Abstract:To our knowledge the cheese industry both Nationally and Internationally, is aware of the loss in colour of pre-packaged internally mould ripened blue cheeses (e.g. The American blue cheese AMABlu -Faribault Dairy Company, Inc.); however, after reviewing data published to date it suggests that no work has been undertaken to explain why this phenomenon is occurring which makes the work detailed in this paper novel. The amount and vivid colour of blue veins of internally mould ripened cheeses are desirable quality characteristics. It is therefore important that there is a sufficient amount of veining and that it maintains its blue appearance to be appealing to consumers therefore leading to maximised sales potential and profit for the manufacturing company.The work undertaken in this study determined that the factors for optimum in vitro growth of Penicillium roquefortii (strain PRB6) were: a temperature of 20 °C ± 1 °C, pH of 6.0 ± 0.1, and a relative humidity of 70 %. Optimum in vitro growth mimicking the conditions typically found in pre-packed blue cheeses, and using 06/09/2010Page 2 of 14 lactose as the sole carbon source, was facilitated by a gas mixture of 5 % Oxygen/0 % carbon dioxide/balance nitrogen).Further in vitro studies have also shown that the increasing 'in pack' carbon dioxide concentration not only depresses the growth of P roquefortii but also affects immature conidiospore pigmentation (no effect has been seen on mature conidiospore pigmentation).The implications of this study suggest that the majority of pre-packed internally mould ripened blue cheeses on sale in supermarkets are packaged in inappropriate materials. For some cheeses (e.g. the Roquefort-type cheeses) this is not an issue since these are packed in a much more mature state and some loss of veining colour is not appreciably noticeable; however, for less mature cheeses (i.e. those intended to continue maturing 'in-pack') any loss in colour has a significant impact on the cheese as well as on consumer perception.