Food Biochemistry and Food Processing

544 Part V: Fruits, Vegetables, and Cereals

CANNEDTOMATOES

Tomato, Lycopersicon esculentumMill, belongs to
the family of Solanaceae. Tomato is a major vegeta-
ble crop in North America. In North America alone,
over 15 million metric tons of processing tomatoes
are produced. Canned tomatoes are prepared from red
ripe tomatoes as whole, diced, sliced, or in wedges.
The fruit may or may not be peeled, but stems and
calices should be removed. Canned tomatoes may
be packed with or without added liquid. Calcium
salts, varying from 0.045 to 0.08% by weight of the
finished products, can also be added. Other ingredi-
ents 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. Unpeeled toma-
toes are labeled accordingly. Stewed tomatoes are
canned tomatoes containing onion, celery, and pep-
pers (Anon. 1993). The flowchart for the manufac-
ture of canned tomatoes is as follows: fresh toma-
toes →sorting →washing →re-sorting, trimming
→peeling →final inspection →cutting (except for
whole tomatoes) →filling →exhausting →steam-
ing and thermal processing. Most of the operations
are similar to the ones described for canning in gen-
eral.

Peeling

Tomatoes for canning are peeled with hot water,
steam, or lye. Tomatoes are passed through a boiling
water bath or live steam chamber for 15–60 seconds,
depending on the variety, size, and ripeness of
the fruit. A temperature of 98°C or above is recom-
mended. The hotter the water, the shorter is the scald-
ing time. Boiled tomatoes are immediately cooled in
cold water and then peeled by hand or with a ma-
chine. Hand peeling is labor intensive and is com-
pleted 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 scrub-
ber 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 remove the residual lye,
treatment with up to 10% of citric acid is recom-

mended. Lye treatment creates more pollutants and

loss of soluble solids that either of the other two

methods.

Inspection

The final inspection after peeling is very important

for the grading of the finished product. Canned

tomatoes are graded in categories: A, B, C, or first-,

second-, or third-class product. The purpose of the

final inspection is to remove any visible defects in

the products including residual peel fragments and ex-

traneous vegetable materials, and to check the whole-

ness of the product (when necessary).

Cutting

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

or 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 calci-

um 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 of C. 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, three-quarter filled cans with peeled

tomatoes are spiced with dehydrated onion, garlic,

chopped celery, and green bell pepper dices, as well

as tablets made of citric acid and a mixture of salt,

sugar, and calcium chloride. The can is then filled up

with tomato juice until an acceptable level of head-

space is reached.

Exhausting

The purpose of exhausting is to create enough vacu-

um in the can to avoid fast deterioration of the can-

ned product during the summer season. A minimum

temperature of 71°C at the center of a can after the

completion of exhaust is recommended (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