Food Biochemistry and Food Processing

22 Biochemistry of Fruit Processing 521

ing; (3) cutting, trimming, and coring; (4) blanching;
(5) packaging and sealing; and (6) heat processing or
freezing
Freezing is generally superior to canning for pre-
serving the firmness of fruits. In order to minimize
postharvest changes, the freezing process is done at
the harvest site for certain fruits. Fruit to be frozen
must be stabilized against enzymatic changes during
frozen storage and on thawing. The principal en-
zymatic changes are oxidations, causing darkening
of color and the production of off-flavor. An impor-
tant color change is enzymatic browning in apples,
peaches, and bananas. This is due to the oxidation of
pigment precursors such as O-diphenols and tannins
by enzymes of the group known as phenol oxidases
and polyphenol oxidases. Frozen fruits are produced
for home use, restaurants, baking manufacturers, and
other food industries. Depending on the intended end
use, various techniques are employed to prevent oxi-
dation.
The objective of blanching is to inhibit various
enzyme reactions that reduce the quality of fruits.
Fruits generally are not heat blanched because the
heat causes loss of turgor, resulting in sogginess and
juice drainage after thawing. An exception to this is
when the frozen fruit is to receive heating during the
baking operation, in which case calcium salts are
added to the blanching water or after blanching. Cal-
cium salts increase the firmness of the fruit by form-
ing calcium pectates. For the same purpose, pectin,
carboxymethyl cellulose, alginates, and other col-
loidal thickeners can also be added to fruit prior to
freezing. Instead of heat treatments, certain chemi-
cals can be used to inactivate oxidative enzymes or
to act as antioxidants. The major antioxidant treat-
ments include ascorbic acid dip and treatment with
sulphites. Other methods for the prevention of con-
tact with oxygen using physical barriers (sugar syr-
up) can be used.
Commonly dissolved in sugar syrup, vitamin C
acts as an antioxidant and protects the fruit from
darkening by subjecting itself to oxidation. A con-
centration of 0.05–0.2% ascorbic acid in syrup is
effective for apple and peach. Peaches subjected to
this treatment may not darken during frozen storage
at 18°C for up to 2 years. Because low pH also
helps in delaying oxidative processes, vitamin C and
citric acid may be used in combination. Further-
more, citric acid removes cofactors of oxidative
enzymes such as copper ions in polyphenol oxidase.

Sulphite is a multiple function antioxidant. Bi-

sulphites of sodium or calcium are used to stabilize

the color of fresh fruits. Like vitamin C, sulphite is

an oxygen acceptor and inhibits the activity of oxi-

dizing enzymes. Furthermore, sulphite reduces the

nonenzymatic Maillard-type browning reactions by

reacting with aldehyde groups of sugars so that they

are no longer free to combine with amino acids. In-

hibition of browning is especially important in dried

fruits such as apples, apricots, and pears. Sulphite

also exhibits antimicrobial properties. The disadvan-

tage of sulphite is that some people are allergic to

this chemical, which makes its use restricted; for

example, FDA prohibited the use of sulphite in fresh

produce and limited the residual sulphite in pro-

cessed products to 10 ppm, with appropriate labeling.

Sugar syrup had long been used to minimize oxi-

dation before the mechanisms of browning reactions

were understood, and still remains as a common

practice today. Application of sugar syrup provides a

coating to the fruit and thus prevents contact of the

cut surface and oxidizable components with atmos-

pheric oxygen. Sugar syrup also increases the sen-

sory attributes of fruits by reducing the loss of vol-

atiles and improving the taste.

Vacuum treatment is used in combination with

chemical dips in order to improve their effects. While

fruits are submerged in the dip, vacuum is applied to

draw air from the fruit tissue, allowing better pene-

tration.

In order to reduce the cost of handling and ship-

ping, some high-moisture fruits are pureed and con-

centrated to two or three times their natural solids

content. Many others are dried for various purposes

to different moisture levels. The majority of fruits in-

cluding apricot, apple, figs, pears, prunes, and raisins

are sun dried. Sulphite is commonly used to preserve

the color when fruits are dried under high tempera-

tures that do not inactivate the oxidative enzymes.

NONENZYMATICBROWNING

Also called Maillard reaction, nonenzymatic brown-

ing is of great importance in fruit processing. During

the Maillard reaction, the amino groups of amino

acids, peptides, or proteins, react with aldehyde

groups of sugars, resulting in the formation of brown

nitrogenous polymers called melanoidins (Ellis 1959,

deMan 1999). The velocity and pattern of the reac-

tion depend on the nature of the reacting compounds,