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Abstract
Saskatoon berry (Amelanchier alnifolia) is a
high moisture content fruit crop, harvested at 75-80% moisture content (w.b.)
and dried to lower moisture contents for a safe, extended shelf-life and for
further usage in food applications. Saskatoon
berries were dried from an initial moisture content of 75.5% to around 25%
(w.b.) using microwave-vacuum drying. In this study, saskatoon berries were
dried at different microwave power levels P10 (745 W), P7 (514 W) and P5 (374
W) and vacuum pressure levels V20 (67.0 kPa) and V10 (33.5 kPa). The vacuum
pressure levels did not greatly influence the drying time of the saskatoon
berries compared to the microwave power levels. Increasing the microwave
power level produced faster drying times for the samples. After 12 min of
drying at power level P10, the moisture contents of the samples at vacuum
pressures of 33.5 and 67.0 kPa were 29.74 and 31.24%, respectively. However,
after drying for 12 min at vacuum pressure of 33.5 kPa, the moisture contents
at power levels P10, P7 and P5 were 29.74, 51.41 and 61.95%,
respectively. Polynomial and
exponential models were fitted to the drying data. The exponential model
provided a better fit for the drying data at the lower vacuum pressure, with
higher R2 and lower SE compared to the polynomial model. On the
other hand, the polynomial model provided a better fit for the drying data at
the higher vacuum pressure. The vacuum pressure and the microwave power
levels did not significantly change the yellowness of the samples at
indicated by the ∆b values. However, microwave-vacuum drying produced
significant color changes in the L (white = 100 to black = 0) and a (green =
-a to red = +a) values as indicated by ∆L and ∆a. Total color
difference was dependent on microwave power and vacuum pressure levels. Water
activity of the samples after drying ranged from 0.61 to 0.75 for moisture
content range of 15.68 to 29.03%.
Key Words: Microwave,
vacuum, drying, saskatoon berry, colour, water activity, quality
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