11.5.5 Colour and flavour
During ripening (Figure 11.16) most fruit change colour. Their bright colour which evolved to attract dispersal agents such as birds, browsing animals and primates now becomes a particularly important visible indicator of maturity and ripeness. Pomefruit, stonefruit and strawberries provide good examples where colour is a prime indicator of ripeness.
By analogy with senescence in most green tissues such as leaves, colour change by fruits involves chlorophyll loss and an increase in production of yellow, orange, red or purple pigments. Yellow, orange or red pigmentation, as seen in oranges and tomatoes, arises from conversion of chloroplasts to chromo-plasts (Figure 11.16). New proteins are formed as end-products of the phytoene pathway and lead to accumulation of yellow-orange carotenoids or red lycopene. Red and purple pigments of the type seen in grapes and boysenberries result from products of the anthocyanin pathway, and are located in vacuoles. Both types of pigment can occur in the same fruit.
Not all fruit show deﬁnite colour changes as they ripen, so other indices of their physiological state are needed. Next to colour, humans rely on aroma to detect when fruit is ready to eat. Later stages of ripening in almost all fruits are accompanied by a production of volatile compounds (volatiles), creating much of the attractiveness to birds, fruit bats, browsing animals and insects. Maturity at harvest strongly influences the capac-ity of a fruit to produce a characteristic suite of volatiles, and some fruits can only be harvested at about the time when the ﬁrst volatiles are already being produced.
These volatiles join with sugars and acids in creating the flavour proﬁle that is unique to each fruit (Figure 11.17). In general, one or two key compounds are regarded as characteristic for fruit of given species or cultivars, and are often used in synthetic mixtures to represent that commodity. Flavour agents for most fruits contain a mixture of volatile acids, aldehydes, alcohols, esters, terpenoids and aromatics. Because human taste sensations and experiences play such an important part in characterising these compounds, a vocabulary has developed to describe their sensory nature. The terms mostly used are ones which relate a particular flavour sensation to that of a widely available standard, and have led to terms like ‘buttery’, ‘grassy’, ‘floral’, ‘vanilla’ and ‘citrus’.
Surprisingly little is known about biosynthetic pathways or control processes for production of flavour compounds. Aside from ethanol and acetaldehyde, which are generally produced from sugars by anaerobic respiration, many are derived from fatty acids which form part of the membrane lipids. Lipases or hydrolases initially cleave off the fatty acids, then lipoxygenases and/or isomerases and/or lyases produce aldehydes such as E-2-hexenal. These compounds have ‘grassy’ aromas reminiscent of green peppers, and are also produced in leaves as a response to wounding. Alcohol dehydrogenases can then transform aldehydes to the corresponding alcohols, which also contribute to ‘green’ aromas. Esterases then probably act on alcohols to form esters, which generally contribute ‘fruity’ and ‘sweet’ characteristics. Such esters often become prominent during over-ripeness and senescence.
Even less is known about the terpenoids which also contribute to a total flavour proﬁle. Some come from glycosidic precursors through action of a glucosidase, or may result from acid catalysis when compartmentation between vacuolar and cytoplasmic contents is lost on completion of ripening.
Examples of fruit where important volatiles have been identiﬁed include: mango — ocimene, myrcene and dimethylstyrene; muskmelon (cantaloupe) — ethyl-2-methyl butyrate, 3-methyl butyl acetate, ethyl butyrate, ethyl hexanoate, hexyl and benzyl acetate and some nine-carbon alcohols and aldehydes; apple and pear — two- to six-carbon esters such as butyl ethanoate, 2-methyl butyl ethanoate and hexyl ethanoate; strawberry — furaneol.
Speciﬁc volatiles are especially important in wine grapes where an individual volatile can become the dominant charac-teristic used in marketing a speciﬁc wine type. Examples include the ‘grassy’ character of methoxypyrazine in Sauvignon Blanc, the ‘richness’ of b-damacenone in red wine, or the ‘foxy’ character of methyl anthranilate produced by Vitis labruscana.
Kiwifruit appear to lack fingerprint volatiles. Instead, they are characterised by a balance of esters, aldehydes and alcohols in which branched volatiles or terpenoids are poorly re-presented (Figure 11.17; Paterson et al. 1991). Ethyl butanoate and E-2-hexenal contribute most to an overall impression, with butanoate having a positive effect and aldehydes a negative effect on consumer response to flavour intensity. A characteristic kiwifruit aroma and flavour requires both compounds plus hexanal but other compounds not yet defined are also needed.