(Bórquez, Melo, & Saavedra, 2015; C

(Bórquez, Melo, & Saavedra, 2015; Chong, Figiel, Law, & Wojdyło,
2013; Cui, Xu, & Sun, 2003, 2004a,b; Cui, Xu, Sun, & Chen, 2005,
2006; Cui et al., 2008; Li, Xu, & Sun, 2007). MVD integrates the
advantages of microwave heating and vacuum drying, which
allows water to be removed in a lower temperature and thus produces
high quality dried products. The major shortcoming of MVD
lies in a non-uniform heating property, leading to local overheating
in the product (Vadivambal & Jayas, 2010). Sample damage (burning
or browning) would occur if the hot spots on the test samples
are continually heated by microwave. Thus, quality supervision of
the final product or product being processed during microwave
drying is essential.
For dried food commodities, moisture content is one of the most
important quality parameters. Fruits are generally dried to 16–25%
moisture level (Desrosier & Desrosier, 1977) in order to inhibit the
growth of microorganisms and molds. The measurement of
moisture content conventionally employs oven-drying methods,
Karl-Fischer titration, or distillation approaches, which are
sample-invasive and time-consuming. Since the O–H functional
group of water specifically absorbs energy at certain wavebands
during electromagnetic radiation, spectroscopic methods were utilized
for moisture content determination (Magwaza et al., 2011;
Sinija & Mishra, 2008). However, the current spectrometers are far
more useful in providing average information of the whole sample
due to the spot-detecting nature, and therefore are not suitable for
monitoring the uneven moisture distribution of the sample.