Biosynthesis of Chiral Flavor and Aroma Compounds in Plants and Microorganisms

Abstract

Capillary gas chromatographic determination of optical purities, investigation of the conversion of potential precursors, and characterization of enzymes catalyzing these reactions were applied to study the biogenesis of chiral volatiles in plants and microorganisms. Major pineapple constituents are present as mixtures of enantiomers. Reductions, chain elongation, and hydration were shown to be involved in the biosynthesis of hydroxy acid esters and lactones. Reduction of methyl ketones and subsequent enantioselective metabolization by Penicillium citrinum were studied as model reactions to rationalize ratios of enantiomers of secondary alcohols in natural systems. The formation of optically pure enantiomers of aliphatic secondary alcohols and hydroxy acid esters using oxidoreductases from baker's yeast was demonstrated.The world-wide trend to "natural" flavor and aroma has significantly increased interest in biogenetical pathways leading to volatiles in natural systems. For chiral compounds the exploration of potential biosynthetic routes is even more important, because chemical syntheses are often difficult and expensive; in many cases however sensory qualities of enantiomers are different (1-3). In our current studies of chiral volatiles in plant and microbial systems we use different analytical approaches, (a) Capillary gas chromatographic separations of diastereoisomeric derivatives are used to determine the configurations of chiral constituents at trace levels, (b) Chemically synthesized (labeled) precursors are added to fruit tissues and microorganisms. Their transformation into chiral constituents is investigated by means of capillary gas chromatography/mass spectrometry; the stereochemical course of these metabolizations is followed, (c) Enzymes catalyzing the stereospecific conversion of precursors to chiral compounds are isolated and characterized; commercially available enzymes are investigated as model systems to elucidate the stereochemical course of biogenetical pathways. The combination of these methods revealed some new aspects of the biosynthesis of chiral compounds in natural systems.

Assignment of distinctive volatiles, descriptive sensory analysis and consumer preference of differently ripened and post-harvest handled pineapple (Ananas comosus [L.] Merr.) fruits

“…Additionally, two diastereomers of methyl 3-acetoxy-2-methylbutanoate were identified as discriminative volatiles formed during post-harvest storage of pineapple fruits. These findings are in good agreement with our previously published results [13,15] and other studies on the post-harvest flavour genesis of pineapple fruit [11,28]. Sea-freighted pineapples stored for post-harvest flavour genesis (GR 25 dah) were additionally characterised by elevated concentrations of hexanal (see Table 2), exhibiting green and grassy notes [23].…”

Volatile constituents of green and ripened pineapple (Ananas comosus [L.] Merr.)

“…In ripened pineapples, ethyl acetate (33.5%), threo-butane-2,3-diol diacetate (13.0%), and 3-hydroxy-2-butanone (8.7 %) were the major constituents. Engel et al (1989) demonstrated that the concentrations of hydroxy and acetoxy acid esters increased during pineapple ripening. 3-Hydroxy-2-butanone has been reported as a main volatile constituent in a vacuum steam distillate from canned pineapple juice The descriptive analysis was done with an aqueous solution of each compound at the level of 1-10 ppm.…”

Volatile Compounds in the Skin and Pulp of Queen Anne's Pocket Melon

“…Alpha) ( 17), but this is the first time these compounds are identified in pocket melon. Engel et al (22) and Umano (24) have shown that the concentrations of hydroxy and acetoxy acid esters increased during pineapple ripening. Nevertheless, according to Wyllie and Leach (8), 2,3butanediol diacetate has a sweet odor and appears to have high odor threshold and, therefore, would not be expected to have a significant role in the aroma of melon (cv.…”

“…Among them, the levels of acetoxy and hydroxy esters were particularly predominant in skin. Although these compounds are generally found in tropical fruits such as passion fruit, pineapple, and mango ( − ), 2,3-butanediol diacetate ( 28 , 32 ) and its precursor 3-acetoxy-2-butanol ( 36 , 37 ) have been previously identified in Japanese melon (cv. Golden Crispy) () and in C. melo var.…”