Studies On Proper Harvest Maturity Control And Postharvest Quality Keeping Of Fresh Lettuce

Fresh lettuce is an important vegetable and a traditional medicine with high nutritional value and commercial importance for human life due to its unique flavor and phytochemicals like antioxidant compounds.This study aimed to investigate the preharvest and postharvest quality change of fresh lettuce by biochemical and multispectral image visualization analysis. In order to find out the quality change, a lot of research were studied, including the effect of harvest maturity on nutritive quality (total phenolic, ascorbic acid and monomeric anthocyanin content) and sensory quality (texture and color), the identification of harvest maturity, quality maintenance and browning inhibition of fresh-cut lettuce by allicin treatment during storage, the nondestructive and intuitive determination of green color quality in lettuces and soybean leaves using multispectral imaging. The findings from this study are expected to provide a theoretical basis and technical support for quality keeping of lettuce. The main results are shown below.For proper harvest maturity control, firstly, the accumulation of total phenolic, ascorbic acid and chlorophyll content in the lettuce increased gradually over the course of the maturation process, reaching its accumulation peak at the mature stage. Although not as dramatic as hydrophilic antioxidant activity, a remarkable increase and decrease in lipophilic antioxidant activity was observed at three maturation stage. The hydrophilic and lipophilic antioxidant activity reached the maximum activity in lettuce over the course of the maturation process, which may be caused by the highest levels of phytochemical antioxidants (total phenolic, ascorbic acid etc.) in the mature lettuce. Sencondly, There were highly significant (p< 0.01) decreases in the levels of total phenolics, ascorbic acid, total monomeric anthocyanin and chlorophyll content in immature lettuce as compared with the mature lettuce. The decrease of phytochemical antioxidants (total phenolic, ascorbic acid and total monomeric anthocyanin) content leads to highly significant (p< 0.01) decreases of hydrophilicand lipophilic antioxidant activity in immature lettuce. Thirdly, the total phenolic, ascorbic acid and chlorophyll degradation was initiated when the lettuce progressed to the senescence stage. However, the senescence did not induce the degradation of total monomeric anthocyanin (p>0.05), but make the hydrophilicand lipophilic antioxidant activity decrease in senescence lettuce. Leaf senescence leads to metabolism and remobilization of the nutrients from senescing cells to developing tissues, which enhances nutrients synthesized in immature lettuce and nutrients degenerated in senescence lettuce. Since phytochemical antioxidants determining the lettuce quality (color, taste, nutritional value), those vegetables with higher phytochemical antioxidants content will have higher quality. In addition, the increase and decreases in total monomeric anthocyanin were different from the enhancement in total phenolics, which would indicate that monomeric anthocyanins are not the main phenolic compounds in lettuce.For the identification of harvest maturity, the findings show that the correct identification rate of partial least squares-discriminant analysis (PLSDA) and (support vector machine) SVM all were 100%, meanwhile, the correct identification rate of back propagation neural network (BPNN) were 95% and 100%, respectively, indicating the effectiveness of these these methods in maturity identification for mature and senescence lettuce. Moreover, the discrimination result of BPNN model was more superior to PLSDA and SVM model, principal component analysis (PCA) also enables to separate these lettuces of three maturity stage.For visualization of green color quality, these maps of the spatial and concentration distribution of chlorophyll help us to understand the change of chlorophyll concentration in lettuce leaves during different maturation stages. As maturity advanced (from immature to mature stage), an increase in chlorophyll concentration for the whole lettuce leaves, with each pixel colors gradually turning red. The mature lettuce leaves contained more regions of red than the immature lettuce leaves, especially at the edge of lettuce leave. When the lettuce progressed to the senescence stage, however, chlorophyll degradation was initiated. The senescence lettuce leaves contained less regions of red than the mature lettuce leaves especially at the edge of lettuce leave, and the red areas were clustered distribution suggesting that the chlorophyll degradation is not synchronized in different regions of cell. Of note, the synthesis and degradation of chlorophyll in the whole lettuce leaves had an’edge effect’, in other words, the amount of chlorophyll synthesis and accumulation and the degradation is the most at the edges of leave during development process of lettuce. The green color quality of soybean has a circadian rhythm which can be observed by multispectral imaging.For browning control of fresh-cut lettuce, during shelf life, combining 0.5% allicin with vacuum packaging reduced the content of total phenols and inhibited polyphenol oxidase (PPO), peroxidase (POD) and phenylalanine ammonia-lyase (PAL) activity and chlorophyll degradation, and then restrained tissue browning of fresh- cut lettuce.