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Author Topic: The chemistry of coffee - What happens to coffee when it "dissolves"?  (Read 9289 times)

Offline DoctorBeaver

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OK, so I make myself a cup of coffee. If I'm using ordinary bleurgh-brand instant muck I add milk & sugar. Sometimes I add dried milk or CoffeeMate.

Let's assume that I use the same amount of water whether I add fresh milk or dried. So, adding fresh milk means I am adding to the volume of liquid in my nice "World's Greatest Beaver" mug. Therefore the coffee powder & sugar have more liquid throughout which to disperse.
If, on the other hand, I use dried milk, not only is there less liquid, but more dry stuff too. Obviously this means that the liquid is taking up more dry stuff.

So, what I want to know:-

1) Do the coffee, sugar & dried milk bond with the water or are they merely suspended?
2) How does using fresh milk affect it?
3) Why is it that no matter how much dried milk or coffee I add the liquid doesn't get any thicker? All I get is a sludge at the bottom of my mug.
« Last Edit: 19/05/2007 11:45:35 by chris »


 

another_someone

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I believe that sugar will dissolve, while coffee is a suspension.

Milk is a hybrid - the fat is suspended, while the sugars are dissolved.  Thus, how much of the milk is dissolved and how much is a suspension depends on whether you are using full fat milk (high fat, low sugar) or skimmed milk (low fat, high sugar).  Dried milk is, I suspect, low fat.
 

Offline eric l

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Your three dried products (coffee, sugar and milk) are all three produced from dispersions or solutions in water, but were "dried" in different ways.  This may influence the speed of dissolving / dispersing, and also the capacity to "bind" water by hydratation or forming of double layers around dispersed products.
  • The instant coffee is probably freeze dried, starting from coffee industrially prepared from ground beans.  Part of it (= some of the chemical components) will dissolve, others will disperse, fixating some of the water
  • The sugar will be the result of a crystallization out of a saturated solution, followed by filtration or centrifugation and drying.  Sugar will dissolve - up to the point of saturation that is, if more sugar is added, it will just disperse
  • The milk powder is probably the result of spray drying.  Again, it will bind some water as it disperses.
So we have a dispersion, but with some products dissolved in the liquid phase.

Now viscosity is a complicated matter, and the rules governing the viscosity of a solution are quite different from those governing the viscosity of a suspension.
In a solution, viscosity is a function of the molecular weight of the dissolved product and of its concentration.  Low molecular weight products like NaCl (salt) have almost no influence on viscosity, whatever the concentration is.  Sugar is a dimere (one molecule of glucose + one molecule of fructose).  It will have some influence on viscosity, which will increase more as you dissolve more of it.  But it will not increase to the same degree as with e.g. gum arabic, which is also a polymer carbohydrate, but a long chain molecule.
In a suspension, viscosity depends on the amount of liquid phase available to separate the particles of dispersed material, and prevent them from colliding (which would limit flowability).  This will be expressed by "total volume", "sediment volume" and "free volume".  I think the notions are clear enough not to need further explanation.
Viscosity increase becomes spectacular once the sediment volume becomes higher than the free volume of the liquid phase.
REMARK :  A visible increase in viscosity would mean an increase by a factor 10, rather than an increase with 10 %.  An increase with 10 % would be measureable with the proper instruments, but never visible.

"Bound" water (fixed in double layers around dispersed particles) would be part of the sediment volume rather than of the free volume.

Anyway, I do not think that you make your coffee in such a way that you have more sediment than free liquid, so I do not expect that you would see any "visible" influence on viscosity.

Sorry if this has become lenghtier than I planned.  I hope a cup of coffee will pull you through 
 
 

Offline DoctorBeaver

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Cor... I never knew there was so much to it!!  :o

 

Offline DrDick

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REMARK :  A visible increase in viscosity would mean an increase by a factor 10, rather than an increase with 10 %.  An increase with 10 % would be measureable with the proper instruments, but never visible.


Actually, you can quite easily notice differences much less than 10x, even as low as 10% differences.  You have to look at the absolute viscosity differences as well.  So you might not notice a difference between 1 cPs and 5 cPs (a five-fold change), but a 100 cPs to 150 cPs change (a 50% increase) would be noticeable.

For some reference points:
water = 1 cPs
motor oil = around 20 cPs
ethylene glycol (antifreeze) = around 175 cPs
shampoos, dishwashing liquids = 500 - 1000 cPs
molasses = 3000 - 10000 cPs

Another interesting fact is that, while salt has little effect on the viscosity of water, it has a dramatic effect on the viscosity of detergent formulations, first increasing the viscosity at lower concentrations (~0.1-1%), then decreasing it at higher concentrations (> ~1%).  Without salt in your shampoo, it would be very runny.  Put too much in and it becomes runny again. (Try this at home, kids! :) but use a small portion - you don't want to ruin the whole bottle)

Dick
 

Offline DoctorBeaver

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Dick - thanks, that's really interesting about salt & shampoo. Are there any other combinations of solid & liquid that get more then less viscous as more solid is added?
 

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