The quality of a cosmetic product, in the same way as to other kind of products, is initially defined by the manufacturer that chooses the features that a product should present. On the other hand, the quality control of a product aims to verify if all of these defined features are in accordance with the standard definitions and if it will be maintained during the shelf life of the product (Shewhart, 1980).
The quality control of cosmetics is important to ensure the efficacy and safety of products and its raw-materials. Due to the rapid growth that cosmetic industries have exhibit all over the world, efficient, low cost and rapid methods to assay cosmetics’ quality control are a priority. Some current techniques used by the cosmetic industry can be applied to the evaluation of cosmetics’ quality control in an efficient manner, such as: rheology, sensory analysis and small angle X-ray scattering (SAXS).
Sensory analysis is a powerful tool, since there is no equipment able to measure the human feelings. It applies in experimental design and statistical analysis to obtain information about a product in relation to what people feel when use or consume a product, in other words, it is used to indicate consumer acceptance of a particular product. It can be understood as the discipline that interprets, assess and measures characteristics of a product, after stimulating people in relation to their vital senses, as vision, touch, smell and taste (Stone et al., 1992). It is widely used in food industry and recently, it has also been applied in the cosmetic industry (Almeida et al., 2008; Aust et al., 1987; Backe et al., 1999; Lee et al., 2005; Parente et al., 2005; Wortel et al., 2000).
The sensory analysis can be applied in the research and development of a new cosmetic (Isaac et al., 2012), in controlling the manufacturing process to evaluate raw materials’ quality and, even, to make possible the substitution of a raw-material of a product that is traditional in the market without changes in the product’s features (Meilgaard et al., 1991; Muñoz et al., 1993).
The application of sensory analysis could be related to the product control, referring to the storage, packaging and maintenance of sensory quality in relation to time and temperature (Muñoz et al., 1993), since these factors can change a sensory attribute that the product present originally (Zague, 2008) and people who participates of the sensorial panel could realize the changes in the sensorial attributes. Another function of this important tool is to performance comparative tests between competing products.
Another tool that could be applied to evaluate cosmetics’ quality control is the rheology, which studies the flow and deformation of fluids. It has been used in research laboratories and industries as a tool for characterizing ingredients and products, and to predict the performance of products and consumer acceptance.
Rheology has been widely used because, by means of this tool, the researcher can determine physicochemical properties of a product. Constructing a rheogram, it is possible to check the flow curve, evaluate if there is a yield stress and a hysteresis area, which appears to be related to the release of drugs and actives. It is also possible to construct a creep and recovery curve obtaining information about viscoelasticity of each system.
Specifically, in relation to the quality control of cosmetics, specifically, rheology can be applied to help in determining the stability of products by means of the apparent viscosity measured periodically in a determined period exposing the samples to stress conditions (high and low temperatures, solar irradiation), and to monitor the flow characteristics during the shelf life or in the stability assay of a product.
The SAXS technique have being used for the analysis of cosmetics, in order to evaluate the presence of liquid crystalline structures, called liquid-crystals, which are known to increase the stability of formulations becoming, therefore, desirable in cosmetics (Makai et al., 2003).
Combining these three tools, it is possible to test the quality of cosmetics with a rich range of data, and obtain a deep characterization of the system. The results contribute to determining product use, or even, they provide indication of what need to be done to develop a product with predetermined characteristics.
2. Sensory analysis
Sensory analysis is defined by Piana et al. (2004) as the examination of a product through the evaluation of the attributes perceptible by the five sense organs (organoleptic attributes), such as color, odor, taste, touch, texture and noise, allowing the establishment of the organoleptic profile of diverse products, including cosmetics.
The sensory analysis was first applied to the food industry, but the high advance in other areas, such as the cosmetic and pharmaceutical industries, and the important da ta obtained with the sensory analysis, demanded this useful technique to describe what the consumers feel.
An important advantage of the use of sensory analysis in the quality control of a cosmetic product is that it yields a complex analysis in relation to all sensorial attributes that a product could present, it means that, the volunteer who participates of the sensorial panel is able to give information about the fragrance, the sensation, the appearance, the consistence, and other features that this person experience when use such product. The description of these characteristics by means of equipment would be an arduous work and would provide not sufficient or not valuable data when compared to the data provided by the human senses. Beyond that, the acquisition of this equipment could be of high cost when compared to the sensory analyses’ costs (Ross, 2009).
The association of data obtained from sensory analysis and instrumental analysis (especially physicochemical analysis) provides great information and a more complete profile of the product (Ross, 2009).
The sensorial features of a formulation are mainly related to the raw-materials and package (Dooley et al., 2009). The raw materials influence directly in what the consumer feels when applies the cosmetic. The emollients, for example, are raw-materials of marked influence in the tactile sense (Parente et al., 2008; Gorcea and Laura, 2010). Other raw-materials are available at the market and are commercialized to be used in formulations as sensorial modifiers. The main representatives of this kind of product are the silicones and Polymethyl Methacry? late (Ozkan et al., 2012).
Rheology is a tool widely applied in the food, petrochemical and pharmaceutical industries, but to the cosmetic industry it is incipient yet. Until now, the majority of cosmetic industries use viscometers to guarantee that the viscosity of different batches of a product is maintained.
Rheology can influence the diffusion coefficient, altering the release and permeation of cosmetics active substances (Welin-Berger et al., 2001; A-sadutjarit et al., 2005; Vasiljevic et al., 2006). Some authors have related the influence of rheological characteristics on the release profiles and consequently in the permeation of active substances in the skin; thus, the addition of thickening agents or attainment of a weak-gel because of physical entanglement of polymer chains must be considered in the choice of cosmetics bases (Spiclin, et al., 2003). Thus, rheology can help in the assay of release and permeation in the skin.
4. Small Angle X-ray Scattering (SAXS)
The use of this technique in determining the quality control of a cosmetic is closely related to the stability of the product, which could be improved with the presence of liquid crystals.
Liquid crystals are described as a state of matter between solids and liquids, it means that, they are fluid like liquids but are organized like solids, being called mesophases (Marsh, 1973; Kelker and Hatz, 1980; Müller-Goymann, 2004). These organization contributes to the highly stability of systems
The formation of liquid crystals in emulsions could be induced by some components present in this system, such as surfactants (Müller-Goymann, 2004). So, what happens is that it is possible to find a peculiar system that is not a simple emulsion and not a genuine liquid crystal, but an emulsioned system that contains liquid crystals, commonly lamellar structures, that are formed around of the inner phase of the emulsion (Oka et al., 2008), making difficult the coalescence, flocculation and the separation of the oily and water phases, what makes the system formed more stable than a simple emulsion (Figures 18 and 19). Flocculation is defined as the formation of aggregates of droplets of an emulsion under the influence of interparticle colloidal forces which are net attractive (Dickinson, 1992) and the formation of lamellar structures avoid or prevent the occurrence of this phenomenon. The formation of lamellar structures is essential to obtain emulsified oil/water systems finely dispersed, with balanced hydrophilic-lipophilic properties, resulting in minimal interfacial tension between aqueous and oily phases, thus contributing to the stability of the system (Engels et al., 1995). Previous studies have also shown that it is possible to make correlation between SAXS and rheological analysis, since were verified that the thicker the interlamellar water layers, the higher the viscosity of the cream (Eccleston et al., 2000). Thus, liquid crystals could be responsible by the emulsion stabilization and by the increasing in the viscosity (Klein, 2002), being the presence of this structures desirable in cosmetic emulsions which could be an indicative of quality of them.
In cosmetics, other kinds of systems could be used, such as genuine liquid crystals aiming to explore its characteristics of controlled delivery systems.