Using Tissue Ammonia and Fermentative Volatile Concentrations as Indicators of Beneficial and Stressful Modified Atmospheres for Leafy and Floral Vegetables

Ammonia, a product of protein catabolism associated with senescence of leafy green vegetables, is toxic to plant cells if not efficiently re-incorporated by glutamate synthetase. In experiments on various intact and fresh-cut leafy and floral vegetables (broccoli, cauliflower, romaine lettuce, spinach and sugar snap peas) at 5-7.5 degrees C, tissue ammonia concentrations were measured as a possible indicator of stressful modified atmospheres and compared with the time course of changes in concentrations of the fermentative volatiles ethanol and acetaldehyde. In spinach, increases in ammonia were observed with high CO2 atmospheres and were associated with darkening of leaf tissues. In cauliflower florets, increases in ammonia and tissue discoloration were associated with the increases in fermentative volatiles in 15% CO2 atmospheres, but minimal changes in ammonia and the fermentative volatiles occurred in air or non-damaging low O-2 atmospheres with 7.5% CO2. Increases in ammonia concentrations in sugar snap peas were also associated with increases in fermentative volatiles in 12 and 18% CO2 atmospheres with 3% O-2; these conditions resulted in browning discoloration. The same CA conditions resulted in no damage and only small increases in ammonia and fermentative volatiles in broccoli florets. In cut romaine lettuce, atmospheres with 9% but not 6% CO2 resulted in notable increases in ammonia, but ammonia was not much affected by differences in O-2 concentrations (0.2, 1.0, or 21%). Increases in fermentative volatiles only occurred in cut lettuce stored in 0.2% O-2 alone or in combination with high CO2. These results indicate that ammonia may be a useful indicator of stress in modified atmosphere storage of leafy tissues. In cauliflower and spinach, atmosphere conditions that resulted in increases in ammonia also resulted in increases in fermentative volatiles. In romaine lettuce, ammonia concentrations indicated stressful atmospheres that were not associated with increases in fermentative volatiles. In other vegetables (sugar snap peas), increases in ammonia and fermentative volatiles had different time courses.