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A modified nitrate reductase test for raw refrigerated milks

Authors: Patricia C. Loane


The original Greiss-Llovsay method for detecting nitrate reductase activity of micro-organisms has been modified for daily routine milk quality control programs by (a) incorporating preliminary incubation of milk sample at 15°C ± 0.5°C for 22 h ± 1 h prior to further incubation at 30°C ± 1°C before subsequent reaction and colour development; (b) addition of penicillin prior to preliminary incubation for selection and gram-negative bacteria; and (c) substitution of the non-carcinogenic reagent alpha-naphthol for alpha-naphthylamine used in the original method.

Bacterial counts of flora, both total, as denoted by the standard plate count, and psychotrophic, were used in the determination of a suitable standard for the time of nitrate reduction (viz. greater than 5 hours).

Results reveal that the method as used and the standard adopted as suggested combine to form an efficient means of reflecting milks of poor bacterial quality.

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