From Milk to Dairy Products 61
Fat globule crystallization
Cream storage/aging at low temperature aims to induce the partial
crystallization of fat, thereby promoting phase inversion. By strict control of
the thermal cycle, it also adapts the consistency of butter to the seasonal and
geographical variability in milk fat composition. In practice, there are two
types of aging:
– low-temperature aging for winter cream,
whereby the cream is
immediately cooled to 6–7°C allowing the formation of many small fat
crystals;
– high-temperature aging for summer cream whereby temperatures are
adapted to obtain large fat crystals.
After aging, the solid and liquid fat ratio in cream is relatively stable.
Cream fermentation
Cream fermentation aims to acidify cream, allowing the development of a
marked and typical aroma, promote phase inversion by decreasing the surface
potential of fat globules at low pH, and ensure biological protection against
microorganisms that can degrade butter. The major disadvantage of cream
fermentation is that it generates an acidic and
aromatic by-product after
churning (buttermilk), which is difficult to stabilize and process further. This
was the driving force behind the developments in butter technology and the
introduction of the NIZO process.
Inoculation of cream with 3–5% lactic acid bacteria is achieved using a
dosing pump. It can be performed either at the beginning of cream aging
(before fat globule crystallization), resulting in pH values below 5.0, or after
fat globule crystallization. Currently, the desired final acidity of butter is
significantly less than it was in the past. Fermentation
is usually carried out
below 15°C for 10 – 12 hours. When the pH reaches a value close to 5.5–5.8,
ripening is slowed by cooling the cream to 8°C. Butter produced in such a way
has a storage pH ranging from 5.2 to 5.6.
Phase inversion
Phase inversion involves transforming ripened cream, an oil-in-water
emulsion, into butter, a water-in-oil emulsion. Phase inversion is performed by
churning, or vigorous agitation, at a temperature corresponding to the
optimum ratio of crystallized and liquid fat (normally 10 – 13°C for churning
62 Handbook of Food Science and Technology 3
winter cream and 7 – 10°C for summer cream). During churning, air bubbles
are incorporated into the cream. The air bubble interface is first stabilized by
(non-homogenized) fat globules. When they become insufficient in number to
cover
the newly-created interface, the foam collapses causing a rapid
convergence of fat globules [VAN 01]. Coalescence of fat globules is
promoted by the reduction of its surface charge depending on cream acidity
and the presence of fat crystals deforming the fat globule surface. The release
of liquid fat contained in the fat globules causes an agglomeration of fat
crystals, intact fat globules and fat globule fragments in the form of granules.
When a sufficient amount of liquid fat has been released, the granules are
converted into butter grains in which droplets
of buttermilk and small fat
globules are dispersed. The emulsion is then rapidly reversed and the
buttermilk is expelled. After washing and optional salting, the butter is
kneaded to compact the butter grains and ensure homogenization by evenly
distributing the aqueous phase and salt.
Conventional churning is carried out batch-wise in a barrel churn rotating
about a horizontal axis. It is generally filled to 40–50% of its volume with
ripened cream. Rotation ensures the incorporation
of air into the cream and
phase inversion. The churn has an outlet for releasing butter-cream and wash
water. Continuous churns, or butter-making machines, operate according to
the same principle as conventional churns, but without interruption. A butter-
making machine consists of a cooled cylinder containing a rotating beater that
incorporates air resulting in phase inversion and a tilted kneading cylinder
containing two counter-rotating augers that compress and release the butter.
The butter is generally washed and kneaded under vacuum to limit the risk of
oxidation.
1.3.5.3.
NIZO butter
The NIZO (Netherlands Institute for Dairy Research) method is used to
prepare butter from cream that has not been fermented. Apart from this
exception, all other processing stages remain the same as for the continuous
manufacture of butter. Acidifying and flavoring agents (NIZO mixture) are
added at a rate of approximately 0.8 – 1.25% to sweet butter after kneading.
The NIZO mixture is prepared under aerobic conditions by vigorously mixing
around 40% of a lactic acid concentrate (lactic acid content of the concentrate
close to 18%)
obtained by culturing
Lactobacillus helveticus
in whey, and
around 60% of a mesophilic lactic starter (
Lactococcus lactis
,
Lactococcus
cremoris
and
Lactococcus diacetylactis
). Intense oxygenation of the mixture is
From Milk to Dairy Products 63
favorable for diacetyl production, characteristic of the butter aroma. The final
mixture is very acidic and no longer contains live bacteria. This first injection
is followed by a second injection of live bacterial culture consisting of
acidifying strains of
Lactococcus lactis
and
Lactococcus cremoris
as well as a
different aromatic strain of the previous culture,
Leuconostoc cremoris
.
Leuconostoc cremoris
is able to consume excess acetaldehyde, responsible for
the “yoghurt” taste in the NIZO mixture (Figure 1.28).
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