Variation in gastric pH may determine kiwifruit's effect on functional GI disorder: an in vitro study

Abstract

Consumption of kiwifruit is reported to relieve symptoms of functional gastrointestinal (GI) disorder. The effect may be related to the proteases in kiwifruit. This in vitro study aimed to measure protein hydrolysis due to kiwifruit protease under gastric and duodenal conditions. A sequence of experiments incubated meat protein, with and without kiwifruit, with varying concentrations of pepsin and hydrochloric acid, at 37 °C for 60 min over the pH range 1.3-6.2 to simulate gastric digestion. Duodenal digestion was simulated by a further 120 min incubation at pH 6.4. Protein digestion efficiency was determined by comparing Kjeldahl nitrogen in pre- and post-digests. Where acid and pepsin concentrations were optimal for peptic digestion, hydrolysis was 80% effective and addition of kiwifruit made little difference. When pH was increased to 3.1 and pepsin activity reduced, hydrolysis decreased by 75%; addition of kiwifruit to this milieu more than doubled protein hydrolysis. This in vitro study has shown, when gastric pH is elevated, the addition of kiwifruit can double the rate of hydrolysis of meat protein. This novel finding supports the hypothesis that consumption of kiwifruit with a meal can increase the rate of protein hydrolysis, which may explain how kiwifruit relieves functional GI disorder.

Figure 1

Effect of KF (kiwifruit protease) concentration (mg/digest) on protein hydrolysis of 600 mg FDM (freeze dried meat) measured by percentage of Kjeldahl nitrogen (N) in the supernatant after 60 min incubation in 20 mL 0.022 M HCl at 37 °C.

Figure 2

Effect of incubation-time on hydrolysis of FDM (freeze dried meat) by KF (kiwifruit protease) measured by the percentage of ammoniacal nitrogen in the supernatant of the hydrolysate. The reaction mixtures contained 600 mg FDM, 20 mL of 0.022M HCl and 100 mg KF.

Figure 3

Effect of pepsin concentration on protein hydrolysis of 600 mg freeze-dried meat at two acid concentrations (0.022 M and 0.074 M HCl) measured by percentage of ammoniacal nitrogen in the supernatant of the hydrolysate after 60 min incubation at 37°C. Points on the graph are means of duplicate digests and error are standard deviations

Figure 4

Effect on the hydrolysis of 600 mg of freeze-dried meat (FDM) after 60 min primary digestion with pepsin alone (red bars), or in combination with 100 mg of KF (purple bars). The green bars indicate the hydrolysis effect in 0.022 M HCl of 100 mg of KF on 600 mg of FDM in the absence of pepsin. Incubations each contained 20 mL 0.022 M HCl, 600 mg of FDM, plus various combinations of pepsin ranging from 0 to 30 mg/digest alone or in combination with 100 mg KF. Bars are means of duplicates and errors are standard deviations.