Internal temperature of soybeans. The moisture contents of
soybeans were 9.6% in unsoaked beans, 38.2% after 1 h soaking,
and 51.9% after 5 h soaking, respectively. Effects of microwave
roasting (cv. Okuhara) were compared on the internal
temperature of whole soybeans at different moisture contents
(Fig. 1). A linear relationship occurred between the
exposure time and internal temperature in the first 4 min of
roasting. Thereafter, the temperature of soybeans of lower
moisture content (9.6%) increased more rapidly than that for
the other two experimental groups. However, an increase in
soaking time gradually decreased the final temperature of
soybeans after sufficient roasting: namely, the final temperature
of soybeans soaked for 5 h was below 110°C, whereas
the final temperature of samples not soaked was more than
160°C. The higher the moisture content of soybeans (38.2 or
51.9%), the lower the internal temperature of soybeans over
the same roasting period after 4-min exposure time. A dark
brownish color and burnt odor occurred after 12 min of heating
in soybeans that were not soaked, but was only slight after
20 min in soybeans of 38.2 and 51.9% moisture. The results
were also supported by an increase of the absorbance at 420
nm (cv. Tsurunoko) shown in Figure 2. However, there were
no significant differences (P > 0.05) in the temperature and
color of soybeans during microwave roasting among the three
soybean cultivars (data not shown)