Quality of Lettuce Stored in a Low Oxygen Closed System.

Abstract

For the low cost vegetable transportation, the applicability of a low O2 closed system was examined under ambient temperature. Heads of leaf lettuce grown under the controlled climate of a commercially operated plant factory individually placed in airtight containers filled with normal air or with low O2 mixed gas (5% 02+95% N2) were stored for 11 d and 13 d at 15°C and for 7 d at 20°C. Populations of mesophilic aerobic bacteria from levels of 107 to 109 CFU/g were detected at the end of storage on lettuce heads in control treatment (normal air). Contrary to this, final populations on lettuce heads in low O2 treatment were 102- to 105-fold less than those in control treatment. As the final populations were less than 1 x 107 CFU/g, leaves of lettuce heads in low O2 treatment were not spoiled for all storage experiments. Supplementary examinations indicated that low O2 treatment was not effective to suppress bacterial populations on lettuce heads having large populations as about the level of 105 CFU/g or more at the beginning of storage. Hence, the low O2 closed system can be applied exclusively to the leaf lettuce grown in plant factory that can normally assure small populations of bacteria. Adding to this bacterial point of view, cumulative quantity of CO2 showed a reduction in respiration of lettuce head in low O2 treatment. Moreover, large amount of ascorbic acid was lost in control treatment as compared to that in low O2 treatment. However, no significant difference was observed in weight loss regardless of treatment.