This work proposes the use of an electronic tongue based on flow injection coulometry for the rapid determination of the antioxidant capacity of fresh lettuce. The e‐tongue consisted of a series of 16 porous carbon electrodes, each poised at a fixed potential from +100 to +850 mV. Each injection leaded to a characteristic hydrodynamic voltammogram, whose profile reflects the composition of antioxidants. The correlation between the peak area recorded by each sensor and the 1,1‐Diphenyl‐2‐picryl‐hydrazyl (DPPH) assay was maximum ín the range of potentials between +400 and 750 mV (R2>0.97). Accordingly, the charge measured provided a direct and simple index of the antioxidant capacity. The practical utility of such index was initially demonstrated by determining the best extraction conditions. This consisted in freeze‐drying of lettuce followed by methanolic extraction. Later, the e‐tongue was used to evaluate the effect of storage (one week at 5 °C) on lettuce. The e‐tongue revealed that lettuce lost up to 25 % of their initial antioxidant activity during storage. However, when lettuce samples were pre‐treated with fast cooling or vacuum cooling, the decrease of the antioxidant index was limited to 14 and 15 %. Overall, the e‐tongue is a rapid, simple and sensitive method for the determination of the antioxidant capacity of fresh lettuce samples. Indirectly, these findings suggest also that lettuce may serve as potential dietary sources of natural phenolic antioxidants.
Artificial neural networks (ANNs) demonstrated sensitive results in predicting final temperature and weight loss percentage of commercial vacuum cooling process. According to the results for final temperature, ANNs showed better prediction performance than multiple linear regression in all criteria, including an adjusted R‐squared (R2adj) of .932 and root mean square error (RMSE) of 0.579. In addition, the predicted values of weight loss percentage from ANN models were in good agreement with all experimental data (Radj2 = .82 and RMSE = 0.286). The process parameters from proper ANN model was subsequently used to investigate the effect of vacuum cooling on the qualities of baby cos lettuce during storage compared with the non‐precooled samples. The results suggested that vacuum cooling was an effective method for extending shelf life of baby cos lettuce from 9 to 16 days at 4°C. Qualities of fresh lettuce vacuum cooled using selected process parameters simulated from proper ANN model were significantly better than the non‐precooled sample during storage (p ≤ .05).
Practical Applications
Vacuum cooling widely considered the best precooling technique for horticultural produce. However, vacuum cooling also has some disadvantages including weight loss and freezing injury occurring during the vacuum cooling process, due to the setting of inapplicable parameters before the vacuum cooling process such as final pressure and holding time. Experimental study of the optimum vacuum cooling parameters for baby cos lettuce in each season (winter or summer) takes a long time and is expensive. Therefore, the present study emphasized the effects of vacuum cooling parameters setting on final temperature and weight loss percentage of baby cos lettuce. Prediction methods using artificial neural network (ANN) allow for vacuum cooling processes based on experimental research to be recommended as feasible at an industrial scale.
Fresh-cut leafy vegetable has an image as a healthy, nutritious, and delicious product. However, the product still faces the challenge of quality retention and short shelf life, especially in tropical climate regions. Enzymatic browning in fresh-cut leafy vegetables is considered one of the most important attributes limiting the shelf life of the product. The hurdle approach using commercial vacuum precooling in combination with modified atmosphere packaging (MAP) as an alternative to the use of chemical preservatives to prevent enzymatic browning, an undesirable attribute that is easily detected by consumers, was investigated. The hurdle technology exhibited synergistic effects on fresh-cut lettuce, namely frillice iceberg, romaine, and red oak, in slowing down cut-surface browning, maintaining quality, delaying microbial growth, and extending shelf life of salad products at the retail level. The findings of the study verified the potential of the hurdle approach in delaying the effect of cutting as well as extending shelf life of the product stored at 4 ± 1 °C with 85% RH from three days to nine days with an additional unit cost of 1.05%. Therefore, our hurdle approach is anticipated as the practice with non-chemical and economical approach in the supply chain of the fresh-cut, leafy vegetables industry.
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