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Sealing a Bag.

Depending on the need of the amount of gas exchange, we manufacture several types of filters for our bags. The choice of filters depends on application, and other variables. In other words, how much oxygen is needed by the mycelium growing substrate, and when is oxygen needed during the growing process defines the best filter requirement. The cultivation process of whether the sawdust block is browned inside the bag or outside the bag helps to determine what filter to use.

Since filter bag cost is a major part of cost of goods sold, most clients tell us that by using a less expensive filtered bag even though time to strip the bag off is a day or two longer, than a more expensive bag, it is worth while to save the cost difference. On the other hand, some growers wish to maintain all other variables constant but wish to choose the filter best suited for their growing condition, our different filter choice help them to reach their optimum condition.

The first requirement for a filter is to provide or maintain a clean atmosphere in the bag. From many technical articles, the use of filters that have a pore opening of maximum 0.1 micron will exclude most contaminants detrimental to specialty mushroom cultivation and spawn production However in practice, heat distorts most filter materials to a certain extent. Autoclave heat of 250-260 °F will reduce pore size on some filter materials and enlarge pore size in some other types. Usually 10 - 50% pore size change can be expected. We have developed several types of filters that are not affected by sterilization heat. The second requirement for a filter is gas transmission. This requirement is influenced by the percentage of pores per given area. Due to methods of manufacture, some parts of the filter material may be dead space. This dead space is an area where no gas exchange occur. This area generally is used to bond the layers of some filters .

Since some breathable membranes are very soft and fragile and is unsuitable for practical machining, therefore a second layer to support the membrane layer is laminated one way or another to the membrane to give the final filter material strength for machining. Filters are usually bonded onto bags by heat, which is generated by heating cartridges or other means. We do not use adhesive. In order to bring out the maximum efficiency of a filter, we developed machinery that opens the largest possible hole whenever is needed to expose the largest area of filter for gas transmission.

Therefore our hole under the filter is square as against round holes. Our filters are sealed by a square die. In mushroom cultivation processes, where the bag is taken off the block in 30 days or less, the filter gas exchange rate plays little effect to the growing process. To simulate natural conditions, as well as to enhance filter efficiency, the grower may lower the night time incubation temperature by say 5 degrees than the day time temperature in the room. In this way the temperature differentiation will help the filter to breath.

It is erroneous to believe that the bigger the filter on a bag the better it may be. For example, if one tries to use bags such as those used by some spawn producers to cultivate shiitake. Those bags have filter materials that occupy almost half of one side the bag face. The breathing holes of these bags are small pin holes. One will not have a healthy mycelium growth and no fruiting will result. This is because, even though the filter material is big in area, the breathing holes are extremely minute. Not enough gas transmission will result. However, for mycelium to incubate in spawn production, it is necessary to have uniform gas transmission evenly distributed for all the substrate in the bag, and yet minimum humidity should be lost through breathing holes. This is the reason for a large filter with a large number small breathing holes. There are other ways to achieve this balance by using smaller filters and therefore lower the price of bags. Filters made not specifically for mushroom cultivation do not provide test data relevant to mushroom production.

Data of filters after heating by autoclaves are more meaningful to mushroom cultivators and spawn producers. Claims of some filters developed for battery separation especially in regards to specific gas transmission rates for oxygen and carbon dioxide are not done under conditions of mushroom cultivation. For spawn bags, we produce filters that give sufficient gas exchange but control water vapor evaporation through the filter so there will be a minimum of drying of spawns right below the filter. The bag also must withstand storage temperatures of the cold room for extended length of time. Bag bottoms are sealed by an impulse sealer.

All bags are individually checked before packing and shipping.

Filter types

Some commonly available filters are of the following construction :

  • Non woven polypropylene, such as Unicorn type P.
  • Non woven polyethylene such as Tyvek,
  • Laminates of polypropylene non woven and polypropylene membrane such as Celgard, and Unicorn type 14, and Snowpro laminates,
  • Laminates of polypropylene non woven with PTFE such as Unicorn Type B and type T filters,
  • PP controlled open cell foam, and such proprietary filtered bags such as bags by Mycelia of Holland, and some companies in Japan.

Non woven PP

There are a number of standard commercial production methods employing PP fiber to produce a filter material. Usually the final process is pressing an accumulated fiber bed by heat and pressure under two steel rollers. The controlled temperature and pressure and the quantity per sq. area of resin fiber will give the final pore size and uniformity. This filter is characterized by its pore size will close gradually with increase of temperature.

Tyvek and similar PE filter materials

Tyvek is a product produced by Du Pont and there are several grades of Tyvek most suitable for use as filters. The best are medical grades, however if one holds Tyvek against light one will see the unevenness of fibers. In practice a difference in thickness also is measurable. Tyvek is manufactured with HDPE, and therefore does not withstand high sterilizable temperatures. Users usually reduce sterilization temperature and increase time to compensate acceptable sterilization. Usually PE bags use Tyvek as filter material, as HDPE material is readily heat sealable on PE bags.

Polypropylene non-woven laminated with polypropylene membrane.

Celgard is a material first developed for use in battery cell separation applications. There are several grades of Celgard generally used for mushroom bag filtration use. Membrane is a material fabricated so that a thin PP film is incorporated with micron sized holes. This property is utilized to allow breathing of fungus during incubation. The laminate of this PP membrane onto a substrate to give the finished product mechanical strength and machinability. Lamination process is by heat and calendering of diamond pattern steel rollers, therefore a large percentage of Celgard is void of pores and amounts to over 50% of a given area. This reduces efficiency of filtration. Unicorn filter type 14 uses a material similar to Celgard. This material also suffers to a certain extent temperature sensitivity, that is under higher temperatures, the pores of the material closes, but much less than straight PP non woven as described above. The advantage of this material is for such mushroom species as shiitake, so that during incubation it requires retention of Carbon Dioxide, therefore a reduced pores and low efficiency of filtration are ideal applications.

Polypropylene non woven laminated with PTFE.

PTFE is a high temperature plastic material and can be fabricated into thin membrane form with micro sized holes. These processes are well documented in numerous patents since 1970's. By laminating with polypropylene non woven this laminate can thus be heat sealed onto PP bags. Because the controlled pore size is on the PTFE, normal sterilizing heat will not reduce or increase the pore size. This laminate is most suitable for use for spawn production.