214 Chapter 9
to infl ate the fl exible membranes and squeeze
out the residual oil fraction. The high pres-
sure used in dry fractionation of milk fat
helps ensure a high yield from the process.Plasticizing
If the anhydrous milk fat product is to be
melted prior to use, then the temperature
treatment pre - or post - packing has no effect
on the functionality of the product. If,
however, the milk fat product is to be used in
a solid or semi - solid state, for example,
incorporated into bakery goods, then the way
in which it is worked and cooled (plasticized)
markedly affects its functionality. Texturiza-
tion of hard milk fat fractions for inclusion
in pastries and croissants is achieved by
physically reducing the size and increasing
the number of milk fat crystals using high -
pressure scraped - surface heat exchangers in
combination with pin workers and setting
(resting) tubes.
Agitation continues during crystalliza-
tion to ensure the numerous small crystals
formed remain discrete and independent. The
texturized product is fi rmer with increased
plasticity and the combination of cooling
and working units can be adjusted to produce
a range of products. At one extreme, textur-
ized cake - making products, which soften
rapidly when worked, and with good cream-
ing and aeration properties, can be pro-
duced, while at the other extreme, products
for pastries that can be rolled and worked
with little appreciable softening or brittle-
ness may be prepared. Emulsifi ers such
as mono - or di - glycerides also may be added
to texturized milk fat fractions blended spe-
cifi cally for cake - making to further improve
functionality.Manufacture of Ghee
Ghee is similar in composition to AMF
(Table 9.3 ), but is produced by a high -
temperature process that confers a character-
istic fl avor. There are many indigenousthe temperature of the molten fat is much less
than the thermodynamic equilibrium tem-
perature. Slow, steady crystal growth is
encouraged, with careful temperature control
and minimal agitation to increase selectivity
and avoid secondary nucleation, which would
lead to an increase in small crystals and high
viscosity, thus retarding crystal growth. A
slow rate of cooling results in regular milk
fat crystals that are easily fi ltered.
The semi - solid milk fat slurry is separated
by vacuum or pressure fi ltration but entrain-
ment of liquid fat within the crystal fraction
decreases fi ltration quality and impairs the
properties of the fraction. Processing factors
also affect the effi ciency of this part of the
process, and these have been investigated by
Vanhoutte et al. (2002, 2003). They reported
that longer crystallizer residence times
induced lower crystal growth and led to
longer fi ltration times and lower oil entrap-
ment, although yield was not affected. Higher
agitation rates decreased fi ltration quality and
increased yield of the high - melting fraction.
Processing parameters such as fractionation
temperature, crystallizer residence time, and
agitation rate can be manipulated to produce
fractions with different melting properties.
The membrane press fi lter is the preferred
fi ltration technique in dry fractionation.
Standard membrane press fi lters operate up
to maximum pressures of 400 to 800 kPa and
provide better separation than vacuum fi lters.
The general principle of membrane fi lters is
that a membrane is used to press the crystal
slurry against (for example) a fl at plate
arrangement. The crystal slurry is held in a
pouch between the membrane and plate and
is pumped into the pouch under pressure.
This squeezes out the majority of the liquid
soft fraction and further pressure is then
applied to squeeze out most of the remaining
oil. The plates used to fi lter milk fat fractions
are fl exible membranes with a dimpled
surface for easy drainage, unlike the rigid
plate system used in the edible oil industry.
Pressures up to 3,000 kPa are routinely used