Chemistry Reference
In-Depth Information
(Jenness and Patton, 1959). The decrease in potential can be identified by the
change in colour of certain dyes added to milk, e.g., methylene blue. These
dyes are reduced to colourless forms with time as the bacteria grow in milk.
Therefore, such changes in colour can be used as an index of microbial
contamination and form the basis of the methylene blue and resazurin reduc-
tion tests for the bacterial quality of milk.
Copper and iron have strong catalytic effects on oxidative processes.
The catalytic effect of copper is up to 100 times greater than that of iron (King
and Dunkley, 1959). Consequently, contamination of milk with copper or
iron increases the E
h
. The initial stage of the reaction involves the oxidation of
ascorbic acid. Vahcic et al. (1992) derived mathematical relationships
between E
h
and the copper, iron and ascorbic acid contents of 72 milk
samples. A discussion of redox reactions and photo-oxidation was presented
by Walstra and Jenness (1984).
15.4.
Surface and Interfacial Tension
At an interface between liquid and air, the behaviour of molecules of the
liquid is different from that of similar molecules in the bulk phase. Mole-
cules in the bulk phase are subjected toattractiveforcesequallyinall
directions by other molecules of the liquid whereas molecules at the surface
or interface experience a net attractive force directed towards the bulk
phase. This inward attraction reduces the number of molecules at the sur-
face, resulting in a decrease in interfacial area. The forces causing a reduc-
tion in surface or interfacial area arereferredtoassurfacetensionor
interfacial tension (g), respectively. The surface tension acting on a liquid
can be considered in two ways, either as a force per unit length acting on a
given length of surface (N m
1
) or as the work required to increase the area
of a surface by a unit amount, referred to as interfacial free energy (J m
2
or
Nm
1
). Surface tension is a characteristic property of a liquid and varies
greatly for different liquids, or for the same liquid but at different degrees of
purity or at different temperatures. With an increase in temperature, the
kinetic energy of the molecules increases and cohesive forces between them
decrease, resulting in a decrease in interfacial tension. For example, the
surface tension of water decreases from 72.6 to 58.9 mN m
1
,withan
increase in temperature from 20 to 1008C.
The presence of surface-active agents, or surfactants, influences the
surface tension of water. Surfactants contain both hydrophobic and hydro-
philic groups and adsorb at the interface with their hydrophobic groups
oriented towards the non-aqueous phase and their hydrophilic groups pointed
towards the aqueous phase. The surfactant molecules form a monomolecular
