Same time after being milled and stored, green coffee becomes normally subject to a phenomenon of discoloration or whitishness of the grains, which causes a considerable loss of its commercial value. In the present paper, which is the first to be published on this question, are reported the results of various experiments carried out in order to investigate the cause of this phenomenon and the influence of some factors on its manifestation. According to these results it may be clearly concluded that the injuries to the grains, such as those which normally result from their mechanical milling, are the indirect cause of the coffee discoloration. Among the studied extrinsic factors, the relative Humidity of air showed to be the most effective of them. The higher the air humidiry, especially on levels above 80%, the more rapid and intense was the manifestation of the phenomenon.