When I put some cream in my coffee, I’m mixing a base with an acid. Tastes good too.
The coffee seems to lighten in color disproportionately to the amount of cream, though. Is there anything going on besides the mixing of solids to make the seemingly-drastic color change?
You’re just seeing the fat disperse. If you put skim milk in your coffee, it hardly lightens it at all.
The opacity of cream or whole milk is vastly greater than the opacity of coffee. You can see often see light through a clear glass carafe of coffee but can rarely see any light through even a small glass of milk. The fact we normally serve coffee in an opaque-sided vessel and milk in a clear-sided vessel serves to disguise this difference.
Once mixed, the much greater opacity of the cream/milk produces a *greater than you expect *color change. The visual effect is even stronger when mixed in an opaque vessel. Like a typical coffee cup.
Where’s the base? Coffee is acidic, around pH 4, and milk or cream is 6.4 or so. You’re making the mixture a little less acidic, but I don’t think any neutralization is happening.
If you were thinking dairy is basic, that’s a common misconception. But consider this: major components of dairy are the fats (made up of fatty acids), and lactic acid and so forth.
Coffee can taste bitter, like a base, also, but that’s due to compounds that cause a certain astringency (like tannins in wine). The darker the roast, the more of those kind of compounds are formed.
More on that: Coffee Chemistry and Bitter Coffee
Strength of the coffee matters a lot when it comes to the hue of the final mix. Weak coffee will turn completely white while stronger coffee will stay quite brown using the same amount of cream and fresh coffee (old, burnt coffee has a dark hue of its own). I can tell who made the coffee at work based on how brown the cup is after I pour it and mix in the cream, because some people here make absurdly weak coffee.
Also, I’m pretty sure coffee and cream are both acidic overall, though coffee has some alkaline components which give its flavor those bitter overtones.
I had to look it up but pH of cream is
Cream, 20 per cent 6.50 - 6.68
Cream, 40 per cent 6.44 - 6.80
coffee is 5.0 to 5.1
So, yeah, both mildly acidic.
EDIT: …or what Elemenopy said.
Conversely, adding lemon juice to black tea WILL cause it to lighten in colour. IIRC, the citric acid breaks down the tannins which give tea its colour. Delicious too!
I knew that reaction was chemical but didn’t know how. Cool info.
Works the same way with coffee; I just tried it. A drop of concentrated sulfuric acid (I didn’t have any lemon at hand) turned some 2.5 cl of black (actually, more like ebony) coffee into chestnut color. (Didn’t improve taste, though - I spit it out, after confirming the acidity!)
On the other hand, a small amount of potassium hydroxide didn’t change the color of the (instant) coffee at all.
The whitening effect of milk (and a lot of other white things) is about light dispersal. Diluted white is still often really white. Anyone who has ever washed emulsion paint out of a roller will attest to this.