Why won’t my fruit mousse set?

If pineapple, papaya, mango or kiwi are included in the mousse, this leads to a degradation of the gelatin, because these fruits contain a proteolytic enzyme, which causes an almost total loss of gelling strength. However, it is still possible to make a gelatin dessert with these fruits, but only if they are briefly steamed or doused with hot water, since heat prevents the enzymes from becoming active.