Influence of kiwifruit on protein digestion

Green kiwifruit consumption has long been thought to assist in the digestion of food proteins due to the presence of the proteolytic enzyme actinidin. This chapter reviews the recent findings of both in vitro and in vivo studies on the effect of green kiwifruit (which contain the enzyme actinidin) on the gastro- and small-intestinal digestion of a range of common food proteins and protein-rich foods including milk, meat, fish, eggs, legumes, and cereal proteins. Clear evidence is provided that green kiwifruit, and the enzyme actinidin itself, can provide enhanced upper-tract digestion (particularly gastric) of a variety of food proteins, which lends support to a role for dietary kiwifruit as a digestive aid. Kiwifruit influences the digestion patterns of food proteins to varying extents. For some protein sources, digestion in the presence of green kiwifruit resulted in a substantially greater digestion of intact protein, and different peptide patterns were produced compared with those seen after digestion with mammalian digestive enzymes alone. Kiwifruit extract alone (in the absence of other digestive enzymes) has been observed to be capable of digesting some proteins present in foods, particularly those in yoghurt, cheese, fish, and raw eggs. An in vivo (pig) study including a positive control of added actinidin and a negative control where the actinidin in green kiwifruit had been inactivated showed conclusively that actinidin is responsible for the enhanced gastric hydrolysis of food proteins.

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.

Actinidin enhances gastric protein digestion as assessed using an in vitro gastric digestion model

Consumption of kiwifruit has long been claimed anecdotally to assist in gastric digestion. This has generally been assumed to be due to the presence of the proteolytic enzyme actinidin; however, there is little published evidence supporting this assumption. This paper reports the findings of an in vitro study that examined the effect of kiwifruit proteases (actinidin) on the digestion of a range of common food proteins under simulated gastric conditions. An extract from green kiwifruit containing actinidin was prepared. Several protein sources derived from soy, meat, milk, and cereals were incubated in the presence or absence of the kiwifruit extract using an in vitro digestion system consisting of incubation with pepsin at pH 1.9, simulating gastric digestion in humans. The digests were subjected to gel electrophoresis (SDS-PAGE). For some protein sources, simulated digestion in the presence of kiwifruit extract resulted in a substantially greater loss of intact protein and different peptide patterns from those seen after digestion with pepsin alone. As an example, the addition of actinidin extract enhanced the digestion of alpha-, beta-, and kappa-caseins in sodium caseinate by 37, 33, and 48%, respectively. Under simulated gastric conditions, kiwifruit extract containing actinidin enhanced the digestion of some, but not all, food proteins over and above that found with pepsin alone.

Pre-rigor infusion with kiwifruit juice improves lamb tenderness

The ability of pre-rigor infusion of kiwifruit juice to improve the tenderness of lamb was investigated. Lamb carcasses were infused (10% body weight) with fresh kiwifruit juice (Ac), water (W) and a non-infusion control (C) treatment. Infusion treatment had no effect on lamb hot carcass weight, cold carcass weight and chilling evaporative losses. The infused treatment carcasses of Ac and W had lower (P<0.05) pH values than C carcasses during the initial 12h post-mortem. The LD muscles from Ac carcasses were more tender with significantly lower shear force (P<0.001) compared with C and W carcasses during the six days following infusion with the kiwifruit juice. The enhanced proteolytic activity (P=0.002) resulting from the infused kiwifruit juice in Ac carcasses was associated with significant degradation of the myofibrillar proteins, appearance of new peptides and activation of m-calpain during post-mortem ageing. Thus, kiwifruit juice is powerful and easily prepared meat tenderizer, which could contribute efficiently and effectively to the meat tenderization process.