Posts Tagged ‘ice cubes’

Ice cubes and air bubbles

Sunday, April 13th, 2008

Ice cubes are used both to cool drinks, but also to significantly impact the flavour of certain drinks. No matter your motivation, you should never use “old” ice cubes which have been sitting in your freezer for a while. Why? Melt some “old” ice cubes and taste the water. You’ll smell why! The reason is that volatile compounds in your freezer slowly find their way into the ice cubes which for some reason mostly are made in trays without a cover. But as I surfed around, researching this post I discovered that oxo and other producers now sell ice cube trays with lids. That’s a small step forward!

Another thing about ice cubes is that they look nice. I admit that air bubbles can sometimes be quite beautiful (and even artistic when pictured with a macro lens as above), but there are times when I whish I could make perfectly clear ice cubes. At room temperature a certain amount of air is dissolved in water. When you cool water, the solubility of air increases (!), but only until the water starts freezing. At this point the water can no longer keep the air dissolved and a bubble is formed. Vice versa – when you boil water the solubility of air decreases and the dissolved gases escape.

When making ice cubes, the bubbles that are formed can easily escape as long as there is no ice blocking their way. This is sort of a catch 22 situation since the air expulsion is directly related to the ice formation. When making ice cubes in a normal freezer, the ice cubes are cooled from the outside, causing the air to get trapped throughout the ice cube.

Many people have thought about smart ways to achieve this (as a quick patent search shows). There are two strategies to obtain clear ice cubes. Let the gas escape while the water freezes or degas and filter the water before freezing. Icicles are a good example that when running water freezes, it normally produces very clear ice. This is utilized in commercial ice cube makers. Here a “cold finger” is exposed to water that moves. This way bubbles are carried away before they can get trapped. These ice cubes typically are ring or cup shaped. The second method is suggested many places on the net. I’ve listed them here together with some thoughts and discussion.

Degas the water (i.e. remove the dissolved air). This is easily done by boiling the water for a couple of minutes and letting it cool again. Some webpages suggest that the process should be repeated for best results.

Slow cooling
If the water is cooled too quickly, the ice will not be able to push the impurities ahead of the freezing interface. But if an ice cube freezes from all sides it doesn’t really help as the bubbles get trapped in the middle. A drawback with slow cooling is that the solubility of gas will increase when the water is cooled and so it will allow more gas to dissolve before the water freezes. So slow cooling should probably be combined with some kind of gas tight cover.

Directional cooling
I’ve been pondering about making trays with insulated sides and cover and a metal base, thereby utilizing the fact that metals are superb heat conductors compared to plastic, wood or glass. The metal would then serve to conduct away heat from the water. Bubbles would form on the ice front, but they would probably escape, rather than become encapsuled into the ice. I’ve tried to illustrate it here:

Turns out that someone has actually patented something similar where metal “fingers” are used to conduct away heat from the center, giving ring shaped ice cubes. Does anyone know if these were ever made for sale? Perhaps an ice cube tray in aluminum would work if one insulates the top so that the cubes freeze from the bottom and up, keeping the water on top free flowing so bubbles can escape.

Layer-by-layer method
There might be a simple (but time consuming) way of achieving directional cooling: By building up the ice cubes layer by layer. Once the first layer is frozen this will help freeze the next layer from the bottom up and so on. I guess layers of 1-5 mm would work, but this needs more testing. My experiments so far have not been very promising. Plenty of bubbles, even with a layer of only 2 mm.

Particles can act as nucleation sites for air bubbles. To avoid this filter the water and make sure that all the equipment is clean. Also, don’t use a towel to try your equipment as this will probably leave small fibers behind.

Remove salts
Both tap water and bottled water contain trace amounts of salts. When water freezes these minerals are not incorporated into the ice structure. As a consequence the soluble salts will concentrate in the water that’s not yet frozen. In the end there is so little water left that the concentration of the salts becomes sufficiently high so that the freezing point of this remaining water is lower than the temperature in the freezer (meaning that this water won’t freeze). Other salts, especially calcium salts such as calcium carbonate will precipitate. And these particles can act as nucleation sites. If after boiling water there are particles present, these should be filtered away before freezing. The easiest way to get rid of salts is to use distilled water.

I’ve done a couple of experiments and it seems there is no quick fix. The water in the ice cubes pictured above was boiled for several minutes before freezing, but plenty of bubbles formed as you can see. I also tried the layer-by-layer method, but even in a thin layer of only 2-3 mm I could detect many bubbles. So clearly I need to do more experiments.

What are your experiences with making clear ice cubes?

New perspectives on whisky and water

Sunday, June 3rd, 2007


Among dedicated whisky/whiskey drinkers it is customary to add a little water as this “helps to unlock and release the esters, or flavours, from the fats”. Another site claims that dilution helps “breaking down the ester chains and freeing the volatile aromatics”. Does this make sense from a chemical perspective?

When Erik posted me a question some months ago about why we add water to whisky and the chemistry that is involved, I started to speculate about possible mechanisms and discussed them with Erik. Perhaps the most obvious effect is that the alcohol concentration is lowered. High alcohol concentrations anaesthetises the nose and sears the tongue (as the site metioned above correctly states). This is especially true for cask strength whisky which can exceed 60% ethanol. We considered the possibility of a temperature effect. The obvious effect could be achieved by adding water with a different temperature to either cool or warm the whisky. The less obvious effect could be due to a possible release of heat when adding water to a concentrated ethanol solution. Having thought about the different possibilities I did a search and found a very fascinating article: “Release of distillate flavour compounds in Scotch malt whisky”. It was published in 1999, but was new to me and gave me some totally new perspectives on whisky and water. When reading the article, it seems to me that the motivation for adding water to whisky is in fact to mask some aromas and release others!

Malt whisky contains high concentrations of esters and alcohols with long hydrocarbon chains. When water is added, the solubility of these esters and alcohols decreases, and a supersaturated solution results. In extreme cases, the decreased solubility of fat-soluble, volatile organic compounds can lead to clouding due to precipitation of small droplets as seen with anise/liquorise liqours such as Pastis, Pernod, Arak, Raki, Sambuca, Ouzo… (I think I’ll post about that later some time). This can also occur with whiskys that haven’t been chill-filtered. But even in whisky that has been filtered at low temperature a form of “invisible” clouding will occur. The excess of esters and alcohols in the diluted whisky form aggregates (or micelles) which can incorporate esters, alcohols and aldehydes with shorter hydrocarbon chains. Once these compounds are trapped in the aggregates, surrounded by longer chain esters and alcohols, they smell much less since they have a harder time escaping from the liquid! Fortunately, some of the compounds that are trapped have less desireable aromas described as oily, soapy and grassy.

The presence of wood extracts originating from the aging in oak barrels also influences aroma release. One effect is that wood extracts displace hydrophobic (fat soluble) compounds from the surface layer of the whisky (this effect is significant at room temperature when smelling the whisky, less so at 37 °C in your mouth). Furthermore the presence of wood extracts increases the incorporation of hydrophobic compounds into the agglomerates mentioned above.


So far I’ve only discussed the aggregates formed by long chain esters. But studies have shown that when an aqueous solution contains more than 20% ethanol, the ethanol molecules aggregate to form micelles, just like the long chain esters do. These micelles can also trap flavour compounds. Unlike the micelles formed by the long chain esters however, the ethanol micelles break up when diluting the whisky, thus releaseing the entrapped flavour compounds. It is interesting to note that ethanol is less “soluble” in water at high temperatures (ie. the solution is no longer monodisperse). As a consequence, serving whisky “on the rocks” will actually promote the release of flavour compounds from the ethanol micelles. As Mirko Junge commented below, this is one of the very few cases where cooling actually enhances flavour! But the wood extracts found in whisky matured in oak casks supports the formation of ethanol micelles, so as Mirko Junge points out, matured whisky needs more dilution and/or cooling since there are more ethanol micelles.


The over-all effect is a fractionation of volatile compounds upon dilution with water: water insoluble compounds are concentrated in the aggregates (or micelles) of long chain esters, water soluble compounds remain in solution and compounds (probably those which are slightly soluble in water) that were originally trapped in ethanol micelles are liberated.

So after all, the popular notion that addition of water “opens up” the aroma of a whisky is true, but who would have thought that the effect is a combination of “masking” (inclusion of some arome compounds in long chain ester micelles) and “demasking” (opening up of ethanol micelles) and that there even is a temperature effect?

Serving whisky “on the rocks” helps break down ethanol micelles due to the combined effect of cooling and dilution. (Photo by Generation X-Ray at

Feel free to share your experiences with whisky dilution in the comments section below!

(Note: The text has been revised and expanded on June 3rd following the discussion below. Special thanks to Mirko Junge for his valuable comments and for pointing out the importance of the ethanol micelles.)