The process of making spawn remains much the same as what Dr. Sinden first developed. The grain is mixed with a little calcium carbonate, then cooked, sterilized and the cooled. Small pieces of pure culture mycelium are placed in small batches of the grain. Once the small batch is fully colonized, it is used to further inoculate several larger batches of grain. This multiplying of the inoculated grain continues until the commercial size containers, usually a plastic bag with a breathable filter patch, are inoculated. During the colonization of each batch the containers are shaken every few days to further distribute the mycelium around. Temperatures are maintained where the mycelium is growing in a range of 75-76F . Uniformity of the air circulating around the bags is important to insure that all containers are keep at the desired temperature range. The mycelium is sensitive and its fruiting mechanism can be easily damaged at high temperatures.
Before the spawning operation, attempts may be made to improve the substrate moisture. Sometimes water is applied or an pesticide application may be made. Farms with a historical problem of mummy disease may avoid water applications at spawning, because the free water improves conditions for the bacteria to reproduce and spread, thereby increasing the incidence of this disease. On bed farms spawn and supplement is broadcast over the surface of the substrate. Uniformity of the distribution is critical to achieve an even distribution and more even spawn growth and temperatures. On tray or bulk farms, spawn is usually metered into the substrate during the mixing operation. Some growers will bring the spawn brought to room temperature before mixing it into the substrate. Gypsum (1-2% of substrate dry wt.) can be added if substrate that is wet, little greasy and/or not completely conditioned.
Historically, three methods of spawning have been used. However, on most commercial farms the "Through Mixing" method prevails. "Broadcast" and "Ruffling In" spawning were the other two methods. Broadcast spawning consisted of covering the substrate surface with spawn. Whereas "Ruffling In' included the scratching the spawn into the top few inches of substrate.
"Through Mixing" spawning has many advantages over the other methods. Research has shown there are increases in yield. Because the mycelium does not have to grow far to colonize substrate, it reduces spawn-growing time, which is important because spawned substrate is exposed to various types of infection. Another advantage is that spawn grains act as supplemental nutrients for the mushroom mycelium. With through mixing spawning it is important that substrate must be cooled throughout the bed before spawning.
Spawning is the cleanest operation performed on a mushroom farm. All equipment, baskets, tools, etc. should be thoroughly cleaned and disinfected before spawning. Normally it is the first operation of the day when personnel have not been into any of the older houses. After spawning, the beds should be covered with plastic to protect the freshly spawned substrate from air borne contaminants and pest. Unfortunately, for any contaminants present before covering, the moist environment provides ideal conditions for germination and growth. Plastic sheeting is sometimes used to cover the substrate to help maintain substrate moisture.
How Much Spawn?
The amount of spawn used depends on the crop cycle and cost. The spawning rate can be expressed as a unit or quart per so many square feet of bed surface; 1 unit (~ 1 lb or 1 liter) per 6-8 ft. is considered a standard rate on a commercial farm. The rate is sometimes expressed based on spawn weight versus substrate weight; therefore, 2-3 percent spawning rate is equivalent rate. A low spawning rate is about 1 unit for every 12-15 sq. ft. (1-2% Dry Weight), whereas a high rate about 1 unit for every 4-5 sq. ft (>3% of Compost Dry Weight). The more spawn used the better, since it is a cheap supplement, increasing overall production; and the more initial growing points will provide a quicker and more efficient use of substrate nutrients. Both of these factors will improve the colonization of substrate, which also helps insure the mushroom will grow quicker than other fungal competitors. Furthermore, as the spawn rate is increased, more heat is generated and the heat surge occurs earlier during spawn growing period.
During the colonization of the substrate by the spawn (spawn run) is a good time to evaluate the crop and substrate. Spawn growth and the presence or absence of other molds helps to indicate how the substrate preparation process has been carried out. Problems with substrate formulation or process and Phase II composting or conditioning may first develop during the spawn-growing period. Weed and indicators molds may tell the grower how the composting process went and what nutrient was lacking or in excess. These molds may grow on compounds that have not been used by the microbes during the Phase II will often suggest a problem occurred.
Although the type of spawn growth depends on many factors, often it may indicate the nutritional and moisture level of the substrate. The spawn strain itself will vary in their inherent capacity for rapid or slow growth. This variation is a genetic characteristic; therefore, growers should be familiar with the characteristics of the different strains and the suppliers of spawn are a good source of this information. The other obvious important factor that determines the type of spawn growth is the substrate. Substrate element analysis is important but does not always correlate with growth or yield but should be monitored to determine trends in substrate preparation. The lab analysis should be used as guidelines and establishing trends from crop to crop. There is a direct correlation between substrate ammonia content and subsequent growth and yield of mushrooms. Substrate should have less that 0.05% ammonia, dry weight, at spawning time. By smelling, most growers can detect 0.1% ammonia levels, which will restrict spawn growth. Ammonia content above 0.2% will kill spawn. The substrate pH has little to no correlation to spawn growth or yield. Spawn can tolerate a pH in the range of 6.5 to 8.2 and normally it will decreases from 7.5 to 6.0 during cropping. When the substrate nitrogen content is analyzed at this time, it should be in a range of 2.0 - 2.5% on a dry weight basis. A positive correlation of substrate nitrogen and yield has been shown. The greater the nitrogen content, with no ammonia, the better the yield. Nitrogen content has no correlation with spawn growth, since rapid spawn growth has been observed in both high and low nitrogen composts. However, generally a high nitrogen substrate has a slower spawn growth, but fills out and becomes denser. The lipid (fats/oils) content of the substrate will influence both the rate and quantity of spawn growth. More nutritional substrate will support slower and finer texture spawn growth. It is suspected that the thinner strands of spawn are slowly adsorbing nutrients. In less nutritional substrate, spawn growth is more rapid and white with more rhizomorphs, suggesting the spawn is seeking nutrients.
Ideal substrate moisture at spawning varies according to the type of substrate. With horse manure substrate, moisture of 65-72% is normal. With synthetic composts a moisture range of 65-75% is normal. However, there are exceptions to these ranges where spawn growth and yield are better outside these ranges. Mushroom size and quality is affected directly by dry substrate, where dry substrate will produce smaller, off-color mushrooms.
Ventilation and environmental requirements for substrate are not well understood. It is assumed that little oxygen is required within the substrate. Carbon dioxide levels are kept high within the room or at least under the plastic that is used to cover the substrate after spawning. It is known that there is an increased spawn growth rate with increasing CO2 levels to 10,000 ppm. The desired relative humidity is 95% or more in order to preserve substrate moisture. Relative humidity within the room can be maintained by watering the walls and floors. Some high-pressure misting systems have been developed, but they are expensive to purchase and maintain. Steam can also be used to maintain humidity, however it is a source of heat and would increase energy cost and put more demands on the air conditioning system.
During the spawn-growing period, little outside ventilation is used, unless outside air is used as a supplemental source of cooling. During the warmer months outside air is not used, and the room air is re-circulated through the air conditioning units to be cooled. The higher humidity of the outside air requires more cooling capacity.