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Natural fragrances in soaps & cosmetics: consumer interest in natural fragrances has never been stronger. But before you start formulating, here are some points to consider.

FRAGRANCE IS THE single most important consumer-recognized ingredient in soap or cosmetic formulations. Even products that claim to be "unfragranced" frequently contain low levels of "masking" fragrances to cover their base odor. The successful incorporation of a fragrance in a consumer product requires aspects of fragrance stability in the product itself, and its deposition, absorption and evaporation from skin surface after product application. With an ever-increasing consumer population complaining of multiple chemical sensitivity (MCS), the consumer safety concerns for both airborne fragrances and those that come in contact with skin are focal points in product development. (1)

Most commercial fragrances are blends of several chemical moieties. The deposition of fragrance moieties on skin is greatly influenced by the presence of other formula ingredients, including emulsifiers, surfactants, oils, emollients and moisturizers. In soap or other surfactant-based, rinse-off products, the components of a fragrance are not deposited in the same ratio as they are present in the fragrance itself. Surfactants can emulsify the constituents of a fragrance in a variable manner and thus influence their deposition on skin during the washing step of a rinse-off product.

The organoleptic response of a fragrance can vary significantly at each step of its formulation and use. The smell of the fragrance itself, the smell of fragrance formulated in a product, the smell of fragrance deposited on skin from that product, and the change in smell of fragrance on skin due to loss of its constituents from absorption and evaporation during the next several hours can all have an impact on overall perceived attributes of that fragrance. The famous "around-the-clock protection" commercial may not provide consistent around-the-clock fragrance perception!

Mankind has used natural essential oils for fragrancing cosmetics since ancient times. Consumer interest in natural aromas (Table 1) has been rekindled. However, it is widely known that many naturally derived fragrances pose problems in soap and cosmetic formulations due to their chemical reactivity, resulting in product discoloration, malodor formation or instability. An encapsulation method to resolve these issues has recently been patented. Biotechnology is also assisting in the development of new, improved natural aromas. (2)

The chemical constituents of popular essential oils, from almond to zingiber, are now known. (3) In the development of soap and cosmetic products it is advisable to approach reactivity and stability issues from the chemical structures of both the fragrance and other chemical constituents of those formulations. A customer recently demanded a vanilla fragrance with glucosamine (a nutraceutical) added to a skin cream formulation. It was predictable that aldehydic vanilla would react chemically with the primary amine functionality of glucosamine to form a yellow colored Schiff's base, causing gradual product discoloration, loss of vanilla odor and disappearance of glucosamine. This article shall attempt to present the aspects of structure, chemical reactivity and formulation strategy of a limited number of essential oils used in soap, personal wash, hair care and cosmetic products. The chemical concepts discussed in detail for vanilla can provide a guideline for other fragrances and essential oils used in cosmetic products.

Vanilla's Not So Vanilla

Natural vanilla, one of the most important natural aromas used in the food and fragrance industry, is extracted from the cured pods of the flowers of Vanilla planifolia. More than 12,000 tons of synthetic vanillin are produced annually from byproducts of the petrochemical and wood pulping industries. New methods are constantly being sought to manufacture vanillin (3-methoxy-4-hydroxybenzaldehyde), the most important organoleptic component in vanilla, due to the escalating costs of producing natural vanilla. (5) Vanillin contains both an aromatic aldehyde and a phenolic group. The aldehyde group is inherently reactive toward a host of ingredients commonly used in soap and cosmetics formulations. This includes primary and secondary amines, amino acids, protein hydrolysates, active methylene esters, thiols, bisulfites and conditions of alkaline pH. (6)

Vanilla, vanillin and ethyl vanillin are all easily oxidized in the presence of air due to their phenol and aldehyde groups. Metal ions, such as copper and iron, further catalyze this reaction. The use of a reducing agent, such as sodium metabisulfite, as a stabilizer may appear to slow this oxidation. In reality, sodium metabisulfite is reversibly reactive to the aldehyde group of vanillin and its analogs. The "non-discoloring" vanillin substitutes that are available commercially usually lack the "true vanilla" organoleptic attributes. This chemical reactivity of vanillin is also shared by almond oil that contains benzaldehyde as its principal component. Microencapsulation or chemical entrapment procedures offer little help in solving these stability-related issues. (7)

Cosmetic Uses

Vanillin and its analogs are best formulated in an acidic pH range. Ammonia, primary and secondary amines, amino acids, protein hydrolysates, proteins, ammonium-type surfactants, reactive methylene emollient esters, mercaptans, oxidizing ingredients and aldehyde-reactive additives must be avoided. Dihydroxyacetone, a popular self-tanning ingredient, is reactive with vanillin under the usual acidic pH conditions of sell-tanning formulations. This exemplifies the wide spectrum of pH range under which vanillin may show reactivity with other ingredients present in a formulation. Body splash products that contain a high percentage of alcohols can undergo acetal formation with vanillin under acidic conditions. The addition of alpha-hydroxy acids (AHAs) to those formulations can further accelerate this reaction.

In shampoos, shower gels or other cosmetic products based on surfactants that also contain vanilla or vanillin-type fragrance, the innocuous replacement of sodium salts with ammonium lauryl sulfate or ammonium laureth sulfate may result in the eventual loss of vanilla odor. This is due to slow chemical reaction of the aldehydic group of vanillin with ammonia in ammonium cation. The addition of skin and hair conditioning protein hydrolysates, which are almost always rich in amino acids, can result in the chemical reaction of vanillin with the amino acids to produce colored Schiff's bases.

Incorporating vanillin into bar soap formulations, both opaque and translucent types, offers unique challenges. In bar soap and liquid soap formulations that contain alkali metal and ammonium salts of fatty acids, vanillin can undergo a slow chemical reaction (Cannizzaro reaction) which transforms two molecules of vanillin self-reacting to produce one molecule each of vanillic acid and vanillic alcohol, both of which lack vanillin's characteristic sweet odor. This is due to the inherently alkaline nature of those soap ingredients. Emollient esters and fragrance fixatives, especially those with an active methylene group, can undergo Knoevenagel/Aldol condensation reaction with vanillin resulting in the loss of vanilla odor and formation of colored byproducts in alkaline soap formulations. Acetyl acetonate esters, malonate esters, phenyl acetic acid esters and other similar emollient esters should be avoided with vanillin under alkaline pH conditions. Ethyl vanillin, a common vanillin substitute, is also reactive chemically in a similar manner.

The bars based on syndet ingredients or certain non-soap detergents do not produce Cannizzaro reactions due to their generally acidic nature. The surfactant-based transparent bars that contain ammonium lauryl sulfate or ammonium laureth sulfate may have stability concerns due to the reactivity of vanillin with ammonia or ammonium cation, as already discussed. Bars made from a combination of soap and syndet surfactants may also have stability problems due to their alkaline pH.

The phenolic group of vanillin is slightly acidic in nature. In opaque soap, translucent soap and certain liquid soap formulations that are inherently alkaline, the deposition of vanillin on skin is poor due to greater water solubility of vanillin in such alkaline media. The deposition of vanillin from superfatted or syndet-based bars is comparatively superior due to acidic nature of those formulations. In cosmetic formulations, the acidic pH, lipophilic additives and absence of any primary and secondary amines and ammonium cations can provide both a slower absorption into skin and a prolonged, slow release of vanilla fragrance into the air. The safety of essential oils deposited on skin should be carefully evaluated, as a number of natural essential oils are allergenic or may cause contact dermatitis.

Other Essential Oils

Here's a look at some more essential oils that have cosmetic applications:

Almond: Benzaldehyde is the principal component of all natural almond fragrances. The formulation and stability issues of almond essential oil are very similar to those of vanillin. However, benzaldehyde is not as easily oxidized, and its oxidation product is usually (colorless and odorless) benzoic acid, which has preservative properties. The presence of proteins, protein hydrolysates and amino acids in a formulation can lead to Maillard reaction with almond oil, resulting in discoloration. (8)

Sandalwood oil: Sandalwood is the most valuable tree in the world. As with gold, platinum and diamonds, it owes its value to a demand based on ritual, fashion and scarcity--it is the stuff of mystery and intrigue. Natural sandalwood oil, obtained from sandalwood, is very expensive. The chemical constituents of sandalwood oil have been identified. The lower cost synthetic equivalents have been difficult to obtain due to specific chemical structural attributes required for natural sandalwood aroma. (9) The perception of chemical senses of taste and smell is a complex neurological function that is not equivalently functional in humans. (10)

Cinnamon oil: Cinnamaldehyde (cinnamic aldehyde) is the principal constituent of this essential oil that is also used as a flavorant. In cosmetic formulations the chemical reactivity of cinnamaldehyde is similar to that of vanillin. Although cinnamaldehyde has been reported to possess strong antifungal activity, its allergenic and skin irritant properties may pose formulation concerns in cosmetics. (11)

Lemon oil: The skin absorption of d-Limonene, a major consitituent of lemon, lime, and orange oils, has been studied, d-Limonene has also been shown to enhance the dermal absorption of certain pharmaceutical ingredients from topical preparations. Geraniol and citral, two minor components in lemon and lime oils, have been reported to impart contact dermatitis. The cosmetic use of lemon oil should, therefore, be carefully specified for its geraniol and citral content, d-Limonene can undergo auto-oxidation in the presence of air, forming a variety of oxygenated terpenes that can cause strong contact dermatitis, d-Limonene should be avoided in cosmetic formulations that may contain oxidizing ingredients. The cosmetic formulations that contain d-Limonene can be protected with BHT for auto-oxidation, provided oxidizing additives are absent. (12)

Peppermint oil: Throughout history different mint species have been used across the globe for their medicinal and culinary properties. At the end of a large meal, after-dinner mints are frequently served. Peppermint reduces indigestion and colonic spasms by reducing the gastrocolic reflex. The medicinal applications of peppermint have recently been discussed. Menthol is the principal constituent of peppermint oil. Polyphenolic and hydrocinnamic derivatives are minor constituents. Menthol is a popular fragrance ingredient in cosmetics, either alone or in combination with other fragrance oils. Most common essential oils, including peppermint, can cause contact dermatitis. (13) Menthol is one of the most stable essential oils used in cosmetic formulations. It is incompatible with oxidizing additives used in certain acne and disinfection compositions (peroxides, hypochlorites and chlorine dioxide).

Lavendar is Popular

Lavender oil: Lavender is used in aromatherapy as a holistic relaxant and is said to have carminative, anti-flatulence and anticolic properties. Its sedative nature, on inhalation, has been shown both in animals and man. Lavender oil and dried parts of lavender plant are the most widely used materials in aromatherapy. The composition of lavender oil has been established by modern analytical techniques. The contact dermatitis with lavender plant parts has recently been discussed. (14)

Rose oil: One of the most romantic natural essential oils used in Eastern cultures, rose oil contains more than 40 chemical compounds. Rose oils from various species of rose differ in minor constituents. The commercial importance of rose has led to its DNA-mapping. The oxidizing ingredients can cause the oxidation of alpha-phenyl ethanol, a major constituent of rose oil, into phenyl acetic acid, resulting in foul odor formation. (15)

Fragrance oils: Most commercial fragrance oils are blended from several organoleptic constituents. The attributes of each major constituent of such fragrance oils should be considered, as discussed above for individual essential oils, in order to provide a meaningful assessment of their performance, stability and safety issues. An innocuous replacement of one fragrance for another in an otherwise acceptable product can lead to an unstable or poorly performing product. In cosmetic product development, the understanding of the constituents of a blended fragrance and the diluents and solubilizers used in its compounding are of commercial significance.
Table 1: Popular Essential Oils in
Soap and Cosmetic Formulations

Common Name
Vanilla
Almond

Sandalwood Oil
Cinnamon Oil
Lemon Oil

Peppermint Oil
Lavender Oil
Rose Oil

Botanical Source
Vanilla planifolia
Prunus dulcis

Santalum album
Cinnamomum zeylanicum
Citrus sinensis

Mentha piperita
Lavandula angustifolia
Rosa hybrida

Key Chemical
Constituents
Vanillin, Ethyl vanillin
Benzaldehyde,
mandelonitrile
[alpha]- and ([beta]-Santalol, lanceal
Cinnamaldehyde
d-Limonene, Geraniol,
Citral
I-Menthol
Linalool
Citral, [beta]-phenyl ethanol,
other


Acknowledgements:

The group efforts of Mr. Gary Grayson, Mr. Jesus San Miguel, and Mrs. Lori Murphy are sincerely appreciated.

References:

(1.) P.J. Frosch et al, ed., Fragrances: Beneficial and Adverse Effects, Springer-Verlag (1998).

(2.) D. Anderson and G. Frater, Fragrance Precursors, U.S. Patent 6,306,818; R.G. Berger, Aroma Biotechnology, Springer-Verlag (1998); A. Gabelman, ed., Bioprocess Production of Flavor, Fragrance and Color Ingredients (1994).

(3.) K.H. Kubecza and V. Formacek, Essential Oils Analysis by Capillary Gas Chromatography and Carbon-13 NMR Spectroscopy, 2nd Ed., John Wiley (in print, May 2002); E. Guenther, The Essential Oils: Individual Essential Oils of the Plant Families, Krieger Publishing (1992).

(4.) S. Gupta, Nutraceuticals-based Topical Delivery Systems. Nutraceuticals World, 54 (November 2001); S. Gupta, The Role of Phytopharmaceuticals in Topical Pain Relief, Happi, 110 (December 2001).

(5.) J. Rabenhorst et al., Process for the Preparation of Vanillin and Microorganisms Suitable Therefore, U.S. Patent 6,133,003; G. Van Den Ouweland et al., Flavor Enhancing Methods, U.S. Patent 6,287,620.

(6.) M.B. Smith and J. March, March's Advanced Organic: Chemistry, 5th ed., Wiley-Interscience, NY, USA (2001).

(7.) M.A. Porzio et al., Encapsulation Compositions, U.S. Patent 6,187,351.

(8.) C. Severini et al., Agric. Food Chem., 48, 4635 (2000).

(9.) J.E. Fox, Biologist (London), 47, 31 (2000); G. Buchbauer et al., Eur. J. Med. Chem., 36, 673 (2001); G. Buchbauer et al., Comput. Aided Mol. Des., 6, 583 (1992); Z. Wang et al., Zhongguo Zhong Yao Za Zhi, 16, 40, 64 (1991): J.A. Bajgrowicz et al., Enantiomer, 5, 225 (2000); R. Eilerman et al., U.S. Patent 4,960,946.

(10.) L.M. Bartoshuk et al., Chemical Senses, Annual Rev. Psychol., 45, 419 (1994).

(11.) H.B. Singh et al., Allergy (Denmark), 50, 995 (1995); T. Sparks, West. J. Med. (U.S.), 142, 835 (1985); F. Speer, Am. Fam. Physician (U.S.), 13, 106 (1976).

(12.) R. Schafer et al., Arzneimittelforschung, 32, 56 (1982); I. Diez et al., Eur. J. Drug Metab. Pharmacokinet., 23, 7 (1998); A.T. Karlberg et al., Contact Dermatitis, 36, 201 (1997); A.T. Karlberg et al., Ann. Occup. Hyg., 38, 199 (1994).

(13.) L.I. Spirling et al., J. R. Soc. Health, 121, 62 (2001); F. Duband et al., Ann. Pharm. Fr., 50, 146 (1992); C.J. Fleming et al., Contact Dermatitis (Denmark), 38, 337 (1998).

(14.) M. An, J. Chromatogr A (Netherlands), 917, 245 (2001); M. Sugiura et al., Contact Dermatitis. 43. 157 (2000); S. Varma et al., Contact Dermatitis, 42, 309 (2000); M. Lis-Balchin et al., Phytother. Res., 13,540 (1999).

(15.) H.J. Kim et al., J. Chromatogr A, 902. 389, 2000); S. Watanabe et al., Biosci. Biotechnol. Biochem., 65, 442 (2001); H. Iwata et al., Gene, 259, 53 (2000); M. Rollet et al., Ann. Pharm. Fr. (France), 33, 531 (1975).

Shyam Gupta is director of R&D at Arizona Natural Resources, Phoenix, AZ, a contract manufacturing company specializing in consumer oriented cosmetics and toiletries. He is also a consultant in skin care cosmetics based on technologically advanced ingredients (www.bioderminc.com). Tel: (602) 569-6900, E-mail: shyam@aznat.com or shyam@bioderminc.com
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Date:May 1, 2002
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