Tomatoes - Crop Post-Harvest: Science and Technology - page 11

are other enzymes that can contribute to fruit softening.
Suppression of a ripening-related β-galactosidase in
tomato reduced both galactose solubilisation and fruit
softening (Smith et al . 2002). Expansins do not have
hydrolytic activity, but are thought to influence the hydro-
gen bonding between the cellulose and hemicellulose cell
wall components, causing cell wall swelling and increased
porosity that allows degradative enzymes to more easily
access their substrates (Brummell 2006). Suppression of a
ripening-related expansin in tomato resulted in reduced
pectin depolymerisation while overexpression resulted in
increased depolymerisation and corresponding changes
in fruit softening (Brummell et al . 1999).

(1984) identified 130 volatiles in tomato fruit, but
determined, using the gas-sniff method, that the most
important for tomato aroma are hexanal, trans -2-hexenal,
2- isobutylthiazole, 2-methyl-2-hepten-6-one, geranylace-
tone and farnesylacetone, and that the concentration of
these volatiles increased with ripening. Tomato volatiles
are formed by different pathways including oxidative
carotenoid breakdown (Buttery et al . 1988), de-amination
and de-carboxylation of amino acids (Yu et al . 1968)
and lipid oxidation (Hatanaka et al . 1986). Aroma volatiles
in tomato are affected by several factors including cultivar,
growing conditions, management practices and post- harvest
handling conditions.
A relationship exists between tomato fruit colour and
its volatile composition, especially those formed by the
oxidation of carotenoids. Several other correlations were
shown between taste descriptors and other fruit components
(Baldwin et al . 1998). Off-flavours are formed in tomatoes
picked green and ripened off the plant, and were related to
higher concentrations of some volatiles such as 2-methyl-
1-butanal. Bruising and other physical damage were found
to cause more off-flavour and less 'tomato-like' flavour
(Kader et al . 1978c; Moretti et al . 2002).

Flavour
Tomato flavour is a very important quality component. It
is the perception of many taste and aroma constituents,
and is affected by several factors. Sugars (mainly fructose
and glucose in standard tomatoes, but some sucrose in
cherry tomatoes) and acids (citric and malic) and their
interactions are the most important factors responsible
for sweetness, sourness and overall flavour intensity in
tomatoes (Malundo et al . 1995; Stevens et al . 1977b,
1979). High sugar content and relatively high acid content
are required for best flavour; high acid with low sugar
content will produce a tart tomato and high sugar with low
acid will produce a bland taste; a tasteless, insipid flavour
is the result of low sugar with low acid.
The pericarp portion of the fruit usually contains more
reducing sugars and less organic acids than the locular
portion, and therefore cultivars with large locular portions
and high concentrations of acids and sugars usually have
better flavour than those with small locular portions (Stevens
et al . 1977a). The sugar content, mainly in the locule walls,
reaches a peak when tomatoes are fully ripe; malic acid
decreases quickly as the fruit turn red, while the citric acid
content is rather stable throughout the ripening period
(Hobson & Grierson 1993). Fruity flavour, which best
describes tomato flavour, was linked to increased levels of
reducing sugars and decreased glutamic acid content (Bucheli
et al . 1999). It has been suggested that changes in acid and
sugar levels in ripening tomato are independent of ethylene
and CO 2 production (Baldwin et al . 1991; Jeffery et al . 1984).
Aromatic (volatile) compounds are numerous in tomato
fruit (Buttery et al . 1971). Some of the volatiles that
were correlated with tomato aroma include n-hexanal,
trans -2-hexenal, β-ionone, 1-penten-3-one, 3-methyl butanal,
3-methyl butanol, cis -3-hexen-1-ol, 2-isobutylthiazole
and some unidentified C 12 -C 16 volatile compounds
(Buttery et al . 1988; Dirinck et al . 1976). Hayase et al .

Nutritional and health values
Tomato and tomato-based products are considered healthy
foods because they are low in fat and calories, cholesterol
free, and a good source of fibre, vitamins A (β-carotene
and some other carotenoids are pro-vitamin A) and C,
lycopene and potassium (Yahia et al . 2005). The interest in
the nutritional and health benefits of tomato fruit and their
products has increased greatly over the past two decades
(Geeson et al . 1985; Giovannucci & Clinton 1998; Guester
1997). Vitamin C content in tomato (230 mg kg −1 ) is not as
high as in several other fruits, but its contribution is very
important due to the extensive use of tomato in the diet of
many cultures. A 100 g tomato can supply about 20% and
40% of the adult US recommended daily intake of vitamins
A and C, respectively. The selection of tomato genotypes
that are rich in vitamins A and C has been accomplished,
and cultivars with very high vitamin A content have been
developed, although their orange colour was not highly
accepted by consumers. Epidemiological studies indicated
that tomato fruit had one of the highest inverse correlations
with cancer risk and cardiovascular disease, including
stroke (Giovannucci et al . 1995).
Lycopene, the principal pigment responsible for the
characteristic deep-red colour of ripe tomato fruit and
tomato products, is a natural antioxidant that can prevent
cancer and heart disease (Shi & Le Maguer 2000).