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25 Biochemistry of Milk Processing 485reasons for this trend include consumer demand for minimally
processed food products, and ongoing challenges in satisfacto-
rily processing certain food products without significant loss of
quality using existing technologies (e.g., deterioration of nutri-
ent content on heating fruit juices, problems with the safety of
shellfish).
One new process that has received particular attention during
the last decade is high pressure (HP) treatment. The principle of
HP processing involves subjecting food products to a very high
pressure (100–1000 MPa, or 1000–10,000 atm), typically at
room temperature, for a fixed period (e.g., 1–30 minutes); under
such conditions, microorganisms are killed, proteins denatured
and enzymes either activated (at low pressures) or inactivated
(at higher pressures). The principal advantage of HP processing,
which does not rupture covalent bonds, is that low molecular
weight substances, such as vitamins, are not affected and hence
there are few detrimental effects on the nutritional or sensory
characteristics of food.
Although it was first described for the inactivation of microor-
gansisms in milk in the 1890s, by Bert Hite at the Agricultural
Research Station in Morganstown, West Virginia, lack of avail-
able processing equipment meant that HP remained unexploited
in the food industry for most of the twentieth century. Only in the
late 1990s were HP-processed foods launched, such as shellfish
in the United States, fruit juice in France and meat in Spain.
To date, no HP-processed dairy products are available, prob-
ably due, at least in part, to the complexity of the effects of HP
on dairy systems, which necessitate considerable fundamental
research to underpin future commercial applications.
In short, HP affects the properties of milk in several, often
unique, ways (for reviews, see Huppertz et al. 2002, Trujillo
et al. 2002). Key effects include the following: Denaturation of the whey proteins,α-lactalbumin (α-la)
andβ-lg, at pressures≥200 or≥600 MPa, respectively, and
interactions of denaturedβ-lg with the casein micelles.
Increased casein micelle size (by∼25%) after treatment at
250 MPa for≥15 minutes (Huppertz et al. 2004a) and re-
ductions in micelle size by approximately 50% on treatment
at 300–800 MPa.
Increased levels of non-micellarαs1–,αs2–,β- and
κ-caseins after HP treatment at≥200 MPa.
Inactivation of indigenous alkaline phosphatase and plas-
min at pressures≥400 MPa; lactoperoxidase is not inacti-
vated by treatment at≤700 MPa.
Reduced rennet coagulation time of milk and time required
for the gel to become firm enough for cutting, and enhanced
strength of the rennet gel, following treatment at 200 MPa
for up to 60 minutes or 400 MPa for≤15 minutes.
Increased yield of rennet-coagulated cheese curd, in partic-
ular after treatment≥400 MPa.
Increased rate and level of creaming in milk, by up to 70%,
after HP treatment at 100–250 MPa, whereas treatment at
400 or 600 MPa reduces the rate and level of creaming by
up to 60% (Huppertz et al. 2003).
Reduced stability of milk to coagulation by ethanol
(Johnston et al. 2002, Huppertz et al. 2004b),HP treatment can also be applied to cheese (for review see
O’Reilly et al. 2000a). A patent issued in Japan in 1992 sug-
gested that the ripening of Cheddar cheese could be signifi-
cantly accelerated by treatment at 50 MPa for 3 days; however,
subsequent research (O’Reilly et al. 2001) has failed to substan-
tiate this claim. There have been studies on the effects of HP
on the ripening of many other cheese varieties; varying degrees
of acceleration have been reported, for example Saldo et al.
(2002) reported effects of HP treatment on ripening of cheese
made from caprine milk. Of particular interest may be posi-
tive effects of HP treatment on the functionality of Mozzarella
cheese (Johnston et al. 2002, O’Reilly et al. 2002a). HP may
also be used to modify the microbial population (either starter
or contaminants) in cheese (O’Reilly et al. 2000b, 2002b, Voigt
et al. 2010).
In the last few years, the first commercial processes apply-
ing HP technology in the dairy sector have appeared, particu-
larly for high-value functional foods (Kelly and Zeece 2009).
Fonterra in New Zealand have proposed the commercial appli-
cation of HP treatment for processing of bovine colostrum, with
HP achieving a commercially relevant shelf life while retaining
functional activity; to achieve a comparable shelf life using heat
treatment would require heating of sufficient severity to result
in loss of functionality of IgG through denaturation. In addi-
tion, Fonterra have patented a process whereby the shelf life
of probiotic yoghurt may be extended by HP treatment to in-
activate spoilage microorganisms while leaving active specially
selected baro-resistant strains of probiotic bacteria, and a pro-
cess for inactivating microorganisms in preparations containing
lactoferrin.
Another technology that may be of potential interest for
dairy processing in the future is high-pressure homogenisation
(HPH), which works like conventional homogenisation, but at
significantly higher pressures, up to 250 MPa; a related pro-
cess, microfluidisation, is based on the principle of collisions
between high-speed liquid jets. As well as reducing the size
of oil droplets in an emulsion, HPH may inactivate enzymes
and microorganisms and denature proteins in food. Many ef-
fects of HPH are due to the extremely high shear forces en-
countered by a fluid being processed; however, there is also
a significant heating effect during the process. Recent studies
have indicated that HPH significantly inactivates bacteria in raw
bovine milk, and affects its rennet coagulation properties, fat
globule size distribution and enzyme profile (Hayes and Kelly
2003a, b).
Another novel process that may be applied to dairy products
is pulsed electric field treatment (which inactivates microor-
ganisms but has relatively few other effects; Datta and Deeth
2002b). Some other processes, such as ultrasonication, irradia-
tion, addition of antimicrobial peptides or enzymes and addition
of carbon dioxide, have been studied for potential application to
milk (Datta and Deeth 2002a). Arguably, HP processing is the
most likely of the novel processes to be adopted by the dairy in-
dustry in the near future, due to the availability of equipment for
commercial processing, but scale of available equipment com-
bined with high cost will remain a significant hurdle (Patel et al.
2008).