An apparatus and method for increasing Vitamin D content in mushrooms is disclosed. A mushroom slurry of comminuted or pulverized mushrooms or mushroom parts and liquid, such as water, is passed under a UV light source and subjected to one or more pulses of UV light. The slurry may be conveyed to the UV light source by a vibrating conveyor. After UV light exposure, the treated slurry may be dried and ground into a powder, or the treated slurry may be filtered and the insoluble portion may be dried and ground into a powder. The irradiated mushroom powder has a mass fraction of Vitamin D2 of at least 2500 IU/gram of powder, and more preferably at least 20,000 IU/gram of powder. Irradiated mushroom powder may be incorporated into consumable food product for humans or animals, and/or may be incorporated into topical preparations for cosmetic use.