Sensory in Cosmetics Deciphered

The five senses are constantly at work to connect the human body with its environment by examining and controlling objects, people and circumstances. As a consequence, they induce logically or emotionally driven changes of behavior. People perform several types of sensory assessment every day, both consciously and subconsciously. The results of these evaluations determine their behavior, preferences, social life and purchasing.

One may ask how the perception involved in a sensory response can be translated by a test instrument, when this response involves the transmission of signals to the brain via the nervous system after physiological and behavioral contact. This is the task of sensory analysis, a scientific discipline created to measure, analyze and interpret the sensorial observations collected by sensory organ contact with objects, products or ingredients.

The sensory world is fueled by research conducted in different disciplines such as biology, neurobiology and sense chemistry. It extends to many industrial sectors where perception elements are crucial, including cosmetics. To express how important the schematic sensory path is in cosmetic formulations, and illustrate what its profile looks like, it is important to develop a strategy for sensory analysis that is easily described to young cosmetic formulators.^1–3

Cosmetics are a formulation art whereby the formulator selects well-identified ingredients from a universe of possible substances and combines them in precise ratios to obtain a functional complex. In order to achieve successful cosmetic systems, four elements of formulation must be considered: safety, stability, efficacy and sensory, which are applied to the identification and assembly of ingredients. As always, cosmetic formulation strategy has many factors, and this column will establish the role that sensory plays during the cosmetic formulation process.

Subjective Sensory Evaluation

The complicated nature of sensory communication makes it seem as though formulators are not provided instruments for sensorial analysis during product development. However, all formulators have them—their senses—and they are efficient and handy. Through memory and technical skill, formulators conduct a thorough investigation of a product through simple examination on the bench. Before use, they examine the product in its container for parameters such as appearance, shine and homogeneity. Then the product is applied, usually to the inside of the forearm, where they investigate its firmness, ease of spreading, silky or velvety feel, stickiness, etc. Finally, immediately or a few minutes after application, formulators evaluate testing parameters like the residual feel and skin smoothness.

Use of one’s own senses is a simple, quick and cheap method that is sensitive to small differences, and a number of product samples can be compared in just a few minutes. Unfortunately, subjective evaluations are expressed with words and not numbers; therefore, no statistical analysis can be applied for an objective comparison. Moreover, this analysis can be influenced by the formulator’s experience and mood. Finally, formulators can often lose objectivity when comparing their own formula to an industry benchmark. Whether this comparison is affected positively or negatively, it certainly can be biased. In spite of these drawbacks, one’s senses are useful tools for driving formulation development and design strategy.^4–7 Qualitative vs. Quantitative Imagine the daily application of a cosmetic such as a cream to the skin.

Aside from the enjoyment experienced by the user, the perceived result derives from the interplay of fundamental qualitative and quantitative sensory variables. First, the tactile variables are bound to physical actions of compression, flow and deformation of the product; i.e., firm, greasy, oily, slippery, waxy, pasty, light, spreadable and lubricious. Then, the user experiences the qualitative side of perception; i.e., silky, velvety, sticky and even. Finally, the user experiences the amount and type of residue perceived on the skin after massage. There are also thermal perceptions that involve a feeling of warm, cool or wet. Important olfactory perceptions are bound to the development of volatile substances from the product and to their intensity; i.e., the low or high capability to stimulate the olfactorymucosa. Visual sensory characteristics relate to luminosity, brightness, color and opacity, both for the product and for the skin that has been treated with a cosmetic. Moreover, the container might give tactile, visual and sometimes even auditory perceptions, such as the smooth gliding of a lipstick case during the forward movement of the stick.^{8, 9} Following is an analysis of each sense’s role in cosmetic formulation.

Scent

Indeed, fragrance plays the main role in connecting the consumer’s sense of smell with a product. It is designed to nonverbally communicate emotional and intensive perceptions, to hide off base odors, and to complete and integrate with the labeled descriptions and claims. The quantitative side of these odoriferous substances includes their compatibility with the base ingredients, safety, solubility in the formula, the odor development curve during application time, influence on color in the formulation, long-term stability, compliance with regulations and cost.

The qualitative sensory side relates to the category of notes—e.g., citrus, chypre, flowery and herbal—connecting the “correct” mental association with the color of the product and its label and packaging; the capability of the fragrance to express abstract or practical concepts like “vegetal, rich, fresh, luxury and anti-aging;” and the release of an olfactory symphony during and after the product use.

For example, the fragrance in a shampoo must be easily recognizable by sniffing the bottle; should develop mild cleansing notes when massaged into the hair with hot water; and leave a pleasant, discrete, residual note on the hair that can be detected by others in close encounters. Moreover, it should fragrance the bathroom and shower in a pleasant, reinvigorating or relaxing way.

As always, the well-known sensorial dose/response curve shown in Figure 1">Figure 1 applies to fragrancing cosmetic products. Here, the initial response is almost flat because the low dose of ingredient confuses the senses, which are influenced by the base signals. Human senses cannot detect sensory signal differences at low concentrations. At a threshold value, however, the perception is clearer and a linearity zone begins, relating the ingredient content in the formula with the sensory answer. At the top of the curve, a saturation profile appears, at which point increasing the sensory ingredient concentration would not increase perception by the senses. This is an important level, as it can help the formulator avoid excessive amounts of costly sensory ingredients such as fragrances.

On the other side of the odor domain, ingredients that adsorb the off-odors of cosmetic ingredients are frequently required due to small traces of oxidized vegetal oils, protein blends or vegetal extracts. An extreme de-aeration procedure could also be used, where the emulsified system is put under a proper vacuum in its hot phase (70–75°C) for a few minutes to eliminate all traces of foul-smelling volatile substances. Alternatively, synthetic zeolites can be incorporated for their crystalline lattice structure, which traps small volatile molecules and absorbs odors.

Sight

The success of emulsions in cosmetics also is due in part to their appearance. An emulsion is created by blending yellowish oils and light colored aqueous phases into a light white/ivory, shiny cream. When the emulsification procedure instead results in a deeper yellow, green or brown shade, the formula is generally considered unacceptable by most companies. The formulator must then reconsider the selection of ingredients in the two emulsified phases. There are few ingredients that may make the formula color lighter; however, a special grade of hydro-dispersible micronized titanium dioxide can be used to reduce the visible impact of brown ingredients^a.

In other cases, oil and aqueous phases of different colors in a formulation are used to attract consumers. Some makeup removers, for example, are sold in transparent bottles so the separation of the two phases is visually apparent. Then by shaking the bottle gently, the consumer sees a temporary stable emulsion with a color derived from the combination of the two layers. In a few minutes, the two layers separate again completely. In makeup products, this same layering effect is used for making creamy lip gloss more attractive. All makeup products are the triumph of visually appealing techniques, from laser-carved compact powders and high relief eye shadows, to multicolored lipsticks.

Another recent range in pearl pigments improves the appearance of creams, imparts luxurious color to shampoo or a homogenous look to skin, and provides luminosity to the skin gently and without influencing the color of the base. This is accomplished through optical interference, and these pigments consist of synthetic mica (fluorophlogopite) coated with iron oxide, titanium dioxide or tin oxide^b.

To modify the appearance of fine and deeper wrinkles, well-known, soft-focus ingredients can be adopted. By changing the reflection of visible rays, they give immediately perceivable anti-wrinkle results.¹⁰ In this way, consumers with mature skin are temporarily convinced about the product efficacy and continue its use long enough to give the active substances time to take effect.

Touch

Selecting thickeners for a cosmetic product can be done sensorially by comparing the texture of gels prepared with their aqueous dispersions. The texture perceived through touch is the result of a combination of low-shear viscosity, visco-elastic behavior, resistance to penetration and friction coefficients. Since the perceived texture of a formula is the result of the ingredient combination, knowing the skin feel of each rheologically important raw material allows the formulator to discriminate which formulation changes result in the desired feel.

The oil phase composition can strongly influence spreadability and absorption speed, which are integral in a consumer’s evaluation of emulsions. Thus, a sensory approach to formulating the oil phase should be adopted in order to make the right selection. Once the emulsion category and the product’s key characteristics have been identified, raw materials are selected that can achieve this target. Table 1">Table 1 is a general illustration of this method, with some examples of specific parameters required by emulsions.

In formulating the oil phase, one should consider the bio-mechanical state of the epidermis, i.e., moisture, elasticity, softness and shine levels; the type of sensory signals sent during/after application, i.e., lubrication, friction, stickiness and absorption; and the required final skin feel. These parameters are interdependent. For example, a high spreading rate refers to raw materials that provide a final dry feel. In this case, a balanced blend of high and medium diffusion oils are used to avoid sudden sensory gaps during application.

Alternatively, combinations of high and medium spreading-rate oils are adopted. Experimental measures have shown that an increase in molecular weight of the oil corresponds to an increase in sensory oiliness/greasiness and to a decrease in the spreading value (SV), i.e., mm2 of skin wet by the oil in normal conditions over 10 min, as shown in Figure 2">Figure 2.

The oiliness perception of oils with decreasing molecular weight can be ordered as follows, from most to least oily: isostearyl isostearate, decyl oleate, isopropyl stearate, isopropyl myristate and dibutyl adipate. The molecular structure of oils determines their interactions with the skin. For example, a branched-chain ester, especially if unsaturated, helps to improve the cutaneous barrier and provide a smooth skin perception since more water is retained. Branched chain molecules exhibit high ease of application and silky feel.

In a paper published in 1992, many sound oils were subdivided according to two key sensory parameters; spreadability (S), which is important during the initial phase of distribution over the skin; and lubricity (L), another sensory parameter related to long-term massageability.¹¹ The researchers organized the world of emollients into four main categories according to the possible combination of these two variables by order of appearance and magnitude. Selecting oils belonging to specific divisions, e.g., high S and low L, helped to quickly adjust the formula feel. The selection of vegetables oils for cosmetic formulae is often hindered by a oily sensory profile. In the past few years, some vegetable oils and butter from Asia, Africa and South America have been introduced that combine functionality and sensory elements. Moringa oil, jojoba esters, cherry oil and olive oil-derived butter are some of the recently discovered or revamped ingredients that impart a light and velvety feel. In some formulations such as those with high sun protection, the raw materials used, i.e., UV filters, can have a strong influence on the feel of the formula. Consequently, the formulator’s goal of creating a soft, pleasant feel is challenged. Ideally, the product feel should be light if the consumer’s compliance is a key requirement for efficient sun protection. To develop a light, pleasant formulation with these raw materials, a wide range of sensorial modifiers and powder fillers could be used; and depending on the physical interactions with the vehicle, different raw materials can provide different sensorial attributes.

Starting with inorganic fillers, boron nitride could offer a wide range of sensorial characteristics, such as soft focus effects and heat dispersion abilities. In a series of experiments, boron nitride powder was compared with the fillers polymethyl methacrylate, mica, talc, bismuth oxychloride and nylon-12 to assess features concerning spreadability, skin coverage degree and homogeneity, and skin softness.¹² Boron nitride and bismuth oxychloride performed best in all evaluated characteristics, as no statistically significant differences were perceived. Solid fillers also can be used to reduce the oily perception of a formulation. The high oil-absorbing capacity of nylon-12 achieves a smooth, dry final feel, with the proper grade selection.

The tactile choice of a suitable sensory modifier can become even more complex if the addition of silicones is considered. Hundreds of fluid silicones, silicone gums or powders may be used in order to achieve a soft-focus effect or luxurious feel. Formula 1">Formula 1 shows a cream incorporating sensory modifier mixtures to impart a velvety and silky feel. The use of polymeric emulsifiers like ammonium acryloyldimethyltaurate/VP copolymer can reduce the amount of poly-ethoxylated emulsifiers needed in a cosmetic formulation and, consequently, reduce their braking influence on the final skin feel. The oil phase is simple, as the only oil selected is a light ester. The focus in this formula, nearly 40% w/w, is on a combination of volatile silicone (polysilicone-11) and two silicone polymers (C30–45 alkyl cetearyl dimethicone crosspolymer) dispersed in a volatile silicone (polymethyl methacylate) with two different synthetic fillers (HDI/trimethylol hexyllactone crosspolymer). A near infinite list of alternative sensory modifiers exists in the market including new methacrylate polymers, sodium polyacrylate with matte and satin effects, and new mixtures of silicone polymers that obtain high sensorial performances in aqueous systems.

Makeup products are often ahead of other product categories with sensory-driven breakthroughs. For instance, trendy mousse foundations comprise a highly efficient coloring foundation with a combination of sensory attributes. They are soft to the touch, like a colored foam on the fingers, after the pick-up phase; velvety and powdery during the application; and they leave an evanescent and silky after feel on the skin. Formula 2">Formula 2 is an example of a typical mousse foundation. The majority of this foundation formula is sensorial ingredients, along with 10–13% pigments coated with methicone for better skin feel and 1% fragrance. The ingredient selection accounts for performance attributes such as skin coverage, homogeneity and pigment adhesion. These attributes are guaranteed by the careful selection of modified silicone polymers like PEG/PPG-20/15 dimethicone and dimethicone/bis-isobutyl PPG-20 crosspolymer.

Cream-to-powder and powder-to-cream foundations often draw the attention of consumers and marketing. This successful magic metamorphosis is sensorially driven. Like all cosmetic sensory experiences, it is created by the combination of functional and tactful ingredients, as shown in Formula 3">Formula 3.

The creamy appearance of a mousse foundation is achieved here through the combination of two esters and a synthetic triglyceride (tricaprylin) that disappear during spreading due to skin absorption, leaving a powdery feel provided by five fillers—dimethicone/vinyl dimethicone crosspolymer, talc, mica, corn starch modified and silica.

Systemic Approach

While formulating complex sensory systems, a mathematical approach may be applied, requiring a combination of sensory evaluations and formulation hypotheses. The strategy starts with a qualitative screening, after which the formulator can identify, in an example, one possible thickener, three different oil phases and two feel modifying ingredients. The problem to be solved by this method is to determine the quantitative combination of such ingredients to provide the best feel.¹³ The same approach, based on chemiometric analysis,¹⁴ often is used in the food field for the selection of the right taste for market success.

References

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4. RC Hootman, Descriptive Analysis Testing, chapter 10 in Sensory Evaluation Techniques, Third Edition, MC Meilgaard, GC Civille and BT Carr, CRC Press Inc: Boca Raton, FL USA (1992)
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7. ISO 6658, Sensory analysis—Methodology—General guidance, International Organization for Standardization (2005)
8. ASTM E2454–05(2011): Standard guide for Sensory Evaluation Methods to Determine the Sensory Shelf Life of Consumer Products, ASTM E1490–03: Standard Practice for Descriptive Skinfeel Analysis of Creams and Lotions, ASTM E2082–06: Standard Guide for Descriptive Analysis of Shampoo Performance, ASTM E1958–07: Standard guide for Sensory Claim Substantiation, ASTM International
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14. R Todeschini, Metodologie della Ricerca Sperimentale, Dipartimento di Scienze dell’Ambiente e del Territorio, www.disat.unimib.it/index.php?option=com_content&view=article&id=288&Itemid=287&lang=it (Accessed Aug 6, 2012)