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

29 Biochemistry of Vegetable Processing 575

Table 29.6.Relative Content of Vitamins in Peas

During Processing

Vitamins, Percentage of Original

Processing C B1 B2 Niacin

Fresh 100 100 100 100

Blanched 67 95 81 90

Blanched/frozen 55 94 78 76

Blanched, frozen, cooked 38 63 72 79

heating, meaning too little heating is ineffective, whereas heat-

ing too much on the other hand, 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

peroxidase as a universal indicator is that it sometimes overes-

timates heat requirements, which may vary from one product to

another. Blanching prior to freezing has the advantages of sta-

bilizing color, texture, flavor, and nutritional quality, as well as

helping the destruction of microorganisms. Blanching, however,

can cause deterioration of taste, color, texture, flavor, and nutri-

tional quality, because of heating (Table 29.6.). There are three

ways of blanching a produce including blanching using water,

steam, or microwave. Blanchers need to be energy efficient, to

give a uniform heat distribution and time, to have ability to keep

quality of the produce while destroying enzymes and reducing

microbial load. Blanching using water is done at 70–100◦Cfora

specific time frame, giving a thermal energy transfer efficiency of

about 60%, versus 5% for steam blanchers. Time-temperature

combination is very important in order to inactivate enzymes

and keeping the quality of vegetables. Effects of blanching on

plant tissues include alteration of membranes, pectin demethyla-

tion, protein denaturation, and starch gelatinization. Microwave

blanching gives similar result as water blanching, but the

loss of vitamins is higher than in steam and water methods

(Table 29.7.)

Canning Procedure

Depending on their pH, vegetables can be grouped into four cat-

egories (Banwart 1989), including high-acid vegetables with pH

Table 29.7.Effect of Blanching Methods on Vitamin C

Content

Vitamin C (mg/100g Fresh Weight)

Asparagus Beans Peas Corn

Water 35.7 22.5 15.6 15.8

Steam 35.3 23.3 11.0 13.6

Microwave 18.9 13.1 9.3 12.9

<3.7, acid vegetables with pH 3.7–4.6, medium-acid vegetables

with pH 4.6–5.3, and low-acid vegetables with pH>5.3.

The purpose of canning is to ensure food safety and high

quality to the product, as the growth of several pathogens

may compromise these parameters. Clostridium botulinum

(C. botulinum), an anaerobic and neuroparalytic toxin-forming

bacterium, is a primary safety concern in hermetically sealed,

canned vegetables. Other important spoilage organisms include

Clostridium sporogenesgroup including putrefactive anaerobe

3679,Clostridium thermosaccharolyticum,Bacillus stearother-

mophilusand related species (Stumbo et al. 1975). Although

C. botulinumspores do not produce toxin, the vegetative cells

formed after germination produce a deadly neurotoxin in canned

low-acid vegetables. Botulism, the poisoning caused by the

C. botulinumis mostly associated with canned products in-

cluding vegetables. Canned vegetables are commercially sterile,

meaning that all pathogens and spoilage organisms of concern

have been destroyed. The product may still contain a few micro-

bial spores that could be viable under better conditions. During

the processing of low-acid vegetables, it is necessary to provide

a margin of safety in the methods schedule. This is achieved ac-

cording to a “12D” process. D-value for a given temperature is

taken from a thermal death time (TDT) curve. D-value is the time

in minutes required to kill 90% of a bacterial population. The

assumption is that by increasing this time by 12, any population

ofC. botulinumpresent in the canned product will decrease by

12 log cycles. This process time allows for adequate reduction

of bacterial load to achieve a commercially sterile product.

t=D(loga−logb)(1)

where

t=heating time in minutes at a constant lethal temperature

D=time in minutes to kill 90% of a bacterial population

loga=log of initial number of viable cells

logb=log of number of viable cells after timet

ForC. botulinum, withD=0.21 at 121◦C, the 12D is equal

to 2.52 minutes (12×0.21 min), which means, if a can contains

one spore of C. botulinum with this D value, then it can be seen

from the above equation that 2.52=0.21 (log 1−logb), or

logb=−2.52/0.21=−12; thereforeb= 10 −^12. The probability

of survival of a singleC. botulinumspore in the can is one in 10^12

(Hersom and Hulland 1980). In canning low and medium-acid

vegetables, where the destruction of spores ofC. botulinumis

the major concern, a 12D process (2.52 minutes or 3 minutes at

121 ◦C) is the minimum safe standard for the “botulinum cook”

(Banwart 1989).

Canned Tomatoes

Tomato,Lycopersicon esculentumMill., belongs to the family of

Solanaceae. Tomato is a major vegetable crop in North America.

In North America alone, over 15 million Mt of processing toma-

toes are produced. Canned tomatoes are prepared from red ripe

tomatoes as whole, diced, sliced, or wedges. The fruit may or

may not be peeled but stems and calices should be removed.

Canned tomatoes may be packed with or without an added