Modeling ammonia accumulation and color changes of arugula (Diplotaxis tenuifolia) leaves in relation to temperature, storage time and cultiva

Leafy greens produce ammonia under stressful conditions during postharvest handling. The objective of this study was to understand the relationship between ammonia accumulation and color variation of arugula leaves in relation to cultivar and storage temperature and time. Leaves of five cultivars ('Bellezia', 'Grazia', 'Letizia', 'Tricia', and 'Wild Thing') were washed, centrifuged, placed in unsealed plastic bags, and stored at 0, 5 or 10°C for 28, 20 and 12 days, respectively. Visual quality score, color (hue angle) and ammonia accumulation were monitored every 4 or 8 days, depending on temperature. Ammonia increased from 11-15 μg g-1 fresh weight (FW) to 150-220 at 0°C, 340-450 at 5°C and >590 μg g-1 FW at 10°C by the end of storage-life. While there were some differences among cultivars, temperature played the major role in ammonia accumulation. Color varied due to leaf yellowing, which increased with increasing temperature. Both hue angle and ammonia changes were fitted with traditional (first- and zero-order kinetics) and Weibullian models, with the latter explaining a higher percentage of experimental data variance. Moreover, considering that time was the common variable between the two kinetics, a new mathematical equation describing ammonia versus hue angle for each cultivar/temperature was obtained. In all cases, there was a good correlation between ammonia content and color changes, with more accurate results at 5 and 10°C (r>0.98), since color at 0°C was quite stable (r>0.72). These results demonstrate that ammonia may be a good indicator of senescence in arugula, since it correlated well to color change with storage temperature and time.