23 Biochemistry of Vegetable Processing 543duce: water, steam, or microwave. Blanchers need to
be energy efficient, give a uniform heat distribution
and time, and have the ability to maintain the quality
of the produce while destroying enzymes and reduc-
ing microbial load. Blanching using water is done at
70–100°C for a specific time frame, giving a thermal
energy transfer efficiency of about 60%, versus 5%
for steam blanchers. The time-temperature combina-
tion is very important in order to inactivate enzymes
while maintaining the quality of the vegetables. Ef-
fects of blanching on plant tissues include alteration
of membranes, pectin demethylation, protein denat-
uration, and starch gelatinization. Microwave blanch-
ing gives a result similar to that of water blanching,
but the loss of vitamins is higher than in steam and
water methods (Table 23.5.)
Canning Procedures
Depending on their pH, vegetables can be grouped
into four categories (Banwart 1989): (1) high-acid
vegetables with pH 3.7, (2) acid vegetables with
pH 3.7–4.6, (3) medium-acid vegetables with pH
4.6–5.3, and (4) low-acid vegetables with pH 5.3.
The purpose of canning is to ensure food safety
and high quality of the product, as the growth of sev-
eral pathogens may compromise these parameters.
Clostridium botulinum (C. botulinum), an anaerobic
and neuroparalytic toxin-forming bacteria, is a pri-
mary safety concern in hermetically sealed, canned
vegetables. Other important spoilage organisms in-
clude theC. sporogenesgroup including putrefactive
anaerobe 3679,C. thermosaccharolyticum,Bacillus
stearothermophillus,and related species (Stumbo et
al. 1983). AlthoughC. botulinumspores do not pro-
duce toxin, the vegetative cells formed after germi-
nation produce a deadly neurotoxin in canned low-
acid vegetables. Botulism, the poisoning caused by
theC. botulinumis mostly associated with canned
products including vegetables. Canned vegetables
are commercially sterile, meaning that all pathogens
and spoilage organisms of concern have been des-
troyed. The product may still contain a few micro-
bial spores that could be viable under better condi-
tions. During the processing of low-acid vegetables,
it is necessary to provide a margin of safety in the
methods schedule. This is achieved according 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 in-
creasing this time by 12, any population of C. botu-
linumpresent in the canned product will decrease by
12 log cycles. This process time allows for adequate
reduction of bacterial load to achieve a commercial-
ly sterile product.t D (log a log b) (2.1)where t is heating time in minutes at a constant le-
thal temperature, D is the time (in minutes) required
to kill 90% of a bacterial population, log a is the log
of the initial number of viable cells, and log b is the
log of the number of viable cells after time t.
For C. botulinum, with D 0.21 at 121°C, 12D is
equal to 2.52 min (12 0.21 minutes), which means
that if a can contains one spore of C. botulinumwith
this D value, then from the above equation, 2.52
0.21 (log 1 log b), or log b 2.52/0.21 12;
therefore b 10 -12. The probability of survival of a
single C. 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 of C. botulinumis the major concern, a 12D
process (2.52 or 3 minutes at 121°C) is the mini-
mum safe standard for the “botulinum cook” (Ban-
wart 1989).Table 23.4.Relative Content of Vitamins in
Peas during Processing
Processing Vitamins, % of original
C B1 B2 Niacin
Fresh 100 100 100 100
Blanched 67 95 81 90
Blanched/ frozen 55 94 78 76
Blanched, frozen, 38 63 72 79
Cooked
Table 23.5.Effect of Blanching Methods on
Vitamin C
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