Abstract
THE PRESERVATIVE PROPERTIES OF HOPS.-For many years it haj been assumed that the preservative properties of hops are directly proportional to the percentage soft resin content. In view, however, of the objections inherent in Brown's method of evaluating the former, it is welcome to find in a recent paper by Mr. A. Chaston Chapman (Jour. Inst. Brew., 1925, xxxi. 13) the description of an improved biological method. To varying amounts of a i per cent, aqueous infusion of the hops, in a medium of nutrient agar, were added a few drops of a culture of an organism isolated from raw sugar, and specially sensitive to the inhibitory effects of hop extract. After incubation for 18 hours at 37° C. on petri dishes, the mixtures were arranged in order of bacterial development, the end-point being represented by a dish containing few or no colonies, followed by one containing a large number. The end-points were sharp and concordant, and within reasonable limits, were independent of the strength of the infusion and of the number of successive digestions of the same simple. The author suggests that the preservative material, though highly insoluble, is being produced continuously during the extraction. Whereas no quantitative connexion between the soft resin content and the preservative value is apparent, experiments on the cold and ordinary storage of hops show that both these properties diminish at a slower rate in the former case. Apparently the two properties are associated in some way not yet clear, the quality rather than the quantity of preservative material being the important factor. Methods of estimating resin content by direct extraction are criticised, and an improvement giving higher results is suggested for soft resins. These are now extracted in a Soxhlet apparatus by petroleum ether from the total resin solution spread on an Adams milk-analysis paper.
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Research Items. Nature 115, 244–246 (1925). https://doi.org/10.1038/115244a0
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DOI: https://doi.org/10.1038/115244a0