4 BULLETIN 839, TJ. S. DEPARTMENT OF AGRICULTURE. 



starch grains are dissolved, or the preparation "cleared," as is com- 

 monly stated. Vigorous heating of the slide is to be avoided in order 

 to prevent burning of the material before the preparation has been 

 sufficiently cleared. After gentle heating, the slide is quickly trans- 

 ferred to the stage of the microscope, where it is allowed to remain a 

 short time before counting is begun. The cold stage causes the larger 

 part of the air bubbles that may be present in the preparation to dis- 

 appear, the very few that remain not hindering in the enumeration 

 of the bran particles and hairs. 



Careful adherence to the details of this technique is necessary to 

 insure a suitable slide for counting. If a slide is improperly prepared, 

 the resulting count probably will not be representative of the flour 

 under consideration. 



COUNTING BRAN PARTICLES AND HAIRS. 



A thorough acquaintance with the histology of the wheat grain is 

 essential before attempting an examination of flours. Any standard 

 work on microscopy or plant anatomy of the common food products 

 contains adequate descriptions of the tissues of the wheat berry in 

 various sections. The following brief description of the anatomy of 

 the wheat berry 1 is given for the purpose of indicating the tissues 

 which are depended upon for judging a flour with respect to its offal 

 content. 



The wheat grain is, botanically, the fruit of various subspecies and 

 varieties of the genus Triticum. This grain or fruit consists of a 

 series of tissue systems, the outermost of which is the pericarp which 

 is composed of three layers, the epicarp, mesocarp, and endocarp. 

 The pericarp is essentially the fruit coat or matured ovary wall. 

 Within the pericarp is the testa (or spermoderm), rather yellowish- 

 brown in color, and easily distinguished in either cross or surface 

 sections under the microscope. Within the testa is a layer of rectan- 

 gular cells (in transverse section) known as the aleurone layer, contain- 

 ing protein material but no starch. This is essentially the outer layer 

 of the endosperm or albumen of the seed. The remainder of the grain 

 within the aleurone layer consists of very thin-walled parenchymatous 

 cells packed full of starch grains. The small embryo, or germ, is 

 located at the end opposite the bearded apex. A crease or groove 

 passes longitudinally from the base of the grain to the apex. 



The essential purpose of milling is to produce the finely ground 

 endosperm or starchy portion of the wheat grain as free as possible 

 from bran particles, hairs, and germ tissues. These bran particles, 

 hairs, and germ tissues are known as offal in milling terminology. The 

 wheat offal, therefore, consists primarily of all the tissue elements of 



1 A. L. Winton. The Microscopy of Vegetable Foods, 2d ed., pp. 65-73. 1916. 



