where the active growing yeast starts to utilize bioethanol as their carbon source in order to survive. In contrast to pH 5, pH 4 is shown to be unfavourable for SSF performance. Bioethanol yield under pH 4.5 is comparable to pH 5 with value of 94.17% at 300 min but at a much lower production rate. This observation revealed that the acidic environment could reduce the activity of yeast. pH between 5.5 and 6 show almost similar trends as pH 5 but production rate is slightly lower after 120 min. This may be attributed to the changes of yeast metabolic pathway where nutrients were consumed to produce side products such as lactic acid (Lin et al., 2012).