BLBS102-c25 BLBS102-Simpson March 21, 2012 13:23 Trim: 276mm X 219mm Printer Name: Yet to Come
482 Part 4: MilkToday, there are two principal classes of non-butter 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 advantage, and (2) reduced-fat
products containing varying levels of milk fat (Frede 2002b).
Such products may be manufactured either in a modified con-
tinuous buttermaking system (butter technology) or in a votator
scraped-surface cooler system (margarine technology). Gener-
ally, all products are made by preparation of aqueous (e.g., skim
milk or cream) and lipid phases, each containing the appropriate
ingredients, followed by mixing and phase inversion of the ini-
tial o/w emulsion 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/or proteins, may
be added for this purpose (Keogh 1995, Frede 2002b).
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 generally 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 the technology of spreads include
the production of triple-emulsion (o/w/o) products (in which
the droplets of aqueous phase in the product contain very small
fat globules) (Frede 2002b). These systems have three distinct
phases, and need two or more emulsifiers and/or the use of
proteins or polysaccharides as stabilisers/thickeners, as they can
be quite unstable due to the presence of at least two interfaces
(Kanouni et al. 2002, O’Regan and Mulvhill 2010).Production and Fractionation of Milk FatThe most purified form of milk fat commercially produced is an-
hydrous milk fat (AMF), which consists of>99% triglycerides,
and is produced in many countries. The production of AMF re-
quires the removal of water and water-soluble components of
milk, and may be achieved either by successive high-speed cen-
trifugation, with phase-inversion at high fat levels, or by starting
with butter, which is melted and centrifuged to recover the pu-
rified fat phase. Lipid oxidation is a significant concern during
AMF manufacture, and the presence of oxygen during the pro-
cess must be avoided.
AMF may be fractionated to yield fractions of milk fat with
different desirable properties (Illingworth et al. 2009), usually
on the basis of melting point. The most common industrial pro-
cesses for milk fat fractionation involve cooling melted milk fat
to temperatures where crystallisation of some of the fat occurs,
facilitating the removal of a high melting point fraction (stearin
crystals) from the low melting point fraction (olein). This re-
moval may be by centrifugation or filtration through plate-and-
frame membrane units. This basic fractionation process may
be improved through the use of detergents or solvents, such as
acetone.
There are also a number of methods for removal of cholesterol
from dairy products, either through enzymatic, microbial, chem-ical (solid–liquid extraction, complexation) or physical means
(distillation and crystallisation, supercritical fluid extraction).
There have been several studies of the application of such
methods for the removal of cholesterol from dairy products,
including homogenised milk, butter and butter oil, and reduced-
cholesterol dairy products have appeared on the marker since
the 1990s; however, greater consumer preference for reduced-
fat versus specifically reduced-cholesterol products may limit
their widespread appeal (Sieber & Eyer 2011).Lipid OxidationLipids with double bonds (i.e., unsaturated fatty acids) are inher-
ently susceptible to attack by active O 2 , that is, lipid oxidation.
Oxidation can give rise to a range of undesirable flavour com-
pounds (such as aldehydes, ketones and alcohols that cause ran-
cid off-flavours), and possibly result in toxic products. Oxidation
is dependent on factors such as availability of oxygen, exposure
to light, temperature, presence of pro- or anti-oxidants and the
nature of the fat (O’Brien and O’Connor 1995, 2002, 2011).
Milk fat contains a high level of monounsaturated fatty acids
(although less than many other fats), but a low level of polyun-
saturated fatty acids. It is susceptible to oxidation but raw milk
samples differ in susceptibility to oxidation as follows:
Spontaneous: Will develop oxidised flavour within 48 hours
without the addition of pro-oxidant metals, such as iron or
copper
Susceptible: Will not oxidise spontaneously, requires con-
tamination by pro-oxidant metals
Non-susceptible: Will not oxidise even in the presence of
iron or copperThe reasons for the difference between milk samples are not
clear, but are likely to be related to lactational and dietary factors,
and the enzyme xanthine oxidase in milk may be critical.
Of particular current interest is the oxidation of the unsatu-
rated alcohol, cholesterol, in milk; consumption 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 product, but have been detected
in WMP, baby foods, butter and certain cheese varieties, albeit at
levels that probably do not pose a risk to consumers (Sieber et al.
1997, RoseSallin et al. 1997). The formation of COPs is influ-
enced by factors such as temperature and exposure of the food
to light (Angulo et al. 1997, RoseSallin et al. 1997, Hiesberger
and Luf 2000).Ice CreamIce cream, probably the most popular dairy dessert, is a frozen
aerated emulsion. The continuous phase consists of a syrup
containing dissolved sugars and minerals, while the dispersed
phase consists of air cells, milk fat (or other kinds of fat)
globules, ice crystals and insoluble proteins and hydrocolloids
(Marshall 2002). The structure of ice cream is particularly com-
plex, with several phases (e.g., ice crystals, fat globules and air