2.4. Determination of ascorbic acid
AMetrohm 746 VA Trace Analyzer (Herisau, Switzerland) equipped
with aMetrohm 747 VA stand was used for the polarographic determination.
The working electrode was a Metrohm multi-mode electrode
operated in the droppingmercury mode. A platinumwire counter electrode
and a saturated calomel reference electrode were used. The following
instrumental conditions were applied: DP50, mode DME, drop
size 2, drop time 1 s, scan rate 10 mV/s, and initial potential −0.10 V.
Beverage (5 mL) was diluted to 25 mL with the extraction solution
(oxalic acid 1% w/v, trichloroacetic acid 2% w/v, sodium sulfate 1%
w/v). After vigorous shaking, the solution was filtered through a folded
filter(Whatman no. 1). Oxalic acid (9.5 mL) 1% (w/v) and 2 mLof acetic
acid/sodium acetate 2 M buffer (pH=4.8) were added to an aliquot of
0.5 mL of filtrate and the solution was transferred to the polarographic
cell. Determinationswere carried out by using the peak height and standard
addition method in accordance to Barba, Esteve, Tedeschi,
Brandolini, and Frigola (2013).