Thermal Food Processing

406 Thermal Food Processing: New Technologies and Quality Issues
degradation involves the loss of phytol to form chlorophyllide, loss of Mg 2+ to
form pheophytin, loss of Mg 2+ and phytol to form pheophorbide, and loss of Mg 2+
and the carbomethoxy group to form pyropheophytin. 80 The conversion is
enhanced by extended heat treatment, acidity, and storage. 81 The most common
change that occurs in green vegetables during thermal processing is the conversion
of chlorophyll to pheophytins, causing a color change from bright green to olivebrown,
which is undesirable for consumer acceptability. 34,82–84
Various methods to retain green color have been proposed. Controlling pH,
followed by HTST processing, showed better retention of the green color of
vegetables. 85–88 Color retention was superior in most of these processes immediately
after thermal processing, but the retained chlorophyll degraded rapidly
during storage. Greater stability of chlorophyll in blanched spinach puree was
found when surfactants were added. However, the protective effect of surfactants
was lost at or above 100°C, indicating its unsuitability for heat-sterilized food
products.
The tristumulus L value described the color change of mushroom, 89 while the
a value represented the pigment content of many vegetables during thermal
processing. The total pigment content of sweet potato and squash correlated well
with the tristimulus a value. 90–92 The color ratio (a/b) is measured routinely as
the quality index in tomato, orange, and red pepper processing industries. 93
13.4.4.2 Kinetics of Color Degradation of Vegetables
during Thermal Processing
The color degradation kinetics of vegetables is complex, and dependable models
to predict experimental color change are limited. Kinetics of pigment and color
degradation of vegetables during thermal processing has been studied by numerous
researchers. 8,84,93–96 The major finding of these studies is that both pigment
and color degradation during thermal processing follow first-order reaction
kinetics.
Chlorophyll a degrades faster than chlorophyll b. 85 Degradation of chlorophyll
a is 2.5 times faster than chlorophyll b at 37°C and a water activity of 0.32.
Thermal degradation of chlorophylls and chlorophyllides in spinach puree was
studied at 100 to 145°C for a retention time of 2 to 25 min and 80 to 115°C for
2.5 to 39 min. Reaction kinetics revealed that both chlorophyll a and chlorophyllide
a degraded more rapidly than the corresponding b form. 97
The tristimulus color ratio (a/b) has been used to determine reaction kinetic
parameters for the discoloration of some vegetables. 98 The process activation energies
for change in visual green color using rate constants were determined at various
temperatures. The activation energy values were found to be in the range from
33.14 to 43.38 kJ/mol for asparagus, green beans, and green peas, respectively.
Since the color a value represents only major pigment, it therefore does
not represent the total color change of vegetables during thermal processing.
In practice, any change in the a value is associated with a simultaneous change
in L and b. Representation of quality in terms of total color may therefore be