Abstract
The globally rising prevalence of vitamin D deficiency was observed due to an increase in in-door life, the use of sunscreen, and inhibited vitamin D absorption in obese populations. Whereas vitamin D is reported in relation to skeletal growth, diabetes prevention, obesity prevention, dermatitis prevention, immune modulation, and anti-tumour effects. There are two types of vitamin D commonly exist in the human body and contribute to health-related effects: ergocalciferol (vitamin D2) and cholecalciferol (D3). Vitamin D3 can intrinsically be synthesised in the human body from 7- dehydrocholesterol upon UV exposure to the skin. Due to the restriction of natural sources of vitamin D in the human diet, as a result of the evolving lifestyle, vitamin D supplementation and the fortification of food products have gained public attention. Unlike the animal-sourced vitamin D3, vitamin D2 mostly exists in fungi which commonly present as mushrooms and yeast in the kitchen. Edible mushrooms are stated to be abundant in ergosterol (provitamin D2), the precursor of vitamin D2. As ergosterol is photo-converted into vitamin D2, there are small amounts of vitamin D2 found in wild mushrooms. But for the cultivated mushrooms, due to the requirement of a dark environment for cultivation, the vitamin D2 content from them is often negligible. Therefore, the fortification of vitamin D2 in cultivated mushrooms by applying an artificial UV source has been widely researched. The main objective of this study was to investigate the optimal UV irradiating conditions for the maximum yield of vitamin D2 in two types of edible mushrooms in New Zealand. White button mushroom (Agaricus bisporus) is the most widely cultivated mushroom species in the world, which is mostly sold in fresh form in supermarkets. Shiitake mushroom (Lentinula edodes) originated from East Asia, which is commonly sold as frozen or air-dried forms in New Zealand. When purchased from supermarkets, vitamin D2 content in white button mushrooms was 3.86 µg/g dw, while the shiitake mushrooms contained 3.67 µg/g dw. A dose-response analysis was performed to predict the optimal production of vitamin D2 by a maximum UV-B intensity of 835 µW/cm2 and exposure duration up to 90 minutes. With 835 µW/cm2 irradiation for 90 minutes, vitamin D2 level in white button mushrooms reached 33.46 µg/g dw. For shiitake mushrooms, the maximum vitamin D2 of 28.95 µg/g dw was reached under 835 µW/cm2 for 82 minutes. By consuming the mushrooms under optimised irradiating conditions, approximately 0.5 g of the mushroom powder could meet the recommended vitamin D daily intake for adults (15 µg/day). Vitamin D2 is not the only compound contributing to the nutritional value of mushrooms. Flavonoids, phenolic compounds, and β glucan are also rich in edible mushrooms contributing to antioxidant activities. Investigating the effect of UV-B irradiation on the antioxidant activity of freeze-dried 2 mushroom powders was the second objective of this study. In DPPH free radical scavenging assay, the antioxidant activity of white button mushrooms increased from 386.6 µg Trolox equivalent (TE) /g dw to 2595.9 µg TE/g dw under optimised irradiating from the dose-response study. Shiitake mushrooms obtained an increase from 248.3 µg TE/g dw to 2028.3 µg TE/g dw under optimal UV-B irradiation. ABTS assay showed higher TE values in the irradiated mushroom samples at 4301 µg TE/g dw from the white button and 3292 µg TE/g dw from the shiitake mushroom. FRAP assay exhibits the ferric-reducing power that irradiated white button and shiitake mushrooms approached 9863 µmol Fe2+ equivalent/g dw and 8416 µg µmol Fe2+ equivalent/g dw respectively. The third aim of this study was to assess the stability of the mushroom vitamin D2 and antioxidant activity enhanced by UV-B irradiation. After 90 days of storage in the freezer (-18 ℃), fridge (+4 ℃), and at room temperature (20 ℃), both vitamin D2 level and antioxidant activity of the freeze-dried mushroom powders showed significant reduction. Storage temperature showed a negative correlation to the retention of both vitamin D2 content and antioxidant activity. Also, white button mushrooms exhibited higher retention than shiitake mushrooms in both vitamin D2 and antioxidant activity. The highest retentions were from irradiated white button mushrooms stored at -18 ℃, which obtained 85.3%, 84.5%, 94.6%, and 86.8% retentions in vitamin D2, DPPH radical scavenging activity, ABTS radical scavenging activity, and FRAP ferric reducing power respectively. The optimised UV-B irradiation added significant value to freeze-dried mushroom powders that both solve the deficiency in vitamin D and provide an extra antioxidant effect. Also, the form of freeze-dried mushrooms offered decent stability under cold storage.