For the development of a single linear regression model for FFA concentration, solutions of OA in toluene (0.25–2.50 g L−1 at 0.25 g L−1 intervals) were prepared, since OA is a major FFA in Japanese rice (Kitta et al., 2005). The OA sample solution at 2.50 g L−1 was prepared using an analytical balance, and was diluted to obtain the others.