Agriculture Reference
In-Depth Information
carotenoids were found in the stem end than in the
blossom end of the fruit (Ellis & Hammer 1943). Lycopene
concentration in tomatoes was higher in summer than
in the winter (Heinonen
et al
. 1989). Tomatoes picked
green and ripened in storage usually have lower levels
of carotenoids than vine-ripened fruit (Gould 1992),
probably because the fruit ripened in storage are not
exposed to the most conducive temperatures for pigment
synthesis. Colour development may also be limited by
environmental conditions when tomatoes are ripened on
the plant (Brandt
et al
. 2006), but post-harvest conditions
can be optimised to favour lycopene synthesis. Lycopene
formation is promoted by ethylene (Jeffery
et al
. 1984)
and is inhibited by ethanol (Saltveit & Mencarelli 1988).
Lycopene content in tomato can also be enhanced by
fertilisers, proper harvest time, and varietal selection
(Lampe & Watada 1971; Mohr 1979).
Several subjective scales and colour charts have been
developed to classify ripeness according to fruit colour
(Plate 2.2). There are six classes of tomato ripeness
beginning with mature green that are recognised and used
almost all over the world (Table 2.1). Objective measures
of tomato colour are also available, including light
reflectance and light transmittance techniques and
chemical pigment determination (chlorophyll, lycopene,
β-carotene). An estimation of lycopene content was
correlated with colour measurements (a
*
, a
*
/b
*
, and [a
*
/b
*
]
2
)
using a portable chroma meter (Arias
et al
. 2000).
Several defects can occur after harvest due to mishandling
(Plate 2.8) such as scuffing, cuts and punctures, vibration
and compression injuries, abrasions and decay development
(Olorunda & Tung 1985). Physical damage can also
increase ethylene production (MacLeod
et al
. 1976) and
therefore can accelerate fruit ripening and favour decay
development.
Dry matter content
Dry matter represents about 5-7.5% of tomato fruit,
of which about 50% is reducing sugars while protein,
pectins, celluloses, hemicelluloses, organic acids, pigments,
vitamins, lipids and minerals represent the remaining half
(Petro-Turza 1986). Fruit with high dry matter content
usually also have higher content of soluble solids, mostly
consisting of sugars and acids, and thus have better taste
and flavour (Hao
et al
. 2000b).
Firmness
Tomato firmness is closely related to quality and ripeness,
and it is important in determining shipping ability and
post-harvest life. Tomato fruit that can maintain good
firmness beyond the table-ripe stage can be picked at a
more advanced ripeness stage, and therefore can develop
better flavour. The preference is for tomato fruit that are
firm and without tough skin, and that do not lose too
much juice upon slicing. Several factors can affect tomato
firmness including cultivar, water content and turgor,
cell wall composition and integrity, ethylene, temperature,
relative humidity, irrigation and mineral nutrients.
Objective measurement of tomato firmness can be destruc-
tive using resistance to force of penetration (fruit firmness
testers, penetrometers), shearing (shear press), cutting,
compression or their combination (Barret
et al
. 1998).
A nondestructive method that includes the measurement of
resistance to compression force applied at a single point
or at multiple points was reported by Kader
et al
. (1978b).
Tomato firmness is related to the integrity of the cell
wall tissues, the elasticity of the pericarp tissue and the
activity of enzymes involved in fruit softening, including
the degradation of pectins. Polygalacturonase (PG), which
depolymerises pectin, is one of the important enzymes
thought to be involved in cell wall degradation and in
fruit softening. However, gene repression and inhibition
of the accumulation of PG mRNA and its enzyme activity
cannot prevent significant fruit softening (Sheehy
et al
.
1988). This suggests that PG is not the only factor involved
in fruit softening. Pectinases are responsible for most of
the de-methylation of cell wall pectins, and are thought to
facilitate cell wall hydrolysis by PG. The β-galactosidases
Size and shape
Fruit size is also important, but preference for different
sizes varies among cultivars, among consumers and
according to the intended use of the fruit. Fruit shape varies
between cultivars, which can be spherical, oblate, elongated
or pear-like. Shape has no direct effect on fruit ripeness
and flavour. However, an angular shape is undesirable
because it reflects immaturity or puffiness (Plates 2.5 a, b;
Kader 1984). Shape defects are commonly due to poor
pollination and irregular development of some locules.
These defective fruit are commonly discarded during
harvest or during packinghouse grading operations. Minor
defects that would not detract from eating quality are
commonly considered to be acceptable. However, serious
defects can detract from quality, cause shrivelling and
enhance susceptibility to decay (Bender
et al
. 1992). Some
of the defects that are known to occur before harvest
include sunscald, insect damage, puffiness, catfacing
(a puckered malformation with brown scarring at the fruit
blossom end; see Plate 2.7), goldfleck or pox syndrome,
radial and concentric growth cracks and irregular ripening.
