From Milk to Dairy Products 59
using a two-stage homogenizer. The homogenization pressure of the first
stage, between 13.5 and 20 MPa, is used to create the interface and decrease
fat
globule size; the second stage, set at a homogenization pressure of 10 –
20% of that of the first stage, separates the aggregates of fat globules formed
during the first stage.
Cream is heat-treated in plate heat exchangers (before or after the
homogenization); the exchange area is about three times larger than that used
in the treatment of milk due to the lower heat transfer coefficient in cream. In
addition, temperatures are higher generally due to higher microbial load in
cream and to the high thermal resistance of microorganisms in the presence of
fat: the intensity of the heat treatment increases with increased fat content. In
the case of pasteurization, the time/temperature combination is approximately
50–10 s/80–100°C. Heat treatment is particularly challenging, since cream is a
fragile emulsion and rapid temperature variations can significantly alter the
properties of the emulsion.
Ripening
Ripened or sour cream is a thick cream. Pasteurized
cream is inoculated
with up to 0.5% starter culture consisting of a combination of acidifying,
aromatic (
Lactococcus lactis
subsp.
Lactis cremoris
,
Lactococcus lactis
subsp
Lactis diacethylactis
,
Streptococcus thermophilus
) and sometimes thickening
strains (
Leuconostoc
), which produce exopolysaccharides generating thick
creams at less acidic pH. The ripening phase takes 12 – 18 h at temperatures
between 12 and 22°C. Acidification causes the gradual destabilization of
casein micelles, some of which are adsorbed
on the surface of the
homogenized fat globules, resulting in the formation of a network of proteins
and fat globules, and a thickening of the cream. The most significant changes
of texture occur at a pH below 5.0–5.2.
Whipping
Whipped cream is a foamed emulsion in which air bubbles are
incorporated into a network of partially coalesced fat globules; in the presence
of emulsifiers (mono- and diglycerides) and stabilizers (gelatine, carrageenan,
etc.), this network ensures the rigidity and stability of the foam.
Cream intended for whipping is first homogenized, which increases the
number of fat globules, and heat-treated before being refrigerated (4 – 10°C)
for several hours (approximately 20 h) to promote fat globule crystallization.
60 Handbook of Food Science and Technology 3
During the aging of cream at low temperature, emulsifiers gradually displace
adsorbed proteins from the homogenized fat globule surface, thereby reducing
fat globule stability [GOF 17]. During whipping, the collision of destabilized
fat globules promotes partial coalescence. Partially coalesced homogenized fat
globules move to the air interface and form a
network that stabilizes air
bubbles. In addition, stabilizers increase the viscosity of the non-fat phase and
limit drainage by interaction with proteins of the non-fat phase and adsorbed
proteins on the fat globules.
1.3.5.2.
Butter
Butter consists of a continuous liquid fat phase in which triglyceride
crystals, small fat globules, aqueous phase droplets and air bubbles are
dispersed (Figure 1.27). It is made from cream,
typically pasteurized,
containing 40 – 50% fat, which is traditionally ripened (cultured butter) and
then churned to induce phase inversion. Ripening includes two combined
operations:
– fat globule crystallization to develop the rheological properties of butter;
– cream fermentation to develop aroma and decrease the pH of cream.
These combined operations occur in cultured butters obtained by traditional
batch churning or a continuous manufacturing process (Fritz process). The
manufacture of cultured butters has gradually been replaced by the NIZO
method, which is more flexible and economical,
whereby fat globule
crystallization and the production of flavor and acid are separate.
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