ZARITZKY Noemi Elisabet
capítulos de libros
Predictive Equations to Assess the Effect of Lactic Acid and Temperature on Bacterial Growth in a Model Meat System. Chapter 24
Food Engineering: Integrated Approaches
Lugar: New York, USA; Año: 2008; p. 345 - 359
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 could 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 they could even prevent microbial growth to occur almost completely.  Muscle pH variation is highly dependent on the tissue glycogen level at the 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 inhibitory effect because of the decrease in pH; this acid could act both on the meat muscle flora itself and on that of the grease, 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 the lactic acid addition has been widely reported (Nakai & Siebert, 2004). The use of organic acids as decontaminants is an emerging preventive procedure, despite the fact that this application is not new. However, for reasons of solubility, taste and low toxicity, the short chain organic acids are more commonly used as preservatives or acidulating systems. Nevertheless, despite its effectiveness as decontaminant, acetic acid has been considered unacceptable because of its pungent odor and its provoking discoloration of the meat surface. Gil & Badoni, (2004) have studied the effect of a 0.02% peroxyacetic acid solution, 0.16% sodium chloride with 2 and 4% lactic acid on the natural flora of the distal surfaces from chilled beef carcasses. Peroxyacetic acid and the acidified sodium chloride solution had little effect on the number of aerobes, coliforms or E. coli on meat, and were less effective than 4% lactic acid for reducing the number of bacteria on meat. Lactic acid is an acceptable decontaminant because it is a natural, non-toxic, physiological product produced naturally in meat products, and it offers the possibility of reducing the contamination of carcasses, cuts and products. According to Snijder et al., (1984), the use of lactic acid as a terminal decontaminant in combination with the perfect slaughter line hygiene produces both bactericidal and bacteriostatic effects which result in extended shelf lives of meat. Cudjoe, (1988), studied the effect of lactic acid sprays on meat keeping qualities during storage. The spraying of the meat surface of a skinned cow head with 1% V/V lactic acid resulted in a significant reduction in total viable counts of bacteria during storage at 4, 15 and 20ºC. Shelf lives of all sprayed heads were observed to have been extended to about three days at 4ºC and to one day at both 15 and 20ºC. Predictive microbiology is a powerful tool for predicting microorganism growth rate under ambient conditions, with the aim of guarantee food quality, thereby determining its effective life. One of the most frequently used mathematical models is that of Gompertz (Gibson et al., 1988; Giannuzzi et al, 1998), which describes the micro-organism response under different factors combinations (Andrés et. al. 2001). It permits estimate parameters such as lag phase duration (LPD), specific growth rate (m) and maximum cell concentration (MDP) of micro-organisms under such conditions. The objective of this work was to analyze and mathematically model the effect of storage temperatures (0, 4 and 10ºC) on the growth of three micro-organisms, 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.