Citric and oxalic acids have no significance effect on the
antioxidant activities of lettuce. These acids reduced pH
sharply, so antioxidant activity may decrease slightly as a
result of pH drop. Low pH may also decrease the activity
of PPO, so that polyphenols cannot be oxidised. Ascorbic
acid also reduced pH but is different from the other acids,
as it has a reductant feature. Ascorbic acid is a common
component of vegetables and has strong radical-scavenging
activity. After vegetables are cut and exposed to oxygen, it
is oxidised to dehydroascorbic acid and has no radicalscavenging
activity