472 Part IV: Milk
by Novo Nordisk); these have 5–20 times the flavor
intensity of mild Cheddar cheese.
MILK LIPIDS
PRODUCTION OFFAT-BASEDDAIRYPRODUCTS
Milk fat is a complex mixture of triglycerides, which
in milk, is maintained as a stable oil-in-water (o/w)
emulsion by the presence of the milk fat globule
membrane (MFGM), which surrounds milk fat glob-
ules and protects the lipids therein from mechanical
or enzymatic damage.
One of the oldest dairy products is butter, a fat-
continuous (water-in-oil, w/o) emulsion containing
80–81% fat, not more than 16% H 2 O, and usually,
1.5% added salt. Production of butter from cream
requires destabilization of the emulsion and phase
inversion, followed by consolidation of the fat and
removal of a large part of the aqueous phase (Frede
and Buchheim 1994, Frede 2002a); this is somewhat
analogous to the manufacture of cheese, but the key
component to be preferentially concentrated is fat
rather than protein, and destruction of the factor re-
sulting in its stability in milk is achieved by mechan-
ical rather than enzymatic means. The manufacture
of butter has been the subject of several recent
reviews (e.g., Keogh 1995, Lane 1998, Ranjith and
Rajah 2001, Frede 2002a).
In traditional butter manufacture, cream is
churned (mixed) in a large partially filled rotating
cylindrical, conical, or cubical churn, which dam-
ages the MFGM. Mechanically damaged fat glob-
ules become adsorbed on the surfaces of air bubbles
(flotation churning) and gradually coalesce, being
bound together by expressed free fat, to form butter
grains. Eventually, the air entrapped within such
grains is expelled, and churning generally progress-
es until the grains have grown to approximately the
size of a pea (Frede and Buchheim 1994, Vanapilli
and Coupland 2001). In a batch process, growth of
grains is monitored visually and audibly (impact of
masses of grains on the churn walls); at this point,
the buttermilk (essentially skim milk but with a high
content of sloughed milk fat membrane components
such as phospholipids) is drained off and the butter
worked by repeated falls and the resulting impaction
within the rotating churn. After a set time, salt is
added and worked throughout the butter.
With the exception of very small scale plants, most
dairy factories today use continuous rather than
batch butter makers. In the most common system, the
Fritz system, each stage of the process occurs in a
separate horizontal cylindrical chamber, with prod-
uct passing vertically between stages. In the churning
cylinder, rotating impellers churn the cream very rap-
idly, and in the second stage butter grains are consol-
idated and the buttermilk drained off. In the final
stage, the butter mass is worked; this chamber is
sloped upwards and, while augers transport the butter
and squeeze it through a series of perforated plates,
the buttermilk drains off in the opposite direction.
Midway through the working stage, salt (generally as
a concentrated brine) is added, and becomes distrib-
uted in small moisture droplets with a high local salt
concentration, which acts to flavor and microbiologi-
cally stabilize the butter. The moisture present in the
final droplets comes originally from milk serum,
with a small contribution from water added in brine.
The by-product buttermilk is rich in milk fat glob-
ule membrane materials, including phospholipids,
and is used as an ingredient in many food products;
however, due to its high content of polyunsaturated
fatty acids, it deteriorates quite rapidly due to oxida-
tion (O’Connell and Fox 2000). The buttermilk may
contain high levels of biologically active (e.g., anti-
carcinogenic) membrane-derived lipids, including
glycosphingolipids and gangliosides (Jensen 2002).
Buttermilk isolates may also have potential applica-
tion as emulsifying agents (Corrideg and Dalgleish
1997).
In some countries, a significant amount of butter
is produced from cream that has been ripened with
lactic acid bacteria (lactic or fermented butter). Bac-
terial acidification enhances the keeping quality of
the butter and changes the flavor, through produc-
tion of diacetyl. However, the production of lactic
butter leads to the production of acidic buttermilk as
an unwanted by-product, and in recent years alterna-
tive technologies have been developed for the pro-
duction of lactic butter. One of the most successful
is the NIZO method, in which a sweet (nonlactic)
cream is used, and an aromatic starter and concen-
trated starter permeate are added to the butter grains
midway through the process. This process leads to
production of normal buttermilk and gives a well-
flavored product, which is very resistant to autoxida-
tion (Walstra et al. 1999).