Additionally, BC production using WBY treated by ultrasonica- tion (pretreatment 3) displayed the highest yield (3.89 g/L) of all four pre-treatments. Furthermore it was clearly higher than that from pre-treatment 1 using 0.1 M NaOH (2.33 g/L), despite the fact that they have almost the same sugar concentration after hydrolysis. It can be concluded that the harsh alkali used in pre-treatment 1 might degrade other nutrients in the waste beer yeast which are beneficial for G. hansenii CGMCC 3917 during BC production, while those nutrients are still present after ultrasonication pre- treatment.