| Crystallisation
When fats are cooled down from a high temperature (so completely
melted fat) to temperatures below the melting point, crystals are
formed; this can occur in different crystal modifications:
- Alpha-solids, formed when fat is quickly cooled down (seconds);
these solids are unstable and will convert in more stable form
as
- Betha-prime solids.
- Betha solids; these are the most stable modification.
Cooling of chocolate (with cocoa butter or cocoa butter equivalents)
is more delicate for getting the right crystal form; tempering is
then applied to create the right crystal form and to speed up the
crystallisation.
In the margarine production undercooling is applied by rapid cooling
in SSHE (surface scraped heat exchanger); then residence time has
to be given to convert alpha solids in betha prime solids and to
make more betha-prime solids.
Recrystallisation can occur; an example is crystallised partially
hardened Rapeseed or Canola. These fats, although present in the
betha-prime solids form, can recrystallise (esp. at variation of
temperature) into betha form (resulting in large crystals in needle
form). This recrystallisation process can be diminished or retarded
by making the fat blend more complex e.g. with higher levels of
C16.0 in a mainly C18-composition.
The crystallisation rate of a fat depends on:
- Rate of cooling.
In drum cooler or SSHE the cooling rate is fast; in a few seconds
the melted fat can be cooled far below the melting point.
If cooling in a tank is applied (sometimes with fractionation)
then the cooling time can take hours.
- The degree of undercooling, also depending on the rate of cooling.
- The type of triglycerides.
The alpha solids are formed rapidly in all cases that fast cooling
is applied but the conversion into betha prime solids strongly
depends on the type of triglycerides present.
- The steepness of the N-line or actually the amount of triglycerides
crystallised at a temperature change.
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