Evaluation of extraction parameters for total oxalate determination in spinach using Design of Experiment analysis

The interaction of pH, temperature and time on the acid extraction of total oxalates from homogenised fresh stems and leaves of spinach (Spinacia oleracea L.) were investigated. The extractions were carried out using HCl solutions between pH 0.93 and 5.81 and temperatures ranging from 25 to 95 °C. The extraction times ranged from 15 to 120 min. The released oxalate was measured using a standard HPLC method. A Design of Experiments (DOE) method using Design Expert DX9 was used to determine the optimum extraction parameters. Increasing the extraction time from 15 to 120 min had no significant effect while increasing the extraction temperature from 25 to 95 °C significantly (P < 0.01) increased the recovery of total oxalates. Reducing the pH of the extraction solutions from 5.81 to 0.93 significantly (P < 0.001) increased the total amount of oxalate extracted. There was no significant correlation between the pH of the extraction solutions and the extraction temperatures. The highest amount of extracted oxalate content in this experiment (856.57 mg/100 g FW) was obtained using an HCl solution of pH 0.93 (0.1 M HCl) at 95 °C.