Figs. 4–8 show the electrical conductivity changes
with field strength obtained for the different products
tested. For all cases electrical conductivity increases
with temperature although it may not be a linear relation.
A linear relation between electrical conductivity and
temperature is evident for the strawberry pulp P1,
strawberry filling and strawberry–apple sauce (Figs. 4,
7 and 8) but this aspect changes for the strawberry pulp
P2 and topping (Figs. 5 and 6) where a second order
polynomial relation has been obtained. This second order
polynomial relation may be due to the presence of air
in the products. In an unpressurized heater, if air is
occluded in the sample, the air bubbles will expand
with temperature, with the pressure remaining constant.
If ideal gas behaviour is assumed, then, the expansion
will be linear in temperature. This would mean that
bubble volume would increase linearly with temperature.
However, the important parameter for electrical conductivity
is the cross-sectional area of the bubbles in a
plane perpendicular to the electric field. This will
increase roughly as a two-thirds power of the volume.
Thus air (which can be roughly considered to be of zero
electrical conductivity) will increase in area, tending to
reduce the electrical conductivity of the mixture. However,
the continuous phase in itself will increase in
electrical conductivity with the temperature, so the net
effect will depend on the relative contribution of the
components. Thus, as temperature increases, the in
electrical conductivity of air may not be linear with
temperature, so a non-linear model may well be justified.
The increase of electrical conductivity with field
strength is clear for both strawberry pulps (P1 and P2)
and strawberry filling. The heating process causes
membrane destruction and consequently the free water
content increases (Bean, Rasor & Porter, 1960; Halden,
De Alwis & Fryer, 1990; Sasson & Monselise, 1977).
The field strength application results in increasing fluid
motion through the capillaries, which is directly proportional
to electrical conductivity (Halden et al., 1990).
On the other hand, the effect of field strength is not as
evident for the strawberry topping or strawberry–apple
sauce and this behaviour may be either due to the small
quantity of strawberry in products’ formulation or to the
fact that their previous processing already lead to
membrane destruction so the increase of temperature