Agriculture Reference
In-Depth Information
15
15.1
Introduction
15.2
Postharvest Respiratory
Metabolism
Postharvest losses of fresh horticultural
products usually exceed 25% of the total
production and are caused by inappropriate
control of the physical, physiological and
microbiological conditions during storage
and commercialization (Lioutas, 1998).
The weight loss after harvest of fresh
horticultural products is caused, mainly, by
water loss through evaporation, which
depends on the temperature and humidity
of the surrounding environment and on the
temperature of the product. The respiratory
processes also contribute to the weight loss,
but to a lesser extent, and are quite depend-
ent on the temperature, increasing with it.
Vegetable water loss causes a quality
decrease in the form of product wilt, discol-
oration and loss of firmness. This water loss
in some fruit vegetables, such as tomato, orig-
inates in the peduncles mainly, because the
skin is practically impermeable, being possi-
ble to compare a tomato fruit to a container
filled with water, because the water content
may be as high as 95% (Scheer, 1994). Other
vegetables such as cucumber, whose skin is
much more permeable, are more sensitive to
dehydration. Therefore, cucumbers are usu-
ally packed inside a plastic film to limit the
postharvest water losses, as it is also common
with leafy vegetables (Photo 15.1).
The respiration process involves the com-
bining of oxygen (O
2
) from the air with
organic molecules in the plant tissues (usu-
ally one type of sugar), downgrading them
to form several intermediate compounds
and, eventually, CO
2
and water. The energy
produced during respiration is used for
other metabolic processes.
Most of the postharvest technology is
directed towards decreasing respiration and
other metabolic reactions, to maintain the
quality of the product by manipulating the
external environment. In general, the shelf
life of the storage of products varies inversely
with their respiration rate, so products with
a lower respiration rate have a longer shelf
life and vice versa.
The most important factors affecting
postharvest are: (i)
temperature
; (ii)
humid-
ity
; (iii)
composition of the atmosphere
; and
(iv)
physical stress
(Saltveit, 2003a).
Within the physiological range of most
crops (between 0 and 30°C) an increase in
temperature causes an exponential increase
in respiration; that is, the respiratory rate is
doubled, for every 10°C increase in temper-
ature (Van't Hoff law).
High temperatures outside this interval
(0-30°C) induce a decrease in respiration
