Effect of medium incubation temperature on total viable counts of beef and sheep carcases

Summary

​The goals of this study were to determine the performance of various domestic and exports standard methods for meat Total Viable Count - TVC - and to investigate if a TVC method could be used to estimate Pseudomonas spp. counts on meat carcases.
The performance of two TVC media at different temperatures and incubation times was measured for sponge samples of beef and sheep chilled carcases of varying ages. The results were analysed to determine equivalence of methods that may impact national and international marketing of beef and sheep products.
Samples were collected from four abattoirs in Tasmania between November 2006 and January 2007. Sponges were used to collect the carcase surface samples from two sites on beef carcases and three sites on sheep carcases. The samples were tested for TVC by one Petrifilm - AOAC 990.12 - and one agar method - AS 5013.5. All visible colonies were counted at 24, 48 and 72 hours. A subset of 73 carcases were also tested for Pseudomonas spp.  
For both agar and Petrifilm there was no difference in log10 colony forming units/cm2 (log CFU) between 25ºC and 30ºC incubation.
Petrifilm incubated at 35ºC for 48 hours - AOAC 990.12 - consistently produced lower results, ~0.5 log CFU, than agar incubated at 30ºC for 72 hours (AS 5013.5) and agar incubated at 25ºC for 72 hours (Meat Standards Australia committee method).
In addition, incubation time affected TVC levels, in that log CFU increased between 24 and 72 hours. The effect of incubation media was not significant when measured at the same temperature. Pseudomonas spp. counts increased with TVC.
On the basis of this study, it can be concluded that 35ºC produces significantly lower estimates of chilled beef and sheep carcase TVC compared to 25 or 30ºC. In addition, preliminary data indicate that TVC levels might be used to predict concentrations of Pseudomonas spp.