h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I don't know anyone has ever noticed this phenomenon, but it seems that when you buy coffee (or tea) from coffeeshops such as Costa or Starbucks, when you put your sugar in, it just sinks right down to the bottom instead of dissolving into the coffee or tea, even if you actually stir it. Instead, it just forms a sugary solution at the bottom of your cup.

Why does this happen with coffeeshop coffee (or tea) and doesn't happen when you make tea or coffee at home?

~Acetegan

Probably down to temperature and agitation of the liquid.

After all your average Starbucks latte isn't exactly the hottest liquid known to man, and no comparison to a spoon full of instant coffee and a fresh boiled kettle at home.

Next there's the stirring thing. Your Costa's little wooden stick isn't going to get the liquid moving around anything like as much as a steel teaspoon from your kitchen cupboard.

might there also be something to do with how much 'stuff' is already dissolved (or held in suspension) in the water? I remember my primary school teacher doing an experiment to see whether those who put 6 teaspoons of sugar in their tea were wasting it or not by doing so with a cup of hot water. All the sugar dissolved. But then I thought - that isn't like for like cos tea already has stuff dissolved in it. Same for coffee.

Not necessarily. 'Proper' (i.e. not instant) teas and coffees are infusions rather than solutions so there's no reason why it would affect the ability of the water to dissolve the sugar.

Traveller in Time drinking black coffee
"Reasons to repeat the experiment using different blends of tea and coffee . "

Mr. Dreadful: Infusion is the method by which the chemicals are extracted from the coffee grounds, but once they are extracted, they are part of a solution. A Starbucks coffee is, as we know, much stronger than home brew, which indicates that the solution is more heavily saturated. A saturated solution will not dissolve sugar crystals.

Coffee shop coffee tends to have far more milk in it than instant you'd amke at home too so will have much higer fat content - would that make a difference?

I'm fairly sure milk fat is not water soluble, which is why milk homogenization was so difficult to achieve. So adding milk fat doesn't affect the solution. However, if you're adding a lot of milk, then you're taking up space that would otherwise have gone for hot water, so you're thinning out the mixture that way. The less hot water, the less capacity it has for dissolving sugar.

So the answer is yes, it would make a difference... but probably not as much of a difference as the strength of the coffee itself. After all, those Starbucks cups tend to be quite a bit bigger than what you serve yourself at home.

Who would have thought coffee &/or tea with sugar and milk would be so complex? I see so many variables that it would be tough to identify one as the main factor without extensive existential experimentation.

How about....could the sugar in the coffeeshops be different than most people have at home? Like larger crystals that take longer to dissolve? Some coffee shops offer a less refined sugar product that is slightly brown. That's probably not as immediately soluble as standard, fine-grained sucrose.

This might be a matter of kinetics rather than thermodynamics, i.e. the sugar may be just as soluble in the coffeeshop coffee but it dissolves more slowly. And who has the time to stir coffee in such urban environs?

Could unconscious human behaviour be at work? Since one doesn't pay any extra no matter how much sugar one puts in, perhaps one tends to add more? Or if it comes in packets it may be there is more sugar in the packet than one would usually add, and it's hard to add only half a packet and (wastefully) toss out the rest.

We could probably come up with a dozen other ideas but I'm leaning toward the more-concentrated-coffee theory at the moment.

even water at freezing point will dissolve twice its weight of sucrose given sufficient stirring

at boiling point it will dissolve nearly five times its weight

so a couple of teaspoons in a standard cup will not get you anywhere near saturation

but ..... crystal size makes a big, big difference to dissolving rate

Have you ever thought of giving up adding sugar to drinks?

Ah, but I simply cannot go without sugar in my coffee. XD *Laughs* It's not as if I drink coffee too often, mind -- it's a bit pricey. I prefer to make coffee when I'm at home using a cafetiere. But sometimes... sometimes, I just get really tired or need a sweetened coffee to soothe my frazzled nerves.

The question I had asked comes from one such day of requiring coffee to ease such frazzled nerves, and noticing this about the sugar.

Thank you all for the interesting answers, btw. =)

~Fif