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Microbial Safety of Meat: Indicator Organisms and Carcass Sampling

It is essential to control the quality and safety of fresh meat/meat products to ensure meeting both legal regulations and consumer demands. HACCP and QM are two typical controlling procedure commonly implemented in food industry for manufacturing meats. HACCP is normally conducted to audit / identify hazards for raw meat primarily with respect to chemical and physical and microbiological aspects.

Indicator organisms

The presence of pathogenic microorganisms such as Clostridium perfringens, Staphylococcus aureus in meat and meat products can be the main contributory factors to the food poisoning. Other major group of pathogenic organisms which are significantly in association with meat consist of Salmonella, Escherichia coli, Campylobacter, Yersinia enterocolitica and Listeria monocytogenens can also be used as indicator organisms. Pseudomonas spp. are the most responsible aerobic organisms for food spoilage while anaerobic spoilage organisms include psychrotrophic Enterobacteria and Shewanella putrefaciens.

Sampling of carcass – General concept

Sampling of carcass is typically conducted by swabbing an area (initially using wet cotton subsequent with using dry cotton wool swab). Each swab should then be homogenised with sufficient volume of diluent followed by pour or spread plate technique for each swab. Advantages of swabbing technique include less time-consumption and simplicity in addition to the fact that meat remains intact without damage.

There has been an increasing trend towards utilising relatively abrasive swab including cellulose acetate sponges or surgical gauze. This method is believed to be more convenient in addition to providing more accurate results compared to those with cotton wool swab. As the existing bacteria on the meat surface are not uniformly distributed, increasing the size of sample will correspondingly increase the probability. Generally, an area of approximately 100cm
can be the optimum size for swab sampling.

Currently enumeration of microorganisms is carried out by “HGMF” (hydrophobic grid membrane filtration) instead of plate counting. It is a labour saving technique. The most possible bacteria population can be detected by HGMF. It provides more precise analysis of some microorganisms such as E coli.

Sampling of carcass is usually conducted by one of the following methods:

i) The most commonly used technique for sampling carcass involves random selection of areas which are assumed to be more prone to contamination. It is required to take the samples of at least three locations of each carcass for this type of analysis. This method has the advantage of simplicity and it has been approved as the most suitable method for raw meat microbiology. However, insufficient data in relation to the distribution of bacteria on carcass and consequent confusion of analysis may occur. In addition, the selected areas for sampling might not represent the largest harboured bacteria on the carcass.

ii) Alternatively, sampling plan can be carried out using standard deviation and determination of average log. Standard deviation is determined by bacterial quantities in a series of more than 20 samples. Sample areas are randomly selected on a random selection of carcasses. The obtained results of the mean log and standard deviation are subsequently evaluated for the log of “arithmetic mean numbers” of bacteria. This processing plan specifies the highest acceptable population (on average). In the incidence of loss of process control, the area of sample influenced by the process failure can be recognized providing the sampling process is routinely monitored; thus the defective system of the process can be detected and restored.

Predictive microbiology

Predictive microbiology is based on certain mathematical formulas to determine micro-organisms activity with respect to pH, salt and water content or to analyse the parameters when processing and storage of meat are concerned. Scientific-based models are chosen to evaluate their effects on micro-organisms development.

There are a few computerised methods used for predictive microbiology including Food Micro Model and MFS Pathogen Modelling Program. The latter can be used for both aerobic and anaerobic growth for some models. The Food Micro Model is mostly in association with the influence of Co2 concentration for organisms including Verocytotoxigenic Escherichia coli, Bacillus cereas and Listeria monocytogenes. The analysed prediction of the pathogens characteristics is usually compared with the actual activity of pathogens from which the determined validation of the food model can be achieved.

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