Agriculture Reference
In-Depth Information
commonly develop in early spring greenhouse crops,
mostly due to low light intensity and inappropriate mineral
nutrition, which reduce the carbohydrate supply to fruit.
food safety). The most commonly used appearance quality
indices include: (1) uniform colour: orange-red to deep
red, and no green shoulder; (2) uniform shape depending
on type: round, globe, flattened globe or oblong and
(3)  freedom from defects such as stem-end scars, growth
cracks, sunscald, catfacing (Plate 2.7), insect injuries,
bruises and mechanical injury.
High-quality tomatoes should have red colour, firm but
juicy texture and good taste and flavour. High sugars and
relatively high acids will result in good flavour, while low
sugar content and low acids will result in poor flavour
(Malundo et  al . 1995; Stevens et  al . 1979). Gel formation
in  the locules of the tomatoes is important for good fla-
vour.  Although appearance quality is important, increasing
attention is given to other quality components such as flavour
and nutritional aspects. Tomato quality components are
influenced by genetic and environmental factors (tempe-
rature, light, nutrients, water supply, etc.) and post-harvest
handling (Brecht et al . 1994; Soto-Zamora et al . 2000).
Solar injury (sunscald, sunburn or sunscorch)
This is a common form of heat injury. When tomatoes are
exposed to direct solar radiation, fruit temperature may
increase by 10°C or more above the ambient (Grierson &
Kader 1986). If the fruit temperature exceeds 30°C for a
long period, the affected part of the fruit becomes yellow-
ish and remains so during ripening (Plate 2.6). When the
temperature of an exposed fruit portion exceeds 40°C, it
becomes white and sunken. Tomatoes at the mature-green
stage are especially susceptible. Affected areas may later
develop Alternaria and Cladosporium rots.
Watery fruit
This disorder results from a massive influx of water into
the fruit, due to excessive root pressure, which can increase
the volume of the cells and may even damage them (Dorais
et al . 2001). This disorder reduces the organoleptic quality
and the shelf life of the fruit. Preventative measures include
avoidance of over-irrigation before the end of the day, the
development of a strong root system and the reduction of
root pressure by maintaining the plant leaf area index at a
reasonable level during summertime.
Colour
Colour is one of the most important quality components of
horticultural crops. Tomato fruit are available in different
colours including red, pink, yellow and orange. External
colour of tomato fruit is the result of both flesh and
skin colours. A pink tomato may be due to colourless skin
and red flesh, while an orange tomato may be due to yellow
skin and red flesh. Chlorophyll in green fruit is replaced in
ripe tomatoes by oxygenated carotenes and xanthophylls,
the most abundant of which are lycopene (red) and its
precursor phytoene (colourless). Lycopene is responsible
for the red colour of tomatoes and is important for
human health due to its antioxidant activity. Therefore, its
degradation is important from the standpoint of both
sensory quality and health. Lycopene in fresh tomato fruit
occurs mostly in the all-trans configuration, and the main
causes of its degradation during processing are isomeriza-
tion and oxidation (Shi & Le Maguer 2000). Isomerisation
converts all-trans isomers to cis -isomers due to additional
energy input, and results in an unstable, energy-rich state.
The lycopene content of fresh tomato fruit is usually
about 30-50 mg kg −1 , while deep red varieties contain
more than 150 mg kg −1 , and yellow varieties contain only
about 5 mg kg −1 (Hart & Scott 1995). Tomato lycopene
synthesis is favoured by constant exposure to tempera-
tures from 12°C to 21°C and is inhibited at temperatures
above 30°C (Tomes 1963). The amount of lycopene in
fresh tomato fruit depends on the variety, ripeness stage
and environmental conditions under which the fruit
ripened. Higher concentrations of lycopene and other
Chilling injury
Tomato plants and fruit in the field are subject to chilling
injury at temperatures below 10°C (Barger et  al . 1952;
Morris 1953), but chilling injury is not normally a serious
pre-harvest problem because tomatoes are typically grown
in areas and seasons in which chilling temperatures are not
normally encountered. In addition, tomatoes can tolerate
significantly lower night temperatures because of the
rewarming that occurs during the daylight hours. It has
been reported that tomatoes harvested in the morning
when  cool are more chill resistant during storage than if
harvested in the afternoon when warm (Saltveit & Cabrera
1987). If daily high temperatures remain below the chilling
threshold temperature for some time, however, then
pre-harvest chilling may occur. Morris (1954) showed
that  susceptibility of tomato fruit to post-harvest chilling
injury was correlated to the number of hours below 15.6°C
during the week before harvest.
Quality components and indices
Tomatoes are commonly selected by consumers on the
basis of appearance, but repeated purchase will depend on
flavour and quality (taste, texture, nutritional value and
Search WWH ::




Custom Search