why is it difficult to separate optical isomers of glucose

We know that the left handed sugar cant be metabolized by any organism (correct me if i am wrong) on earth hence many researchers are all trying to produce left handed sugar in a commercially viable way so that it can be used as an artificial sweeter…why is this process not easy?.Simply make a racemic mixture of glucose, put same harmless bacteria in that mixture they will soon consume all R-sugar and only L-sugar will be left.Then we can easily remove the bacteria .What makes the above process difficult?