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If I put small, separate piles of instant black coffee powder, sugar, powdered creamer, and even something very ionic like table salt out in humid air and come back in a few days, only the instant coffee will have absorbed enough water to change to a sticky tar-like blob while the other three won't change their appearance.

Is it known what substance it is in the instant coffee powder that makes it hygroscopic and even deliquescent; giving it the ability to almost "make itself into coffee" by removing so much water from the air that it dissolves?

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  • $\begingroup$ The other three piles serve as everyday-chemistry equivalents of controls. $\endgroup$
    – uhoh
    Commented Nov 18, 2018 at 7:58
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    $\begingroup$ Interesting. I suspected that instant coffee would start off drier and therefore would be more avid for water, but it seems instant coffee can have as much as 2-3% moisture content, while table sugar is limited to at most 0.04%. Either the surface area to mass ratio plays a large role, or instant coffee really contains more inherently hygroscopic substances. $\endgroup$
    – Nicolau Saker Neto
    Commented Nov 18, 2018 at 8:53
  • $\begingroup$ @NicolauSakerNeto The power is very low density and therefore quite porous, but I don' t know if that's got anything to do with it or not. $\endgroup$
    – uhoh
    Commented Nov 18, 2018 at 8:59
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    $\begingroup$ How about doing an experiment, for science? Take some sugar and salt and pulverise them as finely as possible in a blender, then spread them out a bit on a plate and check if they now also deliquesce? Another thing to keep in mind is that commercial sugar and salt are intentionally sold impure; among other things, they contain anticaking agents, such as tiny amounts of silica, exactly to avoid deliquescence which is rather inconvenient in storage. $\endgroup$
    – Nicolau Saker Neto
    Commented Nov 18, 2018 at 9:08
  • $\begingroup$ @NicolauSakerNeto 2) I try to avoid doing experiments that are not testing a well formulated hypothesis. My current observations are from everyday spillage, not an experiment. 1) I don't have any of that stuff handy ;-) $\endgroup$
    – uhoh
    Commented Nov 18, 2018 at 9:25

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The behavior strongly depends on the temperature and humidity in which the coffee was kept and on the specific type of coffee.

From Tea and coffee powders, M. Huang, M. Zhang, in Handbook of Food Powders, 2013:

Being hygroscopic, instant coffee particles are susceptible to the action of moisture – that is, they absorb moisture from the air. If the moisture content increases to 7–8%, the powder or granules can become a paste or solid mass (Clarke, 1987).

Clarke (R.J. Clarke, in Encyclopedia of Food Sciences and Nutrition (Second Edition), 2003) writes:

Instant coffee is relatively hygroscopic, easily picking up moisture from the atmosphere and caking at about 7–8% w/w moisture content. For example, to keep instant coffee below 5% (dry basis) moisture content, the relative humidity of the air with which it is in contact must be below 35–40%, though the precise value depends upon the nature of the instant coffee in question, primarily due to differences in porosity of the particular particles made. It is necessary, therefore, that jars of instant coffee be well sealed prior to sale.

My answer is admittedly incomplete. As pointed out in comments to the post and in the above quotes, porosity (characterized by e.g. the mean particle surface-to-volume ratio) influences condensation, but so do the intrinsic chemical properties of the particle surface.

The chapter by Huang and Zhang referenced above provides the following table (Table 20.1) detailing the composition of instant coffee powders:

Constituent Content(% dry weight basis)

Minerals 7.6–14.6

Reducing sugars (glucose) 3.2–13.1

Galactomannan 2.4–10.5

Low molecular organic acids 12

Brown pigments 15–28

Caffeine 2.5–5.4

Trigonelline 1.56–2.65

Source: Belitz et al. (2009).

Simple sugars are often hygroscopic as may be organic acid compounds, and caffeine. In particular the presence of glucose (part of "invert sugar") could account for much of the absorption of water. In addition, minerals such as various potassium salts (which make up much of the mineral content of processed coffee beans) are known to be deliquescent.

The Belitz reference appears to be: Belitz et al. (2009). Coffee, tea, cocoa. In Food Chemistry (4th Edition), ed. H.-D. Belitz, W. Grosch, and P. Schieberle, 938-970. Germany: Springer. However haven't accessed the original pubs due to paywalls.

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    $\begingroup$ It's certainly good to have citations confirming that coffee powder does this, thank you! Do you think it's possible to dig up a full citation for Belitz et al. (2009)? $\endgroup$
    – uhoh
    Commented Nov 19, 2018 at 3:21
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    $\begingroup$ I'm guessing it's Belitz et al. (2009). Coffee, tea, cocoa. In Food Chemistry (4th Edition), ed. H.-D. Belitz, W. Grosch, and P. Schieberle, 938-970. Germany: Springer. $\endgroup$
    – Buck Thorn ♦
    Commented Nov 19, 2018 at 13:37
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    $\begingroup$ It's a complicated problem but because of its economic importance I'm sure it's been exhaustively studied. You may need to read a lot to disentangle all of the factors. In fact the presence of minerals is perhaps a challenge as this is ambiguous. The refs I found did not detail the nature of the salts, and perhaps that matters wrt deliquescence. Fun problem... $\endgroup$
    – Buck Thorn ♦
    Commented Nov 19, 2018 at 15:45

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