The case of pink oyster mushrooms
Mushrooms are a good source of vitamin D for vegetarians. However, though most cultivated mushrooms have almost no vitamin D2, they have abundant ergosterol, a precursor of vitamin D2. Ergosterol becomes vitamin D2 only when exposed to UV light,
What is the optimum of UV radiation needed to convert the ergosterol in the mushrooms into vitamin D2?
To find out, researchers from the Gandhi Krishi Vigyana Kendra, Bengaluru recently performed experiments using the pink oyster mushroom, Pleurotus eous.
Pink oyster mushrooms have high ergosterol content. Moreover, these mushrooms have more aroma and flavour than the more commonly cultivated button mushrooms. They retain a firm and shreddable texture that absorbs flavouring agents well when cooked making them a good choice for curries, stir-fries, and rice dishes.
The researchers investigated methods to increase vitamin D₂ levels in pink oyster mushrooms, using ultraviolet light of 254 nanometre wavelength: ergosterol absorbs this specific energy to convert to vitamin D₂.
The intensity of light radiation received by a surface is inversely proportional to the square of the distance from the light source. By keeping the distance strictly fixed at a little more than half a metre across all experiments, the researchers ensured that every mushroom sample received the exact same amount of UV-C energy per second. This guaranteed that any measured changes in vitamin D₂ levels were caused only by the duration of exposure.
To determine the time for maximum vitamin D production before overexposure to light degrades the vitamin, the researchers tested eight different durations, 0 to 70 minutes, in 10-minute increments. They kept one untreated group to act as baseline control.
To measure the increases in vitamin D content, the researchers homogenized 100 grams of mushrooms from each sample, added potassium hydroxide and ethanol, and heated the mixture. Potassium hydroxide converts fats into soap and ethanol helps release the sterols bound inside the cell membranes. Then, the researchers extracted the non-saponifiable sterol fraction using n-hexane, an organic solvent. After evaporating the solvent, they re-dissolved the clean residue in methanol a high-performance liquid chromatography-grade mobile phase.
High-performance liquid chromatography is the gold-standard method for accurately separating, identifying, and quantifying structurally similar molecules such as ergosterol and vitamin D₂. In this method, the liquid sample is pumped under high pressure through a specialized column packed with a stationary phase material. Ergosterol and vitamin D₂ travel through this column at different speeds based on their chemical properties. So they exit the column after distinct retention times. As each compound elutes, the UV detector records its absorption at a specific wavelength – typically around 264 nanometres for vitamin D₂ and 282 nanometres for ergosterol – producing peaks on a chromatogram whose area directly corresponds to the compound’s concentration.
Known standard concentrations of pure vitamin D₂ and ergosterol were run under identical conditions to calibrate the system. This allowed the researchers to convert peak area values into precise microgram quantities per gram of mushroom.
They found that exposing oyster mushrooms to UV-C radiation dramatically increases their vitamin D₂ content compared to untreated mushrooms. As exposure time increased, vitamin D₂ levels rose significantly till about 50 minutes. Beyond this duration, the conversion rate levelled off.
Having detected the exact time for UV radiation to maximise vitamin D content, the researchers tackled the next question: how stable is the vitamin D₂ content in the mushroom during storage?
The researchers stored the mushroom samples in polypropylene bags to protect them from moisture loss. The bags were kept at 4 °C to simulate commercial storage.
The researchers monitored the stability of vitamin D in the mushrooms for twenty days. High performance liquid chromatography helped them track the rate of vitamin D₂ degradation over storage time. Periodic testing revealed that while vitamin D₂ naturally decays over time due to oxidation, the UV-treated mushrooms retained a substantial portion of vitamin D₂ throughout the 20-day period. Storing the fortified mushrooms in sealed bags under refrigeration preserved most of the vitamin D₂ content of the pink oyster mushroom.
Button mushrooms are more commonly found in Indian markets than oyster mushrooms which have shorter shelf life. Oyster mushrooms are best cooked fresh, or have to be dried and powdered for long-term storage.
The research by Gandhi Krishi Vigyana Kendra, Bengaluru provides tips that mushroom farmers can use to make oyster mushrooms more available to consumers.
Journal of Pure and Applied Microbiology, 20(3):2278-2287(2026);
DOI: 10.22207/JPAM.20.3.18
Reported by Gita Madhu
Freelance writer, Goa
Notes for mushroom farmers:
Pink oyster mushrooms are more nutritious than button mushrooms. Besides having higher protein content than button mushrooms these mushrooms contain higher levels of -glucans which help reduce low density cholesterol and stimulate immune cell function.
They produce statins such as lovastatin, which help manage blood cholesterol levels. The pink pigmentation is due to higher phenolic carotenoids and content, which offer higher antioxidant and anti-inflammatory properties.
Button mushrooms require low temperatures and specialized climate-controlled units, making them expensive to cultivate in warm regions of India without refrigeration. Pink oyster mushrooms are tropical fungi that thrive natively in warm, humid conditions requiring less energy and fewer resources to grow.
Pink oyster mushrooms grow rapidly on agricultural residues such as paddy or wheat straw, sugarcane bagasse, and coconut coir dust. Unlike button mushrooms, they do not need composted manure or heavily processed soil substrates.
To replicate the experiment’s exact conditions, mushroom growers need a 254 nanonometre UV-C germicidal tube, often labeled as TUV or HNS quartz glass tubes. Standard germicidal tubes cost between ₹300 and ₹1,200 per tube depending on brand and Wattage.
A fixture with a reflector is useful to reduce waste of energy. A simple wall/stand-mountable batten costs roughly ₹500 to ₹1,500.
UV-blocking safety goggles and gloves are essential, to ensure that the people involved are not exposed unnecessarily to UV rays. Direct exposure to 254 nanonometre UV-C causes skin burns and eye damage, The safety gear may cost around ₹300 to ₹800.
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