Thermal Food Processing

404 Thermal Food Processing: New Technologies and Quality Issues
vegetables 54,57–59 and is being practiced commercially for canned snap beans and
canned cauliflower, tomato, potato, and carrot. 60,61 Maximum firming effects of
these vegetables could be obtained by blanching at 55 to 80°C for times ranging
from several minutes to several hours. For sweet potato, blanching at 62°C for
90 min resulted in maximum firmness, 59 while high-temperature blanching disrupted
cell integrity and cell adhesion, and reduced tissue rigidity. Blanching at
55°C for an hour produced maximum firmness of jalapeno. 54
Several researchers have emphasized the benefits of HTST blanching. Carrot
tissues subjected to high temperature for a short time (HTST) (100°C, 0.58 min)
retained a firmer texture than those subjected to a low temperature for a long
time (LTLT) (70°C, 71.10 min). 43 Galacturonic acid and sugar content of pectin
extract during blanching of carrots were determined, and immunocytochemistry
experiments elucidated changes in the cell membrane. Blanching decreased galacturonic
acid and total sugar contents for all treatments. Carrots subjected to HTST
blanching contained higher galacturonic acid and sugars in pectins than carrots
blanched by the LTLT process.
Two-stage blanching was found to be effective to maintain firmer texture of
green beans. Instead of single blanching at 93°C, preliminary blanching at 70°C
followed by high-temperature blanching resulted in firmer texture of green beans.
During low-temperature blanching, pectinase enzymes partially demethylate the
pectins. This leaves –OH sites free on the pectin chain to cross-link with other
pectin molecules via a calcium bridge, resulting in a firmer texture. 62,63 The beans
are given high-temperature blanching in a second stage, which inactivates the
enzymes.
Several researchers have investigated the changes in pectic substances of
processed vegetables. 64–67 Results indicated that cooking-associated texture loss is
often related to the dissolution of pectins. Cooking or blanching has also been
found to activate enzymes. 68–70 The demethoxylation and depolymerization of
pectic substances were catalyzed by pectin methylesterase (PME) or pectin polygalacturonase
in blanched carrots. At 50 to 80°C, PME hydrolyzed the ester bond
to yield free carboxyl groups and to release methoxyl groups. These free carboxyl
groups were then cross-linked by salt bridges with calcium ions that were present
in the tissue. The cross-linking bridges were often found to result in a firmer texture
of fruits and vegetables. 70 The demethoxylation of pectins in blanched vegetables
can also be caused by chemical saponification. 70 Chemical saponification or enzymatic
deesterification produces a random deesterification of pectic polymers. 70,71
13.4.2.4 Sensory and Nutritional Quality
Ascorbic acid is one of the most heat labile nutrients and easily oxidizable by
the naturally occurring enzyme system, ascorbic acid oxidase. Retention of ascorbic
acid decreases with increased temperature and duration of blanching. In the
case of the potato, significant differences in respect to ascorbic acid retention
were observed during blanching at 80 and 93°C. 72 Boiling water treatment and
microwave blanching of frozen beans showed the same reduction of ascorbic acid