ABSTRACT
Introduction ........................................................................................................ 133
Ice Nucleation in Bulk Water........................................................................... 134
Experimental Determination of the Sample Freezing ................................. 137
Calorimetry ................................................................................................. 137
Microscopy.................................................................................................. 139
Experimental Results......................................................................................... 139
Bulk Water................................................................................................... 139
Bulk Water þ NaCl .................................................................................... 140
Dispersed Water ......................................................................................... 143
Raw Bean..................................................................................................... 144
Blanched Bean ............................................................................................ 146
Blanched Bean þ Immersion þ Freezing ............................................... 147
Extracted Juice ............................................................................................ 148
Solid Residue .............................................................................................. 148
Conclusions......................................................................................................... 149
Acknowledgments ............................................................................................. 150
References ........................................................................................................... 150
Away to preserve vegetables vs. time is first to blanch them and afterwards
to cool them in order to transform available liquid water into ice. This
process is called “freezing” and has been established as an excellent method
for preserving high quality in foods (Reid, 1983). Nucleation (formation of
ice crystals) and crystal growth are the two major thermal events of the
freezing process. Crystal growth depends on three factors: rate of reaction at
the crystal surface, diffusion rate of water, and rate of heat removal (George,
1997). Freezing food materials rapidly leads to a large number of nucleation
sites and to small ice crystals. Then after a freeze/thaw cycle, the food
product characteristic changes tend to be less than those obtained at slow
freezing rates. In the aim of setting up an ameliorated process, we have
studied the conditions of ice nucleation and ice melting in order to define
and determine the parameters needed for the process such as temperature
and energy freezing. Because water is the predominant constituent in most
foods, water content significantly influences the thermophysical properties
of food and their behavior during the freezing process. For fruits and
vegetables, the average value of moisture content (percent by mass) is
around 85% varying from 59% for garlic to 96% for cucumbers (ASHRAE,
2002). For these products, water contents vary with the cultivar as well as
with the stage of development or maturity. Although it is not yet clearly
understood how water is distributed within vegetables, it is known that
water could appear as bulk water like a continuous medium outside the cells
and as dispersed within the cells. Water is not pure and the influence of
solute has also to be taken into account.
