Food Biochemistry and Food Processing

23 Biochemistry of Vegetable Processing 541

VEGETABLE PROCESSING

HARVESTING ANDPROCESSING OF
VEGETABLES

The quality of the vegetables constantly varies de-
pending on the growth conditions. Since optimum
quality is transient, harvesting and processing of sev-
eral vegetables such as corn, peas, and tomatoes are
strictly planned. Serious losses can occur when har-
vesting is done before or after the peak quality stage.
The time of harvesting vegetables is very important
to the quality of the raw produce, and the manner of
harvesting and handling is critical economically. A
study on sweet corn showed that 26% of total sugars
were lost in just 24 hours by storing the harvested
corn at room temperature. Even when stored at low
temperatures, the sugar loss could reach 22% in 4
days. Peas and lima beans can lose up to 50% of
their sugars in just 1 day. Losses are slower under
refrigeration, but the deterioration of the sweetness
and freshness of the produce is an irreversible
process. It is assumed that a part of the sugars is
used for respiration and starch formation in com-
modities such as corn, whereas the sugars are con-
verted to cellulose, hemicellulose, and lignin in the
case of asparagus. Each type of vegetable has its
optimum cold storage temperature, which may vary
between 0 and 10°C. Water loss is another problem
that reduces the quality of the produce. Continued
water loss due to transpiration and drying of cut sur-

faces results in wilting of leafy vegetables. Hermetic

packaging (anaerobic packaging) does not prevent

water loss, but instead creates conditions that pre-

vent deterioration due to an increase in the level of

carbon dioxide and a decrease in the oxygen level.

In order to keep consistent produce quality, many

processors monitor growing practices so that harvest

and processing are programmed according to the

capacity of the processing plant.

PREPROCESSINGOPERATIONS

Vegetables can be processed in different ways in-

cluding canning, freezing, freeze-drying, pickling,

and dehydration. The operations involved in the pro-

cessing depend on the type of vegetable and the

method to be used. After harvest, the processing steps

involved in canning are washing, sorting and grad-

ing, peeling, cutting and sizing, blanching, filling and

brining (brining is very important for filling weight

and heat transfer), exhausting (helps maintain high

vacuum; exhaust temperature in the center of the can

should be about 71°C), sealing, processing (heating

cycle), cooling, labeling, and storage.

Harvesting

The decision to harvest should be based on experi-

ence and on objective testing methods. It is recom-

mended that the vegetables be harvested at optimum

Table 23.3.Percentage Composition of Vegetables

Component

Food Carbohydrate Protein Fat Ash Water

Cereals
Maize (corn) whole grain 72.9 9.5 4.3 1.3 12
Earth vegetables
Potatoes, white 18.9 2.0 0.1 1.0 78
Sweet potatoes 27.3 1.3 0.4 1.0 70
Vegetables
Carrots 9.1 1.1 0.2 1.0 88.6
Radishes 4.2 1.1 0.1 0.9 93.7
Asparagus 4.1 2.1 0.2 0.7 92.9
Beans, snap, green 7.6 2.4 0.2 0.7 89.1
Peas, fresh 17.0 6.7 0.4 0.9 75.0
Lettuce 2.8 1.3 0.2 0.9 94.8

Source:Food and Agriculture Organization (FAO).