Throughout the centuries it has been the practice in various fermentation-based processes to add back a proportion of the previously produced food to the new batch, so-called back slopping. What of course this did was to seed the fermentation with the preferred micro-organism, and for many foodstuffs this organism is a lactic acid bacterium. Such bacteria are only weakly proteolytic and lipolytic, which means that they are quite 'mild' with respect to their tendency to produce pungent flavours. They are also naturally present in the intestine and the reproductive tract, so it is no surprise that nowadays we talk of probiotics and prebiotics in the context of enriching the level of lactic acid bacteria in the gut. Probiotics are organisms, notably lactobacilli and bifidobacteria, which are added to the diet to boost the flora in the large intestine. For example, they are added to yoghurt. Prebiotics are nutrients that boost the growth of these organisms.
Like the brewing and baking yeasts, lactic acid bacteria tend to be GRAS, although some strains are pathogenic. Joseph Lister isolated the first lactic acid bacterium in i873. This organism that we now refer to as Lactococcus lactis is a species of great significance in the fermentation of milk products.
There are i6 genera of lactic acid bacteria, some i2 of which are active in a food context. They are Gram-positive organisms, are either rod-shaped, cocci (spherical) or coccobacilli. For the most part they are mesophilic, but some can grow at refrigerator temperatures (4°C) and as high as 45°C. Generally they prefer a pH in the range 4.0-4.5, but certain strains can tolerate and grow at pHs above 9.0 or as low as 3.2. They need preformed purines, pyrimidines, amino acids and B vitamins. Lactic acid bacteria do not possess a functional tricarboxylic acid cycle or haem-linked electron transport systems, so they use substrate level phosphorylation to gain their energy.
As we saw previously, their metabolism can be classified as either homofer-mentative, where lactic acid represents 95% of the total end products, or heterofermentative, in which acetic acid, ethanol and carbon dioxide are produced alongside lactic acid.
Lactic acid bacteria produce antimicrobial substances known as bacte-riocins. For the most part, these are cationic amphipathic peptides that insert into the membranes of closely related bacteria, causing pore formation, leakage and an inability to sustain metabolism, ergo death. The best known of these agents is nisin (discussed earlier), which has been used substantially as a 'natural' antimicrobial agent. Lactic acid bacteria also produce acids and hydrogen peroxide as antimicrobials.
The most notable species within this genus is L. lactis, which is most important in the production of foodstuffs such as yoghurts and cheese. It is often co-cultured with Leuconostoc.
There are two sub-species of L. lactis: Cremoris, which is highly prized for the flavour it affords to certain cheese, and Lactis, in particular L. lactis ssp. lactis biovar. diacetyllactis, which can convert citrate to diacetyl, a compound with a strong buttery flavour highly prized in some dairy products but definitely taboo in most, if not all, beers. The carbon dioxide produced by this organism is important for eye formation in Gouda.
Leuconostoc mesenteroides, with its three subspecies: mesenteroides, cremoris and dextranicum, and Leuc. lactis are the most important species, especially in the fermentation of vegetables. They produce extracellular polysaccharides that have value as food thickeners and stabilisers. These organisms also contribute to the CO2 production in Gouda.
Oenococcus oeni (formerly Leuc. oenos) plays an important role in malolactic fermentations in wine.
These are mostly pathogens; however, Streptococcus thermophilus is a food organism, featuring alongside Lactobacillus delbrueckii ssp. bulgaricus in the production of yoghurt. Furthermore, it is used in starter cultures for certain cheeses, notably Parmesan.
There are some 60 species of such rod-shaped bacteria that inhabit the mucous membranes of the human, ergo the oral cavity, the intestines and the vagina. However, they are equally plentiful in foodstuffs, such as plants, meats and milk products.
Lb. delbrueckii ssp. bulgaricus is a key starter organism for yoghurts and some cheeses. However, lactobacilli have involvement in other fermentations, such as sourdough and fermented sausages, for example, salami. Conversely, they can spoil beer and either fresh or cooked meats, etc.
Pediococcus halophilus (now Tetragenococcus) is extremely tolerant of salt (>18%) and as such is important in the production of soy sauce. Pedio-cocci also function in the fermentation of vegetables, meat and fish. On the other hand, Pediococcus damnosus growth results in ropiness in beer and the production of diacetyl as an off-flavour.
These faecal organisms have been isolated from various indigenous fermented foods; however, no positive contribution has been unequivocally demonstrated and their presence is debatably indicative of poor hygiene.
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