Do bacterial spores in milk powders survive UHT processing?

Sterility failure is the major microbiological hazard faced by UHT milk manufacturers. Failure to achieve a sterile product has two main causes: spore survival through the heat treatment process and post-heat recontamination. Many UHT milk manufacturers believe that reducing the spore levels in milk powders used for recombining reduces the chance of spore survival. In reality, the cases of sterility failure caused by heat treatment survival are extremely rare. Nevertheless, this belief has led to the adoption of unnecessarily strict mesophilic and thermophilic spore specifications and the unnecessary rejection of quality powders. This project investigated aspects of the recombining and UHT processes that govern whether spores in the original milk powder can survive through to the final UHT milk. The work showed that spore-forming bacteria are able to grow during the holding period between milk powder recombining and UHT processing. However, it is the vegetative cells that increase, not bacterial spores, providing this holding period is less than 8 h. Therefore, assuming that good manufacturing practices are applied, there should be no increase in spore level during recombining and holding compared with the level in the original milk powder. Trials performed on a UHT pilot plant achieved greater than 10 log reductions at 125 ¦C per 4 seconds of a variety of Bacillus spore types tested. It is estimated that this equates to at least a 13 log reduction at 140 ¦C per 4 seconds (i.e. normal UHT treatment conditions). In practical terms this means that, if a powder used for recombining contained 1000 spores per gram, it would result in the production of UHT milk with a contamination failure rate of one pack in 100 million 1 L packs. This work demonstrated that there is negligible risk of microbiological failure of UHT milk from the spores in milk powders that meet current specifications (200 per gram). Indeed, powders that have higher spore counts (e.g. 1000 per gram) may be just as suitable for UHT milk manufacturing purposes.