The International Jewish Cook Book: JELLIES AND PRESERVES
Much waste of sugar and spoilage of jellies can be avoided by using a simple alcohol test recommended by the Bureau of Chemistry, United States Department of Agriculture. To determine how much sugar should be used with each kind of juice put a spoon of juice in a glass and add to it one spoon of ninety-five per cent grain alcohol, mixed by shaking the glass gently.
Pour slowly from the glass, noting how the pectin--the substance in fruits which makes them jell--is precipitated. If the pectin is precipitated as one lump, a cup of sugar may be used for each cup of juice; if in several lumps the proportion of sugar must be reduced to approximately 3/4 the amount of the juice. If the pectin is not in lumps, the sugar should be one-half or less of the amount of juice.
The housewife will do well before making the test to taste the juice, as fruits having less acid than good tart apples probably will not make good jelly, unless mixed with other fruits which are acid.
Recipe from the "The Jewish Manual" book (Free Electronic Text).
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