Food Biochemistry and Food Processing (2 edition)

BLBS102-c29 BLBS102-Simpson March 21, 2012 13:27 Trim: 276mm X 219mm Printer Name: Yet to Come

29 Biochemistry of Vegetable Processing 573

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

and orange) and development of anthocyanins as in eggplant

(red, blue and purple). Vegetables such as carrots are especially

rich in beta-carotene, and red beets owe their red color to be-

tacyanins. Tomatoes are a rich source of the red carotenoid ly-

copene. Anthocyanins belong to the group of flavonoids, occur

as glycosides, and are water-soluble. They are unstable and eas-

ily hydrolyzed by enzymes to free anthocyanins. The latter are

oxidized to brown products by phenoloxidases. The colors of an-

thocyanins are pH dependent. In basic medium, they are mostly

violet or blue, whereas in acidic medium they tend to be red.

Several exotic new vegetables with different colors have been

introduced recently, that include purple carrots and tomatoes

containing anthocyanins, yellow, and green cauliflower (broc-

coflower) and so on with added characters than their parental

crops.

Phenolic Components

Phenolic compounds found in vegetables vary in structure from

simple monomers to complex tannins. Under most circum-

stances they are colorless, but after reaction with metal ions,

they assume red, brown, green, gray, or black coloration. The

various shades of color depend on the particular tannins, the

specific metal ion, pH, and the concentration of the phenolic

complex. Phenolics, which are responsible for the astringency

in vegetables, decrease with maturity, because of the conver-

sion of astringent phenolics from soluble to insoluble form (egg

plant, plantain).

Carbohydrates

The carbohydrate content of vegetables is between 3% and 27%

(Table 29.5). Carbohydrates vary from low molecular weight

sugars to polysaccharides such as starch, cellulose, hemicellu-

lose, pectin, and lignin. The main sugars found in vegetables are

glucose, fructose, and sucrose. In general, vegetables contain

higher amount of polysaccharides (starch). Root vegetables are

rich in starch (e.g., taro and colocasia, various yams, tapioca,

sweet potato, potato). Plantains are also rich in starch. Fruits of

the Artocarpaceae family (jack fruit, bread fruit) are also rich in

starch prior to their ripening, when they are used as vegetables.

Turgor and Texture

The predominant structural feature of vegetables is the presence

of parenchyma cells that are assembled into metabolically and

functionally important regions for their function. In leaf tissues,

the parenchymal cells are organized to obtain photosynthetic ef-

ficiency. In root tissues, the cells are loaded with starch granules.

The texture of vegetables is largely related to the elasticity and

permeability of the parenchyma cells. Cells with high content

of water exhibit a crisp texture. The cell vacuoles contain most

of the water of plant cells. The vacuolar solution contains dis-

solved sugars, acids, salts, amino acids, pigments and vitamins,

and several other low-molecular-weight constituents. The os-

motic pressure within the cell vacuole, and within the protoplast

against the cell walls, causes them to stretch slightly in accor-

dance with their elastic properties. These processes determine

the specific appearance and crispness of vegetables. Damaged

vegetables lose their turgor after processing and tend to become

soft unless precautions are taken in packaging and storage.

VEGETABLE PROCESSING

Harvesting and Processing of Vegetables

The quality of the vegetables constantly varies depending on

the growth conditions. Since the optimum quality is transient,

harvesting and processing of several vegetables such as corn,

peas, and tomatoes are strictly planned. Serious losses can

Table 29.5.Percentage Composition of Vegetables

Component

Food Carbohydrate Protein Fat Ash Water

Cereals

Maize (corn) whole grain 72.9 9.5 4.3 1.3 12

Earth vegetables

Potatoes, white 18.9 2.0 0.1 1.0 78

Sweet potatoes 27.3 1.3 0.4 1.0 70

Other Vegetables

Carrots 9.1 1.1 0.2 1.0 88.6

Radishes 4.2 1.1 0.1 0.9 93.7

Asparagus 4.1 2.1 0.2 0.7 92.9

Beans (snap−, green−) 7.6 2.4 0.2 0.7 89.1

Peas, fresh 17.0 6.7 0.4 0.9 75.0

Lettuce 2.8 1.3 0.2 0.9 94.8

Source: Food and Agriculture Organization (FAO).