Thermal Food Processing

510 Thermal Food Processing: New Technologies and Quality Issues
in the beans increased as the roast progressed, and that its formation in the FIR
roaster is more abounding than that of the hot-blast roast. In order to draw out the
aroma component to the maximum, the coffee beans need to be heated uniformly
and at the appropriate heating speed.
16.4.1.3 Roasting of Green Tea
Ninety percent of the tea produced in Japan is green tea, which is produced by
drying the tea leaves and roasting them through a firing process. In this firing
process, the tea leaves are dried to a moisture content between 3% and 5% (wet
basis), and at the same time the fragrant aroma that is characteristic of green tea
is drawn out. The aroma that is special to the fired tea is thought to be created
when the leaves are dried and heated to between 100 and 120°C. Because this
aroma is not produced when the temperature of the leaves is less than 100°C, the
tea has an unpleasant smell before being fired. 15
Lower-grade tea leaves that contain a lot of stems have a green smell and are
strongly bitter and astringent. This problem of bitterness and astringency is eliminated
in hoji-cha, which is roasted tea. Toasting the green tea to a temperature of
around 200°C, and thus changing the flavor component, is called the torrefaction
process of tea. This torrefaction breaks down the tea polyphenol, which is the
component that causes the bitterness and astringency of the tea, causing the
transpiration of the green smell component and forming the torrefaction flavor
component (types of pyrrole and pyrazine). 15
Conventionally, in the firing and torrefaction processes, gas has been used as
the heat source for direct heating or hot-air heating. As a substitute for hot-air
heating, equipment using FIR or microwaves and FIR in combination 15,27 has
been developed. Takeo 15 conducted composition analysis and sensory tests on the
tea and indicated that the FIR heating methods could heat the tea leaves more
uniformly, preventing the scorching that could occur from partial overheating and
making it possible to produce tea with a better flavor.
16.4.1.4 Roasting of Sweet Potato
It is well known that the taste of sweet potato and chestnut roasted by heated
pebbles, which emit FIR, is better than those roasted by other methods. 28 As a
reason for this, when sweet potato or chestnut is roasted by FIR, it seems to
become more sweet and delicious, since the FIR energy permeates to the food
inside, and it uniformly heats the sweet potato inside. However, as mentioned in
Section 16.3, the permeability of the FIR is small in the food, which mainly
contains water. Hashimoto et al. 29 estimated the absorption coefficient of the sweet
potato from that of water and the drying sweet potato, which was measured
individually. As a result, they estimated that the depth in which the irradiated
energy to the sweet potato attenuated to 1% was 0.1 to 0.22 mm. Therefore, in
the case of sweet potato, the permeability of FIR is not large.