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A Study of the Methods of Estimation of Carbohydrates, especially in Plant-extracts: A New Method for the Estimation of Maltose in presence of other Sugars.

Published online by Cambridge University Press:  27 March 2009

William A. Davis  and
Arthur John Daish
William A. Davis
Affiliation:
(Rothamsted Experimental Station.)
Arthur John Daish
Affiliation:
(Rothamsted Experimental Station.)
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Extract

Certain sources of error encountered in the estimation of sugars in plant extracts are dealt with. Large errors in the gravimetric method may be obtained unless special care is taken in purifying the asbestos by boiling for at least 30 minutes with 20% sodium hydroxide. Weighing the reduced copper as cuprous oxide is likely to give rise to large error, and a process of weighing as cupric oxide, with certain precautions, is recommended.

The volumetric methods of Ling and of Bertrand have been studied; the former is preferable in all respects to the latter, which we regard as only roughly approximate.

In dealing with plant extracts, owing to the accumulation of sodium acetate in the solutions analysed, inversion by citric acid of lower concentration than 10% is generally incomplete. Inversion by invertase is, however, not interfered with by this salt. To estimate cane sugar inversion both by invertase and 10% citric acid is recommended. No loss of sugars occurs owing to the use of basic lead acetate as has been sometimes stated; the supposed loss is probably due to incomplete inversion caused by the presence of sodium acetate.

It is shown by a detailed study of the action of dilute hydrochloric acid on different sugars that it is impossible completely to hydrolyse maltose at either 70° or 100° without simultaneously destroying large quantities of laevulose or dextrose.

The only available method for the accurate estimation of maltose consists in the employment of special maltase-free yeasts, such as S. exiguus, S. marxiarnus or S. anomalus, introducing a correction (for pentoses, etc.) obtained by a special fermentation with baker's or brewer's yeast.

6. A scheme for the quantitative estimation of sugars in plant material is given.

Type
Research Article
Information
The Journal of Agricultural Science , Volume 5 , Issue 4 , October 1913 , pp. 437 - 468
Copyright
Copyright © Cambridge University Press 1913

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References

page 437 note 1 Trans. Roy. Soc., 1893, 63, 604.Google Scholar

page 437 note 2 Trans. Roy. Soc., 1897, 71, 72—123.Google Scholar

page 437 note 3 Biochem. J., 1912, 6, 1.CrossRefGoogle Scholar

page 438 note 1 Ost, (Chem. Zeit., 1897, 21, 613Google Scholar) in reply to Brown, Morris and Millar's criticism of the value he had assigned to the specific rotatory power of maltose, based on a method in which the hydrated substance was weighed and the rotation for the anhydrous substance derived from this, threw doubt on their values, alleging that, when maltose is heated to a temperature above 95°, even in vacuo it begins to decompose, although without showing any external signs of change, the decomposition being indicated only by a falling off of rotatory power. According to Ost the values of the solution-densities and reducing powers given by Brown, Morris and Millar would therefore be only approximately correct (“konnen principiell nicht als exakt gelten”) as slight change had probably occurred in the material used (dextrose, laevulose as well as maltose). This statement is reproduced in von Lippmann's Chemie der Zuckerarten (3rd edition, p. 1468), where preference is given to Ost's values of solution-densities.

We have not specially investigated this point but may point out that Ost makes use of a strange argument in support of his case, and refutes himself, when he maintains that the indication of decomposition having occurred in maltose is a lowering of the specific rotatory power and yet contends that Brown, Morris and Millar's high value for the specific rotation at 15·5° (137·93) as compared with his own (137·46) was due to this cause; had decomposition occurred a lower value would be expected. Schulze, E. (Chem. Zeit., 1902, 26, 7)Google Scholar: on the other hand maintains that maltose hydrate can be completely dehydrated at 100° in a current of air without any decomposition occurring, and Ling, Eynon and Lane (7th International Congress App. Chem., 1910, 1, 137) confirm Brown, Morris and Millar's tables of solution'densities. Our own results were also always in full accord with them.

page 439 note 1 In Moissan's, Traité de Chimie Minérale (Vol. v. p. 459Google Scholar) the oxidation of cuprous oxide on heating in air is said to be incomplete on the authority of Grunhüt, (Chem. Zeit., 1894, 18, 447Google Scholar), Nihoul, (Chem. Zeit., 1894, 18, 831Google Scholar) and Killing, (Zeit.angew. Chem., 1894, 431Google Scholar). As we shall show, this is erroneous, unless the blow-pipe flame is used.

page 443 note 1 Using invertase in the form of autolysed yeast the ratio of weighed varies between 1·060 to 1·090 according to the quantity used; with prepared diastase in starch transformations it is slightly higher (1·105—1·106) and with malt extract it again gives low figures similar to those obtained with autolysed yeast.

page 445 note 1 In all cases our numbers refer to true c.c. at 15° C.

page 447 note 1 Throughout the percentage of citric acid we give refers to the percentage of the ordinary crystalline acid, C6H8O7 + H20.

page 450 note 1 No loss of sugar is therefore caused by the use of basic lead acetate, as has sometimes been stated to be the case. This supposed loss has been probably due to incomplete inversion, brought about by the presence of sodium acetate. Parkin has also shown that no loss occurs by a series of special experiments.

page 462 note 1 Baker, and Dick, (Analyst, 1905, 30, 79CrossRefGoogle Scholar) have suggested the use of S. marxianus for detecting maltose in presence of dextrose, by the increase of specific rotation and drop in reducing power which occur in fermenting the mixed sugars with this yeast; they fermented, however, only for a relatively short time and did not completely remove the dextrose as we have done, so as to make the method a quantitative one.

page 464 note 1 The pentoses present in plant extracts are apparently not fermented by either baker's yeast or the special maltase-free yeasts we have used. Experiments on this point are still in progress.

page 466 note 1 The polarisation of these dilute solutions is usually small and it is therefore necessary to take the reading with a long tube (at least 200 mm. in length), with an instrument reading accurately to , the temperature being maintained constant at 20° C. within . It is an easy matter, using a Lowry thermo-regulator, and circulating the water by means of a small pump, to keep the temperature constant to ; but we find that differences of temperature less than hardly make a perceptible difference in the readings with such dilute solutions as we have worked with.