Food Biochemistry and Food Processing (2 edition)

BLBS102-c29 BLBS102-Simpson March 21, 2012 13:27 Trim: 276mm X 219mm Printer Name: Yet to Come

574 Part 5: Fruits, Vegetables, and Cereals

occur when harvesting is done before or after the peak qual-

ity 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 temperature, 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 reduction in

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 commodities such as corn, whereas the sug-

ars are converted 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◦C and 10◦C.

Water loss is another problem, which reduces the quality of the

produce. Continued water loss due to transpiration and drying

of cut surfaces results in wilting of leafy vegetables. Hermetic

packaging (anaerobic packaging) does not prevent water loss,

but instead creates conditions that prevent 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 the growing practices so that harvest

and processing are programmed according to the capacity of the

processing plant.

Preprocessing Operations

Vegetables can be processed in different ways including canning,

freezing, freeze-drying, pickling, and dehydration. The opera-

tions involved in the processing depend on the type of vegetable

and the method to be used. After harvest, the processing steps

involved in canning are washing, sorting and grading, peeling,

cutting and sizing, blanching, filling and brining (brining is very

important for filling weight and heat transfer), exhausting (help

maintaining high vacuum; exhaust temperature in the center

of the can should be about 71◦C), sealing, processing (heating

cycle), cooling, labeling, and storage.

Harvesting

Decision to harvest should be based on experience and on ob-

jective testing method. It is recommended that the vegetables

be harvested at the optimum maturity and processed promptly

(Luh and Kean 1988, Woodroof 1988).

Sorting and Grading

This operation is done using roller grader, air blower, rod shaker,

or any mechanical device, followed by sorting on conveyor belts.

Electronic sorting is commonly used recently to remove vegeta-

bles affected by diseases and insects.

Washing

Vegetables are washed to remove not only field soil and surface

microorganisms, but also fungicides, insecticides, and other pes-

ticides. There are laws specifying the types and maximum levels

of contaminants that are permitted. In order to remove dirt,

insects, and small debris, vegetables are rinsed with water or

with detergent in some cases. Mechanically harvested tomatoes,

potatoes, 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 cleaners can be used for peas and other small veg-

etables, whereas fragile vegetables such as asparagus may be

washed by gentle spraying belt.

Peeling

Several methods are used to remove skins from vegetables in-

cluding lye, steam, and direct flame. Lye peeling of mechanically

harvested tomatoes and potatoes is a common practice. Vegeta-

bles with loosened skins are jet washed with water to remove

skins and residual sodium hydroxide. Steam is used to peel

vegetables with thick skin such as red beets and sweet potatoes,

whereas for onions and pepper 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 precise 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 dif-

ferent shapes along the length of the vegetable. Brussels sprouts

are trimmed by hand by pressing the base against a rapidly ro-

tating knife. Olives are pitted by aligning them in small cups,

and mechanically pushing the plungers through the olives.

Blanching

The purpose of blanching is to inactivate the enzymes present

in the vegetables. Since many vegetables do not receive a high-

temperature heat treatment, heating to a minimal temperature

before processing or storing inactivates the enzymes responsible

for changes in texture, color, flavor and nutritional quality of the

produce. Several enzymes are responsible for the loss of quality

in vegetables. The deterioration of the cell membrane caused

by the action of phospholipase D and lipoxygenases account

for the flavor development in vegetables (Pinhero et al. 2003,

Oke et al. 2003). Proteases and chlorophyllases contribute to the

destruction of chloroplast and chlorophyll. 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 activity of enzymes that

destroy the vitamins. Ascorbic acid oxidase can cause a decline

in the level of vitamin C.

The blanching process also reduces the microbial load of veg-

etables and renders packaging into containers easier. To evaluate

the effectiveness of blanching, indicator enzymes such as cata-

lase and peroxidase are traditionally used. The reason for using

indicators is that blanching is not a process of indiscriminate