THE ACTION OF LIPOXIDASE IN FROZEN RAW PEAS^a

Abstract

Raw vegetables which are held in frozen storage for extended periods show a progressive decrease in quality, characterized by development of off-flavors.and changes in the composition of lipid matter. One of the major changes occurring during the frozen storage of raw peas and other r a~7 vegetables is the development of peroxides in the lipids (2, 3 ) . There was a loss of chlorophyll and carotene when frozen raw peas were held for periods from one to 6 years at -17.8" C. (3,8,9). I n contrast, blanched peas of the same lots stored under identical conditions did not show this loss of chlorophyll, or changes in coniposition of the lipid materials.The enzyme lipoxidase has been postulated as being a causative agent for chlorophyll breakdown during frozen storage of raw peas (8). The primary action of lipoxidase is to cause a peroxidation of the double bonds of certain unsaturated fatty acids. A concurrent action involves the oxidatioii of certain unsaturated plant pigments such as chlorophyll and carotene (6).To prove that lipoxidase is responsible for peroxidation of lipids and chlorophyll destruction during storage it is necessary, first, to demonstrate its presence in raw peas, and second, this enzyme must be isolated from raw peas, purified, and allowed to react with its substrates in such a manner that the effects of its action may be determined quantitatively.This paper reports the demonstration of the presence of lipoxidase in frozen raw peas, its partial purification, and its action in bringing about the peroxidation of pea lipids and the destruction of chlorophyll in blanched peas.

ENZYME ACTION AND OFF‐FLAVOR IN FROZEN PEAS II. THE USE OF ENZYMES PREPARED FROM GARDEN PEASa

“…I t was impossible to determine when this deterioration of lipase had occurred, but the fact that very little increase in acid number was observed in lipase-treated samples may be best explained by the lack of extensive lipase action during storage. In contrast the pancreatic lipase used in the previous study (14) retained 96% of its original activity during 17 months of frozen storage and produced very large increases in titratable acidity. The finding of positive peroxide numbers, substantial losses of chlorophyll, and lowered flavor scores in lipoxidase-treated pea samples was taken as further substantiation of the role played by lopoxidase in production of off-flavors and other deteriorative quality changes during frozen storage (10,13,14).…”

“…The action of pea catalase in producing off-flavor when added to blanched peas was certainly much more profound than the corresponding action of liver catalase had been in the previous study (14). The sample containing added pea catalase preparation possessed the most disagreeable off-flavors of any samples in this present study.…”

“…Increases in acid numbers were quite meager when compared with the very large acid numbers caused by the addition of pancreatic lipase in the earlier experiments with commercial enzymes (14). Catalase-treated blanched peas showed an increase in acid number, as did the lipase-treated peas.…”

“…Samples of blanched peas to which native pea enzymes had been added were subjected to taste panel evaluation for flavor and color after 18 months of storage at -17.8" C. In each case evaluation was based on comparisons with the flavor and color of blanched peas with no added enzymes, which had been stored under similar conditions. As in the previous study (14) in which cornmercially obtained enzymes were employed, addition of enzymes to blanched peas produced off -flavors ranging in intensity from mild to extremely objectionable. Mild off-flavor was produced by added peroxidase.…”

“…The color change brought about by lipoxidase action through conversion of chlorophyll to colorless products (14) was again demonstrable when lipoxidase-containing samples were mixed with linoleic acid during determination of residual lipoxidase activity. The pea tissue fragments of such samples showed a marked degree of bleaching when compared with blanched pea control samples.…”