The antioxidant activity of the essential oils was determined as previously
described (Magwa et al., 2006). Briefly, a medium composed
of technical agar, beta-carotene (sigma) and linoleic acid
(sigma) was prepared and poured in petri dishes and kept in the
dark. After the medium has set, holes (4 mm diameter) were
punched using a borer and the oil (25 l) was transferred into the
holes and the petri dishes were then incubated at 45oC for 4 h. A
zone of colour retention around the hole following incubation noted
essential oils with antioxidant properties. The zone diameter was
measured using vernier calipers after the oil has been withdrawn
from the hole. Absolute alcohol was used as a negative control and
the ascorbic acid (10 mg/ml) was used as a positive control.