Tomatoes - Crop Post-Harvest: Science and Technology - page 12

Although, unlike some other carotenoids, lycopene has no
pro-vitamin A activity, it does exhibit a physical quenching
rate constant with singlet oxygen almost twice as high as
that of β-carotene. Increasing clinical evidence supports
the role of lycopene as a micronutrient with important
health benefits, due to its role in the protection against a
broad range of epithelial cancers (Shi & Le Maguer 2000).
The serum level of lycopene and the dietary intake of
tomatoes have been inversely correlated with the incidence
of cancer (Helzlsouer et al . 1989; Van Eenwyk et al . 1991).
Protection for all sites of digestive-tract cancers (oral cavity
and pharynx, esophagus, stomach, colon, rectum) was
associated with an increased intake of tomato-based foods,
and an increased supply of lycopene (Franceschi et al .
1994). People who ate at least one serving of tomato-based
product per day had 50% less chance of developing
digestive tract cancer than those who did not eat tomatoes
(Franceschi et al . 1994). The intake of lycopene has also
been associated with a reduced risk of cancers of sites other
than the digestive tract, such as the pancreas and the bladder
(Gerster 1997). Older subjects who regularly ate tomatoes
were found to be less likely to develop all forms of cancer
(Colditz et al ., 1985). A study at the Harvard School of
Public Health carried out on 48 000 men for four years
reported that men who ate ten or more servings of tomato
products (such as tomatoes, tomato sauce and pizza sauce)
per week had up to 34% less chance of developing prostate
cancer (Giovannucci et al . 1995).
Lycopene has a protective effect on oxidative stress-
mediated damage of the human skin after irradiation with
UV light (Ribaya-Mercado et al . 1995). In addition, it was
found to prevent the oxidation of low-density lipoprotein
(LPL) cholesterol and to reduce the risk of developing
atherosclerosis and coronary heart disease (Agarwal &
Rao 1998); the daily consumption of tomato products
providing at least 40 mg of lycopene was enough to
substantially reduce LPL oxidation. Lycopene is recog-
nised as the most efficient singlet oxygen quencher among
biological carotenoids (Di Mascio et al . 1989, 1991).
Lycopene has also been reported to increase gap-junctional
communication between cells and to induce the synthesis
of the gap junction protein connexin-43 (Zhang et al .
1992), which is involved in intercellular communication.
Fresh tomato fruit contains about 7.2 to 200 mg of lycopene
per kg of fresh weight, which accounts for about 30%
of the total carotenoids in plasma (Stahl & Sies 1996). In
contrast to other pigments such as β-carotene, lutein,
violaxanthin, auroxanthin, neoxanthin and chlorophylls a
and b, which accumulate in inner pulp and in the outer
region of the pericarp, lycopene appears only at the end

of the maturation period, and almost exclusively in the
external part of the fruit (Laval-Martin et al . 1975). Other
tomato components that can contribute to health include
flavonoids, folic acid and vitamin E (Dorais et al . 2001).

Safety factors
Tomatine, a steroidal glycoalkaloid, accumulates in
developing fruit of all tomato genotypes, and causes
bitterness when fruit are harvested immature. However,
during ripening tomatine concentration in the fruit declines
to about 400 mg kg −1 (FW), which is considered to be a
safe amount given that the LD 50 value for tomatine is
500 mg kg −1 body weight (Davies & Hobson 1981) and it
would thus be necessary for a person to consume at least
one tomato per kg of body weight to approach a dangerous
tomatine level. Dehydrotomatine is another glycoalkaloid
found in tomato at a concentration of 1.7 to 45 mg kg −1
(FW) in green fruit, declining to 0.05 to 0.42 mg kg −1 in
red fruit (Friedman & Levin 1998). These glycoalkaloids
are considered to function in defensive mechanisms that
protect the plant against insects and pathogens. It has been
suggested that low concentrations of some of these alka-
loids might have health benefits. For example, Friedman
et al . (1997) reported that feeding commercial tomatine to
hamsters induced a significant reduction in plasma LPL
cholesterol, and this reduction was higher when the
animals were fed a high-tomatine green tomato diet than
when fed a low-tomatine red tomato diet.

POST-HARVEST PRACTICES AND PROBLEMS

Fruit maturity and ripeness
Maturity at harvest and the harvesting operation can
influence post-harvest tomato quality (fruit taste, firmness
and shelf life), and the incidence and severity of physical
injuries, which, in turn, can adversely affect tomato quality.
A six-class classification of tomato fruit from mature
green to fully ripe (Table 2.1) has been widely adopted.
For greenhouse tomatoes, the earliest stage for harvest
is the breaker stage, but field-grown tomatoes are often
harvested at the mature-green stage. Tomatoes harvested
at the mature-green stage will ripen adequately, but
immature-green fruit will ripen very poorly, and will
develop poor quality post-harvest. Mature-green tomatoes
are somewhat difficult for pickers to detect (difficult to
distinguish from immature-green fruit). Besides the
characteristics listed in Table 2.1, identification of mature
tomatoes can also be aided by the following characters:
(1) some cultivars turn whitish-green while others show
certain coloured streaks at the blossom end, (2) waxy gloss