Amino Acids as Precursors of Volatile Components in Tomato Fruit

The composition of tomato fruit at nine different maturity stages was studied. The dry matter content decreased gradually, whereas the alcohol soluble material and non-protein nitrogen increased with ripening. Total nitrogen decreased up to the large green stage and then increased during ripening. Starch accumulated until nearly the large green stage and then diminished rapidly. Of the two components of starch, amylose decreased much faster than amylopectin, and the ratios of amylopectin and amylose thus increased during ripening. Among the amino acids, glutamic acid was presented in an exceptionally high concentration, and its concentration doubled as the fruit ripened. A concomitant decrease in the concentration of several other amino acids was also noted. The possibility of amino acids serving as precursors for the synthesis of volatile aroma components in tomato is discussed. Evidence is then presented to demonstrate that crude enzyme preparations from fresh tomatoes can produce carbonyl compounds in the presence of some amino acids. The enzymatic activities of the preparations from field-grown tomatoes were generally higher in converting alanine and leucine than those of the preparations from greenhouse-grown tomatoes. Preparations from young fruit had lower activity despite the high protein concentration. The activity of the enzyme extracts increased with development stages of the fruit, and different amino acids showed different patterns in the response to enzyme preparations. The enzymes are sensitive to pH changes and there is a marked difference in activity between the soluble and the insoluble fractions of the extracts. Analyses were subsequently carried out by gas chromatography for the volatile components enzymatically produced from amino acids by tomato extracts. Alanine, leucine, and valine were found to be the most important precursors. Chromatograms differed with enzyme preparations and with substrate amino acids used. More volatile components were produced with enzymes prepared from red-ripe tomatoes than with those from green tomatoes. The enzyme preparations from greenhouse - grown ripe tomatoes were found capable of producing 3-methyl-butanal in the presence of L-leucine. The produciton of 3-methylbutanal was further confirmed using (14C)-L-leucine as substrate. When preparations from young green tomatoes were used, the enzymatic reaction products contained propanal. This indicates that different enzyme preparations varied their activity in converting the same substrate amino acid. Further experiments have shown that the enzyme extracts from field-grown red ripe tomatoes produced propanal and some unidentified carbonyl compounds from alanine, whereas they produced several alcohols from leucine and valine. Addition of two amino acids as substrates sometimes resulted in new volatile components. The production of volatile components from amino acids appears to involve transamination. This was demonstrated by the analysis of glutamic acid formed during the enzymatic reactions. A decrease in the substrate amino acid and a concomitant increase in glutamic acid was observed in the reaction mixtures. Based upon the reaction products that were identified, some hypothetical pathways of the biosynthesis of a few volatile components have been suggested