Analysis of the Causes of Contaminant Microorganism Contamination in Edible Mushroom Spawn Production

Right now is the critical period for mushroom growers to make high-temperature mushroom spawn. To improve the success rate of cultivation and economic returns, contamination by foreign microorganisms must be prevented during spawn production. The causes of spawn contamination are as follows:

1. Incomplete sterilization: Incomplete sterilization of the growing substrate will cause contamination by foreign microorganisms, and contamination typically appears within 3-5 days. Incomplete sterilization is caused by the following reasons:

1) The specified pressure and temperature are not reached during sterilization: Generally, the mycelium and spores of contaminant microorganisms can be killed after 1-2 hours at a pressure of 1 MPa and a temperature of 121°C, or at normal pressure and 100°C for 6-8 hours. When using high-pressure sterilization, the key is to fully exhaust the cold air from the pot; otherwise, even if the specified pressure and temperature are reached, it cannot be guaranteed that all contaminant microorganisms will be killed. When using normal-pressure sterilization, the sterilization chamber should preferably be built in a round shape so that the temperature inside the stove rises evenly, and the top of the stove should be built in an arched shape, so that condensed water flows down along the walls and will not wet the spawn bottles (bags). In addition, insufficient steam and low temperature are also factors in incomplete normal-pressure sterilization.

2) The influence of the properties of the growing substrate: The type of materials required for spawn production, the microbial load in the growing substrate, the moisture content, and the pH can all affect the sterilization effect. During sterilization, the corresponding sterilization time should be selected according to the different growing substrate formulations. After the growing substrate is prepared, it must be sterilized promptly to avoid massive reproduction of microorganisms, which would increase the microbial load in the substrate and also cause nutrient loss in the substrate. The moisture in the growing substrate should be uniform and appropriate, so that the sterilization effect is good. "Sandwich material" (growing substrate containing dry material that has not absorbed water) should be avoided in the growing substrate. The steam in the sterilization pot has weak penetrating ability through sandwich material, and sterilization dead corners are likely to occur, making complete sterilization impossible. In addition, the pH of the growing substrate also affects the sterilization effect; acidic growing substrate requires a shorter sterilization time than neutral and slightly alkaline substrate.

3) Improper stacking of spawn bottles (bags): The way spawn bottles (bags) are stacked in the sterilization pot can affect the sterilization effect. Certain gaps should be left during arrangement to allow steam circulation and temperature rise. When sterilizing growing substrate contained in spawn bags, because plastic bags tend to deform and soften after being heated, if the filling is not tight, the gaps between the spawn bags will be reduced when they are stacked and pressed together, affecting the sterilization effect.

2. Non-standard inoculation operations: The inoculation process for edible mushrooms requires strict aseptic operation. If the operation is not standardized, it is very easy to cause contamination of the growing substrate by foreign microorganisms. Contaminant microorganisms are generally found in the air of the inoculation room (tent), on the surface of the spawn bags, and on the body surface of the inoculation personnel. The characteristic of growing substrate contamination caused by non-standard operations is that it occurs at the opening of the spawn bag or at the inoculation point. Therefore, inoculation must be carried out strictly in accordance with aseptic operating procedures, and strict disinfection treatment must be carried out before inoculation.

3. Contaminant microorganisms carried in the stock spawn: The stock spawn used for expansion propagation must be pure spawn, so as to ensure the uniformity of the spawn. If the stock spawn itself is not pure, it can cause contamination on a batch scale. Its characteristic is that after inoculation, a batch of spawn is contaminated in a concentrated manner, and the contaminated site is at the inoculation point, appearing within 3-5 days after inoculation. Therefore, when making spawn, we must use excellent stock spawn with strong vitality, suitable spawn age, and strong stress resistance.

4. The culture room is not clean: If the culture room is not clean, the air is not circulating, and the room is humid, contamination by foreign microorganisms can also occur. In this case, the contamination rate is relatively low when mycelium just begins to grow, but as the culture time is prolonged, the contamination rate will gradually increase. A large amount of contamination appears relatively late, generally appearing 10 days after inoculation.