Effect Of Processing Technologies On Quality And Antioxidant Activity Of Green Asparagus Juice

Green asparagus (Asparagas Officinalis L.) is very popular with consumers for its appealing flavor, high vitamins, minerals and other nutritive compounds. It is a vegetable with low calorific energy and high nutritive value. The harvest period of green asparagus is relatively concentrated. After harvest, the vegetable rapidly lignifies, senesces and loses its commodity due to its high respiration rate. It has2-3days shelf life under ambient temperature. It is urgent to deeply develope asparagus product to extend supply time except for prolonging its shelf life by controlling storage condition. Asparagus juice is a relatively new product. Blanching, squeezing, enzymatic treatment, and pasteurization are usual unit operations during asparagus juice processing. These processing treatments affect the quality and flavor of fruit and vegetable juices. However, there is no systemic study on the effect of processing technologies on quality and antioxidant activity of asparagus juice. The present study was designed to investigate the quality, antioxidant activity and juice processing characteristics from different green asparagus species in different harvest period. The studies were primarily carried out about effects of processing technologies, such as juice extraction methods, enzymatic clarification treatment, vacuum concentration, thermal and ultra-high pressure pasteurization, on the quality, antioxidant activity and flavor of green asparagus(Asparagas Officinalis L. cv. Grande) juice. In addition, the effects of storage temperature, light condition and package materials on color and juice stability of green asparagus juice during storage time were also explored. The aim is to provide scientific basis for the development of green asparagus juice product. The results were as follows:1. Significant differences were found in quality, antioxidant activity and juice processing characteristics of green asparagus from different species and different harvest periods. The quality indexes a*, vitamin C, lignin, microelements, amino acids or total flavornoids and total phenolics contents of green asparagus were evaluated by principal component analysis (PCA) and membership function in fuzzy mathematics. The quality of ’Grande’green asparagus was the best. Then, it was’Altas’,’UC301’,’UC157’and ’UC800’. The quality of’Grande’green asparagus harvested on May was the best. Secondly, it was respectively green asparagus harvested on April, July, and June. Generally, the quality of green asparagus harvested in spring was better than that one in autumn. Great antioxidant activity was found in green asparagus’Grande’and’UC157’. Then, it is’Altas’,’UC301’and’UC800’.’Grande’ green asparagus harvested on May showed greater antioxidant activity than those one on April, July, and June.’Grande’green asparagus was appropriate for juice processing due to its high juice yield, soluble solids, and better color stability and minor sediment in its juice during storage time.’Grande’green asparagus harvested on May was appropriate for juice processing due to better color stability and minor sediment in its juice during storage time.2. Blanching at91℃for3min could inactive peroxidase (POD) activity and preserve the color of green asparagus. Juice extraction technology significantly affected the quality and antioxidant activity in asparagus juice. Juice extraction for blanching and breaking could preserve the color of green asparagus juice and improve the stability of vitamin C. Juice extraction for blanching, breaking and cellulase treatment increased the soluble solids, total sugar, total flavornoids contents and improved antioxidant activity in green asparagus juice. The soluble protein and total phenolics contents in green asparagus juice were decreased by these two juice extraction technologies.3. Cellulase and pectinase treatment conditions for green asparagus juice were optimized by using response surface methodology (RSM). The results showed the response and variables were fitted well to each other by multiple regressions and the proposed second-order polynomial model was adequate. According to response analysis, the optimal enzymatic treatment condition for cellulase was concentration of0.07%, incubation temperature of42.96℃, and pH of5.03, and that for pectinase was concentration of1.45%, incubation temperature of40.56℃and pH of4.43. The clarity, juice yield and soluble solid contents in asparagus juice were significantly increased by enzymatic treatment at the optimal conditions.1,1-diphenyl-2-picryhydrazyl (DPPH) radical scavenging capacity was maintained at the level close to that of raw asparagus juice.4. Condensation technology significantly affected the quality and antioxidant activity in green asparagus juice. The greenness, soluble protein, total sugar, and total phenolics contents in green asparagus juice were increased as soluble solids contents were increased. And while the lightness, vitamin C, total flavornoids contents, and antioxidant activity were decreased. Volatile flavoring compounds enriched with SPME were identified by GC-MS. The results indicated that aldehydes were main volatile flavoring compounds in green asparagus juice. Secondly, alcohols, ketones, compounds containing nitrogen, and phenols were found in green asparagus juice. Few esters and organic acids were also contained in green asparagus juice. Condensation treatment significantly affected volatile flavoring compounds in green asparagus juice. The main volatile flavoring compounds, such as aldehydes, alcohols, and ketones, showed significant loss when soluble solids content was10%. Further concentrating treatment increased flavoring compounds. When soluble solids content was30%, green asparagus juice show the best flavor due to its high flavor compounds contents, such as alcohols, aldehydes, ketones, and other compounds. Green asparagus juice showed poor flavor due to its low flavor compounds contents as soluble solids contents was45%.5. Thermal pasteurization (TP) at121℃for3min could effectively kill microbials to the undetectable level and prolong the storage time of green asparagus juice. TP treatment effectively inhibited the decreases of soluble protein and total sugar contents. The decreases in total phenolics, total flavomoids contents and DPPH radical scavenging capacity were inhibited by TP treatment. The results showed that TP treatment was an effective technology for extending the storage time and keeping the quality of green asparagus juice. However, TP treatment had negative effects on the quality indexes of green asparagus juice, especially vitamin C and greenness. High pressure pasteurization (HPP) treatments preserve better greenness, vitamin C, rutin and antioxidant activity than TP treatment. Alcohols and ketones contents were kept at a higher level and aldehydes contents were significantly increased by HPP treatments. HPP treatments for400-600MPa could ensure the safety of green asparagus juice and effectively keep the quality and antioxidant activity. The results identified by GC-MS showed that aldehydes were main volatile flavoring compounds in green asparagus juice. The representatives were hexanal,2-heptenal, pentanal,2-butenal or2-methyl-2-propenal,2-pentenal,2-octenal,2-hexenal, nonal, and octanal. The representatives of alcohols were1-octen-3-ol and nonanol. The representatives of ketones were1-octen-3-one,3-octanone,6-methyl-5-hepten-2-one, geranyl acetone, β-ionone. In other compounds, methoxy-phenyl-oxime was high, then2,4-di-tert-amyl phenol and2-pentyl-furan. Ester was mainly isobutyl phthalate. Organic acid was hexanoic acid.6. Storage temperature, light and package materials obviously affected the color stability of green asparagus juice during storage time. The decreases of chlorophyll, vitamin C, total phenolics and reducing sugar contents packaged in glass bottle were inhibited at refrigeration and light-resistant condition. At the same condition, the increase of carotenoids contents and the production of polymeric color were prevented and the color stability of green asparagus juice was effectively kept. The principle component in sediment of asparagus juice was sugar. Secondly, protein, pectin, and total phenolics were contained in sediment. Fructose was the main sugar in sediment of green asparagus juice. Glucose was also related to the production of sediment. Phenolics compounds in sediment were phenolics acids and flavornoids. The decreases of phenolics acids contents were observed in green asparagus juice after storage. Thus, it is inferred that these phenolics acids, such as chlorogenic acid, caffeic acid, and ferulic acid, were probably related to the formation of sediment. The histidine and lysine were decreased in green asparagus juice after storage. These two arnino acids had high proportion in sediment. It is inferred that they probably interacted with polyphenolics compounds. The proline in sediment of green asparagus juice was high. However, the proline content was not decreased after storage. Thus, it is probably that the formation of sediment in green asparagus juice was not directly connected with proline.