[graham.d (OP)]

I notice that if you allow moisture to get inside a jar of freeze dried instant coffee it causes the grains to stick together. This may be expected if the substance is hygroscopic, and it probably is. But what seems strange is that after this has occurred it takes longer to dissolve the grains in hot water when you make a cup of coffee. Why would this be so?

[tommya300]

I notice this also happening when I microwave the water first and level a spoon full of freeze dried coffee into it. No matter how I submerge the grans and stir the water, if I walk away a minute it is all homogenized.

I think that each crystal has air pockets and when the outer solid gets dampen the outer solid forms a jacket around center until it becomes saturated and disolved.

[Bored chemist]

When the little grains stick together the bigger grains you end up with have less surface area. Since they water can only dissolve the surface it takes longer when there's less surface to act on.

[Joe L. Ogan]

I agree with Bored Chemist.  As a matter of fact, I was getting ready to say something similar but the BC has said it much better.  Thanks for comments.  Joe L. Ogan

[tommya300]

When the little grains stick together the bigger grains you end up with have less surface area. Since they water can only dissolve the surface it takes longer when there's less surface to act on.

If small grains make bigger grains wouldn't the surface areas increase too?
Something sort of like accretion?

First the original freeze dried coffee crystal is porous. When dampened they begin to obsorb water to fill these pores in the attempt to disolve.
If the water touches more surfaces at a time, they will clump together and form a larger surface area I call a jacket of sorts, creating an internal air pockets.
 The added water has a slower time being obsorbed, displacing the air of the pores.
 Since the surface area has been increased the time for the water to penetrate has increased.

[Joe L. Ogan]

The surface area has not been increased, it has been decreased by the sides that are stuck together.  Thanks for comments.  Joe L. Ogan

[tommya300]

 

The surface area has not been increased, it has been decreased by the sides that are stuck together.  Thanks for comments.  Joe L. Ogan

I see the 2 small crystals merge to make one making the surface area smaller in comparing the 2 crystals. Now the volume on that newly formed crystal is doubled, increasing the time to disolve.

[graham.d (OP)]

Thanks for the theories guys. My observations would seem to suggest that it is not to do with the grains sticking as the undissolved grains floating in the cup are much the same size as those in dry freeze dried coffee. In any case, if irregular shaped things stick together the total surface area decreases, but not necessarily by much; these are not spheres merging to form another sphere.

[Geezer]

I wonder if there is some sort of bonding between the coffee and the water in the damp coffee?

What happens if you premix the coffee with a small amount of water so that it's liquid, then dump it into the hot water in the cup? Does it mix more or less quickly compared with adding dry coffee to the cup? The dry coffee may actually disperse more rapidly.

I'd do the experiment myself, but I'm a coffee snob, so I don't allow the instant muck into the house  []

[tommya300]

I wonder if there is some sort of bonding between the coffee and the water in the damp coffee?

What happens if you premix the coffee with a small amount of water so that it's liquid, then dump it into the hot water in the cup? Does it mix more or less quickly compared with adding dry coffee to the cup? The dry coffee may actually disperse more rapidly.

I'd do the experiment myself, but I'm a coffee snob, so I don't allow the instant muck into the house ,  []

The most expensive coffee beans can cost up to $600 a pound
.
 [ Invalid Attachment ]

.
Kopi Luwak  [?]

[Geezer]

A yes. The Bucket List.

[graham.d (OP)]

I wonder if there is some sort of bonding between the coffee and the water in the damp coffee?

What happens if you premix the coffee with a small amount of water so that it's liquid, then dump it into the hot water in the cup? Does it mix more or less quickly compared with adding dry coffee to the cup? The dry coffee may actually disperse more rapidly.

I'd do the experiment myself, but I'm a coffee snob, so I don't allow the instant muck into the house  []

I think you are right. It is my impression that the damp coffee bits have absorbed or adsorbed water on their surfaces and this has changed the nature of the surface to be not so permeable. You can see that the damp coffee bits are more glazed looking than dry ones - maybe crystallised rather than amorphous. This material takes longer to dissolve and protects the drier inner parts until it has done so. The stuff does dissolve eventually but takes a lot of stirring.

[rosy]

Geezer/graham.d, I think you're spot on. The nature of the freeze-drying process is that it leaves tiny pores in the freeze-dried material whence the water has sublimed (glossary - sublimed: like evaporated, but when a solid turns straight to a gas, rather than going via the liquid phase), which means that freeze-dried material rehydrates very readily. Once the coffee gets damp and the outermost layer partially dissolves and closes over the pores, this advantage is going to be lost and the coffee (or whatever) can only dissolve from the outside in.

[graham.d (OP)]

Thankyou Rosy. That explains it.

[Geezer]

Geezer/graham.d, I think you're spot on.

Will we be getting stars then?

[tommya300]

Geezer/graham.d, I think you're spot on.

Will we be getting stars then?

.
 [ Invalid Attachment ]