Short communication: Influence of preserving factors on detection of β-lactamase in raw bovine milk.

The refrigeration (4, 8, 24, or 48 d), freezing (1, 7, or 30 h), thawing (25, 40, or 60°C), and usage of preservatives (sodium azide, potassium dichromate, sodium thiocyanate, bronopol, and methanol), of raw bovine milk were investigated for β-lactamase activity. The β-lactamase activity in all samples was assessed by using the test strips, and the samples with the preservatives were further assessed for β-lactamase activity using the cylinder plate method. Results showed that the refrigeration, freezing, and thawing of raw bovine milk samples had no influence on the assay of the β-lactamase activity. The addition of sodium azide or potassium dichromate as a sample preservative failed the test of β-lactamase activity, whereas the addition of sodium thiocyanate, bronopol, or methanol had no influence on the test. In conclusion, sodium azide and potassium dichromate were not suitable preservatives for assessing β-lactamase activity in raw milk when the cylinder plate method was used.