Coliforms In Dairy Industry

by Aljosa Trmcic
Postdoctoral Associate
Food Safety Laboratory and Milk Quality Improvement Program

Coliforms are a group of bacteria which by definition are aerobic to facultative anaerobic, Gram negative, non-sporeforming rods capable of fermenting lactose to produce gas and acid within 48h at temperatures between 32°C and 37°C. Coliforms are regulated in all Grade A products by the Pasteurized Milk Ordinance (PMO) and also in other dairy products by different federal and state laws. Besides meeting the regulatory demands, coliforms are often tested for in the food industry to determine general microbiological condition of the food, water or environment (i.e., hygienic quality). In pasteurized food products, testing for coliforms is most often used to detect pasteurization failure, post-pasteurization contamination or coliform contamination that was too high in the initial raw material for pasteurization to completely eliminate.

Coliform concept is a very old concept and testing for it a part of tradition. The concept of coliforms was first developed to be used in water and represented E. coli and similar bacteria of fecal origin. Today, 100 years after the birth of this Coliform concept, we know that not all bacteria described by this definition are fecal bacteria. There was an attempt to up-date the concept by adding the definition for fecal coliforms, coliforms that can grow at temperatures between 44.5 and 45.5°C. Again, the definition did not capture the group completely as there are some, so called thermotolerant coliforms, which are associated with also other environments besides fecal. Today, a generally excepted separation of coliforms would be, (i) Thermotolerant fecal coliforms, (ii) Thermotolerant coliforms present in both fecal and other environments, and (iii) Cold-tolerant, environmental coliforms. This cold-tolerant group represents the most problematic coliform group in dairy industry, especially in fluid milk production.

Coliform concept is a very simple concept, on the other hand it is also a concept which is hard to define at a genus or species level, but rather at strain level. For example of five Escherichia species listed in Bergey’s Manual of Systematic Bacteriology (2005) only three are able to utilize lactose. Also, within representative species (E. coli), there is an entire group of metabolically inactive strains of which only 25% can utilize lactose. A well-defined group of bacteria is bacterial family Enterobacteriaceae and most of the coliforms are represented by bacteria from this family. There is a number of different non-coliform representatives of the family Enterobacteriaceae which are responsible for spoilage of milk and dairy products. This bacterial family also includes some genera like Salmonella and Shigella which are well known as pathogens. A lot of dairy industry laboratories are adjusting their testing methods to also capture this relevant group of bacteria, from both safety and quality aspect.

Coliforms are only a part of the quality problem in fluid milk production. Although coliforms are known to contaminate fluid milk and cause rapid spoilage of it, the most common post-pasteurization contamination of fluid milk is caused by different Pseudomonas species. Pseudomonas is another Gram-negative genus which is not detected when testing for coliforms or family Enterobacteriaceae. If testing for coliforms would be replaced by testing for the entire group of Gram-negative bacteria, all bacteria which are affected by pasteurization would be targeted and with this the sensitivity of the test would be improved. Together with having a more sensitive indicator of pasteurization success, all of the individual members of the group that cause the actual problems in fluid milk production would be targeted.

While coliforms have bene used for a long time, dairy companies may want to re-evaluate why they are testing for coliforms and how this testing informs decision making. If, for example, the goal of a testing program is to detect post-pasteurization contamination, a test for family Enterobacteriaceae or preferably all Gram negative bacteria may be more appropriate.

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