Gamma Decalactone Production
The synthesis of y-decalactone using intact microorganisms is a particularly good example of the use of biotransformations in the production of flavours because it required the development of truly innovative technology and has made a big commercial impact. The need for a natural source y-decalactone can be illustrated by a chemical analysis of strawberries. At least 350 molecules are thought to contribute to the taste of strawberry, including y-decalactone and over 100 esters.
- Fig. 10. Flow diagram of the manufacturing process for 5'nucleotide-rich yeast extract as a flavour exhancer for foods (from [9])
This molecule is also an important component of many fruit flavours and is an especial key element of peach and apricot flavours. However, head-space analysis of the chemicals released by strawberries shows that y-decalactone is rapidly lost once the fruit has been picked, presumably due to metabolism by enzymes activated by the trauma of harvesting. This 30-fold reduction is an Important cause of the loss of strawberry-flavour quality post-harvest. In contract, a similar head-space analysis of the aroma chemicals released by growing and harvested ripe peaches showed a 16-fold rise in y-decalactone in the harvested fruit [10]. Similar changes (both increases and decreases) have been found in the amounts of aroma chemicals produced by flowers when the air surrounding growing and picked flowers is sampled and analysed.
Thus, y-decalactone is important in creating high quality fruit flavours. Although y-decalactone can be synthesised easily by conventional chemistry, the consumer demand for 'natural' flavours created a need for a biotechnologi-cally produced material. In addition, y-decalactone is a chiral molecule, and molecules extracted from fruit have a defined stereospecificity. The (R)-isomer predominates in peaches and most other fruit, but appreciable amounts of the (S)-isomer are found in some varieties of mango. Chemical synthesis of y-decalactone only yields a racemic mixture. Therefore, a microorganism capable of forming optically active y-decalactone was sought by a number of companies by screening naturally occurring microorganisms. This search was aided greatly by the earlier observation by Japanese workers that an unlikely source, castor oil, provided a very good precursor molecule [11].
Castor oil is predominantly an ester of ricinoleic acid, an unsaturated, CI8, 12-hydroxy fatty acid, which contains a CIO, 4-hydroxydecanoic acid substructure. Heating 4-hydroxydecanoic acid under conditions of acid pH results in the formation of y-decalactone in almost quantitative yields from the ricinoleic acid that is metabolised. A variety of microorganisms, chiefly yeast's was found that has lipase activity to hydrolyse the castor oil, that was able to tolerate the fatty acids produced, and, most importantly, could carry out partial P-oxidation of ricinoleic acid to form 4-hydroxydecanoic acid. As a result, a process involving the growth of a specially selected yeast strain in a medium containing peptone, yeast extract and ricinoleic acid, followed by treatment of the fermentation broth by pasteurisation, acidification, clarification by centrifu-gation or microfiltration extraction with butylacetone, separation, solvent extraction, distillation and fractionation has been developed. This approach has been patented, scaled up to yield up to 10 g product per litre and, for several years has successfully produced y-decalactone for a wide range of commercial uses (Fig. 11) [12-16],
Subsequently, this method has been extended and improved. Esters of ricinoleic acid, prepared by transesterification, are better precursors because they form less foam and emulsion in the fermenter than castor oil. Mixtures of y-decalactone and unsaturated lactones with new organoleptic qualities have also been made. Addition of glycerol also stimulated y-decalactone formation. Initially this was thought to be an effect of water activity, but in fact the glycerol was acting as a metabolic precursor of lactone formation. Quite different approaches have also been explored, such as the preparation of 5-decanolide and 5-dodecanolide by the use of S. cerevisiae to saturate unsaturated lactones that can be extracted from Massoi bark oil [17],
Using the hydroxyacid approach, yeasts have been used to produce 8-lactones from 11-hydroxypalmitic acid obtained from sweet potatoes or jalap resin (Fig. 11). In this process, the yeast has an absolute requirement for an odd number of carbon atoms between the carboxyl group of the fatty acid and its hydroxyl group [18]. An alternative approach for biolactone production is to produce the hydroxy fatty acid intermediate by microbial hydroxylation of a fatty acid. Mucor species can produce 11-hydroxdodecalactone from dodecanoic acid or its esters, rather than having to rely on a preformed source of the hydroxy fatty acid intermediate. Similarly, octalactones can be formed by the fermentation of coconut oil caprylic acid (w-octanoic acid) by Mortierella species to form the readily lactonised y-hydroxyoctanoic acid or ester [19],
Sterilization
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