Increase of vitamin D2 by UV-B exposure during the growth phase of white button mushroom (Agaricus bisporus)

BACKGROUND Mushrooms are the only non-animal food source of vitamin D. Wild mushrooms have naturally high vitamin D(2) content, and cultivated mushrooms produce vitamin D(2) from ergosterol when exposed to supplementary UV-B during the post-harvest phase. OBJECTIVES This study investigated the effects of providing supplementary UV-B during the growth phase on vitamin D(2) formation and the interactions with growth of mushrooms, as compared to supplementary UV-B during the post-harvest phase or exposure to sunlight for both cultivated and wild mushrooms. METHODS Experiments were carried out with exposure to supplementary UV-B just prior to harvest in the range of 0-2,400 mJ cm(-2). Mushrooms grew for 2 days with or without repeated UV-B exposure each day. Vitamin D(2) and growth rate were determined. In addition, some mushrooms were post-harvest treated by exposure at 200 mJ cm(-2) supplementary UV-B or natural sunlight, prior to vitamin D(2) determination. RESULTS The content of vitamin D(2) was 0.2-164 µg 100 g(-1) fresh weight, and there was a linear relationship between UV-dose up to 1,000 mJ cm(-2) and vitamin D(2) content. The fast growth rate of the mushrooms diluted the vitamin D(2) from 24 to 3 µg 100 g(-1) within 2 days of exposure at 200 mJ cm(-2). Following repeated UV-B exposure, vitamin D(2) increased to 33 µg vitamin D(2) 100 g(-1). Growth was unaffected by UV-B. Post-harvest exposure to supplementary UV-B resulted in a higher vitamin D(2) content of 32 µg 100 g(-1) compared to the 24 µg 100 g(-1) obtained from exposure to UV-B during the growth phase. In contrast, wild and cultivated mushrooms with and without exposure to sunlight had vitamin D(2) content in the range of 0.2-1.5 µg vitamin D(2) 100 g(-1). CONCLUSIONS This study showed that mushrooms with a well-defined content of vitamin D(2) can be obtained by exposure to supplementary UV-B just prior to harvest.