Prevalence and growth characteristics of Bacillus sporothermodurans in UHT milk

The purpose of this paper is to evaluate the prevalence of Bacillus sporothermodurans in UHT milk brands in South Africa and to analyse the level of proteolysis in UHT milk due to the growth of B. sporothermodurans during short-term and long-term storage.

Different brands and batches of commercially available retail UHT milk packages were bought from different supermarkets, during different seasons and tested for the presence of B. sporothermodurans. Sterile UHT milk was spiked with B. sporothermodurans vegetative cells and incubated at 37°C for up to 172 hours. Total plate counts, pH, spore counts, UHT milk proteolysis and the headspace volatiles dynamics were analysed at different intervals.

The contamination of retail UHT milk packages by B. sporothermodurans was found to be prevalent. The growth of B. sporothermodurans in spiked UHT milk reached a maximum of 1.9×105 cfu/ml; however, the significant proteolytic activity in UHT milk due to B. sporothermodurans only occurred long after the exponential growth phase had been attained. Furthermore, the growth of B. sporothermodurans in UHT milk did not lead to significant changes in the headspace volatile profiles of spiked UHT milk samples. Proteolytic activity in retail UHT milk packages, contaminated with B. sporothermodurans, was significantly higher when the use-by dates were reached.

Significant proteolysis in UHT milk means the assurance of high-quality UHT milk with extended storage stability for up to 10-12 months is compromised. Proteolysis of casein may lead to rapid sedimentation in UHT milk compared to UHT milk without sedimentation.

This paper is of interest to manufacturers because it raises the awareness that UHT milk containing B. sporothermodurans may not have the same storage stability when compared those without B. sporothermodurans. The presence of B. sporothermodurans in commercial UHT milk packages may lead to international and national trade restrictions for manufacturers.