Postharvest Physiology and Storage - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 29

always direct. Garcia and Barret (2010) highlighted that
PPO activity is particularly high in mushroom, banana, ap-
ple, pear, potato, avocado, and peach, which are susceptible
to a higher degree of browning.
PPO activity can also vary among the fruit and fruit
parts. In banana, higher activity of PPO is found in the
peel than in the flesh, while the opposite is true for apples
and pears (Garcia and Barret, 2010). Teixeira et al. (2006)
found differences in PPO activity in star fruits of seven
genotypes, they also found differences in their browning
susceptibility measured as changes in luminosity of the
fruit flesh ( L ∗ , CIELAB color scale). Secondary effects of
browning reactions include off-flavors and losses of the
fruit nutrient quality (Vamos-Vigyazo, 1981).
The rate at which browning occurs depends on the fruit,
PPO activity, substrates, contact between PPO substrates,
and external conditions (atmosphere composition, temper-
ature, and other factors). Nonetheless, fruit cultivars with
low PPO activity and phenolic compounds content are
less sensible to browning, and thus they are preferred for
processing.
Other enzymes related with browning of whole, cut, or
processed fruits are phenylalanine ammonia lyase (PAL),
important for phenolic compound biosynthesis and induced
by tissue wounding and peroxidase (POD) that has been
associated with physiological stress and infections, though
its participation in enzymatic browning reactions is not
clear (Garcia and Barret, 2010).
Enzymatic activity can be reduced by lowering posthar-
vest handling temperatures, which also reduce metabolic
rates and help extend fruits' shelf life. Limitation of oxygen
availability by using modified or controlled atmospheres
can also help to delay or diminish browning throughout
storage.

Food processing operations also cause considerable me-
chanical damages to fruit tissue. As the peel is removed and
the fruit is cut, the respiration rate increases. Such response
to injuries is especially important for fresh-cut produce,
compared with other processed fruit products, for which
chemical and thermal treatments are applied to inactivate
enzymes. The severity of the tissue damage depends on the
equipment used to prepare the fruits for processing (blades,
abrasion peelers, and others), the size of the fruit pieces, the
direction of the cuts, and even fruits' maturity stage. The
largest and the most severe mechanical damage, the fastest
deteriorative reactions, occur in the fruit, which favor qual-
ity attribute losses.

POSTHARVEST STORAGE

Cooling and cold storage
Fruits should be handled and stored at their recommended
storage temperature, which in most of the cases is below
15 ◦ C for tropical and subtropical fruits (Table 2.5). When
harvested fruit arrives at the packing or processing facili-
ties, their internal temperature (field heat) usually is a few
degrees above ambient temperature, and it has to be re-
duced as soon as possible to stabilize the fruit and extend
its shelf life. Once the fruit is cooled down, modified and
controlled atmosphere can be used as a supplemental tool
to maintain the fruit quality longer.

Cooling methods with air and water
Some cooling can be achieved by storing the fruits in the
shade in well-ventilated areas, as they wait to enter into the
process or packaging lines.
In conventional cooling (room cooling), the product is
exposed to cold air in a controlled temperature room. It
is a method suitable for most fruits, which are not highly
perishable, since cooling rate is low. In forced-air cooling
systems, cold air is forced to pass through the containers,
achieving a better contact of the cold air with the fruit sur-
face and thus accelerating the temperature reduction up to
10 times as compared with room cooling (Thompson et al.,
1998). This system is much faster than conventional cooling
and can be used for almost every fruit, though care should
be taken since it can cause excessive water losses in some
products.
Cold water has a large capacity to remove the field
heat from the fruits; hydrocoolers work by fruit immer-
sion or by water shower over the product. Cooling times are
4-10 times faster than those with forced-air coolers, though
cooling time will depend on the product size, shape, and
surface-to-volume ratio. It is suitable to control water

Mechanical damage and wounding response
Postharvest handling and processing operations cause me-
chanical damage in fruit tissues even when the fruit is han-
dled carefully. Such damage can include open injuries, but
also bruises and cracks which deteriorate product quality.
Damage can affect fruit surfaces or internal tissues located
even several millimeters deeper from the site of the im-
pact. Mechanical damages favor contact between PPO and
phenolic substrates favoring browning reactions, and other
product deterioration reactions, and also facilitate micro-
bial spoilage. The living tissues respond to wounding by
increasing respiration rate, ethylene production, fruit tissue
metabolism, and producing phenolic compounds that might
contribute to product discoloration.