molds grow slowly in laboratory culture in comparison to bacteria. Thus, they are enumerated by a plate count procedure that uses agar supplemented with bacteria-inhibiting agents. Chloramphenicol, rose bengal, and dichloran are common selective agents in these agars. Spread or pour plates, incubated at 25°C for three to seven days, are recommended (ISO 7954).

Why should I test them?

The microbes present in a product have a considerable effect on its quality. The types and number of microorganisms present influence the sensory properties (such as taste, aroma, texture and color) and shelf life of the product. Food producers may choose to test for some of these microorganisms for insight into quality changes in the product; for this reason, certain yeasts and molds are designated as quality indicator organisms. Such quality indicators are often used to ensure that the product is microbiologically stable and acceptable in terms of the sensory properties listed above. The growth and number of a quality indicator is inversely proportional to the quality of the product in question. While the indicator should be present in all products whose quality is to be assessed, its growth should be unaffected by other microbial populations present. There should furthermore be relatively simple methods available for detection, differentiation and quantitation. A good example is yeast and mold counting. 

What methods do processors use to test?

Culture techniques rely on growing a particular microbial population to observable levels; this is a time-consuming process and involves producing ideal growth conditions, such as temperature, oxygen content and pressure in liquid or on solid media containing specified nutrients. The resultant colonies may be counted on solid media following incubation. This technique has its drawbacks, as it assumes that each colony is derived from an individual cell and that the incubation conditions would allow for the recovery of all cells present. Despite these limitations, plate count techniques remain the gold standard in qualitative microbiology. Counts may also be achieved in liquid media using most probable number (MPN) techniques. Unlike these more traditional methods, reporter assays assess the microbial population size through the metabolic activity of the cells. The population does not necessarily have to multiply to visible colonies before the measurement takes place. Such techniques include colorimetry; impedance/conductance; ATP-based tests and turbidometry.

There’s more to come! The series continues with a look at total count.

This article was originally published in International Food and Meat Topics, vol. 29, no. 6 (December 2018).