Bacteriological studies on the extension of the shelf life of raw minced beef

Abstract

Two hundred and thirty four samples of raw minced beef were subjected to storage at 0° and 7°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 effects of these treatments on the bacterial population in the samples was studied after storage intervals of one, two, four, seven, 11 and 17 days. The numbers of aerobic bacteria, Enterobacteriaceae, lactic acid bacteria, Brochothrix thermosphacta, psychrotrophic bacteria and several indirect determinations of pseudomonads were used to assess the microbiological quality of the minced beef. Determinations of pH, redox potential and. lactic acid concentration were used in conjunction with microbial levels and evaluations of physical and sensory characteristics to determine shelf life of the raw minced beef under various conditions of storage and treatment. One hundred and twenty eight representative psychrotrophic spoilage isolates were identified in conjunction with seventy one selectively isolated lactic acid bacteria. Results indicated that lowered storage temperatures produced significant increases in the shelf life of the minced beef as was evident by significant reductions in the levels of most spoilage microorganisms. In addition it was demonstrated that all forms of packaging and/or additive treatment were greatly aided in their ability to prolong shelf life by a lower storage temperature. This finding led to the conclusion that temperature control around 0°C was the central element in achieving shelf life extension of a highly perishable product such as raw minced beef. Packaging and additive treatments enhanced the effect of low storage temperatures. Compared to aerobic packaging (control), all other treatments led to substantial increases in shelf life, especially when samples were stored at 0°C. Identification of psychrotrophic spoilage isolates revealed a predominance of Gram-negative organisms (63, 3%), most of which were classified as pseudomonads (71, 5%), the rest being Enterobacteriaceae. Among the Gram-positive isolates, lactobacilli predominated (44, 7%), with the rest belonging to the coryneform group and the genera Ku.JLtf'U,Q, MicJtococcLW and SbteptococcLW. Identification of lactic acid bacteria isolates revealed the presence of three predominant species, Lactobac,i,le.LW ~ake (33,8%), Lactobac,i,ULW cUJtvatU/2 (22,5%) and Lactobae,1,U,LW bavaJticLW (21,1%). Various other Lactobac~ and Leucono~toc species made up the remainder of isolates in small proportions. Commonly observed oxygen relationships of typical meat spoilage bacteria 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 aerobically packed samples. These apparent anomalies could be attributed to the specific type of microenvironment prevailing in raw minced beef.