QotW: Why does coffee dissolve so fast?

07 July 2020

COFFEE-BEANS

A cup on its side, with coffee beans spilling out of it

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Question

How do coffee granules dissolve so well compared to, say, soup in a bag for example? The difference is amazing, coffee is dissolved with minimal or no stirring!

Answer

Adam Murphy poured over the answer with Cambridge University chemist Ljiljana Fruk...

Adam - Yeah, when you pour in that water, the instant coffee disappears, ready for that first sip

[SIPPING NOISE]

Ugh, I don’t like coffee.

Ljiljana - But, thankfully, University of Cambridge chemist Ljijana Fruk does, and knows how it dissolves so quickly

Ahhh.. that beautiful smell of coffee in the morning, and that caffeine kick  that helps us to get to that early morning zoom meeting. But sometimes, we just do not have time for the coffee to get brewed and we need some more  instant. Instant coffee!

And making a nicely soluble instant coffee required a bit of chemical engineering.

Coffee granules we dissolve in hot water are nothing else but brewed coffee, which was dehydrated: the water was removed either by process of spray- or freeze drying. These processes result in fine powder, which is not particularly soluble: it binds the water and clumps, and  it doesn't really look nice. And it also doesn't dissolve readily. 

Adam - That means you need to do something to those particles to make them dissolve so quickly, but what could possibly jump in here?

Ljiljana - Enter granulation: a process by which fine powder particles are made larger, they agglomerate into bigger and more porous particles which have larger surface and are easier for the water to flow. There is a size, which is considered very good in terms of solubility of granules,  and this is usually around 250 micrometers. 

Adam - That explains coffee, but what about instant soup. Which I do like.

Ljiljana Soup is a bit different. It contains many different ingredients. All of these: many ingredients with different properties, use of binders, presence of starch makes uniform granulation more difficult, and results in instant soup that requires hot water and mechanical mixing to dissolve, unlike our homogeneous and granulated instant coffee

Adam - Thanks to Ljiljana for that solution. Next week, we’ll be answering this question, from Carol

Carol - The government has provided  more than a billion items of PPE to hospitals. There have been 130,000 covid cases in hospitals, about 10,000 items of PPE per patient. Can you find out why so much?

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