Food Biochemistry and Food Processing (2 edition)

BLBS102-c27 BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come

534 Part 5: Fruits, Vegetables, and Cereals

volume, tomato, orange, banana and grape are the major fruit

crops used for consumption and processing around the world

(Kays 1997).

BIOCHEMICAL COMPOSITION

OF FRUITS

Fruits contain a large percentage of water, which can often ex-

ceed 95% by fresh weight. During ripening, activation of sev-

eral metabolic pathways often leads to drastic changes in the

biochemical composition of fruits. Fruits such as banana store

starch during development, and hydrolyse the starch to sugars

during ripening, that also results in fruit softening. Most fruits

are capable of photosynthesis, store starch and convert them to

sugars during ripening. Fruits such as apple, tomato, grape and

so on have a high percentage of organic acids, which decreases

during ripening. Fruits also contain large amounts of fibrous

materials such as cellulose and pectin. The degradation of these

polymers into smaller water-soluble units during ripening leads

to fruit softening as exemplified by the breakdown of pectin

in tomato and cellulose in avocado. Secondary plant products

are major compositional ingredients in fruits. Anthocyanins are

the major colour components in grape, blueberry, apple and

plum; carotenoids, specifically lycopene and carotene, are the

major components that impart colour in tomato and watermelon.

Aroma is derived from several types of compounds that include

monoterpenes (as in lime, orange), ester volatiles (ethyl, methyl

butyrate in apple, isoamyl acetate in banana), simple organic

acids such as citric and malic acids (citrus fruits, apple) and

small chain aldehydes such as hexenal and hexanal (cucumber).

Fruits are also rich in vitamin C. Lipid content is quite low in

fruits, the exceptions being avocado and olives, in which tri-

acylglycerols (oils) form the major storage components. The

amounts of proteins are usually low in most fruits.

Carbohydrates, Storage and Structural

Components

As the name implies, carbohydrates are organic compounds con-

taining carbon, hydrogen and oxygen. Basically, all carbohy-

drates are derived by the photosynthetic reduction of CO 2 to

the pentoses (ribose, ribulose) and hexoses (glucose, fructose),

which are also intermediates in the metabolic pathways. Poly-

merisation of several sugar derivatives leads to various storage

(starch, inulin) and structural components (cellulose, pectin).

During photosynthesis, the glucose formed is converted to

starch and stored as starch granules. Glucose and its isomer fruc-

tose, along with phosphorylated forms (glucose-6-phosphate,

glucose-1,6- diphosphate, fructose-6-phosphate and fructose-

1,6-diphosphate), can be considered to be the major metabolic

hexose pool components that provide carbon skeleton for the

synthesis of carbohydrate polymers. Starch is the major storage

carbohydrate in fruits. There are two molecular forms of starch,

amylose and amylopectin and both components are present in the

starch grain. Starch is synthesised from glucose phosphate by the

activities of a number of enzymes designated as ADP-glucose

pyrophosphorylase, starch synthase and a starch-branching en-

zyme. ADP-glucose pyrophosphorylase catalyses the reaction

between glucose-1-phosphate and ATP that generates ADP-

glucose and pyrophosphate. ADP-glucose is used by starch syn-

thase to add glucose molecules to amylose or amylopectin chain,

thus increasing their degree of polymerisation. By contrast to

cellulose that is made up of glucose units inβ-1,4-glycosidic

linkages, the starch molecule contains glucose linked byα-1,4-

glycosidic linkages. The starch branching enzyme introduces

glucose molecules throughα-1,6- linkages which further gets ex-

tended into linear amylose units withα-1,4- glycosidic linkages.

Thus, the added glucose branch points (α-1,6-linkages) serve as

sites for further elongation by starch synthase, thus resulting in

a branched starch molecule, also known as amylopectin.

Cell wall is a complex structure composed of cellulose and

pectin, derived from hexoses such as glucose, galactose, rham-

nose and mannose, and pentoses such as xylose and arabinose,

as well as some of their derivatives such as glucuronic and

galacturonic acids (Negi and Handa 2008). A model proposed

by Keegstra et al. (1973) describes the cell wall as a polymeric

structure constituted by cellulose microfibrils and hemicellulose

embedded in the apoplastic matrix in association with pectic

components and proteins. In combination, these components

provide the structural rigidity that is characteristic to the plant

cell. Most of the pectin is localised in the middle lamella. Cel-

lulose is biosynthesised by the action ofβ-1,4-glucan synthase

enzyme complexes that are localised on the plasma membrane.

The enzyme uses uridine diphosphate glucose (UDPG) as a

substrate, and by adding UDPG units to small cellulose units,

extends the length and polymerisation of the cellulose chain. In

addition to cellulose, there are polymers made of different hex-

oses and pentoses known as hemicelluloses, and based on their

composition, they are categorised as xyloglucans, glucoman-

nans and galactoglucomannans. The cellulose chains assemble

into microfibrils through hydrogen bonds to form crystalline

structures. In a similar manner, pectin is biosynthesised from

UDP-galacturonic acid (galacturonic acid is derived from galac-

tose, a six carbon sugar), as well as other sugars and derivatives

and includes galacturonans and rhamnogalacturonans that form

the acidic fraction of pectin. As the name implies, rhamnogalac-

turonans are synthesised primarily from galacturonic acid and

rhamnose. The acidic carboxylic groups complex with calcium

that provide the rigidity to the cell wall and the fruit. The neu-

tral fraction of the pectin comprises polymers such as arabinans

(polymers of arabinose), galactans (polymers of galactose) or

arabinogalactans (containing both arabinose and galactose). All

these polymeric components form a complex three-dimensional

network stabilised by hydrogen bonds, ionic interactions involv-

ing calcium, phenolic components such as diferulic acid and

hydroxyproline-rich glycoproteins (Fry 1986, Negi and Handa

2008). It is also important to visualise that these structures are

not static and the components of cell wall are constantly being

turned over in response to growth conditions.

Lipids and Biomembranes

By structure, lipids can form both structural and storage

components. The major forms of lipids include fatty acids,