Not all gelatin is equal
Recipes call for gelatin in many forms: sheets of different grades, grocery powder in envelopes, or professional powder of a stated bloom strength. Bloom measures how firm a gel is, and using a different gelatin without adjusting the amount can leave a dessert runny or rubbery.
What bloom means
The bloom value is the force, in grams, needed to press a standard plunger 4 mm into a gel made and chilled under standard conditions. Typical values:
- Bronze sheets: about 140 bloom, sheets of about 3.3 g
- Silver sheets: about 160 bloom, about 2.5 g
- Gold sheets: about 200 bloom, about 2 g
- Platinum sheets: about 240 bloom, about 1.7 g
- Grocery powder: about 225 bloom, sold in 7 g envelopes
- Pastry powder: often 250 bloom
Sheet weights vary a little between brands, so check the package.
The conversion
Gel firmness depends on the concentration squared times the bloom. To keep the same firmness:
Weight needed = weight in the recipe × √(bloom in the recipe ÷ bloom of your gelatin)
Example
A recipe uses one 7 g envelope of grocery powder, about 225 bloom, to set 2 cups of liquid. You have gold sheets of 200 bloom:
- Weight needed: 7 × √(225 ÷ 200) = 7.4 g
- Number of 2 g gold sheets: 3.7, so about 4 sheets for a slightly firmer set
- Concentration in 500 ml: about 1.5%
Using sheets and powder
- Sheets: soak in plenty of cold water for 5 to 10 minutes, squeeze out, then stir into warm liquid until melted.
- Powder: sprinkle over 3 to 5 times its weight of cold water, let it swell for 5 minutes, then melt.
- Never boil gelatin, which weakens it.
- Raw pineapple, kiwi, papaya and fresh ginger contain enzymes that prevent setting; cook them first.
- Acid, alcohol and a lot of sugar soften the set, so recipes for them use a little more.
For vegetarian desserts, agar-agar sets differently and at much lower amounts, so it is not converted with this formula.