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Post-harvest losses of both fresh and freshly processed agricultural produce have been estimated to be as high as 25 to 40% of the total production. To counteract post-harvest losses, the use of modified atmosphere (MA) storage techniques has increased for bulk storage, but also for consumer-sized packages. Moreover, MA packaging (MAP) complies with recent trends in the use of healthy convenient foods, that are minimally processed and less heavily preserved as compared with traditionally processed foods. Research on these and related issues has been published in a 96-page thesis from Wageningen Agricultural University. One of the objectives of the study was to obtain information on the impact of refrigerated MA storage conditions on the growth of microorganisms on minimally processed, MA packaged produce. Since initial studies substantiated the possible hazard which can be posed by psychrotrophic pathogens, the use of biopreservation for adequate control of these microorganisms was investigated. In this respect the focus was on lactic acid bacteria because they occur naturally on fresh and minimally processed vegetables, and are able to produce a variety of antimicrobial substances, including bacteriocins. The thesis deal with the following areas: MAP of vegetables, and biopreservation by the use of lactic acid bacteria; the influence of oxygen and carbon dioxide on the growth of prevalent Enterobacteriaceae and Pseudomonas species isolated from fresh and MA stored vegetables; growth of phychrotrophic foodborne pathogens in a solid surface model system under the influence of carbon dioxide and oxygen; vegetable-associated Pediococcus parvulus produces pediocin PA-1; a role for oblique peptides in pore formation by bacteriocins of lactic acid bacteria; interactions of nisin and pediocin PA-1 with closely related lactic acid bacteria that manifest over 100-fold differences in bacteriocin sensitivity; biopreservation for the control of Listeria monocytogenes on minimally processed, MA stored vegetables.
The improvement of the safety and quality of ready- to-eat foods is of major interest to both the consumer and the food industry. Research has been carried out into the development of edible protective films and coatings for foods. These are applied directly to the surface of a food product and protect against microbial spoilage and loss of intrinsic product quality. The coatings are fully biodegradable (unlike many plastic packaging materials) and create a modified atmosphere around the product, (eg a carrot or fruit). Wheat gluten, pectin or beeswax, with alcohol as solvent, are examples of materials that can be used as coatings. Obviously the coatings must be clear (not opaque) and flexible; they must also be resistant to breakage and abrasion and so the composition of the film-forming solution is important. The permeabilities of the coatings to oxygen and water vapour can be manipulated to fall within the ranges of the conventional plastic packaging materials which means that the conditions within a prepack can be reproduced around individual units of food product (ie a cocoon-like effect). The edible coatings are conducive to the use of natural antimicrobials or antioxidants as these can be included in the coating and are concentrated at the produce surface which is the place where protection is needed. This means that only very small amounts of additives are required. Aspects currently being researched in this project include the rate of diffusion of preservatives through coatings prepared from pectin and other polymers and also the microbial and physical stability of the coatings themselves.

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