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

576 Part 5: Fruits, Vegetables, and Cereals

liquid. Calcium salts varying from 0.045% to 0.08% by weight

of the finished products can also be added. Other ingredients

such as organic acids, spices, oil, and flavorings can be added

up to 10%. There are three categories of canned tomatoes. The

label tomatoes are valid only for peeled and canned tomato. Un-

peeled tomatoes are labeled accordingly. Stewed tomatoes are

canned tomatoes containing onion, celery, and peppers (Anon

1993). The flowchart for the manufacture of canned tomatoes is

as follows: Fresh tomatoes→Sorting→Washing→Resorting

→Trimming→Peeling→Final Inspection→Cutting (except

for whole tomatoes)→Filling→Exhausting→Steaming and

Thermal processing. Most of the operations are similar to the

ones described for canning in general.

Peeling

Tomatoes for canning are peeled with hot water, steam, or lye.

Tomatoes are passed through boiling water bath or live steam

chamber for 15–60 seconds, depending on the variety, size, and

ripeness of the fruit. The temperature of 98◦C or above is recom-

mended. The hotter the water, the shorter is the scalding time.

Boiled tomatoes are immediately cooled in cold water and then

peeled by hand or with a machine. Hand peeling is labor in-

tensive and is completed by removing the core with a knife.

Machine peeling is faster and is done by scrubbing or cutting

and squeezing. For lye treatment, a solution of hot 14–20% soda

caustic (sodium hydroxide-NaOH) is normally used in two ways.

Tomatoes could be immersed in the solution or sprayed. Then,

a scrubber removes the disintegrated skins. Lye peeling is less

labor intensive, but uses more water than hot water and steam

peeling. Since the peeled tomatoes are thoroughly rinsed to re-

move the residual lye, treatment with up to 10% of citric acid is

recommended. Lye treatment creates more pollutants, and loss

of soluble solids compared to the other two methods.

Inspection

The final inspection after peeling is very important for the grad-

ing of the finished product. Canned tomatoes are graded in cat-

egories, A, B, C or 1st, 2nd, or 3rd class product. The purpose

of the final inspection is to remove any visible defects in the

products including residual peel fragments, extraneous vegetable

materials, as well as checking the wholeness (when necessary).

Cutting

Peeled tomato may be cut into halves, slices, dices, wedges when

necessary, using cutter, slicer, or dicer before filling in the can.

Filling

Filling of tomato in cans can be done by hand or by machine.

The best quality whole tomatoes are filled by hand and filled

up with tomato juice. Softening during heating can be avoided

by addition of calcium chloride or calcium sulfate, in the form

of tablets or mixed in tomato juice and dispensed in each can.

In order to inhibit the action ofC. botulinum, a required pH

less than 4.6 is secured by the addition of citric acid in the

form of tablets in each can. For stewed tomato, 3/4 filled cans

with peeled tomatoes are spiced with dehydrated onion, garlic,

chopped celery, green bell pepper dices, as well as tablets made

of citric acid and a mixture of salt, sugar, and calcium chloride

into the can. The can is then filled up with tomato juice until an

acceptable level of headspace is reached.

Exhausting

The purpose of exhausting is to create enough vacuum in the

can, in order to avoid fast deterioration of the canned product

during the summer season. A minimum temperature of 71◦C

at the center of a can after the completion of exhaust is rec-

ommended (Luh and Kean 1988). Exhaust is normally done in

steam chambers. The exhaust time in the chamber depends on

the size of the can and varies from 3 minutes for 300 mm×

407 mm to 10 minutes for 603 mm×700 mm cans. Exhaust

is also done by mechanical vacuum closing machines (Lopez

1987, Gould 1992).

Processing Time of Canned Tomatoes

The processing time of canned tomato depends on many fac-

tors including the pH of the canned tomato, the major spoilage

microorganisms of concern, the size of the can, and the type

of retort (sterilizer). Organisms of concern in canned tomatoes

areClostridium pasteurianumandClostridium butyricum,as

well asBacillus coagulans(Bacillus thermoacidurans). Spores

of butyric acid anaerobes are destroyed at 93.3◦C for 10 minutes

when the pH is higher than 4.3, and after 5 minutes, when the pH

is between 4 and 4.3. Canned tomatoes can be processed with

a rotary sterilizer, a conventional stationary retort or in an open

nonagitating cooker. In general, the processing time is longer

if the cooling is done by air instead of with water. For a rotary

sterilizer the processing time at 100◦C for a can of 307 mm×

409 mm may be reduced by 9 minutes or shorter if air cooled,

and by 13 minutes or shorter if water cooled. For the same size

of can at 100◦C an open nonagitating cooker or a conventional

retort would require a processing time of 35–55 minutes. The

can center temperature should reach 82◦C, when air cooled, and

90 ◦C when water cooled (Lopez 1987). The retort time depends

on the size of the can, the fill weight, and the initial temperature

of the canned product.

Tomato Juice Processing

Processing

Tomato juice is defined as the unconcentrated, pasteurized liq-

uid containing a substantial portion of fine tomato pulp extracted

from good quality, ripe, whole tomatoes from which all stems

and unnecessary portions have been removed by any method

that does not increase the water content, and may contain salt

and a sweetening ingredient in dry form (Canadian Food and

Drugs Act). Only high-quality tomatoes should be used for juice

production. Tomatoes are important source of vitamins A and