Gelatin (sometimes gelatine) is a common gelling agent and thickener that most people are familiar with. It is flavorless, colorless, and brittle when dry. In its pure form, gelatin comes either as a gelatin sheet or as powder. Generally, it is made from animal bones and collagen, the most common source being animal skin.
Sheet Gelatin, also called Leaf Gelatin, works like granular gelatin found in your local grocery store, but in a different form. Rather than a powder, it takes the shape of thin sheets or leaves of gelatin film. The sheets dissolve more slowly than the granulated form, but also produce a clearer gelled product.
Commercially produced gelatin has been available since the early 1800s (with a British patent for its manufacture issued in the mid-1700s). This gelatin was always in sheets or strips. In 1890, Charles Knox developed the world’s first pre-granulated gelatin which possessed several advantages (for the home cook of that day): it dissolved faster in water and was easy to measure with measuring cups.
Gelatin sheets, also called leaf gelatin, are made from gelatin that is dried in a thin flat piece. The sheets dissolve more slowly than the granulated form, but there is no chance of undissolved granules in the dish. In addition to ease of measuring, most professional chefs use gelatin sheets because it results in a clearer, more transparent final product with a purer taste. European recipes typically call for the use of leaf gelatin.
To make things more complicated, there are different types of gelatin sheet, each with different gelling strengths or “bloom”. I’m going with silver sheet gelatin, which I’ve used most often.
To use it you just put the gelatin sheets in a bowl filled with cold water. After they’ve been submerged about five minutes, they’re re-hydrated. Hold them gently in one hand, and slick the water off the sheets with the fingers of your other hand. The gelatin dissolves quickly when stirred into a warm liquid or heated over a double boiler.