As the Maillard reaction is known to be favoured during
the dehydration process systems [13] under conditions of
pH ranging 5–7, with intermediate moisture content and
temperatures over 50 C using long processing times, a few
sets of simple experiments were performed on pear tissues
to relate the occurrence of Maillard compounds with the
reddish brown colouration.
In order to test the influence of pH on the development
of the colour of the pears, pieces of freeze-dried pulp of a
fresh pear were immersed in buffer solutions in the range
of the pH scale. It was observed that the tissues immersed
in the solutions with higher pH became darker than the
other ones. The major colour variations were observed in
the interval of pH 4–6, whereas at pH 5.0, the colour of the
tissue approached the reddish brown colour characteristic
of the traditional product (Fig. 5a).
The freeze-dried pear tissues that have been immersed in
the buffer solution at pH 5.0 were placed (in a wet form) in
two plastic containers, one exposed to sunlight and the
other one was placed in the dark. This experiment allowed
to observe that the sample exposed to sunlight presented an
increase in the intensity of the colour when compared with
the sample placed in the dark (Fig. 5b). Because water
condensation was observed over the walls of the containers
where the tissues were exposed to the light and not in the
control ones, it was hypothesised that the increase in
temperature of the wet tissues could be an important factor
for pear tissues colouration. To test this hypothesis, pieces
of freeze-dried pear tissues were immersed in the buffer
solution at pH 5.0 and the wet material was cut in two
halves. One part was placed in a Petri dish that was closed
to preserve the moisture and the other one was left in direct
contact with the air. Both samples were placed in an oven
at 30 and 50 C. It was observed that the tissues that were
maintained closed, keeping the moisture, presented more
intense colourations and the colour intensity was higher for
50 C than for 30 C (Fig. 5c).
These results allow concluding that the pH, the drying
temperature and the tissues moisture are important
parameters for the development of the characteristic colour
of the traditional product.