Chemistry Reference
In-Depth Information
layer on the surface until the surface is fully covered. Usually, this results in a
reduction in interfacial tension that can be expressed by the Gibbs equation
d g
ΒΌ
RT Gd ln a
(5)
where G is the excess concentration of solute at the interface over that in the
bulk solution and a is the activity of the solute in the bulk phase. Many
proteins are good surfactants and may be adsorbed at the interface to form
either a monolayer or a multilayer, more or less irreversibly, depending on the
individual protein (Walstra and de Roos, 1993). When protein molecules are
adsorbed at the interface, they may unfold and rearrange their conformation
or even denature to suit the new environment; Gibbs' equation no longer
applies (Walstra and Jenness, 1984).
Methods for determining surface tension were described and reviewed
by Harkins (1952) and Whitnah (1959). These are based on the following
principles: (1) the increase in the height of liquid in a capillary, (2) the weight
or volume of drops formed when a given amount of liquid is allowed to flow
from a capillary tip, (3) the pressure required to force a bubble of gas through
a nozzle immersed in the liquid and (4) the force required to pull a ring or
plate free from the surface of a liquid. Methods involving a ring or plate have
been used most commonly for milk and milk products.
The surface tension of milk is a fundamental physical property that
relates to the stability of foams, emulsions and films; it affects fractionation,
concentration and drying processes. Milk contains several surface-active
components (casein micelles, phospholipids, whey proteins and fatty acids)
that can readily adsorb at an air-water interface and reduce surface tension;
salts and lactose do not contribute to surface tension (Walstra and Jenness,
1984). Jenness and Patton (1959) reported surface tension values for several
fractions from milk, with rennet whey, skim milk, whole milk, 25% fat cream
and sweet-cream buttermilk having values of 51-52, 52-52.5, 46-47.5, 42-45
and 39-40 mN m
1
, respectively.
The surface tension of milk decreases during the time required for mea-
surement until it reaches a base value, although the real equilibrium is never
reached or is reached very slowly (Michalski and Briard, 2003). Values in the
range 40-60 mN m
1
(average
52)at208C have been reported (Aschaffenburg,
1945; Dunkley, 1951; Watson, 1958; Parkash, 1963; Sharma, 1963; Calandron
and Grillet, 1964; Bertsch, 1983; Kristensen et al., 1997; Michalski and Briard,
2003), depending on the measurement technique. The surface tension decreases
with increase in temperature for both skim milk and whole milk (Table 15.2).
Other factors that influence the surface tension of milk include fat
content, homogenization and temperature history. Surface tension decreases
with increasing fat content up to
4% but no further decrease occurs at higher
