Freeze distillation of hard apple cider: A phase diagram demonstration

This is an in-class demonstration for introductory Petrology

This demo can be used as a first introduction to eutectic binary phase diagrams, or as a focus for in-class discussion after they have been introduced.

I do this demo during one of the first few weeks of Petrology, as part of a unit on binary phase diagrams.

This demonstration and class discussion is simply to help develop 'phase diagram thinking' and to explore the binary eutectic system.

"It's indeed bad to eat apples, it's better to turn them all into cider"
Benjamin Franklin

Apple cider was an important staple in colonial America, and in New England it was commonly used to make hard apple cider, also known as applejack. During the winter freeze distillation was used to concentrate the small amounts of alcohol in fermented cider was concentrated to make a drink with up to 20-40% alcohol. When temperatures drop below 0°C water ice begins to form and is removed by the producer, increasing the alcohol content of the remaining cider. In northern areas with cold winters making applejack was a common cottage industry in the 18th and 19th centuries, so much so that it was used as an early description of fractional crystallization and differentiation in igneous systems in the geological literature:

A familiar instance is the freezing of weak alcoholic liquids. A bottle of wine or a barrel of cider exposed to a low temperature deposits nearly pure ice on the walls, while a stronger liquor may be tapped from the center. If a still lower temperature were applied the central and more fusable portion would also solidify. Such a mass would be, so far as I can see, a very perfect analogue to a laccolite.

Becker, G. F. (1897). Fractional crystallization of rocks. American Journal of Science, 4th series, 257-261.

Becker's example of a freezing barrel of cider as an analogue for a zoned magma chamber probably speaks to an expectation that his late 19th century readers would be familiar with the practice. With the recent upswing in hard apple cider popularity in the early 21st century most students are familiar with alcoholic ciders and similar drinks. The link to craft brewing and the favorite drink of the Founding Fathers makes for an effective and interesting introduction into how binary phase diagrams work, and is a good starting point to thinking about fractional crystallization.

Brady (2004) describes hard apple cider and freeze distillation as one of his examples of 'kitchen chemistry' binary phase diagrams (http://serc.carleton.edu/NAGTWorkshops/petrology/teaching_examples/25370.html), along with other classroom-friendly binary systems such as brine and popsicles. Other good in-class and lab exercises for teaching binary phase diagrams can be found here: http://serc.carleton.edu/research_education/equilibria/index.html.

Brady (2004) notes practical difficulties in using freeze distillation of alcohol-water mixtures in a classroom setting, because after freezing the liquid-ice mixture is difficult to separate because the ice crystals are small and the interstitial liquid is dispersed between them. I also found this to be the case when freezing alcohol-water mixtures in a home or laboratory freezer. However, I discovered that freeze distillation works well as a class interactive demonstration when I froze the sample overnight during the winter outside– the slow growth of ice at slightly higher temperatures than most freezers forms fairly large crystals and a more easily separated liquid (Figure 1).

As a starting material I use a 12 oz bottle of Angry Orchard® hard apple cider, which has an alcohol content of 5%. Overnight freezing yielded a ice-cider mixtures with ice:liquid ratios ranging from 1:3 to 6:1. In general the ice formed bladed crystals at the top of the jar used for freezing. The ice and cider are relatively easy to separate, except for samples collected at <-13°C when >ca. 75% ice forms, which tends to trap a lot of cider between ice crystals (Figure 2). Temperature logs (from a weather station seven km away) for seven successful trials show the ice and cider percentages at the morning temperature when the experiment was examined (Figure 3).

If you teach in an area that allows you to freeze apple cider outside and bring it into the classroom I would suggest this method over using a freezer. I have found that cider frozen overnight can be stored in a freezer until it is time for class, however, because the outside-formed ice is already separated from the cider and the ice can still be (relatively) easily removed.

Outside-frozen hard apple cider turns out to be a great demonstration in a classroom setting. The ice can be removed from the cider using a kitchen sieve, and the hard apple cider had a beautiful amber color and a very strong smell of alcohol. I use this demonstration as a way to explore the ethanol-water phase diagram (Brady, 2004) and to introduce different ways of thinking about liquid and solid compositions during crystallization. In my experience it is obvious to a class of college students how taking newly-formed ice out of the barrel of cider makes the remaining cider more alcoholic, and this makes for a fun and accessible entrée to understanding fractional crystallization.

Hard apple cider makes a good in-class demonstration, but I have not been successful in using this method to build a binary phase diagram (cf. Brady, J Geoscience Education 1992). I think that it is impossible to quantitatively remove all the cider from between the ice crystals, and with the changing temperatures of the experiment (especially near the end when it is brought into a warm building) the resulting data are too imprecise. Out of seven trials, only two were within 5% of the ethanol content predicted by the liquidus at the temperature to which the sample was frozen. Although the experiments with <75% ice were broadly consistent with the ethanol-ice liquidus, they couldn't be used to determine it with any accuracy.

A few words of caution: It appears that using freeze distillation to increase the alcohol content of any beverage but beer (which is regulated separately) is illegal in the US without a permit. That being said there is a very active community of home brewers online who have differing opinions on this issue. One of the slightly pedantic arguments made is that freeze distillation is not a form of distillation at all, but rather should be called freeze concentration because the alcohol is not being removed (as in a still), but is being concentrated. Indeed, this real distinction is perhaps the best reason to encourage students to 'not try this at home', because along with sugars and ethanol toxic trace methanol will also be concentrated by freeze distillation, perhaps to dangerous levels.

Phase diagrams from kitchen chemistry: http://serc.carleton.edu/NAGTWorkshops/petrology/teaching_examples/25370.html
Teaching phase diagrams: http://serc.carleton.edu/research_education/equilibria/index.html
Making a binary phase diagram as a class or lab activity:
http://www.science.smith.edu/~jbrady/Papers/Halite_Ice.pdf