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Point of Brew -- Michael J. Lewis

Tuesday, 29 January 2019 01:38
Antarctic coastal glacier Antarctic coastal glacier

Ice is a lot cooler than we think.

I have seen ice sheets melting in the Arctic and in Antarctica and the massive retreat of glaciers in New Zealand, Africa, Europe and North and South America; I have read of warming and rising oceans and the death of corals and experienced droughts and read of floods and monster destructive storms. There is no doubt our heat-engine planet is warming, revving up; it seems to me that understanding and mitigating the worst of warming’s effect might equal the importance of avoiding nuclear war.

There is a tiny squib in this week’s TIME magazine reporting that Antarctica has lost some 250 billion tons of ice per year since 2009. That’s 2,500,000,000,000 tons or two-and-a-half trillion tons in the decade. This rate of loss appears to be accelerating.

Of course ice is very good at cooling things down.

For many years brewers used an ice-bank for cooling boiling wort (malt extract) down to fermentation temperature; these days, in modern busy-busy breweries, that is done in two stages first with cold water to recover a useful volume of hot water then with cold glycol to reach the required fermentation temperature of say 10oC. But the old-fashioned ice-bank had many advantages in its time, not the least of which was that it could do the whole cooling job in a single pass.

This is because ice has a particular property that makes it unusually efficient as a coolant. I have not seen this property mentioned in global warming literature though I am quite sure it is part of the calculations that climate scientists make.

When ice warms up it melts from the solid state to the liquid state; engineers call that a phase change. It turn out that the mere act of changing from solid ice to liquid water at 0oC absorbs a good deal of energy.              

An out-moded but, in this context, useful measure of heat energy is a calorie (cal). That is the amount of heat required to raise the temperature of one gram of water one degree centigrade. That is called sensible heat and can be measured with a thermometer. Incidentally, this is not the so-called food calorie or large calorie (Cal) which is the energy required to raise the temperature of a kilogram of water one degree centigrade. So a Cal is 1,000 cals.

Melting a gram of ice at 0oC to a gram of water at 0oC, although there is no temperature change, requires almost 80 calories (the latent heat of fusion). That is, the mundane act of melting ice at 0oC, uses enough energy to raise the temperature of the same amount of water to 80oC. And so melting a trillion tons of Antarctic ice absorbs enough energy to heat a trillion tons of water from 0oC to 80oC.

That’s hot enough to make tea.

This extraordinary ability of ice at the phase change to absorb heat, of course, is why the old-time brewers found an ice-bank such a good way of cooling hot wort to fermentation temperature. In the same way, I suppose, the melting of polar ice-caps and glaciers around the world has somewhat mitigated the effects of global temperature rise.

Now then, we cannot melt 250 billion tons of ice every year indefinitely. The last mile of the last glacier will melt at some point. Then there will be no buffer between us and warming that must then inevitably accelerate.

Maybe that is part of the tipping point that climate scientists warn of.

Water is the most common molecule on the planet and is a remarkable substance. On the plain face of it water, good old common-or-garden H2O, should not be a solid or liquid under ordinary terrestrial conditions, but a gas with a boiling point of maybe minus 100oC.  The secret is that the bonds between the Hs and O act like little magnets (dipoles). So, like magnets everywhere, water molecules can link up with other water molecules to form liquid water that may be thought of as a mixture of very large molecules made from this linking together of many tiny H2O molecules.

In ice this linkage is fully developed and so ice is an expanded form of water and, being less dense than water, floats. For ice to melt enough energy must be put into the ice to break apart many of these tiny magnetic links. That’s the discussion above.

Another phase change is converting water into steam. Much more energy must be put into liquid water at 100oC to change it into a gas (steam at 100oC) by breaking all the tiny magnetic linkages; this may be the origin of the proverb that “a watched pot never boils”. Importantly steam carries an enormous amount of heat energy that is used in all sorts of ways in many industries. For example, steam is used to boil brewer’s wort for an hour or so as part of normal beer manufacture.   

And that’s a good thing.

 

Michael J Lewis MUG Picmonkey

 

Michael J. Lewis, Ph.D., is professor emeritus of brewing science at the University of California, Davis, and the academic director and lead instructor of UC Davis Extension’s Professional Brewing Programs. Lewis has been honored with the Master Brewers Association of the Americas’ Award of Merit and the Brewers Association’s Recognition Award. He is an elected fellow of the Institute of Brewing & Distilling. He is also a recipient of the UC Davis Distinguished Teaching Award.

Last modified on Tuesday, 29 January 2019 01:51

 

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