FLOUR	61
effected by means of air mixed with oxides of nitrogen (nitrogen peroxide,
etc.) and is detected as follows :
^Reagent. Sulphanilic acid (0-5 gram) and a-naphthylamine (o-i gram)
are each dissolved in 150 c.c. of 30% acetic acid and the solutions then
mixed. The vessels in which the solutions are prepared should previously
be washed with acetic acid. In some cases the mixture exhibits a faint
red coloration, which is removed by addition of zinc dust ; also after long
standing this coloration may appear, but it is prevented by the presence of
zinc dust.
Procedure. From 3 to 5 drops of this reagent are placed on a quantity
of the flour spread out and pressed flat, as in determining the colour (see 2,
above) and any coloration noted. With bleached flour a pink or red colour
appears after a few seconds or, in any case, before the lapse of a minute,
whereas untreated flour is not coloured.^1
14. Practical Baking Tests. — Those most generally employed are as
follows :
(a)	water-absorbing power.   The absorbing power of 100 parts of
flour towards water is determined and hence its behaviour as regards forma-
tion of dough.   The determination is made as follows :
A definite quantity of the flour (about 100 grams) is placed in a porcelain
dish and a small depression made on the surface by means of a convex
object. 25 c.c. of water are added and the whole mixed with a glass
rod to make a paste adherent to the rod. This dough is then placed
on the palm of the hand sprinkled with the flour and more flour added and
mixed until a dough is obtained which does not adhere to the finger. The
dough is then weighed (P) and the water used calculated by means of
the formula, A ~ — -A_ ...   Three measurements should be made and the
P— 25
mean of the results taken.
This test gives only relative values, and for a determination more in con-
sonance with practice, a larger quantity of water — say 10 litres— is taken and
doughed with the necessary amount of flour.
(b)	baking test.   This is best made on 10-15 kilos of flour by a person
experienced in practical baking.
For laboratory tests use may be made of a small jacketed metal oven
containing heavy mineral oil and coated with asbestos board ; it is heated
by gas and serves to bake loaves of about 150-^200 grams prepared "by
doughing i kilo of flour with the required amount of water (about 500 c,c.),
i% of sodium chloride and 20 grams of beer (or pressed) yeast being added
and the whole left to rise for 2 hours in an oven at 30-33° C.
The loaves prepared on either the large or the small scale are examined _f
as to their external appearance, and in section to ascertain their porosity,
the thickness of the crust, the adherence of the latter to the dumb, etc.,
and also as to their objective characters. Further, since the value of a
flour for bread-making increases with the volume of the loaf, the latter is
often determined. To this end loaves are prepared as above from 100-200
Bw&wald and Neumann ;  Ztitxhf. f. das gu, GG&ewtewesm, 1909, p. 135.