2 BULLETIN 47, HAWAII EXPERIMENT STATION. 
DETERMINATION OF PECTIN IN FRUIT JUICES. 
Careful search through literature failed to give any quick, prac- 
tical method for determining the pectin content of fruit juices. As 
a rule the pectin content is not determined, and the quantity present 
is indicated by specific gravity or by means of the Brix hydrometer. 
The Brix reading indicates at once the proportion of sugar that 
should be added to the juice in accordance with tables that have 
been worked out for apple juice, but since such tables are of no 
value in connection with other fruits a separate table must be con- 
structed for each specific fruit. 
Home economics text and cook books commonly advocate the 
method of adding fruit juice to alcohol in a tumbler to determine 
the pectin content of a juice. The appearance of the precipitated 
pectin when poured out is taken as an indication of the content, 
the juice being said to contain a high percentage of pectin when 
the precipitate pours out in a single lump, only a medium quantity 
when there are several small lumps, and not enough pectin for 
jelly making when there are no lumps. Many substances, such as 
magnesium sulphate, can be used in place of alcohol to precipitate 
the pectin. This method, while valuable for the housewife, was not 
deemed sufficiently accurate to be of use in connection with the 
present investigation. 
Quantitative methods entailing precipitation, filtration, weighing,, 
and ignition require too much time for completion to be of practical 
use in jelly making. 
By means of a cylinder, graduated to 25 cubic centimeters and 
fitted with a plunger, 2 a pipette, to deliver the juice to the alcohol 
drop by drop, and a piece of silk bolting cloth about 4 inches square,, 
the writer developed a method for determining the pectin content 
of fruit juices. (PI. I, Fig. 1.) After 20 cubic centimeters of 95 per 
cent ethyl alcohol were measured into tha cylinder, 10 cubic centi- 
meters of the juice to be tested were added drop by drop to the 
alcohol. The contents of the cylinder was then filtered through the 
silk bolting cloth, which was rolled back and forth in the hand 
until the pectin precipitate gathered into a compact mass and the 
alcohol ceased to run out. 3 As soon as the pectin mass had been 
rolled into cylindrical shape it was slipped into the cylinder and 
shaken to the bottom. The plunger was then placed on top of the 
mass and the volume in cubic centimeters, or " pectin number," was 
noted. The operation required only three to five minutes with most 
of the juices tested, and duplicate results were had within a quarter 
of a cubic centimeter. 
The amount of alcohol which the pectin mass will retain depends 
of course on the pressure exerted. The purpose of rolling on the 
silk cloth is to remove the excess alcohol with no pressure at all 
other than that exerted by its own weight. The purpose of the 
plunger is not to exert pressure, but to make possible an accurate 
reading of the pectin number. 
The relationship existing between the "pectin number" and the 
percentage of pectin was determined by ascertaining the alcohol 
precipitate, that is, the impure peetin, for which purpose the method 
2 A Nessler tube having the proper diameter can be used as the plunger. 
3 In this operation time'ean be saved by placing the cloth upon a folded towel to absorb the alcohol, but, 
the results obtained are not nearly so accurate as when no towel is used. 
