How do you prepare this extract? I think we need more information to answer the question.

I recover water from garlic with a French press.

Ok, maybe you should try to dry your extract and redissolve it in methanol. Did you check some paper in literature?

I've checked and in literature they freeze-dry the sample but I'have to use the extract in this way. I'm thinking to use FRAP method to overcame this problem or to dilute the extract before the DPPH analysis.