Thermostable Deoxyribonuclease Content and Enterotoxigenicity of Cheddar Cheese Made with Sub-Normal Starter Activity

Abstract

The minimum detectable concentrations of purified thermostable deoxyribonuclease (nuclease) in Tris buffer and in Tris buffer containing 0.1% bovine serum albumin were 300 and 10 ng/ml, respectively. Recovery rate of nuclease added to cheese samples was 57 ± 18%. The trend of changes in nuclease concentration in unsalted and salted Cheddar cheese made with sub-normal starter activity, during production and storage at 11 and 4 C for 6 weeks, was similar to that of enterotoxin-A. Correlations between Staphylococcus aureus count, nuclease and enterotoxin concentrations were not significant under all experimental conditions. Nuclease was always detected in cheese containing enterotoxin, and also in cheese containing S. aureus in numbers below those required for producing detectable amounts of enterotoxin. In some instances, uncharacteristic results were observed on the nuclease assay reagent; however, these were easily distinguishable. The nuclease test appeared to be specific for S. aureus when concentrations of > 20 ng/20 g of cheese were considered a positive result. Nuclease showed stability to the extensive biological activity of the cheese during extended storage. It is recommended that the nuclease test be done routinely in factories processing cheese, made with sub-normal starter activity, immediately before processing.