208
ANALYTICAL EDITION
forming a colorless solution, for the most part tannins from ripe seeds are converted into more complicated compounds (lignin), and the remaining tannins become colored substances with a phlobaphene character, passing with more difficulty into the water extract than the tannins of hulls from unripe seeds. Burrs (experiments 21 and 23) did not show any increase in furfural yield over their weight before treatment with water, but taking into consideration that a considerable part of the substances apt to form furfural have passed into the water extract one has to admit the influence of tannins on the yield of furfural here. Where tannins are present in a substance to be examined, it is necessary to consider the possibility of decrease in yield of furfural, and hence a decrease in pentosan content, determined according to Tollens. The influence of tannins is shown by experiments to determine furfural yield from a sample after removing tannin extract with water and comparing the furfural yield to the weight of substance before extraction. If there is an increase in furfural yield after removal of tannins their influence on the determination of pentosans according to Tollens becomes apparent. If the yield of furfural is lower after extraction with water, it is possible that substances forming furfural (as with burrs) belong to the group of substances readily soluble in water. In this case it would prove useful to determine the furfural yield from a water extract after precipitating the tannins.
Vol. 6, No. 3
The determination of pentosans according to Tollens is quite insufficient for accurate work with substances containing tannins (and perhaps also lignin) and must be supplemented by the extraction of tannins from the substance and by direct hydrolysis of pentosans to xylose. The pentosan content (furfural-forming substances in general) is to be determined as a total of three determinations-in water extract (after precipitation of tannins), in acid extract, and in the residue after hydrolysis with acid. The influence of lignin on the determination of pentosans according to Tollens can as yet be only supposition, but a t the first opportunity the authors plan to investigate by experiment the existence of such an influence. The importance of tannins in the determination of pentosan content is of greater influence than all the other known errors of the Tollens method; it leads to lowered results, whereas the other defects of the method give exaggerated results.
LITERATURE CITED (1) Freudenberg, K., and Harden, M., Ber., 60B, 581-5 (1927).
(2) Haar, A. W. van der, “Anleitung aum Nachweis, zur Trennung und Bestimmung der reinen und aus Glukoaiden u. s. w. erhaltenen Monosaccharide und Aldehydsauren,” p. 63, Gebriider Borntraeger, Berlin, 1920. (3) Hesa, K., “Chemie der Zellulose und ihrer Begleiter,” p. 123, hkademisohe Verlagsgesellschaft, Leipaig, 1928. (4) Phillips and Weihe, IND.ENQ.CHEM.,23, 286-7 (1931). RECEIVED January 12, 1933.
Filtering Fruit Juices and Plant Extracts J. ALFREDHALL^ AND WILLARD E. BAIER,California Fruit Growers Exchange, Ontario, Calif.
T
HE large-scale laboratory filtration of fruit juices and aqueous extracts of plant material is often a very troublesome operation when suction flasks and Biichner funnels only are to be used. Some years ago the authors in the Research Laboratory of the California Fruit Growers Exchange developed a method for orange juice that has since been successfully applied in many instances. I n effect, the operation combines ordinary laboratory suction filtration with the principles of industrial filtration. The filter is prepared as follows: A piece of filter cloth if available, or a fairly heavy piece of muslin, is cut to fit loosely into the bottom of a Biichner funnel of suitable size. A slurry of paper pulp of a type commercially available for filter mass is prepared in water. A thin layer of the pulp slurry is poured onto the filter and distributed evenly, and then suction is applied to drain the mass thoroughly, care being
FIGURE1. CROSSSECTIONOF FILTER
taken, however, that the pulp does not become compressed too tightly. Now a dilute suspension of a commercial filter aid, such as Filter-Cel, is prepared and, without disturbing the pulp mat, a thin layer of the diatomaceous earth is allowed to spread out on the pulp. Moderate suction is then applied t o pull this down on the pulp. These operations may be repeated several times, building up alternate layers of pulp and filter aid until the funnel is about half full, if 8 large quantity of liquid is to be filtered. 1
Present address, Forest Produots Laboratory, Madison, Wis.
There are two objects of the alternate layers of paper pulp and filter aid. Extremely fine particles are easily retained by the filter, so that brilliantly clear filtrates result. By using and removing successively the paper-pulp filter mats, a large quantity of liquid may be filtered with the minimum of dilution or loss in starting the filtering operation.
FILTRATION The liquid to be filtered is agitated with an amount of filter aid appropriate to the material. About 250 cc. were found sufficient for a liter of orange juice. I n pouring the mixture of liquid and filter aid onto the filter, ohre must be taken not to disturb the mat. It is advisable to place a small watch glass in the center and to pour upon it. Filtration is begun with only as much vacuum as is required to maintain a flow through the mat. Suction is increased gradually as the mat clogs. A cake of diatomaceous earth will build up in the funnel. When this becomes too thick for convenience, the filter may be sucked fairly dry without raising the vacuum, and, with suction off, the built-up cake and the top layer of pulp may be removed from the next thin layer of filter aid. Filtration may be continued as before. Operating in this manner, the authors were able to filter with an 8-inch (20-cm.) Biichner funnel as much as 20 liters of orange juice through one prepared filter mass a t a rate of about 15 liters per hour. The method has since been applied to a variety of products, such as pectin solutions and tannin extracts, without difficulty. Various grades of natural and calcined diatomaceous earth filter aids are available. The use of higher grades of material may be desirable in some cases. R ~ ~ C E IJanuary V ~ D 22, 1934.
