Thermal Food Processing

Thermal Processing of Vegetables 401
attributes of thermally processed vegetables are color, aroma, taste, and texture,
while hidden quality attributes like nutritional values and safety (chemical and
microbiological) remain the most challenging concern in processed vegetables.
Color plays an important role in appearance, processing, and acceptability of
vegetables. When a vegetable is exposed to light, about 4% of the incident light
is reflected at the outer surface, causing specular reflectance or gloss, and the
remaining 96% of incident energy is transmitted through the surface into the
cellular structure of the product, where it is scattered by the small interfaces
within the tissue or absorbed by cellular constituents. 29 Recently, imaging technology
has been introduced for more precise color measurement of fruits and
vegetables, and multi- or hyperspectral cameras permit rapid acquisition of
images at many wavelengths. 2 This type of imaging provides information about
the spatial distribution of constituents (pigment, sugar, moisture) in vegetables
near the surface of the product.
Rheology, the study of the deformation and flow of matter, has been extensively
applied to vegetables in an effort to understand the relationship between structure,
texture, and changes taking place during processing. The mechanical properties of
vegetables have been widely studied following the same technique applied for nonbiological
materials. It helps to understand the mechanical behavior of vegetables
to some extent; however, vegetables as a biological material differ from nonbiological
materials in many respects. Various researchers have defined the term texture of
vegetables in different ways. 30 Sensory analysis of vegetable texture in combination
with mechanical measurement could represent the vegetable texture more precisely.
The application of the principle of hazard analysis and critical control points
(HACCPs) has done much to focus and formulize the process of demonstrating the
safety of processed vegetables. It is generally agreed that verification processes are
necessary to assess the effectiveness of the HACCP plan and to confirm that the
food safety system, once implemented, adheres to the HACCP plan. An optimum
time–temperature combination (TTC) is an important critical control point (CCP)
that has to be controlled in order to guarantee the microbiological safety of processed
vegetables. The safety of the sterilization process can be evaluated according
to the lethality achieved and the microbiological risk alteration 31 of the target
microorganisms that survive the thermal treatment.
13.4.2 EFFECT OF BLANCHING ON QUALITY OF VEGETABLES
13.4.2.1 Enzymes
Peroxidase (PO) is considered to be the most heat stable enzyme in vegetables
and is therefore used as the indicator of blanching efficiency. The Food and Drug
Administration (FDA) recommends its inactivation to reduce quality loss during
storage of processed foods. 32 Thermal inactivation of PO depends on the nature,
thickness, and geometry of the vegetables, and the applied time–temperature
combination. Heating times can be significantly reduced if an individual quick
blanching process is followed. The first-order reaction kinetics can adequately
describe PO inactivation. 33