Statistical analysis provides guidance in scotch tasting.
SCOTLAND, BLESSED LAND OF WHISKEY! There it is called scotch, and there are several kinds. Single malts are made from fermented barley at a single distillery; blended whiskeys are mixtures of different kinds of whiskey that may come from different parts of the country. Obviously the single malts are preferred by connoisseurs, who scrutinize them with regard to five crucial qualities: nose, color, body, mouth, and finish.
Does the provenance of a single malt determine its organoleptic qualities? If it does, can types of scotch be associated with particular environments (or terroirs)? To find out, François-Joseph Lapointe and Pierre Legendre at the University of Montreal analyzed data collected by taster Michael Jackson, who has described, tasted, and judged the roughly 330 single malt whiskeys produced by 109 of the most prestigious distilleries in Scotland.
Characteristics of Single Malts
Statistical analysis of single malts proceeds from the fact that their distinctive characteristics assume several forms. Thus the nose may be aromatic, peaty, light, sweet, fresh, dry, fruity, grassy, salty, sherry-flavored, spicy, or rich. The body may be smooth, medium, full, round, honeyed, light, firm, fat, and so on. What is the best way to divide the population of single malts into groups of analogous individuals in order to determine whether they come from the same terroir? The University of Montreal statisticians simplified the problem by considering only one scotch per distillery, for a total of 109 samples.
To quantify the various sensory impressions, or characteristics, produced by these scotches, they assigned to each one a value of 1 if it was present in a single malt and 0 if not. They then displayed the statistical data in the form of a table by arranging the individual scotches in rows and the characteristics in columns.
Next they calculated the numerical distance between pairs of single malts by dividing the number of characteristics common to the two whiskeys by the total number of characteristics identified in either one of the two; the distance between them therefore is equal to 1 minus this parameter. The set of these distances was then used to form a new matrix, where the value of a given compartment represents the distance between two whiskeys linked with one another by row and column. The smaller this distance, the closer the whiskeys to each other.
In order to classify individual whiskeys, Lapointe and Legendre partitioned the population by aggregating the closest individuals in pairs. This aggregate is then considered to be a new individual that replaces the two aggregated individuals, whose characteristics are averaged to arrive at the value of the new individual’s characteristics. Proceeding in this fashion until all individuals have been related to one another as members of a single family, one winds up with a dendrogram (or tree diagram). Different partitions can be obtained by making a “cut” at a given distance from the root: The nearer to the root the cut is made, the smaller the number of classes.
In Search of Class
The dendrogram obtained for single malts divides into two branches. On one side are golden whiskeys having a dry, smoky body. On the other are amber whiskeys that have a light body, smooth in the mouth with a fruit finish. The first group, consisting of 69 whiskeys, was then subdivided into amber scotches, which are full-bodied, fruity, fat, and spicy, and golden scotches, which have a smooth and light body with a grassy finish. The farther away one goes from the root, the smaller the groups.
Let’s come back to the geographic component. Scotland is divided into three scotch-producing regions: the Highlands, the Lowlands, and the Isle of Islay. These three regions are divided into thirteen districts. The researchers constructed a new table in which the 109 scotches were again arranged by row and column. The compartment at the intersection of a row and column contains a 0 if the corresponding scotches are from the same district, otherwise a 1.
Comparing this matrix with the one that generated the dendrogram, one observes that the division into regions of production corresponds to the tree diagram division, whether six or twelve groups are considered. These groups correspond to terroirs. What is more, this distribution confirms that the water, soil, microclimate, temperature, and overall environment are indeed the determining factors of the characteristics of single malt whiskeys, as upholders of the terroir theory maintain. The secrets and traditions of individual distillers account for only small differences by comparison.
Finally, the Canadian researchers sought to relate the five types of characteristic to one another. It was clear that nose, color, body, and mouth are not independent characteristics: The color of a scotch is related to its nose and its body, and the nose is related to the mouth and, to a lesser degree, the body. The finish, on the other hand, which is to say the impression that is left in the mouth when one has finished drinking, depends on neither the nose, the mouth, the body, nor the color. The familiar method of tasting that consists of spitting out the beverage after its color, nose, body, and mouth have been judged therefore is open to criticism because it neglects a fundamental parameter.