IE57106B1 - Continuous process for the partial sterilisation of horticultural material - Google Patents

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 beneficial organisms unharmed. The horticultural material is passed through a tunnel (2) and is exposed to radio frequency 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 exposed to the waves for a period long enough to sterilise the material, before allowing it to cool to ambient temperature.

Description

THE MINISTER OS' AGRICULTURE, FISHERIES AND FOOD, OF GT. WESTMINSTER HOUSE, HORSEFERRT ROAD, LONDON SWIP 2ΑΞ, ENGLAND. the university of Sheffield of mappin street, Sheffield SI 3JD, ENGLANDPrice eop VUWIlNUOU5...n,lliU?AirriAL_^J’Ei{lLtSAUON The present invention relates to a continuous process for the a sterilisation of horticultural materials in particular for the partial sterilisation of mushroom casing, and to an apparatus for carrying out the process r, It is frequ· ntly necessary in horticulture to sterilise hortlcu) tur. I materials, eg. substrat.es such as soils, peat, rockwool fibre etc o fertilisers to remove harmful organisms such as flies, fungi and m tes. in some cases however it is desirable fo only partially .· ferilise the material, so as to remove harmful organisms without kil ing beneficial organisms, particularly certain bacteria.

This is espi cially so in mushroom production, where partial sterilisation of mushroom 'casing* is carried out.

The production of mushrooms has three distinct phases. In the first, the cultivation step, pure culture spAwn is introduced into prepared beds of mushroom compost. For good growth of the mushroom mycelium throughout the compost, the culture spawn should be broken into small pieces and, when the temperature of the compost is about 30°C, pressed into the beds about 25cm apart. The growth of mycelium through the compost is described as a ‘run*. This period usually lasts from 15 to 30 days, during which time conditions of temperature, humidity and ventilation are kept nt an optimum, whilst 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) mushrooms form on the surface of the compost, but they are heavy, and if there is nothing to support them they may fal I over and break * the 'roots’ through which it derives sustenance.

I (b) The surface of the compost dries out very readily, and it is extremely difficult to replace the evaporated water without killing the spawn. The casing layer prevents drying out. (c) Vegetative mycelium is encouraged to Fruit when it enters a medium deficient in food, it attempts to ensure its survival by producing fruit containing spores. A suitable casing material provides this medium.

An ideal casing material is one which has the following characteristics: (a) It absorbs water quickly and releases it slowly. (b) Its water holding capacity is such that it can be watered without sealing off the compost. (c) Its texture is not substantially altered by watering. (d) it. is wither acid nor alkaLine, but neutral. (e) It must contain a bacterial flora which promotes fruiting, (f) It is I roe from disease organisms and insects. (g) Et is (i*ee from undeconiposed vegetable matter (which is susceptible to attack by undesirable moulds).

The most commonly used casing material is a mixture of peat and chalk or lime, but other materials which have been used or contemplated arc peat alone (if neutral) weathered mushroom compost, recycled paper ° pulp, recycled sugar beet lime, and some soils. Research continues to find other materials, , ι a About 5 weeks after casing, the first ‘flush’ of mushrooms arc ready for collection and the final stage of’ cropping and packing.

During the entire cultivation process the presence of fungi and invertebrate nests, such as flies, mites and nematodes must be minimised. For mushroom casing, the control of these nests and fungi has traditionally been achieved by steam treatment of the casing prior to use. However there are a number of disadvantages associated with the use of steam. First the capital cost is high. Second, uniformity of treatment is practically impossible since a temperature differential in the casing will occur. Thirdly a wide variation in sample temperature occurring when steam is used will mean that some bacteria which are beneficial to mushroom growing, as well as pests will be destroyed.

There has been some discussion of the possibility of the use of radiofrequeney radiation to destroy microorganisms in soils, but there is considerable uncertainty as to the mechanism of destruction, and to the optimum conditions for destruction. For example Baker and Fuller, Phytopathology, 59. 193197 (1969) concludes that the efficiency of destruction is very dependent on soil moisture content, whereas Ferris, American Phytopathological Society, 2k, 121Ί26, (ΙθβΜ) 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 radiofrequeney radiation on the novel medium of mushroom casing. l· It is one object of the present invention 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 object to provide a sterilisation apparatus to carry out that process.

According to one aspect of the present invention there is provided a continuous process for the sterilisation of horticultural material comprising continuously passing wet horticultural material through a tunnel end exposing at least a part of tne material within the tunnel to radiofrequency electromagnetic waves, the length of exposure to the waves being such that fungi and invertebrate pests are destroyed.

In a preferred form of the process, more suited to but not exclusively for the 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 sterilise the material or casing, and subsequent to the exposure maintaining the material or casing substantially at that temperature for a predetermined time in a region not exposed Co the waves, before allowing the material or casing to cool to ambient temperature.

Sterilisation in this latter form of the process may be partial or complete depending upon the combination of temperature and time. A suitable temperature for both is 90 - 100°C. The time for which the temperature is maintained may be between 3θ seconds and 20 minutes.

The latter time is generally sufficient for complete sterilisation at the quoted temperatures, and for partial sterilisation of mushroom casing a time of around 2 minutes is generally sufficient. * 35 According to a second aspect of the invention, there is provided a steriliser for horticultural material (as defined above) i adapted for continuous operation comprising a set of radiofrequeney electromagnetic wav»; applicators and between the applicators a tunnel which at least betwi^cn the applicators is of a low loss dielectric material, the tunnel being adapted for passage of material therethrough and also adapted to prevent water vapour escaping from the tunnel in the region of the applicators.

In a preferred form of the steriliser, the tunnel extends downstream beyond the appLicators and in the extended region at least is adapted to substantially maintain the temperature of heated material contained therein for a predetermined time.

The steriliser facilitates the performance of the process of the invention and the preferred form of the steriliser is particularly intended for the preferred form of the process and for partial sterilisation. The radiofrequency electromagnetic waves may be microwaves but are preferably radiofrequency waves of frequency 13 to 100 MHz, The use of such waves for Lhe sterilisation of horticultural materials has a number of advantages over the use of steam. Firstly, all parts of the exposed material are treated simultaneously and thus at the same rate, enabling uniform treatment.

Secondly radiofreqvencies are cheap to produce, the equipment necessary being simple and commercially available. A number of rf frequencies are allocated by national laws to food industry use, and of these 27.12 MHz 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 preferred process and steriliser of the invention is their energy efficiency. By maintaining the temperature of the material without the need for further exposure to waves, the power input may be dedicated solely to initially raising the 10 temperature to the desired range, without energy waste in simply boiling off water. The parameters of electrical power, dimensions of the tunnel flow rate of the material etc discussed below have been devised by the inventors to optimise energy efficiency and hence a running costs.

Heating by rf electromagnetic waves relies upon nonconducting / materials absorbing waves passing through then and converting the energy absorbed into heat. The amount of energy absorbed is given by the following equation: 1 PBbs = 2nf£(itrtancrE2 Watts m'3 where f= frequency (Hz) E= electrical field strength (Vm'1) 8.854 x 10‘12 {Fm -*) fc, = relative dielectric constant tancr» the loss tangent 'Hie factors €r and tancr* depend upon the material, its temperature and usually vary with frequency. It can be seen that the higherCrand tan c* are the greater the energy absorbed for particular values of f and E. For this reason materials such us mushroom casing must be wet for rf heating as its tantf’value is very low when dry.

The process and steriliser of the invention have been found to be suitable for use with horticultural materials with a wide range of water contents, and water contents from 20 up to 200 wt% have been successfully sterilised. An optimum water content for mushroom casing partial sterilisation is 100 wt$, ie a 1:1 wt. ratio of dry casing to contained water. The sterilised output may of course have its water content adjusted to a useful level, eg by addition of extra water. β It is desirable that the period of exposure of the wet horticultural material to the rf waves is just sufficient to heat the material to 90 i - 100°C. If the period is longer, then energy is wasted in simply boiling off water without any further rise in temperature.

Furthermore, if this water evaporates, the material will dry out and the efficiency of heating will decrease, so the temperature may drop.

It has been found to be beneficial however to reach the temperature of - 100°C about 85% of the way through the heating period in some applications.

Using a commercially available 20 KW rf generator suitable heating may be achieved in a layer of wet horticultural material of maximum thickness 10 cm, a preferred thickness being 7-5 ~ 10 cm. The air gap between the applicators and the material should be the minimum possible to avoid power loss without causing arcing. A power density of up to 60 KW is preferred, with a maximum of 90 KW m'? for such a thickness. A period of exposure to the rf waves of around 1-2 minutes under these conditions is generally adequate for Lhe heating to 90 - 100°C, but the precise conditions may be determined by experiment. These exposure conditions may be achieved by a suitable combination of flow speeds and length and width of the exposure region.

The choice of rf wave generator and applicators will be entirely conventional. Generators 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 lower capacity in parallel, so that if the generator breaks, the process need not be halted completely. It is generally advisable and is often a legal requirement that the generator and applicators are shielded to avoid exposure of workers to radiation. The amount 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 embodiment the rf applicators consist of two parallel plates of metal eg. copper placed on opposite 6 sides of the tunnel. 1’ The tunnel may be of any convenient cross sectional shape, but a preferred shape is rectangular. The tunnel should be steam-tight to retain water within the horticultural material and to prevent sleum contacting (.he applicators.

The tunnel in the region of the applicators should be constructed from any low loss dielectric material, 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 horticultural 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 integru.1 tho 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 a temperature drop of 10 - 15°C may be acceptable in some applications. The extended region of the tunnel, if not integral may for example be of insulated aluminium.

The length of the extended region is determined only by practical limits, but a minimum of £&, 0.5 ® 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 may be determined by the method for detecting harmful organisms described below.

It is also desirable to extend the tunnel upstream of the applicators so that steam given off as the wet horticultural material is heated may preheat the incoming material, further reducing energy waste. An upstream extension of 0.5 - 1.0 m has been found adequate in a tunnel of cross section 30 * 10 cm.

It should be noted that unlike the steam sterilisations of the prior i 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, horticultural material is passed continuously through the tunnel. This may be achieved for example by a conveyor belt or an archimedean screw passing along the tunnel. Those should be made of low dielectric constant materials to e avoid heating. Alternatively the tunnel may be placed at an angle 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 the horticultural material to avoid air spaces in the tunnel. Other methods of passing the material through the tunnel will be apparent to those skilled in the art.

The speed at which the 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, water content etc. Using a rectangular tunnel 30cm x 10cm deep, a 20 KW rf generator, two rectangular applicators 2m x 30 cm, a conveyor belt speed of 1 m min1 was found to be entirely adequate.

After the^orticuitural material has passed through the process of the invention^is desirable to cool it as quickly as possible eg. by water 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 tests may be detected in the processed horticultural material, in the case of pests by visual inspection, or in the case of fungi by germination followed by visual inspection. Other methods of detection will be known to those skilled in the art.

In the case of mushroom casing, the presence of pests or fungi may be r detected simply by use in the cultivation of mushrooms, when symptoms £ of infestation will be quickly manifested if pests or fungi are present. Such symptoms are widely recorded, see for example 'Mushroom Growing Today, 5th edn.. 1966, chapters 17 and 20*.

The invention will now be described by way of example only with particular reference to Figures 1 and 2 in which: Fig 1 Shows a schematic perspective view of a tunnel and applicators according to the invention, and Fig 2 Shows the tunnel and applicators in position in a steriliser.

Referring to the Figures a mushroom casing partial steriliser is shown generally at (1). A glass tunnel (2) of rectangular cross section 30 x 10 cm passes between two parallel rectangular copper applicator plates (3,4). These plates (3,4) 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 commercially available 20 KW, 27 MHz rf generator (5) supplied by Induction Heating Equipment Ltd, Horsham, Sussex, UK, and the applicators (3*4) are connected to the output of the generator (5) by copper strip connectors. A region (6) of the tunnel (2) Is covered with a thermal insulating material, glass wool, and the two ends (7,6) of the tunnel (2) project outside the body of the rf generator (5). A conveyor belt (9) passes through the tunnel (2). The insulated region (6) is downstream and about 2m long. The upstream end (8) is about lm long.

In use, mushroom casing (10) from a feed hopper (11) is passed onto the conveyo* belt (9). A tensioner (not shown) was found to be necessary t<» compensate for expansion of the belt (9)· A belt speed of 1 m s'1 was used. Casing on the belt (9) enters the tunnel (2) and is then irradiated with rf waves at 27.12 MHz passing through the tunnel from the applicators (3,4). It was found to be necessary to 'tune' the position of the applicators (3,4) to achieve maximum absorption of rf waves by the casing (10), as measured as a maximum anode current reading, by raising and lowering the upper applicator (3) using an electric servo motor (not shown). This tuning was easily automated.

During the process steam is generated within the tunnel (2) and moves in both directions along the tunnel (2). In the upstream direction the steam preheats casing moving towards the rf field, whilst in the downstream direction the steam maintains the treated casing at a high temperature, the bulk of the temperature loss being reduced by the insulator (6).

The treated casing, in which both pests and fungi have been destroyed but beneficial bacteria are unharmed passes out of the end (7) of the tunnel (2} and is deposited into the collection vessel (12).

Results A trial was performed to illustrate the growth of mushrooms on three 20 casing mixtures, (a) An uncontaminated, untreated peat/chalk mixture (b) An untreated peat/chalk mixture contaminated with Vorticillium 25 funsiculflL^ (c) A peat/chalk mixture initially contaminated with Verticillium 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. (b) Very few mushrooms of acceptable quality were produced due to dry bubble disease caused by VerticiIlium fungicola. 95* of the mushrooms were infected with this disease. j (c) The mushrooms produced showed no visible differences from the commercial yield and no sign of disease. Colonisation of the casing layer took place more quickly using this treated casing than when the i untreated uncontaminated casing was 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 m2.

Claims (28)

1. A continuous process for the sterilisation of horticultural material comprising continuously passing wet horticultural material through a tunnel and exposing at least a part of the material within the tunnel to radiofrequency electromagnetic waves, the length of exposure to the waves being such that fungi and invertebrate pests are destroyed. I

2. A continuous process for the sterilisation of horticultural material comprising continuously passing the wet material through a tunnel and exposing at least a part of the material within the tunnel to radiofrequency electromagnetic waves, the length of exposure to tho waves being such as to heat the material to a temperature which will sterilise the material and subsequent to the exposure maintaining the material substantially at that temperature or allowing the temperature to fall by nt the most 15°C for a predetermined time in a region not exposed to the waves.

3. A process as claimed in claim 2 wherein the temperature is 90-100 U C.

4. A process as claimed in claim 2 or claim 3 wherein the predetermined time is in the range 30 seconds - 20 minutes.

5. A proces; as claimed in any one of claims 1 to 4 wherein the horticultural material is a horticultural substrate.

6. A process according to any one of claims 1 to 4 wherein the horticultural material is a fertiliser.

7. A process as claimed in any one of claims 1 to 4 wherein the horticultural material is peat.

8. A process as claimed in any one of claims 1 to 0 wherein the horticultural material is mushroom casing.

9. A process according to any one of the preceding claims wherein the sterilisation is partial.

10. A process as claimed in any preceding claim wherein the £> radiofrequeney electromagnetic waves are 13 ~ 100 MHz.

11. A process as claimed in claim 10 wherein the power density to which the horticultural material is exposed is up to 9θ KW m' 2 . ίθ

12. A process as claimed in claim 11 wherein the power density is up to 60 KW m’ 2 .

13. A process as claimed in any one the preceding claims wherein the water content of the horticultural material is 20 - 200 wt^.

14. A process as claimed in any preceding claim wherein the period of exposure of the horticultural material to the waves is substantially just sufficient to heat the material to 9θ “ W0”C with minimum evaporation of water. □ O

15. A steriliser for horticultural material adapted for continuous operation comprising a set of radiofrequeney electromagnetic wave applicators and passing between the applicators is a tunnel which at least between the applicators is of a low loss dielectric material, 09 c 3 the tunnel being adapted for passage of material there through and also adapted to prevent water vapour escaping from the tunnel in the region of the applicators.

16. A steriliser as claimed in claim 15 and wherein the tunnel extends downstream beyond the applicators and in the extended region at least is adapted to substantially maintain the temperature of heated material therein or allowing the temperature to fall by at most 15°C for a predetenained period of time. 35

17. A steriliser as claimed in claim 16 wherein the predetermined time is in the range 3θ seconds to 20 minutes.

18. A steriliser as claimed in claim l6 or claim 17 wherein the temperature is in the range 90° - 100°C. 5 19.

A steriliser as claimed in any of the horticultural material is a horticultural claims 15 substrate. to 18 wherein the 20.

A steriliser as claimed in any one of horticultural material is a fertiliser. claims 15 to 18 wherein the 1© 21.

A steriliser as claimed in any ono of horlicultural material is peat. claims 15 to 18 wherein the 15 22.

A steriliser as claimed in any one of horticultural material is mushroom casing. claims 15 to 18 wherein the 2,3.

A steriliser as claimed in any one of claims 15 to 22 wherein the radiofrequency electromagnetic waves are of frequency 13 - 100 MHz.

24. A steriliser as claimed in any one of claims 15 to 23 wherein the layer of horticultural material is 7-5 “ 10 cm thick.

25. A steriliser as claimed in any one of claims 15 to 24 wherein the power density of the waves is up to 90 KW m' 2 .

26. A process as claimed in any one of claims 1 to 14 substantial ly as hereinbefore described with reference to the accompanying drawings

27. A steriliser as claimed in any one of claims 15 to 25, substantially as hereinbefore described with reference to the 30 accompanying drawings.

28. A process for sterilising horticultural material which uses a steriliser as claimed in claim 27.