Studies On Active Compositions In Fermented Broth Of Aspergillus Niger For Removing Naringin And Limonin From Citrus Juice

Naringin and limonin are main bitter compounds in citrus fruit,which hinder the development of citrus processing products due to that no practical technology for removing naringin and limonin have been estabilished yet.Our early study have found that the concentrations of naringin and limonin in fermented broth decreased significantly when Aspergillus niger was cultured with the extract of Guanxi pummelo segment and its albedo as the carbon source.The active composition in fermented broth of Aspergillus niger for removing naringin and limonin was analysed by modern seperation technology in the present thesis,providing references for the processing of citrus fruit.The main results were shown as following.(1)The concentrations of naringin and limonin in citrus juice decreased significantly as well as the bitter taste after citrus juice was treated with fermented broth of Aspergillus niger,and it demonstrated there were active compositions in fermented broth of Aspergillus niger which could remove naringin and limonin from citrus juice.(2)After treated with fermented broth of Aspergillus niger,naringin in citrus juice was hydrolysed into prunin and naringenin,but no metabolites of limonin has been detected.(3)Naringinase was purified from fermented broth of Aspergillus niger by the sequent procedure of ammonium sulphate precipitation,ion exchange chromatography and gel chromatography.After activity identification,the active compound in fermented broth of Aspergillus niger for removing naringin from citrus juice was just naringinase.The naringinase had the molecular weight of 293 kDa and it contained four subunits.(4)The effective treatments for purified naringinase removing naringin from citrus juice were stated as below: 0.012 IU of naringinase per mililiter juice,waterbath for 90 min,optimum temperatures of 50-60 °C,optimum pHs of 3.0-5.0.After the treament,the removal ration of naringin reached as high as 97.40%.(5)No enzyme for removing limonin in citrus juice was purified from fermented broth of Aspergillus niger,however ammonium sulfate was found to decease limonin significantly.And heat treatment was the essential condition for ammonium sulfate removing limonin from citrus juice.(6)The optimun temperatures and pHs for ammonium sulfate to remove limonin from citrus juice were 60-80 °C and 4.0-6.0 respectively,the removal ration of limonin reached up to 63.68%.(7)For ammonium sulfate,sulfate ion plays the key role in removing limonin from citrus juice.The merchanism of sulfate ion to decrease limonin was that sulfate ion changes the equilibrium shifing direction of different forms of limonin in citrus juice,which leads to limonin in juice supernatant transfering into juice precipitate as solid state.In conclusion,the thesis has demonstrated the active compostions in fermented broth of Aspergillus niger for removing naringin and limonin were naringinase and sulfate ion,and revealed the treatment conditions for naringinase and sulfate ion to remove naringin and limonin respectively.The results provided the reference for the processing of citrus juice.