Food Biochemistry and Food Processing

518 Part V: Fruits, Vegetables, and Cereals

and improved color, whereas high calcium content
reduces the incidence of physiological disorders and
improves the quality of fruits. Phosphorus is a con-
stituent of several metabolites, and nucleic acids and
plays an important role in carbohydrate metabolism
and energy transfer.

DIETARYFIBER

Dietary fiber consists of cellulose, hemicellulose,
lignin, and pectic substances, which are derived from
fruit cell walls and skins. The dietary fiber content of
fruits ranges from 0.5 to 1.5% by fresh weight. Be-
cause of its properties, which include a high water-
holding capacity, dietary fiber plays a major role in
the movement of digested food and in reducing the
incidence of colon cancer and cardiovascular dis-
ease.

PROTEINS

Fruits contain less than 1% protein (as opposed to
9–20% protein in nuts). In general, plant protein
sources provide a significant portion of the dietary
protein requirement in countries where animal pro-
teins are in short supply. Plant proteins, unlike ani-
mal proteins are often deficient or limiting in one or
more essential amino acids. The green fruits are im-
portant sources of proteins as they contain enzymes
and proteins associated with the photosynthetic
apparatus. Enzymes, which catalyze metabolic pro-
cesses in fruits, are important in the reactions in-
volved in fruit ripening and senescence. Some of the

enzymes important to fruit quality are

• Ascorbic acid oxidase:Catalyzes oxidation of
  ascorbic acid and results in loss of nutritional
  quality.


• Chlorophyllase:Catalyzes removal of the phytol
  ring from chlorophyll; results in loss of green
  color.


• Polyphenol oxidase:Catalyzes oxidation of
  phenolics, resulting in the formation of brown-
  colored polymers.


• Lipoxygenase:Catalyzes oxidation of
  unsaturated fatty acids; results in off-odor and
  off-flavor production.


• Polygalacturonase:Catalyzes hydrolysis of
  glycosidic bonds between adjacent
  polygalacturonic acid residues in pectin; results
  in tissue softening.


• Pectin esterase:Catalyzes deesterification of
  methyl groups in pectin; acts in conjunction with
  polygalacturonases, leading to tissue softening.


• Cellulase:Catalyzes hydrolysis of cellulose
  polymers in the cell wall and therefore is
  involved in fruit softening.


• Phospholipase D:Initiates the degradation of
  cell membrane.

LIPIDS

In general the lipid content of fruits is very small,

amounting to 0.1–0.2%. Avocado, olive, and nuts

are exceptions. Despite the relatively small amount

Table 22.2.Sugar Composition of Selected Fruits

Sugar (g/100 mL of juice)

Fruit Sucrose Glucose Fructose Sorbitol

Apple 0.82
0.13 2.14
0.43 5.31
0.94 0.20
0.04
Cherry 0.08
0.02 7.50
0.81 6.83
0.74 2.95
0.33
Grape 0.29
0.08 9.59
1.03 10.53
1.04 ND
Nectarine 8.38
0.73 0.85
0.04 0.59
0.02 0.27
0.04
Peach 5.68
0.52 0.67
0.06 0.49
0.01 0.09
0.02
Pear 0.55
0.12 1.68
0.36 8.12
1.56 4.08
0.79
Plum 0.51
0.36 4.28
1.18 4.86
1.30 6.29
1.97
Kiwifruit 1.81
0.72 6.94
2.85 8.24
3.43 ND
Strawberry 0.17
0.06 1.80
0.16 2.18
0.19 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 (less than 0.05 g/100 ml).