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 D2 content, and cultivated mushrooms produce vitamin D2 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 D2 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 D2 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 D2 determination.Results: The content of vitamin D2 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 D2 content. The fast growth rate of the mushrooms diluted the vitamin D2 from 24 to 3 μg 100 g 1 within 2 days of exposure at 200 mJ cm 2. Following repeated UV-B exposure, vitamin D2 increased to 33 μg vitamin D2 100 g 1. Growth was unaffected by UV-B. Post-harvest exposure to supplementary UV-B resulted in a higher vitamin D2 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 D2 content in the range of 0.2–1.5 μg vitamin D2 100 g 1.Conclusions: This study showed that mushrooms with a well-defined content of vitamin D2 can be obtained by exposure to supplementary UV-B just prior to harvest.