542 Part V: Fruits, Vegetables, and Cereals
maturity and processed promptly (Luh and Kean
1988, Woodroof 1988).
Sorting and Grading
This operation is done using a roller grader, air blow-
er, or any mechanical device, followed by sorting on
conveyor belts. Electronic sorting is commonly used
recently to remove vegetables affected by diseases
and insects.
Washing
Vegetables are washed to remove not only field soil
and surface microorganisms, but also fungicides, in-
secticides, and other pesticides. There are laws spec-
ifying the maximum allowed level of contaminant.
In order to remove dirt, insects, and small debris,
vegetables are rinsed with water, or with detergent
in some cases. Mechanically harvested tomatoes, po-
tatoes, red beets, and leafy vegetables are washed
with fruit grade detergents. The choice of washing
equipment depends on the size, shape, and fragility
of the particular type of vegetable. Flotation clean-
ers can be used for peas and other small vegetables,
whereas fragile vegetables such as asparagus may be
washed by gentle spraying while being transported
on conveyer belts.
Peeling
Several methods are used to remove skins from veg-
etables including lye, steam, and direct flame. Lye
peeling of mechanically harvested tomatoes and po-
tatoes is a common practice. Vegetables with loos-
ened skins are jet washed with water to remove skins
and residual sodium hydroxide. Steam is used to
peel vegetables with thick skins such as red beets
and sweet potatoes, whereas for onions and peppers
direct flame or hot gases in rotary tube flame peelers
are used.
Cutting and Trimming
Cutting, stemming, pitting, or coring depends on the
type of vegetable. Asparagus spears are cut to pre-
cise lengths. The most fibrous part is used for soup
and other heated products where heat tenderizes
them. Green beans are cut by machine into several
different shapes along the length of the vegetable.
Brussels sprouts are trimmed by hand by pressing
the base against a rapidly rotating knife. Olives are
pitted by aligning them in small cups and mechani-
cally pushing plungers through the olives.BlanchingThe purpose of blanching is to inactivate the en-
zymes present in the vegetables. Since many vegeta-
bles don’t receive a high temperature heat treatment,
heating to a minimal temperature before processing
or storing inactivates the activity of enzymes re-
sponsible for changes in the texture, color, flavor,
and nutritional quality of the produce. Several en-
zymes are responsible for the loss of quality in
vegetables. The deterioration of the cell membrane
caused by the action of phospholipase D and lipoxy-
genases account for the flavor development in veg-
etables (Pinhero et al. 2003, Oke et al. 2003, Kruger
et al. 1991). Proteases and chlorophyllases contri-
bute to the destruction of chloroplast and chloro-
phyll. Changes in texture occur due to the activity of
pectic enzymes and cellulases. Color deterioration
occurs due to the activity of polyphenol oxidase,
chlorophyllase, and peroxidase (Robinson 1991).
Changes in nutritional quality can occur by the ac-
tivity of enzymes that destroy the vitamins. Ascorbic
acid oxidase can cause a decline in the level of vita-
min C.
The blanching process also reduces the microbial
load of vegetables and renders packaging into con-
tainers easier. To evaluate the effectiveness of blanch-
ing, indicator enzymes such as catalase and peroxi-
dase are traditionally used. The reason for using
indicators is that blanching is not a process of indis-
criminate heating: too little heating is ineffective,
whereas heating too much negatively impacts the
freshness of certain vegetables. The choice of an
indicator depends on the vegetable being processed.
For example lipoxygenase may be an ideal indicator
for peas and beans. The problem with using peroxi-
dase as a universal indicator is that it sometimes
overestimates heat requirements, which may vary
from one product to another. Blanching prior to
freezing has the advantage of stabilizing color, tex-
ture, flavor, and nutritional quality, as well as help-
ing destroy microorganisms. Blanching, however,
can cause deterioration of taste, color, texture, fla-
vor, and nutritional quality, because of heating
(Table 23.4.). There are three ways of blanching pro-