Recovery of maltol through aqueous extraction

Disclosed is a process for recovering maltol from water-insoluble source material containing maltol, comprising extracting the maltol with a hot aqueous solution containing a solute which increases the immiscibility between said source material and said solution.

FIELD OF THE INVENTION 
This invention is related to a process for obtaining maltol. 
Maltol (2-methyl-3-hydroxy-4-pyrone) is a heterocyclic aroma chemical used 
extensively in flavor and fragrance compositions. It is naturally 
occurring in numerous plant species, especially in coniferous trees such 
as Larix and Abias spp. 
The presence of maltol in various plant sources has been known for many 
years and considerable efforts have been made to develop a sensible method 
for its commercial recovery. The existing techniques are, however, rather 
complex and the use of the resulting maltol is cost-prohibitive. 
The solubility of maltol in ethylene glycol at ambient temperatures exceeds 
4%. This completely prohibits economical maltol recovery from dilute 
mixtures, effectively eliminating virtually all natural sources. Moreover, 
there is a limiting practical consideration being that the crystallization 
of maltol from ethylene glycol at ambient temperatures is very slow. At 
very low temperatures the viscosity of ethylene glycol also considerably 
hampers filtration of maltol from ethylene glycol/maltol mixtures. 
Ethylene glycol derived maltol is also unsuitable for food application, 
since the removal of toxic ethylene glycol contamination from maltol is 
rather difficult. 
Maltol can be obtained in very small amounts from the destructive 
distillation products of wood, and by a partially synthetic process from 
kojic acid, which is obtained from fermentation media. However, maltol, 
obtained therefrom, is still quite expensive. 
Maltol has been reported to be in the bark of some species of larch trees. 
Maltol is present in larch bark in combined form to an extent varying from 
about 0.1 percent to about 2 percent by weight depending upon the bark 
layer and the season of harvest. The richest supply of maltol is found in 
the bark of roots of the larch trees although, for practical reasons, not 
much root bark is harvested. Large quantities of larch trees and bark 
containing maltol exist and are available primarily in the northwest part 
of the United States and southwest Canada. The bark is available at 
sawmills where it is stripped off of larch trees and stored in a pile, 
there to be burned for fuel or otherwise used if economical processes for 
recovering useful components therefrom can be found. 
It is also known that maltol is present in various parts of coniferous 
species and found in rather large concentrations in the oleoresin 
extracted from fresh foliage of balsam fir (Abias balsamea L.). 
U.S. Pat. No. 5,221,756 discloses that sufficiently pure maltol can be 
effectively recovered from the said resin through the co-distillation with 
a suitable hydrocarbon and, in particular, alpha-pinene. The process of 
co-distillation, although effective, requires a rather complex 
technological set-up, application of vacuum, high pressure steam and 
necessary handling of flammable liquids. During co-distillation maltol 
crystallizes directly from the gaseous phase in a microcrystalline form. 
Thus maltol obtained from the co-distillation process retains substantial 
quantities (30 to 40%) of the hydrocarbon which complicates further 
purification. 
On the other hand, fir balsam resin is a valuable product for the perfume 
industry for its fine and delicate organoleptic qualities. During 
co-distillation this material can suffer from long exposure to high 
temperatures which alters the organoleptic profile of the resin and 
consequently reduces its value as an aroma ingredient. 
Alternative methods for the recovery of maltol especially adapted to its 
separation from oleoresin of coniferous species, suffer from various 
practical shortcomings. 
Oleoresin with high maltol content can only be recovered from fresh fir 
foliage through extraction by a relatively polar water-immiscible solvent. 
This is due to the fact that maltol is insoluble in hydrocarbons, which 
eliminates all this class from the list of potential extractants. On the 
other hand, application of water miscible solvents such as alcohol and 
acetone is also excluded since such solvents will dissolve all the water 
contained in the fresh plant material. This results in an extracted 
product in which the water content can reach 50-60%, leading to undesired 
difficulties in recovering the maltol from the blend of solvent and water. 
Processes in which the extractant is limited to water-immiscible solvents 
for the oleoresin result in obtaining water insoluble oleoresin that 
contains maltol. 
It is known that maltol is substantially soluble in hot water (about 20% at 
boiling) while its solubility at low temperatures is reduced (about 0.7% 
at 0.degree. C). However, it is possible to extract maltol from fir 
oleoresin by pure hot water and subsequently to recover maltol from the 
aqueous solution. Such an extraction needs to be conducted at elevated 
temperature since at room conditions the resin is extremely viscous. 
Unfortunately, the specific gravity of the resin is very close to that of 
water. Intensive mixing of hot water and resin results, therefore, in 
formation of a stable emulsion which makes the separation of the liquid 
phases rather difficult. 
Thus, there remains a need for an efficient, practical process for 
recovering maltol from source material, particularly where the source 
material is oleoresin. 
BRIEF SUMMARY OF THE INVENTION 
According to the present invention, maltol is recovered from source 
material, particularly water insoluble source material, and especially 
from oleoresin of coniferous species. In particular, maltol is recovered 
from fir balsam resin. The source material is extracted with a hot aqueous 
salt solution, and specifically, a hot, aqueous solution of sodium 
chloride. This is followed by the recovery of maltol from the aqueous 
solution and, in particular, by direct crystallization of maltol from said 
aqueous phase.

DETAILED DESCRIPTION OF THE INVENTION 
Source material that can be treated in the process of the present invention 
includes virtually any maltol-containing matter that contains 
water-insoluble components. Preferred source material includes 
oleoresinous fractions that can be obtained by prior treatment of plant 
material, such as plant matter from balsam fir, larch, or other coniferous 
plant matter. An example of a particularly preferred source material is 
oleoresin, such as balsam fir resin. 
The source material can contain water, but the water content is preferably 
low, e.g., up to about a few weight percent. 
The source material is extracted with an aqueous solution. The solution 
contains dissolved therein one (or more) solutes which acts to increase 
the specific gravity of the water, and has (or have) the property that it 
(or they) increase the immiscibility between the water-soluble material 
and the aqueous phase. Immiscibility can be increased in the sense of 
reducing the solubility of the source material other than maltol in the 
aqueous phase, by reducing the solubility of the aqueous phase in the 
source material, and/or by reducing the tendency of the two phases to form 
a third phase or stable emulsion. Immiscibility can also be increased in 
the sense of decreasing the settling time, that is, the time required for 
a mixture of the source material and the aqueous phase to form two 
distinct, cleanly defined layers. 
The solute material also needs to have the property that at low 
temperature, i.e., at a temperature in the range of 0.degree. C. to 
20.degree. C., it is more soluble in water than the maltol is. Preferably, 
the solute material is dissolvable from the recovered maltol; and it is 
preferred that the solute material be compatible with uses for the maltol 
that require contact with the human body, including ingestion by humans. 
Suitable solute materials include nonionic and ionic compounds. Exemplary 
of nonionic materials are sugars and sugar alcohols such as dextrose, 
glucose, sucrose, fructose, mannitol and sorbitol. More complex 
carbohydrates and polysaccharides are included as well. Other nonionic 
materials include lower alkanols containing up to 6 carbon atoms, glycols 
containing up to 6 carbon atoms, such as propylene glycol, diethylene 
glycol, and water-soluble longer chain poly(alkoxy) compounds wherein each 
alkoxy unit is ethoxy or propoxy. 
Preferably, the solute materials should not act as surfactants which would 
lessen the immiscibility of the source material and the aqueous phase. 
Other suitable solute materials are ionic compounds, that is, salts. 
The salt is preferably sodium chloride, for reasons of effectiveness as 
well as economy. Other water-soluble inorganic and organic salts can also 
be employed, notably any water-soluble halide (e.g., fluoride, chloride, 
bromide, iodide) salts of alkali metals and water-soluble halide salts of 
calcium, magnesium or other alkaline earth metals. Other useful anionic 
groups include carbonates, bicarbonates, nitrates, sulfites, sulfates, 
phosphates, and organic anions preferably including alkanoates containing 
up to 6 carbon atoms, such as acetate and propionate, and polybasic 
organic anions such as citrate. Terms such as "sulfate" and "phosphate" 
are intended to include any analogs having a valence from -1 to the 
maximum; thus analogs such as HSO.sub.4.sup.- and H.sub.2 PO.sub.4.sup.- 
are intended to be included. Other useful cationic groups include ammonium 
and metals such as aluminum, manganese, iron, cobalt, nickel, copper and 
zinc. Mixtures of two or more salts can also be used. 
The concentration of the solute material in the solution will depend 
somewhat on the identity of the source material and the identity of the 
solute material being used. Generally, solutions containing about 1 wt.% 
to about 20 wt.% are useful. Solute concentrations of 1 wt.% to 10 wt.% 
are preferred, and especially concentrations of about 4 wt.% to about 7.5 
wt.%. 
The amount of solute present should be sufficient to increase the specific 
gravity of the aqueous phase enough to improve the ease of separating the 
phases. Too low a solute content is undesirable because subsequent 
separation of the aqueous and resinous phases is difficult. Too high a 
solute content is undesirable because it would lessen the solubility of 
the maltol in the aqueous phase. 
When sodium chloride is being used, amounts of about 5 to about 10 wt.% are 
useful, generally about 5 to about 7.5 wt.%. 
The solution with the solute material is contacted together with the source 
material in a vessel, such as an agitated tank, in a manner to provide 
close contact between the solution and the source material. Contact is 
preferably increased by thoroughly mixing the materials together. The 
ratio of solution to source material is generally about 100:1 to about 
1:100. It is a straightforward matter to determine the optimum ratios for 
any given source material and conditions. The ratio will of course depend 
on the solute content of the solution. 
The solution as it contacts the source material is preferably at elevated 
temperature. While temperatures above about 20.degree. C. are useful, 
temperatures of at least 50.degree. C. or even at least 80.degree. C. to 
100.degree. C. are more useful as greater amounts of maltol can be 
extracted. 
The solution and the source material are preferably maintained in close 
contact for a length of time (up to several hours) sufficient to reach 
equilibrium of maltol between the source material and the solution. 
Following the contact, the solution and the source material are allowed to 
settle, the aqueous solution is separated, and preferably filtered. The 
solution can then be treated to recover the maltol therefrom. For 
instance, the solution can be cooled (preferably to near 0.degree. C.) so 
as to crystallize solid, crude maltol. The maltol can be recovered, 
washed, or otherwise further purified as necessary and used as a flavoring 
agent and/or an aroma agent. This aspect of the process takes advantage of 
the higher low-temperature solubility of the solute material. 
The process of the present invention has several advantages. In particular, 
it affords improved yields of maltol compared to extractive processes not 
using an aqueous solution as described herein. The solution is also 
believed to have improved selectivity in that it solubilizes the maltol 
yet solubilizes less of other coproducts present in the source material. 
This feature eases the subsequent purification of the maltol following 
recovery thereof from the solution. Also, this process does not introduce 
reagents that could contaminate the maltol or cause it to be unacceptable 
for use in food or personal care products. It also does not create an 
environmental hazard. Such solute that remains in the product is easily 
washed cut of it. The process also does not expose the maltol to 
conditions which could adversely affect the maltol itself, as by thermal 
or chemical decomposition or otherwise. If fir balsam is used as source 
material, it retains its organoleptic properties. 
The invention is further described in the following examples: 
EXAMPLE 1 
973.6 grams of resin which contained 8.92% maltol was loaded into a 
cylindrical jacketed steam heated glass separatory vessel. The resin was 
sequentially extracted using a mechanical stirrer with two portions of 7% 
sodium chloride solution in water. The volume of the aqueous phase was two 
liters each time. The temperature of the resulting extraction mixture was 
maintained at 90.degree. C. and the mixing time was three hours for each 
extraction stage. 
After each stage, the phases were allowed to separate. The hot aqueous 
portions were drained, filtered and placed into a refrigerator to 
crystallize out maltol. Precipitated crystals of maltol were filtered out 
at 4.degree. C. 48.4 grams of crude maltol were recovered which still 
contained about 15% of moisture. The residual concentration of the maltol 
in the resin after the two aqueous extraction stages was found to be 1.86% 
which corresponds to recovery of 80.6%. The residual maltol content in the 
mother-liquor was found to be 0.7%. 
The recovered maltol was a reddish macro crystalline mass. This material 
was re-crystallized twice from 90% aqueous methanol in the presence of 
1000 ppm of EDTA (ethylene diamine tetraacetic acid) resulting in snow 
white flavor grade maltol. 
Simple mechanical agitation of resin in an aqueous salt solution, such as 
carried out in accordance with Example 1 above, can require a substantial 
time for equilibrium to be reached. In order to speed up the establishment 
of the equilibrium and increase the efficiency of maltol recovery by 
aqueous extraction, a special mixing/settling device was designed and 
constructed. It is schematically presented in the Figure and described as 
follows. 
Both resin and aqueous phase are loaded into blending/settling tank (1) 
where the mixture is warmed up under stirring to about 100.degree. C. 
Reflux condenser (6) prevents water and essential oil (naturally found as 
a component of the resin) from escaping the system. The hot blend of resin 
and aqueous phase is taken from the bottom of the tank by the pump (2) and 
transferred to a heat exchanger/static mixer (3), and then enters 
contactor (4) packed with sturdy high surface, low resistance 
Teflon-coated packing material. This material is well wetted by the resin 
which forms a thin layer moving on the surface of the packing material 
surrounded and tightly contacted by the aqueous phase. The emerging liquid 
is returned to the tank through the open valve (7). When equilibrium is 
established the valve (7) is closed and the mixture is pushed out from the 
contactor back into tank (1) by opening air pressure valve (11) and (12). 
All wetted parts were Teflon coated. 
At this point valve (9) is closed. The tank mixer is stopped and the liquid 
phases are allowed to separate in the tank (1). The valve (9) is then 
opened, and the separated aqueous phase is pushed out through valve (9) by 
air pressure through valve (10); it passes through the contactor, which in 
this case acts as a filter and retains on its surface the micro-inclusions 
of the resin. Through open valve (8) the aqueous phase which contains 
maltol is dropped into chilled vessel (5), where maltol crystallizes out 
under slow mixing. Crude maltol is obtained from the resulting slurry by 
filtration. 
EXAMPLE 2 
185 kg of fir balsam resin which contained 5.0% maltol was loaded into tank 
(1) of the above-described system. 370 kg of 7% sodium chloride solution 
was added. This solution was used in a previous extraction and was already 
saturated with maltol at 3.degree. C. This corresponds to 0.6% maltol in 
the aqueous phase. The mixture was warmed up to 95.degree. C. At that 
point pump (2) was turned on and the mixture recirculated through the 
contactor for one hour. The system was stopped for a short time to take a 
sample of the aqueous phase. After that the recirculation continued for an 
additional two hours, whereupon it was stopped and a sample of the aqueous 
phase was again taken. The content of maltol in the aqueous phase after 
one hour of extraction was found to be identical to that taken after three 
hours. This means that the equilibrium was established in one hour or 
less. 
The mixture was pushed out of the contactor into tank (1) and was allowed 
to settle for three and one-half hours. The aqueous phase thus formed was 
then transferred into chilled vessel (5) and allowed to cool down under 
stirring until the temperature reached 3.degree. C. 
The extraction was repeated under identical conditions. However, for the 
second time fresh 7% sodium chloride solution was used. The residual 
concentration of the maltol in the resin after two extractions was found 
to be 1.18% which corresponds to 77.3% recovery. 9.4 kg of crude wet 
maltol were recovered. The water content was found to be 24%. 
The maltol content in the spent fir resin can be further reduced through 
additional extraction. Substantial quantities of maltol remain in the 
mother-liquor. This can be reduced either by decreasing the 
crystallization temperature, changing the concentration of the salt 
solution used for the extraction, vaporizing mother-liquor, or any other 
conventional method.