Effects of water activity on the flavor characteristics and drying quality of asparagus lettuce: The alteration of cell wall structure and water distribution.

Reducing the water activity (aw) of vegetables is an effective approach to extending their shelf life, nevertheless, excessively low aw levels may lead to undesirable effects such as texture hardening, nutrient degradation, and loss of flavor quality. This study investigated the effects of different aw levels on the sensory quality, flavor, water migration, cell wall polysaccharide content, related enzyme activities, and microstructure of asparagus lettuce, and identified aw = 0.6650 (group D) as the optimal level. Group D exhibited a significant increase in bioactive compounds, suggesting that moderate reduction of aw had a beneficial impact on nutritional quality. Moreover, group D exhibited richer aroma profiles, with enhanced fruity and vegetable notes in asparagus lettuce. The reduction of aw altered water distribution and microstructure, thereby influencing mechanical properties. Compared with fresh asparagus lettuce, the water-soluble pectin (WSP) content decreased by 54.34 %, whereas the ionic-soluble pectin (ISP) and covalently-soluble pectin (CSP) increased by 0.22-fold and 1.35-fold, respectively. These changes were accompanied by elevated activities of pectin methylesterase (PME) and polygalacturonase (PG). Transmission electron microscopy further demonstrated that moderate aw reduction preserved the integrity of the cell wall structure. These findings confirm that moderate reduction of aw enhances the nutritional quality, flavor characteristic, and texture of asparagus lettuce, providing a theoretical basis for vegetables drying processes.