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. 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 (FW), 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.