A Knauer Smartline System chromatographer (Knauer, Berlin, Germany) with a column for organic acids (BioRad HPX-87H
300 7.8 mm) was employed at 41 C. The flow-rate was 0.6 ml/min. The eluent used was 5 mM H2SO4 (pH 2) and the
detection was carried out with an RI detector Knauer K-2301.