These Brazilian authors looked at the differences in polyphenol content between juices from grapes grown conventionally or organically - and you'll be surprised what they found.
For the full text, go to this site and click on article # 28: http://www.sciencedirect.com/science?_ob=PublicationURL&_cdi=5036&_pubType=J&_version=1&_urlVersion=0&_userid=4421&md5=f2593c05c8e8c4db2fd7268e286c0fa5&jchunk=45#45