Postharvest Physiology and Storage - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 19

measurements used for harvesting are the time elapsed from
fruit set, light or heat hours in the fields during the fruit
development, respiration rate, and physical and chemical
attributes of the fruit (size, weight, shape, skin thickness,
fruit firmness, soluble solids content, acidity, aroma synthe-
sis, starch hydrolysis, chlorophyll degradation, carotenoids,
and anthocyanins synthesis, etc.).
For most fruits, sugar and acid content are very impor-
tant. In general, sweetness increases (measured as soluble
solids content or ◦ Brix) and acidity decreases (% titratable
acidity) as the fruit ripens, and their ratio is directly related
to taste and acceptability, and consequently, such ratio is
used as a quality and harvesting indicator. However, for
most tropical and subtropical fruits, more than one indica-
tor should be used to define the time of harvest and quality of
the fruit.
Product sensitivity to bruises and other physical injuries
also changes as the fruit ripens, as mechanical properties
of fruits vary due to inherent changes in cell walls, mem-
branes, and tissues. Unripe fruits usually can resist larger
impact, compression, vibration, and puncture forces than
mature fruits and thus can be handled more easily without
causing mechanical damages.
Harvesting indices of fruits for fresh consumption might
vary considerably from those used for processing. For the
fresh market, the fruit should resist postharvest handling
and arrive to the final consumer with its fresh appearance,
whereas for the processing industry, quality requirements
can be very variable, depending on the end product, pro-
duction yields, and product resistance to process opera-
tions. They can vary from very firm fruit, able to resist
handling and high-temperature processes without chang-
ing their shape, to fully ripe and very juicy fruits, with
increased flavor compounds content.
Table 2.2 shows some maturity/harvesting indices com-
monly used for fresh tropical and subtropical products con-
sumed as fresh produce, which could be useful, though
some adjustments might be necessary to account for
the final use of the product and cultivar differences, as
mentioned above.

Fresh fruits are perishable products, composed of liv-
ing tissues. Respiration is commonly used as an indicator
of their metabolic activity, though many changes in color,
composition, texture, and sensorial characteristics occur si-
multaneously.

Respiration
Respiration, one of the most important processes in fresh
fruits, is the conversion of sugars in the presence of O 2
to CO 2 , water, and energy. The rate of respiration varies
among fruits and their maturity stage. Usually, the higher
the respiration rate, the shorter the shelf life of a fresh
product is (Saltveit, 2002).
Respiration rate is mainly affected by temperature, at-
mospheric composition, and mechanical damage. For every
10 ◦ C temperature increase, biological reactions involved in
the respiration processes are increased by a factor of 2. The
reduction of oxygen or the increase in carbon dioxide con-
tent in the atmosphere surrounding a fruit can reduce its rate
of respiration and increase the shelf life of some fresh fruits.
However, the beneficial effect varies among fruit, as well as
the minimum oxygen concentration which the product can
tolerate without fermentative reactions or anaerobic res-
piration. Conversely, mechanical damages caused during
harvesting or postharvest handling of the fruit can increase
respiration and other metabolic reactions, accelerating the
fruit deterioration.
Other factors which influence the rate of respiration are
the stage of maturity of the fruit, water stress, light, growth
regulators,

pathological growth, and chemical stresses

(Saltveit, 2002).
Fruits can be classified into climacteric and nonclimac-
teric, based on their respiration pattern. For climacteric
fruits, an abrupt increase in the respiration rate is observed
during ripening; it reaches a maximum (climacteric peak),
followed by a rapid decrease. In contrast, nonclimacteric
fruit respiration rates show very little change during ripen-
ing (Tucker, 1993). Table 2.3 Shows a list of tropical and
subtropical climacteric and nonclimacteric fruits. Respira-
tion rates of selected tropical and subtropical fruits at 20 ◦ C
are shown in Table 2.4.
Climacteric peak cannot necessarily correspond to the
fruit maximum edible quality; however, these fruits must
reach their physiological maturity before harvest to develop
all desirable characteristics as they ripen.

POSTHARVEST PHYSIOLOGY
Postharvest physiology relates to functions and processes
happening in the fruits and the related chemical and biolog-
ical changes occurring after harvest. It studies the produce
changes once it is separated from the plant, how such al-
terations are accelerated or controlled during postharvest
handling, and how they can affect the quality of the fruit
during storage, distribution, and processing.

Ethylene production and response
Ethylene, C 2 H 4 , is a naturally occurring gas which par-
ticipates in various biological processes in the fruits. It is
colorless and effective at very low concentrations. It can