Food spoilage—interactions between food spoilage bacteria
Introduction
Preservation of foods has, since the beginning of mankind, been necessary for our survival. The preservation techniques used in early days relied—without any understanding of the microbiology—on inactivation of the spoiling microorganisms through drying, salting, heating or fermentation. These methods are still used today, albeit using less and less preservation and combining various lightly preservation procedures to inhibit growth of microorganisms. Spoilage is characterised by any change in a food product that renders it unacceptable to the consumer from a sensory point of view. This may be physical damage, chemical changes (oxidation, colour changes) or appearance of off-flavours and off-odours resulting from microbial growth and metabolism in the product. Microbial spoilage is by far the most common cause of spoilage and may manifest itself as visible growth (slime, colonies), as textural changes (degradation of polymers) or as off-odours and off-flavours. Despite chill chains, chemical preservatives and a much better understanding of microbial food spoilage, it has been estimated that 25% of all foods produced globally is lost post harvest or post slaughter due to microbial spoilage (Anonymous, 1985).
This review gives a brief introduction to the specific spoilage organism concept and an overview of what is currently known about spoilage microbiology of different food products. The microorganisms dominating a product can be predicted and qualified guesses can be made in terms of the organisms causing the spoilage by understanding how the major preservation parameters affect microbial selection. Clear patterns emerge, independent of raw material and processing, in terms of which organisms dominate and spoil food products as a function of, e.g. origin, substrate base, temperature, pH, aw and atmosphere. We believe that such understanding of the microbial ecology is essential if the excessive post harvest and post slaughter losses are to be reduced. Also, knowledge of the microorganisms involved in spoilage and the metabolites associated with spoilage is needed to develop microbiological and chemical methods for evaluation of quality and shelf life.
The growth and activity of spoilage microorganisms is mostly described and studied as function of substrate base and of chemical and physical parameters such as temperature, pH, aw and atmosphere. The importance of these conditions for the selection of a spoilage microflora cannot be underestimated, however, it is becoming increasingly clear that in some situations also interactive behaviour between the microorganisms determines selection and/or metabolism and subsequently spoilage. The major part of this review gives 3 examples of microbial interactive behaviour of potential importance in microbial food spoilage, namely (i) antagonism, (ii) metabiosis and (iii) cell-to-cell communication.
Section snippets
The concept of specific spoilage organisms
Each and every food product harbours its own specific and characteristic microflora at any given point in time during production and storage. This microflora is a function of raw material flora, processing, preservation and storage conditions. Despite the variability in all of the three, some very clear patterns emerge, and based on knowledge of a few chemical and physical parameters it is possible with great accuracy to predict which microorganisms will grow and dominate in a particular
Food spoiling microorganisms
Almost all groups of microorganisms harbour members that under some conditions can contribute to spoilage of foods. Theoretically, one can assume that all microorganisms are initially present on a food product where after a selection occurs—based primarily on nutrient composition and on the chemical and physical parameters. Similar microfloras emerge in different food products under the same conditions despite the heterogeneity in the outset. Thus Pseudomonas spp. and a few other Gram-negative
Interactions between food spoiling bacteria
The selective conditions imposed on the food microbial community by physical and chemical parameters, as outlined in Table 1, undoubtedly are the most important in terms growth and selection of microorganisms. However, microbial food spoilage is a process involving growth of microorganisms to numbers (107–109 cfu/g) at which the microorganisms also must be assumed to interact and influence the growth of one-another (Boddy and Wimpenny, 1992). The interactions between microorganisms may be
Concluding remarks
Many foods spoil due to microbial degradation with their metabolites being the cause of the off-flavours or the textural changes resulting in sensory rejection. Despite the wide range of raw materials, the different processing parameters and the diverse array of storage conditions, very similar microfloras will develop in products with similar physical and chemical characteristics. Only a fraction of the microorganisms present in spoiling food products cause the spoilage characteristics. These
Acknowledgements
The authors and the studies on the involvment of AHLs in food quality deterioration was supported by research grants from the Danish Research Council for the Technical Sciences [no. 9700726 and 26000134] and by the Danish Research Academy [PhD grant for L.R.]. We thank Morten Hentzer for assistance with confocal scanning laser microscopy. We extend our thanks to Claus Reesbøll and librarian Søren Tørper Christensen, who helped us get additional references during the revision—for which we were
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