If you have a gelatin of a certain strength and need to know how much to multiply the amount in a recipe calling for a different strength, you need to use a little formula to convert:
M_b = M_a * (B_a/B_b)
Where M represents mass, B is bloom strength, the b subscript the bloom strength you want to convert to and a subscript the one you are called to use.
To simplify this, I made a table of conversion factors, since the ratio of bloom strengths doesn't change, but the masses do. To use this table, multiply the mass of the original amount of gelatin by the conversion factors below to get the mass of the gelatin you want to use. First, choose the row. This is the strength of gelatin you want to convert from. Then, go right across the columns to find the gelatin strength you're converting to.
|Name (Bloom)||Bronze (125-155)||Silver (160)||Gold (190-220)||Platinum (235-265)|
For example, suppose I have a recipe calling for 21 grams of Platinum strength gelatin, but I have Silver strength. To find out how much gelatin I need, I find platinum on the left most column, then I move across the table to find silver. The gelatin conversion factor is 1.47. Amount I need is equal to 1.47 times 21. I need 30 grams of silver strength gelatin.
Occasionally you'll see the bloom strength given directly. Then you can use the formula above.
Also, you may see sheets of gelatin rather than weights. In this case, use the table below to calculate the amount of gelatin per sheet.
I'd like to thank Martin Lersch of khymos.org for publishing a guide to hydrocolloids. Please visit his site.