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.

Abstract

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.