By F. C. Walter, Berne, Switzerland
From The Brewers’ Journal – April, 1918
Little is known concerning the dexrins present in beer. Since in mashing saccharification is carried to the stage at which iodine produces no coloration, only the lower dexrins can occur in beers. The isolation of dextrins from beer in a state of purity is practically impossible with the means at present available, and the author therefore attempted to gain some information respecting their nature by indirect methods.
The specific rotatory power of the dextrins of beer was first estimated. For this purpose, beer was fermented as far as possible with bottom-fermentation yeast, which does not ferment dextrins. The rotation of the product was considered due to dextrin, together with small quantities of gums, pentosans, etc. The liquid was then heated with hydrochloric acid to hydrolyse the dextrins, and the amount of dextrose formed was found by determination of reducing power before and after fermentation and calculated to dextrin. Corrections were made for the influence of the pentosans present on the rotatory power. The results indicate that the specific rotatory power of the dextrins of pale beers varies between 164 deg. and 170 deg., the average from 172 deg. To 178 deg., with an average of 176 deg. In pale beers dextrins were found to constitute about 55 percent; in dark beers about 57 per cent of the extract.
Lintner and Dull, and more recently Biltz, have pointed out that the viscosity of dextrin solutions varies with their molecular weight. The author made measurements of the viscosity of beer freed from alcohol and carbon dioxide, and of dextrin fractions precipitated therefrom by alcohol. An Ostwald viscosimeter was employed, and the measurements were made at 17.5 deg. C. The viscosity values given represent the time of low of the liquids under investigation, divided by that of pure water (104 seconds), and multiplied by the density of the liquids. The values found for pale and dark beers, freed from alcohol and carbon dioxide, and reduced to a gravity of 2 deg. Balling were 1.163 and 1.049 respectively, but as other extractive substances besides dextrins contribute towards the viscosity of beers, these values represent only upper limits for the beer dextrins. Five successive precipitations were carried out on beer by means of alcohol; the viscosity values of the fractions, in 2 per cent solutions, were respectively 1.572, 1.164, 1.076, 1.087, and 1.076. The first two values are higher than those for beer extract itself, and these fractions doubtless contained the greater part of the gums, proteins, etc., of the beer, which are less soluble in alcohol than the dextrins. The similarity between the values for the last three fractions renders it probable that the true viscosity of the dextrins of beer lies between 1.07 and 1.09. According to Biltz, such a viscosity is shown (in 2 per cent solutions at 25 deg. C.) by the lower achroödextrins of molecular weight between 2000 and 4000. Attempts were made by various methods to remove the colloidal extractive matters other than dextrins, either from fractionation; these attempts were only partially successful, but in some of them the viscosity of the first dextrin fraction was reduced to 1.18 or 1.19, and that of the second fraction to 1.07 – 1.08, i.e., in line with that of the later fractions.
Molecular weight determinations were made with dextrin fractions from beers, but values were obtained ranging from 400 to 1100, i.e., far below those which, according to Biltz, correspond with the viscosities found, and more in line with those given by Lintner and Dull for Achroödextrin II, or even lower dextrins. Since the dextrin fractions obtained by the author, however, contained more than 1 per cent of mineral matter, and in one case 2.6 per cent, the values found for the molecular weight were probably much lower than the truth.
nota bene – achroödextrin is a dextrin which doesn’t give a color with iodine.
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