20 Biochemistry of Milk Processing 477- Inactivation of indigenous alkaline phosphatase
and plasmin at pressures 400 MPa;
lactoperoxidase is not inactivated by treatment at
700 MPa. - Reduced rennet coagulation time of milk,
reduced 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 particular 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. 2004b) - Reduced stability of milk to coagulation by
ethanol (Johnston et al. 2002, Huppertz et al.
2003).
HP treatment can also be applied to cheese (for
review see O’Reilly et al. 2000a). A patent issued in
Japan in 1992 suggested that the ripening of Ched-
dar cheese could be significantly accelerated by
treatment at 50 MPa for 3 days; however, subse-
quent research (O’Reilly et al. 2001) has failed to
substantiate this claim. There have been studies on
the effects of high pressure on the ripening of many
other cheese varieties; varying degrees of accelera-
tion have been reported [e.g., Saldo et al. (2002)
reported effects of HP treatment on ripening of
cheese made from caprine milk]. Of particular inter-
est may be positive effects of HP treatment on the
functionality of mozzarella cheese (Johnston et al.
2002, O’Reilly et al. 2002). HP may also be used
to modify the microbial population (either starter
or contaminant) of cheese (O’Reilly et al. 2000b,
2002).
Another technology which may be of potential
interest for dairy processing in the future is high-
pressure homogenization (HPH), which works like
conventional homogenization, but at significantly
higher pressures, up to 250 MPa; a related process,
microfluidization, is based on the principle of colli-
sions between high-speed liquid jets. As well as re-
ducing the size of oil droplets in an emulsion, HPH
may inactivate enzymes and microorganisms and
denature proteins in food. Many effects of HPH are
due to the extremely high shear forces encountered
by a fluid being processed; however, there is also a
significant heating effect during the process. Recent
studies have indicated that HPH significantly inacti-
vates bacteria in raw bovine milk and affects its ren-
net coagulation properties, fat globule size distribu-
tion, 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 microorganisms but has relatively
few other effects; Deeth and Datta 2002). Some other
processes, such as ultrasonication, irradiation, addi-
tion of antimicrobial peptides or enzymes, and addi-
tion of carbon dioxide, have been studied for poten-
tial application to milk (Datta and Deeth 2002).
Arguably, HP processing is the most likely of the
novel processes to be adopted by the dairy industry
in the near future, due to the availability of equip-
ment for commercial processing.CONCLUSION AND SUMMARY
Milk is a very complex raw material, with consti-
tuents and properties that are sensitive to applied
stresses such as heat and changes in pH or concen-
tration. Changes that can occur include inactivation
of enzymes, mineral distribution and equilibria, and
denaturation of proteins. The extent of such changes
and the consequences for the properties and stability
of milk depend on the severity of the treatment.
As well as physicochemical changes that occur
during processing, many dairy products involve com-
plex enzymatic pathways, such as those involved in
cheese ripening; exogenous enzymes may be added
to milk to achieve a variety of end results. Milk can
also be fractionated by any of a range of complex
technological processes to yield a broad portfolio of
food ingredients.
In conclusion, the processing of milk represents
perhaps one of the most complex fields in food sci-
ence and technology, and while many underpinning
principles have been characterized, much research
remains to be done in several areas.REFERENCES
Andrews AT, Anderson M, Goodenough PW. 1987. A
study of the heat stabilities of a number of indige-
nous milk enzymes. J Dairy Res 54:237–246.
Angulo AJ, Romera JM, Ramirez M, Gill A. 1997.
Determination of cholesterol oxides in dairy prod-
ucts. Effect of storage conditions. J Agric Food
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