Nitrogen lactic acid bacteria

Lactose is readily fermented by lactic acid bacteria, especially Lactococcus spp. and Lactobacillus spp., to lactic acid, and by some species of yeast, e.g. Kluyveromyces spp., to ethanol (Figure 2.27). Lactic acid may be used as a food acidulant, as a component in the manufacture of plastics, or converted to ammonium lactate as a source of nitrogen for animal nutrition. It can be converted to propionic acid, which has many food applications, by Propionibacterium spp. Potable ethanol is being produced commercially from lactose in whey or UF permeate. The ethanol may also be used for industrial purposes or as a fuel but is probably not cost-competitive with ethanol produced by fermentation of sucrose or chemically. The ethanol may also be oxidized to acetic acid. The mother liquor remaining from the production of lactic acid or ethanol may be subjected to anaerobic digestion with the production of methane (CH4) for use as a fuel several such plants are in commercial use. 

Penicillium caseicolum produces an extracellular aspartyl proteinase and a metalloproteinase with properties very similar to those of the extracellular enzymes produced by P roqueforti (Trieu-Cout and Gripon 1981 Trieu-Cout et al. 1982). Breakdown of casein in mold-ripened cheese results from the synergistic action of rennet and the proteases of lactic streptococci and penicillia (Desmazeaud and Gripon 1977). Peptidases of both lactic acid bacteria and penicillia contribute to formation of free amino acid and nonprotein nitrogen (Gripon et al. 1977). 

Together with proteins and peptides, amino acids constitute the main components of the nitrogenous fraction of musts and wines. They are also the most studied and best known nitrogenated components in wines. Free amino acids in musts are of paramount importance. They constitute a source of nitrogen for yeasts in alcoholic fermentation, for lactic acid bacteria in malolactic fermentation, and can also be a source of aromatic compounds (Kosir and Kidric, 2001). In certain cases, some amino acids... 

Collar, C., Mascaros, A. and Barber, B. (1990) Biochemical evolution of nitrogen compounds during fermentation of wheat doughs containing pure cultures of lactic acid bacteria. Zeitschrift Lebensmittelforsch. u. Forsch. 190, 397-400. 

Cytoplasmic granulations can be revealed by specific coloration techniques. They are insoluble reserve substances of an organic nature polymers of glucose or of the polyester of j6-hydroxybutyric acid. These reserve substances accumulate in the event of a nitrogen deficiency, when a source of carbon is still present. Inclusions of volutin (a polymer of insoluble, inorganic phosphate) are characteristic in lactic acid bacteria, especially certain species of the genus of strictly homofermenta-tive Lactobacillus. Volutin comprises a phosphate reserve available for the synthesis of phosphory-lated molecules such as nucleic acids. 

Amino acids and sometimes peptides supply lactic acid bacteria with their assimilable nitrogen. Amino acid requirements vary with respect to the species and even the strain. These acids can be strictly indispensable or simply growth activators. 

Thiamine diphosphate is a particularly interesting substance because it illustrates a chemical evolution from lactic acid bacteria to yeasts and then to higher animals. It converts pyruvic acid, which is an a-keto-acid, into the equivalent of a 3-keto-acid (with C=N instead of C=0), which can then lose CO2 by decarboxylation. We know that the hydrogen atom next to nitrogen in the thiazole ring is acidic and easily removed because if we shake thiamine with D2O, we get rapid exchange of that atom (Figure 2.11). 

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