The influence of extrinsic factors on the microbiological spoilage pattern of ground beef

Two hundred and thirty four samples of raw minced beef were subjected to storage at 0 and 7 degrees C over a period of 17 days. The samples were subjected to four different treatments where the controls (Treat 1) were aerobically packed. The vacuum-packed samples (Treats 2 and 3) differed only by the addition of 0.5% L-(+)-ascorbic acid to the Treat 3 samples. Treat 4 represented aerobically packed samples to which a commercial 'colour retainer' was added. The microbiological results showed clearly that temperature control around 0 degree C was the central element in achieving shelf life extension of raw minced beef. Vacuum packaging and additive treatments enhanced the effect of low storage temperatures. Identification of 128 psychrotrophic spoilage isolates revealed a predominance of Gram-negative bacteria (63%), most of which were classified as Pseudomonas spp. (72%), the rest being Enterobacteriaceae. Among Gram-positive isolates, lactobacilli and yeasts predominated (45% and 28%, respectively). Commonly observed oxygen relationships were not found in this study. Pseudomonads proliferated even in vacuum packaged samples throughout the entire storage period, whilst lactic acid bacteria were also found in high numbers in aerobically packaged samples. Identification of 71 isolates of lactic acid bacteria revealed a strong predominance of Lactobacillus saké (34%), followed by L. curvatus (23%), L. bavaricus (21%) and L. alimentarius (10%). Significant numbers of lactobacilli were isolated from samples in all treatment groups, including the aerobically packed categories.