Study On Chitosans And Oligochitosans Anti-oxidative Activity And Anti-browning Mechanism Of Apple Juice

Apple juice, due to its nutritional value as well as pleasant sensory qualities, is a popular social beverage. But the browning occurred in the apple juice process, can not only affect the color and flavor of fruit juice, but also greatly compromise its nutrition;Therefore, the use of anti-browning agents is required. However, market trends on food preservation are limiting the use of synthetic additives and the search for natural anti-browning products is increasing in correspondence to industrial and consumer demands. Chitosan (CS) as a kind of natural alkalescency polysaccharides with active hydroxyl and amino groups not only has antioxidant but also can refresh fruit. So it's a significant task for us to research the anti-browning of CS in the apple juice.Firstly, the different molecular weight chitosan samples were prepared by the method of acid degradation in the HCl + Ethanol mixtures. Chitosans with four different molecular weights (C4:94 KDa, C5:160 KDa, C6:320 KDa, C7:560 KDa) were prepared by degradation of the initial specimen (Mw: 1186 KDa), and the DD of them were above 90%. The water-soluble oligochitosans (OCS) with 1 KDa (C1), 2 KDa (C2) and 5 KDa (C3) were purchased from Dalian Glycobio Co., Ltd. (Dalian, China). Their DD were also above 90%.The antioxidant activities of these CS and OCS were evaluated by determination of scavenging of hydrogen radical, superoxide radical, DPPH radical, as well as the reducing power and the total antioxidant activity in linoleic acid emulsion system. Low molecular weight oligochitosans (C1-C3) were more effective scavengers of the radicals than those high molecular weight chitosans. At 2 mg/mL, the hydroxyl radical scavenging activities of C1 and C2 were 98.35% and 96.52%, and that of C3 was 57.64%. However, C4-C7 were only in the range of 39.68% - 43.04%;When the concentration of samples was 10 mg/mL, C1 exhibited higher 76.54% scavenging ability against superoxide radical compared to that 28.32% of C7. The order of scavenging activity against superoxide radicals was: C1> C2 > C3 > C4 > C5 > C6 > C7;The three low molecular weight chitosans C1-C3 exhibited higher DPPH radical scavenging activity than four high molecular weight chitosans C4-C7. At 1 mg/mL, the scavenging abilities of C1-C3 were 65.52%, 59.41% and 53.59%, respectively.However, those of C4-C7 were only 19.38%, 14.85%, 13.34% and 10.53%. The reducing power of the low molecular weight oligochitosan is clearly more pronounced than that of high molecular weight chitosan. At a concentration of 5 mg/mL, the values of reducing power of C1-C3 were 1.337, 1.075, and 0.600, respectively. In contrast, those values of C4-C7 were nearly zero.The strong correlation between the total antioxidant activity and the molecular weight of chitosans in linoleic acid emulsion system was also found. Among the seven types of chitosans, C1 had the highest scavenging ability against radical, the reducing power and the total antioxidant activity.Browning of apple juice gradually increased with the storage time extending from 0 to 48 h. C1 inhibited the browning of apple juice during storage with a concentration-dependent mode at 1.0, 2.0 and 4.0 mg/mL. For example, at 0h, the browning inhibition rate of C1 was 65.24%,74.87% and 82.73%, respectively. The contents of VC and reduced sugars in the apple juice treated with C1 were almost unchanged, indicating that C1 did not affect the quality of apple juice when inhibiting browning. But the contents of proteins and total phenols decreased markedly. This may be because C1 and proteins and total phenols flocculate by the electrostatic adsorption in order to prevent the post-haze in apple juice.Then the inhibition mechanism of C1 was studied. The apple juice treated with C1 could decrease the content of O2-·; delay the activities of SOD and POD, and maintain them at high level. Besides, C1 has a strong inhibition of polyphenol oxidase (PPO).The experiment showed that all chitosans exhibited antioxidative activities in vitro. Low molecular chitosans were more effective in the antioxidant than high molecular chitosans. C1 could decrease the browning of apple juice and it did not affect the quality of apple juice when inhibiting browning. The inhibition mechanism of C1 may be it could decrease the content of O₂^-·, delay the activities of antioxidase and inhibit the activities of polyphenol oxidase (PPO).