Luigi Rigano – Institute of Skin and Product Evaluation (ISPE), Italy SENSORY EVALUATION
Development via quantitative sensorial evaluation
During the product development phase of cosmetics, a whole set of evaluation procedures is applied in order to check the effective result of the various trials in comparison with the requirements of the hypothesised characteristics. Some of them are essentially instrumental (e.g. pH, viscosity, density) microbiological or belong to the bioengineering evaluations. Some others are sensorial (appearance, flow, spreading over the skin). While the first group of evaluations has technological relevance (in other words, necessary to understand the average physico-chemical, microbial or functional behaviour of products) the other is related to perceptive characteristics, that cannot be measured with standard devices or easily expressed with numbers.
The way of providing this type of evaluation with the characteristics of scientific measurements (i.e. reproducibility and repeatability, possibility of comparison with previous values, statistical significance and reliability) is the task of sensorial analysis.
Its applications in the development of a successful cosmetic product are many but, until now, only very few big industries are using this tool in a systematic way. Some examples in which sensorial analysis demonstrates its usefulness can be taken from everyday problems of the cosmetic industry: how to select among a set of formulations, having the same stability (and costs) the one having quicker superficial absorption? Which amount of perfume
gives the best performances at low costs? How to control if the industrial production run has the same perceived characteristics as the laboratory sample? How to qualify the supplier of an ingredient in a successful product, without changing its perceived characteristics? How to formulate a product sensorially very similar to the main competitor on the market? Which controls are more suitable during the product stability evaluations? Finally, is moisturising power increasing by 10% (instrumentally measured) the hydration level of our epidermis perceivable by consumers? This paper will try to give answers to some of these questions.
Clues in sensorial analysis We use sensorial evaluations throughout our life: we smell, touch, look, listen, taste; our senses are essential for the control of (and survival in) our environment, the maintenance of our life and social behaviour. The external perceptions reaching our brain are decoded into three main components.
Firstly comes the type of perception. Our brain identifies the key structure of the stimulus: thermal, pressure, evenness, friction, noise, odour, visual impact. This is the qualitative variable. The second is the intensive one, related to the amount of stimulus we perceive: how intense is a yellow shade, how soft is a cream, how black is Vegemite, how much time a perfume requires to fade are typical examples. The last one is the hedonic one,
or in other words the pleasure or disgust that these sensations can induce, because of our personal history, attitudes, culture, experience, taste, mood.
In normal life we do not make a clear separation between these variables, but we use verbal or mental expressions for their combination, as they are decoded by our brain.
The expert’s evaluation In the cosmetic field it is a common technique for the testing of new raw materials or new formulations to spread them over one hand, then massaging carefully. This is one example of the so- called ‘expert’s evaluation’. It is a truly sensitive method that can detect even small differences, but it has its drawbacks. The result is variable according to the expert’s experience (there are no two experts with the same brain and experience) and it cannot give numerical values. It is also impossible to reproduce with the same result on different days, and it cannot be compared during time. In other words, it is an instant evaluation, which is valid in that very moment but has no statistical significance. This means that the expert evaluates the products by a mental comparison with his internal standards, which are variable according to his mood, the knowledge of the test product and affective impulse for the internally prepared formulations. This type of evaluation is nevertheless useful to select among serial formulation
April 2012 PERSONAL CARE 107
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