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Published online by Cambridge University Press: 15 May 2009
The action of air containing nitrogen peroxide upon flour, in quantities up to 300 c.c. of nitrogen peroxide to one kilogramme of flour, may be summarised as follows:—
I. The golden-yellow tint of the flour is destroyed. Immediately after bleaching no difference in tint due to excess of the bleaching agent could be observed with Lovibond's tintometer, but on keeping for several days the more highly bleached samples became decidedly yellow, while those treated with 30 to 100 c.c. of nitrogen peroxide per kilogramme became still whiter, the maximum of bleaching effect being attained within these limits.
II. The amount of nitrous acid or nitrites present in a freshly bleached flour is approximately proportional to the amount of nitrogen peroxide employed, and corresponds to about 30% of the total nitrogen absorbed, rising to 40% in the more highly bleached samples. After the lapse of several days, the proportion of nitrites present decreases considerably in the higher concentrations, but remains very nearly the same in the more slightly bleached samples.
III. Approximately 60% of the total nitrogen introduced as nitrogen peroxide into the flour during bleaching can be recovered as ammonia a short time after bleaching by reducing the aqueous extract of the flour with a copper-zinc couple, and may be assumed to be present in the flour as nitric and nitrous acids or as nitrates and nitrites. After keeping the bleached flour for some days the amount of nitric acid extracted with cold water decreases. Experiments with pure glutenin and gliadin indicated that in certain circumstances nitric acid may be withdrawn from solution or “adsorbed” by these proteins.
IV. In highly bleached flour a considerable increase in the amounts of soluble proteins and soluble carbohydrates takes place. If one kilogramme of flour is bleached with 300 c.c. of nitrogen peroxide, the amount of soluble nitrogen is doubled. This appears to be due almost entirely to the solubility of gliadin in nitric acid of certain concentrations. The simultaneous increase of soluble carbohydrates would seem to point to an intimate relationship between the gliadin and certain carbohydrates in flour.
V. If highly bleached flour is allowed to stand for some time after bleaching, the oil undergoes very considerable alteration and acquires the characteristics of an oxidised oil. About 6 to 7% of the nitrogen introduced as nitrogen peroxide during bleaching is absorbed by the oil.
VI. The absorption of nitrogen peroxide by flour does not appear to be accompanied by the production of free nitrogen, nor was any evidence obtained of the formation of diazo-compounds.
VII. Sodium nitrite was found to exert no inhibitory action on the digestion of soluble starch by saliva, but the rate of digestion was greatly retarded if the starch had been previously treated with nitrogen peroxide gas. Bleaching was found to exercise an inhibitory effect on the salivary digestion of flour.
page note 171 1 It is not easy to obtain distilled water quite free from nitrites. In examining commercial samples of flour it is preferable to use ordinary tap water as this can generally be obtained free from any traces of nitrites.
page note 172 1 In this connection it is interesting to note that bleaching may exercise a considerable preservative action on flour. In a series of extracts containing no toluol the unbleached or slightly bleached samples became sour very much sooner than those which had been highly bleached. (Cf. Fleurent, E.Bull. Soc. Chim. [iii.], xxxv. pp. 381–396. 1906.)Google Scholar
page note 173 1 Cf. Purvis, J. E. and Courtauld, R. M., Proc. Camb. Phil. Soc. xiv. pp. 441–446. 1908.Google Scholar
page note 175 1 The action of extremely dilute acids and alkalies on the gluten of flour has been investigated by Wood, and Hardy, (Proc. Roy. Soc. 1909, Series B. lxxxi. pp. 38–43)CrossRefGoogle Scholar, and will be referred to again when dealing with the action of dilute nitric acid on pure glutenin and gliadin.
page note 177 1 The letters C.I.indicate the methods of expression recommended by the International Conference for the Unification of Methods of Analysis (Paris, 1910).
page note 178 1 Egoroff, (Journ. Russ. Phys. Chem. Soc. 1903, xxxv. pp. 973–997)Google Scholar, has shown that small quantities of nitrogen peroxide act on oleic acid giving an additive product, and it is only where this latter is present in a certain proportion, for the formation of which a certain minimum quantity of nitrogen peroxide is required, that the isomeric changes of oleic into elaidic acid can take place.
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