The relationship 01 the distribution of pectic substances to tissue softening was exanined in ripening mangos at four stages of ripeness. Water-soluble and alkali-soluble pectin declined and ammonium oxalate soluble pectin increased as the mango lost its firmness and became soft. Polygalacturonase and cellulase activities of cell wall preparations ir creased markedly during ripening. The decline in alkali-soluble peci in and the increase in polygalacturonase activity correlated well wit11 the loss of firmness. Alkali-soluble pectin declined slowly in rip : mangos stored at 4°C. This decline correlated with loss of fnmne:s of the stored mangos. The cellulase activity of cell wall preparatio 1s from ripe mangos increased during 4°C storage and the increase correlated with the decrease in firmness.

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Representative samples of fresh orange juice (OJ) with various flavor scores were assayed for peroxidase activity. In thk assays, pphenylenediamiue (PPDA), the hydrogen donor, was oxidized by H, 0, after a short time lag (about 1 min) caused by interaction of ascorbic acid in the juice with a PPDA-oxidation intermediate. A search was made for compounds that are native substrates of OJ peroxidase. Thus, the peroxidase activity of a protein fraction precipitated from neutralized OJ with ammonium sulfate and dialyzed free of ascorbic acid was tested with H,O, and a number of hydrogen donor compounds that are normal juice constituents. Ascorbic acid was very reactive, as were the phenolic acids (caffeic, gentisic and coumaric). The flavonoids, criodictyol, hesperidin and naringin were unreactive. Reduced nicotinamide adenine dinucleotide (NADti) was also reactive in the presence of hydroquinone and other compounds that mediate electron transfer through intermediate states. The dialyzed protein fraction also catalyzed the oxidation of pyridoxal-PO,. indoleacetic acid. dihvdroxvmaleic acid, and NADH-; pcresol l& 0, plus Mn++. Although dJ appears to contain many compounds that are reactive with peroxidase, their reactivities in OJ are apparently very slow due to the level of H, 0,. Concentrations of ascorbic and caffeic acids did not change in OJ incubated at 30°C for 4 hr. Processing conditions that increased peroxidase activity and pulp content in OJ decreased quality of the juice.

Stored carrots vacuum packed in plastic shrink bags showed signijkant variations in the concentration of certain important secondary compounds.in the acetylene group, 18 compounds were isolated and measured. Only eight of these compounds have previously been reported as carrot constituents. The concentrations of 10 of the 18 increased with storage time, and six were present only in stored samples.

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The quality of citrus fruit represents the sum total of fruit development--fruit set, growth, tissue differentiation and ripening on the tree. During this development period compounds are formed that are responsible for the color and flavor of the ripe fruit. Many of these compounds have been identified, and research is now directed toward identifying the enzymic reactions that regulate their biosynthesis through metabolic pathways. Stewart (1) recently reviewed the carotenoid pigments identified as present in citrus and assessed their contribution to citrus color. Reported with that review in the same journal was the mode of action of bioregulators in controlling carotenoid biosynthesis through enzyme inhibition (2). Many terpenoids, aliphatic esters and aliphatic aldehydes in citrus fruit have been identified, but the pathways for their biosynthesis have not yet been determined (3). Bruemmer et al. (4) recently reviewed the working hypotheses on the mechanism for regulating the biosynthesis of citrus acids and concluded that supportive data were inadequate to identify the enzyme reactions that regulate acid metabolism in citrus.The quality of extracted citrus juices depends on enzyme reactions that occur not only in the fruit during the development period, but also in the juice during processing.When juice is extracted from citrus fruit, enzymes are released from their normal restraint in the cell.Several of these enzymes catalyze reactions that adversely affect taste and appearance of the juice. Unless the reactions are controlled, the juice products will not meet the standards of quality set up by the USDA Food Safety and Quality Service.The two reactions of commercial importance are the hydrolysis of pectin to pectic acid, which clarifies juice, and the lactonization of limonoic acid Α-ring lactone to the bitter compound, limonin. Research efforts to identify and characterize the reactions, to isolate and purify the enzymes, and to develop methods to control the reactions are described in this review. This chapter not subject to U.S.