control devices not shown to provide for the desired heating of the supply air various optional or accessory devices and systems are adapted to be incorporated into the system of the invention to effect differing desired treatments of the malt in the course of movement through the dryer units a first such option is in the form of a sulfur dioxide injection system functioning to inject controlled amounts of sulfur dioxide gas into the dryer units to thereby expose the malt as it is being dried to the sulfur dioxide gas for the purpose of eliminating or minimizing the amount of naturally occurring carcinogens which may be present in the malt a source of sulfur dioxide gas is shown in the form of a supply tank 62 connected by way of conduits 64 and 66 to each of the dryer units 2 4 6 and 8 suitable shut off valves and ancillary controls not shown normally are provided in the various inlet or supply lines 64 66 to permit controlled amounts of sulfur dioxide to be selectively applied to the various fluid bed dryer units 2 4 6 and 8 to carry out the desired degree of sulfur dioxide gas absorption by the malt as it moves through therethrough any unabsorbed sulfur dioxide will pass out of the fluid bed dryer units through the previously described exhaust lines 46 48 58 and 60 associated with the dryer units 2 4 6 and 8 respectively and will ultimately pass into and through filter 52 in a preferred form of the invention filter 52 is of the activated carbon type designed to selectively filter out substantially all the sulfur dioxide gas from the air drawn out of the dryer units a further option incorporated into the illustrated embodiment of.