Note: this is a first draft and I may or may not take the time to clean it up later.
WTF is bruised gin? Exactly. What indeed. A twitter friend is training to be a bartender and mentioned that if you shake a martini, you will bruise the gin. That sent me on a quest for knowledge. What I found was a lot of anecdotes, myth, and misinformation.
Depending on who you ask, bruised gin has a different flavor, or texture, or appearance and is caused by shaking the martini mixture instead of stirring it. I had to wade through a lot of James Bond references and his preference for a shaken cocktail. Here’s a quick guide to what I found in about an hour’s worth of research.
The only scientific research I found on the subject is a paper in the 18 December 1999 issue of the British Medical Journal titled “Shaken, not stirred: bioanalytical study of the antioxidant activities of martinis” which found that shaken martinis had more antioxidants than those that were stirred. However the paper did not explore whether or not this had any effect on the taste, appearance, or texture of the drink.
All of the other readily Googled information was strictly an opinion of its author. As might be expected, they were prone to draw conclusions without controlling for all the relevant variables. I may make the same mistake, but I’m at least attempting to approach this from a technical standpoint. Keep in mind that’s it’s been 25+ years since I studied chemistry, physics, and thermodynamics. With that caveat, onward we go.
In mixing a martini, the general process is to add the gin, vermouth, and ice to a mixing vessel. Shake or stir to combine the ingredients, then strain the liquid into a chilled glass. So, how does the choice of shaking versus stirring affect the outcome?
Taste could be affected by the addition of water from the melted ice, temperature of the final product, and potentially the difference in antioxidants. Many commenters stressed that the shaken martini would be colder than the stirred version. That may be true, but it is dependent on the mixing time (assuming all other variables are constant). The temperature of the drink is determined by how much energy is transferred from the warmer gin and vermouth to the colder ice. Shaking the mixture will allow more of the liquid to come in direct contact with the surface of the ice than will storing. If the mixing time is held constant, then the shaken drink will be colder. The key point is the serving temperature of the drink, not how the temperature was achieved. Also, this colder drink will include more water. To make the liquid colder, more energy was transferred to the ice, which melts it to water. One poster insisted that a stainless steel (or even better, silver) shaker would make the drink colder that would a glass shaker. That’s just completely wrong.
Texture and appearance can differ due to small bits of ice and micro bubbles that could be present in the shaken drink. The action of shaking the mixture can break of bits of ice and aerate the liquid. No big mystery here. Just like a fresh soda will taste different than a flat soda, the martini with the micro bubbles will have a different texture and taste. The ice bits and micro bubbles can also give the drink a cloudy appearance.
I can’t address the question of whether or not the difference in antioxidants has a perceptible effect on the drink. I may do more research on that point.
In writing this my focus shifted a bit from just gin to the martini in general. The martini includes vermouth as a key ingredient and I didn’t do any research on “bruised” vermouth.