Agriculture Reference
In-Depth Information
2.2 Developmental characteristics of fruits and vegetables
Fruit ripening is characterized by several marked physiological and biochemical changes
resulting in the coordinated development of complex characteristics. Following pollination
and fertilization, the fruit develops in size leading to the ripening process, which results in
the development of ideal organoleptic characters such as taste, color, and aroma that are
important quality-determining features. Fruits that are used as vegetables are harvested early
prior to their ripening. The physiological process of ripening occurs rapidly when the fruit
is mature, and beyond a certain stage, harvested fruits undergo rapid deterioration in quality.
Ideally, fruits are harvested at an optimal physiological stage or maturity characteristic to
the type of fruit, after which appropriate storage procedures can be adopted for preserving
the shelf life and quality of the fruits. Fruits do not ripen fully showing the appropriate
quality characteristics if picked at a young stage before the attainment of physiological
maturity. Citrus fruits are allowed to fully ripen before they are harvested. Avocado fruits
do not ripen if left on the tree and start ripening only after harvest. Irrespective of the
nature of the produce whether it is fruits, vegetables, or flowers, various technologies such
as cold storage, controlled atmosphere storage, and inhibition of hormone and enzyme
action are adopted to slow down the metabolic processes to provide an optimal quality
produce for marketing and consumption. Advances in the biochemistry and molecular
biology of the fruit ripening process have enabled the development of biotechnological
strategies for the preservation of postharvest shelf life and quality of fruits, vegetables, and
flowers.
Several metabolic changes are initiated after the harvest of fruits and vegetables. In
the case of vegetables, harvesting induces stress responses through reduced availability of
water and nutrients, wounding, and exposure to shelf life enhancing storage methods such
as cooling. In most cases, these changes help the produce to enhance the shelf life. In the
case of fruits, an increase in the biosynthesis of the gaseous hormone ethylene serves as the
physiological signal for the initiation of the ripening process. In general, all plant tissues
produce a low, basal, level of ethylene. During the ripening process, some fruits evolve
large amounts of ethylene, sometimes referred to as an autocatalytic increase in ethylene
production, which occurs in conjunction with an increase in respiration referred to as the
respiratory climacteric. Fruits are generally classified into climacteric or nonclimacteric
types on the basis of the pattern of ethylene production and responsiveness to externally
added ethylene. The climacteric fruits characteristically show a marked enhancement in
ethylene production and respiration, as noticeable by the evolution of carbon dioxide. By
contrast, the nonclimacteric fruits emit a considerably reduced level of ethylene. (For a list
of fruits showing climacteric or nonclimacteric pattern of ripening, see Kays, 1997, General
Reading.) In climacteric fruits such as apple, pear, banana, tomato, and avocado, ethylene
evolution can reach 30-500 ppm/(kg h) (parts per million, microliter per liter), whereas
in nonclimacteric fruits such as orange, lemon, strawberry, and pineapple, ethylene levels
usually range from 0.1 to 0.5 ppm/(kg h) during ripening. Climacteric fruits respond to
external ethylene treatment by an early induction of the respiratory climacteric and accel-
erated ripening in a concentration-dependent manner. Nonclimacteric fruits, on the other
hand, show increased respiration in response to increased levels of ethylene concentration
without showing acceleration in the time required for ripening. Vegetables produce very low
amounts of ethylene most of them with less than 0.1
μ
L/(kg h), with slightly higher levels as
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