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'Adapted from Bergey's Manual of Systematic Bacteriology, Volume 2, 8th Ed. 2Under anaerobic conditions

3Some strains reduce nitrate under low glucose, high pH conditions 4Formerly Micrococcus varians

'Adapted from Bergey's Manual of Systematic Bacteriology, Volume 2, 8th Ed. 2Under anaerobic conditions

3Some strains reduce nitrate under low glucose, high pH conditions 4Formerly Micrococcus varians

Box 6—1. Micrococcaceae, Nitrate, and "Old World" Sausage

In the United States, most fermented sausage starter cultures contain only lactic acid bacteria. The fermentation temperatures are high and fermentation times are short. In contrast, for many European products (and a few in the United States), the microbial flora contain species of Staphylococcus, Micrococcus, and Kocuria in the Family Micrococcaceae (Hammes and Hertel, 1998;Tang and Gillevet, 2003). In Europe, fermentation temperatures are low and fermentation times are long.

Not coincidently, nitrite salts are, by far, the most common curing agents used in fermented sausage manufacture in the United States, whereas in Europe, nitrate salts are more common. These differences reflect not only the different manufacturing requirements, but also the desired quality attributes relative to "new world" and "old world" sausage-making technologies and styles.

To achieve the expected color, flavor, and antimicrobial property peculiar to all cured meats (whether fermented or not), either nitrate or nitrite must be added as a curing agent. However, it is the nitrite form, and not the nitrate, that actually reacts with the meat pigments and provides the curing effect. If nitrate is used as the curing agent, it must first be converted to nitrite (Figure 1). This conversion is simply a reduction reaction catalyzed by the enzyme nitrate reductase.

Nitrate

Myoglobin (Mb) (Purple-red)

reductase v NO3--► NO2- -¡h+ NO H+ ► NO-Mb heat ► Nitrosyl myochromogen

Nitrate Nitrite Nitric oxide Nitrosyl i myoglobin O2 | H2O2

(Bright red)

Gray, Green, Brown

Figure 1. Nitrate and nitrite reactions in meat. Nitrate is first reduced to nitrite by nitrate reductase, an enzyme produced by strains of Micrococcus and Staphylococcus. Nitrite is further reduced to nitric acid, which then reacts with myoglobin at low pH to form nitrosyl myoglobin. When heated, nitrosyl myochromogen (also called nitrosyl hemochromogen) is formed, giving a pink color. The latter can be oxidized to form an undesirable gray, green, or brown appearance.

Box 6—1. Micrococcaceae, Nitrate, and "Old World" Sausage (Continued)

In general, bacteria that produce nitrate reductase are respiring aerobes that use nitrate as an electron acceptor during growth under anaerobic or low oxygen conditions.Among the organisms having high nitrate reductase activity are species of Staphylococcus, Micrococcus, and Kocuria. Thus, if the sausage formulation contains nitrate, then its conversion to nitrite will depend on the presence of nitrate reductase-producing strains of these bacteria, either those naturally present in the raw meat or added in the form of a starter culture. In the latter case, it might seem rather odd to add staphylococci to food, since some coagulase-positive staphylococci (e.g., Staphylococcus aureus) are pathogenic and produce exo-toxins. However, the strains of Staphylococcus carnosus and Staphylococcus xylosus used for sausage fermentations are coagulase-negative and non-toxigenic. Still, despite this caveat, the use of these strains in sausage fermentations is not acceptable in some areas.This also points out why it is important to accurately identify strains of Staphylococcus that might end up in a starter culture (Morot-Bizot et al., 2004).

If nitrate must be converted to nitrite to produce cured sausage, then the obvious question to ask is why add nitrate in the first place? Why depend on the nitrate-reducing bacteria to perform this important function? In other words, why wouldn't all sausage manufacturers simply add nitrite as the curing agent, as is widely done in the United States? To answer these questions, one must consider what other functions the micrococci and other nitrate-reducing bacteria might perform in fermented sausage, other than to reduce nitrate. It turns out there are several.

The actual nitrate-to-nitrite reaction occurs rather slowly during the sausage fermentation because the prevailing conditions (low temperature and low pH) are less than optimal for nitrate reductase activity. It can take several weeks for the conversion of nitrate to nitrite.This affords the micrococci, kocuriae, and staphylococci the opportunity to secrete lipases and other enzymes that ultimately generate flavor precursors in the meat. If, instead of ripening the sausage at 20°C, it was fermented at 38°C to 40°C, the lactic acid bacteria would produce acid too quickly, and the non-lactic acid bacteria would be inhibited and unable to reduce nitrate or otherwise influence the properties of the finished product. It is also argued that slow conversion of nitrate to nitrite enhances color development (although why this should be the case is not understood).

Among the flavor compounds produced by staphylococci, micrococci and kocuriae are metabolic end products resulting from protein and fatty acid metabolism. Most strains are lipolytic and many are also proteoyltic. In addition, amino acid metabolism may also generate flavor and aroma compounds. For example, it was recently reported that metabolism of leucine by Staphylococcus xylosus resulted in formation of various metabolites, such as 3-methylbutanol and 2-methylpropanol,that contribute to the flavor properties of fermented meats (Beck et al., 2004).

References

Beck, H.C.,A.M. Hansen, and F.R. Lauritsen. 2004. Catabolism of leucine to branched-chain fatty acids in

Staphylococcus xylosus. J.Appl. Microbiol. 96:1185-1193. Hammes, W.P., and C. Hertel. 1998. New developments in meat starter cultures. Meat Sci. 49 (Supple. 1): S125-S138.

Morot-Bizot, S.C., R. Talon, and S. Leroy. 2004. Development of a multiplex PCR for the identification of Staphylococcus genus and four staphylococcal species isolated from food. J. Appl. Microbiol. 97:10871094.

Tang,J.S., and P.M. Gillevet. 2003. Reclassification of ATCC 9341 from Micrococcus luteus to Kocuria rhi-zophila. Int.J. Syst. Evol. Microbiol. 53:995-997.

Although some strains may also produce cata-lase or catalase-like enzymes that degrade hydrogen peroxide, the production of these enzymes cannot always be counted on to inactivate accumulated peroxides.

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