CA1298952C - Continuous process for the partial sterilisation of mushroom casing - Google Patents

Abstract

ABSTRACT
A process and apparatus are provided for the sterilisation of horticultural material, which may be substrate, fertiliser, peat or especially mushroom casing. The sterilisation may be complete or may be partial, removing only harmful organisms whilst leaving benefic-ial organisms unharmed. The horticultural material is passed through a tunnel and is exposed to radiofrequency electromagnetic waves for a period sufficient for sterilisation. In a preferred process and apparatus the horticultural material is heated to a suitable temperature by the waves and is then maintained in a region not exp-osed to the waves for a period long enough to sterilise the material, before allowing it to cool to ambient temperature.

Description

---`` 12~8~52 CONTINUoUS P~OCESS FOR THE PARTIAL ST~ ISATION
_ _ _ _ ______ uF l~'US~O~M C SlN~

The present invention relates to a continuous process for the sterilisation of horticultural materials in particular for the partial sterilisation of mushroom casing, and to an apparatus for carrying out the process.
It i8 frequently necessary in horticulture to sterilise hortieultural materials, eg substrates such as soils, peat, roc~wool fibre etc or fertilisers to remove harmful organisms such ag flies, fungi and mites. In some cases ho~ever it is degirable to only partially sterilise the material, so as to remo~e harmful organ-3 isms without killing beneficial organisms, partieularly cer~ain baeteria. Thig is egpecially so in mushroom production, ~here partial sterilisation of mushroom 'easlng' is carried out.
The produetion of mughroomg hag three d~stinct phases. In the first, the culti~ation step, pure culture spa~n is introdueed into prepared beds of mush-room eompost. For good ~ro~th of the mushroom myeelium throughout the eompost, the eulture gpa~n should be bro-~en into small pieees and, when the temperature of the compost i8 about 30 C, pressed into the beds about 25em apart. The gro~th of myeelium through the compo~t i8 de~eribed as a 'run'. Thi6 period usually last# from 15 to 30 days, during ~hich time condition8 of temperature,-humidity and ventilation are kept at an optimum, ~hilst _ .~ O

12~8352 pests infecting the compost are kept to a minimum.
The next stage in the process is the casing step.Once the mycelium has penetrated throughout the compost, the bed is moistened and a thin layer of mushroom casing is spread over the surface. This is done for a number of reasons:
(a) ~iushrooms form on the surface of the compost, but they are heavy, and if there is nothing to support them -they may fall over and break the 'roots' through ~hiChit derivessustenance.
(b) The surface of the compost dries out very readily, and it is extremly difficult to replace the evaporated water ~ithout ki~ling the spawn, The casing layer preve-nts drying out.
(c) Vegetative mycelium is encouraged to fruit when it ent~rs a n~edium deficient in food, it attempts to ensure its survival by producing fruit containing spores. A
suitable casing material provides this medium.
~n ideal casing material is one which has the follo~ing characteristics:
(a) It absorbs water guickly and releases it slowly, (b) Its water holding capacity is such that it can be watered uithout sealing off the compost.
(c) Its texture is not substantially altered by ~at-ering.(d) It is neither acld nor alkaline, but neutral.
(e) It must contain a bacterial flora ~hich promotea fruiting.
(e) It iæ free from disease organisms and insects.
(f) It is free from ~ndecomposed vegetable matter 12~ S2 (which is susceptible to attack by undesirable moulds).
The most commonly used casing material is a mixt-ure of peat and chalk or lime, but other materials which have been used or contem?lated are peat alone (if neutral) weathered mushroom compost, recycled paper pulp, recycled sugar beet lime, and some soils. Research continues to find other materials.
About 5 ~eeks after casing, the first 'flush' of mushro~ms are ready for collection and the final stage of cropping and packing.
During the entire cultivation process the presence of fungi and invertebrate pests, such as flies, mites and nematodes must be minimised. For mushroom casing, the control of these pests and fungi has traditionally been ` 15 achieved by steam treatment of the casing prior to use.
However there are a number of disad~antages as60ciated with the use of steam. First the capital cost is high.
Second, uniformity of treatment is practically impossible since a temperature dif$erential in the casing will occur, Thirdly a wide variatlon in sample temperature occuring when stean iB used will mean that some bacteria which are beneficial to mushroom growing, as well as pests will be destroyed.
There has been some discugsion of the possibility of the use of radiofrequency radition to destroy micro-organlsms in soils, but there is considerable uncertalnty as to the mechanism of destruction, and to the optimum conditions for destruction. For example Baker and Fuller, Phytopathology, 59, 193-197 (1969) concludes that the ef~'iciency of destruction is very dependent on soil ~2~ 52 moisture content, whereas Ferris, American Phytophathological Society, 74, 121-126 (1984) concludes that soil moisture content has no effect. Whilst the experiments described in these publications have been carried out on soils, no suggestion has ever been made as to the effect of radiofrequency radiation on the novel medium of mushroom casing.
It is one object of the present inventio~ to provide a process for the sterilisation of horticultural materials (as described above) and especially for the partial sterilisation of mushroom casing.
It is a further ob~ect to provide a sterilisation apparatus to carry out that process.
Accordlng to one aspect of the present invention there is provided a continuous process for at least partially sterilising wet horticultural material comprising the steps of:
(a) providing an open ended tunnel having radio frequency wave applicator means positioned at least above and below a central portion of the tunnel, said tunnel being defined by steam tight walls constructed of low loss dielectric material in at least a central portion of said tunnel; (b) continuously passing wet horticultural material through said tunnel; and (c) simultaneously exposing at least a part of said wet horticultural material to radio frequency waves in a frequency range of 13 to 100 MHz and at a power density of up to 90 KWm 2 to heat said material to a predetermined temperature of 90-100C, wherein said wet horticultural material is exposed to said radlo frequency waves and heat generated by said waves for a period of time in the range 30 seconds to 20 minutes.

. ~
~ ~ 4 1298~S2 In a preferred form of the process, more suited to but not exclusively for ~he partial sterilisation of horticultural material and especially for mushroom casing, the wet material or wet casing is continuously passed through a tunnel and at least part of the material or casing is exposed to radiofrequency electromagnetic waves, length of exposure to the waves being such as to heat the material or casing to a temperature which will 4a 12~ 9SZ

steriliæe the material or casing, and subsequent to the exposure maintaining the material or casing substantially at that temperature for ~ pre-determined time in a region not exposed to the waves, before allowing the material or casing to cool to amb-ient temperature.
Sterilisation in this latter form of the process may be partial or complete depending upon the combination of temperature and time. ~ suitable temperature for both is 90 - 100 C. The time ~or which the ternperature is maintained may be between 30 seconds and 20 mlnutes. The latter time is generaliy suf~icient for cor.plete steril-isation at the quoted temperatures, and for partial sterilisation of mushroom casing a time of around 2 ; 15 minutes is generally suf$icient.
~ ccording to a second aspect of the invention, there is provided a steriliser for horticultural mater-ial (as defined above) adapted for continuous gperation comprising a set of radiofrequency electromagnetic wave applicators and between the applicators a tunnel which at least between the applicators is of a lo~ loss die-lectric material, the tunnel belng adapted for passage of materlal therethrough and also adapted to exclude water vapour emanating from the tunnel, from the appli-cators-In a preferred form of the steriliser, thetunnel extends do~nstream beyond the applicators and in the extended region at least is adapted to substantiallY
maintain the temperatur.e of heated materlal contained therein for a predetermined time.

12~95Z

The steriliser facilitates the perfo~mance of the process of the invention and the preferred form of the steriliser is ~articularly intended for the preferred form of the process and for partial sterilisation.
The radiofrequency electromagnetic ~aves may be microwaves but are preferably radiofrequency waves of frequency 13 to 100 M~z. The use of such ~aves for the sterilisation of horticultural materials has a number of advantages over the use of steam. ~irstly, all parts of the exposed material are treated simultaneously and thus at the same rate, enabling uniform treatment.
Secondly radiofrequencies are cheap to produce, the equipment necessary being simple and commercially available. ~ number of rf frequencies are allocated by national la~s to food industry use, and of these 27.12 liXz is preferred.
Thirdly, and as a consequence of the uniform treatment, the process is easily controllable to achieve conditions suitable for partial sterilisation.
A particular advantage of the prefer~ed proc-ess and steriliser of the invention is their energy efficiency. By maintaining the temperature of the mater-ial without the need for further exposure to waves, t4e power input may be dedicated solely to initially rais-~5 ing the temperature to the desired range, ~ithout ener-gy waste in simply boiling off water. The parameters of electrical po~er, dimensions of the tun~el flow rate of the material etc discussed below have been devised by the inventors to optimise energy efficiency and hence running costs.

7 12~.952 - Heating by rf electromagnetic waves relies upon non-conducting matexials absorbing waves passing through them and converting the energy absorbed into heat. The amount of energy absorbed is given by the follo~ing equation:

2 _~
5 P - 2~f~0~tan ~ E Watts m abs where f_ ~requency (Hz) E= electric field strength (Vm ~c~ 8.854 x 10 (Fm , - relative dielectric constant tan o the loss tangent The factorsrand tan o depend upon the material, its temperature and usually vary with frequency. It can be seen that the higher~and tan ~ are the greater the energy absorbed for particular values of f and E. For this reason materials such as mushroom casing must be wet for rf heating as its tan ~ value is very low ~nen dry.
The process and steriliser of the invention have been found to be suitable for use with horticultural materials with a wide rang~ of water contents, and ~ater contents from 20 up to 200 wt% have been succ-essfully sterilised. An optimum water content for mush-room casing partial sterllisation is 100 wt% , ie a 1:1 wt. ratio of dry casing to contained water. 'rhe sterilised output may of course have its ~ater content adjusted to a useful level, eg by addition of extra water.
It is desira~le ~hat the period of exposure of the wet horticultural material to the rf ~aves is just \~

8 12~

sufficient to heat the material to 90 - lOO~C. If the per-iod is longer, then energy i6 wasted in simply boiling off water without any further rise ln temperature. Furthermore, if this water evaporates, the material will dry out and the efficiency of heating will decrease, so the temperaturc ~ay drop. It has been found to be beneficial however to reach the temperature of 90 - 100C about 85~oOf the way through the heating period in some applications.
Using a commercially available 20 KW rf gener-ator suitable heating may be achieved in a laysr of wet horticultural m~terial of maximum thic~ness 10 cm, a pre-ferred thic~ness being 7.5 - 10 cm. The air gap between the applicators and the material should be the minimum possible ~o av~i~ power luss wit~l~u~ c~s~ rcln~.
j po~er aen~ ty of up to 60 KW m 2 is preferred, with a m~d~um OI 90 Kn m, for such a thicknes~. ~ period of ex~osure to the rf waves of around 1 - 2 minutes under these conditions is genarally adequate for the heating to 90 -100 C, but the precise conditions may be determined by experiment. These exposure conditions may be achieved by a suitable com.bination of flow speeds and length and width of the exposure region.
The choice of rf wave generator and applicators will be entirely conventional, Gene~ators of 20 and 50 KW
output are available commercially. The use of a larger generator and larger dimensions of tunnel may increase throughput of material but it is generally more convenient to use a number of sterilisers of lo~er capacity in para-llel, so that if the generator breaks, the process need 9 1~ 952 not be halted completely. It is generally advisable and is often a legal requirement that the generator and ap?licat-ors are shielded to avoid exposure of workers to radiation.
The a~ount and type of shielding will be governed by local laws.
The applicators may for example be a number of bars or plates surrounding the tunnel. In a preferred embodi-ment the rf applicators consist of two parallel plates of metal eg copper placed on opposite sides of the tunnel.
The tunnel may be of any convenient cross sectional shape, but a preferred shape is rectangular. The tunnel should be steam-t~ght to retain water within the hortic-ultural material and to prevent steam contacting the app-licators.
The tunnel in the region of the applicators should be constructed from any low loss dielectric ~aterial, that is any material with a low dielectric constant. Typical of such materials, which are essentially transparent to rf waves are sodaborosilicate glass, certain polymers such as PTFE, polyethylene, polystyrene, polypropylene, certain ceramics and silicon resin bonded fibreglass.
The extended regiOn of the tunnel where the hort-icultural material is not exposed to rf waves may be of any convenient material, and may be integral with the tunnel region between the applicators and so made of the same material. If the two regions are not inte gral the join should be steam-tight. The extended region should either have good insulating properties or be covered with an insulating material to retain the temperature, although 10 l2~æ

a temperature drop of 10 - 15 C may be acceptable in some a~?plications. The extended region of the tunnel, if not integral ~ay for example be of insulated aluminium.
The legth of the eY.tended region is determined only by practical limits, but a minimum of ca. 0,5 m has been found desirable to avoid contact of the applicators by steam. Determination of a suitable length to achieve a desired sterilisation time as discussed above for a given flow rate and cross section ray be determined by the meth-od for detecting harmful organisms described below.
It is also desirable to extend the tunnel up-stream of the applicators so that steam given off as the wet horticultural material is heated may preheat the in-coming material, further reducing energy waste..An upstr-eam extension of 0.5 - 1.0 m has been found adequate in a tunnel of cross section 30 x 10 cm.
It should be noted t~.at unlike the steam ster-ilisations of the prior art, the degree of heating by the steam generated in the present process and steriliser is not sufficient to kill bacteria present in mushroom casing.
In the present process and steriliser, hortic-ultural ~aterial is passed contiuously through the tunnel.
This may be achieved for example by a conveyor belt or an archimedean scre~ passing along the tunnel. These should be made of lo~ dielectric constant materials to avoid heat-ing. Alternati~ely the tunnel may be placed at an anglr to the ground and the material passed through the tunnel by the pull of gravity, optionally encouraged by agitators.
It is desirable that the tunnel is substantially filled by ll 12~952 the horticultural material to avoid air spaces in the tunn-el. Other methods of passin~ the material through the tunnel r~ill be ap?arent to those skilled in the art.
The speed at ~rh;ch tlle horticultural material is passed through the tunnel will be determined in particular by the size of the tunnel, the shape of the tunnel, the power input, rrater content etc. Using a rectangular tunnel 30cm x lOcm deep, a 20 KW rf generator, trro rectangular a?plicators 2m x 30 cm a conveyor belt speed of 1 m min uas found to be entirely adequate, ~ fter the horticultural material has passed thr-ough the process of the invention i~ is desirable to cool it as quickly as possible eg by ~ater spraying of the downstream end of the tunnel so as to avoid destruction of beneficial organisms in a partial sterilisation process, and to permit early handling.
The presence of harmful fungi and pests may be detected in the processed horticult~ral material, in the case ~f pests by visual inspection, or in the case of fungi by germination follo~rcd by visual inspection- Other methods of detectlon Prill be known to those skiiled in the art.
In the case of mushroom casing, the presence of pests or fungi may be detected simply by use in the cultivation of mushrooms, when symptoms of infestation rrill be quickly manifested if pests or fungi are present. Such symptoms are ~idely recorded, see for exam~le l.ushroom Growing Today, 5th edn., 1966, chapters 17 and 20.
The invention rrill no~ be described by way of example only with particular reference to Figures 1 and 2 12~

in whicll:
Fig 1 ~hows a schematic perspective view of a tunnel and ap~licators according to the invention, and Fig 2 Sho~s the tunnel and ap~licators in position in a steriliser.
Referr~ng to the Figures a musllroom casing part-ial steriliser is shown generally at (1). A glass tunnel (2) of rectangular cross section 30 x 10 cm passes betw--een two parallel rectangular copper applicator plates (3,~). These plates (3,~) are positioned above and below the central portion of the tunnel (2). The tunnel (2) and applicators (3,4) are within the shielded body of a comm-ercialy available 20 1~l, 27 MHz rf generator (5) supplied by Induction Heating Equipment Ltd, Horsham, Sussex, ur~, and the applicators (3,4) are co~:nected to the output of the generator (5) by copper strip connectors. ~ region (6) of the tunnel (2) is covered with a thermal insulat-ing nlaterial, glass wool, and the two ends (7,8) of the tunnel (2) project outside the body of the rf generator (5).
conveyor belt (9~ passes through the tunnel (2). The insulated region (6) is downsteam and about 2m long. The upstream end (8) is about lm long.
In usel mushroom casing (~) from a feed hopper (11) is passed onto the conveyor belt (9). ~ tensioner (not shown) was found to be necessary to compensate for expa-nsion of the belt (9), ~ belt speed of 1 m s was used.
Caæing on the belt (9) enters the tunnel (2) and is then irradiated with rf waves at 27.12 MHz passing through the 13 1 2 ~ ~ 952 tunnel from the applicators (3,4). It ~as found to be nec-essary to 'tune' the position of the applicators (3,4) to achieve ma~:imum zbsorbtion of rf waves by the casing (10), as r,!easured as a maximum anode curr~nt reading, by raising and lo~ering the upper applicator (3) using an electric servo motor (not sho~n). This tuning ~as easily automated.
During the process steam is generated within the tunnel (2) and moves in both directions along the tun-nel (2). In the upstream direction the steam preheats casing moving towards the rf field, wllilst in the do~nst-ream direction the steam maintains the treated casing at a high temperatureJ the bulk of the temperature loss being reduced by the insulator (6).
The treated casing, in which both pestæ and fungi hav~ been destroyed but beneficial bacteria are un-harmed passes out of the end (7) of the tunnel (2) and is deposited into the collection vessel (12).
R _ults A trial was performed to i11ustrate the gro-~th of mushrooms on three casing mix~ures, (a) An uncontaminated, entreated peat~chalk mixture.
(b) An untreated peat/chalk mixture contaminated with Verticillium fungicola.
_ _ _ _ (c) A peat/chalk mixture initially contaminated with Verticilllum fungicola as in (b) but exposed to rf treatment in the manner described herein prior to use-Results were as follows:

(a) The mushrooms produced showed no visible differences from the commercial yield.

12~ 52 1~

(b) ~'ery few mushrooms of acceptable quality were produced due to dry bubble disease ca~sed by Verticillium fungicola-95% of the mushrooms were infected ~ith this disease.
(c) The ~ushrooms produced showed no visible differences from the commercial y~eld and no sign of disease. Colon-isation of the casing layer took place more quick]y using this treated casing than ~hen the untreated uncontaminated casing ~as used, and the yield obtained was in some cases greater by ca. 20% than that obtained using uncontaminated untreated casing. Typical yields were 16 kg m

Claims (13)

1. A continuous process for at least partially sterilizing wet horticultural material comprising the steps of:
(a) providing an open ended tunnel having radio frequency wave applicator means positioned at least above and below a central portion of the tunnel, said tunnel being defined by steam tight walls constructed of low loss dielectric material in at least a central portion of said tunnel;
(b) continuously passing wet horticultural material through said tunnel; and (c) simultaneaously exposing at least a part of said wet horti-cultural material to radio frequency waves in a frequency range of 13 to 100 MHz and at a power density of up to 90 kWm-2 to heat said material to a predetermined temperature of 90-100°C, wherein said wet horticultural material is exposed to said radio frequency waves and heat generated by said waves for a period of time in the range 30 seconds to 20 minutes.

2. A process according to claim 1 wherein as a result of heating said wet horticultural material to said predetermined temperature, steam is generated ; the process including the further step of utilizing said steam downstream of said radio frequency wave applicator means to maintain the wet horticultural material at a temperature within about 10 to 15°C of said predetermined temperature without utilization of additional heating means.

3. A process according to claim 1 or claim 2 wherein as a result of heating said horticultural material to said predetermined temperature steam is generated; the process including the further step of utilising said steam upstream of said radio frequency wave applicator means to preheat the wet horticultural material.

4. A process according to claim 1 wherein said power density is up to 60 KW m-2.

5. A process according to claim 1 wherein said horticultural material is selected from the group consisting of horticultural substrate, fertiliser, peat and mushroom casing.

6. A process for partially sterilizing wet mushroom casing material comprising the steps of:

(a) providing an open ended tunnel having radio frequency wave applicator means positioned at least above and below a central portion of the tunnel; said tunnel being defined by steam tight walls constructed of low loss dielectric material in at least a central portion of said tunnel;
(b) continuously passing wet mushroom casing material through said tunnel; and (c) simultaneously exposing at least a part of said wet mushroom casing material to radio frequency waves in a frequency range of 13 to 100 MHz for a period of 1-2 minutes to heat said wet mush-room casing material to a temperature of 90 to 100°C, wherein said wet mushroom casing material is exposed only to said radio frequency waves and heat generated by said waves for a period of time in the range 30 seconds to 20 minutes sufficient to destroy fungi and invertebrate pests and wherein as a result of heating said wet mushroom casing material to said predetermined tempera-ture, steam is generated; the process including the further step of utilizing said steam upstream of said radio frequency wave applicator means to preheat the wet mushroom casing and downstream of said radio frequency wave applicator means to maintain the wet mushroom casing material at a temperature within about 10 to 15°C
of said predetermined temperature without utilization of additional heating means.

7. Apparatus for at least partially sterilizing wet horticultural material comprising:
an open ended tunnel having a channel extending therethrough, said tunnel defined by steam tight walls, constructed of low loss dielectric material in at least a central portion of said tunnel;
radio frequency wave electromagnetic applicator means positioned at least on opposite sides of said central portion of said tunnel for heating wet horticultural material passing through said tunnel to a predetermined temperature in the range 90 - 100°C, said tunnel having no other heat generating means associated therewith;
and wherein said radio frequency wave applicator means is capable of emitting RF radiation at a frequency of 13 to 100 MHz, and of applying an RF power density of up to 90 KW m-2; and means for continuously passing the wet horticultural material through said tunnel.

8. Apparatus according to claim 7, said tunnel being provided with a first extended region upstream of said radio frequency wave appli-cator means adopted to expose incoming material when in the said first extended region to steam given off from the heated material.

9. Apparatus according to claim 7 or claim 8 said tunnel being provided with a second extended region downstream of said radio fre-quency applicator means and wherein the said downstream extended region includes insulation means for maintaining the wet horticultural material at substantially said predetermined temperature and is adapted to expose outgoing material to steam given off said heated material.

10. Apparatus according to claim 7 wherein said radio frequency wave applicator means is capable of applying an RF power density of up to 60 KW m-2.

11. Apparatus according to claim 7 wherein said radio frequency wave applicator means comprises a pair of parallel plates.

12. Apparatus according to claim 7 wherein said tunnel is sub-stantially rectangular in shape, with a cross-sectional dimension of about 30 x 10 cm2.

13. Apparatus according to claim 7 wherein said radio frequency wave applicator means comprises a 20 KW, 27 MHz generator.