Effects of guaraná concentrations on phase lag of conjugated dienes formation during serum oxidation. Serum (diluted 100-fold) in PBS 10 mM (pH7.4) was incubated at 37°C for 180 min in the presence of 30 μM CuSO4. Conjugated diene formation was measured by determining the absorbance at 245 nm every 20 min. Experiments were repeated three times, showing similar results. * p < 0.05 compared to control with copper and without guaraná (black bar).