The differences in electrical conductivity values are
reflected in the heating curves presented in Fig. 9.
Strawberry topping has considerably lower electrical
conductivity (5% of the electrical conductivity of P1
and 19% of the electrical conductivity of P2) and
a consequently lower heating rate (0.3 8Csy1 comparing
to 1.60 8Csy1 for P2 and 6.22 8Csy1 for the pulp P1)
that would indicate the use of a different ohmic heater
When comparing samples treated with ohmic heating
alone (OH) and with a combination of conventional
heating and ohmic heating (CHqOH) or ohmic heating
plus ohmic heating (OHqOH) it is evident that the
electrical conductivity decreases after the first thermal
treatment for both cases (Figs. 10 and 11). The raw
samples (OH) present the highest electrical conductivity
value while the lowest value is found after two ohmic
cycles. These results are opposite from the ones reported
by Wang and Sastry (1997) in vegetable samples. The
structural changes caused by heating that should lead to
an increase of electrical conductivity are probably counterbalanced
by water evaporation during the heating process,
reducing the fluid motion and the electrical
conductivity values. This has been confirmed by the
measurement of the Brix values before and after the
treatment (Brix increased from 14.5 to 16.0 for pulp P1
and from 26.5 to 28.5 for pulp P2). The decrease in
electrical conductivity with thermal treatments is less
evident for P1 probably because starch gelatinization
(2.5% wyw) and the resulting change in water binding
capacity, thus preventing its loss to the atmosphere; also,
viscosity may increase, which may reduce electrical
conductivity. In formulation P2 there are no hydrocolloids
present and therefore dehydration is facilitated.
The sample P1 pre-treated by conventional heating
showed higher values of electrical conductivity than
when pre-treated by ohmic heating. This finding is
consistent with work relating to starchy materials. Previous
work with starchy materials (Wang & Sastry,
1997) has shown that preheating by conventional methods
may result in greater starch gelatinisation and a
greater subsequent increase in electrical conductivity,
than ohmically heated samples (where heating is very
rapid and sufficient time may not be available for
complete gelatinisation). On the other hand, P2 presented
higher electrical conductivity when preheated by
ohmic heating. This behaviour can be explained by
different formulation and lower electrical conductivity
values than P1 so the ohmic process might induce
structural changes causing the obtained increase.