Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/1996
Publication Date: N/A
Interpretive Summary: A number of off-flavors in soybeans prevents the expanded use of soybeans as food. Pentenal and ethylvinylketone are among these off-flavors, the latter being described as "raw bean" odor. The specific steps in the biosynthetic pathway were identified for the first time. Both pentenal and ethylvinylketone originate from a polyunsaturated fatty acid, linolenic acid, through the action of enzymes. This work gives a rationale basis to eliminate these odors from soybeans, such as breeding low linolenic acid varieties or genotypes devoid of the enzymes involved.
Technical Abstract: Previous work [Salch et all., Plant Physiol. (in press)] showed that under anaerobic conditions and the presence of a polyunsaturated fatty acid, soybean (Glycine max L. cv Century) lipoxygenase isoenzymes converted 13(S)-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid, a lipoxygenase- catalyzed oxidation product of linolenic acid, into 1-penten-3-ol, 2(Z)- penten-1-ol, and 13-oxo-9(Z),11(E)-tridecadienoic acid. It seemed plausibl that the reported "raw bean odor," ethylvinylketone, previously isolated from soybean homogenates [Mattick and Hand, J. Agric. Food Chem.17, 15-17 (1969)] could arise from 1-penten-3-ol by oxidation catalyzed by soybean alcohol dehydrogenase and NAD+. In this communication it was shown that both ethylvinylketone and 2-pentenal are produced by a soybean preparation after anaerobic incubation with 13(S)-hydroperoxy-9(Z),11(E),15(Z)- octadecatrienoic acid and linolenic acid, and it was also demonstrated that tNAD+ stimulated the formation of 2-pentenal. Further work showed the presence of two separable isoenzymes of alcohol dehydrogenase in soybeans, and that these isoenzymes in the presence of NAD+ were capable of utilizing both 1-penten-3-ol- and 2(Z)-penten-1-ol as substrates. However, one of the "leaf alcohols," (2E)-hexen-1-ol, was a preferred substrate over both pentenols as demonstrated by its lower Km value. The Km determined for 2(Z)-penten-1-ol was significantly lower than for 1-penten-3-ol. Thus, in terms of substrate preference indicated by Km, the order was 2(E)-hexen-1- ol> 2(Z)-penten-1-ol> 1-penten-3-ol.