The colour of the reaction mixture was measured using a colour
difference metre (NE 4000, Nippon Denshoku, Tokyo, Japan). The
results are expressed as L⁄ (lightness) and b⁄ (+b⁄ = yellowness,
b⁄ = blueness) according to the methods specified in the equipment
manual. The instrument was standardised with a standard
white tile. Measurements were made three times for each sample,
and results are expressed as mean ± standard error.