130 J O U R N A L OF T H E A. S. S. B. T.
Table 1.—Extraction of Araban from Sugar Beet Pulp.
fails to remove pectin. It is of interest to n o t e t h a t the p u l p becomes slimy a n d difficult to filter w h e n extracted at pH 9. T h i s m i g h t be an explanation for the observed d e t e r i o r a t i o n of p u l p at high battery supply water tem- p e r a t u r e s . T h e battery supply water is usually alkaline because of con- densate r e t u r n , a n d this, coupled with the h i g h e r t e m p e r a t u r e , causes the d e g r a d a t i o n of pectin a n d the solution of a r a b a n . F r o m these results it would seem desirable to control the alkalinity of the battery supply water to i m p r o v e the pressability of the p u l p a n d to minimize the extraction of a r a b a n .
Extractions also have been m a d e with saturated lime water at 100°.
T h e arabinose o b t a i n e d was 13.5 p e r c e n t in 40 minutes, a n d 18.5 percent in two hours. Since this is close to the total arabinose in the p u l p , lime water was chosen as the extracting m e d i u m because of simplified control of conditions, ease of removal of excess calcium, a n d because the pectic acids p r o d u c e d form insoluble lime salts.
Purification and Isolation
T h e p r e p a r a t i o n of a r a b a n was u n d e r t a k e n in o r d e r to study the effect of this material on purification a n d crystallization studies in o u r processing laboratory. For o u r purposes it was desirable to o b t a i n p u r e a r a b a n , or the p u r e s t arabinose-containing polysaccharide possible, in o r d e r to isolate the effects d u e to a r a b a n from those of the o t h e r colloidal materials in the beet.
A l t h o u g h the presence of a high p r o p o r t i o n of arabinose in a gum from sugar beet p u l p was k n o w n as early as 1873 ( 1 ) , the first r e p o r t of the isolation of an a r a b a n from sugar beets was by G a p o n e n k o v (5) in 1936.
T h i s m a t e r i a l was o b t a i n e d by extraction of h y d r o p e c t i n by 70 percent e t h a n o l a n d yielded 98 p e r c e n t arabinose on hydrolysis. It is claimed by Schneider a n d Bock (6) t h a t this extraction p r o c e d u r e , which has been used by o t h e r workers, actually extracts only the lower-molecular-weight arabans, a n d i n d e e d the m a t e r i a l isolated by G a p o n e n k o v was fractionated by p r e c i p i t a t i o n with 80 percent, 75 percent, a n d 70 p e r c e n t alcohol into fractions with molecular weights of 5,700, 6,600, a n d 7,700 respectively. In 1945 I n g e l m a n (3) purified an a r a b a n from sugar beets by dialysis and electrophoresis a n d d e t e r m i n e d the m o l e c u l a r weight to be a b o u t 10,000, a n d the specific r o t a t i o n to be — 1 2 9 ° . H i r s t a n d J o n e s (7) have reported on the structure of sugar beet a r a b a n . It is a highly b r a n c h e d chain of L-arabo-furanoside residues, 2/3 of the u n i t s linked t h r o u g h positions 1 a n d 5, the r e m a i n d e r b e i n g t e r m i n a l g r o u p s l i n k e d t h r o u g h positions 1 and 3.
VOL. IX, No. 2. July 1956 131 We have found that extraction of p u l p with lime water and precipita-
tion of araban with alcohol yields an impure araban, as noted by Hirst and Jones. We have resorted to fractionation of the acetylated araban followed by dc-acetylation to give a purified araban.
In a typical isolation. 120 grams of sugar free beet p u l p was extracted for two hours at 100° in 1500 ml. of saturated lime water. T h e extract was treated with CO., to pH 8, filtered, and the residual ions removed with ion exchange resins. After concentration to 200 ml., araban was precipitated with 9 volumes of 95 percent E t O H , washed, and dried. Yield was 17.5 grams of a r a b a n : 14.9 grams of this crude araban was acetylated by the method of Carson and Maclay (8) . using formamide as a dispersing agent. T h e product was precipitated in ice water, washed, and dried. The yield was 23 grams of almost white material, anhydroarabinose 48 percent, calc. for araban diacetate 60.2 percent.
Crude araban acetate prepared in this m a n n e r was fractionated by two methods, column fractionation on charcoal or fractional precipitation from acetone by the addition of petroleum ether. Details of the fractionations are being published elsewhere (9) . Columnar fractionation was somewhat superior to the fractional precipitation.
To recover araban from the acetate, samples were treated with IN KOH for several days at room temperature until solution was complete.
KOH was neutralized with, excess acetic: acid, then 8 volumes of E t O H were added. T h e precipitated araban was reprecipitated from aqueous solution to remove KOAc and dried. T h e yield was essentially quantitative. Data on fractions from a charcoal column are given in T a b l e 2. T h e specific rotation is the best measure of the purity of the fractions. Fractions 0 and
11 were obtained by elution of the column with acetone-chloroform mixtures, fraction 20 by elution of the column with dioxane. T h e purities reported were calculated from the pentose values by the orcinol-ferric chloride method (10), winch has consistently given results about 5 percent to 10 percent higher than p a p e r chromatographic determination. A second fractionation, starting with an 89 percent araban acetate, gave fractions with a maximum purity of 95 percent, and a specific rotation of — 1 2 2 ° . T h i s material has not been deacetylated. Although even the best araban from these fractions still shows evidence of the presence of galactose and possibly a trace of rham- nose, it is probably suitable for use in studying the effect of araban on processing characteristics of sugar beet juices.
It is probable that the arabinose in sugar beet pulp is present largely as a true araban, since the yield of araban acetate with a purity of greater
132 J O U R N A L OF T H E A. S. S. B. T.
t h a n 90 p e r c e n t was a b o u t 35 grains from 75 grams of starting material.
T h e possibility of the presence of an arabo-galactan is certainly n o t excluded by these results, however, for n o n e of the p r e p a r a t i o n s were completely free of galactose, which is the p r i n c i p a l i m p u r i t y in t h e various materials p r e p a r e d in this study.
Extraction of Commercial Grade Araban
T h e r e is a n o t h e r aspect to the a r a b a n in sugar beet p u l p ; namely, its value as a by p r o d u c t . A l t h o u g h a r a b a n from sugar beets is low in molecular weight, it may be useful, just as are o t h e r p l a n t gums a n d mucilages, as an adhesive, a dispersing agent for emulsions, as a s u s p e n d i n g a g e n t in lotions, creams, etc. T h e drawback to the use of a r a b a n from sugar beet p u l p is the expense of the usual alcohol p r e c i p i t a t i o n . If Ca ( O H )2 is used as the extracting m e d i u m , the pectin is degraded to pectic acids which form in- soluble calcium salts a n d can be separated by filtration. T h e excess calcium may be removed, along with the proteins, by c a r b o n a t i o n in the usual m a n n e r . A r a b a n can t h e n be o b t a i n e d by e v a p o r a t i o n of the filtrate. This p r o c e d u r e has been followed in this laboratory to o b t a i n crude araban as a light-brown powder. T h e use of carbon may be necessary to produce a light-colored p r o d u c t . T h e cost of this o p e r a t i o n would be very low, since existing e q u i p m e n t in the factory could be used for almost the e n t i r e process.
Summary
T h e isolation of a r a b a n from sugar beet p u l p has been described, and the conditions u n d e r which a r a b a n is extracted from the p u l p have been investigated. T h e use of a r a b a n as a by p r o d u c t of the p r o d u c t i o n of sugar is discussed, a n d an economical m e t h o d for its p r e p a r a t i o n is o u t l i n e d .
References
(1) SCHEIBIER 1873. U e b e r das V o r k o m m e n der A r a b i n s a u r e (Gummi) in d e n Z u c k e r r u b e n u n d u b e r d e n Arabinzucker (Gummizucker) . Ber.
6:612.
(2) H A R R I S , W. A., N O R M A N , L. W., T U R N E R , J. H., H A N E Y , H. F., a n d COT- TON, R. H. 1952. M e t h a n o l - i n s o l u b l e m a t e r i a l in sorgo a n d beet juices.
Ind. Eng. C h e m . 44:2414.
(3) I N G E L M A N , B. 1945. Results of sugar research in U p p s a l a . Socker Handl.
1: 179 ( C A . 39: 5525 1945).
(4) M C C R E A D Y , R. M. a n d M C C O M B , E. A. 1954. Q u a n t i t a t i v e determination of sugars on p a p e r c h r o m a t o g r a m s by a reflectance m e t h o d . Anal.
C h e m . 26: 1645.
(5) G A P O N E N K O V , T. K. 1936. Physicochemical p r o p e r t i e s of a r a b a n . Colloid J. (USSR) 2: 561, (C.A. 3 1 : 1649 1937).
(6) SCHNEIDER, G. G. a n d BOCK, H. 1937. U b e r die K o n s t i t u t i o n der Pectin- stoffe. Ber. 70: 1617.
(7) H I R S T , E. L. a n d J O N E S , J. K. N. 1948. Pectic substances pt. VIII.
T h e a r a b a n c o m p o n e n t of sugar beet pectin. J. C h e m . Soc. 1948: 2311.
(8) CARSON, J. F. a n d M A C L A Y , W. D. 1946. Acylation of polyuronides with f o r m a m i d e as a dispersing agent. J.A.C.S. 68:1015.
(9) GOODBAN, A. E. a n d O W E N S , H. S. C h r o m a t o g r a p h i c fractionation of sugar beet a r a b a n . M a n u s c r i p t in p r e p a r a t i o n .
(10) FORNELL, W. R. a n d K I N G , H. K. 1953. T h e simultaneous determina- tion of p e n t o s e a n d hexose in m i x t u r e s of sugars. Analyst 78:80.