SUMMARY
The influence of air temperature, microwave application and vacuum
impregnation or pulsed vacuum osmotic dehydration as pre-treatments on
convective drying and rehydration kinetics of apple and strawberry and also on
some characteristics of the dried and rehydrated samples has been studied. It
was observed that air drying at 50ēC was faster than this at 30ēC, which implies
a lower degree of sample’s browning. Also, the higher air temperature induces a
greater change on the pectin solubility, as a result of an increase on the water
soluble fraction and a decrease on both the oxalate soluble fraction and the
residual pectins, which implies a higher mechanical resistance of the dried
samples. Microwave application to hot air drying provokes a great reduction on
process time and changes on pectin solubility similar but greater than those
observed when increasing the air temperature on the convective drying. Hence,
when applying microwaves, dried strawberry and apple showed a higher
mechanical resistance and moreover, strawberry kept better the initial shape.
However, the structural damage associated to dehydration is reflected on the
lower mechanical resistance of the corresponding rehydrated samples and also
in a lower capacity to retain the liquid phase. On the other hand, microwave
application induces an increase in luminosity of the strawberry, which shows an
anthocyanin content loss. Related to pre-treatments, vacuum impregnation of
apple increases translucency and and remarks the colour changes that take
place during drying, although the osmotic pre-treatment did not affect
significantly its colour. Both kinds of pre-treatments implied higher process
times when using convective drying but no significant differences were found
when applying microwaves. Nevertheless, both promoted pectic solubilization
which, together with the higher sugar content when dried, could explain the
higher mechanical resistance observed on the pre-treated samples. At the
same time, the sugar added during the osmotic pre-treatment contributes to a
better mechanical response of the tissue when rehydrated and to a lower
decrease on the sample volume after drying. Taking into account all these
considerations, it is recommended to use these pre-treatments and microwave
application to hot air drying in order to obtain dried apple or strawberry with a
high mechanical resistance, although colour changes would be greater in these
cases, especially in apple. The higher the sugar introduced on the product with
the pre-treatment, the higher the resistance of the dried fruit to fracture, but
there is also a higher probability for obtaining a product at rubbery state. If the
objective of the process is mainly to obtain a product with good response to
further rehydration, osmotic pre-treatment is recommended as well, but not
microwave application to convective drying.