The batter was prepared in a mixer (Kenwood Major Classic, UK), in
which the egg whites were whisked for 2 min at top speed. The
sugar was added and mixed in for 30 s more at top speed. The egg
yolk, half the milk and the citric acid were then added and the
mixer was set to speed 3 for 1 min. The flour, sodium bicarbonate
and cocoa fibre (or cocoa powder, depending on the formulation)
was added and the batter was beaten for an additional minute at
speed 3. Lastly, the mixer speed was increased to setting 4, the rest
of the milk was added and the oil was gradually dripped in. The
batter was beaten for 3 min at speed 4 until smooth