Food Chemistry

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, per-
oxidases 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 semi-
ripe 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 uni-
form and are affected by type and ripening stage
of fruits.

18.1.3.3.3 Lipids

Little is known about changes in the lipid frac-
tion. 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 ripen-
ing of fruits. Lemons, as already mentioned, are
an exception. The proportion of various acids
can change. In ripe apples malic acid is the ma-
jor 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 pyru-
vic acid, and can produce acetone through ace-
toacetic acid. Acetone is formed abundantly dur-
ing ripening:

(18.44)

The synthesis of ascorbic acid is also of import-

ance. 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 chloro-

phyll 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 forma-

tion 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 com-

pounds 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.