Mushrooms are not only a nutrient-rich food but also a livelihood source with healthy protein content. These are considered a product of an upgraded value chain. For example, rice straw mushroom production can add USD 50–100 profit to a hectare of rice production. These upgraded value chains of rice and rice straw also led to a significant reduction in rice straw burning.
World commercial mushroom production is about 8 million tons. Out of many varieties, edible mushrooms, such as rice straw mushroom (also called white button mushroom), and oyster mushroom are widely cultivated in tropical countries such as China and Vietnam.
Rice straw mushroom and oyster mushroom constitute 38% and 24% of the world’s mushroom production, respectively. These mushrooms can be grown by using different substrates including biomass basal substrates, such as sawdust, rice straw, wheat straw, and cottonseed hull. The biological efficiency (ratio between mushroom weight at harvest and dry weight of substrate used for mushroom cultivation) of rice straw mushroom is 5–10% , while that of oyster mushroom is 30–130%, depending on the substrates.
These mushrooms are not only a nutrient-rich food but also a livelihood source with healthy protein content. These mushrooms are considered a product of an upgraded value chain. For example, rice straw mushroom production can add USD 50–100 profit to a hectare of rice production. These upgraded value chains of rice and rice straw also led to a significant reduction in rice straw burning.
However, fresh paddy straw mushroom and oyster mushroom, which are highly perishable commodities with high moisture content (MC) ranging from 75% to 90% on a wet basis, can only be maintained for less than two days in ambient temperature in tropical countries without losing quality (e.g., 28–35 °C). Excess mushroom from the fresh markets is therefore processed into salted-canned and dried forms.
Dried mushrooms can be further processed into powder and used in food processing, as a substitute for meat, and for fortification of bakery products. Drying is also considered as a cost-effective method compared to the other preservation methods as it can increase the storability of mushroom up to more than a year with airtight packages.
Mechanical and industrial drying technologies have also been developed for mushroom drying. Convective hot-air, microwave-convective heated-air, and a combination of fast heating of microwave and low-temperature convective drying technologies provide acceptable quality of dried mushroom. Some more advanced technologies were also introduced and investigated, such as osmo-air drying, low-temperature drying, freeze-drying, and fluidized-bed drying.
On the other hand, sun drying is commonly used by small resource-poor growers as it requires zero or insignificant investment costs. However, this practice results in poor-quality products as it is strongly affected by the environment and weather. The product is also often contaminated by dirt, insects, bacteria, etc.
The constraints of sun drying can be addressed by solar drying, which does not require electric grid power and fuel, particularly for a high-MC product. Hybrid-solar drying and solar-assisted heat pump drying were introduced as advanced technologies for mushroom drying. However, these technologies usually are characterized by high investment and operating costs.
This research aimed at developing a farm-scale solar drying technology for mushrooms. We modified the Solar Bubble Dryer (SBD) originally developed for paddy drying. An experiment was conducted using oyster mushroom to identify characteristics of the drying process based on the reduction of mushroom moisture content corresponding to the specific ambient temperature, relative humidity, and solar radiation.
In addition, energy efficiency, greenhouse gas (GHG) emission, and cost-benefits were also analyzed for drying 1 kg of oyster mushroom. The effects of climatic factors (i.e., radiation, ambient temperature, and RH) on drying time, capacity, and cost benefits were taken into account through a sensitivity analysis.
The adaptation of SBD and drying performance profile resulting from this research illustrated that it is a promising technology for mushroom drying at the farm scale. The SBD with a perforated floor added ensured the quality without any requirement of mixing or turning the mushroom during drying. Mushroom MC was reduced from 90% down to 40–60% within 2–4 hours, corresponding to the drying rate at this stage of 10–20% h−1. At the next stage of reducing MC down to 8–10%, the drying process took about 4–6 h corresponding to the drying rate of 2–10%/hr
The drying process caused a slight reduction of N contents from 5.1% N (dry matter) down to 4.8% (dry matter). However, the color of the dried mushrooms still remained white-cream. The drying process required 1.27 kWh (or 4.57 MJ), emitted 0.33 kg CO2e, and created an input cost of USD 1.86 per kg of dry product.
The SBD can reach a capacity of 500 kg of dry mushroom per year. For a specific case in the Philippines, it can generate a net profit of USD 468–1468/year and the investment will break even at 1.3–4.0 years corresponding to the selling price of dry mushroom of USD 10–12/kg. With the assumption of the drying capacity changing from 400 to 600 kg of dry mushroom per year as affected by climate factors, the net profit would range from USD 678 to 1,258 kg/year.
Read the study:
Van Hung N, Fuertes LA, Balingbing C, Paulo Roxas A, Tala M, Gummert M. (2020) Development and Performance Investigation of an Inflatable Solar Drying Technology for Oyster Mushroom. Energies.