Use of natural antimicrobials for control of Escherichia coli O157:H7 in ground beef and dry fermented sausages.

摘要

Lactobacillus (Lb.) reuteri (which produces an antimicrobial reuterin by anaerobic fermentation of glycerol) and allyl isothiocyanate, AIT (extracted from horse radish) are two potent antimicrobials with activity against Gram positive and Gram negative bacteria. These two antimicrobials were studied for their direct inhibitory effects against Escherichia (E.) coli O157:H7 in refrigerated ground beef. In addition non-deheated mustard flour as a natural source of allyl isothiocyanate was used as an ingredient and studied for its action against E. coli O157: H7 in ground beef. Different microencapsulation methods and encapsulating material for protecting Lb. reuteri against simulated gastric juice were evaluated and an optimum method and material were determined. The suitability of Lb. reuteri and Bifidobacteria ( B.) longum cultures as probiotic cultures in dry fermented sausages was studied with or without protecting the cells by microencapsulation. The effects of these probiotic organisms against E. coli O157:H7 in dry fermented sausages were also investigated.;The antimicrobial effect of deheated and non-deheated mustard flour at 10 and 20% in refrigerated ground beef inoculated with a five strain cocktail of E. coli O157:H7 was studied and compared with the effects of pure AIT at 1300 ppm. Non-deheated mustard flour at 20% eliminated E. coli O157:H7 after day 3 whereas AIT and 10% non-deheated mustard flour required 15 and 18d, respectively, to eliminate inoculated E. coli O157:H7 from ground beef. Deheated mustard flour at both 10 and 20% failed to eliminate E. coli O157:H7 by the end of storage. Total bacterial numbers were significantly reduced ( P 0.05) when 20% non-deheated mustard flour or AIT were used, indicating potential extension of ground beef shelf life.;A study was undertaken to determine the most suitable method and wall material for microencapsulation of Lb. reuteri and resulting cell viability were studied during challenge with simulated gastric juice. Four wall materials, namely alginate, alginate plus starch, kappa-carrageenan with locust bean gum, or xanthan with gellan and two methods of microencapsulation (extrusion or phase separation by emulsion formation) were studied for their ability to protect cell viability during gastric acid challenge.;Experiments were also conducted to find whether Lb. reuteri viability was better protected in dry fermented sausages by direct addition or after microencapsulation in alginate wall material with microcapsules prepared by either extrusion or emulsion. Pediococcus (P. ) pentosaceus and Staphylococcus (S.) carnosus were used as starter cultures for fermentation. The pH of sausages dropped from 5.8 to 4.8 by the end of fermentation and the aw dropped from 0.97 to 0.89 at the end of drying in all treatments. There were no significant differences among treatments in terms of pH, aw or in P. pentosaceus and S. carnosus numbers. Microencapsulation was found to maintain Lb. reuteri viability during processing of dry fermented sausages.;Allyl isothiocyanate and Lb. reuteri were added separately and in combination and examined for their action against a cocktail of5 strains ofE. coli O157:H7 in ground beef stored at 4°C over a period of 25 d. Lb. reuteri at both levels in the presence of glycerol killed E. coli O157:H7 at both inoculated levels before day 20. AIT eliminated E. coli O157:H7 when initially present at 3 log cfu/g and reduced viability > 4.5 log cfu/g when initially present at 6 log cfu/g by the end of the storage period. The combination of Lb. reuteri with AIT did not yield synergistic or additive effects against E. coli O157:H7.;The effect of probiotic B. longum and Lb. reuteri both as planktonic and microencapsulated cells either alone or in combination were studied as natural antimicrobials against a 5 strain cocktail of E. coli O157:H7 in dry fermented sausages. Numbers of planktonic Lb. reuteri and B. longum decreased during sausage processing, but microencapsulation was shown to protect them significantly during the manufacture of dry fermented sausages. Microencapsulation of probiotics like Lb. reuteri and B. longum may be employed for supplementing dry fermented sausages with probiotics but their value as antimicrobials againstE. coli O157:H7 is limited. (Abstract shortened by UMI.)