Food Biochemistry and Food Processing

520 Part V: Fruits, Vegetables, and Cereals

nents are oxidized by polyphenol oxidase enzyme in
the presence of O 2 to brown products according to
the following reactions:

Phenolic components are responsible for the
astringency in fruits and decrease with maturity,
because of their conversion from soluble to insolu-
ble forms, and metabolic conversions.

CELLSTRUCTURE

The fruit is composed of three kinds of cells: par-
enchyma, collenchyma, and sclerenchyma. Pa re n -
chyma cellsare the most abundant cells found in
fruits. These cells are mostly responsible for the tex-
ture resulting from turgor pressure. Collenchyma
cellscontribute to the plasticity of fruit material.
The collenchyma cells usually are located closer to
the periphery of fruits. Collenchyma cells have
thickened lignified cell walls and are elongated in
their form. Sclerenchyma cells,also called stone
cells (e.g., in pears), are responsible for the gritty/
sandy texture of pears. They have very thick ligni-
fied cell walls and are also responsible for the
stringy texture in such commodities as asparagus.
For some commodities such as asparagus, the num-
ber of sclerenchyma cells increases after harvest and
during handling and storage.

Monophenol O+⎯ 2 Polyphenol oxidase⎯⎯⎯⎯⎯→quinonne H

O

2

2

Polyphenol oxidase

+

+⎯⎯ →

O

14,,diphenol ⎯⎯⎯⎯⎯ 1,4 quinoine H+ 2 O

FRUIT PROCESSING

HARVESTING ANDPROCESSING OFFRUITS

Harvesting is one of the most important of fruit-

growing activities. It represents about half of the cost

involved in fruit production because most fruit crops

are still harvested by hand. Improvements are needed

for the development of mechanical harvesters. Pro-

duction of fruits of nearly equal size that are resistant

to mechanical damage during harvest is one area

addressed by breeding technology. Certain fruits that

are not to be processed are harvested with stems in

order to avoid microbial invasion. Examples of this

include cherries and apples.

Harvested fruit is washed to remove soil, microor-

ganisms, and pesticide residues and then sorted ac-

cording to size and quality. Fruit sorting can be done

by hand or mechanically. There are two ways of mech-

anically sorting fruits: (1) sorting with water, which

takes advantage of changes in density with ripening,

and (2) automatic high-speed sorting, in which com-

pressed air jets separate fruit in response to differ-

ences in color and ripeness as measured by light

reflectance or transmittance. When not marketed as

fresh, fruits are processed in many ways including

canning, freezing, concentration, and drying.

FREEZING ANDCANNING OFFRUITS

For freezing and canning of fruits, the general se-

quence of operations include (1) washing; (2) peel-

Table 22.3.Organic Acids of Selected Fruits

Organic Acid (mg/100 mL of juice)

Fruit Citric Ascorbic Malic Quinic Tartaric

Apple ND tr 518
32 ND ND
Cherry ND tr 727
20 ND ND
Grape tr tr 285
58 ND 162
24
Nectarine 730
92 114
6 501
42 774
57 tr
Peach 140
39 tr 383
67 136
28 ND
Pear 109
16 tr 358
72 121
11 tr
Plum ND tr 371
16 220
2ND
Kiwifruit ND tr 294
24 214
68 ND
Strawberry 207
35 56
4 199
26 ND ND

Source:Reprinted with permission from Van Gorsel et al., J Agr Food Chem, 1992, 40:784–789. Copyright, American
Chemical Society.
ND Not detected, tr (10 mg/100 mL).