Chemistry Reference
In-Depth Information
increase in cross-linking of whey proteins and development of non-enzymatic
browning were attributed to the reaction of autoxidation products (free radi-
cals) with amino groups in proteins, explaining the insolubility arising in milk
powders during long-term storage (Stapelfeldt et al., 1997). The decrease in the
solubility of proteins in whey protein powders was shown to be caused mainly
by the formation of intermolecular disulphide bonds, under conditions where a
decrease in T
g
results from hydrolysis of the proteins and increase of water
content of the powder during storage (Zhou and Labuza, 2007).
11.7.
Conclusion
In dairy products, as in foods in general, the implications of water are multi-
ple. Water activity and glass transition are the two concepts that are most
commonly referred to for explaining the role of water in many aspects of the
product characteristics and behaviour during processing and storage. The
relevance of both approaches, thermodynamics and material science, has
been strongly questioned. They currently appear, however, as forming a
coherent knowledge base in ''Food Material Science'' (Karel, 1999). Accord-
ing to this author, ''Perhaps the only concept in food technology with equal or
greater impact was the development of heat processing criteria based on heat
transfer parameters combined with kinetic equations relating rates of thermal
inactivation of microorganisms to temperature''. The applicability and limits
of both approaches are being recognized. Their respective relevance is limited
to water content/temperature domains depending on the product and on the
objectives (ISOPOW, 2000). Moreover, whereas the usefulness of these con-
cepts is to provide guidelines for product and processing design, abusive
generalization must be avoided. Examples may be, concerning a
w
, the need
to consider specific solute effects or additional effects due to molecular
mobility. Concerning the glass transition-associated concepts, WLF kinetics
should be applied only when conditions are fulfilled (i.e. constant concentra-
tion). It should also be remembered that the molecular mobility of all con-
stituents does not always obey WLF kinetics or that other parameters than
mobility may play a role in reaction kinetics. One must remark that water
retention, which is of particular importance for dairy products, cannot be
predicted based on a
w
or water dynamics. Water interactions with other
constituents have only an indirect role on this characteristic, which is con-
trolled mainly by the product structure (ISOPOW, 2000).
Many questions remain to be answered. Properties of water itself are
not fully understood, even though the ''anomalies'' in its physical properties
do not mean that water behaviour is unique when compared to other liquids
(Angell et al., 2000). The situations in which the properties of water are the
