How does one make an oil-in-water emulsion that is stable during
storage but that can be whipped into a topping? The first question
then is: what happens during a whipping process that results in a
suitable topping? Several product and process variables affect the
result.All of these examples have in common that knowledge of physical
chemistry is needed to understand what happens and to solve the problem.
Physical chemistryprovides quantitative relations for a great number
of phenomena encountered in chemistry, based on well-defined and
measurable properties. Its theories are for the most part of a physical
nature and comprise little true chemistry, since electron transfer is
generally not involved. Experience has shown that physicochemical aspects
are also of great importance in foods and food processing. This does not
mean that all of the phenomena involved are of a physical nature: it is
seen from the examples given that food chemistry, engineering, and even
microbiology can be involved as well. Numerous other examples are given
in this book.
The problems encountered in food science and technology are
generally quite complex, and this also holds for physicochemical problems.
In the first place, nearly all foods have a very wide andcomplex
composition; a chemist might call them dirty systems. Anyway, they are far
removed from the much purer and dilute systems discussed in elementary
textbooks. This means that the food is not in thermodynamic equilibrium
and tends to change in composition. Moreover, several changes may occur
simultaneously, often influencing each other. Application of physicochem-
ical theory may also be difficult, since many food systems do not comply
with the basic assumptions underlying the theory needed.
In the second place, most foods are inhomogeneous systems.
Consequently, various components can be in different compartments,
greatly enhancing complexity. This means that the system is even farther
removed from thermodynamic equilibrium than are most homogeneous
systems. Moreover, several new phenomena come into play, especially
involving colloidal interactions and surface forces. These occur on a larger
than molecular scale. Fortunately, the study of mesoscopic physics—which
involves phenomena occurring on a scale that is larger than that of
molecules but (far) smaller than can be seen with the naked eye—has made
great progress in recent times.
In the third place, a student of the physical chemistry of foods has to
become acquainted with theories derived from arange of disciplines,asa
look at the table of contents will show. Moreover,knowledge of the system
studied is essential: although basic theory should have universal validity, the