The most important group of bacteria used as SC is LAB. In fact, among commercially available SC, only a few are non-LAB. They receive this name due to their ability to convert fermentable carbohydrates in lactic acid via either homofermentative or heterofermentative metabolism. They are characterized by Gram-positive microorganisms that share several physiological and biochemical traits. In general, they are non-sporulating, catalase-negative, acid-tolerant and grow over a wide temperature range, although most LAB are either mesophiles or moderate thermophiles (Zhang et al. 2013). Likewise, they vary with respect to salt tolerance, osmotolerance, aerobiosis, and other environmental conditions, accounting, in part, for the diversity of habitats with which they are associated (Giraffa 2012).

Taxonomically, LAB species are found in two distinct phyla, namely Firmicutes and Actinobacteria. Within the Firmicutes phylum, LAB belong to the Lactobacillales order and include, among others, the following genera: Aerococcus, Alloiococcus, Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Oenococ- cus, Pediococcus, Streptococcus, Symbiobacterium, Tetragenococcus, Vagococcus, and Weissella, which are all low-GC content organisms (31-49%) (Giraffa 2012).

Seven of the LAB genera (Lactobacillus, Lactococcus, Leuconostoc, Oenococcus, Pediococcus, Streptococcus, and Tetragenococcus) are widely used in many industrial applications, especially as SC in fermented foods such as fermented milks, cheeses, butter, as well as in fermented meat products such as salami and fermented vegetables. Due to their ability to hydrolyze proteins, sugars and lipids, they have great influence on the sensory profile of the final product, resulting in improved texture, flavour and aroma (Pescuma et al. 2008).

In sourdough bread, for example, SC strains of Lactobacillus sanfranciscansis ferment maltose and lower the dough pH via production of lactic acid, but the culture also produces acetic acid and other flavour and aroma compounds. In contrast, one of the LAB included in sausage SC, Pediococcus acidilactici, essentially performs the production of lactic acid and reduces the meat pH to a level inhibitory to undesirable competitors (Hutkins 2006). For decades, LAB have been used in food preservation. They play a role in the conservation recognized and microbiological safety of fermented food, thereby promoting microbial stability of the final product. The protective effects are promoted by producing organic acids, CO2, ethanol, hydrogen peroxide and diacetyl, antimicrobial components, such as fatty acids and bacterio- cins (Giraffa 2012).

Although LAB are the most important group of SC bacteria, other non LAB also are included in various SC. In the cheese industry, for example, Propionibacterium freudenreichii subsp. shermanii is used to manufacture Swiss type cheeses and Brevibacterium linens is used in the production of surface ripened cheeses. On the Actinobacteria phyla, the genera relevant to fermented foods include Bifidobacterium, Kocuria, Staphylococcus and Micrococcus. Bifidobacterium actually serve as functional (probiotic purposes) culture. Species of Kocuria and the Staphylococcus/ Micrococcus group are used in fermented sausages with only one purpose, enhancing the desired flavour and color via production of the enzyme nitrate reductase.

It is worth emphasizing that fermented foods may contain many other microorganisms, whose presence occurs as a result of inadvertent contamination. So, the production of quality fermented foods requires close attention to the characterization, differentiation and maintenance of bacterial cultures involved in the production process.

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