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Safety and hygiene management together with thermal processing largely contribute to lower microbial populations in foodstuffs, thereby ensuring quality and safety of the final product. However, recently concerns about spore contamination in chilled and minimally processed foods and in milk, have increased despite the existence of various quality and safety management systems. Studies have shown that some Bacillus species have survived commercial sterilisation as well as Ultra High Temperature (UHT) processing. This survival can be attributed to the ability of microorganisms to adapt to various stress conditions over time. For example, the ability of Bacillus and Paenibacillus to form endospores as a mechanism for adaptation has emerged as one of the greatest obstacles in extending the shelf life of milk, especially under circumstances where the spores are heat resistant and where upon germination these organisms are able to grow under a diverse range of temperatures as well as pH. The research of Mugadza and Buys, therefore, aimed at isolating, characterising and identifying microorganisms from extended shelf life (ESL) milk processing as well as during chilled storage up to the end of shelf life.
Milk samples were collected at three critical sampling locations (raw milk tank, pasteurised milk tank and packaged ESL milk). Samples were analysed for total aerobic plate count and an average count of 6 log cfu/ml was obtained for raw milk, down to 3 log cfu/ml for pasteurised milk and 4 log cfu/ml for packaged ESL. Coliforms were observed in raw milk only. Packaged milk samples were obtained and stored at 4oC for 21 days. Analysis was done at day 0, 7, 14, 18 and 21 for total aerobic plate count and a gradual increase in microorganisms from 2.5 log cfu/ml to slightly above 4 log cfu/ml was noted along the shelf life. Morphological characteristics and biochemical tests were also done for the isolates. Bacillus pumilus dominated the spore formers followed by Bacillus subtilis. Among the non-spore formers was Artherobacter species, E. faecium and S. capitis which could be an indication of poor hygiene and post process contamination, but also of great concern with regards to milk spoilage.
Equipment surfaces are recognised to be a major source of contamination of processed milk with both spoilage and pathogenic microflora. One of the reasons for contamination of ESL milk could be attachment of persistent spores to such surfaces. The aim of the Khoza and Buys study, therefore, was to evaluate the ability of B. cereus and M. luteus to form biofilms on SS-306 stainless steel strips.
The bacterial strains were isolated from the nozzles of aseptic filling machines. A continuous flow reactor system was used to grow biofilm of B. cereus and M. luteus in skim milk. The skim milk was inoculated with a spore suspension of B. cereus and a bacterial suspension of M. luteus. The bacterial suspensions were run separately over a period of 22 hours at 37°C. Stainless steel strips were submitted to Scanning Electron Microscopy after 22 hours. The results suggested that spores of B. cereus and M. luteus can attach to stainless steel and form biofilms. The results confirmed that one of the reasons of contamination of ESL milk could be the ability of B. cereus and M. luteus to form biofilms. Over and above the fact that the spores of B. cereus can lead to spoilage of milk and milk products, a concern is that the toxins produced by some of the strains of B. cereus are detrimental to human health.
References:
D. T. Mugadza and E. M. Buys, 2014. Diversity of Spore formers and Non – sporeformers in Extended Shelf Life Milk. Proc. of the SASDT Conference 2014, Cape Town.
S. Khoza and E.M. Buys, 2014. Attachment and biofilm formation by B. Cereus and M. luteuson stainless steel strips. Proc. of the SASDT Conference 2014, Cape Town.