Agriculture Reference
In-Depth Information
Recommendations on RH have been made for most commodities
(Table 5.4), with variation in the recommendations from dif erent sources.
These dif erences in recommendations may refl ect general conclusions for
a particular commodity group or specifi c observations. There is fairly good
agreement as to the degree of water (or mass) loss from initial fi eld condition
before a commodity shows wilting symptoms (Robinson
et al.
, 1975; Sastry
et
al.
, 1978). The wilting symptoms most often reported are less gloss, wrinkling
or fl accidness. Maximum permissible losses can vary from ~10% for onions,
~7% for papayas and other tropical fruit to ~3% for lettuce. The corresponding
loss rates are 0.02% initial mass/day/mbar WVPD: 3.6, 0.5 and 7.5,
respectively (Table 5.3). This loss can be modifi ed by postharvest handling
practices such as packaging, waxing and wax removal during washing. This
loss criterion occurs at about double the loss required for the fi rst visible
symptoms to appear and integrates the overall loss rate, which normally
declines with storage. Mass loss is linear and related to WVPD; hence loss can
be reduced by lowering the WVPD via reducing air temperature, increasing
humidity or creating a barrier to water loss. High air fl ow may be necessary
during cooling; once cool, RH is crucial in determining rate of moisture loss.
Moisture loss from pre-climacteric avocado, mango, banana, plantains
and pear fruit hastens ripening (Fig. 5.4). There is a linear negative
relationship between water loss and green life of avocado, banana and mango
(Fig. 5.4). For example, the green life of bananas is about 22 days at 20°C with
95% RH and ~16 days at 13% RH. There is also about a 50% reduction in the
postharvest life of custard apple at low RH and 65% reduction for plantains.
There are numerous studies relating reduced chilling injury symptom
development in sensitive tropical commodities to high RH. Lemon chilling
injury symptoms in the peel are reduced at high RH, though RH had no impact
on fl esh chilling injury symptoms.
Mechanical injury
Mechanical injury causes unsightly blemishes and favours decay by breaking
the cuticle, which also favours water loss. Three types of mechanical injury
are recognized as occurring preharvest, at harvest and during handling; they
are impact, compression and abrasion. Often personnel handling fruit do
not recognize that they are causing injury as the symptoms do not become
obvious until the fruit ripens. Examples of this would be abrasion injury to
green bananas, which turn black as the fruit ripens, and in papaya, which
appears as sunken green areas. Impact and compression injury is sometimes
not visible on the skin but is recognized as water-soaked areas in the fl esh. The
water-soaked areas are often only on one side of the fruit and are caused by the
kinetic energy of the impact being dissipated in the fruit cells, which rupture.
