250 Produce Degradation: Reaction Pathways and their Prevention
that should be inhibited or eliminated. It is thus reasonable to state that the type of
equipment used, blanching method, time, temperature, maturity, and cultivar of the
produce would influence the degree to which the nutritive composition changes.
8.10.1 BLANCHING
One of the ancient and most often used heat treatments of fresh fruits and vegetable
is blanching. This is a short heat treatment of fruits and vegetables to kill microor-
ganisms on the surface of the produce and inactivate some of the thermolabile
enzymes that cause quality deterioration. Blanching also helps to expel gases from
the produce, in preparation for canning. Blanching can be done by immersing the
produce in hot water (water blanching), passing a stream of steam through the
produce (steam blanching), or microwaving the produce for a short time (microwave
blanching). The influence of blanching on the degradation of the nutritive compo-
sition varies from product to product. Water blanching has been shown to be asso-
ciated with enormous loss of the water-soluble vitamins such as ascorbic acid,
thiamin, riboflavin, and niacin. Steam and microwave blanching has been shown to
be more effective in the retention of the water-soluble vitamins (Mosha et al., 1995).
For example, green vegetables blanched by the microwave method showed signifi-
cantly higher retention (40 to 60%) of carotene and water-soluble vitamins relative
to conventional hot water blanching. It was also more effective in inactivating
antinutritional factors such as trypsin and chymotrypsin inhibitors than was hot water
blanching (Mosha et al., 1995; Mosha and Gaga, 1999). Broccoli packaged in plastic
bags and microwave-blanched for 20 to 30 sec indicated 100% inactivation of
catalase and peroxidase enzymes. There was also 100% retention of ascorbic acid
relative to 37 to 100% retention in steam blanching and 24 to 93% retention in water
blanching. In these cases, the losses of vitamin C were mostly due to leaching rather
than to destruction (Salunkhe, 1974).
Although heat treatments are generally effective in inhibiting enzymatic reac-
tions and in reducing microbial loads, they are rarely used for stabilizing minimally
processed fruits and vegetables due to their negative effects on flavor, texture, and
fresh-like quality. Conversely, the development of suitable heat treatments associated
with a low negative impact could be of great interest in the production of minimally
processed fruits and vegetables with a longer shelf-life that could be used as semi-
manufactured and final products. Low temperature blanching treatments that are
associated with activating pectin methylesterase and cross-linking with cations have
shown a good retention of color and texture in fresh fruits and vegetables (Bourne,
1987). Therefore, the type of heat treatment needs to be considered on multiple
levels for optimum product quality (physical and nutritional), as well as safety, and
would obviously be product- and process-specific.
8.10.2 REFRIGERATION AND FREEZING
Cold storage is one of the oldest methods for preserving shelf life and nutrients in
fresh fruits and vegetables. It involves keeping the produce at low temperatures
(at least between 0 and 10°C) (Salunkhe, 1974; Paull et al., 1997). Refrigeration
helps to minimize water loss from the produce and slows down the rate of respiration,