The water-content water activity relationship of cured tobacco and water relations of associated spoilage fungi

The relationship between moisture content and water activity (aw) in cured tobacco was significantly influenced by sugar content. Overall, high sugar tobaccos such as Oriental and Virginia had a higher moisture content at any given water activity compared to low sugar tobaccos such as Burley. Virginia and Burley were both predominantly colonised by Aspergillus and Penicillium spp. Of these, about 80% of isolates could germinate at between 0.75 and 0.85 aw, equivalent to moisture contents of between 18% and 24% in Burley and between 22% and 31% in Virginia. Growth of the dominant Aspergillus and Penicillium spp. was much slower on Virginia and Burley tobacco extract than on malt extract agars over the range 0.85 to 0.98 aw. For some species the optimum aw for growth on tobacco extract medium was altered from that on the richer malt extract agar and for some there was also a significant difference in growth between Virginia and Burley extract agars. The mould-free storage periods for five different tobacco types was influenced by aw. Visible moulding occurred within 7-14 days at 0.85-0.90 aw but only after about six months at 0.70-0.75 aw. There were some differences in rate of moulding between tobacco types as well as in the range of fungi isolated at different aw storage levels.