Defects and Spoilage of Fermented Meats

Defects of fermented meats can occur before, during, or after manufacturing. Like all fermented foods, the production of high quality products depends largely on the microbiological quality of the raw ingredients. Perhaps this is even more so for fermented sausage, since the starting material, meat, is raw and cannot be heat-processed to inactivate spoilage or other undesirable microorganisms. Thus, any organisms present in the raw meat will be present in the sausage batter and may even survive fermentation.

Of course, fermentation acids kill or inhibit many organisms, and, if the sausage is cooked after fermentation, most of the remaining organisms will be killed. However, spoilage products may have already been produced. For example, psychrotrophic bacteria, such as Brochothrix thermosphacta and various Pseudomonas spp., may produce ammonia and other volatile off-odors and flavors in the meat during storage even before fermentation. Lipases and other enzymes may also be produced by these organisms prior to fermentation, resulting in rancid, "cheesy,"or bitter end-products.

The other main group of spoilage organisms are lactic acid bacteria.These bacteria are part of the natural meat microflora, and are quite tolerant of the barriers that ordinarily control microbial activity in fermented sausage (i.e., low pH, low aw, low Eh, nitrite, salt, etc.). Under appropriate conditions, lactic acid bacteria can cause flavor, color, and tactile defects, and, as discussed in Box 6-3, some strains are also re sponsible for foodborne disease, due to their production of biogenic amines. Of course, the causative organisms are not necessarily endogenous to the product, because the starter culture itself is comprised of lactic acid bacteria, including L. sake, L. curvatus, L. plantarum, P. acidilactici, and other species capable, in theory, of producing these defects.Therefore, it is important that screening and selection of strains for starter cultures be based, in part, on their ability (or inability) to produce undesirable end-products.

Among the spoilage products formed by lactic acid bacteria, hydrogen peroxide is probably the most problematic. It is produced primarily by lactobacilli, but only by specific strains, and only under specific conditions. Oxygen is required for production of hydrogen peroxide by lactic acid bacteria; it serves as a reactant in the hydrogen peroxide-generating reactions and also induces expression of the enzymes involved in these reactions. Thus, minimizing exposure to air during mixing and subsequent steps is critical.

Once formed, hydrogen peroxide participates in several undesirable reactions. First, it reacts with heme-containing pigments, especially nitrosyl myochromogen, formed as a result of curing.When the heme iron is oxidized by peroxide, the desirable pink color is lost and an undesirable green pigment is formed (Box 6-1). Second, hydrogen peroxide can form hydroxyl radicals (e.g., O-) that serve as initiators of lipid oxidation reactions.

One way to limit the formation of hydrogen peroxide in fermented meats is to include cata-lase-producing strains in the starter culture.Al-though some lactic acid bacteria produce a small amount of catalase or a pseudo-catalase, micrococci (present in some cultures) produce much greater levels of this enzyme. Another less common defect caused by lactic acid bacteria is slime formation. In particular, strains of L. sakei have been shown to produce exo-polysaccharides (i.e., slime) in vacuum-packaged meat products, and have occasionally been implicated in this form of spoilage in fermented sausages.

In general, microbial spoilage can best be prevented by keeping psychrotrophic bacteria out of the raw meat and keeping other potential spoilage organisms out of the finished fermented product via comprehensive sanitation programs. Few organisms should be present in cooked fermented sausages, provided postprocessing contamination does not occur.Vac-uum or modified atmosphere packaging with carbon dioxide inhibits aerobic psychrotrophs, but not facultative and anaerobic spoilage organisms, such as B. thermosphacta and lactic acid bacteria. Antimicrobial agents, including organic acids and bacteriocins, may be effective against these bacteria, as described previously.

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