Use of petrifilm to evaluate the microflora of frozen dessert mixes
1989
Smith, L.B. | Zottola, E.A. | Fox, T.L. | Chausse, K.
Ninety vanilla frozen dessert mix samples were analyzed to determine total microbial populations and coliforms present in samples inoculated with a coliform isolated from raw milk. Standard methods agar (SMA, PCA, Difco) and violet red bile agar (VRBA, Difco) as well as Petrifilm (PSM, PVRB) were used for plating of samples. Standard VRBA 1:10 method produced significantly higher counts of colony-forming units (CFU) than PVRB 2:3 and VRBA 2:3 methods. VRBA 2:3 colony-forming unit counts were also significantly higher than those on PVRB 2:3, but both methods showed a moderately strong linear relationship. Repeatabilities of all three coliform plating methods (VRBA 1:10, VRBA 2:3, and PVRB 2:3) were acceptably low. Less than 10% of samples plated on SMA and PSM resulted in total aerobic colony-forming units in the countable range, making evaluation of data difficult and resulting in a lack of strong linear relationship between PSM and SMA. An additional 70 local retail store samples containing naturally occurring coliforms were evaluated using PVRB 2:3 and VRBA 1:10 methodology, confirmed in brilliant green lactose bile broth (BGLB, Difco) and compared to standard VRBA 1:10 previously analyzed. All methods were equivalent for mean log coliforms, i.e., 1.38, 1.33, and 1.31 for PVRB 2:3, VRBA 2:3, and VRBA 1:10, respectively. Petrifilm methods were comparable to standard methods for enumerating coliforms in frozen dairy products, and would be a valid alternative to standard coliform and total plate count methods.
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