The success of a cosmetic product on the market is largely dependent on the consumers’ perception and the organoleptic profile of the product [1
]. Therefore, sensory analysis of such a product is a mandatory process that determines market approval [2
]. In 1979, sensory analysis was invented in France by Gonnet and Vache using conventional methods; these methods were later developed in Italy into a more sophisticated protocol involving extensive training, selection of panelists and sensory descriptive terms, before being absorbed by other European countries [3
]. A general guidance for sensory analysis was then set up by the International Organization of Standardization, Geneva, ISO [8
The sensory assessment method outlined by the ISO standard allows the qualitative and quantitative description of the attributes of a cosmetic product, hence providing accurate measurements [9
]. It is a widely used tool during the product development stage, involving the adoption of a simple descriptive lexicon, a controlled environment, and 10–20 extensively trained panelists or judges that qualify the products provided based on their honest verbal perceptions (i.e., feel, fragrance and appearance), as well as quantifying the test products by assigning scores to each perception or attribute on a scale. A statistical (ANOVA) tool is then applied to compare the attributed scores and performance evaluation of the individual judges to assess data reproducibility and quality, respectively [9
Sensory evaluation study performed by Gilbert et al., on eight oil-in-water cosmetic creams, using a set of panelists, successfully described perception terminology in three different stages of simple descriptive lexicons—appearance, pick-up and rub-out—to help provide information on the identity and quality of the creams [11
]. Another study performed by Montenegro et al., used the standard ISO and three-stage simple descriptive lexicon method to assess sensory attributes, however, the result showed a number of variations in the data obtained from panelists; for example, 50% of the panelists labelled three test products as oily, while the other 50% labelled the same products as non-oily [12
]. The difference proved that this method is not 100% accurate due to individual preferences or limitations in sensory skills. Other limitations include the lack of adequate analytical information to back claims. It is also extensively time consuming (ranges from 10 to 120 hours based on sample nature), and expensive to acquire and maintain well-trained judges for both small and big companies and for academic research purposes, wherein time luxury and the availability of funds cannot be afforded [13
]. Therefore, the need for an inexpensive, less time consuming and a more quantitative approach is essential.
The rheometer is a laboratory equipment that provides quantitative information on a product’s attributes and/or qualities, by measuring flow (viscometry test, i.e., yield stress) and deformation (oscillatory test, i.e., strain/stress amplitude sweep and frequency sweep) behavior of a sample [14
]. Yield stress is an important rheological parameter that allows the investigation of the critical value or amount of applied force needed to cause the structured cream to flow out of a plastic tube or be dispensed from a bottle, i.e., stress required to trigger flow. Beneath this critical value, the cream is said to deform elastically, like a solid, but flows like a liquid above the critical value [15
An oscillatory rheological test that measures the degree of linearity of the formulation is the strain (stress) amplitude sweep test, a good first step in determining the viscoelastic characteristic of the cream. The linear viscoelastic region (LVR), which is the region in which a sample is capable of maintaining its structure when force is applied (the line perpendicular to the shear strain axis), gives information on cream structure/firmness, i.e., the longer the LVR, the more firm/structured the cream, while the shorter the LVR, the less firm it appears [19
]. Another oscillatory rheological test is the frequency sweep test, providing structural identity, i.e., is the cream more elastic/bouncy, just like a solid or viscous like thin oil/water. The identity of the cream at a strain below the critical strain allows the assessment of the effect of colloidal forces, as well as particles and droplets interaction; the dispersed particles and/or globules are expected to float and not form sediment when the elastic (storage) modulus, G’, is greater than viscous (loss) modulus, G”, at a low frequency [20
]. A structured or solid-like cream shows an elastic modulus or component, G’, nearly independent of frequency, while the more dependent G’ is on frequency, the more liquid the cream. The cream is said to be non-sticky when there is no crossover of the elastic G’ and viscous G” moduli, and sticky in nature when a crossover occurs [21
The association between rheological measurements and the adhesive ability (tackiness) of pressure-sensitive adhesives on the skin is well-known [19
] and evidences that there is a correlation between user trial data with rheological measurements. The objective of this study was to expand this association to a wider range of sensorial attributes by developing a standard, simple and reliable method for the quantitative assessment of the sensorial attributes of O/W cream formulations by correlating simple sensory lexicons to viscometry (yield stress) and oscillatory (amplitude and frequency sweep) rheological parameters.
As consumer perception of a cosmetic product is an important determinant of market approval and success, sensory assessment is consequently a mandatory step in the claims substantiation stage of a product’s launch to the market.
In this study, we report a new test protocol, which correlates the rheological parameters of semisolid formulations (creams) with their sensorial characteristics such as pourability, firmness, elasticity and stickiness. This protocol avoids the time, costs and subjectivity associated with qualitative user-trials; it is a quantitative method that can be used for the creation of sensorial radar diagrams for cosmetic and personal care semisolid formulations.
One limitation of the protocol is the inability of rheological measurements to reveal sensory attributes like odour, color, glossiness and oiliness. This limitation can be compensated by using other relevant analytical laboratory meters in conjunction with the rheological measurements.