THE COXSISTEXCY OF PEc’rm GELS iPRELIMINARY COMMUS ICATION) BY J.4S
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Just what chemical substances are necessary and how thej7 combine to form a fruit jelly of the proper consistency has been quite a subject of in\-estigation and argument. Fremy ( I) claimed t h a t jell)- formed from a change of pectin into pectic acid. He considered pectic acid the essential part of jell)- as fruit juice spontaneously gelatinized when pectin was transformed t o pectic acid by the action of a n enzyme pectase This jell>-, howe\.er, has no connection with fruit jelly for three reasons. , I . When a pectic acid solution and sugar are mixed no -jelly forms, and secondly, pectic acid together with sugar and another acid or salt gives a sprupy liquid -1 jelly of the proper consistency does not form because pectic acid is riot soluble in this medium 3 -in analysis of fruit jelly does not disclose any pectic acid H e considered Tschirch t 2 i eiperimented with c:irrants jell>- as due to a pectin and sugar yolution or a combination between pectin and sugar \\-ithout sugar he could obtain no jell?- -1s a disproof of this theor!- we have the experience of von Fellenberg Vpon dialyzing currant jelly, he observed t h a t it attracted water quickly The liquid increased in the apparatus and in fifteen hours the jelly dissolved S o clear solution resulted, howex-er, flocks of a body similar to cellulose remained which could be removed b>- filter or centrifuge. n’hen the clear filtrate was dialyzed, flocks were formed during dial>-siswhich were not present beforehand in the jelly These flocks dissolved in a sugar solution and on successive heating and cooling a jelly formed These flocks were considered b>Tschirch as pectin But this substance is not taken as pectin by \-onFellenberg I 3 I first because the filtrate free from these 1 Contrihutioii from the Rudolph Spre Physiological Laboratory and the Enology Laboratory ot the L-niversitl of California
flocks when heated with sugar gives a jelly Secondly. this flocking body in currants is not present in other fruits Thirdly it difTers from the pectin of \ o n Fellenberg in not containing methoyyl groups Lastly, Tromp de Haai and Tollens ( 9 ) analJ-zed this flocculating hodj to find it elidently a carbohjrdrate containing j 4 4 percent carbon, and 5 O j percent hydrogen I t containi, therefore a higher percentage of carlmii than either cclluloie or pectin .* 1 ~ prove 1 that pectin p l a j i an important riile in jelly formation voii Fellenberg dissoli ed quince jell? in boiling \I ater From thi5 mi\ture lie precipitated pectin 11) adding ould form onl? a 5~ rupy solution, alcohol The filtrate i~ not jell) If the precipitated pectin 11 ere added. jell! formed .I iirnilar result n a i ohtained when purified pectin w a i added to quince SJ rup .Ipurified pectin dtid sugar solution alone ga\ e no jelly Pectin froni varioui fruiti and arying quantities when added t o '1 iugar solution resulted in s ~ r u p sonly He concluded, therefore, the fortnation of jell? is due to other su1:stances than pectin and iugar If jell? formation lw considered as a coagulation it might be iupposed that A jell! nould result b j addition 0 ; mineral salt. which coagulate pectin to a solution of pectin and sugar. This was not the case for when ferric chloride was added to a solution of pectin and sugar. flocks of a pectin-iron coagulum separated, no jelly formed Von Fellenberg obtained better results by using organic salt\, z c , malatei of calcium, magnesium, and aluminum These gaxe jellies when added to a solution of pectin and sugar Such jelliei nere not as stiff as those formed by the addition of pectin to pectin-free fruit 5yrups. I n summarizing his work with jellies, Yon Fellenberg concluded t h a t besides sugar and pectin certain salts are necessary to jelly forination H e considered the possibilitj of other parts of fruit juice playing active riiles in jelly making ;t pre\ ious inx estigator to von Fellenberg on jelly making, Miss Goldthwaite 11), noticed that jelly formation would
occur when 0. j percent tartaric acid solution. I percent pectin, and three-fourths \-olume of sugar were heated for not inore than fifteen minutes. She considered the presence of free acid necessarj- for jelly. In fact she claimed that neutral juice would not gelatinize. In considering earlier researches I lielie\.e t h a t the malates of calcium, magnesium and aluminum may owe their 1-irtue in gelatinization t o the presence of the hydrogen ion. Acid malates may ha1.e been present as impurities in the salts used by von Fellenherg. I t is doubtful if neutral malates would aid appreciably in gelatinization. In commencing work on jell!- I coilsidered the principal factors in securing a fruit jellIi7 of the proper consistent!-. pectin, sugar, and acid. The pectin u ~ e dw a s ohtained from Villa Franca and Eureka lemon rinds 1))- the \-on Fc~l1enl)erg process. 4 kg of lemon rinds were boiled in a reflus c o ~ i denser 6-8 times (zcniinute 1:eriodsi \vith 4 - 6 liters 01' OLj percent alcohol. This process tended to free them from coloring matter, acids, etc. RetIveeii each 1)oiling the material was pressed in a screw press. Three liters of water were adc1t.d to the final press cake and the misture heated in an autocla\-c for an hour a t I I G ' C. A l tthe end of this period the contentc; were pressed and the liquid was filtered through a Seitz asbestos filter until clear. The pectin was obtained from the filtrate by adding a double \-olurne of alcohol to which c'c o f concentrated hydrochloric acid had been added per liter. The coagulum was pressed in canton flannel arid kneaded in several changes of alcohol until the acid reaction disappeared. Finall!. the pectin was washed with ether and dried in a vacuum desitcator o\-er sulphuric acid. Dr. Robertson suggested that a method might be evol\-ed for the determination of pectin through use of its refracti\-e index. The refractive index of a 2 percent water solution of lemon pectin was found to lie 0.00130 per g percent when observed in a Pulr'rich refractometer which indicates to within I ' of the angle of total reflection. A sodium flame was employed as a source of light. The solution was adjusted to
the temperature of the room so t h a t the temperature of the refractometer prism and t h a t of the solution dsed would be the same. It has been shown by Robertson i 5) t h a t in alkaline or acid aqueous solutions of proteins the change in the refractive index of the solvent is directly proportional t o the concentration of the dissolved protein. I n respect t o its refractive index pectin acts similarly t o protein. This relation can be expressed by the equation: n-nl
=
a X c
when 7% is the observed refractive index of the pectin solution, w1 t h a t of the solvent in which the pectin is dissolved, c the percentage of pectin in solution, and a a constant, expressing the change in the refractive index of the solvent by the addition of I g of pectin per IOO cc. In the viscosity experiments a Redwood viscosimeter was used. The results will be found in Table I. The results show: I . T h a t all three of the substances tested, when in aqueous solution, independently increase the viscosity of distilled water. 2 T h a t mixtures of any two or all three of the substances result in an increase of viscosity greater than the \-iscosities independently. 3 . T h a t jelly will form when acid, pectin, and sugar are in solution of certain concentration 3. T h a t jelly will form from a solution of 3 g percent pectin and G j g percent sugar. j. The viscosities of the separate substances are not additil-e. A4ccordingto Hardy (61 and Robertson ( 7 ) jelly formation is analogous in some instances to emulsification and may, therefore. be dependent on surface tension phenomena. From the Tolman ( 8 ) theory of colloids, in the case of a lyophilic colloid in equilibrium with a dispersing medium, an increase in the concentration of the hj-drogen ion in the dispersing medium lessens surface tension and hence increases dispersion. Thus an increase in hydrogen ion concentration would result in an increase in the total surface of the pectin and consequently the viscosity of the solution could be increased. Still further additions of acid might lead t o such high degrees of dispersion
The Consistency qf Pectin Gels
COWP
c e
l 3 ~
I
N
N
N
.N -. N.
3 3 3
1
63 7
t h a t the condition of affairs in so-called true solutions would be approached and thus the viscosity again decreased. The function of sugar ma?- lie t h a t of a dehydrating agent. It also imparts a certain amount of viscosity to t h e jelly presumably by combining the water which would otherwise lead t o fluidity. Glycerol acts similarly. The comparative anhydrous state of jell?- may be shown by the addition of a small amount of either cupric chloride or cobalt chloride t o the jelly. Green and red colors, respectively, result. which are the anhydrous colors of these salts. Upon dilution these colors change t o hlue and pink. the respective hydrous colors of the salts. If the function of sugar is principallj- that of dehydration then a higher concentrat,'on of pectin together with thc same concentration of acid and no sugar should gi1.c. a jelly of equal firmness to the one in which sugar is present. Indeed experimental results indicate such to he the case.
Conclusions I , The author considers pectin. acid, and sugar the principal factors in fruit jelly making. 2 . Pectin was obtained from lemon rinds b y boiling in several changes of alcohol, heating in an autoclave for I hr. a t 110' C in the presence of water, filtering. precipitating by a double volume of acidified alcohol, removal of acidity with alcohol, and ether, drying in 1-acuum desiccator over sulphuric acid. 3 . The combined refractive index of Villa Franca and Eureka lemon pectin in water solution was o.oo13c per g percent. 4. The results of the viscosity experiments indicate t h a t : i d ) Pectin, acid, and sugar independentlj- or combined increase the \-iscosity of water. ( b ) Jell?- will form from a mixture of acid, pectin and sugar when in certain concentration. (i) Jelly will form when pectin and sugar are dissoh-ed in certain concentration.
id) The viscosities of the various substances are not additive I C ) The increase of viscosit? may lie due t o the presence of the hydrogen ion T h a t sugar ma!- ha\-? two functions, t h a t of a (-1 dehydrating agent and of increasing the viscosity 'The nork i i being continued I would like t o express m! gratitude to Professor T Brailsford Robertson and -Assistant Professor 1T.m Tr Cruess, under 11 horn thi5 I\ ork wa5 done L ~ :
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R I B I,I OG I
