The PM method for sensory evaluation of wine
The workflow of PM is illustrated step by step in Figure 1. Panellists receive all the wines simultaneously (Figure 1, Step 1) and are asked to focus on the sensory similarities and differences between wines. The wines are positioned, according to similarities and differences on a flat surface, such as a tablecloth, white paper or computer screen (Figure 1, Step 2). Each panel member decides how similar or different the wines are according to his/her own discretion. Similar wines are placed close to each other and different wines far apart. The distances between wines indicate the degree of similarity; the bigger the sensory differences between wines, the further apart the wines will be placed from each other. Panellists can taste each wine as many times as they want. After deciding on the final positions of the wines on the paper, tasters make a small cross and write down the three digit code of the wine on the paper to indicate the position of the wine (Figure 1, Step 3).
How do we analyse PM data and interpret the graphs
The X and Y co-ordinates of the positions of all the wines on the paper are measured (Figure 1, Step 4). These co-ordinates from all the tasters are recorded in a table (Figure 1, Step 5). The number of times a term is used by all the tasters to describe a wine, is then counted. When many terms are used by the tasters, similar terms are combined in the data analysis stage, to reduce the number of terms displayed on the graphs, so that the most important information obtained with the PM sample descriptions is shown. The sum of all the tasters’ descriptive data is calculated after reducing the terms and added to the data table as extra columns (Figure 1, Step 5). The statistical method most frequently used to analyse PM data is Multiple Factor Analysis (MFA). This technique takes all the individual tasters’ placing configurations of wines on the paper into account. The graphical presentation shows the most prominent sensory characteristics of the different wines or groupings of wines (Figure 1, Step 6). Other statistical techniques that can be used to analyse PM data include DISTATIS, INDSCAL, as well as General Procrustus Analysis (GPA). The graphs produced by these methods are similar to MFA graphs. There is expertise available at IWBT-DVO, Stellenbosch University to help readers interested in doing these techniques.
Main similarities and differences between the PM and Sorting methods
During Sorting (refer to Part 2 of the series on wine sensory evaluation), the taster has to decide whether the differences between wines are big enough for the wines to be placed in different groups. With PM, the taster does not have to make this decision. PM is particularly suitable for a set of wines where the sensory properties change gradually and no distinct groupings of wines are perceived. Pagès and co-authors suggested a method referred to as Sorted Napping in 2010. This method is a combination of Sorting and Napping. The tasters group the products, while at the same time position the products on the “tablecloth” or paper. Groups are indicated by circling products that belong together. The distances between the groups give an indication of the differences and similarities between the groups. The further apart groups appear from each other, the larger the differences between groups. The closer groups are to each other, the more similar they are. The differences between products within the same group can also be indicated with distances on the paper in the same manner.
An important difference between Sorting and PM is that with PM each wine is usually described separately, while with Sorting the samples belonging to the same group share the same descriptors as the group. Again, as with Sorting, the PM panel leader can decide whether a predetermined list of terms should be used for description, otherwise tasters can use their own terms.
Choosing a PM panel
A wide range of tasters can be used for PM, including wine experts and ordinary consumers. Although the training of a panel is not required for PM/Napping, researchers have obtained good results with trained panels. Such an example is the sensory method development done by Louw and co-authors (2013) for brandy profiling, a study which showed that panel training improved the results. Several studies on wine compared results obtained by non-expert consumers to that of expert consumers. When selecting a panel, the information required from the tasting should always be kept in mind. Trained panels and experts consistently provide more in-depth and technical descriptions of the wines, and can also describe smaller differences between wines more accurately, than non-expert consumers.
Helpful tips to overcome difficulties related to PM
PM is a useful technique commonly used as sensory technique for food profiling. It can be used for the evaluation of colour, aroma, taste, as well as mouth-feel. The quality of the data is better and the task is easier for tasters, when only one of these modalities is evaluated in a tasting session. It is therefore sometimes better to arrange separate sessions for aroma and taste, when it is necessary to take both aspects into account during profiling. This variation of the method is called partial mapping or Napping.
When people who do not regularly participate in wine tasting are used as sensory panellists, it is essential to ensure that they understand the instructions. A “practice session” before the formal sensory evaluation, where tasters practise the mapping exercise, can improve the quality of the data and ease the task for the tasters. The number of wines that can be evaluated with most rapid sensory evaluation methods are limited, because tasters are presented with all the wines at the same time.
Polarised projective mapping is a variation of PM where predetermined wines are used as reference points (also called poles), in order to merge datasets obtained from various tastings. The reference wines are tasted at all tastings and their positions on the plane or paper are fixed and the same for all sessions.
The last article in this series, Part 4, will discuss Check-all-that-apply (CATA) and a comparison of the practical aspects and requirements of all the methods discussed in this series, DA and the rapid methods, is also given, so that the person in the industry can choose the best method for his/her sensory tasks.
This research was funded by Winetech, project IWBT W13/02: “Rapid descriptive sensory methods for wine evaluation – special focus on further optimisation of rapid methods and streamlining of workflow”. The Institute for Grape and Wine Sciences (IGWS), Stellenbosch University, NRF and THRIP are acknowledged for financial support.
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Consultation services offered by the Wine Sensory Facility, IWBT-DVO, Stellenbosch University:
- Selection of appropriate sensory methods.
- Guidance with selection, training, testing of panellists.
- Experimental design and practical workflow.
- Statistical analysis and interpretation of data.
– For more information, contact Jeanne Brand at firstname.lastname@example.org, Hélѐne Nieuwoudt at email@example.com or contact the authors at firstname.lastname@example.org.