| ?-poly-L-lysine??-PL?is a homo-poly-amino acid produced by microbial fermentation,?-PL is a basic polyamide that consists of 25-30 residues of L-lysine with an ?-amino group-?-carboxyl group linkage.Because of its broad antibacterial spectrum,high safety,strong thermal stability and good water solubility,it has been widely used as a natural food preservative in Japan,Korea,Europe and other countries and regions of the food industry.In addition,it is also widely used in medicine,electronics and materials because it has the characteristic of polycation,it is one of the new industrial biotechnology products with potential market value.In this study,separation and extraction of ?-PL were used as the object of study,to ensure the successful implementation of ?-PL separation and extraction trial production and to improve the original extraction process as the purpose.The selection of membrane separation equipment and the process parameters in ?-PL extraction were studied.The effective of ultrafiltration technology used in the impurities removal was discussed,and a new process of ?-PL extraction based on two-stage ultrafiltration was proposed.The main findings are as follows:?1?A solid-liquid separation method of ?-PL fermentation broth with centrifugation combined ultrafiltration was established.First,the bacterial removal of the diluted fermentation broth was carried out by using the ultrafiltration with tubular membrane,and serious membrane fouling and unrecoverable membrane flux proved that the solid-liquid separation of ?-PL fermentation broth could not be achieved by ultrafiltration only.Secondly,the solid-liquid separation method of disc centrifuge combined with tubular membrane ultrafiltration was established by the optimization of the disc centrifuge conditions and ultrafiltration conditions of selected membrane.The optimal conditions were as follows: the dilution rate of fermentation broth was 1:4,the feed flow was 3.6 m-3·h-1,the pH was adjusted to 9.0,and the molecular weight cutoff of membrane was 200 KDa.The average ?-PL yield of two batches of solid-liquid separation in trial production reached 92.31%,the average purity reached 35.28%.?2?The method of removing salt from ?-PL ion exchange eluent by nanofiltration was established.To develop the process of desalination in the separation and purification of ?-poly-L-lysine,the nanofiltration was used in this study.Firstly,several nanofiltration membranes were evaluated,then the operation parameters were optimized through one-factor-at-a-time experiment using model solution as object.Finally,the eluent of ion exchange step was desalinated by nanofiltration at the optimal condition.The results showed that SP800 was the best choice used for desalination in terms of permeate flux,?-PL retention ratio and NaCl removal ratio.Meanwhile,the optimal desalination conditions were as follows: the pH was 9.0,and the maximum ?-PL concentration was concentrated to about 100 g·L-1,while the permeate conductivity was controlled less than 0.3 mS·cm-1 through adding deionized water.To evaluate the efficient of the developed desalination process,1000 L ?-PL eluent from ion exchange step was employed for desalination by nanofiltration at the optimal condition.Finally,the NaCl removal ratio and the ?-PL loss ratio were reached 96.43% and 0.77%,respectively;and the ?-PL purity and ash content were attained 96.21% and 1.02%,respectively.This is the first report on the desalination of ?-PL solution using nanofiltration,it would be helpful to the production of ?-PL in industry.?3?The effective removal of impurity in the trial production was achieved.By analyzing the change trend of impurity in the extraction of the laboratory and the trial production,it was found that the impurity can not be effectively removed in the nanofiltration and the molecular weight of the impurity was about 1000-2200 Da.Then,the removal of impurity by different ultrafiltration membranes was investigated.It was found that the 3 KDa membrane could effectively remove the impurity.When 12 3KDa membranes?each membrane area is 26.5 m2?were used to dispose solution from ion exchange step?solution volume was 10.5 m3;the concentration of ?-PL was 5.02 g·L-1?,the experimental results show that the average flux is 9.08 m3·h-1 and the impurity could be totally removed after 160 min of ultrafiltration.The loss of ?-PL is 19.11%,the purity of the sample is 98.89% and the ash content is 0.99%.?4?A new process of ?-PL extraction based on two-stage ultrafiltration was studied.Considering the defects of existing process,such as large extraction scale and long operation time,etc.the idea of two-stage ultrafiltration used to concentration and decolorization was proposed.The effect of the ultrafiltration membrane?10 KDa,8 KDa,5 KDa?on the yield of ?-PL and decolorization were investigated.The 8 KDa ultrafiltration membrane was found as the second-stage ultrafiltration membrane;Through the pH value investigation,it was found that when the pH value of the liquid was adjusted to 9.0,the effect of concentration and decolorization was the best.Based on the existing extraction process and the experiment of different combinations,it w found that the new process based on two-stage ultrafiltration is better than the original process,and the yield of ?-PL is 70.22% and the purity is 99.13%. |
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