4.4. Vitamin C Analysis
To quantify the VC (ascorbic plus dehydroascorbic acids) present in serum blood, 0.5 mL serum was mixed with 1 mL of a 20 g/L DL-dithiothreitol solution for 2 h at room temperature and in darkness. Afterwards, 1 mL of this mixture was extracted with 2.25 mL 0.1% oxalic acid for 3 min and immediately filtered through a 0.45 mm membrane filter before injection. The HPLC (Jasco, Madrid, Spain) conditions were: Ultrabase-C18, 5 mm (4.6 × 250 mm) column (Análisis Vínicos, Tomelloso, Spain); mobile phase 0.1% oxalic acid, volume injection 20 µL, flow rate 1 mL/min, detection at 243 nm and at 25 °C. A standard solution (Panreac, Barcelona, Spain) was prepared. The results were expressed as mg of VC per L of serum. The VC content of the juices served to the participants was also analyzed as described by Galindo et al. [