Three body lotions (water-in-oil type) differing only in the emulsifier content were subjected to rheological measurements and sensory analysis with the aim to obtain coupling between the selected rheological characteristics and sensory factors. Both methods proved different behaviour of the samples caused by the difference in the emulsifying compound. To detect the relations between rheological (application of the power law model) and sensory variables, the four most important characteristics from sensory assessment (ease of pouring from the bottle, ease of spreading on a palm, thickness, and ease of spreading on back of hand) were selected. A close coupling (and hence mutual substitution) was found between consistency parameter
Cosmetic lotions are formed by two immiscible liquids with different polarities, and each emulsion is exposed to appearance of various types of instabilities. Creaming, sedimentation, flocculation, Ostwald ripening, coalescence, and phase inversion (Tadros [
Theoretical determination of rheological characteristics is rather complicated. Pal [
For evaluation of these properties, sensory analysis is taken as a primary tool differentiating between consumer-friendly and negative attributes of the cosmetic lotions usually used in the form of body lotions or cleansing lotions for face. From the viewpoint of product’s sales potential, sensory characteristics occupy a privileged position (Lukic et al. [
From the previous, it is apparent that rheological and sensory properties are coupled, not detached. As stated by Karsheva et al. [
Rheological analysis is quite extended in recent research of cosmetic emulsions, but only some studies include the connection with sensory analysis. Nakagawa and Ueda [
An important term in sensory analysis of cosmetics is represented by skin feeling. Brummer and Godersky [
If we take into account the time and cost of sensory analysis and volunteer’s subjectivity, it seems that potential use of instrumental measurements correlating with sensory properties is very perspective (Lukic et al. [
The study of influence of a concrete ingredient on both rheological and sensory properties is relatively rare. Lukic et al. [
Rheological measurement can be also used for the determination of the optimal content of the active ingredient as applied by Abu-Jdayil et al. [
The objective of this study is to specify the influence of a small change in the composition of a cosmetic emulsion to its rheological and textural properties. To detect this, three cosmetic emulsions, water-in-oil body lotions, differing in a ratio of emulsifier were prepared. Samples were analyzed by rheological procedures and sensory analysis, and the particle size was evaluated. The results were compared with the ingredients of the sample to assess the role of the emulsifier. Afterwards, a relationship among detected characteristics was specified to find out a connection among the used methods.
Three samples of body lotions, denoted as BL1, BL2, and BL3, were prepared. Each sample contains 5 wt.% of emulsifiers (different for different samples) and 95 wt.% of other components (identical for all three samples). The composition of the samples is documented in Table
Ingredients of the samples.
Ingredients | Ingredients (INCI name) | Supplier | Content [wt.%] | ||
---|---|---|---|---|---|
BL1 | BL2 | BL3 | |||
Emulsifiers | Details in Table |
5 | 5 | 5 | |
Isopropyl myristate | Isopropyl myristate | OLEON Scandinavia N.V | 6 | 6 | 6 |
Paraffin oil | Paraffinum liquidum | Hansen and Rosenthal | 15 | 15 | 15 |
Glycerin | Glycerin | Neuber | 3 | 3 | 3 |
Jojoba oil |
|
Gustav Hess | 0.2 | 0.2 | 0.2 |
Coenzyme Q10 | Ubiquinone | EISAI, Co., Ltd. | 0.025 | 0.025 | 0.025 |
Magnesium sulfate | Magnesium sulfate | Brenntag CR | 0.7 | 0.7 | 0.7 |
Allantoin | Allantoin | Merck | 0.1 | 0.1 | 0.1 |
D-panthenol | Panthenol | Roche vitamins | 0.5 | 0.5 | 0.5 |
Nipasept | Methyl, ethyl, propylparaben | Jan Dekker | 0.3 | 0.3 | 0.3 |
Neolone 950 | Methylisothiazolinone | Rohm and Haas | 0.1 | 0.1 | 0.1 |
Fragrance | Parfum | Quest Int. | 0.15 | 0.15 | 0.15 |
Citric acid | Citric acid | Brenntag CR | 0.5 | 0.5 | 0.5 |
Water, distilled | Aqua | 68.425 | 68.425 | 68.425 |
The structure of all three body lotions differed only in the composition of an emulsifier. In all three cases, the emulsifier consisted of the three same basic components (see Table
Emulsifiers contained in the samples.
Component of emulsifier | Component of emulsifier (INCI name) | Supplier | Weight ratio | ||
---|---|---|---|---|---|
BL1 | BL2 | BL3 | |||
Arlacel 989 | PEG-7 hydrogenated castor oil | ICI Surfactants | 3/5 | 2/5 | 1/5 |
Arlatone T | PEG-40 sorbitan peroleate | ICI Surfactants | 1/5 | 1/5 | 1/5 |
Span 60 | Sorbitan stearate | Cognis | 1/5 | 2/5 | 3/5 |
A semi-plant mixer BECOMIX RW15 (A. Berents, Stuhr, Germany) was applied for the preparation of the samples. This mixer includes vacuum processing vessel with homogenizer and jacket enabling heating or cooling. It has a batch capacity of 12 litres, with a minimum capacity of 1-2 litres.
Rheological measurements were carried out with a rotational rheometer RheoStress 300 (Thermo Scientific, Karlsruhe, Germany). A cone-and-plate system with sensor C60/2° (cone diameter 60 mm, cone angle 2°) was used. In contrast to a plate-and-plate arrangement, the chosen cone-and-plate arrangements respect non-Newtonian course of measured characteristics and, hence, provide more responsible data. All measurements were carried out at 25°C.
During the emulsification, the mixing intensity corresponded with level 3, and homogenization proceeded 5 minutes at level 5 using the mixer Becomix RW15 (levels 1 and 5 correspond to preset minimum and maximum revolutions, resp.). The water phase was added into the oil phase, and to obtain higher degree of homogenization, temperature was set to 75°C. To prevent evaporation, thermal sensitive fragrance was added at a reduced temperature of 45°C. Total amount of one sample attained 10 kg; consequently, it was filled into 250 mL plastic (PE) bottles and stored at room temperature.
Both rheological and sensory approaches were applied for characterization of the samples. These two attitudes are not strictly separated but just, on the contrary, coupled. Sensory analysis is time and cost demanding; in addition, there is a certain influence of volunteer’s subjectivity. Inputs from the sensory analysis can be in some range replaced by the rheological characterization, which represents substantially cheaper and effective solution. To verify this approach, loosely said an acceptable substitution of sensory analysis by the rheological one, for the problem studied (emulsifier alteration) the following steps were carried out: rheological measurement determining a relation between shear stress and shear rate (the flow curve) in the range of shear rate 10−2–600 s−1. Shear rate continually increased from the minimum to maximum (upcurve) and decreased back to minimum (downcurve). Typical shear rates for pouring from a bottle terminate at approximately 100 s−1, where by coincidence typical shear rates for spreading of lotions start; see, for example, Ward et al. [ sensory evaluation was carried out under standard conditions as specified by the ISO standards (ISO 8589, ISO 8586.1, and ISO 6658). Seventeen trained panellists completed a special questionnaire concerning 11 parameters (including colour, shine, appearance of the surface, feelings during rub-in, and frequency of the use of body lotion). Each parameter was rated on a category scale with predefined descriptive terms. The four most important parameters (the ease of pouring from the bottle, the ease of spreading on a palm and on a back of hand, and thickness) involved, except a category scale, a graphic unstructured one as well. For these four parameters, there were adjusted counter-equivalent rheological qualities measured by the rotational rheometer.
Data processing of both rheological and sensory measurements, and their coupling is presented in Section
The flow curves relating shear stress
The flow curves of BL1, BL2, and BL3 in the range of shear rate 10−2–600 s−1.
It is apparent that relatively small change in the compositions of BL1, BL2, and BL3 results in substantially different behaviour of flow curves. The change in emulsifier composition is reflected not only in a course of the upward curves but especially in their downward behaviour. This confirms a nonnegligible impact of the emulsifiers used on the overall manifestation of the individual products.
For characterization of flow curves in upward direction, the power-law model was used as follows:
Parameters of the power law model.
|
|
|
---|---|---|
BL1 | 28.12 | 0.46 |
BL2 | 13.90 | 0.52 |
BL3 | 11.45 | 0.46 |
Approximation of the flow curves (upward direction) of BL1, BL2, and BL3 in the range of shear rates 10−2–600 s−1 by the power law model.
Table
Results of the sensory analysis.
Assessed property | Descriptive terms | The most frequent descriptions |
---|---|---|
Frequency of the use of BL | Daily, very often, often, sometimes, rarely, never | Often, sometimes |
Pouring from the bottle | Very easy, easy, difficult, very difficult | Very easy, easy |
Shine of the BL | Very shiny, shiny, matt, uneven | Very shiny, shiny |
Surface of the BL | Smooth, grainy, bubbles, separated oil/water, other inhomogeneity | Smooth, bubbles |
Ease of spreading on a palm | Optimal, satisfactory, bad, very bad | Optimal, satisfactory |
Thickness | Thin, optimal, too thick | Optimal |
Ease of spreading on a hand | Optimal, satisfactory, bad, very bad | Optimal, satisfactory |
Feelings during rub-in | Cooling, smooth, warm, rough, scratchy, burning, drying out | Cooling, smooth, warm |
Fragrance | Extremely nice, very nice, nice, neutral, unpleasant, unsavory | Very nice, nice |
Absorption | Very quick, good, difficult, very difficult | Good, difficult (BL1) |
Interest in using the BL | Daily, very often, often, sometimes, rarely, never | Sometimes, rarely, often |
Ease of spreading is evaluated at two places—on the palm and on the back of hand due to different body temperature. This dual assessment of spreading was proved to be useful (see Morávková and Stern [
The four properties (pouring from the bottle: P, ease of spreading on a palm: SP, thickness: T, and ease of spreading on back of hand: SH) range to the most important characterization of the body lotions studied. Their more detailed evaluation is presented in Table
Results of the sensory analysis, selected parameters (mean values, evaluated at scale 0–100).
100 responds to | P | SP | T | SH |
---|---|---|---|---|
Very difficult | Optimal | Too much thick | Optimal | |
BL1 | 30 | 65.5 | 54.5 | 66.5 |
BL2 | 24.5 | 73.5 | 40.5 | 79 |
BL3 | 24 | 85 | 36.5 | 86 |
Considering ease of spreading (both places), the samples sort BL1, BL2, and BL3 from bad to optimal. The same order settles in the thickness category, BL1 being the most thick, BL2 the less, and BL3 the least. Reversely assessment of the pouring from the bottle gives similar values; only lotion BL1 slightly divides with more difficult pouring.
One of the main purposes of the emulsifier is to decrease the interfacial tension and thus help to form the interface between the continuous oil phase and water droplets. This effect should eliminate the breakdown processes in the emulsion. In the presented samples, there is a supposed homogeneity in coating of the interface by an emulsifier as its amount attains 5 wt.%. This results in no breakup or tip-dropping of the droplets.
The microstructure of the samples was observed by the laboratory microscope DSML (Leica, Germany) integrated with digital camera. Figure
Measured droplet size.
Droplet size mean value ( |
Standard deviation ( |
|
---|---|---|
BL1 | 7.12 | 3.79 |
BL2 | 5.53 | 1.66 |
BL3 | 5.36 | 3.27 |
Microstructure of samples BL1, BL2, and BL3 (from left to right).
In the following, the linear relations between the sensory variables and rheological parameters obtained by applying the power law model are determined; see Figures
Correlation coefficients among rheological and sensory parameters.
|
|
Droplet size | |
---|---|---|---|
P: pouring the lotion from a bottle |
|
−0.4731 |
|
SP: ease of spreading on a hand | 0.8822 | 0.0592 | 0.8581 |
T: thickness |
|
−0.3474 | 0.9921 |
SH: ease of spreading on a back of hand | 0.9746 | −0.2045 | 0.9625 |
Droplet size |
|
−0.4621 | — |
Statistically significant relations are bold.
Relationship between sensory parameters (and droplet size) and consistency parameter
Relationship between sensory parameters (and droplet size) and flow behaviour index
As already mentioned above, sensory analysis is time and money consuming, and it is also influenced by volunteer’s subjectivity. The above analysis enables eliminating evaluation of the ease of pouring of a bottle and thickness as these properties can be estimated directly from the rheological measurements through the parameter
Three cosmetic lotions, water-in-oil type body lotions, were prepared with different composition of the emulsifier. The other ingredients were identical. All samples were characterized by rheological analysis and sensory profiling. Rheological analysis proved that a small change in emulsifier composition reflects on the behaviour of the lotion; it is illustrated by the differences in the measured flow curves. To characterize the flow properties of the samples, the power law model was applied. The consistency parameter
To detect the relations between rheological parameters obtained by the power law model and sensory variables from the analysis, the linear approximation was carried out. The correlation coefficients describe the relevance of the relations. Tight relations were found for the following couples: droplet size, pouring from the bottle;
The founded close relationships allow the replacement of time and money consuming sensory assessment by fast instrumental analysis.
A future step in analysing the above studied body lotions will be also based on oscillatory measurements completing a rheological approach to this topic. This approach provides a very useful means for characterizing textural and sensory properties as recently documented, for example, by Lukic et al. [
The authors wish to acknowledge the Grant Agency CR for the financial support of Grant Project no. 103/09/2066.