This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Olsen states that the abnormally high jelly strength with pectin heated to only 55°, shown in Table 22, is not the result of the time factor alone. To prove this he recovered the pectin from duplicate batches of jelly, one of which had been heated to 100° and poured; the other poured at 50°C. The recovered pectins were remade into jelly at 50°. Both gave jellies testing as high in strength as the original jelly poured at 50°C. This proved that the lesser strength of the pectin heated to 100° was not due to hydrolysis of the pectin but the effect of temperature upon the gel structure. From these results Olsen concludes that "the structure of jelly is fundamentally different when slowly set from the hot solution than when rapidly set from the cool sirup. If we assume that pectin exists in two states of hydration; that is, if the amount of water bound by the pectin fibrils differs depending upon the temperature at which the pectin is precipitated, then a ready explanation is at hand."
These results are not to be interpreted as showing that hydrolysis of the pectin does not occur with long boiling but that none takes place with a short-boil process.