Bloom

patent drawing

A measure of the gel strength of gelatin, reflecting the average molecular weight of its constituents. The higher the Bloom number the stiffer the gelatin and, in general, the more expensive it will be. The name is from Oscar T. Bloom, inventor of the Bloom gelometer.

The word is found both capitalized and in all lowercase in publications by knowledgeable experts in the field. It is sometimes preceded by a “g”, for gram, the reason for which is explained below, and sometimes followed by “number”, as in “Bloom number”.

To make a test, a 112-gram sample of 6.666% w/w gelatin gel is prepared in a standardized container and conditioned following a highly standardized time and  temperature regime. After a number of hours the sample is brought to 10°C and an instrument measures the force needed to push a plunger 12.5 millimeters in diameter 4 millimeters into the gelatin. This force is produced by dropping shot into a cup in a controlled manner until the plunger reaches the 4-mm depth.  It is the weight of a mass, and that mass, expressed in grams, is the Bloom number. So if it takes 250 grams of shot to depress the plunger 4 millimeters into a sample of gelatin, that is 250 Bloom gelatin.

In Europe, following the British Standard, the bottom edge of the plunger was rounded (0.4 mm radius). In North and South America, Japan and some other locations the plunger has a square, 90° lower edge (the “AOAC” plunger). This difference produced a very slight difference in results. The Gelatine Manufacturers of Europe and the Gelatin Manufacturers Institute of America mutually agreed that, as of 1 July 1998, the AOAC plunger would be used everywhere.

Gelatin used in food usually runs from 125 Bloom to 250 Bloom; the unflavored gelatin sold in supermarkets is at the higher end of this range. The highest grade in commerce is around 300 Bloom, which is used, for example, by special effects makeup artists to create prostheses, such as fake wounds. Gelatins with low Bloom numbers are used, for example, to clear cloudiness in beverages.

A frequent problem is knowing how much gelatin of a particular Bloom number is needed to substitute for a given quantity with a different Bloom. For example, in some culinary uses 1.54 pounds of 125 Bloom gelatin may be substituted for 1 pound of 250 Bloom gelatin. Unfortunately, no formula for making such a conversion will give satisfactory results in all the many uses to which gelatin is put. Check with your supplier.

United States Patent No. 1,540,979.
Machine for Testing Jelly Strength of Glues, Gelatines, and the Like.
June 9, 1925.

BS 757: 1975.
Methods for sampling and testing gelatine.
British Standards Institution, 1975.

No longer an active standard.

Carl R. Fellers and Francis P. Griffeths.
Jelly-Strength Measurements of Fruit Jellies by the Bloom Gelometer.
Industrial and Engineering Chemistry, August 1928, page 857.

An example of the use of the device from the period of its invention.

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