MICROBIAL BIOTECHNOLOGY: SCOPE, TECHNIQUES CONTD
PREPARATION OF FERMENTED FOODS:
The use of microorganisms to produce fermented foods has a very long history. Microbial fermentation is essential to production of wine,beer,bologna, buttermilk, cheeses, kefir, olives, salami, sauerkraut, and many more . The metabolic end products produced by the microorganisms flavor fermented foods. For example, mold-ripened cheeses owe their distinctive flavors to the mixture of aldehydes, ketones, and short-chain fatty acids produced by the fungi. Lactic acid bacteria are widely used to produce fermented foods. These organisms are also of particular importance in the food fermentation industry because they produce peptides and proteins(bacteriocins) that inhibit the growth of undesirable organisms that cause food spoilage and the multiplication of food borne pathogens. The latter include Clostridium botulinum(the cause of botulism) and Listeria monocytogenes (which produces meningoencephalitis, meningitis, perinatal septicemia, and other disorders in humans).
Nisin, an antimicrobial peptide produced by strains of Lactococcus lactis, is widely used as a preservative at low concentrations (up to 250 ppm in the finished product) primarily in heat-processed and low pH foods. Nisin inhibits the growth of a wide range of Gram-positive bacteria, including Listeria, Clostridium, Bacillus, and enterococci, but is not effective against Gram-negative bacteria, yeasts, and molds. The antibacterial activity of nisin is the combined outcome of its high affinity interaction with lipid II at the outer leaflet of the bacterial cytoplasmic membrane and permeabilization of the membrane via pore formation.
Nisin is designated as a Generally Regarded as Safe (GRAS) food preservative in the United States and in many other countries around the world. It is used in many food products, including pasteurized cheese spreads with fruits, vegetables, or meats; liquid egg products; dressings and sauces; fresh
and recombined milk; some beers; canned foods; and frozen dessert.
LACTOBACILLUS SAKEI: A PROMISING BIO PRESERVATIVE
L. sakei, a psychrophilic lactic acid bacterium, was first isolated from sake, a Japanese rice beer that is produced partly by lactic acid fermentation. Subsequently,L. sakei strains were found to dominate the spontaneous fermentation of meat in the manufacture of salami and other dry fermented sausages. Such strains are also major components of the microbial flora of processed food products stored at cold temperature. L. sakei starter cultures have come to be widely used in the manufacture of fermented meats, and this organism has been shown to prevent the growth of spoilage organisms and pathogens.L. sakei is also a transient inhabitant of the human gut. A number of other lactic acid bacteria are either transient or permanent members of the human gastrointestinal flora, including Lactobacillus acidophilus. In that setting, these organisms – called probiotic species – stimulate the immune response and suppress the growth of potentially pathogenic bacteria. Recently, the genome of L. sakei23K, isolated from
a French sausage, was completely sequenced and was 43% identical to L. acidophilus. There is much interest in using safe bacteria as bio preservatives, and for the various reasons outlined above,L. sakei is an excellent candidate. The availability of the complete genome of L. sakei23K allows one to formulate testable hypotheses as to the attributes of this organism that enable it to flourish on fresh meat and to survive stressful conditions it encounters during meat fermentation and storage. Such challenges include high levels
of oxidative stress, high salt, and low temperatures. The L. sakei genome codes for four proteins predicted to be involved in cell–cell interaction and in binding to collagen exposed on the surface of
meat. Such proteins are absent from other lactobacilli. Two other gene clusters are predicted to function in the production of surface polysaccharides that may contribute to the attachment of the bacterium to the meat surface. These protein and polysaccharide surface components might mediate the
aggregation of L. sakei and formation of a biofilm on the meat surface that would exclude other microorganisms. Meat undergoes auto proteolysis on aging with release of amino acids.L. sakei is auxotrophic for all amino acids (except glutamic and aspartic), and thus the meat surface is an excellent ecological niche. Meat storage frequently requires refrigeration and salts (up to 9% NaCl).
L. sakei is well adapted to both low temperature and the osmotic stresses
encountered at high salt concentrations. It has a larger number of putative
cold stress proteins than other lactobacilli. It also has uptake systems for the
efficient accumulation of osmo- and cryoprotective solutes such as betaine
and carnitine.L. sakei is also well equipped with enzymes that detoxify reactive oxygen species such as superoxide or organic hydroperoxides generated during meat processing. Finally,L. sakei requires and takes up both heme and iron from the meat. The competition for iron may represent yet another important factor in the ability of L. sakei to exclude other organisms from the meat surface.
Cited By Kamal Singh Khadka/ Shailendra Parajuli
Msc Microbiology, TU.
Assistant Professor In Pokhara University, Pokhara Bigyan Thata Prabidhi Campus, PNC, LA , NA.