Effect of Evacuation Rates on Vacuum Precooling of Iceberg Lettuce and Its Post-storage Quality
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Abstract:
The techniques of slow air precooling and vacuum precooling with different evacuation rates (pressure reduction rates at A-0.126 min-1, B-0.185 min-1, and C-0.315 min-1) were applied to precool iceberg lettuce, and all samples were packaged after precooling and stored in a cold room (5 ± 2 ℃). The effects of different precooling methods on iceberg lettuce were evaluated by measuring related indices for precooling and storage processes. The experimental results showed that vacuum precooling at any evacuation rate was more effective in removing heat than in slow air cooling. However, the precooling times of groups B (65 min) and C (70 min) were equivalent, showing no significant difference (p > 0.05). Nearly the same amount of time was required to remove the heat in different parts of the iceberg lettuce when the evacuation rate was the same. After six days of storage, group B showed a significantly lower respiration rate 183.9 mg/(kg?h) and CO2 concentration (7.2%), and significantly higher vitamin C (46 mg/kg) and O2 (2.1%) concentrations at the top of the packaging bag than groups A and C. All differences were significant (p < 0.05). In conclusion, an evacuation rate of 0.185 min-1 is suitable for precooling of iceberg lettuce.
