Vitamin C
From Cookipedia
Use in baking
A single vitamin C tablet, crushed between two tablespoons is a great substitute for citric acid, specifically in recipes for breadmakers that use brown flour or wholemeal flour.
Crushed vitamin C tablets can be used as a substitute for ascorbic acid [E300] in wholegrain bread recipes, where it helps sustain the leavening of bread loaves during baking. It also helps produce the acidic atmosphere in which yeast grows best.