2009_Book_FoodChemistry

846 18 Fruits and Fruit Products
18.1.3.3.2 Proteins, Enzymes
During ripening of some fruits, although the
total content of nitrogen is constant, there is
an increase in protein content, an increase
assigned primarily to increased biosynthesis
of enzymes. For example, during ripening of
fruit there is increased activity of hydrolases
(amylases, cellulases, pectinolytic enzymes,
glycolytic enzymes, enzymes involved in the
citric acid cycle, transaminases, peroxidases
and catalases). Proteinaceous enzyme inhibitors
which inhibit the activities of amylases, peroxidases
and catalases are found in unripe
bananas and mangoes. The activities of these
inhibitors appear to decrease with increasing
ripeness.
The ratios of NADH/NAD + or NADPH/NADP +
pass through a maximum during ripening of fruit.
For example, the values for mangoes are 0.32–
0.67 in the unripe stage, 1.44–6.50 in the semiripe
stage and 0.57–0.93 in the ripe stage. During
ripening of fruit, shifts also occur in the amino
acid and amine fractions. The shifts are not uniform
and are affected by type and ripening stage
of fruits.
18.1.3.3.3 Lipids
Little is known about changes in the lipid fraction.
Shifts in composition and quantity have been
found, especially in the phospholipid fraction.
18.1.3.3.4 Acids
There is a drop in acid content during ripening
of fruits. Lemons, as already mentioned, are
an exception. The proportion of various acids
can change. In ripe apples malic acid is the major
acid, while in young, unripe apples, quinic
acid is dominant. In the various tissues of any
single fruit, various acids can be dominant. For
example, apple peels contain citramalic acid (I,
cf. Formula 18.44) which is formed from pyruvic
acid, and can produce acetone through acetoacetic
acid. Acetone is formed abundantly during
ripening:
(18.44)
The synthesis of ascorbic acid is also of importance.
It takes place in many fruits during ripening
(cf. 18.1.2.7)
18.1.3.3.5 Pigments
The ripening of fruit is usually accompanied
by a change in color. The transition of green to
another color is due to the degradation of chlorophyll
and the appearance of concealed pigments.
Furthermore, the synthesis of other pigments
plays a big role. For example, the lycopene
content of the tomato increases greatly during
ripening. The same applies to the carotenoid
content of citrus fruits and mangoes. The formation
of anthocyanin is frequently enhanced by
light.
18.1.3.3.6 Aroma Compounds
The formation of typical aromas takes place
during the ripening of fruit. In bananas, for
example, noticeable amounts of volatile compounds
are formed only 24 h after the climacteric
stage has passed. The aroma build-up is affected
by external factors such as temperature and
day/night variations. Bananas, with a day/night
rhythm of 30 ◦ C/20 ◦ C, produce about 60% more
volatiles than those kept at a constant temperature
of 30 ◦ C. The synthesis of aroma substances is
discussedinsection5.3.2.