Predictive Equations to Assess the Effect of Lactic Acid and Temperature on Bacterial Growth in a Model Meat System

Meat microflora is mainly composed of Acinetobacter, Moraxella, Brochothrix termosphacta, Lactobacillus, Pseudomonas and Enterobacteriaceae family genera, such as Klebsiella sp. and E. coli. In natural conditions meat pH can range from about 6.0 (being close to the optimum level for most pathogenic and alteration-causing bacteria) to values close to 5.5, at which microbial growth rate decreases significantly. Combining low pH with other factors such as low temperatures can almost completely prevent microbial growth from occurring. Muscle pH variation is highly dependent on the tissue glycogen level at the time of slaughter.

Weak organic acids tend to be more effective as antimicrobials than strong acids because they acidify the interior of the cell (Anderson et al., 1987). Antimicrobial activities exerted by organic acids depend upon i) pH reduction, ii) minimizing dissociation of the acid and iii) maximizing toxicity of the acid molecule (Anderson et al., 1987). Lactic acid produces an inhibitory effect because of the decrease in pH; this acid could act both on the meat muscle flora itself and on that of the fat, although such antimicrobial effect varies, not only according to the type of acid used, but also according to the microbial variety to be treated. Sometimes it could be bacteriostatic and sometimes it could have a bactericidal action. High efficiency in meat surface sanitization due to lactic acid addition has been widely reported (Nakai and Siebert, 2004).

The objective of this work is to analyze and mathematically model the effect of storage temperatures (0°C, 4°C and 10°C) on the growth of three microorganisms isolated from beef samples: Klebsiella sp., E. coli and Pseudomonas sp., inoculated in a culture broth with different concentrations of lactic acid leading to pH values ranging between 5.6 and 6.1.