6 Gaps in Your Environmental Testing Program: #4 Psychrotrophic bacteria

Psychrotropic bacteria are especially problematic for foodstuffs and beverages such as raw meat and milk stored at low temperatures for longer periods of time.

What are psychrotrophic bacteria?

Psychrotrophic bacteria are bacteria that can grow at temperatures as low as 4 °C with optimal and maximal growth temperatures above 15 °C. This characteristic makes such microbes especially problematic for foodstuffs and beverages such as raw meat and milk stored at low temperatures for longer periods of time. The psychrotrophic groups of bacteria most commonly found in food are the genera Pseudomonas, Aeromonas, Achromobacter, Serratia, Alcaligenes, Chromobacterium and Flavobacterium spp., as well as Gram-positive genera such as Bacillus, Clostridium, Corynebacterium, Streptococcus, Lactobacillus and Microbacteria.

Why should food producers care about psychrotrophic bacteria?

Psychrotropic bacteria are adulterants and can significantly diminish the quality and, in particular, the shelf life of food. Chilled production facilities and storage tanks offer the perfect environment for the multiplication of these bacteria species. In chilled milk, for example, Pseudomonas fluorescens can produce both proteases and lipases. Hence, Pseudomonas is regarded as a species as typically responsible for technological difficulties, as the proteases and lipases they produce can cause milk fat to degrade, giving milk a greyish color and bitter taste. In vacuum-packed, refrigerated raw meat, the microflora is dominated in most cases by psychrotrophic lactic acid bacteria. Psychrotrophic Pseudomonas species are the microorganisms most often responsible for spoilage in aerobically stored chilled meat. It is well known that Pseudomonas species are very robust and able to withstand stressful environmental conditions that would inhibit the growth of other spoilage microorganisms.

How can food producers detect psychrotrophic bacteria?

ISO standards for the enumeration of psychrotrophic bacteria (ISO 17410:2019) require an incubation of 10 days at 6.5 °C. This is not a practical timeline for analyzing a product before it is sent to the vendor or customer. There is also a standard method (defined in ISO 8552) that merely estimates the quantity of psychrotrophic bacteria; this needs 25 hours to get results. However, this is not a rapid method, and is not suitable for cleaning verification as it only provides an estimate of contamination.  Flow cytometers include psychrotrophic bacteria in their counts and can give results within minutes. However, most cytometers are expensive and bulky and are used mainly in the milk industry to test large batches of milk simultaneously. In their current state, they are not applicable to in-process hygiene management or other industries such as meat production.

This article originally appeared in International Food & Meat Topics 31 (4).