Why does milk become bitter if you add kiwi?
If you combine milk, yogurt or curd with fresh kiwifruit, it will become bitter and viscous.
The reason for this unpleasant effect is due to the proteolytic enzyme actinidin of the kiwifruit. This enzyme breaks down milk proteins (casein) into smaller sub-units (peptides, amino acids), which are responsible for the bitter taste. Pineapple and papaya can also make milk taste bitter because of its proteolytic enzymes bromelain (in pineapple) and papain (in papaya).
If the gelatin does not get thick...
This may also be attributed to the compounds of the mentioned exotic fruits. Actinidin and the other enzymes "attack" the protein structure of the gelatin, so that it doesn't thicken up.
The enzymes of the fruits are inactivated by scalding them briefly with hot water. Treated like this, the gelatin thickens up despite of the presence of kiwi and pineapple. In muesli or cereals this would be a bit too much effort and the heat-sensitive vitamins would also suffer. Here it helps to add the kiwifruit just before consumption or to serve them separately.
But you can also take advantage of those proteolytic properties of exotic fruits: e.g. if you put kiwi or pineapple slices onto the surface of meat. This breaks down the connective tissue and makes the meat more tender. In the industry, tenderizers like bromelain and papain are used most often.
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