20 Biochemistry of Milk Processing 473The high cost of butter was the original reason
that led, in the second half of the 19th century, to
the development of alternatives to butter, such as
margarine; consumer preference for reduced-fat
(i.e., higher perceived “healthiness”) products has
strengthened this trend. A further significant disad-
vantage of butter for domestic applications is the
fact that at typical refrigeration temperatures, butter
behaves essentially as a solid and is not easily
spreadable; moreover, at room temperature it oils off
and exudes water. The spreadability of butter may
be increased by blending with vegetable oils, or by
modifications such as interesterification (Marangoni
and Rousseau 1998).
Today, there are two principal classes of nonbut-
ter, milk fat–based spreads: (1) full-fat products
with partial replacement of milk fat by another (e.g.,
vegetable) fat, with physicochemical, rheological,
economic, or dietary advantages, and (2) reduced-
fat products containing varying levels of milk fat
(Frede 2002b). Such products may be manufactured
either in a modified continuous butter-making sys-
tem (butter technology) or in a Votator scraped-
surface cooler system (margarine technology). Gen-
erally, all products are made by preparation of aque-
ous (e.g., skim milk or cream) and lipid phases, each
containing the appropriate ingredients, followed by
mixing and phase inversion of the initial o/w emul-
sion to a final w/o emulsion. Reduction of the fat
content of the mix requires increased attention to the
structural characteristics of the aqueous phase,
which must increasingly contribute to the texture
and body of the product. Aqueous phase structuring
agents, such as polysaccharides and proteins, may
be added for this purpose (Keogh 1995, Frede
2002).
Butter is a relatively stable product; the small size
and high salt content of the water droplets make
them an inhospitable environment for microbial
growth. Lower-fat spreads, while also being gener-
ally stable products, may have preservatives, such
as sorbates, incorporated into their formulation to
ensure shelf life and safety (Delamarre and Batt
1999).
Some new developments in spread technology
include the production of triple-emulsion (o/w/o)
products (where the droplets of aqueous phase in the
product contain very small fat globules) (Frede
2002b).
LIPIDOXIDATIONLipids with double bonds (i.e., unsaturated fatty
acids) are inherently susceptible to attack by active
O 2 , that is, lipid oxidation. Oxidation can give rise
to a range of undesirable flavor compounds (such
as aldehydes, ketones, and alcohols, which cause
rancid off-flavors) and possibly result in toxic
products. Oxidation is dependent on factors such as
availability of oxygen, exposure to light, tempera-
ture, presence or pro- or antioxidants, and the exact
nature of the fat (O’Brien and O’Connor 1995,
2002).
Milk fat contains a high level of monounsaturated
fatty acids (although less than many other fats), but a
low level of polyunsaturated fatty acids. It is suscep-
tible to oxidation, but raw milk samples differ in
susceptibility to oxidation, according to the follow-
ing classifications:- Spontaneous:Will develop oxidized flavor
within 48 hours without the addition of
prooxidant metals, such as iron and
copper. - Susceptible:Will not oxidize spontaneously,
requires contamination by prooxidant
metals. - Nonsusceptible:Will not oxidize even in the
presence of iron or copper.
The reasons for the differences between milk
samples are not clear, but are likely to be related to
lactational and dietary factors, and the enzyme xan-
thine oxidase in milk may be critical.
Of particular current interest is the oxidation of
the unsaturated alcohol, cholesterol, in milk; con-
sumption of cholesterol oxidation products (COPs)
in the diet is tentatively linked to the incidence
of artherosclerosis (Kumar and Singhal 1991), and
hence these products are regarded as potential health
hazards. COPs are not found in all dairy products,
but they have been detected in whole milk powder,
baby foods, butter, and certain cheese varieties,
albeit at levels which probably do not pose a risk to
consumers (Sieber et al. 1997, RoseSallin et al.
1997). The formation of COPs is influenced by fac-
tors such as temperature and exposure of the food to
light (Angulo et al. 1997, RoseSallin et al. 1997,
Hiesberger and Luf 2000).