According to the quality parameters determined in the present
study, the application of US during convective drying is an adequate
procedure to obtain dried strawberry samples with high
quality and appropriate microbiological stability. Losses of lysine
and arginine due to their involvement in the Maillard reaction
were similar to those observed in conventionally dried (not UStreated)
strawberries. Moreover, the amounts of 2-FM-AA were,
in all cases, lower than those of commercial dried strawberries
and other fruit samples. Values of vitamin C retention were very
high (P65%), even under the most severe conditions used (US at
70 C and 60 W). A combined effect of US and heat was observed,
since the lowest retention of vitamin C was found in US-assisted
dried samples at high temperature. Furthermore, the obtained values
of vitamin C retention and rehydration properties in US treated
strawberries were within the ranges reported in the literature for
convectively-dried fruits and vegetables. Finally, in samples treated
at 70 C with and without US, during the storage at ambient
temperature, no changes in the microbiological counts were
detected with respect to the initial processed samples, indicating
the stability of US-treated samples for at least 6 months. Moreover,
these samples showed similar evolution in the losses of vitamin C
(approx. 50%) during the storage period. According to the data here
shown, the best processing conditions (temperature and electric
power applied to the air-borne US transducer) could be 50 C and
30 or 60 W of US power. This is the first study on the evaluation
of quality indicators of fruits, particularly strawberries, dehydrated
by US-assisted drying. The results of this work point out that US is
a suitable example of an emerging and environmentally friendly
technology that accelerates the convective drying, allowing the
obtainment of dried strawberries with premium quality.
A