Concentration by Freezing

In furtherance to Winter Food.

While reading the maple syrup section of On Food and Cooking, I came across a shocking bit of information.

Even though North American Indians didn't have metal pots until the Europeans came, they had an ingenious method for reducing maple sap to make syrup. They would leave the sap in the cold air overnight. In the morning there would be ice on top. That ice would be mostly (but not exclusively) water, so in discarding the ice they were left with a higher concentration of sugar in the sap.

After reading this, I immediately turned to the section of the book on distilled spirits, to see if there was any mention of whether this method works to concentrate alcoholic solutions. Sure enough, there was a boxed sidenote titled, "Concentration by Freezing," with references to all kinds of liquors that are made in the frosty outdoors and never see the inside of a still.

On the internet, notably Wikipedia, this process goes by the name "freeze distillation." My dad, a process engineer, takes exception to the term, as distillation separates solutes based on their different boiling points. There is a process called freeze crystallization that separates solutes based on their different freezing points, but that doesn't apply here, either. The large difference between the freezing points of water (0°C) and ethanol (-114°C) is not what makes this process work, as evidenced by the fact that the process can concentrate maple syrup, which has no ethanol. I'm going to use the accurate though somewhat anemic word "concentration."


Process

Put your drink in a bowl and put it in a freezing environment like, say, your freezer. Or maybe your backyard.

When I first read about freeze-concentration, I imagined that there would be a clear, easily separated layer of pure ice on top, and a liquor beneath. This is not what happens. Instead, a very slushy network of ice crystals forms, from which the liquor must be pressed.


Results

My first attempt was with beer. I had never heard of beer being freeze-concentrated, but I thought I should try it, since the AGLC has banned strong beer from stores. I used Big Rock Traditional Ale, which is based on (ahem) traditional English ales. It has a dark caramel colour with a hint of red, and is very clear. It has a slightly fruity nose and tastes of caramel and roasted malt. It's one of my favourite Albertan beers.

I froze 700g of beer and pressed 215g of liquor from the ice, meaning roughly two thirds of the beer was wasted.

• Appearance -The liquor was ever so slightly darker than the original beer.
• Taste - The difference in taste was noticeable, but subtle. The sweetness and the roasted, caramel notes were definitely enhanced, though it didn't taste any more boozy than the original.
• Mouthfeel - The beer lost its carbonation.
• Overall assessment - The results were pleasing, but not nearly good enough to justify wasting two thirds of a beer.

Next I tried wine, a sauvignon blanc that my father made. His wine has a surprisingly deep gold colour, and is dry, tart, and slightly grassy.

I froze 330g of wine, from which I pressed 105g of liquor. Again, roughly 30% yield by weight.

• Appearance - The liquor was just slightly richer in colour than the original. This really surprised me, as the colour difference between freeze-concentrated applejack and the original cider is often very striking.
• Bouquet - Freezing really seems to kill the nose...
• Taste - Here I really started to see the benefits of the process. The wine was much sweeter, more alcoholic, and, interestingly, less acidic.
• Mouthfeel: Inconclusive.
• Overall assessment: I could see myself doing this again, especially if I were using a homemade fruit wine. Once thawed, the lower-concentration liquid that froze still has some flavour (extremely dry, acidic) and could probably be used in cooking.

Future Experiments

In terms of percent of alcohol captured from the original drink, freeze-concentrating is not as effective as distillation, but it definitely has its advantages. It preserves the flavour of the original drink much, much better than distillation, which doesn't capture any sugars.

Freeze-concentrating also has advantages over heat-concentrating (ie. reducing over heat). Concentration by heat introduces dark caramel flavours to the liquid. If you had a chance to taste Indian Head birch syrup when the River Valley Syrup Company was selling at the Old Strathcona market, you know that it is a lot darker in colour and taste than maple syrup. This is because birch sap has a lower sugar concentration than maple, and must therefore be reduced more to produce a comparably sweet product. The prolonged reduction resulted in lots of deeply caramelized sugars. That darkness of flavour was certainly one of the appeals of the product, but it would be interesting to try freeze-concentrating. With a lighter syrup you might be able to to compare the flavour of birch and maple on more level ground.