2009_Book_FoodChemistry

844 18 Fruits and Fruit Products
Fig. 18.9. Respiration rise in tomatoes —— : CO 2 ,
–––:Ethylene
pending on the fruit, this can occur before or after
harvesting. Figures 18.8 and 18.9 show that
such a rise occurs a short time after harvest for
apples and tomatoes and is accompanied by increased
ethylene production.
The climacteric respiration rise is so specific that
fruits can be divided into:
• Climacteric types, such as apples, apricots,
avocados, bananas, pears, mangoes, papaya,
passion fruit, peaches, plums/prunes and
tomatoes; and
• Nonclimacteric types, which include pineapples,
oranges, strawberries, figs, grapefruit,
cucumbers, cherries, cantaloupes, melons,
grapes and lemons.
It should be emphasized that nonclimacteric
fruits generally ripen on the plants and contain
no starch. The differing effects of ethylene
on the two types of fruits are covered in
Section 18.1.4.2.
Fruits can also be classified according to respiration
behavior after harvesting. Three fruit types
are distinguished:
Type 1: A slow drop in CO 2 production during
ripening (as illustrated by citrus fruits).
Type 2: A temporary rise in CO 2 production. The
fruits are fully ripe after this increase
reaches a maximum (e. g., avocados, bananas,
mangoes or tomatoes).
Type 3: Maximum CO 2 production in the fully
ripe stage, until the fruit is overripe
(e. g. strawberries and peaches).
The reason for the increase in CO 2 production is
not yet fully elucidated. Physical and chemical
factors are involved. For example, a change
in permeability for gases occurs in fruit peels.
With increasing age the peel cuticle becomes
thicker and is more strongly impregnated with
fluid waxes and oils. Thus, the total permeability
drops, while the CO 2 concentration
within the fruit increases. Three possibilities
are usually considered for the rise in CO 2
production. The first is related to increased
protein biosynthesis coupled with increased
ATP consumption thus stimulating enhanced
respiration. Secondly, since the respiratory
quotient (RQ) increases from 1 to 1.4–1.6, it
is assumed that the additional CO 2 source is
not due to respiration but to decarboxylation
of malate and pyruvate, i.e. there is a switch
from the citric acid cycle to malate degradation.
Another possibility is the partial uncoupling of
respiration from phosphorylation by an unknown
decoupler.
New concepts involving structural factors suggest
that fruit flesh possesses marked photosynthetic
activity which is then associated with CO 2
uptake. With the onset of ripening, an increased
disorganization occurs in chloroplasts and other
cell organelles. Photosynthetic activity decreases
and finally stops completely. The same is the
case for other synthetic activities. Catabolic
processes, catalyzed by cytoplasmic enzymes,
become dominant. Based on such a perception
(Phan et al., 1975) the “climacteric is seen as an
indication of the natural end of a period of active
synthesis and maintenance, and the beginning of
the actual senescence of the fruit”.
18.1.3.2 Changes in Metabolic Pathways
Metabolic shifts may occur in several fruits during
ripening. For example, during ripening of bananas,
there is a marked rise in aldolase and carboxylase
activities and thus it appears that at this
stage the Embden–Meyerhoff pathway becomes
dominant and the pentose-phosphate pathway is
suppressed.
An increase in malate and pyruvate decarboxylase
activities is observed in apples during
the climacteric stage. The activities drop as