WO2023213954A1

WO2023213954A1 - Apparatus and method for curing plants

Info

Publication number: WO2023213954A1
Application number: PCT/EP2023/061818
Authority: WO
Inventors: Eleandro CRESTANI; Jaime Roque MENEGASSO; Jacson SCHNEIDER; Ricardo Bruning SCHUTZ; Leo Miguel CARMINATTI
Original assignee: British American Tobacco (Investments) Limited; Souza Cruz S.A.
Priority date: 2022-05-04
Filing date: 2023-05-04
Publication date: 2023-11-09

Abstract

An apparatus and method for curing plants are disclosed. The method comprises performing a first, natural, curing stage in ambient conditions, followed by performing a second, artificial curing stage, using a man-made heater and/or forced ventilation source. The transition from the first stage to the second stage is made after the plants attain a substantially final coloration. A mobile structure for curing plants in the field using ambient conditions is also disclosed herein. The mobile structure comprises a framework for supporting multiple plants which hang down from the frame, at least one side of the mobile structure being open for natural ventilation of the plants.

Description

APPARATUS AND METHOD FOR CURING PLANTS

Field of the Invention

The present invention relates to an apparatus and method for curing plants, such as for curing tobacco plants.

Background

Tobacco is an agricultural crop of considerable economic importance, used primarily in the manufacture of cigarettes, cigars, and other such products. Tobacco is grown in more than one hundred (mostly tropical) countries, spread across North and South America, Europe, Africa and Asia, including, for example, Brazil, Italy, Turkey, Pakistan, USA and Tanzania. There are various types (varieties) of tobacco, the three most common types being Virginia, grown frequently in countries like Brazil, China, India, Tanzania and the US; Burley, grown frequently in countries like Brazil, Italy and the US; and Oriental, grown frequently in countries like Greece and Turkey. Cigarettes may be produced containing just one variety of tobacco, e.g. Virginia, or blends of multiple varieties of tobacco.

The post-harvest processing of tobacco leaves typically includes drying (curing) the leaves and removing (separating or defoliating) the leaves from the tobacco plants prior to use in cigarettes. The curing process is performed not only to remove moisture from (dehydrate) the tobacco leaves, but also to achieve desired attributes of sensorial quality due to chemical changes in the tobacco leaves. Virginia tobacco is usually cured in heated barns (so is sometimes referred to as “flue-cured” tobacco), Burley is usually air-cured in barns, while Oriental is usually sun-cured in the open air.

The consumer experience of tobacco typically occurs through smoking a cigarette or cigar, and is characterised by various sensory inputs relating to flavour, taste, aroma, etc. Cigarette manufacturers seek to provide consumers with a consistent and reliable product, including in terms of the various sensory factors mentioned above. However, given that tobacco is a natural product, it may be subject to intrinsic variation between individual plants, combined with additional variations such as differences in growing location, soil and/or details of subsequent processing for the tobacco (such as curing).

A typical approach for air curing tobacco (such as Burley) is to hang the plants in a barn for a curing period of usually 30-45 days, which is then followed by separation of the leaves (defoliation) and baling. The barn may be at least partly open-sided to support ventilation through the plants. Sometimes there is a delay of say 15-20 days between the end of curing and the start of the separation and baling such that total length of the overall process is often around 60 days. This approach however suffers from certain drawbacks, including: a) the interval between curing and separation, etc, may cause a quality loss, since the cured tobacco plants in the barn may still be exposed to natural weather conditions; b) if the crop is rainy (wet), the curing period may have to be significantly extended, which can result in degradation, mould, etc.

Such drawbacks may in turn affect the smoking quality of the resulting tobacco.

Summary

The invention is defined in the appended claims.

An apparatus and method for curing plants are disclosed herein. The method comprises performing a first, natural, curing stage in ambient conditions, followed by performing a second, artificial curing stage, using a man-made heater and/or forced ventilation source. The transition from the first stage to the second stage is made after the plants attain a substantially final coloration. An apparatus may be configured to perform such a method.

A mobile structure for curing plants in the field using ambient conditions is also disclosed herein. The mobile structure comprises a framework for supporting multiple plants which hang down from the frame, at least one side of the mobile structure being open for natural ventilation of the plants.

Brief Description of the Drawings

Various examples and implementations of the present invention will now be described in detail by way of example only with reference to the following drawings:

Figure 1 is a schematic flowchart of an example process for curing tobacco as described herein.

Figure 2 comprises four photographs of example barns which may be used in a first phase of curing tobacco as described herein.

Figure 3 comprises two photographs of example mobile structures which may be used in a first phase of curing tobacco as described herein.

Figure 4 comprises a photograph of tobacco leaves that have attained a brown colour and are ready to transition from the first stage to a second stage of the process for curing tobacco as described herein.

Figure 5 is a diagram showing the demarcation of a tobacco plant into different categories of leaf (according to one known classification scheme).

Figure 6 is a photograph of defoliated tobacco leaves stored in a set of example metal containers (boxes) in the second stage of the process for curing tobacco as described herein. Figure 7 comprises two photographs of an oven for use in the second stage of the process for curing tobacco as described herein, the left-hand photograph showing the oven with the door closed, the right-hand photograph showing the oven with the door open.

Figure 8 is a photograph of an oven including a box such as shown in Figure 6 containing tobacco leaves for curing in the second stage of the process for curing tobacco as described herein.

Figure 9 is a photograph of shackled tobacco leaves such as may be produced by the process for curing tobacco as described herein.

Detailed Description

Figure 1 is a schematic flowchart of an example process for curing tobacco as described herein. At a high level, the process can be considered as comprising two stages (phases), a first stage 10 based on natural or ambient curing/drying, and a second stage 20 based on artificial or forced curing/drying.

In the first stage 10, the tobacco plants are placed into a barn or other suitable structure in operation 110 in order to commence the curing. Typically the barn or other structure will be provided (for example) with shelving or racks, and the tobacco plants hang down from the shelving or racks. In other implementations, another approach may be taken, for example, the tobacco plants may be lain horizontally on such shelving or racks.

The barn or other structure is at least partly open to the outside such that the tobacco plants within the barn or other structure experience (are exposed to) natural or ambient (atmospheric) conditions. For example, one or more walls (sides) of the barn may be omitted to provide ventilation to the tobacco plants within the barn; alternatively (or additionally), one or more walls (sides) of the barns may be formed from netting or other similar material(s) to allow ventilation from the outside to pass through the netting and into (and out of) the barn. Figure 2 comprises four photographs of example barns which provide suitable ventilation via openings, netting, etc, and hence may be used for curing tobacco as described herein during the first stage 10 of the process illustrated in Figure 1.

Figure 3 comprises two photographs of example mobile structures which may be used for curing tobacco as described herein during the first stage 10 of the process illustrated in Figure 1. The mobile structures are formed by open frames (typically of metal) that expose the tobacco plants within the mobile frame to natural (ambient) conditions. The tobacco plants are retained within the mobile frames, for example, by hanging or suspending the plants from shelving, wiring, racking, etc. In other implementations, some or all of the tobacco plants may be lain on shelving or racks within the mobile frame structure.

The mobile frames shown in Figure 3 are relatively lightweight, and may be readily transported around a farm or plantation to where the tobacco plants are harvested. This avoids the need to transport the newly-picked (and hence relatively heavy) tobacco plants around the farm to the nearest barn (such as one of the more permanent structures shown in Figure 2). The mobile frames may also be collapsible, such as by folding or simple disassembly, to support easier transport and/or more compact storage of the mobile frames. It will be appreciated that the barns and mobile frames shown in Figures 2 and 3 are presented as examples of suitable structures for performing the first stage 10 of the curing process under natural conditions as described herein, and the skilled person will be aware of other suitable structures that might be used for this purpose.

As can be seen in Figure 3, the tobacco plants in the mobile frame are still predominantly green, with some tinge of yellow (and the tobacco plants placed in a barn such as shown in Figure 2 would likewise commence with a predominantly green colour). The tobacco plants remain in the barn or other structure (such as shown in Figure 2 or 3) until in step 120 the leaves reach a brown colour, such as shown in the two photographs of Figure 4. Please note that for clarity, Figure 4 shows the leaves after they have been separated from the stalk of the tobacco plant, as described in more detail below, however, this defoliation does not, in itself, affect the colour of the leaves, hence the coloration of Figure 4 is consistent with the coloration at the end of the first stage 10.

(It will be appreciated that the skilled person is already aware of the initial green colour of tobacco leaves as per Figure 3, and a suitable brown colour for cured tobacco leaves as per Figure 4, and accordingly Figures 3 and 4 are provided by way of example only).

The change in colour of the leaves, which transition from green through yellow and orange to brown, reflects chemical changes occurring in the leaves as a result of the curing process, such as starch converting into sugar, and other chemical changes that may affect flavour and other properties of the tobacco. Attaining the desired colour (typically brown, but potentially orange or yellow) at operation 120 represents the completion of the first stage 10 of the curing process described herein and indicates that the tobacco plants/leaves are ready to transition from the first phase 10 to the second phase 20. The duration of the first stage is typically in the range from 12 to 18 days, depending (for example) upon the weather conditions and the type of barns. At the end of this stage the inherent characteristics of the tobacco type in terms of sensory qualities and colour fixing have been formed.

The process of Figure 1 now enters the second stage 20, in which the color fixing has been completed, and the remaining moisture of the ribs and stems is removed from the tobacco plants. The second stage 20 begins with the removal (stripping) of the tobacco leaves from the tobacco plants at operation 130. This process is known as defoliation, and is performed to remove the leaves from a stalk of plant material such as tobacco. The defoliation may be performed manually (by hand) and/or by using a suitable defoliator machine.

Figure 5 illustrates a tobacco plant 510. The leaves of the plant 510 can be assigned to different categories according to the position of the leaf on the stalk 560. For example, in one (known) classification scheme illustrated in Figure 5, the leaves of a tobacco plant may be assigned to one of four different categories: lugs 520 (X), cutters 530 (C), bodied leaves 540 (B) and tip leaves 550 (T). It will be appreciated that other schemes for leaf classification may be used, whether for tobacco or other plants, and these may identify a different total number of leaf categories as appropriate.

The tobacco leaves removed from the plants are separated by leaf position (X, C, T and B) and are stored loose in boxes. Figure 6 is a photograph of an example of defoliated tobacco leaves stored in a set of boxes as per operation 130 of the second stage 20 of the process for curing tobacco as illustrated in Figure 1. The boxes in Figure 6 are shown on their sides, such that the base of each box is to the right of the photograph, one end of each box is to the left of the photograph, and one side of each box is to the top of the photograph.

Figure 6 depicts a total of four boxes 610A, 610B, 610C, 610D in a 2x2 arrangement. In particular, the centre foreground shows a first pair of two boxes, one 610A on top of the other 610B, with their respective side walls in contact. The right background shows (in part) a second pair of two further boxes, again one 610C on top of the other 610D, and again with their respective side walls in contact. The second pair of boxes and the first pair of boxes have their respective end walls in contact. In the following, we will use the reference numeral 610 to refer collectively or generically to boxes 610A, 610B, 610C, 610D and to any other such containers. (It will be appreciated that the boxes shown in Figure 6 are presented as examples of containers for performing the second stage 20 of the curing process as described herein, and the skilled person will be aware of other suitable containers that might be used for this purpose).

The orientation of the boxes 610 on their sides, as shown in Figure 6, makes for easier packing of the leaves into the boxes through the open tops of the boxes, which face sideways. In effect, the leaves are inserted horizontally into the boxes stem-first, i.e. with the leaf stems facing towards the base of the boxes, and the newly inserted leaves lie on top of the previously inserted leaves. When the box has been filled with tobacco leaves, the box is now reorientated, such that the base is located on the ground, and the top of the box faces upwards (as per the orientation shown in Figure 8, as discussed below).

Typically different categories of leaf are stored in different boxes. For example, one of the boxes 610 shown in Figure 6 might be used to store lugs 520 (X), one of the boxes might be used to store cutters 530 (C), one of the boxes might be used to store bodied leaves 540 (B), and the remaining box 610 may be used to store tip leaves 550 (T). The boxes 610 shown in Figure 6 are made of metal, with the side walls and end walls of each box formed by metal rails, and the base of each box formed by a metal rack. The use of the rails and a rack in this manner provides good ventilation of the tobacco leaves stored in the boxes, and hence supports drying of the moisture from these tobacco leaves. In addition, the boxes 610 of Figure 6 facilitate convenient transportation of the tobacco leaves, such as from the barn or other structure where the first phase 10 of natural curing has been performed, to an oven where the second phase of artificial curing 20 is to be performed. The skilled person will appreciate that other designs and/or materials may be used for the formation of such boxes or containers.

Figure 7 comprises two photographs of an oven 710 for use in the second stage of the process for curing tobacco as described herein, the left-hand photograph showing the oven with the door closed, the right-hand photograph showing the oven with the door open. In addition, the right-hand photograph shows a box 610 located in the oven (the box shown in Figure 7 is empty of tobacco leaves).

In the second stage 20 of curing, the tobacco leaves are subject to drying within an oven 710 or other suitable structure to remove remaining moisture from the leaves (especially from the ribs and stems of the leaves) as per operation 140 of Figure 1. The oven 710 provides a sealed and controllable environment for this purpose. For example, the oven 710 typically includes a fan or other device for performing forced ventilation of the tobacco plants using the air in the oven. It will be appreciated that such forced ventilation can help to provide quicker drying than relying only on natural ventilation.

In addition, the oven is generally able to heat the drying air above the ambient temperature. The heating can be achieved using any suitable heat source such as burning oil, gas or wood, electrical or solar power. In the case of solar power, it will be appreciated that the oven is not simply put outside in the sunshine, rather, the solar power may be used to generate electricity or to heat water, etc, and this electricity and/or hot water is then supplied to the oven to heat and dry the tobacco. Accordingly, even in the case of solar power, the heating is not achieved purely in a natural manner, but rather by using a manmade heater that takes solar power as an input (in a similar manner to taking gas or electricity as a input).

Note that such heating not only increases the temperature in the oven, but also reduces the relative humidity, both of which help to accelerate the drying of the tobacco. It will be appreciated that the relative humidity of the drying air may also (or alternatively) be reduced by other measures, such as by including a dehumidifier in the path of the air through the oven, and/or by replacing relatively hot and humid air inside the oven with cooler, less humid air drawn from outside the oven (rather than just continuing to re-circulate the same air within the oven). The oven 710 may include sensors to monitor properties such as temperature and humidity. The sensors can then act as a feedback system for controlling the operation of the oven 710 to ensure that the temperature and/or humidity are kept at levels that support rapid drying of the tobacco leaves but without damaging or degrading the leaves. The drying time for the second stage 20, as per operation 140, is typically 2-4 days in duration, depending upon the condition of the tobacco leaves, the parameters of the oven 710, and other relevant factors such as the target moisture content. For example, it is often desired to retain a certain amount of moisture in the cured tobacco leaves to make them more pliable, and less brittle and liable to damage during handling of the tobacco. On the other hand, retaining an excess of moisture may impair the quality and conservation of the tobacco leaves. It will be appreciated that the skilled person is aware of a suitable level of residual moisture content for the tobacco leaves at the end of curing.

Figure 8 is a photograph of a box such as shown in Figure 6 containing tobacco leaves and located in an oven 710 for curing in the second stage of the process for curing tobacco as described herein. It can be seen that the leaves now have an orientation with their stems at the bottom and the tips of the leaves at the top (in contrast to the sideways orientation shown in Figure 6). In addition, it can be seen that the sides of the oven are provided with rails or supports 801 A, 801 B such that the box 610 is suspended above the floor of the oven. It will be appreciated that such a configuration supports airflow underneath the box and so helps to quicken the drying procedure.

The size of oven used for performing the second phase of the curing may vary according to circumstances. The oven 710 shown in Figure 7 is able to accommodate multiple boxes 610 of tobacco for drying at the same time. However, some installations may utilise smaller or larger ovens. Note that because of the initial drying in the first phase 10, the drying in the second phase 20 is relatively short (typically 2-4 days as mentioned above). This shorter drying time generally allows smaller (and/or fewer) ovens to be used, compared with some other curing processes, while still allowing the same overall throughput of tobacco leaves. For example, if another curing process takes 8 days of oven time compared to the 4 days for the curing process disclosed herein, the oven used for the curing process disclosed herein might only have half the capacity of the oven used for the another curing process in order to achieve the same overall throughput (output) of cured tobacco leaves.

In some implementations, the second phase 20 of the curing, in particular the artificial drying of operation 140, may be performed in a barn with heating and/or ventilation and/or humidity control to provide a suitably controlled environment (rather than in a barn such as shown in Figure 2, in which material inside the barn is exposed to ambient/atmospheric conditions). One example of such a barn that provides a suitably controlled environment for artificial drying is disclosed in WO 2017/103560 (albeit this barn is described in the context of conditioning rather than curing tobacco). The skilled person will be aware of other facilities that may be suitable for performing the second phase of the curing process described herein.

At the end of the second phase, the dry and softened tobacco leaves may be removed from the oven 710 and shackled to provide a grouping of around 25 leaves tied manually together. Figure 9 is a photograph of an example of such shackled tobacco leaves as may be produced by the process for curing tobacco as described herein. Alternatively, the loose leaves may be baled or stored loose and separated by position in a dry location, typically covered with cotton cloth and plastic canvas. It will be appreciated that the details of this subsequent processing of the cured tobacco at the end of the second phase 20 will vary according to circumstances, such as the intended usage of the tobacco, the available facilities on the farm/plantation, and so on.

It has been found from sensory investigations that Burley tobacco cured as described herein provides similar or the same sensory properties as conventional air-cured Burley tobacco. Accordingly, from the perspective of a tobacco consumer, the two curing methods (conventional, and as disclosed herein) produce little or no difference. However, for a tobacco producer, the curing method disclosed herein provides various benefits over the conventional approach for curing.

Thus the approach described herein reduces curing time from 30-60 dates down to typically 14-24 days. It will appreciated that this reduced curing time is attractive for a tobacco producer, since the economic value of the crop can be realised more quickly. In addition, the faster curing can reduce the exposure time to (and hence likelihood of) various problems that might degrade the quality of the crop, e.g. fungal attack, insect attack, rot, storm damage, etc.

A further benefit is that the barn occupancy in the first stage 10 of the curing process disclosed herein is reduced to 12-18 days, compared with 30-45 days for a conventional curing process. This reduction in occupancy time can be used to support a greater crop output for a given number of barns, or a reduced number barns for a given output of crop (or some intermediate arrangement). For example, the reduction in occupancy may render it practical to use the same barn multiple times with the same crop - a first portion of the crop is cured, then a second portion of the crop is cured, and so on (rather than having to cure the first portion of the crop in one barn and the second portion of the crop at the same time in another barn). In other words, the shorter occupancy time can support a sequential rather than parallel approach, and hence a lower spatial density of curing barns is needed across a farm or plantation. In addition, reducing the number (and/or size) of barns used for a given crop helps to reduce costs associated with constructing the barns, and also allows land that would otherwise be used for barns to be converted to grow extra crops. Furthermore, in some implementations, the first stage may be performed using mobile structures (also termed yards) such as shown in Figure 3. Unlike conventional barns, such structures comprise lightweight frames which can be moved around the fields, thereby avoiding the need to transport the cut tobacco plants to a (fixed) barn prior to curing. In addition, the mobile structures can be cleared when no longer needed, and so do not continue to occupy arable land if no longer required.

Accordingly, the benefits of the approach described herein include: *reduced (shorter) exposure of the tobacco to weather conditions;

*fewer barns (because more crops can be processed through existing barns) *quicker supply of the crop to the market

*improved crop stability and consistency in view of reduced exposure to external factors such as harmful weather (e.g. high moisture)

*the ability to use mobile structures for the first stage, rather than necessarily curing the tobacco in conventional barns.

In summary, a method is provided herein for curing plants. The method comprises performing a first, natural, curing stage in ambient conditions, followed by performing a second, artificial curing stage using a man-made heater and/or forced ventilation source. The transition from the first curing stage to the second curing stage is made after the plants attain a substantially final coloration.

The first curing stage may be performed in a barn which is at least partly open to the atmosphere external to the barn, hence providing the ambient conditions of the first curing stage. Alternatively the first curing stage may be performed in a mobile structure located in a field; for example, the mobile structure may comprise an open metal framework configured to support the hanging of plants for curing. Accordingly, plants in the mobile structure are again exposed to the atmosphere to support the first stage of curing in ambient conditions.

It may be possible to fold up or disassemble the mobile structure for ease of transportation and/or storage. For example, the mobile structure might be moved from one field to another field as a crop ripens and is picked at different times. This approach may be more flexible and efficient than the use of fixed barns for curing.

In general, the duration of the first curing stage is in the range 12-18 days, although other ranges are also envisaged according to conditions (e.g. 10-20, 10-16, 40-20, 14-18, 12- 16 days, and so on). As discussed above, the duration of the first curing stage is therefore significantly shorter than the duration of a conventional air-curing process, which has potential benefits such as improved product quality (due to less risk from external factors such as bad weather) and increased economic efficiency.

The first curing stage generally lasts until (and ends when) the leaves of the plants are (fixed) brown. At this point, the main sensorial properties of the tobacco are set or fixed, and it is time to transition to the second curing stage to reduce the moisture content of the leaves, especially from the ribs and/or stems of the leaves of the plants.

In general, the first stage of curing is performed with the plants substantially intact, i.e. with the leaves still attached to the stalk. At the start of the second curing stage, the leaves are defoliated from the stalks, such that the second curing stage is performed on leaves rather than plants. However, in some implementations, the leaves might be defoliated from the stalks prior to (or at the beginning of) the first curing stage, so the first curing stage is also performed on leaves rather than plants. Alternatively, the defoliation of the leaves might be delayed until the end of (or after) the second curing stage, so the first and second curing stages are both performed on plants rather than on leaves. The skilled person may select which approach to adopt according to the circumstances of any given implementation.

The defoliated leaves may be sorted into categories according to position on the stem between the first and second curing stages, based on a classification system (which may be conventional). The second curing stage (and potentially the first curing stage, if relevant ) may be performed on the defoliated leaves separated into categories. This may allow the parameters of the second curing stage, for example the temperature and duration of heating in the oven, to be adjusted or optimized according to the different categories of leaves.

Typically, the defoliated leaves are placed into containers for the second curing stage, with one or more containers then inserted into an oven for performing the second curing stage. The oven may supply both artificial heating and forced ventilation for the second curing stage, and may also include a facility to reduce humidity in support of drying the leaves. The second curing stage terminates when moisture has been removed from the ribs and stems (to a desired level), and generally lasts in the range 2-5 days, although other ranges are also envisaged according to conditions (e.g. 1-6, 1-4, 2-6, 2-4, 3-5 days, and so on).

The second curing stage may be performed in an oven, as noted above, or in a barn or other structure providing a closed (sealed) environment which is controlled in respect of parameters such as temperature, ventilation and/or humidity. For example, the oven, barn or other structure may provide a sealed environment to prevent or restrict the ingress of moisture (such as rain or humidity), unwanted pests (birds, insects, fungi, etc), while helping to maintain the tobacco plants at a high temperature for the heating and drying (curing).

The approach described herein is particularly suited to use with tobacco plants that are conventionally air-cured, such as Burley. The sensory characteristics of tobacco leaves cured according to the approach described herein generally match the sensory characteristics achieved by conventional air curing. Accordingly, the benefits of the present approach, as described herein, can be realised without degradation in product quality.

* * * In order to address various issues and advance the art, this disclosure shows by way of illustration various embodiments in which the claimed invention(s) may be practiced. The advantages and features of the disclosure are of a representative sample of embodiments only, and are not exhaustive and/or exclusive. They are presented only to assist in understanding and to teach the claimed invention(s). It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects of the disclosure are not to be considered limitations on the disclosure as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claims. Various embodiments may suitably comprise, consist of, or consist essentially of, various combinations of the disclosed elements, components, features, parts, steps, means, etc. other than those specifically described herein, and it will thus be appreciated that features of the dependent claims may be combined with features of the independent claims in combinations other than those explicitly set out in the claims. The disclosure may include one or more other inventions not presently claimed, but which may be claimed in the future.

Claims

1. A method for curing plants comprising: performing a first, natural, curing stage in ambient conditions; performing a second, artificial curing stage, using a man-made heater and/or forced ventilation source; and wherein the transition from the first curing stage to the second curing stage is made after the plants attain a substantially final coloration.

2. The method of claim 1, wherein the first curing stage is performed in a barn which is at least partly open to the atmosphere external to the barn.

3. The method of claim 1, wherein the first curing stage is performed in a mobile structure located in a field.

4. The method of claim 3, wherein the mobile structure comprises an open metal framework configured to support the hanging of plants for curing.

5. The method of claim 3 or 4, wherein the mobile structure can be folded or disassembled for ease of transportation.

6. The method of any of claims 1 to 5, wherein the first curing stage lasts in the range 12- 18 days.

7. The method of any of claims 1 to 6, wherein the first curing stage lasts until leaves of the plants are fixed brown.

8. The method of claim 7, wherein the second curing stage beings directly after leaves of the plants are fixed brown.

9. The method of any of claims 1 to 8, wherein at the end of the first curing stage, the ribs and stems of leaves of the plants still retain moisture.

10. The method of any of claims 1 to 9, wherein the first curing stage is performed with leaves still attached to the plants.

11. The method of claim 10, wherein the leaves are defoliated from the plants between the first and second curing stages.

12. The method of claim 11 , wherein the leaves are placed into containers for the second curing stage.

13. The method of claim 11 , wherein the defoliated leaves are sorted into categories according to position on the stem between the first and second curing stages.

14. The method of claim 13, wherein the second curing stage is performed on sets of leaves, each set corresponding to a given category of leaves and being held within a respective container.

15. The method of any of claims 1 to 14, wherein the leaves are shackled or baled after the second curing stage has completed.

16. The method of any of claims 1 to 15, wherein the second curing stage is performed with both artificial heating and forced ventilation.

17. The method of any of claims 1 to 16, wherein the second curing stage includes reducing humidity to facilitate drying.

18. The method of any of claims 1 to 17, wherein the second curing stage terminates when moisture has been removed from the ribs and stems.

19. The method of any of claims 1 to 18, wherein the second curing stage lasts in the range 2-5 days.

20. The method of any of claims 1 to 19, wherein the second curing stage is performed in an oven, barn, or other structure providing a closed and controlled environment.

21. The method of any of claims 1 to 20, wherein the plants are tobacco.

22. The method of claim 21 , wherein the plants are tobacco, such as Burley, which is conventionally air-cured.

23. The method of claim 22, wherein the cured tobacco has sensory characteristics that match conventionally air-cured tobacco.

24. Apparatus configured to perform the method of any of claims 1 to 23.

25. A mobile structure for curing plants in the field using ambient conditions, the mobile structure comprising a framework for supporting multiple plants which hang down from the frame, at least one side of the mobile structure being open for natural ventilation of the plants.

26. The mobile structure of claim 25, wherein the mobile structure comprises an open metal framework configured to support the hanging of plants for curing.

27. The mobile structure of claim 25 or 26, wherein the mobile structure can be folded or disassembled for ease of transportation.