BLBS102-c28 BLBS102-Simpson March 21, 2012 13:54 Trim: 276mm X 219mm Printer Name: Yet to Come
558 Part 5: Fruits, Vegetables, and Cerealsanthocyanins. The latter may be oxidized to brown products by
phenoloxidases after wounding.PhenolicsFruit phenolics include chlorogenic acid, catechin, epicatechin,
leucoanthocyanidins and anthocyanins, flavonoids, cinnamic
acid derivatives, and simple phenols. The total phenolic content
of fruits varies from 1 to 2 g/100 g fresh weight. Chlorogenic
acid is the main substrate responsible for the enzymatic brown-
ing of damaged fruit tissue. Phenolic components are oxidized
by polyphenol oxidase enzyme in the presence of O 2 to brown
products according to the following reactions.Monophenol+O 2Polyphenol oxidase
→ quinone+H 2 O1,4 diphenol+O 2Polyphenol oxidase
→ 1,4 quinone+H 2 OPhenolic components are responsible for the astringency in
fruits and decrease with maturity because of their conversion
from soluble to insoluble forms, and metabolic conversions.Cell StructureThe fruit is composed of three kinds of cells that include
parenchyma, collenchyma, and sclerenchyma.- Parenchyma cells: Parenchyma cells are the most abundant
cells found in fruits. These cells are mostly responsible for
the texture resulting from turgor pressure. - Collenchyma cells: Collenchyma cells contribute to
the plasticity of fruit material. The collenchyma cells
are located usually closer to the periphery of fruits.
Collenchyma cells have thickened lignified cell walls and
are elongated in their form. - Sclerenchyma cells: Also called stone cells, for
example, in pears, sclerenchyma cells are responsible for
the gritty/sandy texture of pears. They have very thick
lignified cell walls and are also responsible for the stringy
texture in such commodities as asparagus. For some
commodities such as asparagus, the number of
sclerenchyma cells increases after harvest and during
handling, and storage.
FRUIT PROCESSING
Harvesting and Processing of FruitsHarvesting is one of the most important of fruit-growing ac-
tivities. It represents about half of the cost involved in fruit
production because most fruit crops are still harvested by hand.
Improvement needs to be done for the development of mechan-
ical harvesters. Production of fruits of nearly equal size, and
resistant to mechanical damage during harvest, is another area
addressed by breeding technology. Certain fruits that are not to
be processed are harvested with stems in order to avoid microbial
invasion. Examples of this include cherries and apples.
Harvested fruit is washed to remove soil, microorganisms and
pesticide residues, and then sorted according to their size andquality. Fruit sorting can be done by hand or mechanically. There
are two ways of mechanical sorting of fruits, sorting with wa-
ter, which takes advantage of changes in density with ripening;
and automatic high-speed sorting, in which compressed air jets
separate fruit in response to differences in color and ripeness
as measured by light reflectance or transmittance. When not
marketed as fresh, fruits are processed in many ways including:
canning, freezing, concentration, and drying.Freezing and Canning of FruitsFor freezing and canning of fruits, the general sequence of oper-
ations includes (1) washing; (2) peeling; (3) cutting, trimming,
coring; (4) blanching; (5) packaging and sealing; and (6) heat
processing or freezing.
Freezing is generally superior to canning for preserving the
firmness of fruits. In order to minimize postharvest changes,
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 enzymatic
changes are oxidations, causing darkening of color and the pro-
duction of off-flavor. An important color change is enzymatic
browning in apples, peaches, and bananas. This is due to the oxi-
dation of pigment precursors such aso-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
oxidation.
The objective of blanching is to inhibit various enzyme reac-
tions 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 blanch-
ing water or after blanching. Calcium salts increase the firmness
of the fruit by forming calcium pectates. For the same purpose,
pectin, carboxymethyl cellulose, alginates, and other colloidal
thickeners can also be added to fruit prior to freezing. Instead
of heat treatments, certain chemicals can be used in order to in-
activate oxidative enzymes or to act as antioxidants. The major
antioxidant treatments include ascorbic acid dip and treatment
with sulfites. Other methods for the prevention of contact with
oxygen using physical barriers (sugar syrup) 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 concentration 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 ◦Cfor
up to 2 years. Because low pH also helps in delaying oxidative
processes, vitamin C and citric acid may be used in combination.
Furthermore, citric acid removes cofactors of oxidative enzymes
such as copper ions in polyphenol oxidase.
Sulfite is a multiple function antioxidant. Bisulfites of sodium
or calcium are used to stabilize the color of fresh fruits. Like
vitamin C, sulfite is an oxygen acceptor and inhibits the ac-
tivity of oxidizing enzymes. Furthermore, sulfite reduces the