Eight plating procedures for the detection and enumeration of coliforms in ice cream and nonfat frozen dessert were compared. The procedures were: (i) direct plating of 1 ml, (ii) 2 ml, or (iii) 3 ml of product with violet red bile agar (VRBA), (iv) direct plating of 5 ml of product with VRBA in a large petri dish, (v) plating 10 ml of a 1:10 dilution of the product across three plates poured with VRBA, (vi) plating 1 ml of a 1:10 dilution with VRBA, and (vii) plating 1 ml of a 1:10 dilution to Petrifilm E. coli Count (PEC) plates incubated for 24 and (viii) 48 h. Three samples of ice cream (high-fat chocolate, high-fat vanilla, and high-fat strawberry) and three samples of frozen dessert (fat-free chocolate, fat-free vanilla, and fat-free strawberry) were selected as representative products and were inoculated with coliforms for use in the study. The data indicated that direct plating of ice cream or frozen dessert was less reliable than plating a diluted product for detection and enumeration of coliforms. Results of platings with VRBA and PEC of 1-ml portions of a 1:10 dilution were closely related to the results of the Standard Methods agar procedure of plating 10 ml of a 1:10 dilution for detection and enumeration of coliforms.
1Silliker Laboratories, Inc., 1304 Halsted Street, Chicago Heights, Illinois 60411
2Microbiology Products/3M Building 270-3N-04, 3M Center, St. Paul, Minnesota 55144-1000
3Statistical Data Services/3M Pharmaceuticals Building 270-3A-01, 3M Center, St. Paul, Minnesota 55144-1000.