Smith - 2003 - Rice origin, history, technology, and production

588 Products and Product Processing
Harvest moisture content, %
Figure 4.3.16, Percentage points of moisture content (MC) removed before
head rice yield reduction occurred in relation to harvest moisture content. (From
Fan et al., 2000b.}
behavior; tliis ongoing work is beyond the scope of this book, but is addressed in
summary at the end of this chapter.
D rying S y stem s
Rough rice drying systems in the United States can be broadly categorized into either
commercial or on-farm systems. In both, rice is dried by passing either ambient or
heated air through tlie rice bulk. The sensible heat of the air evaporates water from
the rice; the evaporated water is carried out of the grain bulk by the airstream as
water vapor. Prior to entering the grain, air is typically heated and the RH is lowered
correspondingly. This increases the drying capacity of the air (i.e., the amount of water
vapor tlaat can be transferred to a unit of air before being saturated). This is why most
drying systems have some form of burner system as part of the fan and ducting system.
A more complete treatise of the psychrometric processes involved in grain drying is
given in Brooker et al. (1974) and Henderson and Perry (1976),
As discussed above, there are limits to the air temperature levels and drying
durations that grain driers can use without incurring kernel quality reduction, particularly
with rice. Excessively high temperatures can result in stress cracking of kernels,
which typically results in lowered HRYs. This situation can exist in either on-farm or
commercial systems, but due to the throughput speed with which commercial driers
are often required to operate, the potential for damage in commercial driers can be
greater.
Commercial systems are inherently larger than on-farm systems and have far
greater drying capacity. Most rice in the United States is dried in commercial facilities.