Detailed Explanation of Key Points in Shiitake Mushroom Drying and Processing Technology

Shiitake mushroom is a world-renowned precious edible fungus and also a traditional export commodity of our country. Shiitake mushrooms contain a unique aroma, are rich in nutrients, and have tender and smooth flesh. They are one of the ingredients commonly used in famous Chinese and foreign dishes and are hailed as the king of vegetables. At the same time, shiitake mushroom is also a famous medicinal fungus, with extremely high medicinal value such as tonifying qi, preventing hunger, and activating wind and breaking blood. Modern medicine has proven that long-term consumption of shiitake mushrooms can strengthen the physique; soften blood vessels, lower cholesterol, and even have anti-cancer and cancer-preventing effects. Therefore, shiitake mushrooms are very sought after on the international market and have become the world's second-ranked edible fungus. The production and processing of shiitake mushrooms can bring enormous economic and social benefits and have broad market prospects.

For the primary processing of shiitake mushroom products, the vast majority of mushroom farmers, specialized households, and small processing units currently adopt relatively extensive traditional processing methods, namely simple sun-drying and earthen kiln drying, and then directly enter the market @#@136@#@. The products processed in this way are neither competitive in the market nor make good use of excellent shiitake mushroom raw materials. After our country joined the WTO, the international market has increasingly higher requirements for product quality. To enable shiitake mushroom products to go abroad and be competitive in foreign markets, it is necessary to actively adopt advanced processing technology and production equipment, establish a complete set of scientific quality control methods and operating standards, and improve the processing quality of products.

1. Material preparation

(1) Harvesting method: Do not water for 1 day before harvesting, and harvest at the appropriate time. Thick mushrooms should be harvested when they are 40% to 50% mature, and generally harvesting should not exceed 80% maturity. When harvesting, select mushroom bodies with no disease and no deformity in which the mushroom veil has naturally ruptured, the cap has not yet opened, the margin is incurved, and the gills are extended. When harvesting, gently pinch the stipe with your fingers and twist to remove it, do not touch the gills, handle gently, and do not pile too thickly. When transporting or placing in bamboo baskets, avoid crushing, deformation, or damage.

(2) Pretreatment: Cut off the mushroom base, leaving the stipe 1 cm to 2 cm long. First sort out flower mushrooms, thick mushrooms, and thin mushrooms, then pack them separately in trays according to variety and size, and expose them to sunlight for 2 h to 3 h to remove part of the moisture. When placing on trays, the caps should face upward and the stipes downward, arranged evenly and upright, and they must not overlap or be piled up, so as to prevent damaging the mushrooms and affecting quality. Shiitake mushrooms must be baked within 6 h after harvesting. If cold storage conditions are available, the storage time can be appropriately extended.

2. Drying room drying

(1) Entering the drying room: After the shiitake mushrooms are placed on trays, put them on the racks and send them into the drying room for baking. Large and thick mushrooms with high moisture content should be placed on the upper layers, and small and thin mushrooms with low moisture content should be placed on the lower layers. Generally, 8 to 10 layers are arranged. If too many are placed, it is easy to make the heating temperature of the materials in the upper, middle, and lower layers uneven. The spacing between each layer should be 30 cm.

(2) Temperature control: Only when the temperature of the drying room rises to 35°C can the shiitake mushrooms be put in for drying. Two wet-bulb thermometers should be hung near the window inside the room. During drying, the temperature must be low first and then gradually increased. Usually, the temperature is increased by 1°C to 3°C per h, and the maximum temperature should be controlled at 80°C to 85°C. The general requirement is baking at 35°C to 40°C for 6 h -> baking at 40°C to 60°C for 8 h to 10 h -> baking at 60°C for 2 h. The higher the moisture content of shiitake mushrooms, the longer the time needed for baking at low temperature. If the temperature rises suddenly at the very beginning of baking, it will inevitably cause the tissue to lose water too quickly, easily causing the shiitake mushroom caps to deform and become not round and neat, the gills to fold over, the caps to crack, the color to turn black, and the enzyme activity to be destroyed, making the shiitake mushrooms lose their original aroma. After the shiitake mushrooms are sent into the drying room, they should be baked continuously until dry. Heating must not be interrupted, and the temperature must not fluctuate, otherwise the shiitake mushrooms will turn black and their quality will decline.

(3) Moisture exhaust: In the drying process of shiitake mushrooms, in addition to strictly controlling the heating temperature, timely moisture exhaust is also an important step. Moisture exhaust holes must be set at the upper part of the drying room. The moisture exhaust hole for a small drying room is 10 cm x 12 cm, for a medium drying room is 15 cm x 20 cm, and for a large drying room is 20 cm x 40 cm. According to need, some can be equipped with 2 moisture exhaust holes. If the surrounding environment is not conducive to moisture exhaust, exhaust fans should be installed, and the moisture exhaust holes should preferably lead directly outdoors. The general principle of moisture exhaust is: in the early stage of shiitake mushroom baking, when the drying room temperature is 35°C to 40°C, all moisture exhaust holes or exhaust fans should be opened; when the temperature rises to 40°C to 60°C, exhausting moisture for 3 h to 4 h is enough. After 60°C, all moisture exhaust holes can be closed and no moisture exhaust is needed. If moisture is exhausted excessively, the shiitake mushrooms are prone to be light-colored and whitish. If the baked shiitake mushrooms have a water-soaked yellow color, it indicates that moisture exhaust is poor, or the furnace temperature is insufficient, especially if heating is stopped midway, which is more likely to cause this phenomenon.

(4) Ventilation: In order to regulate the indoor temperature and ensure hot air circulation in the drying room, the room must be ventilated at any time. Therefore, 2 ventilation holes of 20 cm x 20 cm should be set at the lower part of the drying room. If the air supply is not smooth, a blower should be installed. The air sent to the drying room should preferably undergo purification treatment first.

(5) Quality inspection: When baking reaches 16 h to 18 h, the drying room door can be opened to inspect whether the dryness of the shiitake mushrooms is qualified. During inspection, press with a finger at the junction between the cap and the stipe. If only a mark appears, it indicates that drying is qualified; if it feels soft, and the gills are also soft, then drying must continue. The characteristics of qualified dried products are: they have the special aroma of shiitake mushrooms; the gills are yellow, upright, complete, and not folded over; the moisture content of the mushroom body does not exceed 13%; the shiitake mushrooms retain their original shape, the caps are round and flat, and the natural color is maintained.

(6) Grading and packaging: 1. Sieving. First remove shiitake mushrooms with cap diameters below 3 cm and above 7 cm as well as fragments with a thin iron round sieve, and the remaining part is used as standard-grade product for grading and packaging. 2. Recutting stipes. According to the specified requirements, cut off stipes with unqualified length. 3. Sorting. Pick out damaged mushrooms and deformed mushrooms. 4. Grading. Grade according to the requirements of export products, divided into three major categories: flower mushrooms, thick mushrooms, and thin mushrooms, with a total of 12 grades,

see Table 1. 5. Subpackaging. After grading, weigh and put into plastic bags, and pack into cartons according to the required quality. After shiitake mushrooms are dried, they must be put into plastic bags within 6 h to prevent moisture absorption causing mildew and deterioration. Dried shiitake mushrooms that cannot be subpackaged in time should first be put into large plastic bags and the bag openings tied tightly. It is strictly forbidden to use nylon woven bags or gunny bags and other breathable bags for packaging. The packaged products should be stored in a dry room.

Table 1 Shiitake mushroom grading table

Name|Characteristics|Cap diameter cm|

Large flower mushroom|Large incurved margin, obvious patterns, thick flesh|4-6|

Medium flower mushroom|Thick mushroom body|3-4|

Small flower mushroom|Mushroom body semicircular|2-3|

Flower mushroom pieces|Mushroom body round and neat|1-2|

Large thick mushroom|Abundant mushroom flesh, large incurved margin, cross-sectional measurement >1 cm|4-6|

Medium thick mushroom|Abundant mushroom flesh|3-4|

Small thick mushroom|Thick mushroom flesh|2-3|

Thick mushroom pieces|Thick mushroom flesh|1-2|

Extra-large thin mushroom|Fully opened cap, thin cap flesh|>7|

Large thin mushroom|Flat cap|4-7|

Medium thin mushroom|Flat cap|3-4|

Small thin mushroom|Flat cap|2-3|

3. Dryer drying

For small rural processing enterprises, the tunnel dryer is a relatively ideal dehydration and drying equipment for shiitake mushrooms. At present, a relatively advanced model is the chain-circulation tunnel dryer, which can automatically control temperature, operate continuously, has fast drying speed, and is simple to operate. Multiple fuels such as diesel and raw coal can all be used. The chain-circulation tunnel dryer consists of two parts: mechanical power and thermal power. When working, as long as the speed-regulating motor, blower, and heater are started and the valves are properly adjusted, the material trays can be run in circulation in parallel, at uniform speed, and in multiple layers to dry the products. Because this equipment adopts flowing circulating hanging trays for its drying method, the materials do not need to be turned, and drying is uniform. Therefore, the processed shiitake mushroom products are of good quality and high grade. In addition, the dryer adopts an indirect heating method, with no pollution and high thermal efficiency. Compared with earthen kiln drying, the drying time of the chain-circulation tunnel dryer is shortened by nearly 40%.

4. Use and maintenance of drying equipment

(1) When the shiitake mushrooms dried by the dryer reach the standard and can be taken out of the oven, the fire should be stopped first and then the machine stopped, so as to avoid overheating and deformation of the pipes.

(2) If the furnace chamber is damaged, it must be repaired in time. The thickness of refractory bricks should be 6 cm, and refractory mortar should be used for coating and masonry.

(3) After each furnace batch is dried and taken out and cooled, the smoke-cleaning handles should be pulled back and forth several times in time to prepare for the next furnace drying.

(4) The coal or firewood used should meet certain standards to ensure the time and quality of baking.

(5) Necessary equipment such as blowers and exhaust fans should be maintained and repaired, and a certain number of spare items should be available to prevent impact on normal operation.

(6) Electrical equipment should be standard and safe, and power line connections should be firm and reliable. Normal power supply must be ensured during drying to avoid economic losses caused by temporary power outages.

(7) The chimney must be installed outside the room, with a height above 4 m, to ensure the total draft requirement of the flue. Fire must be prevented.

In the production and processing of shiitake mushrooms, in addition to strict quality management, attention must also be paid to good economic accounting. Shiitake mushrooms must be purchased according to quality standards and dried as soon as they are collected. Finished products and semi-finished products should all be well protected against theft, rats, insects, and moisture. The remaining mushroom stipes and deformed mushrooms with no drying value should be pickled immediately, and later can be processed into products such as shiitake mushroom preserves, shiitake mushroom stipe floss, and shiitake mushroom soy sauce, to increase the development and utilization value of by-products and increase the economic benefits of the enterprise.