Thursday, January 2, 2014

Chemistry of Beer, Part II: Freezing Point Depression and Fractional Freezing

I'm sitting here in Pittsburgh, watching the snow fall and thinking about how cold it is outside.  Often when traveling to visit friends or having friends visit during the winter months, someone will ask a question like, "I have a bottle of wine in the trunk of my car.  Can I leave it in there overnight or will it freeze?"  The answer depends on both the outside temperature as well as on the nature of the wine itself.

Throwing a bottle of white wine or a couple of beers into the freezer to cool them down quickly is a common practice.  Because of this, many people probably already recognize that alcoholic beverages like beer or wine take longer to freeze than water.  However, as anyone who has left beer in the freezer for too long will also already know, beer will freeze.   So will wine.  Perhaps confusingly, other alcoholic beverages like vodka can be kept in the freezer permanently, because they just don't freeze at normal freezer temperatures.

Why does alcohol lower the freezing point of liquids and why do different types of alcoholic beverages have different freezing points?  This is a question that I wanted to explore in this blog post.  The answer has important implications for everyday life that reach far beyond the realm of beer.  Like the previous post on beer, this topic gives us a vehicle to explore some very general scientific principles.


Beer, wine, and other alcoholic beverages are solutions.  They are made up of a variety of chemical compounds dissolved in a solvent.  The chemical compounds dissolved in the solvent are called solutes.  In the case of beer, wine, and liquor, the solvent is water and the solutes are all of the chemicals that make up the flavor and color of the beverages.  This includes sugars, proteins, gasses, and all kinds of other compounds that come from the the grains and the fermentation and/or distillation.  The most abundant solute is ethyl alcohol (also called ethanol), which is the alcohol in beer that is the by-product of fermentation as we previously discussed.

As increasing amounts of solute are dissolved in a liquid, the freezing point of that liquid goes down.  The process by which solutes dissolved in a solvent decreases the freezing point of a solution is called freezing-point depression.  This occurs becasue a solute dissolved in a solution helps to stabilize the liquid phase of the solvent.  A chemist would say that solutes increase the chemical potential of the liquid state.  This means that, compared with the pure solvent (e.g., water) the solution (e.g., beer) must be cooled to a lower temperature in order to freeze, or change from the liquid state to the solid state.

Crystal Structure of Ice (from Wikimedia Commons)
When water changes state from a liquid to a solid, the water molecules orient themselves into a very specific pattern to form an ice crystal.  This pattern is called the crystal structure.  The polar oxygen molecules (shown in red) and non-polar hydrogen molecules (shown in white) orient themselves toward each other in a specific pattern where each water molecule interacts with 4 other water molecules.    

The presence of a dissolved solute actually dillutes the solvent (a chemist would say that it reduces the mole fraction of solvent), which reduces the ability of solvent molecules to come together to form a crystal as well as impedes the ordered arrangement that the water molecules like to get into when forming a crystal.

The freezing point depression in beer, wine, or liquor is primarily a result of the dissolved ethyl alcohol.  The other solutes contribute a little, but they are found in very small concentrations relative to the ethanol.  Nonalcoholic beers (beers with 0.5% alcohol or less) typically have a freezing point near that of pure water, showing that most of the freezing point depression effect is due to the presence of the alcohol.



Shown to the right is a chart of the freezing points of solutions of ethanol in pure water.  Temperature is plotted here using both the Farenheit scale (where pure water freezes at 0 °F at standard atmospheric pressure) on the left  and the Celsius scale (where pure water freezes at 0 °C at standard temperature and pressure) on the right.  As the ethanol concentration goes up (in other words, as the number of ethanol solute molecules in the water solvent goes up), the freezing point goes down.

The dots on the graph show specific freezing points that have been experimentally measured.  Basically, everything at temperatures above the line will be solid and everything at temperatures below the line will be liquid.    



The dip in the curve that you see around 93% ethanol is called the eutectic point; that's important and we'll get to that in a moment.  First, though, let's look more closely at the range of ethanol concentrations found in alcoholic beverages.  

Typical freezer temperature is around 0 °F (-20° C), while a 40% ethanol solution (80 proof) has a freezing point of around -20 °F (-30 °C).  Both wine and beer have much less ethanol in them than most liquors.  Wine has a typical ethanol content of 10-20%, while beer has a typical ethanol content of around 3-8% but sometimes can have up to 15% ethanol.  The equates to freezing points of 23 °F (-5 °C) to 13 °F (-11 °C) for wine and 30 °F (-1 °C) to 21° F (-6 °C) for beer.

The higher concentrations of ethanol in liquors like vodka or whiskey (35-45%) are what allows these liquors to be stored in the freezer or in trunk of your car on a very cold night and not freeze solid.  Wine and beer, however are more tricky.  Depending on the alcohol content and the temperature, they certainly can freeze on a cold winter night or if left in the freezer too long.



Let's get back to that dip in the ethanol/water mixture freezing point curve that you see around 93% ethanol.  This is the mixture of water and ethanol that has the lowest freezing point, called the eutectic point.  A eutectic mixture is a mixture that solidifies or freezes at a lower temperature than any of the pure ingrediants.  The eutectic mixture is special because at this mixture, water and ethanol will both freeze together.  At other points on the graph, reaching the freezing temperature will results in crystals of water forming with less solute (ie, ethanol) than the remaining liquid.  This is because, when ice forms, the water molecules don't want to make room for solute molecules inside of the ice crystals.  This is the basis of a technique called fractional freezing, or freeze distillation.

In fractional freezing, beer, cider, or other fermented beverages are cooled enough to where the water begins to freeze.  It is thus possible to remove some of the water by separating the ice crystals from the remaining liquid containing most of the dissolved solutes and ethanol.  This is a method of concentrating both the flavor and alcohol content.  German brewers use this method to make eisbock beers (typically 10-13% ethanol) by freezing bock beers (a strong type of lager, typically brewed at 6% ethanol) to raise the alcohol content.

The Canadian Labatt brewery initially looked at fractional freezing as a way to concentrate beer for shipping.  The thought was that water could be added back to the beer before it was sold.  However, while this didn't work out as planned, it did start a marketing trend based around fractional freezing.  Beers that are subjected to some degree of fractional freezing are sometimes marketed as "ice" beers, such as Bud Ice, Natural Ice, etc., though often times water is added back to such beers to keep the alcohol content within certain legal requirements.  This is because, in the US, it is illegal to sell anything that could be considered "beer concentrate."

Ice wines also undergo factional freezing, but unlike ice beers, ice wines are made with grape juice that is frozen before fermentation to concentrate the sugars and grape flavor of the juice used to make the wine.

Because the ethanol and water freeze together at the eutectic point, this is the theoretical limit to how much you could concentrate the ethanol in  a solution of ethanol and water using fractional freezing.  However, in reality, fractional freezing is not a very efficient method of concentrating beers and is never used to concentrate beer anywhere near its theoretical maximum.  When the crystals of water freeze during fractional freezing, they do contain some alcohol in side them.  While the alcohol content of the frozen crystals is much less than the alcohol content of the remaining liquid, more and more alcohol and other solutes (including those that make up the flavor of beer) are lost in the ice crystals with each round of fractional freezing.    

The phenomenon of freezing-point depression occurs with all kinds of solutions, not just in alcohol/water solutions.  For the most part, freezing point depression is dependent on the number of solute molecules dissolved in the given volume of solvent (ie, the concentration of the solute).  Freezing point depression is thus usually described in chemistry as a "colligative property," which is a term used to describe a property of a solution that is dependent on the number of solute molecules and not their identity.  However, there are some instances where the nature of the solute does make a difference due to interactions of solute molecules with other solute molecules or with solvent molecules, but for the purposes of this blog post, we can ignore that.

Freezing-point depression is the reason why salt is added to icy roads.  The salt dissolves in the water/ice and lowers the freezing point, keeping the water liquid at colder temperatures and preventing ice from forming.  Because sea water already has a lot of salt in it, it can remain liquid at temperatures well below 32 °F (0 °C).  In general, the more salt that is added to an icy road, the lower the temperature at which ice will form.  However, the lowest freezing point that can be obtained with salt dissolved in water is about 0 °F (-18 °C), and this is only with water containing a lot of salt dissolved in it.  Road salt is thus ineffective if temperatures drop below this point.

P. crucifer (image from US Geological Survey)
Freezing-point depression is also an important chemical concept in the animal kingdom.  One example is the spring peeper frog, Pseudacris crucifer, which is commonly found throughout the US and into Canada.  This type of frog frequently has to survive cold temperatures in the northern US and Canada, and its body has evolved to respond to freezing temperatures by breaking down large amounts of glycogen (a macromolecule made up of large chains of glucose that your body uses to store energy) in its liver.  The breakdown of glycogen results in a large increase in the glucose concentration of its body fluids.  This increase in solute concentration helps to prevent the body fluids of this frog from freezing solid at low temperatures and helps the frog survive.        

Dissolving a solute in a solvent does more than just lower the freezing point.  It can also increase the boiling point of the resulting solution.  This is called boiling-point elevation and occurs when a non-volatile solute (i.e., one that does not escape into the gas phase during boiling) is dissolved in a solvent.  A solution containing a nonvolatile solute has a boiling point higher than that of the pure solvent.

Ethylene glycol is a solute found in car antifreeze/coolant mixtures that carry heat away from the car engine block to the radiator, where the heat can dissipate.  Ethylene glycol protects against both freezing of the antifreeze during the winter months and boiling of the antifreeze due to the high temperatures of the engine.  Pure water has a freezing point of 32 ° F (0 °C) and a boiling point of 212 ° F (100 ° C).  A 50:50 water:ethylene glycol mix can lower freezing point to -35 ° F (-37 ° C) and raise boiling point to 223 ° F (106 ° C).  A 70:30 mixture of ethylene glycol:water can change freezing and boiling points to - 67 ° F (- 55 ° C) and 235 °F (113 ° C), respectively.  Note that the magnitude of the freezing-point depression is typically much greater than the magnitude of the boiling-point elevation, as seen here.

The high concentrations of ethylene glycol in antifreeze make it work really well for increasing the boiling point and decreasing the freezing point of antifreeze, but because ethylene glycol causes damage to the kidneys and other organ systems, it also makes antifreeze very toxic to both humans and other animals.  Thus, antifreeze solutions should always be kept away from children and pets.

The real phenomenon of boiling-point elevation has led to a common misconception that adding salt to a boiling water to cook pasta will speed to cooking by increasing the temperature at which the water boils through boiling-point elevation.  In reality, adding a teaspoon of salt per 32 ounces (approximately 1 liter) of water increases the boiling point by less than 0.5 ° F, which makes negligible difference in cooking.  What it does do, however, is season the pasta so it tastes better.  

Overall, freezing-point depression and boiling-point elevation are important chemical concepts with broad effects on our world.  Freezing-point depression in particular lets brewers make stronger beers but more importantly also lets us melt snow and ice off of roads to travel safely.  As I've said before on this blog, there's a lot of interesting and broadly-applicable science to be learned by looking at some of the basic properties of beer.  


© 2014, TheMadScienceBlog.


Sources
  • W.A. Hardwick.  "The Properties of Beer."  Ch. 19 in Handbook of Brewing.  Ed. Willam Hardwick.  CRC Press.  1994.  
  • E.W. Flick.  Industrial Solvents Handbook.  Fifth Edition.  Noyes Data Corporation.  1998.  [Ethanol/water mixture freezing points came from here]




10 comments:

  1. So will wine. Perhaps confusingly, other alcoholic beverages like vodka can be kept in the freezer permanently, because they just don't freeze at normal freezer temperatures. John Nolan

    ReplyDelete
  2. Thanks! This was fun to read and think about. Very accessible and nicely organized! Also, way to represent the Burgh!

    So, I'm curious, is it possible for the solutes to settle out due to difference in density if the mixture sits long enough? If not, why not? What sort of molecular adhesion exists between the ethanol and water? How strong is such a force?

    ReplyDelete
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