Chemistry Reference
In-Depth Information
offers the advantage of pre-forming protein gels while the product is still in a
tank. The effect of subsequent shearing (which happens during the canning
process) is discussed below.
I.8.
Controlling Lactose Crystal Size by Cooling
Lactose can be supersaturated by increasing the concentration of lactose in
relation to water content by evaporation or by reducing its solubility through
cooling.
When a solution is cooled to produce a supersaturated solution and to
cause crystallization, the heat that must be removed is the sum of the sensible
heat necessary to cool the solution and the heat of crystallization. Theoreti-
cally, the heat of crystallization is equal to the heat of solubilization plus the
heat of dilution, but the heat of dilution is small and can be ignored. The heat
of solubilization for lactose monohydrate is -15,500 kJ mol
-1
or -43.1 kJ kg
-1
.
In order to achieve maximum heat transfer, agitation must be effective.
The most common crystallizer used in SCM manufacture is a simple
open tank or vat in which the solution loses heat to its surroundings. Since
cooling in this fashion is slow and not very effective, large crystals are
generally produced. To reduce crystal size or increase the rate of cooling,
cooling coils or jackets can be added and these crystallizers can be made
continuous.
Continuous crystallizers used in the food industry are usually cylindri-
cal, scraped-surface heat exchangers, similar in design to those for plasticizing
margarine and cooking fats, and for crystallizing ice cream. It is essentially a
double-pipe heat exchanger fitted with an internal scraper. The material is
pumped through the central pipe and agitated by the scraper, with the cooling
medium flowing through the annulus between the outer pipes.
I.8.1.
Seeding/Cooling Temperature
The removal of
-lactose from the solution due to crystallization means
that the ratio between
- and
-lactose changes, so that the solution contains
more
-lactose than
-lactose during crystallization. Due to mutarotation,
the solution of
-lactose becomes supersaturated again, and crystallization
continues. This process continues for as long as the solution is supersaturated.
The rate of mutarotation is influenced directly by the temperature of the
solution; it proceeds faster at high temperatures and very slowly at tempera-
tures near the freezing point. Thus, temperature has the opposite effects on
mutarotation and crystallization. This means that by cooling the concentrate
too fast or to a very low temperature, crystallization proceeds quickly but the
