And it can't set because these fruits contain enzymes called proteases, such as bromelain or papain, which prevent the gelatin from setting. So you cannot just make a straightforward kiwi or pineapple jelly.
Fortunately, science comes to the rescue and explains that it is possible to neutralise the effects of these bothersome proteases. We call this denaturing. All that is necessary is to heat the fruit to 60°C.
So, to make a pineapple jelly, for example, you simply need to heat the coulis over low heat to 60°C, add the gelatine, then leave to cool.
Even so, you should note that for a kiwi jelly, you should take care not to overheat the coulis if you wish to preserve the lovely green colour of the fruit. If it is overheated, the colour will turn dull, then brown. A
thermometer is essential to ensure that the right temperature is reached but not exceeded.
As a last resort, you can use another jelling agent, such as carragheen.