Submitted By:
PATEL RITESH R

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1. Introduction
The production of flavor and aroma chemicals by microorganism has been recognized for over 60 years. Flavor is usually the result of the presence, within complex matrices, of many volatile and nonvolatile components Flavor is usually the result of the presence, within complex matrices, of many volatile and nonvolatile components possessing diverse chemical and physicochemical properties. Whereas the nonvolatile compounds contribute mainly to the taste the volatile ones influence both taste and aroma. A vast array of compounds may be responsible for the aroma of the food products, such as alcohols, aldehydes, esters, dicarbonyls, short to medium-chain free fatty acids, methyl ketones, lactones, phenolic compounds and sulphur compounds .
Since early times, flavour compounds ranging from single to complex substances have been extracted from plant sources. Eventually, after elucidation of their structure, synthetic flavours were produced by chemical synthesis. Nowadays, flavours represent over a quarter of the world market for food additives and most of the flavouring compounds are produced via chemical synthesis or by extraction from natural materials. However, recent market surveys have shown that consumers prefer foodstuff that can be labelled as »natural«. Although flavours may be produced by chemical transformation of natural substances, the resulting products cannot legally be labelled as natural. Furthermore, chemical synthesis often results in environmentally unfriendly production processes and lacks substrate selectivity, which may cause the formation of undesirable racemic mixtures, thus reducingprocess efficiency and increasing downstream costs. On the other hand, the production of natural flavoursby direct extraction from plants is also subject to various problems. These raw materials often contain low concentrations of the desired compounds, making the extraction expensive. Moreover, their use depends on factors difficult to control such as weather conditions and plant diseases. The disadvantages of both methods and the increasing interest in natural products have directed. many investigations towards the search for other strategies to produce natural flavours.
An alternative route for flavour synthesis is based on microbial biosynthesis or bioconversion . The most popular approaches involve the use of microbial culture or enzyme preparations, although plant cell cultures have also been reported as suitable production systems. Microorganisms can synthesize flavours as secondary metabolites during fermentation
Flavours and fragrances find wide application in the food, feed, cosmetic, chemical and pharmaceutical sectors. Many flavour compounds on the market still are produced via chemical synthesis or via extraction from plant and animal sources; however, a rapid switch towards the bio-production and use of flavor compounds of (micro) biological origin – bioflavours – Is observed. The reasons are, among others, the facts that chemical synthesis results often in an environmentally unfriendly production process and in undesirable racemic mixture compounds. Furthermore, the consumer has developed a ‘chemophobia’-attitude towards chemical or synthetic (even nature-identical) compounds, especially when related to food and products used in the home.1–4 Up to now, certain plant and animal sources remain an important source of bioflavours, but these bioactive compounds are often present in minor quantities, making isolation and formulation very expensive, or they are found only in exotic (plant) species.The other bio-route for flavour synthesis is based on de novo microbial processes (fermentation) or on bioconversions of natural precursors using microbial cells or enzymes (biocatalysis).
NATURAL AND NATURE-IDENTICAL FLAVOURS
Natural flavours as ‘flavouring substances or preparations which are obtained by appropriate physical processes or enzymatic or microbiological processes from material of vegetal or animal origin’.Natural flavour’ means ‘ the essential oil, oleoresin, essence or extractive, protein hydrolysate, distillate of any product of roasting, poultry, eggs, dairy products or fermentation products’. In view of the emerging concept of bioproduction of natural flavours, the term ‘natural’ has been clearly defined in the USA as well as in Europe (EC). In the USA a distinction is made between natural and artificial flavour compounds and according to the ‘Code of Federal Regulations’ (1990), the term ‘natural flavour’ means ‘… the essential oil, oleoresin, essence or extractive, protein hydrolysate, distillate of any product of roasting, heating or enzymolysis, which contains the flavouring constituents derived from a spice, fruit juice, vegetable or vegetable juice, edible yeast, herb, bud, bark, root, leaf or similar plant material, meat, seafood, poultry, eggs, dairy products or fermentation products thereof, whose significant function in food is imparting flavouring rather than nutrition’.
The EC Flavour Directive (88/388/EEC) defines natural flavours as ‘… flavouring substances or preparations which are obtained by appropriate physical processes or enzymatic or microbiological processes from material of vegetal or animal origin’.
Both definitions state that natural flavours include products obtained through microbial or enzymatic processes as long as the precursor/raw material be natural and obtained via physical or bio-processes and that the precursor and product can be found in nature or are part of traditional foods. Physical processes for obtaining natural flavours are extraction, distillation, concentration, crystallisation, etc, i.e. from animal sources (e.g. beef, chicken, seafood) or plant sources (e.g. spices, mushroom, citrus, fruits, mints). Products that occur in nature but are produced via a chemical (a non-natural) process are called ‘nature-identical’; this mode of production is no longer accepted as consumer-friendly.
2. FLAVORS AND FRAGEANCE CHEMICALS
3.1 Methyl ketones
All the flavors in demand, there is no doubt that chemicals that contribute to cheese flavors are most important. Starkle first demonstrated the importance of methyl ketones,RCOCH3,and their formation in mold ripened cheese numerous other investigators have attributed the odor and tast of ripened cheese to the presence of methyl ketones, particularly methyl n-pentyl ketone as well as other short chain contribute fruity-spicy notes to fragrances
The formation of methyl ketones from fatty acids was first attributed to penicillium roqueforti mold spores and not to be mycelium.it was subsequently shown by Lawrence and hawke that the p.roqueforti mycelium was capable of converting fatty acids with less than 14 carbon atoms,RCH2CH2CO2H,To methyl ketones.in every case,the acids were oxidized to methyl ketones with one less carbon atom than the original acid.this has been confirmed in the laboratories of the givaudan corporation using a varity of fungi.it is also to possible toconvert vegitable oil and triglycerides to methyl ketones.
3.2 Diacety
Diacetyl, CH3COCOCH3 is a naturally occurring chemical characterized by a powerfull and diffusive odor resembling butter when dilute.it is extensively used in imitation butter and other dairy flavors and in numerous flavors where butter notes are desirable.diacetyl also find limited use in perfumes,primarily in reconstituting essential oils.closely related to diacetyl is acetoin.Acetoin is frequently found with diaaetyl but probably conyributes little or no flavor by itself.for many years it had been assumed that diacetyl was produced from acetoin by microbiological oxidation. numerous recent studies have shown that this is not the case.
The various theoretical pathway to diacetyl synthesis have been summarized by kempler and MCKay the organisms of commercial significance which produce diacetyl are streptococcus lactisssp diacetilactis and several leuconostoc species troller has patented method for increasing the diacetyl production of bacteria such as s.diactilactis,s.cremoris and lactis the use of a humectants such as glycerol or sucrose, which lowers the Aw value of the medium results in greater diacetyl production the production of diacetyl is enhanced by a PH below 5.5,low temperature and aeration a PH below 5.5 flavors citric acid permease activity and restricts diacetyl reductase activity.
3.3 Lactones
Lactones are internal ester of primarily γ-and δ-hydroxy acids.lactones are ubiquitous in food, contributing taest and flavor nuance. numeous odor and taste characteristics have been attributed to lactones. among these are oily-peachy, creamy, fruity, nut-like, coconut, hony, and so on. mega summarized the types of lactones identified in fruits and vegitables. many of these have been chemically synthesized, finding use in variety of artificial flavors. the use. however, of microbially produced natural lactones in flavors would find wide acceptance among consumers.
The substrates include triolein, sebum, lecithin, oleic acid and tween 80. A coconut aroma is highly desired by flavorists. γ-octalactone and γ-nonalactone posssess this aroma. another lactones’ having a coconut odor is 6-pentyl-2-pyrone.this chemical was found by Collins and halim to be the major volatile constituent of the fungus trichoderma viride. the productionof ustilagic acid has been scaled up in 200gal fermenters.yiled of 22gl-1 of fermentation broth have obtained.
3.4 Butyric Acid
Butric acid, CH3CH2CH2CO2H, at low concentration is used to supply butter-like notes to flavors. it find particular application in natural cheese flavors of various products. pentyl butyrate provides a strong, ethereal, fruity, somewhat pungent odor suggestive of pear, pineapple and banana.
Although natural butyric acid as an ester may be found at a concentration of 2-4%in butter, its isolation is an expensive and difficult process. as a result ,the fermentative production of natural butyric acid is a valuable alternative.
Butyric acid is primarily produced by obligate anaerobes of the genera clostridium, butyrivibrio, Eubacterium and Fusarium. The clostridia, particularly C.acetylbutyricum, have been studied in detail. Their ability to produced organic solvents such as acetone and butanol has led to commercial processes which may be modified and adapted to produce butyric acid. besides proper selection of the microorganism, it is necessary to maintain the PH above 5.0 in order to direct the fermentation away from solvent and formation and towards butyric acid formation.
3.5 Isovaleric Acid
Isovaleric acid,(CH3)2CHCH2CO2H,is undoubtedly one of the most offensive odors encountered in the flavor/fragrance industry. Not only does it possess an acid-acrid odor commonly described as ‘locker room ’or ‘dirty feet’, but isovaleric acid has a tenacious affinity for the skin. despite this, in extremely dilute concentration isovaleric acid becomes agreeable and herbaceous. furthermore, its ester find widespread use in the flavor industry. prominent among these are ethyl isovalerate, which possesses a powerful apple-fruity odor and finds application in numerous fruit flavors as well as in candies and chewing gums; isopentyl isovalerate, which has a fruit-apple-raspberry odor and find use in apple flavors and as a modifier in numerous fruit and nut flavors; and isobutyl isovalerate, also apple-raspberry-like and not only in fruit flavprs, but also in perfumes for lipsticks.
Isovaleric acid may be produced synthetically by the oxidation of isopentyl alcohol. subsequent direct esterification leads to the various ester. if natural isovaleric acid is desired, this process is not suitable. two potential methods exist for the microbial production of natural isovaleric acid. the first ,the microbial oxidation of isopentyl alcohol is theoretically possible since the oxidation of terminal alcohol groups to their corresponding carboxyl groups is well known. this procedure is limited both by the toxicity and the low solubility of isopentyl alcohol
3.6 Geosmin
Geosmin, first isolated by gerber and lechevalier, is an earthy-smelling chemical produced by blue-green algae, myxomycetes, actinomycetes and other microorganisms. Geosmin has frequently been found contaminating water supplies. not only does this result in unpalatable water, but also fish and animals drinking the water become unacceptable for consumption. geosmin has two unique properties its odor is detectable in water at a concentration of 0.2µg l-,and it has the ability to ‘fatigue’ the nose rapidly, making organoleptic evaluation difficult. the earthyl notes of geosmin are useful in imparting or modifying amber nots in perfume. at a concentration of 1-100 p.p.m, geosmin is described as being useful for the reconstitution of natural essential oils.
Gerber and Lechevalier produced geosmin in fermenters utilizing Streptomyces griseus LP-16 in a soybeen meal-peptone-salt-glucose medium. yields were 6 mg l-1 after three days fermentation. geosmin was recovered by extracting a steam distillate with methylene chloride and obtaining the geosmin is complicated, its production by fermentation might be a practical commercial process.