This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
If the boiling point of sugar solutions is referred to, p. 49, it is found that a sugar solution containing 60 per cent of sugar boils at 103.0°C, and a 70 per cent solution boils at 106.5°C. A jelly boiled to 103°C. would contain approximately 60 per cent of sugar, and one boiled to 106.5°C. would contain approximately 70 per cent of sugar. Evidently, a jelly cooked to a temperature between these two temperatures would contain between 60 and 70 per cent of sugar.
Cruess and McNair have reported that jellies containing amounts of sugar indicated by boiling to 65° Brix give a good concentration for jelly. A solution of sugar of 65° Brix boils at sea level at 103.9°C. Cruess and McNair state that if, to the boiling point of water of any locality, 3.9°C. or 7.02°F. is added, a suitable boiling point for jelly will be obtained for that locality.
The directions in the laboratory outline state to boil the sirup to a temperature of 103°C. for jelly. This is for a location where the water boils at 99°C. This is sufficient for fruit juices that contain a high percentage of pectin and have a high acidity. This would give approximately 60 per cent of sugar in the finished jelly. Some juices with a low acidity and pectin content will yield better jelly if boiled to a higher temperature than 103°C. A temperature between 104° and 105°C. yields a jelly with a sugar content of about 65 per cent. If they do not jell with a temperature of 105° to 106°C. the juice does not contain sufficient pectin or acid.
Jelly test. The method of housekeepers of determining when jelly is boiled sufficiently is to let a portion of the sirup drop from a spoon. When the sirup "sheets" off the spoon the jellying temperature has been reached.