3.2. UVB-induced production of vitamin D2 by hot-air- and freeze-dried mushrooms
This is the first work that compares the content of vitamin D2 in the three most commonly cultivated mushrooms in the world whose fruit bodies before UVB irradiation were dried by two different methods (hot air- or freeze-drying). The results indicate that the type of material (hot-air- or freeze-dried mushrooms), and the dose of UVB radiation have a significant influence on the level of vitamin D2 production in dried mushrooms. As it can be seen in Table 2, increasing the exposure time of UVB irradiation of dried mushrooms resulted in the steady increase in the amount of vitamin D2 with the exception of oyster mushroom, for which the highest content of ergocalciferol was observed after 15 min in freeze-dried fruit bodies (59.04 μg/g dw) and after 20 min in hot-air dried fruit bodies (40 μg/g dw); the further increase of UVB radiation dose resulted in a decrease of the ergocalciferol content. UVB irradiated freeze-dried fruit bodies contained significantly more vitamin D2 than those which were hot-air dried with a few exceptions. In the case of P. ostreatus only fruit bodies, which were dried by various methods and irradiated with UVB for 30 min did not differ significantly in vitamin D2 content. In turn in dried shiitake mushrooms, which were irradiated between 5 min and 15 min, the level of vitamin D2 was not statistically different for hot-air- and freeze-dried mushrooms. UVB-irradiated freeze-dried fruit bodies of L. edodes for 20 and 30 min contained significantly more vitamin D2 than those dried by convection method. The highest content of ergocalciferol in dried fruit bodies was observed in white button mushroom. Depending on UVB exposure time, freeze-dried fruit bodies of button mushrooms contained from 42.08 μg/g dw to 119.21 μg/g dw of vitamin D2, and fruit bodies dried by convective method contained had from 21.51 μg/g μg to 81.17 μg/g dw of vitamin D2. Generally, freeze-dried mushrooms contained significantly more vitamin D2 than hot-air dried. Although the fruit bodies, which were dried by convection had significantly less vitamin D2, its content is still so high that hot-air dried mushrooms can be regarded as a rich source of ergocalciferol. Moreover, dehydration of mushrooms by convective method is much cheaper than lyophilization and it seems a more economical solution for the possible implementation in the food industry.