Abstract
One hundred samples (50 pork and 50 beef) of mince meat were purchased from the central open market in Athens, and the spoilage bacterial groups total aerobic flora, pseudomonads, Br. thermosphacta, lactic acid bacteria, H2S - producing bacteria and Enterobacteriaceae were enumerated. The prevalence of pathogenic bacteria such as E. coli. Listeria spp. and Salmonella spp was also examined. The total microbial population ranged between 4.6 until 8.5 log 10 cfu/g. In all samples tested pseudomonads were the dominant organisms followed by Br. thermosphacta. 5% of total were positive for Salmonella spp., and 44% were positive for Listeria spp. Of 44 samples in which Listeria spp. were detected, the population was below 10 cfu/g in 42, while in 2 samples it was from 10-100 cfu/g. Only 2 from the 44 samples contained Listeria monocytogenes. The concentration of E. coli, ranged fr-om > 1 to 4.58 log 10 cfu/g and it was present in 74% of samples. Changes in microflora and sensory characteristics of fresh ground meat (beef and pork) with pH from 5.34 to 6.13 were monitored at different storage temperatures (0-20°C). At all conditions tested, pseudomonads were the dominant bacteria followed by Brochothrix thermosphacta while the other bacteria (lactic acid bacteria and Enterobacteriaceae) remained at lower levels. The microbiological and sensory analysis showed that pseudomonads can be used as a good index for spoilage of aerobic stored ground meat. The kinetic parameters (maximum specific growth rate (mumax) and lag phase (lambda)) of the spoilage bacteria were modelled using a modified Arrhenius equation for the combined effect of temperature and pH. Meat pH affected growth of all spoilage bacteria except lactic acid bacteria. The "adaptation work," characterized by the product of mumax and lambda (mumax*lambda) was unaffected by temperature for all tested bacteria but was affected by pH for pseudomonads and B. thermosphacta. For the latter bacteria a negative linear correlation between ln(mumax*lambda) and meat pH was observed. The models were further validated under dynamic temperature conditions using different fluctuating temperatures. Graphical comparison between predicted and observed growth and examination of the relative errors of predictions showed that the models satisfactorily predicted growth under dynamic conditions. Shelf life predictions based on pseudomonad growth showed a mean difference of 13.1% compared to shelf life observed by sensory analysis. The present study provides a "ready to use", well-validated model for predicting spoilage of aerobically stored ground meat. The use of the model in combination with data on the initial microbial level can lead to effective management systems for the optimization of meat quality.