Changes in Hygienic Quality of Vacuum-packed Pork during Storage

Abstract

Napravnikova E., L. Vorlova, L. Malota: Changes in Hygienic Quality of Vacuumpacked Pork During Storage.Acta Vet. Brno 2002, 71: 255‐262. Hygienic quality of boned vacuum-packed pork was monitored during storage by sensory, chemical, and microbiological methods. Pork was vacuum-packed into polyamide /polyethylene (PA/PE) foil 72 h after slaughter. Initial temperature did not exceed 9° C. The meat was stored at 2.5 ± 0.5°C in the dark for 35 days and samples for laboratory examinations were collected at days 14, 21, 28, and 35 to assess appearance, colour, odour, and characteristics of released meat juice, to determine pH and concentrations of ammonia and D- and L-forms of lactic acid, and to enumerate lactacidogenic bacteria. Lactobacillus curvatus and Lactobacillus xylosus were identified as the causative agents of meat spoilage using the API system. The results were subjected to correlation analyses. The shelf life of boned vacuum-packed pork varied between 21 and 28 days, depending on initial pH. The results of correlation analyses indicated that only ripe pork with the optimum pH = 5.8 to 5.95 is suitable for vacuum-packing. In addition to the lactacidogenic bacteria count (LAC), the shelf life can be estimated also from ammonia concentration (LAC/ammonia r = 0.9584, P < 0.05), pH (LAC/pH r = -0.9317, P < 0.05) and the concentration of D-lactate (LAC/D-lactate r = 0.9867, P < 0.01). Pork, vacuum packing, lactobacilli, ammonia, pH, lactic acid Meat spoilage is a complex process in which microorganisms present in the muscular tissue due to secondary contamination during processing are involved, and which depends on ambient temperature. The storage of meat is associated with changes in quality resulting from microbial activity, shift of pH, production of toxic substances, and aberrant odour (Huis 1996). Most prone to spoilage are foods with a high protein content, such as meat, poultry, fish, and milk, which have a high dietetic value, neutral, or lightly acid pH, and a high water content providing favourable conditions for bacterial growth (Huis 1996). Bauer (1995) demonstrated that enzymes of the muscular tissue and bacterial enzymes are involved in chemical changes (production of biogenic amines, changes in colour and fat) occurring during storage. The most important factors influencing the development and growth of spoilage microorganisms on raw meat in cold storage include the initial relative number of psychrotrophic flora and its growth dynamics at low temperatures. Qualitative and quantitative composition of the microbial population responsible for meat spoilage depend on pH, water activity, storage atmosphere, and among-species relations (Gould 1995). Dominant in the microbial population of vacuum-packed meat are lactacidogenic bacteria which participate in the development of an unpleasant odour. Dainty (1996) detected indol and hydrogen sulphide by gas chromatography and identified them as the cause of this sensory defect. The microbial activity results in the production of typical metabolites including invariably the D and L forms of lactic acid. D-lactate is produced by bacteria exclusively and is regarded as an indicator of the growth of specific microbial flora (Sinell and L uke 1979; Schneider et al. 1983). Further metabolites produced by the