A Summary of the article on “Actinidin in Green and SunGold Kiwifruit Improves Digestion of Alternative Proteins — An In Vitro Investigation”
Background
The study by Kaur et al (2022), published in Foods in September 2022, investigates the potential of kiwifruit to enhance the breakdown of plant-based protein sources.
Context and Objective
Alternative proteins, such as those derived from plants, are increasingly popular but there is insufficient knowledge regarding how they are digested in the human tract. Green kiwifruit (Actinidia deliciosa var. ‘Hayward’) is known to contain the proteolytic enzyme actinidin, which has been reported to enhance the upper tract digestion of animal proteins. While other gold varieties lack actinidin, the SunGold (Actinidia chinensis var. ‘Zesy002’) variety contains significantly higher actinidin activity than those others, though its activity is still much lower than that found in the green fruit. The primary objective of the study was to determine the effectiveness of actinidin present in Hayward and SunGold kiwifruit in digesting four alternative protein sources: pea protein isolate (PPI), almonds, tofu, and quinoa.
Methods and Analysis
The protein sources were digested using a three-stage in vitro model simulating oral, gastric, and small intestinal digestion. The study compared controls (no enzymes or digestive enzymes only) against treatments including kiwifruit extract (KE) alone, or KE plus digestive enzymes. Two complementary analytical approaches were used: sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) to observe the disappearance of intact proteins, and ninhydrin-reactive free amino N determination for a quantitative measure of peptide bond hydrolysis.
Key Findings on Protein Breakdown
The findings showed that both Hayward (green) and SunGold (gold) kiwifruit extracts enhanced the breakdown of all studied protein sources, particularly during the gastric digestion phase. This effect is attributed to the broader specificity of actinidin compared to pepsin, allowing it to hydrolyse a wide range of peptide bonds and potentially exposing new sites for further enzyme action. For many proteins, most of the intact proteins disappeared within the first few minutes of gastric digestion when kiwifruit extract was added.
While green kiwifruit extract generally had a greater effect due to its higher actinidin activity, SunGold extract still had significant positive effects. Interestingly, SunGold extract alone (in the absence of digestive enzymes) led to some protein breakdown during the small intestinal phase, an effect not observed with green kiwifruit extract alone.
Protein Specific Results
- Pea Protein Isolate (PPI): Both extracts alone were able to nearly completely digest resistant proteins, such as convicilin subunits, after the first few seconds of gastric digestion. Actinidin, alongside pepsin, considerably enhanced the rate of gastric digestion of pea proteins.
- Tofu: Similar to PPI, significant protein breakdown was observed with both kiwifruit extracts alone during the gastric phase, with effects becoming more pronounced when pepsin was present. The enhancement in the rate of proteolysis during the gastric phase by actinidin led to enhanced overall hydrolysis.
- Almonds: Green kiwifruit extract led to the greater breakdown of polypeptide chains of Pru-du 6, the major allergen in almonds. Almond digests displayed higher and faster release of free amino N compared to soy and pea proteins when exposed to actinidin.
- Quinoa: Cooked quinoa proteins were found to be the least digestible of the proteins tested. Although green kiwifruit extract alone showed enhanced protein breakdown visually during the gastric phase, corresponding positive effects on free amino N release were generally not observed.
Conclusions and Implications
The study suggests that both Hayward and SunGold kiwifruit can improve the breakdown and digestion of alternative proteins when consumed as part of a meal. The significant protein and peptide breakdown observed when kiwifruit extract was incubated alone suggests its potential as a digestive aid for population groups seeking faster and greater protein digestion, such as athletes, the elderly, or those with impaired digestive systems. However, the researchers noted that these results are derived from a model system and require further validation in animal or clinical trials.
References
Kaur, L., Mao, B., Bailly, J., Oladeji, O., Blatchford, P., & McNabb, W. C. (2022). Actinidin in Green and SunGold Kiwifruit Improves Digestion of Alternative Proteins—An In Vitro Investigation. Foods, 11(18), 2739. https://doi.org/10.3390/foods11182739
This post is based on Open Access research and is for informational purposes only.
