Microbial content stability and inhibitory effects of antimicrobials on the shelf-life of high moisture dried fruit

[摘要] ENGLISH ABSTRACT:Drying of fruit is an age-old preservation technique that even to this day,remains popular. On the foreign markets there is a trend towards dried fruitproducts that are softer and more palatable. Based on this, the South Africandried fruit industry has launched a range of soft eating, high moisture driedfruit (HMOF) products. These products have a moisture content of ca 36%(w/w) and a water activity (aw) of ca 0.85, both of which are considerablyhigher than that of traditional dried fruit. It is this higher moisture content thatmakes the product susceptible to serious microbial spoilage and colourdeterioration in spite of the use of sulphur dioxide. Currently, the shelf-life ofthese products is approximately three months and to ensure a quality exportproduct, the shelf-life must be at least nine months.During this study the microbial composition of five South Africancommercially available HMDF products (raisins, prunes, nectarines, clingstone peaches and apricots), was determined. It was found that high moisturedried (HMO) raisins and prunes had the highest bacterial counts, whileapricots were more susceptible to yeast and mould spoilage. The mostfrequently isolated microbes were members of the endospore forming Bacillusgroup and osmotolerant yeasts like Zygosaccharomyces. Members of thegenera Salmonella, Staphylococcus and even anaerobic endospore formerswere present in certain samples of the HMO prunes and raisins. From theseresults it was concluded that the current pasteurisation method of 7 h at 80°Cused by the industry is not effective in successfully eliminating spoilagemicrobes.High moisture dried nectarines and prunes were used in challengingstudies, where four indicator organisms (B. cereus, Z. rouxii, Escherichia coliand Penicillium chrysogenum) were selected to evaluate the shelf-stability ofthese two HMOF products over six months at 5° and 25°C. The resultsshowed higher bacterial counts for the fruit stored at 25°C (1 200 cfu.g ̄ ¹versus 450 cfu.g ̄ ¹ at 5°C), as well as extensive Maillard browning of thenectarines at the higher storage temperature. Furthermore, it was found thatthe pasteurisation method (7 h at 80°C) was not effective in eliminating theendospore former B. cereus, P. chrysogenum and Z. rouxii, which may thuslead to serious spoilage implications. The E. coli strain did not survive ineither of the HMOF products and no counts were found even beforepasteurisation. From these results it was clear that the current pasteurisationmethod of HMOF products is not effective in terms of the prevention ofspoilage, and that storage of the product at lower temperatures would lead toa better quality product.The impact of different preservatives (lysozyme, pimaricin, sorbate,benzoate and an imidazole compound) on the survival and growth of B.cereus and Z. rouxii, were evaluated in nectarine puree prepared from driednectarines. Lysozyme showed the best inhibitory effect (70% inhibition) on B.cereus over a 12 h incubation period, followed by benzoate, sorbate, theimidazole compound and lastly, pimaricin. The data obtained for the inhibitoryeffect of the preservatives on Z. rouxii showed that benzoate and sorbateshowed. the best growth inhibitory effect (100% inhibition after 24 h).Pimaricin gave a 100% inhibitory effect after 48 h while the imidazolecompound gave a 89% inhibition.From the different pasteurisation time/temperature combinationsapplied to eliminate the endopspore former B. 'cereus, it was found thatpasteurisation of 150 min at 90°C showed the best inhibition results.However, the colour of the product was unstable. It was, therefore, decided toalso evaluate the use of modified atmosphere packaging (MAP), as part of theproduct's processing. It was found that the colour of the HMO nectarines wasstable and it is thus recommended that a time/temperature combination of 150min at 90°C together with MAP, is used to ensure a shelf-stable HMOFproduct.

[发布机构] Stellenbosch University