Food Biochemistry and Food Processing (2 edition)

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28 Biochemistry of Fruit Processing 557

Table 28.3.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. 1992. Copyright, American Chemical Society.

ND, not detected; Tr,<10 mg/100 mL.

ketones, organic acids, and so on. Their relative importance de-

pends upon the threshold concentration (sometimes,<1 ppb)

and interaction with other compounds. For some fruits such as

apple and bananas, the specific aroma is determined by the pres-

ence of a single compound, ethyl-2-methylbutyrate in apples,

and isoamylacetate in bananas.

Water

Water is the most abundant single component of fruits and veg-

etables (up to 90% of total weight). The maximum water content

of fruits varies due to structural differences. Agricultural con-

ditions also influence the water content of plants. As a major

component of fruits, water has an impact on both the quality and

the deterioration. Turgidity is a major quality determinant factor

in fruits. Loss of turgor is associated with loss of quality, and is

a major problem during postharvest storage and transportation.

Harvest should be done during the cool part of the day in order

to keep the turgidity at its optimum.

Organic Acids

The role of organic acids in fruits is twofold.

1. Organic acids are an integral part of many metabolic
   pathways, especially Krebs (TCA) cycle. Tricarboxylic
   acid cycle (respiration) is the main channel for the oxi-
   dation of organic acids, serving as an important energy
   source for the cells. Organic acids are metabolized into
   many constituents including amino acids. Major organic
   acids present in fruits include citric acid, malic acid, and
   quinic acid.


2. Organic acids are important contributors to the taste and
   flavor of many fruits and vegetables. Total titratable acid-
   ity, the quantity and specificity of organic acids present in
   fruits and so one influence the buffering system and the
   pH. Acid content decreases during ripening, because part
   of it is used for respiration and another part is transformed

into sugars (gluconeogenesis). The composition of organic

acids in some fruits is given in (Table 28.3).

Pigments

Pigments are mainly responsible for the skin and flesh colors in

fruits and vegetables. They undergo changes during maturation

and ripening of the fruits including, loss of chlorophyll (green

color), synthesis and/or revelation of carotenoids (yellow and

orange), and biosynthesis of anthocyanins (red, blue, and purple)

(Table 28.4.). Anthocyanins occur as glycosides and are water

soluble, unstable, and easily hydrolyzed by enzymes to free

Table 28.4.Anthocyanins of Selected Fruits

Anthocyanin

Fruit Identification Peak Area

Apple Cyanidin 3-arabinoside 22 ± 6

Cyanidin 7-arabinoside or

Cyanidin 3-glucoside

27 ± 15

Cyanidin 3-galactoside 260 ± 69

Cherry Cyanidin 3-glucoside 189 ± 40

Cyanidin 3-rutinoside 1320 ± 109

Peonidin 3 rutinoside 47 ± 36

Grape Cyanidin 3-glucoside 121 ± 33

Delphinidin 3-glucoside 586 ± 110

Malvidin 3-glucoside 2157 ± 375

Peonidin 3-glucoside 478 ± 92

Nectarine Cyanidin 3-glucoside 322 ± 51

Peach Cyanidin 3-glucoside 180 ± 43

Plum Cyanidin 3-glucoside 42 ± 5

Strawberry Cyanidin 3-glucoside 70 ± 18

Pelargonidin 3-glucoside 1302 ± 29

Pelargonidin 3-glycoside 78 ± 9

Source: Reprinted with permission from Van Gorsel et al. 1992.

Copyright, American Chemical Society.