VOL.3, NO.5
THEPECTICSUBSTANCES
505
THE PECTIC STJBSTANCES W. H. DORE,DrvrsroN or PLANT NUTRITION, AGRICULTURAL EXPERIMENT STATION. UNIVERSITYOF CALIFORNIA, BERKELEY,CALIFORNIA
At the Los Angeles meeting of the American Chemical Society last August, a group of about sixty chemists assembled for the purpose of holding a symposium on pectin. The event may he taken as an index of a considerable present activity in research on the pectic substances, and of a widespread general interest in pectin chemistry. The pectic substances have a present claim upon the interest of intelligent people from a t least three points of view: (1) the role of these suhstances in plant life, (2) their value i n animal nutrition, and (3) their possible commerical applications. The role in plant life is placed first, partly because it is the most fundamental of these aspects, and partly because of the probahility that i t has an important bearing upon agricultural practices. The second phase, concerning the nutritive value of pectin, has so far received scant attention, but present tendencies indicate a greater interest in the future. The commercial aspects have attracted much attention within the last few years and it appears probable that the development of the industrial applications of pectin has only just begun. In order to appreciate the significance of any of these three phases, some understanding of the physical and chemical nature of the pectic substances is necessary. Unfortunately onr knowledge in that field is still far from complete, but researches which are now being carried out may be expected Lo reduce this deficiency to some extent. An attempt will be made here to outline some of the facts which have been established up to the present time and to indicate the trend of studies now under way and their application in the three fields mentioned. Chemistry of the Pectic Substances Just a century ago, in 1825, Braconnot1 first isolated the jelly-making substance of frnit juices and called i t pectin. This discovery was followed by the extensive researches of Fremy2 and later by those of S~heibler,~ H e r ~ f e l dtoll , ~ en^,^ and other^.^ The work of these investigators gave considerable information as to the occurrence and chemical nature of the pectic substances and many pectin preparations were made and named. I t is uncertain how many of these preparations were individual substances. The known pectic substances may be grouped into three classes: (1) Protopectin., formerly called pectose, is the insoluble form which occurs in unripe fruits and in most other parts of plants. This suhstance, as its name implies, is the original pectic substance from which the soluble forms are derived. ( 2 ) Pectin is a term used to designate the soluble pectic substances oc-
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curring naturally in the juice of ripe fruits or obtained artificially by treating protopectin with mild hydrolytic agents. (3) Pectic acid is obtained by subjecting pectin to hydrolysis. As will be pointed out later in this article, pectin is an ester which on saponification yields salts of pectic acid. Pectic acid occurs naturally in overripe fruits and vegetables. The terms used to designate the individual pectic substances have been used differently by various investigators and as a result there has been much confusion in the literature. In an attempt to remedy this condition, a committee was appointed a t the Los Angeles Pectin Symposium to study the existing nomenclature and make recommeudations for the uniform naming of pectic substances. The pectic substances were early recognized as closely related to the carbohydrates in that they yielded arabinose and galactose on hydrolysis. The early belief that they were true polysaccharides like starch and cellulose was shown to be erroneous when Tollens6 demonstrated the presence of carboxyl groups. Since this discovery, the pectic substances have been recognized as carbohydrate derivatives possessing acid properties rather than as true carbohydrates. In 1917, von Fellenberg' showed that pectin, on being treated with sodium hydroxide undergoes saponification like the fats, yielding methyl alcohol and the sodium salt of pectic acid, and from this he concluded that pectin is the methyl ester of pectic acid. In the same year Ehrlich' announced the discovery that the carboxyl groups of pectic acid reside in galacturonic acid, a sugar acid derived from galactose by the oxidation of its terminal alcohol to carboxyl according to the equation: COH.(CHOH)CH*OH f On --+ COH.(CHOH)r.COOH Galactose Galacturomc acid
Oxidation of the aldehydic group of the galactose molecule does not occur because of glycosidal linkage, the presence of which was demonstrated by the isolation of a galactose-galacturonic acid among the hydrolysis products of pectin. By another type of hydrolysis, Ehrlich isolated from pectin a product in which four galacturonic acid groups have apparently condensed with the elimination of three molecules of water to give a tetragalacturonic acid. The sodium salt of this acid is obtained by alkaline saponification of the pectin, and methoxyl and galactose groups are simultaneously split off. According to Tutin,s pectin yields acetone as well as methyl alcohol upon saponification; the acetone is believed to result from its isomer, and pectin is regarded by Tutin as ( OH CH2) probably "the dimethyl-isopropenyl ester of pectic acid." On the other isopropenyl alcohol CH~-C
