MALTOL
What is maltol?
SYNTHETIC INGREDIENT FOR PERFUMERY OVERVIEW
Maltol is a naturally occurring organic compound that is used primarily as a flavor enhancer. It is found in the bark of larch tree, in pine needles, and in roasted malt (from which it gets its name). It is a white crystalline powder that is soluble in hot water, chloroform, and other polar solvents. Because it has the odor of cotton candy and caramel, maltol is used to impart a sweet aroma to fragrances. Maltol’s sweetness adds to the odor of freshly baked bread and is used as a flavor enhancer (INS Number 636) in bread and cakes. It is not registered as a food additive in the EU and thus has no E-number. Instead, maltol is registered as a flavor component in the EU. Maltol, like related 3-hydroxy-4- pyrones such as kojic acid, binds to hard metal centers such as Fe3+, Ga3+, Al3+, and VO2+. Related to this property, maltol has been reported to greatly increase aluminum uptake in the body and to increase the oral bioavailability of gallium and iron.
It is known in the European E number food additive series as E636.
Some synthetic derivatives of maltol, developed at the University of Urbino, showed limited in vitro antiproliferative activity towards cancer cells lines, perhaps inducing apoptosis in these cells. [1]
It is widely distributed in Nature and it has been manufactured since late in the 19th century. Only the last few decades have brought the material into commercial production and a reasonable price level. [3]
Discovery
In 1861, maltol was first isolated from the bark of the larch tree (Pinus Larix, Linn.) by the British chemist J. Stenhouse (1861, 1862) during his investigations on the chemistry of tanning. The larch was considered at that time to be pine and its scientific name provided Stenhouse the basis for naming this substance larixinic acid. Stenhouse obtained larixinic acid from larch bark by water extraction, evaporation, and crystallization, followed by sublimation which purified the substance. Care had to be taken to prevent exposure of larixinic acid to iron or iron salts and avoid the formation of a deep-purple-colored liquid, from which it could not be recovered. Physical properties such as melting point, sublimation temperature, and crystalline geometry provided Stenhouse with evidence to distinguish larixinic acid from substances that had been previously isolated from plants. Stenhouse concluded that larixinic acid existed in the bark and was not a product of the isolation procedure. An interesting feature in Stenhouse's original publication was an error in the empirical formula of larixinic acid due to confusion concerning the composition of water (which was thought to be HO) and relative atomic masses. When Stenhouse's combustion data is used with the correct water formula and relative atomic mass values, the empirical formula of larixinic acid (maltol) results. Stenhouse was the first to comment on its pleasant odor (at room temperature) and its slightly bitter and astringent taste in pure form. A description based on taste during Sten- house's period was considered part of the procedure for characterizing new substances. In the late 1800s, the brewing industry introduced a caramel-colored malt, which contained a higher sugar content than ordinary malts. The beer prepared from this malt gave a violet color with ferric chloride. The causative agent, originally thought to be salicylic acid, was determined by Brand (1894) to be a substance other than salicylic acid. This crystalline substance that reacted with ferric chloride to produce the violet color was recovered from malt by sublimation. In spite of a negative reaction with Millon's reagent (a test for phenols such as salicylic acid), its possession of a positive ferric chloride test for phenols and its isolation from malt provided Brand with the basis for naming this substance maltol. Brand showed that maltol was produced during the roasting process in malt production. The correct molecular formula of C6H603 for maltol was determined by Brand who suspected that this formula resulted from the loss of three molecules of water from glucose (or any other hexose). Brand suggested that maltol was 3,6-dihydroxy-7-oxabicyclo [2.2.1]hepta-2,5-diene.
Shortly after Brand's discovery of maltol, Kiliani and Bazlen (1894) oxidized maltol with potassium permanganate and observed water, carbon dioxide, and acetic acid as products. They observed that only one acidic hydrogen and just the monoacyl derivative of maltol could be formed. These observations required the existence of only one hydroxyl group and a methyl group in the structure of maltol, thereby excluding the structure proposed by Brand. By comparisons made with the chemistry of pyrotechnic acid, Kiliani and Bazlen deduced that maltol must be a methylpyromeconic acid.
Several years later, Feuerstein (1901) isolated a substance from the needles of the silver fir (Abies alba, Mill.) and found it to be identical with maltol. This latter observation soon led to the conclusion by Peratoner and Tamburello (1903) that maltol was larixinic acid. Peratoner and Tamburello (1905) later went on to hydrolyze the methyl derivative of maltol with barium hydroxide and analyze the products recovered. In addition, derivatives of maltol were compared with those of pyromeconic acid and it was concluded that the 2-methyl derivative of pyromeconic acid was maltol (3-hydroxy-2-methyl-4-pyrone). [7]
Natural occurrence and formation
Maltol, or 3-hydroxy-2-methyl-4-pyrone, can be found in strawberry, bread, dairy products, cocoa, coffee, barley, filbert, and peanut. [5]
Veltol occurs in pine needles and the bark of young larch trees. It is produced when cellulose or starch is heated and is a constituent of wood tar oils. [6]
Maltol occurs naturally in certain conifers, but it also forms when certain disaccharides are heated (pyrolysis). Maltol has been identified in a wide variety of heated materials such as bread crusts, coffee and cocoa beans, cereals, dried whey, soy sauce, and chicory (Ensminger et al., 1983). Maltol has also been observed to be a product of the alkaline decomposition of the antibiotic streptomycin B (Schenck and Spielman, 1945). Maltose, when heated to 191°C, decomposes, in part, to maltol (Ensminger et al., 1983). This latter observation explains the formation of maltol during the production of malt or baked goods. Reducing sugars, when heated with amino acids, will also dehydrate into maltol by a nonenzymatic browning reaction of the Maillard type (Patton, 1950). Maltol is formed in less extreme pH and temperature conditions when amino acids (catalyst) are present.
Because maltol sublimes at room temperature, it contributes to the odor of baked goods, caramel, and cotton candy. This realization and the thought of replacing maltol "lost" from foodstuffs by sublimation led to its use as a food additive. [7]
Other uses
An interesting feature of Maltol is its effect as a fungus growth inhibitor at concentrations of 0.1% (however, the finished consumer product will never contain such a high concentration of Maltol). [3]
Profile
📂 CAS N° 118-71-8
⚖️ MW — 126.11 g/mol
📝 Odor Type — Gourmand
📈 Odor Strength — Medium, smelling is recommended in solution at 10%.
The tenacity is excellent in spite of the unusually high vapor pressure at room temperature. [3]
👃🏼 Odor Profile — Intensely sweet, fruity, jam-like, Pineapple- Strawberry type flavor with caramellic undertone. Warm-fruity, caramellic-sweet odor with emphasis on the caramellic note in the dry state, while solutions of Maltol show a pronounced fruity, jam-like odor of Pineapple, Strawberry type. Glycol-solutions show more strawberry-like character, and Phenylethylalcohol solutions of Maltol are more balsamic, Pine-like, with fruity undertones. [3]
👅 Flavor Profile — Sweet, cotton candy, caramellic, with jammy fruity and berry notes. [2]
The caramellic effect is predominant at high (20 to 100 ppm) concentrations, while the fruity effect is most attractive at much lower concentrations. The flavor is furthermore strongly dependent upon the presence of Vanillin, Heliotropine, and other flavor sweeteners, some of which act as enhancers upon Maltol. [3]
⚗️ Uses — Maltol is commonly used in perfume compositions, but only in very low concentrations. Its effect in Pine Needle type fragrances is well known and widely used, and its warm-sweetening effect in Rose undertones is also highly appreciated. As a trace component in fruity lipstick fragrances, often with Undecanolide, Ethylmethylphenylglycidate, Ionones, etc. Its main use is, however, in flavor compositions, not only as a fruity component in Pineapple and Strawberry but in general as a sweetener. Its flavor is clearly perceptible at concentrations near 5 ppm, and the concentration in finished products is normally about 5 to 25 ppm in beverages.
ppm in the flavor compositions
40 to 170 ppm in marmalades
100 to 200 ppm in concentrated soups
50 to 250 ppm as a general flavor enhancer.
For “bouquetting” e. g. in dentifrice flavors, minute amounts will suffice, normally equivalent to a few ppm. [3]
Production
Dow Chemical Co., Midland, Mich., was the first corporation to commercially exploit the use of maltol as an agent for improving the flavor and aroma of foods. Maltol, under the trade name Palatone, was introduced in 1942 as a flavor enhancer for fruit flavors (Sanders, 1966). Dow's maltol was recovered from tars obtained as a byproduct of the destructive distillation of wood at its Cliffs Dow facility in Marquette, Mich. (Goos and Reiter, 1946). In spite of the tendency for maltol produced in this manner to contain impurities which adversely affected its use as a food additive, during the 1940s, Dow sold 5 to 6 tons of Palatone annually for about $11 a pound, mainly to the General Foods Corp. (Anonymous, 1962). Because the demand for this product increased and the production was seriously limited by the number of tars available, attempts were made to develop a synthetic method of production. The first successful synthesis method published started with pyromeconic acid, which itself was an expensive material, and gave maltol in low yields (Spielman and Freifelder, 1947). During the 1960s, a series of U.S. Patents were granted to the Chas. Pfizer and Co., New York, N.Y., for the synthesis of maltol from kojic acid (Pfizer, 1977). Kojic acid could be obtained cheaply as a fermentation product of an aspergillus fungus (Beelik, 1956) and allowed the yield of maltol to be increased substantially to a commercially feasible level. The synthetic routes involved the oxidation of kojic acid to comenic acid and the addition of formaldehyde to the 2-position and its subsequent reduction to a methyl group. Decarboxylation could occur before or after the addition of formaldehyde. By substituting acetaldehyde for formaldehyde, an analog of maltol called ethyl maltol (on the model of ethyl vanillin as noted below), was produced (Rennhard, 1971). To date, ethyl maltol has not been observed as a naturally occurring substance (Freydberg and Gortner, 1982). Pfizer produces and markets maltol and ethyl maltol under the trade names of Veltol and Veltol-Plus, respectively. The most significant foreign producers include Firmenich Inc. of Geneva, Switzerland, who several years ago marketed the maltols under the name Corps Praline, and Otsuka Chemical Co., Osaka, Japan, who currently markets these compounds under the trade name Piromaltol and Ethyl Pyromaltol (Stettler, 1988).
The increase in flavor enhancement properties (see below) in the maltol to ethyl maltol transition is analogous to the increase in the vanilla-like flavor of ethylvanillin over vanillin(Hodge,1967). Vanillin, the naturally occurring substance responsible for the flavor/odor of vanilla, contains a methoxy group; ethyl vanillin, a component of most synthetic vanilla preparations, contains the ethoxy group.
A compound isomeric with maltol, thus called isomaltol, has also been found to contribute to the odor/flavor of baked goods (Hodge and Moser, 1961). Many aspects of the history of this compound are similar to that of maltol, including a publication reporting an incorrect structure (Backe, 1910). The correct structure, 3-hydroxy- 2-furyl methyl ketone, has appeared in the literature (Fischer and Hodge, 1964). [7]
Methods of production in brief
(biosynthesis): by alkaline hydrolysis of Streptomycin salts.
(chemical synthesis): from Pyridine via Piperidine to Pyromeconic acid. The 2- Methyl-derivative is Maltol. [3]
isolated from beechwood tar. [6]
Properties
Maltol and ethyl maltol are both slightly acidic substances that form salts with bases. Both compounds are white, crystallize readily, have low melting points, and sublime at room temperature. They possess a cotton candy, caramel-like aroma. The sublimation of these compounds is significant because it is responsible, in part, for their aroma and odor-altering properties. Better retention of maltols is provided if packaging and storage are in tight, inert containers. Ethyl maltol sublimes more readily and has a greater solubility in water than maltol, a factor that might account for its greater flavor-enhancing activity. Because both substances are chelators and readily form complexes (many are colored) with metals, care should be taken to ensure that the maltols or products composed of them are not packaged in certain containers made of metals or some grades of stainless steel; glass or plastic containers are more suitable (Pfizer, 1977).
Judging from nuclear magnetic resonance data, maltol exhibits some aromatic character (Lassack and Pinhey, 1968). [7]
Consumption and toxicity
Maltol and ethyl maltol are added to foods in minute amounts. In 1970, the average daily intake per person in the U.S. of maltol and ethyl maltol was estimated at 0.4 and 0.3 mg, respectively. Combined, this corresponds to about 0.01 mg/ kilogram of body weight for a person who weighs 60 kg (Freydberg and Gortner, 1982).
Studies indicated that ethyl maltol was slightly more toxic than maltol when administered as a single dose to laboratory animals. In repeated doses, however, the opposite was true. When maltol was fed to rats (dogs) at the rate of 1,000 (500) mg/kg/day, there was significant body weight depression, kidney damage, and death among the different individual test animals. Ethyl maltol at the same dosage caused no significant effects on either animal type except for a mild weight loss. Ethyl maltol was fed daily to rats and dogs up to and including 200 mg/kg/day for as long as two years without any adverse toxic effects. The animals mated and no effects on the fertility or offspring development were noted. Neither compound produced any allergic reaction or sensitization (Gralla et al., 1969).
The metabolic fate of maltol and ethyl maltol has been investigated in the dog. Orally dosed animals readily absorbed the materials from the gastrointestinal tract; neither substance was detected in the feces after oral dosing. Within 24 hours of an orally or intravenously administered dose, both substances were rapidly metabolized and excreted in the urine as glucuronide and sulfate derivatives. These metabolic fates are both common to exogenous phenols and related compounds (Rennhard, 1971).
The U.N. Joint FAO/WHO Expert Committee on Food Additives concluded that up to 2 mg/ kg/day (120 mg/day for a 60 kg person) was an acceptable level of consumption of ethyl maltol for humans. This value is many times greater than the current average consumption levels for both compounds (Freydberg and Gortner, 1982).
Both compounds are included in a list proposed by the Flavor and Extract Manufacturers' Association (FEMA) as substances generally recognized as safe (GRAS) for use in foods. Maltol and ethyl maltol have been given the following respective FEMA (or GRAS) reference numbers: 2656 and 3487. The Food and Drug Administration (FDA) includes both compounds in its list of "synthetic flavoring substances and adjuvants" that are safe for use in foods. The Bureau of Alcohol, Tobacco, and Firearms claims that both compounds can be safely added to alcoholic beverages if amounts do not exceed 100 ppm for use as a stabilizing agent or 250 ppm for use as a smoothing agent. Typically, maltol is added to foods at levels ranging from 50 to 200 ppm while ethyl maltol is added at amounts corresponding to the range of 1 to 50 ppm (Pfizer, 1977). [7]
Applications
At the recommended concentrations, the maltols do not contribute a flavor of their own but modify or enhance the inherent flavors of the foods to which they are added. Their diversity of action in modifying flavors offers a variety of routes to the production of new food products with unusual flavor sensations (Pfizer, 1977). The actual mechanism for the flavor-modifying effects of the maltols is unknown (Lindsay, 1985) so food flavor research must proceed in the future in an empirical manner.
Pfizer states that maltol creates a "velvet mouth" sensation, especially in sweet foods (Anonymous, 1962). The commercial trade name "Veltol" might have originated as a contraction of velvet-maltol, which is reminiscent of Brand's original malt-phenol combination suspected of producing maltol's name. Maltol, a potent enhancer, replaced flavor and aroma enhancers which were not as effective in low concentrations. Coumarin, an enhancer widely used in the past, had to be cautiously added so that its powerful natural aroma, resembling that of vanilla beans, would not overpower the desired enhancing qualities. Maltol could easily replace four times its weight of coumarin (Stephens and Allingham, 1968). Since 1954, coumarin has been classed by the FDA as a toxic substance and subsequently was banned as a food additive. Because of its effectiveness and lack of toxicity, maltol filled the void left by the demise of coumarin. Tests conducted by Pfizer (1977) and independent groups have shown that ethyl maltol is two to six times more effective as a flavor enhancer than maltol. In addition, effects not possible to achieve with maltol are observed with ethyl maltol because of its effectiveness at such low concentrations (Pfizer, 1977). Upon realization that the relative costs per weight of maltol and ethyl maltol are comparable, it becomes apparent that ethyl maltol is a more economical and convenient flavor enhancer than maltol (Stephens and Allingham, 1968). The maltols may be added to the food or perfume directly in a dry form or as a solution. Care must be taken to evenly distribute the additive in the final product because minute quantities have such a powerful effect (Stephens and Allingham, 1968).
The greatest application of maltol, and later ethyl maltol, it was discovered, was in synthetic berry and citrus fruit flavorings.
These additives intensify and produce the characteristic fruity flavors of strawberry and raspberry in fruit-flavored drinks and enhance the flavors of orange, pineapple, and black cherry as well. They have been used along with sodium citrate and sodium gluconate to minimize the bitter aftertaste of the artificial sweetener saccharin, used in dietetic products. The maltols are added to other beverages which improve in overall flavor and aroma such as grape and apple juice, fortified wines (port, tokay, muscatel, and sherry), liqueurs (cordial and brandy) ale, and even tomato beverages, of which acidity and aftertastes are also muted (Dow, 1959; Pfizer, 1977).
Flavor in addition to textural qualities such as richness and creaminess is improved upon adding the maltols to ice creams, puddings, frozen custards, and gelatin desserts (Pfizer, 1977).
The maltols will lessen the yeasty taste of baked foods. Although the maltols have been shown to survive baking (Dow, 1959; Lindsay, 1985), they (especially ethyl maltol) should be added as late as possible and at the lowest temperature feasible to minimize losses due to volatilization. They impart a fresh-baked or browned odor to bread and enhance the flavor character of cakes, cookies, pie fillings, tarts, and pastries, especially those which have fruit flavors (Dow, 1959; Pfizer, 1977).
The maltols increases the richness and smoothness of chocolates in addition to improving aroma and mellowing of harsh flavor qualities. Miniscule amounts of the maltols can make a "chocolate candy taste more chocolatey" with a decrease in essential flavor components (Anonymous, 1962; Pfizer, 1977). The maltols have been used in products other than foods where they often mute undesirable flavors and odors. Many tobacco products, perfumes, colognes, baby talcs, shampoos, cough syrups, multivitamin tablets, scented candles, and aftershave lotions contain these modifiers (Pfizer, 1985). When maltol is added, the content of sugar used in some foods can be reduced by 5 to 15% with no apparent loss in sweetness. Maltol itself does not have a sweet taste. For materials composed mainly of sugar (candies, carbonated beverages, and fruit drinks), any reduction in sugar can represent a substantial saving for the manufacturer (Bou- chard et al., 1968). [7]
Notes:
Solubility at room temperature: [3]
1,2% in water,
1,2% in Glycerine,
2,8% in Propylene glycol,
3,3% in Ethyl alcohol,
5,0% in Phenylethyl alcohol,
Very poorly soluble in Terpenes and most other hydrocarbons. The material sublimes at 93° C. and its vapor pressure at room temperature is relatively high. Accordingly, the material evaporates rapidly unless kept in tightly closed containers. [3]
Maltol is unstable in alkaline media, and it discolors in presence of Iron, under influence of air and daylight, but it is quite stable in mild acids. Its vapors are capable of penetrating plastic bags, and if Maltol is stored in plastic bags inside a cardboard or fiber drum which has nails or wire-staples in it, discoloration will appear quickly and very conspicuously near the traces of metal. [3]
Maltol SDS [4]
National Center for Biotechnology Information. PubChem Database. Maltol, CID=8369
Sources:
Wikipedia
Mosciano, Gerard P&F 17, No. 4, 33, (1992)
Perfume and Flavor chemicals 1
Sigma-Aldrich
Chemistry and Technology of Flavors And Fragrances
Common fragrances & flavor materials
Maltol and ethyl maltol - From the Larch Tree to Successful Food Additive, Synthesis of a natural compound led to improved flavor and aroma-enhancing capabilities (Darren T. LeBlanc and Hugh A. Akers)
National Center for Biotechnology Information (2020). PubChem Compound Summary for CID 8369, Maltol. Retrieved November 24, 2020 from https://pubchem.ncbi.nlm.nih.gov/compound/Maltol.