Optimization of Poly(<Emphasis Type="SmallCaps">dl</Emphasis>-Lactic Acid) Degradation and Evaluation of Biological Re-polymerization

Poly(dl-lactic acid) or PLA is a biodegradable polymer. It has received much attention since it plays an important role in resolving the global warming problem. The protease produced by Actinomadura keratinilytica strain T16-1 was previously reported as having PLA depolymerase potential and being applicable to PLA biodegradation, which was used in this work. Therefore, this research demonstrates the important basic knowledge on the biological degradation process by the crude PLA-degrading enzyme from strain T16-1. Its re-polymerization was evaluated. The optimization of PLA degradation by statistical methods based on central composite design was determined. Approximately 6700 mg/l PLA powder was degraded by the crude enzyme under optimized conditions: an initial enzyme activity of 200 U/ml, incubated at 60 °C for 24 h released 6843 mg/l lactic acid with 82% conversion, which was similar to the commercial enzyme proteinase K (81%). The degradable products were re-polymerized repeatedly by using commercial lipase as a catalyst under a nitrogen atmosphere for 6 h. A PLA oligomer was achieved with a molecular weight of 378 Da (n = 5). This is the first report to demonstrate the high efficiency of the enzyme to degrade 100% of PLA powder and to show the biological recycling process of PLA, which is promising for the treatment and utilization of biodegradable plastic wastes in the future.