Isolation of unpleasant flavor compounds in the avocado (Persea americana)

Abstract

Three long-chain C17 aliphatic compounds (a section. The last two compounds are considered 4-keto-2-hydroxy-l-acetate, a 1,2-dihydroxy-4-ace- to be the principal unpleasant flavor compounds. toxy compound, and a 1,4-dihydroxy-2-acetoxy These compounds were shown to be present in compound), each with a terminal acetylenic bond, immature fruit from several different avocado which were isolated from immature avocado seed, varieties, and in commercial samples of crude and flesh, and skin, appeared to be the main constit- rectified fresh oil of unknown origin. uents of an unpleasant “bitter”-type flavor in each he avocado is one of the few cultivated succulent fruits in which fatty oil is the characteristic and predominant T dry constituent (Winton and Winton, 1935), and crude and rectified avocado oils are widely used in the cosmetic industry (Shannon, 1949). The oil content of the avocado has been taken as the most reliable index of maturity, although oil content for satisfactory flesh flavor may vary over a range of 15 x or more for a particular variety and, further, the oil content of different varieties may be as low as 10 % or as high as 30x for acceptable flavor and soft even consistency in flesh of mature ripened fruit (Hope, 1963). It is well known that immature avocado flesh has an unpleasant “bitter”-type flavor, which, while not being particularly intense, nevertheless leaves a distinctive, prolonged aftertaste on the palate. In some varieties of avocado, such as Fuerte, Sharwil, and Hass, this unpleasant flavor is undetectable in mature, ripened flesh, while in other varieties, particularly Zutano, the strong unpleasant flavor in the flesh of the immature fruit is also readily detectable as an aftertaste in the flesh of fully mature fruit before and after postharvest ripening. This apparent decline of the unpleasant flavor with maturity, particularly in varieties such as Fuerte, together with problems of heat-induced off-flavor development in the processing of ripened avocado flesh (Bates, 1970), suggested that if the chemistry of this unpleasant flavor was elucidated, further advances might be made in the maturity assessment and in the processing of the avocado, as well as in the development of improved hybrids, e.g., from Zutano (which is noticeably blemish-resistant) and Fuerte (which is particularly prone to anthracnose). Weatherby and Sorber (1931) reported a bitter-astringent principle in the avocado seed, and an anise-like odor in the seed from Mexican varieties. Unsuccessful attempts were made by Bilger et al. (1932) to isolate the bitter principle. Geissman and Dittmar (1965) isolated a proanthocyanidin of strong astringent flavor from avocado seed. This paper reports the isolation and identification of compounds contributing to the unpleasant flavor in the Zutano avocado.