Food Biochemistry and Food Processing (2 edition)

BLBS102-c28 BLBS102-Simpson March 21, 2012 13:54 Trim: 276mm X 219mm Printer Name: Yet to Come

556 Part 5: Fruits, Vegetables, and Cereals

Table 28.2.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. 1992. Copyright, American Chemical Society.

ND, not detected (<0.05 g/100 mL).

improved color, whereas high calcium content reduces the inci-

dence of physiological disorders and improves quality of fruits.

Phosphorus is a constituent of several metabolites and nucleic

acids and plays an important role in carbohydrate metabolism

and energy transfer.

Dietary Fiber

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. Because of its properties that include a

high water-holding capacity, dietary fiber plays a major role in

the movement of digested food and reducing the incidence of

colon cancer and cardiovascular disease.

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 dietary protein requirement in countries where ani-

mal proteins are in short supply. Plant proteins, unlike animal

proteins, are often deficient or limiting in one or more essential

amino acids. The green fruits are important sources of proteins

as they contain enzymes and proteins associated with the pho-

tosynthetic apparatus. Enzymes, which catalyze metabolic pro-

cesses in fruits, are important in the reactions involved 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 phytol ring from

chlorophyll; results in loss of green color.


 Polyphenol oxidase: Catalyzes oxidation of phenolics, re-

sulting 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.


 Pectinesterase: Catalyzes deesterification of methyl groups

in pectin; Act in conjunction with polygalacturonases, lead-

ing 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 of storage lipids in fresh fruits, they

still possess cell membrane lipids. Lipids make up the surface

wax, which contributes to the shiny appearance of fruits, and

cuticle, which protects fruits against water loss and pathogens.

The degree of fatty acid saturation establishes membrane flu-

idity with greater saturation, resulting in less fluidity. Lipids

play a significant role in the characteristic aroma and flavor of

fruits. For example, the characteristic aroma and flavor of cut

fruits result from the action of the enzyme lipoxygenase on fatty

acids (linoleic and linolenic acids), eventually leading to the

production of volatile compounds. The action of lipoxygenase

is increased after the fruit is cut because there is a greater chance

for the enzyme and substrate to mix together. Lipoxygenase can

also be responsible for the off-flavor and off-aroma in certain

plant products (soybean oil).

Volatiles

The specific aroma of fruits is due to the amount and diversity of

volatiles they contain. Volatiles are present in extremely small

quantities (<100 ug/g fresh wt). Ethylene is the most abundant

volatile in fruit, but has no typical fruit aroma. Characteristic

flavors and aromas are a combination of various character im-

pact compounds, mainly esters, terpenes, short chain aldehydes,