The biochemistry of fermentation Microorganisms Involved In Food

* The biochemistry of fermentation * Microorganisms Involved In Food Fermentations * Chemical aspects of food fermentations • • • Lactic acid bacteria Propionic acid bacteria Acetic acid bacteria Yeasts Fungi

o Fermentation as the result of an anaerobic type of metabolism. The most commonly used microorganisms is glucose. energy substrate among o In the presence of oxygen many microorganisms metabolitze glucose producing water, carrbondioxide and large amounts of energy (ATP) which is used for growth activities. o Such end product metabolites often include large amounts of lactic acid, acetid acid and ethanol as well as minor amounts of other volatile organic acids, alcohols and their esters.

o Fermented food are the result of the activity of a few species of microorganisma among the thousands of known species of bacteria, yeasts and moulds. o Of the food fermenting organisms the lactic acid producing bacteria, acetic acid producing bacteria and some alcohol producing yeasts are most important. o Certain mould species also play a very important role in some fermented foods.

o The lactic acid produced lowers the p. H value of the growth environtment and has a sour taste. It is also inhibitory to the growth of many other microbial species. o Within this group two sub-divisioins are rcogniized, the homofermentative organism and the heterofermentative organism. o Homofermentative species essentially produce only lactic acid from their sugar metabolism whereas the heterofermentative species produce carbondioxide and minor amounts of other volatile acids, alcohols and esters in addition to lactic acid.

o Some important species in this group include : o Streptococcus thermophilus, Streptococcus lactis and St. Cremoris. These are Gram positive coccus shapped bacteria which occur in chains and all are of great economic importance in the dairy industry. o Pediococcus cereviseae. These are Gram positive cocci, typically occuring in pairs or tetrads. Although this species in noted for beer and wine spoilange, it plays an important role in meat and vegetable fermentations. o Leuconostoc mesenteroides, leuconostoc dextranicum. These are Gram positive cocci occuring in pairs or short chains. However, they are very important species in initiating fermentation of vegetables and are also found in fruit juices, wines, and other foods. o Lactobacillus lactis, L. acidophilus, L. bulgaricus, L. plantarum, L. delbrueckii. These organism are Gram positive rod shaped bacteria frequently forming pairs and chains of cells. They are very important in milk and vegetable fermentations.

o Species belonging to this groups are found in the genus are Propionibacterium and are Gram positive rod shaped organism. o They are of significance in food fermentations since they are capable of fermenting carbohydrates and even lactic acid with the production of propionic and acetic acid and carbondioxide. o They are important in Swiss cheese fermentation.

o There are Gram negative rod shaped bacteria found in the genus Acetobacter for example Acetobacter aceti. o Their metabolism is essentially aerobic (unlike the species above) but their main concern with food fermentation is the ability to oxidize alcohol and other carbohydrate substances to acetic acid. o They are used in vinegar manufacture.

o Yeasts carry out classical alcoholic fermentations where the main product of their metabolism is ethanol. o Saccharomyces cereviseae is the most important species involved in the production of alcoholic beverages such as beers and wines and is also used for dough fermentation in bread manufacture.

o Certain moulds are used in the preparation of several of cheese and in the fermentation of many Asian foods such as soy sauce and tempeh. o Species belonging to the genera Aspergillus, Rhizopus, Penicillium are most important in the activities.

o Foods, in general, furnish a satisfactory medium for the growth of a variety of microorganisms. o Fermentations are enzyme-induced chemical alterations in the food; the enzymes involved may be produced by microorganisms or they may be indigenous to the food.

o The character of fermented foods will be determined by the nature and quality of the food itself, the changes that occur as a result of the action of its inherent enzymes, the alterations that occur as a result of microbial fermentation, and the interactions that occur between the products of these activities and constituents of the foods. o Fermentatios by desirable organisms impart flavour, bouquet and texture to the fermented foods. In many lactic acid fermentations, the high acidity, low p. H and low redox potential attained are responsible for inhibition of other organisms and of undesirable chemical changes

o Pure culture fermentations rarely occur naturally. o The broad spectrum of nutrients supplied by foods support the growth of yeasts and many species of lactic acid bacteria so that mixed fermentation normally occur. o Food sugars provide the energy for the microbial metabolic processes, but proteins lipids, vitamins, nucleic acids and minerals are essential for the synthesis of cell constituents. o In addition of food carbohydrate alterations during fermentation, food proteins, fats and nucleic acids may also be degraded with consequent effects upon food flavour, and texture.

o Many of the chemical alterations which do occur in fermented foods cannot be attributed entirely to the action of microorganism and it is believed that food indigenous enzymes are also responsible. Generally, such activity is associated with curing, ripening, and aging rather can actual fermentation.