Chemical characteristics of poplar type propolis of different geographic origin Milena P. Popova, Vassya S. Bankova, Stefan Bogdanov, Iva Tsvetkova, Christo Naydenski, Gian Luigi Marcazzan and Anna-Gloria Sabatini Apidologie, 38 3 (2007) 306 Published online: 14 June 2007 DOI: 10.1051/apido:2007013