Gelatine is a common component in patisserie delicacies but, for the home cook, the gelling agent is a far from an easy ingredient to master. It comes in two forms, powdered and sheet (also called leaf). Although powdered gelatine is easier to find, professionals tend to prefer sheets because they are easier to use, melt more smoothly and the resulting jelly is clearer than when using the powdered form. Both types need to be softened (the term is 'bloomed') in liquid before they can be melted; it's essential that the liquid is cool otherwise the gelatine will melt. Powdered gelatine needs to be sprinkled over the liquid so it's evenly moistened - if you add it all at once, there might be pockets of hard, dry gelatine that will never soften.
Many recipes call for sprinkling a 'package' (whatever that happens to be; it varies according to manufacturer) over a small amount of liquid - often as little as two tablespoons. Usually, though, that's not enough liquid, and the gelatine doesn't soften evenly, so, when it's melted, it's grainy. But you also can't use too much liquid, or it dilutes the flavour of the dish, and it can also mean it won't gel firmly. The general guideline for powdered gelatine is that one pack (seven to eight grams) sets up 500ml of liquid.
Gelatine sheets are also bloomed in cool liquid, although the amount of it doesn't matter because the excess will be squeezed out. Lay the sheets one by one in the liquid, making sure each one is evenly moistened before adding the next. If you were to add many sheets all at once to the liquid, the outside sheets would be moist before the ones in the centre are sufficiently soft. They're ready when they're malleable, so that you can pick them out of the water and squeeze out the excess liquid. There's a fine point between being just soft enough and being too soft, though; if the sheets fall apart, you've overbloomed them and will need to start all over again. It takes only about five minutes to bloom gelatine sheets.
The problem with gelatine sheets is that they come in various sizes and strengths. Different sizes weigh different amounts - and therefore have different 'setting' powers. The strengths are usually called bronze, silver, gold and platinum (or titanium). Many recipes ask for X number of sheets of gelatine, without specifying the type. If you buy boxed gelatine sheets, the instructions should tell you how many sheets are needed to gel a certain amount of liquid, with a range given depending on how firm you want the finished product to be; if you want to mould the product, it needs to be firmer; for softly set dishes, use fewer sheets.
But what if you have a cookbook recipe that indicates only the number of sheets, without giving specifics? I have two types of sheets that I bought from Twins (1/F, 137 Johnston Road, Wan Chai, tel: 8111 3090). They're labelled 'thick' and 'thin', and although their dimensions are the same (8cm x 22cm) the former weighs five grams per sheet, the latter 2.5 grams. If I used five sheets of thick gelatine, when a recipe writer wanted the thin sheets, my dish would be overgelled and therefore rubbery.
So, how to figure it out? A good place to start would be by weighing your gelatine sheets, and using, as a general guideline, eight to 12 grams per 500ml of liquid (this is subject to trial and error). In this week's recipe, on page 60, eight grams of sheets gives a softly set jelly that isn't rubbery.
With all types of gelatine, ingredients such as sugar, milk and alcohol strengthen the gelling power while acidic ingredients and salt lessen it. Certain fruits, such as pineapple, mango, papaya and kiwi, kill gelatine - they contain bromelain or papain, which break down the gelatine, rendering it unable to gel. Overheating the gelatine will also break down its setting power.