This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Jelly deteriorates in flavor on standing. Freshly made jelly has a better flavor and aroma than jelly that has been made and stored for some time. The cooking causes some decomposition of flavoring products. In standing, some of the aromatic substances and flavoring substances are volatilized and some are probably lost by decomposition.
Storage, mold, and sugar concentration. With too high a percentage of sugar, crystallization of the sugar may occur. With too low a concentration, the jelly will mold more readily. It will also ferment more readily than a jelly with a higher concentration of sugar. Singh has reported that jellies made with less than 65 per cent of sugar molded unless the jelly containers were sterilized.
Sterilization of the glasses, and having them dry and hot when the jelly is added, probably aid in preventing mold and also fermentation. Running melted paraffin over the top of the jelly so that it forms a complete seal is an aid in keeping it.