Real-Value & Metaphor Smell includes information on how much a smell sample can be diluted to reach the human nose threshold, the lowest concentration which human noses can detect. The odour index is defined as 10?log10 (dilution rate). This formula was derived by the careful analysis of the human olfactory recognition system [20]. It has been shown that humans can not differentiate the intensities of smells by their liner concentrations. The index 10 implies that most humans can detect smells in 10-fold dilutions, and an index of 20 indicates a 100-fold dilution. Using this index, odour from chewing gum were set around 40 and restrooms odours were set around 30 in our previous study [21]. These odour indices help us record and imagine smell intensities as absolute values. Our second consideration was the criteria of smell specification. We proposed describing this aspect of odour using known smell categories provided by standard gasses (metaphor expression). Using the known smell information provided by standard gases, the amount of accessible information we can use for describing smells can increase dramatically. There are, for example, two common ways to describe the flavour of tea. ‘This flavour is sweet’ and ‘This flavour is like muscat’; the latter metaphor description enables us to imagine the flavour better. We enabled an electronic nose, FF-2A (Shimadzu Corporation, Japan) [21,22], to report on both intensity and specification. The device, therefore, can calculate a virtual odour index in terms of standard gas categories. The FF-2A electronic nose recognized odours and calculate the odour indices as described below. The device contained 10 metal oxide semiconductors sensors with different sensitivities and selectivity for different fragrant substances (Figure 1a) [22]. These sensors were standardized with 9 standard gases (hydrogen sulphide, methylmercaptan, ammonia, trimethylamine, propionic acid, butylaldehyde, butylacetate, toluene and heptane). From the Figure 1. Measurement scheme and calculation methods. (a) Nine standard gases were introduced to the smell sensor array in FF-2A electronic nose system. The FF-2A used multivariate analysis to calculate the vectors of the standard gases. After yeast volatile samples were introduced and their vectors were calculated, the standard gas vectors were used to calculate the indices and similarities of yeast volatile samples. (b) Calculation method of odour index, total odour index and similarity. After all the vectors were calculated (left), the indices were calculated using virtual sample concentrations for each axis (right). Similarity was calculated using the angles between the different sample vectors. doi:10.1371/journal.pone.0007939.g001 PLoS ONE | www.plosone.org 2 November 2009 | Volume 4 | Issue 11 | e7939