Another option is freeze-drying. Starting with a concentrated solution,
freezing will soon lead to a mixture of ice crystals and a sucrose-aroma glass: see
Figure 16.6. Drying should then be done at high vacuum, taking care that the
temperature remains belowTg. The porous glassy product obtained can readily be
ground to a fine powder.
Note Probably, the manufacturer will not use pure sucrose but admix
other sugars, such as a dextrose syrup, which will greatly reduce the
tendency of sucrose to crystallize. That would allow the application of
spray-drying at higher temperatures.16.3 FREEZING OF FOODS
In the food industry, freezing is applied to make ice cream and comparable
products (edible ices or frozen desserts) or as a process step in freeze-
concentration or freeze-drying. The main purpose is to prevent or delay
deterioration of the food. This concerns undesirable changes due to
microbial growth, enzyme action, chemical reactions, or physical processes,
usually involving mass diffusion. Such freezing is applied to natural
products (fish, fruits, vegetables, etc.) as well as to a variety of fabricated
foods. Essential primary changes occurring owing to freezing are
Lowering of the temperature
Freeze-concentration of the aqueous solution
Greatly increasing viscosity and possibly attaining a glassy stateAn important question is, of course, how stable the product is and what
factors determine stability. A problem can be that freezing greatly alters the
consistency of the product and—often in combination with the thawing of
the frozen material—leads to damage of its structure; another question is
thus how to minimize such damage.
Low Temperature. Most aspects were discussed before, especially
in Chapters 4 and 8. Briefly summarizing, the following effects of lowering
the temperature can be mentioned.
The rates of nearly all chemical reactions decrease.
Chemical equilibria can change, since the rates of the reactions leading
to the equilibrium may decrease to a different extent. Some