A Brazilian engineer discovered that glasses with a wider top that narrow to the base are the best for keeping beer cool.
Cláudio de Castro Pellegrini, a mechanical engineer at the Federal University of São João del-Rei, spent a day at the pool when he came across the problem that would inspire a scientific study: his beer was already getting hot.
“In the most critical scenario, like at the beach on a hot day [38 ºC], just 3 minutes can be enough to make the beer undrinkable,” he considers.
To try to alleviate this problem, the scientist got to work and decided to try to understand the ideal shape of glass to keep beer cold. It was initially published in 2019 in the Brazilian Journal of Physics Teaching and was improved more recently, in October 2024.
The first factor is avoid heat absorption as much as possible — that is, use a container with the smallest possible surface area in relation to volume. The author also took into account the fact that the emptier the glass becomes as you drink, the more surfaces of the glass come into contact with the external environment.
In three dimensions, the shape that has the smallest surface area in relation to its volume is the ball. But a spherical cup would not be practical and would not hold upright.
To create the equation, the author had to ignore some issues that could influence the temperature, such as the heat that would be transferred from the person’s hand and the thermal insulating actions of beer foam, says .
However, he added other practical requirements: the glass must be symmetrical and the base of the cup would have to be flat and stable enough for the cup to hold. Pellegrini also assumed that the base of the glass would be a perfect thermal insulator and that the density and temperature of the drink would be uniform.
All of this resulted in the following formula:
T represents the temperature of the beer, ρ the density, V the volume, hCV the convective heat transfer coefficient, cₚ the specific heat capacity, T∞ the ambient temperature and Aₜₒₜ is the exposed surface of the beer.
The engineer initially calculated the quotient Aₜₒₜ /V, equated it to zero and, using the properties of bodies of revolution, calculated a formula for the ideal radius (r) of a perfect beer glass as a function of height (h).
“Want to know what a ‘great of the great of the great’ glass would be like? It would be a evacuated double wall mugor made of porous ceramic which is cheaper, with a shape given by my formula, a handle to isolate heat from the hand and a lid to reduce convection through the opening. It will be very ugly but very good”, reflects Pellegrini.
While this glass, “great of the great” does not yet exist, the formula indicates that the ideal glass must have a wide top and get narrower towards the base.
But the engineer has not yet given up on his search for the perfect glass and intends to include in future projects factors that were not taken into account in this study, such as heat exchange through the bottom and beer foam.
