THOMSON: METHYL RED AS AN INDICATOR 519

NOTES ON FLOUR : 1. ACIDITY OF FLOUR. 2. NATURAL AND ARTIFICIAL BLEACHING OF FLOUR. 3. SULPHATES AND LIME IN FLOUR.

BY R. T. THOMSON.

(Read at the Meeting, November 4, 1914.)

EARLY in 1912 I was engaged in a, case, under the Food and Drugs Acts, in connection with bleached flour, and in this paper I propose to put on record some of the results of experiments made at that time, and since then.

1. ACIDITY ’’ OF FLOUR. In Jago’s “Technology of Bread-Illaking,” and in other works, it is taken for

granted that flour is acid, and i n the work named (1911 edition, p. 773) the use of

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520 THOMSON: NOTES ON FLOUR

phenolphthaleyn as indicator is recommended, and in calculating the result the instruction is in the following terms : (6 Assuming that the acidity of meal or flour is due to lactic acid (as undoubtedly it is in whole or great part),” etc. I n Wood’s ‘‘ The Story of a Loaf of Bread ” (1913, p. 68) it is stated that ‘‘ wheat, like almost all plant substances, is slightly acid ”; and these statements appear to be the last words on the subject at present. The last-named book does not mention the indicator employed, although the former does; but both are inexact or careless in the use of terms which should be defined with accuracy as to what is actually meant. Acidity and alkalinity are words which may mean anything according to the indicator employed, an apposite example of which is fresh milk, which is acid to phenol- phthalein, alkaline to methyl orange, and amphoteric or practically neutral to litmus. Accordingly a bald statement could be made with apparent accuracy that milk is acid, alkaline, or neutral, in conformity with the point of view taken. In flour we have in certain respects a, mixture similar to milk, as i t contains proteins, carbo- hydrates, and phosphatee, so that an unqualified assertion of acidity is unscientific. I n Jago’s volume, besides phenolphthaleln, there is mention of turmeric as an indicator for acid in flour ; but I showed many years since that these two substances behave similarly and belong to the same class, so that there is no corroboration of acidity. In the same volume Planchon is referred to as having made various tests and to have come to the conclusion that the proper way to test the “acidity” of flour is to suspend 5 grms. in water and titrate with standard caustic soda, using phenolphthalein as indicator. Now, there is no valid reason for coming to such a conclusion, as is quite apparent from the results obtained with different indicators. In order to test this point, methyl orange, methyl red, and phenolphthalein, were selected as representing most clearly the three classes of indicators in general use.

A first-grade flour from wheat grown in Scotland was made the subject of experiment, and the titrations were made in three different solutions prepared from 5 grms. of the sample. In the first case 10 grms. were suspended in 400 C.C. of water, and 200 C.C. filtered off after standing for one hour; in the second 5 grms. were suspended in 200 C.C. of water for one hour; and in the third 5 grms. were heated t o boiling with 200 C.C. of water, and allowed to cool for one hour. These solutions were then titrated, with the following results :

Acidity Reaction Alkalinity Ly Phenol- to Methyl by Methyl ph thalein. Red. Orange.

SO, per Cent. NaOH per Cent. Filtered sample . . . ... ... 0,056 neutral 0.042 Unfiltered sample . . . ... 0.114 neutral 0*108 Boiled sample . . . ... ... 0.114 neutral 0.108

The reason for adopting SO, and NaOH as expressing acidity and alkalinity is that SO, (80) is equal to 2NaOH (80), so that the figures are interchangeable.

These results are somewhat startling, and give no support to the assumption that flour is actually acid. I t may be argued that phenolphthalein, being most sensitive to acids, is the proper indicator to use ; but in a complicated mixture like flour there might .be some disturbing element which is apparently acid, whereas,,

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THOMSON: NOTES ON FLOUR 521

like sulphate of ammonia, it may be really neutral. The mineral matter of the flour was treated with hot water, filtered, and titrated with FG hydrochloric acid. The ash insoluble in Walter was dissolved in a measured quantity of the same acid, and titrated back with TG caustic soda. I n each case the results were calculated on the same basis as already described, and the results were as follows :

Per Cent. Water ... ... ... ... ... ... ... .,.. 12-31

Ash soluble in water ... ... ,.. ... ... ... 0.29 Ash insoluble in water . , . ... ... .I. ... 0.08

Total mineral matter ... ... ... ... ... 0.37

Alkalinity of soluble ash to methyl orange ... ... ... 0.042 Alkalinity of insoluble ash to methyl orange ... ... ... 0.038

Alkalinity of total mineral matter ... ... ... ... 0.080 --

Alkalinity of soluble ash to phenolphthaleln ... ... ... neutral Alkalinity of insoluble ash to phenolphthalein.. . ... ... 0.012

These results account to a large extent for the alkalinity of the original flour to methyl orange, but there is still a little, the presence of which cannot be explained satisfactorily. It may be due to the apparent alkaline reaction of some organic ingredient, but it must also be borne in mind that the end-point of the reaction with flour is obscure, and not reliable within narrow limits. As regards phenolphthalein, the mineral matter as a whole is slightly alkaline, and the soluble portion is neutral, which indicates that the ash consists largely of mono-acid potassium phosphate (K,HPO,), or rather the corresponding pyrophosphate. Of course, this does not help to explain the apparent acidity of the flour when phenolphthalein is used as an indicator.

I n addition to the above, twenty samples of flour, the sources of which were wheat from Scotland, America, Canada, Russia, etc., mere examined, and all were found to be alkaline to methyl orange, practically neutral to methyl red and litmus, but acid to phenolphthaleh. The acidity by the last-named indicator varied from 0.096 to 0,136 per cent. of SO,. I t is against all reason to suppose that this apparent acidity is due to the presence of free lactic acid, according to Jago’s confident state- ment, as i t has beenaell known for a long period that lactic acid can be estimated with fair accuracy if litmus, especially in the form of paper, is used as an indicator. I n my paper on methyl red I have shown that fully 98 per cent. of lactic acid can be determined by that indicator, so that it is obvious that free lactic acid cannot exist in flour when it is practically neutral to both litmus and methyl red. In order to corroborate this, flour was extracted with ether and alcohol respectively, the solution evaporated after mixing with a little water, then filtered and titrated, when only minute traces of acid were found. As the average acidity of flour is equal to about 0.25 per cent. of lactic acid, we have now absolute proof that flour does not contain that acid in the free condition, and, indeed, that proportion would make flour quite

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522 THOMSON: NOTES ON FLOUR

sour to the taste. So far the proof of non-acidity is negative, and I am not in a position as yet to state positively what the apparent acidity to phenolphthalein is

When flour becomes 6 ' sour " it shows acidity to methyl red and litmus, and this increases in that condition to phenolphthalein, whereas the alkalinity to methyl orange is not reduced. This is apparent from the following results obtained with a sound flour, and one, of similar composition, which had become sour :

due to.

Per Cent.

... ... ... ... ... ... Water ... Mineral matter ... *.. ... ... ... Alkalinity of filtered solution (methyl orange) ... ... Acidity of filtered'solution (methyl red) ... ...

,, (phenolphthaleyn) ... ...

Acidity ,, Y , I , (methyl red)

Y , ,?

Alkalinity when suspended in water (methyl orange). .. ... ... 9 9 7 7 7 , ,, (phenolphthalein)

Soimd. 12.44 0.36 0.048

neutral 0.060 0*110

neutral 0.112

sour. 13-10 0.35 0.054 0.160 0.312 0.080 0.294 0.455

I t is only necessary to note that, in mite of the production of a considerable proportion of acid, methyl orange still indicates alkalinity; but methyl red shows a, substantial amount of acid, especially when the flour is suspended in water during titration. I t is generally supposed that lactic acid is produced by the souring of flour, but this is doubtful, because, when extracted with alcohol or ether, only a very small proportion of the additional acid was found. Further experiments will be necessary in order to determine what acid or acids have been produced during the passage of sound flour into the sour condition.

Although no definite conclusion has been reached as to what the apparent acidity of flour is due, there can be no doubt that it is not actually acid, except perhaps to a minute extent, and the statement that acidity is undoubtedly due to lactic acid is a mere theoretical fallacy. From the results recorded, it is evident that any appreciable acidity due to the souring of flour can be detected and deter- mined by titration with standard caustic soda, using methyl red solution or litmus- paper as indicator.

2. NATURAL AND ARTIFICIAL BLEACHING OF FLOUR. The discussion on this subject hinges on the theories (1) that natural bleaching

by exposure to the atmosphere is similar to artificial bleaching by nitrogen peroxide, and (2) that these two processes are eesentially different. I n dealing with this question, it will be necessary to know the composition of air as regards nitrites, and to trace the proauction of flour from the grain to the final bleaching. In order to do this, I visited at a flour-mill, and observed the process of milling, procuring samples of the wheat and the different products for subsequent examination.

The first point, however, was to test the air for nitrites, several plans for this being tried; but the best was the following adaptation of published methods: A thoroughly cleaned stoppered bottle capable of holding about 4 litres was procured,

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THOMSON: NOTES ON FLOUR 523

filled with air in the mill by drawing it in with a pump, then 100 C.C. of water containing 2 C.C. of the Griess-Ilosvay reagent were poured into the bottle, and the latter closed. The bottle was now turned about occasionally, so that the liquid spread over the sides, and finaIly the contents were left overnight. Three or four stoppered bottles, capable of holding rather more than 100 c.c., containing different quantities of standard sodium nitrite in 100 C.C. water, and 2 C.C. of the Griess-Ilosvay reagent, were also left overnight, and then compared with the solution from the air, when, by a comparison of the colours, it was possible to determine the amount of nitrite present. The following are the results obtained in the air of the mill referred to :

Mgrm. Grain per in 10 Litres. 1,000 Cubic Feet.

Nitrous anhydride (N203) . . . ... ... 0.0061 0.267 Equal to sodium nitrite ... ... ... 0*0110 0.484

Samples of the wheat, and products of the milling of it, were taken and tested, with the following results :

Colour by Lovibond’s Nitrites as Sodium Tintometer.

Nitrite. A- Part per Million. Yellow. Red.

Wheat ... ... ... 0.16 First-grade flour ... 0.51 0.65 0.32 Second-grade flour ... 0.30 0-84 0-40 Fine thirds ... ... 0.18 Common thirds . . . ... 0.16 Bran ... ... ... 0.20 Germ ... ... ... 0.12

I t will be apparent that the first-grade flour has absorbed 0.35 part per million of nitrites from the atmosphere, and for this to take place 1 pound of flour would have to take up all the nitrites contained in 5 cubic feet of air, and 1 ton would require 11,200 cubic feet. This result is not at all surprising, as the flour falls through the atmosphere in a, fine dust, which would pick up most of the nitrites in the air. As the product becomes coarser, the absorption of nitrites naturalIy becomes less. Of unbleached flour from various sources, the nitrites ranged from 0.10 to 0.62 part per million.

We have next to consider the cause and effect of natural bleaching, or exposure of flour to the influence of the atmosphere under different circumstances, as compared with artificial bleaching with nitrogen peroxide.

A sample of unbleached flour was selected and tested for nitrites, and 24 ounces of it were placed in a thin, slightly glazed paper bag, and kept in a light, well- ventilated room for thirty-four days. At the end of this period + ounce was carefully removed from the surface, and this was called LLfirst layer.” Another sample of + ounce was taken from the flour immediately below where the top layer had been, and this was named ‘‘ second layer.” One ounce of the flour in direct proximity to the sides of the bag was next taken, and the remaining 22 ounces were thoroughly mixed, and the four samples thus drawn were analysed separately. I n addition, a

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524 THOMSON: NOTES ON FLOUR

portion of the remaining 22 ounces was bleached with 5 C.C. of nitrogen peroxide gas per kgrm. of flour, and two quantities of 2 ounces each were exposed to a certain atmosphere for half an hour and two hours respectively, in open Petri dishes 6 inches in diameter, having thus an exposed surface of about 28 square inches, The atmosphere referred to was that of ’an almost hermetically closed safe, about 600 cubic feet in capacity, and in which two jets of gas were burning, with a consumption between them of about 6 cubic feet per hour. The results of the tests were as follows:

Total Colour Yellow Red Nitrites. Colour.

Colour* destroyed. Per Cent.

Original sample before exposure ... 0-34 First layer ... ... ... 3.12 Second layer ... ... ... 2.50 From sides of bag ... ... 2.90

Average analysis ... ... 0.83

After two hours’ exposure in air ... 6.20

kgrm. ... ... ... 3.06

Remaining 32 ounces ... ... 0.58

After half-hour’s exposure in air 3-05

After bleaching with 5 C.C. NO, per

0.86 0.80 0.80 0-80 0.86 0-85 0.80 0.76

0.64

0.36 0.34 0.34 0.34 0.36 0.35 0.34 0-32

0-28

- 6 6 6 0 2 6

11

24 Water ... ... ... ... ... 14.74 per cent. Mineral matter ... ... ... ... 0.42 ,,

The nitrites are stated as sodium nitrite, parts per miIlion. The colour was taken in Lovibond’s tintometer.

I t will be observed that, in the first layer of the flour contained in the bag, the nitrites have risen from 0.34 to 3.12 parts per million, that on the sides of the bag to 2.90 parts, while the second layer is 0.62 less than the first layer, showing that penetration by the atmosphere is slow, or a t least that the nitrites in it are absorbed by a thin layer of the flour. Taking this fall in nitrites as being regular into the centre of the flour, it was found, by taking the measurement of the bag into account, that the influence of the atmosphere as indicated by nitrites had ceased when it had penetrated 4 inch into the flour, the bag being 4 inches in height, 49 inches in breadth, and 28 inches in thickness. From these considerations i t was apparent that three-fourths of the flour had been totally unaffected, and in any event the whole sample, had it been mixed together, would only have contained 0.83 part of nitrites per million. Under comparatively unfavourable conditions, therefore, the absorption of nitrites from the air is very small, and i t was found that a fairly well glazed bag prevented even such absorption for months.

If we now make a comparison of the analysis of the first layer of flour, that of the portion exposed for half an hour in the special atmosphere referred to, and that of the portion bleached with 5 C.C. of NO, gas per kgrm., we find that the proportion of nitrites is practically the same in each. The percentage of colour destroyed, how- ever, is entirely different in each case, that by exposure in the bag in the top layer being only 6 per cent., the same being the case with the flour exposed for half an hour,

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THOMSON: NOTES ON FLOUR 525

whereas by artificial bleaching no less than 24 per cent. has been removed. The same thing can be shown by another example of portions of the same flour being exposed in Petri dishes as before, in different localities, for ten days, the following being the results obtained :

Colour. Colour destroyed. - f l . Nitrites. Yellow. Red. Yellow. Red. Total.

- - Original sample ... ... 0.42 0.64 0.30 - Medium smoky air ... 3.32 0.38 0.24 37 20 34 Smoky air ... ... 4.50 0.38 0.24 37 20 34

Pure smokeless air ... 1.30 0.42 0.26 34 13 27

None of this flour was available for bleaching, but another flour bleached with 2 C.C. of NO, per grm., showed 1.40 parts of nitrites per million, but only 7 per cent. of colour removed, showing clearly that the destruction of 27 per cent. in the pure smokeless air, and only an absorption of 1-30 of nitrites, is due to other causes than nitrous acid, probably chiefly oxidation, assisted by moisture and light. Those who approve of the principle of tampering with flour by bleaching with nitrogen peroxide very ingeniously draw an analogy bet ween this and the natural bleaching, such as has been described, and hold that they are identical, only that the one process is rapid and the other slow. I n face of the results given, it is evident that this is a mere theory, with no data whatever on which to build the top-heavy edifice, but, while the argument so far is convincing, it can be clinched by other data which explain the reason why so much nitrite is present without having the bleaching effect that might be expected.

I t is generally assumed that when flour is bleached by nitrogen peroxide, a mixture of nitric and nitrous acids is first produced, but it is very doubtful whether this reaction occurs before bleaching is effected. The colouring matter of flour does not appear to be intimately associated with any of the constituents, and as bleaching with nitrogen peroxide is practically instantaneous, it is probable that i t acts to a large extent as such, and not after the acids have been formed.

The various flours mentioned above were tested by mixing with water, sodium nitrite, potassium iodide, and a little starch solution (the latter being scarcely necessary, but added for safety), then allowed to stand for several hours, and in some cases for twenty-four hours. I n no case was iodine liberated, which means that no trace even of free nitrous acid was present, as this test is extremely delicate, because there is continuous regeneration of nitrous acid from the nitric oxide at first formed, and the blue colour becomes gradually stronger owing to the accumulative action taking place. I t was found that a flour having an apparent acidity of 0.114 per cent. of SO, to phenolphthalein required the addition of lactic or hydrochloric acid equivalent to 0.08 per cent. of SO, before any trace of iodine was liberated. The acid required to bring the flour into a condition capable of liberating nitrous acid is practically equivalent to the alkalinity of the mineral matter to methyl orange-that is, to the point at which the di-acid phosphate (KH,PO, or other analogous com- pound) is formed. I t was found by experiment that KH,PO, in solution liberated nitrous acid almost immediately, but a mixture of 80 per cent. of that compound

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526 THOMSON: NOTES ON FLOUR

with 20 per cent. of K,HPO, had no action even after twenty-four hours, The.flour in question had an alkalinity of 0.108 per cent. to methyl orange, so that rather more acid is required than four-fifths of the alkalinity of the ash (0.08 per cent.), but not the full amount indicated by methyl orange. Some flours require rather less acid than this, and others rather more, before free nitrous acid would be produced, but acidity equal to fully 700 parts of nitrous acid per 1,000,000 parts of flour, would be required to be absorbed by flour before any of that acid would be in the free condition. Even admitting that atmospheric air is slightly acid, which I have never found it to be, enormous volumes would have to come in contact with flour before there could be any liberation of nitrous acid. As nitrites do not bleach flour, we have reduced to absurdity the plausible theory that nitrous acid is the active agent in bleaching flour exposed to the atmosphere. Since reaching these conclusions Dr. G. W. Monier-Williams published his report (Food Reports, No 19) in which he proves the difference between natural and artificial bleaching, but on different lines from those I have followed. I t is therefore apparent that the supporters of bleaching flour have not proved that the compounds produced are not deleterious, nor have their opponents proved the opposite view. I may note here that L. Weil (ANALYST, 1909, 34, 102) stated that sulphuretted hydrogen gas when passed over bleached flour brought back the original colour. I made some experiments in this direction, leaving the flour in contact with sulphuretted hydrogen for several hours, and once under a sealed bell jar overnight, but the colour had evidently been destroyed, there being no restoration visible.

3. SULPHATES AND LIME IN FLOUH.

The determination of these is important where sulphates are to be allowed for in self-raising flour, or in testing whether a flour contains persulphate of potash. The method of analysis I have found most suitable is to mix 20 grms. of the flour with 250 C.C. of water containing about 15 C.C. of hydrochloric acid of about 1.16 ~ p . gr., heat in a vessel of boiling water until the gelatinous starch becomes liquid, and then boil gently for a few minutes. Now allow the solution to cool, filter off the bulky precipitate, chiefly proteins, wash with dilute hydrochloric acid of the same strength as the sample was dissolved in, heat to boiling, and add a solution containing 1 grm. of barium chloride. Allow to stand for a t least twelve hours, collect the precipitate on a double Swedish filter, wash carefully and thoroughly with cold water, dry, ignite, and weigh. If this process be carried out exactly as described, there will be no difficulty in the filtration, which proceeds rapidly and gives clear solutions. The following are results obtained in Borne of the products of the wheat in which nitrites have already been given :

Per Cent. - / .

Second-Grade Fine Thirds. Coninion Thirds. First -Grade Flour. Flour.

... ... ... so,

CaO Mineral Matter . .

0.011 0.023 0.048 0.061 0.015 0.034 0.060 0.102 0.400 0.820 2.240 3.820

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THOMSON: NOTES ON FLOUR 527

Other samples of flour containing from 0.35 to 0.42 of mineral matter showed from 0.010 to 0.013 per cent. of SO,. These are even higher than the results found by Cripps and Wright (ANALYST, 1914, 429), but our methods of testing differ, and I have not had an opportunity of comparing their process with mine. The sulphates evidently rise and fall with the ash, and the ratio is practically the same for first- grade and second-grade flour, but not for thirds, I t would appear that a fair allow- ance for SO, naturally present in a flour would be one-fortieth of the mineral matter, if no other mineral matter has been introduced.

DISCUSSION.

The PHESIDENT said that, whilst he had personally no experience in regard to the acidity of wheaten flour, he had had a good deal in connection with that of barley flour and malt, and he should have thought that a t the present day the bulk of the acidity of these grain infusions, when fresh, would never be attributed to lactic acid. It had been shown years ago by Fernbach, who was one of the first to investigate the matter, that the greater part of the acidity of these infusions was due to various acid phosphates, and this was confirmed later by Matthews and others. The phosphates most likely to be present were KH,PO, (neutral to methyl orange and acid to phenolphthaleyn) and K,HPO, (alkaline to methyl orange and neutral to phenolphthalein); and a liquid containing a mixture of these two phosphates would clearly be acid to phenolphthaleln on account of the one and alkaline to methyl orange on account of hhe other. The fact that in flour which had become sour the alkalinity to methyl orange remained about the same as in the original flour would seem on the face of it to indicate that the organic acids formed were incapable of converting the one phosphate into the other; but there might be other and more probable explanations. The production on ignition of a mixture of pyrophosphate and metaphosphate, the former alkaline and the latter acid to phenolphthaleln, would suffice to explain the results obtained by the author in his examination of the ash of the flour. The whole matter was rendered more complicated by the probability of the presence in small quantities of other phosphates-e.'g., those of magnesium and calcium. For a long time it was the custom, in examining malt, to return the acidity as lactic acid, but this had been almost entirely abandoned for a good many years. Litmus and certain other indicators were useless in this connec- tion, because they did not yield definite indications with the phosphates referred to. L4s Mr. Thomson had said, the best indicators for the purpose were methyl orange and phenolphthaleyn. With regard to the question of bleaching, he should like to point out what he considered to be the inadvisability, if it could possibly be avoided, of expressing Lovibond tintometer values in terms of mixed series. I t was impossible to make a valid comparison of results expressed in more than one series, because, for instance, 6 yellow + 2 red was not the same as 5 yellow + 3 red. It was better, in his opinion, to try and make an approximate match with a single series whenever possible.

Mr. C. A. SEYLER thought that the increase in acidity on souring must be due originally to formation of an organic acid. The change which took place was

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525 THOMSON: NOTES ON FLOUR

probably due to bacterial action, which must result in decomposition of organic matter, and therefore one would expect some organic acid to be formed, lactic acid being the most probable. Lactic acid was a far more powerful acid than KH2P0,, its dissociation constant being 1380 x 10-7, as against only 2 x 10-7 for KH,PO,. If lactic acid was formed, i t would be capable of converting K2HP04 into KH2P0,, and would then disappear in the form of potassium lactate, which was alkaline to methyl orange ; so that the alkalinity to methyl orange would remain about the same ; but the acidity to phenolphthalein would be increased in proportion to the’ lactic acid formed.

The PRESIDE~T said that he did not suggest that organic acids were not formed ; but it was an undoubted fact that, in the case of malt flour at any rate, the acidity was largely due to these phosphates.

Dr. MONIER-WILLIAMS said that the most important point in MY. Thomson’s observations as to bleaching was the very small penetration, of nitrous acid or nitrites into flour when it was kept in an atmosphere containing the products of combustion ; and the fact that, whereas artificial bleaching resulted in the presence of nitrite together with the destruction of colour, the absorption of nitrite from the air resulted merely in the presence of nitrite without destruction of colour. H e very much doubted whether i t was possible to estimate accurately amounts of nitrite less than 1 part per million, since in order to obtain, with such minute traces, colours which are a t all comparable, the standard nitrite solutions must be prepared with the aqueous extract from a sample of unbleached flour containing no trace of nitrites ; and in practice, this is difficult of attainment. H e thought that in proceedings relating to bleached flour, the question of the deleterious action of nitrites themselves had been given rather too much prominence, because it was open to the defence to say that the actual quantity of nitrite present was too minute to have any physio- logical effect. It was probable that the alterations produced in the body of the flour itself-such as the destruction of the colouring matter-might be of more importance than the presence of minute amounts of nitrite. The colouring matter of flour seemed to be identical with carotin, a highly unsaturated hydrocarbon, which prob- ably played an important part in plant physiology in certain directions connected with plant respiration. Willstatter has shown that the colouring matters of flour: yolk of egg, green leaves, and also of the corpus Zuteum of the animal body, were closely allied, and were presumably derived from the same substance. Until more was known on the subject i t seemed dangerous to say that the destruction of even a small proportion of such a constituent in a foodstuff was without any effect on the properties of the foodstuff itself. With regard to nitrates, of course if flour contain- ing moisture came in contact with nitrous fumes, nitrates as well as nitrites would be formed, but he thought it might be assumed that nitratzs or nitric acid would not affect the colouring matter, which was probably in solution in the oil of the flour. What probably happened was that the gas was absorbed to a small extent by the oil of the flour, I t had been shown that in order to produce the elaidin reaction a certain minimum proportion of nitric oxide must be absorbed by the oil, forming an additive compound first, after which the elaidin reaction proceeded. It seemed very likely that such an additive compound was formed when the gas came in contact

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TWO METHODS OF ESTIMATING WATER IN CRUDE PETROLEUM, ETC. 529

with the oil of the flour, and that the absorbed nitric oxide combined with the colouring matter, quite apart from any subsidiary reactions due to interaction of the excess of gas with the moisture in the flour. I t was interesting to note that practically all flour contained some proportion of nitrites. In the case of untreated flour, ammonium nitrite might be absorbed from the air, and there was also the possibility of the production of nitrites from nitrates by bacterial or enzymic action. On the other hand, if bleached flour were kept for a long time, the nitrite content tended towards a certain minimum-about 1 part per million; so that, whether in bleached or in unbleached flour, a balance seemed to be struck in regard to the pro- portion of nitrite present after the flour had been kept for some time.

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1914

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