Presumptive Coliforms

  • Prepare cheese homogenates and serial dilutions as described in Section A.
  • Place 5 ml of the 0.1 dilution (= 0.5 g of original sample or Omega dilution) and 1 ml of additional dilutions as required into each of duplicate petri dishes and add molten VRB agar. (Note: Do not sterilize VRB agar.) When solidified, pour over layer (5 ml VRB).
  • Incubate at 35C + 10C for 18 - 24 hrs.
  • Count the dark red colonies, at least 0.5 mm in diameter, and record results as coliforms per g of sample.

Samples of cottage cheese and other acid milk products should be plated within 24 hrs. after manufacture because coliform counts decline under acid conditions. Coliforms also decrease in number during aging of ripened cheese varieties.

It must be emphasized that this method provides a presumptive count only. If presumptive counts are consistently high, colonies should be confirmed (see Standard Methods). The Canadian Food And Drug Act and Regulations permit 500 coliforms/g of cheese made from pasteurized milk and 5,000 coliforms/g of cheese made from unpasteurized milk. Permitted counts of Eshericia coli are 100 and 500/g respectively.