IiYDUSTRIAL A N D ENGINEERING CHEJfISTRY
July 15, 1930
329
Carob-Seed Gum' Its Use for the Detection and Estimation of Boric Acid and Borates Ralph Hart THEHARTPRODCCTS C O X P A S S , 1440 BROADWAY, K € w YORK,h-,Y.
AROB-SEED gum, or A method is given for preparing a water solution amounts of tlle gull1 Tl-ere delocust-kernel gum, is a of the gum. The effect of alkali, acids, and boiling on ternlined where possible with cream-colored powder, the viscosity and other properties of the solution is a11 ordinary 25-cc. glass pipet,, described. It is shown that the characteristic boraxwhich swells readily in cold having a long stem. The mixgum gel is very sensitive to the quantity of borax and, tures were boiled and allowed water and dissolves in hot water to form a tasteless and based on that fact, a method has been developed for to stand o v e r n i g h t before colorless mucilaginous liquid. the detection and estimation of boron in solutions. the tests were made. The Upon evaporation of this results are given in Table I. liquid a continuous film, Jvhich is transparent and flexible, Table I-Viscosities of Carob-Seed G u m Solutions remains. Because of t,hese properties the gum has been GUZI v I S C O S I T Y AT l & - L ' l l o c. Per ccnl Seconds used in the sizing of rayon, cotton, and other yarns, as well 14 as in the finishing, printing, and back-filling of fabrics. It & "0 0.50 6% is said to be employed also as a restraining influence in tanning 0.i5 500 and as a thickener for edible sauces. 1 00 Very thick
C
:
3 00
Method of Manufacture
Carob-seed gum is obtained from the hard brown seeds contained in the locust bean, or the fruit of the carob tree (Cemtonia siliqua). According to a recent patent ( 1 ) the endosperm of t,he seed is first separated from its ye1loTV germ and tenacious husk by rolling and crushing, roasted not, to exceed 150" C. to a golden brown color, and then extracted with t\venty times its lx,-eight of boiling rvater. The clear filtered liquid is evaporated and the dried residue finely powdered. Chemical Composition
According to Killiams 14) carob-seed gum is a carbohydrate, more precisely a polysaccharide, and in its chenlical and physical properties closely resembles the starches. It probably belongs to t'he class of polysaccharides known as hemicelluloses. Williams gives the folloiving composition of a commercial sample of the gum, which also varies little from that of the endosperm:
Hexosans: Galactan Mannan Levulan Pentosans
Composition of Carob-Seed Gum Per cenl Proteins 24.84 Iiitrogen 64.39 Cellular tissue Trace Mineral matter
Peu cenl 2.40 0.39 1.46 2.81
4.07
Solid
Preparation of Water Solution
The difficulty in preparing a smooth, uniform solution of the dry gum in water is probabb responsible for the use in the industries of cominercial jellies containing only 2 to 4 per cent of gum. With proper precautions, horever, it is probably easier, 2nd of course much more economical, to use the dry gum directly. If an attempt is made to dissolve the gum by first making a paste with a small quant'ity of TTater, large lumps are formed which become covered with a hard coating of gelatinized gum, through which water penetrates so slowlv that even after 24 hours' standing the interior still remains dry and unjellied. d smooth paste is then almost impossible, even after prolonged boiling. By gradually adding the powder to the cold water while stirring vigorously, better results are obtained. The mixture is then allowed to stand for several hours-if highly concentrated, preferably overnight-and finally brought t o a boil. A smooth paste of any concentration may be prepared by first wetting the dry gum with its o m weight of alcohol or other liquid in which the gum does not jell in the cold. I n diluting the paste it is always best to add a sinal1 quantity of water a t a time, heating and stirring after each addition until uniform, and when the mixture becomes limpid the rest of the v-ater may be added more quickly.
Trade Names
Effect of Alkalies, Acids, and Boiling
Carob-seed gum comes on the market under a variety of names such as locust-kernel gum, locust-bean gum, carobseed gum, carob-bean gum, gum Hevo, gum Gatto, gum Tragon, Jandagum, Lakoe gum, Lupogum, Luposol, Rubigum, Tragarab, Tragasol, etc. It seems that a one-word name for this gum would be desirable and "caroban" is therefore proposed. It suggests its botanical origin and by its ending also its chemical nature.
Alkalies and alkaline salts darken solutions of the gum. Caustic soda, even in traces, is particularly effective in this respect; i t also makes the solution transparent. The writer finds, though contrary to the literature, that heating with caustic soda greatly reduces t'he viscosity of the solution. The viscosity of a solution containing 0 . i 5 per cent dry gum and 0.5 per cent caustic soda, heated to 95-98' C. and cooled to 3&32" C., was found to be 62 seconds, compared with 145 seconds for a similarly prepared solution but without the caustic. The viscosity test was made with the 25-cc. pipet mentioned above. Upon acidifying, the gum solution develops a characteristically sweet odor, which disappears when the solution is made neutral again or alkaline. Acids also whiten the gum solution, but upon boiling seem to hydrolyze it into watersoluble products, reducing t h e viscosity almost to that of water. Thus, the previous experiment was repeated using
Viscous Nature
When dissolved in hot water, carob-seed gum forms very viscous liquids or heavy opaque pastes; these tend to become clear on boiling, but precipitate a heavy flocculent substance upon cooling. A solution containing as little as 0.5 per cent gum is noticeably viscous and a paste of 5 per cent is practically non-flowing. The viscosities of solutions containing various 1
Received Cctober 3, 1929.
,
AAV-4L Y TI C44L EDI T I O S
330
0.5 per cent salicylic acid instead of the caustic, and the viscosity was found t o be 20 seconds, compared with 145 seconds for the gum solution without any chemical and 14 seconds for pure water. Continued boiling likewise exerted a thinning effect. Thus the previous experiment was repeated with the gum alone, but the solution was boiled for 2 hours (the water evaporated being replaced every 15 minutes) ; the viscosity was reduced to 70 seconds as compared with 145 seconds when heated t o 95-98' C. only. Gum-Borax Gel
Bean (2) states that borax renders the gum solution more glutinous and greatly increases its viscosity, even a dilute solution becoming so extremely viscous as to appear semisolid. The jellying effect of borax is confirmed by Williams ( 4 ) , who states that a saturated solution of borax mill convert a 0.5 per cent solution of the gum into a solid jelly, and he proposes this reaction as a means of identifying the gum. The writer finds that the effect of borax on the gum solution, if both are sufficiently concentrated, is to produce a solid, irreversible, transparent gel which is dry, firm, and cohesive, but will no longer stick to glass or other substances. It is consequently doubtful whether borax at any time increases the glutinous character of the solution. The characteristic effect with borax seemed very promising as a sensitive test for boric acid and borates and as a method for determining approximately the quantity of boron in solutions. Hence this reaction was investigated further. A solution of borax was gradually added to preparations ultimately containing from 0.25 to 3 per cent dry gum until compact, coherent gels were formed that no longer wetted the sides of the container. The amount of borax required t o produce these gels is given in Table 11. The jellies that were formed with these quantities of borax liquefied more or less upon heating; if, however, more borax was addedapproximately three times as much-dry, firm, quivering gels were formed that were practically unaffected when heated to 100" C. It seems, therefore, that the ultimate products formed with excess borax are heat-irreversible gels, with the possible exception of very dilute solutions, containing 0.25 per cent gum or less, which liquefy even in the presence of a large excess of borax. Table 11-Borax COSCEKTRATION OF
Pev cent 0.25 0.50
0.75 1.00 2.00 3.00
Necessary for Coagulating Carob-Seed Gum Solutions BORAXREQUIRED TO FOR\% GEL GUM Grams per 100 gvams d r y gum Per cent of soln. =to, 12 =t 50 0.04 8 0.03 4 0.03 3 1 0.02
