Mechanistic insights into gas exchange in postharvest cucumbers through biochemical and physical contributions

<h2>Abstract</h2> We measured the effective diffusion coefficient (<i>D</i>∗) in cucumber fruit using Ne gas. This method involves sealing Ne with the fruit in an airtight container and monitoring its concentration in the container's free volume, allowing us to determine <i>D</i>∗ based on the reaction-diffusion equation with a zero reaction rate. Results revealed a decrease in <i>D</i>∗ over time after harvest, possibly due to reduced porosity and increased apparent gas pathway length within the fruit. We constructed a gas exchange model incorporating <i>D</i>∗ for O₂, converted by Graham's law, and a Michaelis-Menten type O₂ reaction without CO₂ inhibition. The O₂-dependent gas exchange rates were estimated using kinetic constants from measurements taken under 20 % O₂ at 1 d after harvest. These estimates matched satisfactorily with measured values, confirming the model's validity. Numerical simulations unveiled the respective contribution of transport properties and reaction rates to the observed decrease in postharvest gas exchange rate. The decline in <i>D</i>∗ explained approximately 15 % of the reduction in gas exchange rate from 1 to 5 d after harvest. A smaller Michaelis constant and a larger maximum reaction rate than at 1 d after harvest were required to explain the reduced gas exchange rate at 5 d, suggesting that the decline in respiratory substrates and respiration-related enzymes activity primarily governs the postharvest gas exchange rate. Our results clarified the combined effects of changes in gas diffusivity and reaction rates on the gas exchange of the whole fruit, providing mechanistic insights for postharvest handling and storage.