Physiological Grouping
Acid producers
The organisms capable of fermenting lactose to form lactic acid are known as acid producers. Lactic acid coagulates milk by producing precipitation of Casein at 4.6 pH.
Homofermenters: Lactococcus, few Lactobacilli
Heterofermenters: Few Lactobacilli, Lueconostoc spp.,
LAB are classified as heterotrophic chemoorganotrophs i.e. these require the preformed carbon and energy sources. LAB lack cytochrome and cannot use respiratory pathway for energy generation. LAB generally lack metabolic diversity and mainly relies on two types of fermentation for the metabolism of sugars that form the basis of their taxonomic characterization. Based on Embden Meyerhoff Pathway and Phosphoketolase pathways LAB are classified as follows:
·         Homofermentative LAB: These Homofermentative LAB group use Embden Meyerhoff Pathway for hexose sugars utilization and produce 2 pyruvate and 2 ATP molecules. Pyruvate is further converted to lactic acid by the enzyme lactate dehydrogenase and is the only end product. However, under certain conditions the homofermenters can also undergo heterolactic fermentation. For example: during pentose sugar fermentation by LAB.
·         Heterofermentative LAB: This group of bacteria catabolise hexose sugars by Phosphoketolase Pathway and produce an equal amount of lactic Acid, carbon dioxide, acetate and ethanol as the major end products.
·         During this process only one ATP molecule is produced.
Gas producers
Gas producers are the organisms capable of producing CO2 or \ and H2 from lactose fermentations (e.g. E. coli, yeasts, and Clostridium spp.).
Proteolytic
Proteolytic organisms degrade milk proteins into soluble components by enzymes known as proteinases or proteases (e.g. Bacillus spp, Pseudomonas spp.).
Lipolytic
Lipolytic organisms are capable of attacking milk fat by enzymes such as lipases liberating glycerides and fatty acids (e.g. Pseudomonas spp, Achromobacter lipolyticum.
Moulds: Geotrichum candidum, Penicillium roqueforti).
Sweet curdling
Organisms capable of causing curdling of milk by rennin like enzyme before the development of sufficient acidity are known as sweet curdling organisms (e.g. B. subtilis, B. Cereus and Enterococcus liquefaciens).
Ropiness
Ropiness organism produce change in the viscosity of milk or forming threads when the milk is poured from one container to other, due to production of gums, mucins etc. (e.g. Alcaligenes viscosus).
Flavour producing
Many organisms are capable of producing different flavours in milk during their growth. Some of the flavours and associated organisms are listed below:
Fruity Pseudomonas fragi
Malty Lactococcus lactis subsp. lactis biovar maltigenes
Fishy Proteus icthyosmius
Unclean E. coli
Colour fermentations
Many organisms are capable of producing different colours in milk during their growth by producing chromogenic compounds. Some of the colours and associated organisms are listed below:
Yellow:          Pseudomonas synxantha
Blue:              Pseudomonas cyanogenes
Green:            Penicillium roqueforti
Black:             Pseudomonas nigrifaciens




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