Simultaneous Coliform and Aerobic Plate Counts in Raw Milk using the Chromogenic Plate Method
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Abstract:
In order to improve the counting efficiency of coliforms and the aerobic plate count, 2-Nitrophenyl-β-D-Galactopyranoside was selected as the chromogenic substrate in the plate culture medium. The β-Galactosidase encoded by the lacZ gene in coliforms decomposes it to produce yellow compounds, making the coliforms different from other strains. This was used for simultaneous enumeration of coliforms and the aerobic plate count. The medium formula was optimized to evaluate the specificity, accuracy, and application in raw milk. The medium formula was finally determined as follows: casein tryptone 12.5 g/L, yeast extract fermentation 2.5 g/L, potassium dihydrogen phosphate 2.75 g/L, potassium dihydrogen phosphate 1.75 g/L, sodium chloride 5.0 g/L, agar 15.0 g/L, 2-Nitrophenyl-β-D-Galactopyranoside 0.1 g/L; pH value 7.0. Based on this, adding 0.05 g/L Isopropyl-β-D-thiogalactoside or 2.0 g/L lactose enhanced the colony color effect, while glucose inhibited the formation of yellow compounds. In the specificity test, the color of coliforms was yellow to orange, whereas other strains were milky white. In raw milk samples, the coliform and aerobic plate counts in this medium were basically consistent with the national, standard method. In conclusion, the complex chromogenic medium can greatly improve the efficiency of coliform and aerobic plate counts, reduce the detection cost, and provide a feasible scheme for the detection of health indicators in raw milk.
