IN order to follow the changes which take place in the pectic constituents of stored apples, a number of preliminary investigations have been found to be necessary. In the first place a method was required by which pectin could be estimated accurately in dilute solution. This having been established, it has become possible to proceed to an investigation of the means by which the various pectic constituents of the fruit can be separately extracted. The present communication, in so far as the extractions are concerned, is confined to an examination of methods for extracting the soluble pectin of fruits. The difficulties which are encountered in the course of this extraction are almost entirely mechanical-difficulties inseparable from the process of washing out this soluble colloidal material from the insoluble colloids of the cell wall with which it is intimately associated. The clearing up of these mechanical difficulties is a necessary preliminary to any further investigation of methods of extraction, for unless suitable methods of washing out are adopted much soluble pectin may remain behind to form a mixture with the pectin dissolved out by subsequent treatment.It has been the general practice hitherto to estimate the pectin content of solutions by precipitating the pectin with alcohol. This method at its best is inconvenient and lacking in accuracy. The pectin obtained is necessarily of variable composition, since pectin probably exists in a number of forms intermediate between neutral pectin and pectic acid. Moreover, as dilution increases precipitation becomes increasingly difficult, until at very low concentrations no precipitate is obtained, even after prolonged standing with a large excess of alcohol. By the method now adopted the difficulty of precipitation is avoided, for calcium pectate can be made to flocculate from solutions of very low concentration, and a product of definite chemical composition is obtained. A comparison of the results of precipitating pectin with alcohol and as calcium pectate is given in the sequel.
MEASUREMENTS of the hydrogen ion content of the juice of apples were undertaken by one of the authors (D. H.) some years ago in order to ascertain whether any sudden change in the value of this property marked the beginning of breakdown. The experiments were carried out over two seasons and i3o
The substances known as the pectins, which are said to form the middle lamella of plant cells, offer many points of interest. It is supposed that they are readily attacked by various fungi which are the causes of plant diseases, and a knowledge of their chemistry is, therefore, of considerable importance to plant pathologists. They also, under certain conditions, which are not yet clearly understood, form gels, and it was mainly the interest in this particular property which first attracted the authors of this paper to' their investigation. One of them (S. B. Schrvver) had already commenced a systematic chemical research, when the other [Haynes 1914] published a paper dealing with the conditions under which pectic gels were formed, and they decided to continue in partnership their researches, which have, unfortunately, been repeatedly interrupted by other work. Very little of the literature dealing with the subject of the pectins is of recent date, and the statements made by earlier authors are often confused and contradictory. It is not proposed to discuss the literature in any detail in this place, as it is dealt with in an adequate manner in various handbooks .
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