93 



conditions, increase among the Acarids is very rapid at temperatures 

 between 60° and 75° F., less so between 50° and 60°, while between 

 40° and 50° it is slow. How and where wheat becomes infested with 

 Acarids is not yet satisfactorily determined. All evidence points to 

 their occurrence in the wheat of temperate countries only ; within 

 this limit however their geographical range is extremely wide, the 

 same three or four species occurring all round the world. In wheat 

 from very hot countries, such as India, Acarids rarely occur,' probably 

 on account of the low moisture content of such grain. 



Wheat that has become badly infested can be much improved, and 

 fittther damage prevented, by subjecting it to some process that will 

 dry the grain and at the same time kill ofi most of the mites and 

 remove their bodies, such as a blast of hot air followed by cooling. 

 Wheat in bulk can be so treated in such a machine as the " Hess Drier 

 and Cooler," in which the grain is passed from a band conveyor into a 

 garner, whence it falls into a chamber containing a series of horizontal 

 racks or ba^e-plates arranged in a zig-zag manner one above the other. 

 In this chamber the wheat may be brought into contact with a draught 

 of hot air. After a period which varies according to its condition, 

 it is passed into a similar chamber where it is subjected to a draught 

 of cold air and cooled ; it then passes into a hopper beneath, and this 

 discharges on to a band conveyor. Cold air is drawn through the lower 

 chamber by a powerful fan, and the same current passes through the 

 fan and is forced on over a series of steam coils, thence going through 

 the wheat in the upper chamber. 



The hermetical sealing of vessels containing flour was found to 

 destroy Acarids by asph}xiation, but did not destroy all the eggs, as 

 \vhen fresh air was again admitted borings began after a few days. 

 Dessication is apparently more effective in destroying the eggs than 

 hermetical seaHng. 



Prevention from Acarid infestation is best secured by storing flour 

 wrth a moisture content below 11 per cent. For infested flour, heat 

 is probably the best remedy, if applied in time, and for small quantities 

 heating for an hour or more at 150° F. would eliminate further damage. 

 In mills, a lower temperature applied for a longer period would be more 

 practicable. It is found that the lowest absolutely lethal temperature 

 is 120° F., and this must be applied for at least 12 hours to be effective. 

 Lower temperatures than this greatly reduced the numbers of mites 

 but did not ensure the destruction of all eggs. Sifting infested 

 flour through a very fine bolting-silk sieve would remove most of the 

 mites, but the mesh would have to be very fine to prevent the passing 

 of the eggs, which measure only 0*12 by 0*08 mm. 



Two papers are included as Appendices ; the first, by J. M. Beattie, 

 is a preliminary note on samples of flour submitted for bacteriological 

 examination by Professor Newstead, and is chiefly concerned with 

 bacteria ; the second, by A. E. Humphries, is a report on six samples 

 of flour into which mites have been introduced. Examination of these 

 samples lead to the conclusion that mites attack the nitrogenous 

 matter of flour and render the flour in some way, apart from their 

 mere presence, unfit for the manufacture of bread. It uas found that 

 the mites could be removed by sifting the flour through No. 14 silk and 

 that practically no mites reappeared in flour that had passed through 

 the sieve; only fine flour will, however, pass through this fine mesh. 



