Freshness evaluation of leafy vegetables for the sustainable world

Abstract

This keynote will introduce the challenges we have tackled so far. Freshness of fruits and vegetables is often judged by appearance, such as their color change and wilting, and is sometimes classified into the secondary function of food, namely preference characteristics. However, a concept of freshness involves degrees of the fulfillment of primary (nutritional) and tertiary (healthful) functions. Recently, several researchers have tried to define freshness by capturing components that reflect the history of postharvest physiological conditions. These attempts are so-called searches for freshness markers, and some substances or genes have been proposed as the marker candidates so far. Our research team also have attempted to give a scientific definition of freshness. We are focusing on the dysfunction of cell membranes because altered membrane properties are the key event leading to a cascade of unfavorable biochemical reactions culminating in freshness deterioration. In this study, membrane damage during storage was evaluated by the chemical, mechanical, and transport properties of the cell membrane. Hydroponically cultivated spinach leaves (Spinacia oleracea L., cv. ‘Orai’) was stored under 5 %, 10 %, and 20 % oxygen concentration at 20 °C in the dark environment for 4 days. Malondialdehyde (MDA) equivalent, membrane tension at protoplast bursting (γ), and hydraulic conductivity coefficient (Lp) were measured over time during storage. We also tackled nondestructive measurement for membrane damage using near infrared spectroscopy (NIRS). Results showed that low oxygen conditions suppressed the changes in all three measures. MDA equivalent was linearly correlated to the γ. MDA equivalent was a leading indicator of the increase of Lp. These findings indicated that the measurements of membrane integrity would be valuable indicators for the freshness of spinach leaves, and MDA equivalent was the most useful indicator for membrane damage among the measures we investigated because it required protoplast isolation for measuring the γ. NIRS could estimate MDA equivalent with more than 0.6 of R-squares, which provided supporting information to evaluate the remaining shelf life. Our obtained knowledge will serve as a foundation for developing a freshness evaluation theory based on the dysfunction of cell membranes and will contribute to promoting an innovative technology for keeping freshness in fruits and vegetables.