Mechanisms of Food Additives, Treatments, and Preservation Technology 315
10.2.2.2.1 Methods of Blanching
The most frequent commercial methods of blanching are steam and water blanching.
Water blanching is probably the most widely used method. The blanchers consist
of a simple water bath or various continual machines such as tubular blanchers,
rotary screw blanchers, rotary drum blanchers, and thermoscrew blanchers. The loss
of water-soluble nutrients is one of the limitations of water blanching. This may be
reduced by maximizing the produce-to-water ratio, by recycling the blanching water,
or by the use of a salt or sugar solution to reach an isotonic state (Selman et al.,
1983).^ Recycling of blanching water represents a risk of contamination and growth
of thermophilic microflora. The simplest steam blancher consists of a mesh conveyor
belt that carries food through a steam atmosphere in a tunnel. The advantage of
steam blanching is lower leaching losses relative to water blanching. Steam blanch-
ing is therefore used for blanching foods with large area of cut surface. Conventional
steam blanching does not allow the uniform heating of multiple layers of food. In
order to reach the required inactivation effect there is a risk of overheating the edges
and producing changes that affect flavor and texture. This problem was overcome
by the development of steam blanchers in the 1970s that utilize the Individual Quick
Blanch (IQB) system (Lazar et al., 1971). IQB consists of two stages of heating. In
the first stage the food is heated in a single layer to a sufficiently high temperature
to inactivate enzymes. In the second stage (adiabatic holding) a deep bed of food is
held for a sufficient time to allow the temperature at the center of each piece to
increase to reach a combination of temperature and time needed for enzyme inac-
tivation. The heating time is reduced (e.g., for diced carrots it is 25 sec of exposure
to steam, then 50 sec in a deep bed to allow the equilibration of temperature
throughout the carrot pieces. This reduction results in an improvement in the effi-
ciency of energy consumption of 86 to 91% compared to conventional water blanch-
ing (Cummings et al., 1984).
Other methods of heating that have been tested for blanching include microwave
blanching. This seems to be the most promising^ (Dietrich et al., 1970; Devece et al.,
1999; Ramesh et al., 2002) because of the low cost, but it is not currently used on
a commercial scale. Thermoinactivation of enzymes can also be achieved by other
methods of heating. Hot-gas heaters have also been tested. Losses of water-soluble
nutrients were comparable to those obtained by water or steam blanching, but surface
drying and oxidation processes when hot air was used were disadvantages (Fellows,
2000b).
10.2.2.2.2 Effects of Blanching
Enzyme activity increases with temperature until the denaturation temperature is
reached, then dramatically falls zero. The blanching process should be sufficiently
fast (high temperature, short time (HTST)) to protect the labile nutrients and prevent
acceleration of enzymatic activity that takes place at temperatures before denatur-
ation is reached. It is not important for polyphenoloxidases when oxygen is limited
during the process, but it could be important for processes such as the pasteurization
of stewed kernel fruits, products that when heated by a slow process contain more
hydrogencyanide (Voldrˇich and Kyzlink, 1992). Low-temperature blanching (LTB)
can be also used to improve the texture of produce. Moisture heating during the