TLDR

What do the streaks in a nosing glass mean?

Sometimes called "legs", "tears" or "church windows" the streaks in the glass with whisky or wine are often claimed to reveal something about the quality of the spirit but this is a misconception. The shape and consistency of the streaks tell us something about the ethanol content of a liquid, and can indicate sugar but are meaningless in determining the quality of the spirit. However they can give an indication of mouthfeel.

The Long Read

Streaks in the glass: a sign of quality?

We are familiar with the picture of wine and whisky tastings: those who consider themselves to be tastings experts wave their glasses and then examine the streaks forming on the inner glass walls with a critical eye (sometimes called “church windows”, “legs” or “tears”). We are constantly told that these allow us to draw conclusions about the composition or even the quality of the liquid. However what the streaks really tell us about the contents of the glass while interesting is quite limited. For the long answer you have to go back a little further…

Streaks, church windows, tears: many names for the Marangoni effect

Anyone who waves a whisky glass with high-proof content will be rewarded with pretty streaks that the liquid draws on the inside of the glass. This is due to an effect that was first scientifically described by an Italian physicist in 1865: the Marangoni effect, named after its discoverer Carlo Marangoni. Though it’s worth noting this was first explained in 1855 by Glasgow physicist James Thomson. The Marangoni effect describes the behavior of liquids with different surface tensions.

Technical description

Marangoni convection is a flow created by differences in interfacial tension. This leads, among other things, to the stabilization of foams. The cause of the different interfacial tension can be e.g. changes in temperature, the concentration of solutes or the charge density (electrocapillarity) along the interface. As a result, the fluid flows along the interface from the site of reduced interfacial tension in the direction of the locally increased interfacial tension, which can be caused, for example, by a reduced detergent concentration. As a result, the surface tension gradient is reduced. A parameter for the Marangoni convection is the Marangoni number.

Streaks form when alcohol evaporates

To put it very simply, whisky consists primarily of alcohol (mainly high-quality ethanol) and water. In contact with air, alcohol evaporates faster than water. So far so good. So what happens when we wave our liquor goblet? Part of the liquid wets the inner walls of the glass and thus forms a larger surface area to the air. In this area, the alcohol not only evaporates faster than the water, it also evaporates faster than that in the bottom of the glass. As a result, the alcohol content on the glass walls decreases, the water content increases as a percentage and with it the relative density.

The streaks “pull” the liquid upwards

Since the alcohol evaporates more quickly on the glass wall than in the rest of the glass, the surface tension of the liquid increases with the density of the liquid film. This is where the effect described by Mr. Marangoni sets in: the liquid with the higher surface tension “pulls” on the one with the lower surface tension, the streaks no longer run down as quickly as they should. This effect can be observed with the naked eye when streaks begin to form in the glass when panning, which seem to defy the laws of gravity. Because the streaks stick to the glass wall for minutes and form striking structures (the typical church windows with their filigree pillars.

The myth of viscosity

If the streaks were only related to the special viscosity (thickness) or density of a spirit, sweet soda and water should actually form more conspicuous church windows than clear spirit. Because of the lower density of the alcohol (typically around 40% of the total volume), this also has a lower overall density.

The desired drinking alcohol, ethanol is not the only alcohol formed during fermentation, higher alcohols are also produced. Higher alcohols have a longer, more branched molecular structure and make the end product more viscous. However, many of these higher alcohols are undesirable (fusel alcohols) and are removed by either controlling spirit cut or filtration such as in the well-known carbon filtration of many bourbon whiskies. At lower temperatures, the longer alcohols also solidify more quickly and can therefore be filtered out of the product (chill filtration). In theory viscosity would be a useful indicator of this however the impact of this is far less pronounced than can be discerned easily by the naked eye.

So what do the streaks tell us about the liquid?

First of all, that there is alcohol in the glass. The shape and consistency of the streaks tell us something about the ethanol content of a liquid, but do not allow us to make any statements about the quality of a liquid that go far beyond that. More alcohol results in thicker “tears” and pointed-arched “stained glass windows,” while less alcohol results in weaker tears and more rounded arches. The Marangoni effect is also influenced by the shape of the glass : Glasses that taper upwards (like our Schnaps glasses ) increase the formation and adhesion of streaks.

As can be seen from the Marangoni effect, the streaks are a sign of power. But the speed at which the streaks flow down the side of the glass and their thickness also gives you an indication of the whiskies’ character. Although viscosity is a different area, it is clear that higher levels of oak extracts and other oils contribute to striae speed and weight. The streaks therefore help you to get an idea of ​​the strength, character and degree of interaction (age) of the whisky. Simply put, the faster the streaks move, the lighter the whisky will be in the mouth; the thicker and slower it is, the heavier it will feel. The inside of the glass can therefore be described as a kind of replica of the inside of the mouth.

There are two other ways your eyes can help judge a whisky’s character. The first is the “pearl test”, you can also do this by covering the glass and shaking it vigorously. The more persistent these pearls are, the higher the strength of the whisky. A mature whisky will not bubble if it is below 50% ABV. Oak extracts will also play a role. For example, new brands do not form pearls. The beads can therefore give you an indication of the barrel’s influence and strength. Again, it’s an example of the Marangoni Effect.

The ultimate use of sight comes when you add water to a mature whisky. If you gently hold the whisky in your hand, you will observe how vortices form in the liquid. These are called viscimetric vortices (the overall effect is called viscimetry). While there’s debate about this - it could be caused by an exothermic reaction of cold water into the whisky, or it could be a visual manifestation of a pressure wave - it’s another clue to the feel of the whisky. The more persistent the viscometry, the higher the firmness and the thicker the mouthfeel.

Exceptions to the rule

There are exceptions to this. It is notable that mature grain whisky has a lower viscometric content than single malt, leading some viscometric researchers to claim they can estimate the grain content in blends. It has also been noted that Japanese whisky generally has lower viscosities than Scotch.