Method for preparing an aerosol-generating substrate

Abstract

A method for preparing an aerosol-generating substrate, the method comprising: providing a mixture comprising a plant material and a solvent; and adding lactic acid and an alkali to the mixture comprising the plant material and the solvent to form the aerosol-generating substrate, wherein the method comprises heating the mixture comprising the plant material and the solvent before adding the alkali.

Description

[0001] METHOD

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to methods for preparing an aerosol-generating substrate, in particular an aerosol-generating substrate for an e-cigarette.

[0004] BACKGROUND

[0005] E-cigarettes are devices that operate by heating a liquid solution to a certain temperature so that it produces an aerosol that can be inhaled. Typically, e-cigarette devices have a heating element that atomizes a liquid solution called e-liquid. The devices are typically activated by taking a puff or pressing a button.

[0006] E-liquid typically contains one or more active ingredients (such as nicotine), flavourings, and a humectant such as propylene glycol and / or glycerol to retain moisture and create an aerosol when heated.

[0007] E-cigarette users often search for a flavoured e-liquid that does not taste artificial. The nicotine in e-liquids is typically provided in the form of concentrated nicotine, such as nicotine shot, which does not contain any flavour. Flavourings are therefore added to the e-liquid to mimic the taste and flavour of tobacco. However, the tobacco flavour is often still not considered satisfactory. Furthermore, e-liquid with added nicotine or some other active ingredients can only be marketed as a pharmaceutical product in certain markets, which is undesirable as it makes it difficult to market the e-liquid as a reduced risk product.

[0008] For these reasons, e-liquids have been proposed that contain plant material such as tobacco leaves or other parts of the tobacco plant that have been ground into powder form. This would provide the e-liquid with a more authentic flavour. However, a common problem of adding the plant material into the e-liquid is that upon repeated use of the e-cigarette, the heating element quickly becomes fouled, causing an unpleasant smell and burnt taste for the user, and reduced vaporisation of the e-liquid over time. Fouling may also lead to the vaporisation of undesired toxic compounds, which negatively impact consumer safety. The fouling effect is thought to be caused by sugars and other non-volatile compounds extracted from the plant material into the e-liquid which solidify on the heating element of the e-cigarette when the e-liquid is heated. Replacing the heating element is not economical as it is more costly than a pack of traditional cigarettes, leading some users to revert back to their old habits.

[0009] Methods have been proposed for producing refined tobacco extracts in which non-volatile components have been removed. The refined tobacco extracts are added to the e-liquid to provide an authentic tobacco flavour with reduced fouling. However, methods for producing refined tobacco extracts usually require expensive and time-consuming steps, such as distillation and / or the application of solvents that have to be removed later. For example, EP3918927A1 discloses a method for producing e-liquid for vaping which requires extracting a tobacco-containing solvent mixture with a distillation.

[0010] Alternatively, tobacco extracts can be diluted to lower the concentration of the non-volatile components and thereby reduce fouling. However, nicotine and other flavourings then need to be added to the e-liquid to meet the consumer’s need for sufficient tobacco taste and nicotine delivery, which is undesirable as discussed above.

[0011] CN111887471 A discloses a method for extracting 400% tobacco paste from waste tobacco powder. However, the method requires a time-consuming distillation step and the application of petroleum ether which then needs to be removed.

[0012] CN116439403A discloses an atomized liquid comprising nicotine in which lactic acid is added to reduce the accumulation of carbon deposits on the heating element. However, the tobacco flavour provided by the atomized liquid may be insufficient. Furthermore, if the content of lactic acid is high, the pH of the atomized liquid may be low making the overall taste of vaping acidic and inhibiting the aroma effect during the atomization process.

[0013] There therefore remains a need for new methods for producing e-liquids for e-cigarettes. SUMMARY OF INVENTION

[0014] According to a first aspect of the invention, there is provided a method for preparing an aerosol-generating substrate, the method comprising: providing a mixture comprising a plant material and a solvent; and adding lactic acid and an alkali to the mixture comprising the plant material and the solvent to form the aerosol-generating substrate, wherein the method comprises heating the mixture comprising the plant material and the solvent before adding the alkali to the mixture.

[0015] It has surprisingly been found that the methods of the invention produce aerosol-generating substrates having an improved taste and flavour with reduced fouling. By providing a mixture comprising a plant material and a solvent and heating the mixture, desired compounds will be extracted from the plant material into the solvent thereby providing an authentic taste and flavour of the plant material. Furthermore, it has been found that adding lactic acid and an alkali to the mixture reduces the fouling effect caused by sugars and other non-volatile compounds that may also be extracted from the plant material, while also allowing the pH to be controlled to improve the safety and flavour of the aerosol-generating substrate. A benefit of the methods of the present invention is that fouling can be reduced without the need for expensive or time-intensive steps such as distillation or application and removal of other solvents.

[0016] According to a second aspect of the invention, there is provided an aerosol-generating substrate obtainable by the method of the first aspect of the invention.

[0017] BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a graph showing the effect of the number of puffs taken from an e-cigarette on the amount of aerosol generated for the e-liquids of Example 1 and Comparative Examples 1 and 2.

[0019] Figure 2A is a picture of the heating element of an e-cigarette after using the e-liquid of Comparative Example 1. Figure 2B is a picture of the heating element of an e-cigarette after using the e-liquid of Comparative Example 2.

[0020] Figure 2C is a picture of the heating element of an e-cigarette after using the e-liquid of Example 1.

[0021] Figure 3 is a graph showing the effect of the number of puffs taken from an e-cigarette on the amount of aerosol generated for the e-liquid of Example 2.

[0022] Figure 4 is a graph showing the effect of the number of puffs taken from an e-cigarette on the amount of aerosol generated for the e-liquid of Comparative Example 3.

[0023] Figure 5 is a graph showing the effect of the number of puffs taken from an e-cigarette on the amount of aerosol generated for the e-liquid of Comparative Example 4.

[0024] DETAILED DESCRIPTION

[0025] The invention provides methods for preparing an aerosol-generating substrate. As used herein, the term “aerosol-generating substrate” refers to a material that can be heated in an e-cigarette to release volatile compounds and produce an aerosol. The aerosol-generating substrate may be provided in any suitable form. For example, the aerosol-generating substrate may be in the form of a paste which can be heated in an e-cigarette to produce an e-liquid in situ. Alternatively, the aerosol-generating substrate may be in the form of a liquid which can then be used directly as an e-liquid in an e-cigarette.

[0026] The method for preparing the aerosol-generating substrate comprises providing a mixture comprising a plant material and a solvent. As would be appreciated by the skilled person, providing a mixture comprising a plant material and a solvent causes desired compounds in the plant material to be extracted into the solvent. When the resulting aerosol-generating substrate is heated in an e-cigarette, volatile compounds extracted from the plant material will be released and inhaled by the user as part of the aerosol. The aerosol-generating substrate therefore provides an authentic taste and flavour of the plant material. For example, when tobacco is used as the plant material, the volatile compounds released from the aerosol-generating substrate will include nicotine and compounds providing the characteristic taste and flavour of tobacco that consumer’s desire.

[0027] It will be understood that the term “mixture” takes its usual definition in the art, and so refers to a solution or dispersion.

[0028] The mixture comprising the plant material and the solvent may be provided in any suitable way. For example, the mixture may be provided by combining a plant material and a solvent.

[0029] As used herein, the term “plant material” refers to a material comprising part of a plant. Any suitable plant material can be used depending on the desired taste and flavour of the aerosol-generating substrate. For example, the plant material may comprise at least one of tobacco, mint, tea, cannabis, coffee, vanilla, cocoa, ginger, rosemary, eucalyptus, clove, star anise, and fruit. The plant material preferably comprises tobacco.

[0030] The plant material may comprise any suitable part of the plant, for example leaves, stems, roots, bark, seeds, flowers, or mixtures thereof. Furthermore, the plant material may be processed into a more suitable form for use in the method for preparing the aerosol-generating substrate. For example, the plant material may be subjected to milling, pulverization, granulation, filtration, and combinations thereof. These processes may provide the plant material in the form of fine particles with a higher surface area thereby enhancing the solvent extraction process.

[0031] When the plant material comprises tobacco, the tobacco may comprise at least one of tobacco leaf, tobacco lamina, tobacco stem, tobacco powder, tobacco dust, reconstituted tobacco, and tobacco waste.

[0032] It will be understood that the solvent used to form the mixture comprising the plant material and the solvent is a solvent or mixture of solvents suitable for extracting compounds from the plant material. Any suitable solvent may be used, but the solvent is preferably chosen such that it can be used as the base solution of an e-liquid. This simplifies the method for preparing the aerosol-generating substrate since the solvent does not need to be removed and / or other solvents added, in order to provide an aerosol-generating substrate suitable for an e-cigarette. The solvent added to form the mixture comprising the plant material and the solvent therefore preferably comprises at least one of propylene glycol, glycerol (such as vegetable glycerol), and water, more preferably at least one of propylene glycol and glycerol, more preferably propylene glycol and glycerol. In some embodiments, the solvent added to form the mixture comprising the plant material and the solvent may consist of one or more of propylene glycol, glycerol, and water. Preferably, the solvent added to form the mixture comprising the plant material and the solvent consists of propylene glycol and glycerol.

[0033] The solvent can comprise any suitable amount of propylene glycol, glycerol, and / or water. For example, the amount of water in the solvent added to form the mixture comprising the plant material and the solvent may be in the range of from 0 to 100% by weight, preferably in the range of from 0 to 10% by weight. The amount of propylene glycol in the solvent added to form the mixture comprising the plant material and the solvent may be in the range of from 0 to 100% by weight, preferably in the range of from 50 to 100% by weight, more preferably in the range of from 70 to 90% by weight. The amount of glycerol in the solvent added to form the mixture comprising the plant material and the solvent may be in the range of from 0 to 100% by weight, preferably in the range of from 0 to 50% by weight, more preferably in the range of from 10 to 30% by weight.

[0034] In a preferred aspect, the solvent added to form the mixture comprising the plant material and the solvent therefore comprises from 50 to 100% by weight of propylene glycol and 0 to 50% by weight of glycerol, more preferably from 70 to 90% by weight of propylene glycol and from 10 to 30% by weight of glycerol.

[0035] The amount of the plant material used relative to the amount of the solvent will depend upon various factors, such as the type of solvent, the temperature at which the extraction is performed, and the type or form of the plant material. The skilled person can therefore select the most appropriate amounts depending on the circumstances. However, the total amount of the plant material added to form the mixture relative to the total amount of the solvent added to form the mixture may be in the range of from 1 :1 to 1 :100 by weight, preferably in the range of from 1 :1 to 1 :25 by weight, more preferably in the range of from 1 :2 to 1 : 15 by weight, more preferably in the range of from 1 :3 to 1 :7 by weight, more preferably in the range of from 1 :4 to 1 :6 by weight.

[0036] The method for preparing the aerosol-generating substrate further comprises adding lactic acid and an alkali to the mixture comprising the plant material and the solvent to form the aerosol-generating substrate. As the skilled person would appreciate, the lactic acid and the alkali do not need to be added at the same time. The lactic acid and the alkali may be added to the mixture simultaneously, or the lactic acid and the alkali may be added sequentially in any order.

[0037] It has been found that adding lactic acid and an alkali to the mixture produces an aerosol-generating substrate with a reduced fouling effect, while also allowing the pH to be controlled. While the alkali may have some effect on fouling, it is thought that the reduced fouling is primarily caused by the presence of lactic acid in the aerosol-generating substrate which reduces solidification of non-volatile compounds (such as sugars) on the heating element that were extracted from plant material. However, the presence of lactic acid will lower the pH of the aerosol-generating substrate, which could negatively impact the taste and safety of the aerosol-generating substrate. By adding alkali to the mixture, it is possible to neutralize the lactic acid and provide an aerosol-generating substrate with a more suitable pH for use in an e-cigarette.

[0038] The method for preparing the aerosol-generating substrate comprises heating the mixture comprising the plant material and the solvent before adding the alkali to the mixture. Heating the mixture promotes extraction of compounds from the plant material into the solvent, thereby speeding up the extraction process and / or providing an aerosol-generating substrate with an improved flavour and taste. Furthermore, heating the mixture before adding the alkali may be advantageous in avoiding side reactions between the alkali and the plant material, as discussed in more detail below. It is therefore preferred that the mixture comprising the plant material and the solvent is heated before any alkali is added to said mixture.

[0039] It may be desirable to mix the mixture before heating in order to first homogenise the mixture. This reduces the risk of plant material settling at the bottom of the mixture which may be burned when the mixture is heated thereby negatively impacting the flavour and taste of the aerosol-generating substrate. The method for preparing the aerosol-generating substrate may therefore further comprise mixing the mixture comprising the plant material and the solvent before heating the mixture. The mixture may be mixed for any suitable length of time in order to homogenise the mixture. For example, the mixture comprising the plant material and the solvent may be mixed for a time period in the range of from 1 minute to 2 hours before the mixture is heated, preferably in the range of from 10 minutes to 1 hour, more preferably in the range of from 20 minutes to 40 minutes. Preferably, the mixture comprising the plant material and the solvent is mixed at room temperature before the mixture is heated. As the skilled person would appreciate, the term “room temperature” refers to the typical ambient temperature of a room, which may be in the range of from 18°C to 25°C.

[0040] Once the mixture is heated, it may be advantageous to maintain the mixture at the desired temperature for a period of time in order to further promote extraction of compounds from the plant material into the solvent. The method for preparing the aerosol-generating substrate may therefore further comprise the step of:

[0041] (i) maintaining the temperature of the mixture comprising the plant material and the solvent within the range of from 40 °C to 200 °C for a time period before adding the alkali to the mixture, preferably within the range of from 60 °C to 180 °C, more preferably within the range of from 100 °C to 160 °C, more preferably within the range of from 120 °C to 140 °C.

[0042] The temperature of the mixture may be maintained within the desired temperature range for any suitable period of time. For example, the time period in step (i) may be in the range of from 30 minutes to 24 hours, preferably in the range of from 45 minutes to 12 hours, more preferably in the range of from 1 hour to 4 hours, more preferably in the range of from 2 hours to 3 hours. The time period can be selected depending on the desired taste of the aerosol-generating substrate. For example, the mixture can be maintained within the desired temperature range for a longer period of time if a stronger flavour of the plant material is desired. Furthermore, the optimal time period may depend on the temperature at which the mixture is maintained. For example, a higher temperature will speed up the process of extracting desired compounds from the plant material and therefore the mixture can be maintained at the temperature for a shorter period of time. Similarly, the mixture may need to be maintained at a lower temperature for a longer period of time in order to achieve sufficient extraction of compounds from the plant material and achieve the desired taste and flavour of the aerosol-generating substrate.

[0043] In a preferred embodiment, step (i) comprises maintaining the temperature of the mixture comprising the plant material and the solvent within the range of from 120 °C to 140 °C for a time period in the range of from 2 hours to 3 hours before adding the alkali to the mixture. This may be advantageous in increasing the amount of the desired compounds (e.g. nicotine) extracted from the plant material, while minimising the generation of undesirable compounds which may be generated at higher temperatures and may negatively impact the taste of the aerosol-generating substrate.

[0044] Step (i) may further comprise mixing the mixture comprising the plant material and the solvent. This may further promote extraction of compounds from the plant material and reduce the risk of plant material settling to the bottom of the mixture and burning.

[0045] While heating the mixture helps to extract compounds from the plant material, it has been found that side reactions between the alkali and the plant material are more prone at high temperatures, which may negatively impact the stability and taste of the aerosol-generating substrate. It is therefore preferred that the mixture is at a suitably low temperature when the alkali is added in order to reduce the risk of side reactions occurring between the alkali and the plant material. The method for preparing the the aerosol-generating substrate may therefore further comprise the step of: (ii) maintaining the mixture comprising the plant material and the solvent at a temperature of no more than 120 °C, preferably at a temperature of no more than 80 °C, more preferably at a temperature of no more than 50 °C, more preferably at room temperature; wherein the alkali is added to the mixture comprising the plant material and the solvent during step (ii).

[0046] The method for preparing the aerosol-generating substrate preferably comprises both of steps (i) and (ii). In which case, step (ii) of course takes place after step (i). This allows the mixture to be heated at a relatively high temperature for a period of time to achieve the desired level of extraction of compounds from the plant material, while also allowing the mixture to be at a suitably low temperature when the alkali is added in order to minimize side reactions.

[0047] As the skilled person would appreciate, it may be necessary to cool the mixture comprising the plant material and the solvent between step (i) and step (ii) so that the temperature of the mixture is in the desired range during step (ii).

[0048] In a preferred aspect of the invention, the method for preparing the aerosol-generating substrate comprises the steps of:

[0049] (i) maintaining the temperature of the mixture comprising the plant material and the solvent within the range of from 100 °C to 160 °C fora time period before adding the alkali to the mixture; and

[0050] (ii) maintaining the mixture comprising the plant material and the solvent at a temperature of no more than 80 °C, wherein the alkali is added to the mixture comprising the plant material and the solvent during step (ii).

[0051] In a more preferred aspect of the invention, the method for preparing the aerosol-generating substrate comprises the steps of: (i) maintaining the temperature of the mixture comprising the plant material and the solvent within the range of from 100 °C to 160 °C fora time period before adding the alkali to the mixture; and

[0052] (ii) maintaining the mixture comprising the plant material and the solvent at a temperature of no more than 50 °C, wherein the alkali is added to the mixture comprising the plant material and the solvent during step (ii).

[0053] In a more preferred aspect of the invention, the method for preparing the aerosol-generating substrate comprises the steps of:

[0054] (i) maintaining the temperature of the mixture comprising the plant material and the solvent within the range of from 120 °C to 140 °C fora time period before adding the alkali to the mixture; and

[0055] (ii) maintaining the mixture comprising the plant material and the solvent at room temperature, wherein the alkali is added to the mixture comprising the plant material and the solvent during step (ii).

[0056] The lactic acid can be added at any suitable time during the method for preparing the aerosol-generating substrate. For example, in some embodiments the lactic acid is added to the mixture comprising the plant material and the solvent before step (i). However, it is preferred that the lactic acid is added to the mixture comprising the plant material and the solvent during step (ii). This ensures that the mixture is at a suitably low temperature when the lactic acid is added to reduce the risk of side reactions occurring between the lactic acid and the plant material.

[0057] Step (ii) may further comprise mixing the mixture comprising the plant material and the solvent after the alkali (and optionally the lactic acid) has been added to the mixture. This may promote dissolution of the alkali and the lactic acid in the mixture. The mixture comprising the plant material and the solvent may be mixed for a time period in the range of from 10 minutes to 12 hours after the alkali (and optionally the lactic acid) has been added, preferably in the range of from 20 minutes to 6 hours, more preferably in the range of from 30 minutes to 2 hours. After step (ii), it is preferred that the mixture comprising the plant material and the solvent is still maintained at a temperature of no more than 120 °C for the remainder of the process for preparing the aerosol-generating substrate, preferably at a temperature of no more than 80 °C, more preferably at a temperature of no more than 50 °C, more preferably at room temperature. This ensures that the alkali and the lactic acid in the mixture are not subsequently exposed to high temperatures that may increase the risk of side reactions as discussed above.

[0058] It may be advantageous to premix the alkali and / or the lactic acid in a solvent before these compounds are added to the mixture comprising the plant material and the solvent, in order to further promote dissolution of the alkali and the lactic acid. The method for preparing the aerosol-generating substrate may therefore comprise providing an additive mixture comprising the lactic acid and / or the alkali and adding the additive mixture to the mixture comprising the plant material and the solvent. The additive mixture can comprise any suitable solvent. However, the solvent in the additive mixture is preferably the same as the solvent added to form the mixture comprising the plant material and the solvent.

[0059] The method may further comprise mixing the additive mixture before adding the additive mixture to the mixture comprising the plant material and the solvent. This may further promote dissolution of the alkali and the lactic acid. The additive mixture may be mixed for a time period in the range of from 30 minutes to 24 hours, preferably in the range of from 3 hours to 20 hours, more preferably in the range of from 8 hours to 16 hours.

[0060] The additive mixture is preferably maintained at room temperature before the additive mixture is added to the mixture comprising the plant material and the solvent. However, the additive mixture may alternatively be heated before adding the additive mixture to the mixture comprising the plant material and the solvent, which may further promote dissolution of the alkali and the lactic acid.

[0061] It is preferred that the additive mixture comprises both the lactic acid and the alkali.

[0062] It is also preferred that the additive mixture is added to the mixture comprising the plant material and the solvent during step (ii). As discussed above, this ensures that the mixture is at a suitably low temperature when the lactic acid and the alkali are added in order to suppress side reactions.

[0063] It will be understood that the term “alkali” takes its usual definition in the art, and so refers to one or more basic compounds. Any suitable basic compounds may be used. For example, the alkali may comprise at least one compound selected from metal hydroxides and metal carbonates. Preferably, the alkali comprises at least one of sodium carbonate, potassium carbonate, lithium carbonate, calcium carbonate, magnesium carbonate, sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, and magnesium hydroxide. More preferably, the alkali comprises at least one of sodium carbonate and potassium carbonate, even more preferably the alkali comprises sodium carbonate.

[0064] As discussed above, the primary function of the alkali is to neutralize the lactic acid and provide the aerosol-generating substrate with a more suitable pH for use in an e-cigarette. The total amount of alkali added to the mixture will therefore depend on the total amount of lactic acid and any other acids added to the mixture, as well as the desired pH of the aerosol-generating substrate. However, the total amount of alkali added to the mixture may be in the range of from 0.01 to 5.0% by weight relative the total amount of components added to the mixture, preferably in the range of from 0.1 to 2.5% by weight, more preferably in the range of 0.2 to 1 .0% by weight, more preferably in the range of from 0.4 to 0.8% by weight, more preferably in the range of from 0.5 to 0.7% by weight.

[0065] From the perspective of safety and taste of the aerosol-generating substrate, it is preferred that the final pH of the aerosol-generating substrate is approximately neutral, but the invention is not so limited and the skilled person can select any desired pH for the aerosol-generating substrate. However, the final pH of the aerosol-generating substrate may be in the range of from 3.5 to 9.5, preferably in the range of from 5 to 9, more preferably in the range of from 6 to 8.

[0066] The lactic acid may be added to the mixture in any suitable amount in order to reduce the fouling effect of the aerosol-generating substrate. For example, the total amount of lactic acid added to the mixture may be in the range of from 0.01 to 5.0% by weight relative to the total amount of components added to the mixture. However, in order to minimise the fouling effect, the total amount of lactic acid added to the mixture is preferably in the range of 0.1 to 2.0% by weight relative to the total amount of components added to the mixture, more preferably in the range of from 0.5 to 1.5% by weight, more preferably in the range of from 0.6 to 1.1 % by weight, more preferably in the range of from 0.7% to 0.9% by weight.

[0067] One or more other acids in addition to lactic acid may be added during the method for preparing the aerosol-generating substrate. For example, the method may further comprise adding benzoic acid to the mixture comprising the plant material and the solvent. It has surprisingly been found that benzoic acid may improve the taste of the aerosol-generating substrate when used in an e-cigarette. The presence of benzoic acid may also further reduce the fouling effect of the aerosol-generating substrate. The total amount of benzoic acid added to the mixture may be in the range of from 0.1 to 2.0% by weight relative to the total amount of components added to the mixture, preferably in the range of from 0.5 to 1.5% by weight, more preferably in the range of from 0.6 to 1.1 % by weight, more preferably in the range of from 0.7 to 0.9% by weight.

[0068] The benzoic acid may be added at any suitable time during the method for preparing the aerosol-generating substrate. However, it is preferred that the benzoic acid is added to the mixture comprising the plant material and the solvent during step (ii). This will reduce the risk of side reactions occurring between the benzoic acid and the plant material as discussed above. The benzoic acid may be comprised in the additive mixture described above, to assist with dissolving the benzoic acid before it is added to the mixture comprising the plant material and the solvent.

[0069] A benefit of the methods of the invention is that the aerosol-generating substrate is suitable for use in an e-cigarette without further processing because the lactic acid reduces fouling caused by non-volatile compounds that have been extracted from the plant material. As discussed above, the method may produce the aerosol-generating substrate as a paste which can be used directly in an e-cigarette. The method may therefore further comprise storing the aerosol-generating substrate as a paste.

[0070] However, it may be desirable to filter the aerosol-generating substrate to remove solids that may otherwise block the heating element in an e-cigarette. The method for preparing the aerosol-generating substrate may therefore comprise filtering the aerosol-generating substrate to separate liquid and solid fractions. The liquid fraction (i.e. the liquid filtrate) can then be used as an e-liquid in an e-cigarette. The aerosol-generating substrate may be filtered using vacuum filtration, but any suitable separation method for viscous substances could be used.

[0071] It is preferred that the method for preparing the aerosol-generating substrate does not comprise distilling the mixture. Similarly, it is preferred that the method does not comprise adding any other solvents to the mixture in addition to the solvent added to form the mixture comprising the plant material and the solvent (and optionally the solvent in the additive mixture). As discussed above, distillation and application of other solvents is not necessary since the lactic acid sufficiently reduces the fouling effect, and omitting these steps provides a simpler and quicker process.

[0072] The invention also provides an aerosol-generating substrate obtainable by the method of the invention.

[0073] As discussed above, the aerosol-generating substrate can be heated in an e-cigarette to release volatile compounds and produce an aerosol that can be inhaled by the user. In particular, volatile compounds extracted from the plant material will be released and inhaled as part of the aerosol, thereby providing an authentic taste and flavour of the plant material.

[0074] As discussed above, the presence of lactic acid and alkali in the aerosol-generating substrate reduces the fouling effect caused by sugars and non-volatile compounds that may also have been extracted from the plant material, while also allowing the pH to be controlled to improve the safety and flavour of the aerosol-generating substrate. The aerosol-generating substrate preferably comprises the solvent added during the method of the invention. As discussed above, the solvent is preferably chosen such that it can be used as the base solution of an e-liquid. For example, the solvent preferably comprises at least one of propylene glycol, glycerol (such as vegetable glycerol), and water.

[0075] The aerosol-generating substrate may also comprise the plant material added during the method of the invention. Alternatively, the aerosol-generating substrate may be filtered as discussed above and the plant material may be removed as part of the solid fraction.

[0076] EXAMPLES

[0077] COMPARATIVE EXAMPLE 1

[0078] A solvent was prepared comprising 80% by weight of propylene glycol and 20% by weight of vegetable glycerol. Tobacco material (5g) and the solvent (30g) were combined in a ratio of 1 :6 (w / w) to form a mixture.

[0079] The mixture of the tobacco material and the solvent was continuously stirred at room temperature for 30 minutes. The mixture was then heated up to 120 °C while continuing stirring. Once the temperature plateau was reached, the mixture was left stirring for 1 hour. The mixture was then allowed to cool down to room temperature to form a paste having a pH of 5.1. The paste was filtered using vacuum filtration to obtain a liquid filtrate.

[0080] The liquid filtrate was used as the e-liquid in an e-cigarette to investigate the degree of fouling.

[0081] As can be seen in Figure 1 , the amount of aerosol generated by the e-cigarette using the e-liquid of Comparative Example 1 decreased as the number of puffs taken increased. This is indicative of fouling where non-volatile compounds block the heating element leading to reduced evaporation of the e-liquid overtime. This is supported by Figure 2A which is a picture of the heating element of the e-cigarette after use with the e-liquid of Comparative Example 1 , where significant residue has built up on the heating element. Fouling is therefore a significant problem with the e-liquid of Comparative Example 1 .

[0082] COMPARATIVE EXAMPLE 2

[0083] A solvent was prepared comprising 80% by weight of propylene glycol and 20% by weight of vegetable glycerol. Tobacco material (5g) and the solvent (30g) were combined in a ratio of 1 :6 (w / w) to form a mixture.

[0084] The mixture of tobacco material and solvent was continuously stirred at room temperature for 30 minutes. The mixture was then heated up to 120 °C while continuing stirring. Sodium carbonate was added to the mixture in an amount of 2.1 % by weight and the mixture was left for 1 hour at a temperature between 70 and 120 °C. The mixture was then allowed to cool down to room temperature to form a paste having a pH in the range of from 9 to 10. The paste was filtered using vacuum filtration to obtain a liquid filtrate.

[0085] The liquid filtrate was tested as an e-liquid in an e-cigarette in the same manner as Comparative Example 1 and the results are depicted in Figure 1. The aerosol generation profile of the e-liquid of Comparative Example 2 was slightly improved compared to Comparative Example 1 , but the amount of aerosol generated by the e-cigarette still decreased as the number of puffs taken increased. Furthermore, Figure 2B still shows significant build up of residue on the heating element after use of the e-cigarette.

[0086] Fouling is therefore still a significant problem with the e-liquid of Comparative Example 2 where only an alkali was added to the mixture during preparation of the e-liquid.

[0087] EXAMPLE 1

[0088] A solvent was prepared comprising 80% by weight of propylene glycol and 20% by weight of vegetable glycerol. Tobacco material (5g) and the solvent (30g) were combined in a ratio of 1 :6 (w / w) to form a mixture. The mixture of tobacco material and solvent was continuously stirred at room temperature for 30 minutes. The mixture was then heated up to 120 °C while continuing stirring. Sodium carbonate was added to the mixture in an amount of 2.1 % by weight and the mixture was left for 1 hour at a temperature between 70 and 120 °C. The mixture was allowed to cool down to room temperature and lactic acid was then added to the mixture in an amount of 0.9% by weight while continuing stirring. The resulting paste had a pH in the range of from 6.5 to 7.5. The paste was filtered using vacuum filtration to obtain a liquid filtrate.

[0089] The liquid filtrate was tested as an e-liquid in an e-cigarette in the same manner as Comparative Example 1 and the results are depicted in Figure 1 . The amount of aerosol generated by the e-cigarette using the e-liquid of Example 1 was relatively constant across 100 puffs from the device. Furthermore, significantly less residue was visible on the heating element of the e-cigarette after use compared to Comparative Examples 1 and 2, as can be seen in Figure 2C.

[0090] Fouling is therefore significantly reduced by the method of Example 1 where an alkali and lactic acid were added to the mixture during preparation of the e-liquid.

[0091] EXAMPLE 2

[0092] A solvent was prepared comprising 80% by weight of propylene glycol and 20% by weight of vegetable glycerol. Tobacco material (5g) and the solvent (30g) were combined in a ratio of 1 :6 (w / w) to form a mixture.

[0093] The mixture of tobacco material and solvent was continuously stirred at room temperature for 30 minutes. The mixture was then heated up to 120 °C and stirred for a further 3 hours before the mixture was cooled down to 50 °C. 350mg of lactic acid (90% purity, 0.9% w / w) and 210mg of sodium carbonate (0.6% w / w) were added to the mixture and the mixture was stirred for a further 30 minutes. The resulting paste was then filtered to obtain a liquid filtrate.

[0094] The liquid filtrate was tested as an e-liquid in an e-cigarette in the same manner as Example 1 and the results are depicted in Figure 3, which shows that the amount of aerosol generated by the e-cigarette was relatively constant across 100 puffs. Fouling was therefore significantly reduced by the method of Example 2 where an alkali and lactic acid were added to the mixture during preparation of the e-liquid. Furthermore, the alkali and the lactic acid were added to the mixture at a low temperature (i.e. 50 °C) which suppresses side reactions between these compounds and the tobacco material. This improves the stability of the e-liquid.

[0095] EXAMPLE 3

[0096] A solvent was prepared comprising 80% by weight of propylene glycol and 20% by weight of vegetable glycerol. Tobacco material (5g) and the solvent (20g) were combined in a ratio of 1 :4 (w / w) to form a mixture.

[0097] The mixture of tobacco material and solvent was continuously stirred at room temperature for 30 minutes. The mixture was then heated up to 120 °C and stirred for a further 3 hours before the mixture was cooled down to room temperature. 350 mg of lactic acid (90% purity, 0.9% w / w) and 210 mg of sodium carbonate (0.6% w / w) were combined with 10 g of the same solvent described above to form a separate mixture. The mixture comprising the lactic acid and the alkali was left stirring overnight at room temperature before it was added to the mixture comprising the plant material and the solvent. The combined mixture was stirred for a further two hours at room temperature. The resulting paste was then filtered to obtain a liquid filtrate.

[0098] The lactic acid and the alkali were therefore kept at room temperature throughout the entire process. This was found to reduce the risk of side reactions occurring between the lactic acid and the alkali, which would otherwise negatively impact the stability of the e-liquid. Furthermore, premixing the lactic acid and the alkali in a separate mixture was found to more completely dissolve these compounds. Importantly, the fouling effect was still prevented.

[0099] COMPARATIVE EXAMPLES 3 AND 4 To demonstrate the effect of changing the acid, e-liquids were prepared in the same manner as Example 2 except the lactic acid was replaced with benzoic acid (Comparative Example 3) or tartaric acid (Comparative Example 4). The e-liquids were tested in the same manner as Example 2. The results for Comparative Example 3 are shown in Figure 4 and the results for Comparative Example 4 are shown in Figure 5.

[0100] As can be seen from Figure 4, the amount of aerosol-generated varied less with the number of puffs taken compared to Comparative Examples 1 and 2 and so the use of benzoic acid and alkali slightly reduced fouling. However, the amount of aerosol generated still decreased as the number of puffs increased and therefore fouling was still a problem. As can be seen from Figure 5, the amount of aerosol generated reduced significantly as the number of puffs increased and therefore the use of tartaric acid and alkali significantly increased fouling.

Claims

CLAIMS1. A method for preparing an aerosol-generating substrate, the method comprising: providing a mixture comprising a plant material and a solvent; and adding lactic acid and an alkali to the mixture comprising the plant material and the solvent to form the aerosol-generating substrate, wherein the method comprises heating the mixture comprising the plant material and the solvent before adding the alkali to the mixture.

2. The method of claim 1 , further comprising mixing the mixture comprising the plant material and the solvent before heating the mixture, preferably wherein the mixture comprising the plant material and the solvent is mixed for a time period in the range of from 1 minute to 2 hours before the mixture is heated, more preferably in the range of from 10 minutes to 1 hour, more preferably in the range of from 20 minutes to 40 minutes.

3. The method of claim 1 or 2, further comprising the step of:(i) maintaining the temperature of the mixture comprising the plant material and the solvent within the range of from 40 °C to 200 °C for a time period before adding the alkali to the mixture, preferably within the range of from 60 °C to 180 °C, more preferably within the range of from 100 °C to 160 °C, more preferably within the range of from 120 °C to 140 °C; optionally wherein the time period in step (i) is in the range of from 30 minutes to 24 hours, preferably in the range of from 45 minutes to 12 hours, more preferably in the range of from 1 hour to 4 hours, more preferably in the range of from 2 hours to 3 hours.

4. The method of any of the preceding claims, further comprising the step of:(ii) maintaining the mixture comprising the plant material and the solvent at a temperature of no more than 120 °C, preferably at a temperature of nomore than 80 °C, more preferably at a temperature of no more than 50 °C, more preferably at room temperature; wherein the alkali is added to the mixture comprising the plant material and the solvent during step (ii).

5. The method of claim 4, wherein the lactic acid is added to the mixture comprising the plant material and the solvent during step (ii), optionally wherein step (ii) further comprises mixing the mixture comprising the plant material and the solvent after the alkali and the lactic acid have been added to the mixture, preferably wherein the mixture comprising the plant material and the solvent is mixed for a time period in the range of from 10 minutes to 12 hours after the alkali and the lactic acid have been added to the mixture, more preferably in the range of from 20 minutes to 6 hours, more preferably in the range of from 30 minutes to 2 hours.

6. The method of claim 5, comprising providing an additive mixture comprising the lactic acid and the alkali and adding the additive mixture to the mixture comprising the plant material and the solvent, optionally wherein the method further comprises mixing the additive mixture before adding the additive mixture to the mixture comprising the plant material and the solvent, preferably wherein the additive mixture is mixed for a time period in the range of from 30 minutes to 24 hours, more preferably in the range of from 3 hours to 20 hours, more preferably in the range of from 8 hours to 16 hours.

7. The method of claim 6, wherein the additive mixture is maintained at room temperature before the additive mixture is added to the mixture comprising the plant material and the solvent.

8. The method of any of the preceding claims, wherein the alkali comprises at least one compound selected from metal hydroxides and metal carbonates, preferably wherein the alkali comprises at least one of sodium carbonate, potassium carbonate, lithium carbonate, calcium carbonate, magnesium carbonate, sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, and magnesium hydroxide, more preferably wherein the alkalicomprises at least one of sodium carbonate and potassium carbonate, more preferably wherein the alkali comprises sodium carbonate.

9. The method of any of the preceding claims, further comprising adding benzoic acid to the mixture comprising the plant material and the solvent.

10. The method of any of the preceding claims, wherein the final pH of the aerosol-generating substrate is in the range of from 3.5 to 9.5, preferably in the range of from 5 to 9, more preferably in the range of from 6 to 8.11 . The method of any of the preceding claims, wherein the solvent added to form the mixture comprising the plant material and the solvent comprises at least one of propylene glycol, glycerol, and water, preferably at least one of propylene glycol and glycerol, more preferably propylene glycol and glycerol.

12. The method of any of the preceding claims, wherein the plant material comprises at least one of tobacco, mint, tea, cannabis, coffee, vanilla, cocoa, ginger, rosemary, eucalyptus, clove, star anise, and fruit, preferably wherein the plant material comprises tobacco.

13. The method of any of the preceding claims, further comprising filtering the aerosol-generating substrate to separate liquid and solid fractions.

14. The method of any of claims 1 to 12, further comprising storing the aerosol generating-substrate as a paste.

15. An aerosol-generating substrate obtainable by the method of any of the preceding claims.

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