Table 2. Antimicrobial barriers in fermented meats.
Property Level, range, or function pH 4.5-5.5
Nitrite 125 ppm
Box 6-3. Pathogens, Toxins, and the Safety of Fermented Sausage (Continued)
The problem, however, is now more complicated, because it has been recognized recently that some pathogens appear to be tolerant even to multiple barriers. Some strains of E. coli O157:H7, for example, are much more tolerant to low pH and organic acids than are normal E. coli strains. Furthermore, L. monocytogenes is resistant to low pH, low water activity (aw), high salt, and nitrite, and can even grow at refrigeration temperatures.And although there have been few food poisoning outbreaks caused by these organisms in fermented sausage, challenge experiments indicate that E. coli O157:H7, L. monocytogenes, and other pathogens can theoretically survive typical sausage manufacturing procedures.Thus, even the mere presence of these organisms in ready-to-eat meat products would likely initiate a recall. Indeed, the U.S. Department of Agriculture has established rules that now require a minimum five-log reduction of pathogenic organisms during the manufacture of uncooked, ready-to-eat fermented sausage. Manufacturers are also required to develop and implement HACCP plans that describe the relevant intervention steps necessary to produce safe products.
Of course, the simplest solution to many of these potential problem organisms and the most effective way to ensure a greater than five-log reduction would be to include a heating step somewhere during the process. However, many manufacturers are unwilling to accept the changes in the sensory properties caused by heating, and instead rely on ensuring that natural barriers are sufficient. It is also possible to include additional intervention measures into the product or process. For example, meat starter cultures capable of producing bacteriocins and other antagonistic agents that are inhibitory to pathogens are now available (discussed previously).
As noted above, bacterial pathogens are not the only food safety issue of concern to the fermented meats industry. Parasites and viruses can also be present in raw meat, and toxigenic fungi can contaminate sausage during fermentation and ripening.The parasite most commonly associated with meat, and pork in particular, is the tapeworm Trichinella spiralis, the causative agent of trichinosis. Although certified Trichinella-free pork is available in the United States (and in Europe), it is also possible to inactivate this nematode by a freezing treatment, as per USDA guidelines. Otherwise, a cooking step is required to destroy the cysts.
In contrast to parasites, viruses have not been considered as a serious food safety problem in fermented meats, and this is still largely true.Viruses found in meat are not usually pathogenic to humans, they do not replicate, and they are mostly sensitive to the acidification and drying steps used in sausage manufacture. Although not a human pathogen, the spread of the foot-and-mouth virus in the United Kingdom and Europe during 2001 had devastating economic and social effects on all segments of the meat processing industry, even though it posed no threat to humans.
This situation was different from that which occurred in the 1980s and 1990s when "mad cow disease" (or bovine spongiform encephalopathy) appeared in the United Kingdom. In the latter case, the causative agent was an unusual type of infectious protein called a prion that occurred in beef and that was fully capable of causing an extremely rare but fatal disease. The prion is not destroyed by fermentation, heat, or other food processing techniques, so the only means of control is to prevent the transmission of the prion during animal production.
Several different genera of fungi, including species of Penicillium and Aspergillus, are frequent contaminants of fermented meats. For many products, including both whole fermented meats (i.e.,hams), as well as fermented sausages, their growth is encouraged, due to their ability to produce flavor-generating enzymes (as discussed previously). However, some strains isolated from mold-fermented products are capable of producing mycotoxins. Moreover, inoculation of ham and sausage with toxigenic fungi and incubation under optimized conditions results in toxin formation. Despite these findings, the presence of these toxins in mold-fermented meat products appears to occur rarely, if at all, a situation not unlike that for fungal-ripened cheese and other mold-fermented products.
Still, there is much concern about the potential for mycotoxin production in fermented meats, and there is now a trend to use defined, nontoxigenic strains rather than the wild or
Box 6—3. Pathogens, Toxins, and the Safety of Fermented Sausage (Continued)
house strains that have commonly been used. For example, Penicillium nalgiovense, a fungal meat starter culture, has many desirable properties, but since some strains produce mycotoxins, only strains demonstrated to be non-toxin producers are in commercial use. Undesirable mold growth on sausage can also be controlled by antimycotic agents such as sorbic acid and, if permitted, the antibioic pimaricin. Smoke, which is usually applied to high moisture, but not dry products, also contributes antimycotic constituents.
Finally, another biologically-active group of microbial end products found in a wide variety of fermented foods, including fermented meats, are referred to as biogenic amines (Suzzi and Gardini, 2003).These compounds are formed via decarboxylation of amino acids by various bacteria that are commonly found in fermented meats, including lactic acid bacteria, enterococci, Enter-obacteriaceae, and Micrococcaceae. In some individuals, ingestion of biogenic amines results in a particular food poisoning syndrome marked by headache, nausea, and dilation of blood vessels.
In fermented meats, the most common biogenic amine capable of causing food poisoning symptoms is tyramine, derived from the amino acid tyrosine.In general, dry fermented sausages contain about 50 to 300 mg tyramine per kg (Table 3). Histamine, derived from histidine, may also be present, but usually at ten-fold less concentration. However, only when concentrations are very high (> 1,000 mg per Kg of dry weight) do these products pose a health risk in most normal individuals.
Table 3. Biogenic amines in European fermented dry sausages1,2.
Product (Country) Tyramine Histamine Cadaverine Putrescine
Soppressata (Italy) 178 22 61 99
Salsiccia (Italy) 77 0 7 20
Sobrasada (Spain) 332 9 13 65
Fuet (Spain) 191 2 19 72
Salchichón (Spain) 281 7 12 103
Chorizo (Spain) 282 18 20 60
Adapted from Suzzi et al., 2003 2Concentrations given in mg/Kg
Other biogenic amines may also be formed in meat at appreciable levels, including pu-trescine and cadaverine, but these compounds do not generally elicit symptoms described above. Since formation of biogenic amines requires the presence of free amino acids, the amount produced depends on the extent of protein hydrolysis that had occurred in the food. Thus, the longer the meat is aged or fermented, the higher will be the concentration of amino acid substrates and the more likely it is that the product will contain biogenic amines (assuming the relevant decarboxylating enzymes are also present).
Because some lactic acid bacteria have the ability to produce amino acid decarboxylases, starter cultures strains should be screened to eliminate such strains from use. Moreover, it may be possible to use starter cultures that have inhibitory activity against potential bioamine producing bacteria, either by virtue of their ability to produce acids rapidly, produce bacteriocins, or outcompete them for nutrients. Finally, some staphylococci and lactic acid bacteria produce amine oxidases, enzymes that cause oxidative deamination of amines, and therefore, act to detoxify biogenic amines.
Suzzi, G., and F. Gardini. 2003. Biogenic amines in dry fermented sausages: a review. Int. J. Food Microbiol. 88:41-54.
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