During microwave-vacuum puffing of blackcurrant fruit slab,
microwave power showed most significant effect on the changes of
dehydration rate, followed by the effect of initial moisture content of
blackcurrant slab. The effect of vacuum pressure was the least. The
results indicated that the change of volume and shape of the final
slab snack products were affected by operation parameters of the
complex processes. The shrinking was due to the dehydration results
on the sample surface. The expansion caused by moisture
evaporation inside the sample dominated the changes of sample
volumes. During MVP, most of the vapor originated from moisture
content inside berry slab that absorbed microwave energy and
propelled the volume expansion up to a stable state during the early
puffing stage. This stage was followed by a dehydration process to
increase the hardness of the slab and to obtain the desired moisture
content of the blackcurrant snack product. According to the volume
of puffed slabs and ratio of diameter to thickness kept stable after
puffing time of 70 s shown in Figs. 6 and 7, two distinct stages were
observed. One stage was inclined to volume expansion with stable
hardness, and the other stagewas the dehydration stage with stable
volume and shape. Above-mentioned explanations provide the
basis for the models constructions about moisture transfer and
microwave energy absorption and conversion, as well as volume
expansion inside puffed berry slab during MVP process. These results
are in favor of the allocation of MVP parameters to produce
high quality snack product in industrial scale application. According
to single factor result, the sequence of the effect of single factor on
expansion ratio was that initial moisture content > microwave
power > vacuum pressure > puffing time. The interaction between
initial moisture content and microwave power, vacuum pressure
has significant effect on the expansion ratio (Liu, 2010).