Kiwifruit proteins and enzymes: actinidin and other significant proteins.

Protein is a minor but significant component of kiwifruit. Crude protein is typically measured at about 1% of fresh weight; however, soluble protein is much less, around 0.3%. The difference can be accounted for by nonprotein nitrogen and insoluble protein, such as polypeptide chains forming part of the cell wall. Kiwifruit soluble protein is mostly accounted for by the proteolytic enzyme actinidin and its inactive forms, a so-called thaumatin-like protein and an unusual protein called kiwellin, which has no known function. Actinidin is the predominant enzyme in kiwifruit and can play a role in aiding the digestive process. There is also a wide range of enzymes involved in the ripening of kiwifruit, particularly enzymes involved in polysaccharide and oligosaccharide metabolism and in the development of flavor and aroma compounds. Whether the enzymatic actions of these have any effect during the consumption and digestion of kiwifruit is not known, although any noticeable effect is unlikely. Some enzymes are likely to have an effect on flavor, texture, and nutritional values, during storage, processing, and preparation of kiwifruit.