Milk Not "Off" But Clumps in Coffee?

Question

This has happened to me several times over the past year. I pour milk into coffee, and it sort of congeals/clumps up in the coffee. I then smell and taste the milk itself, and it's still good. I taste the coffee - it's fine, except for these nasty clumps, which are not pleasant to drink. 

Any thoughts on why this happens?

Answer

My first thought was that the high acidity level in some coffees could curdle the milk (pure speculation though it has happened to me - the clumping, that is), but wouldn't that make the milk clumps taste gross and not the same? Odd ...

Answer

The clumping is not a function of fat separating, but protein clumping together in curds (curdling). Even though your milk is not "off" yet, as milk ages, the acidity increases, and when you put into something acidic and hot (like coffee), the milk can curdle. Fresher milk will be less vulnerable.

Answer

Could the difference in temperature have anything to do with the clotting? I thought at one time that I'd figured that out.

Answer

I know this is a super old post...but in case anybody Googled it like I did and ends up here... 

Milk is actually a homogenous mixture called a colloid. This is because there are proteins and fats floating around in water in a fairly uniform distribution; the proteins & fats are what makes it milk instead of just water.[1] 

Proteins denature in particular conditions due to the disruption in bonds at different levels of protein organization.[2] In Biology we always look at surrounding conditions to determine the behavior of a thing X (in this case proteins). This is no different. The particular conditions that proteins denature (i.e. lose their structure and change conformation)[2] are temperature and pH. A higher temperature makes proteins change conformation because it cranks up particle movement (called formally kinetic energy)[3] and the bonds get shaken up and split apart. A lower pH (i.e. more acidic which = more hydrogen ions aka H+ aka H3O+)[4] disrupts bonds because the protein's organization actually relies on various charges in its molecules. Some of these charges are attracted to the extra H+ floating around, and some are repulsed.[5] Hence, they move in different directions, and the bonds split up. 

Milk "goes bad" when lactic acid is produced by bacteria that ferment the lactose (a sugar -- a good source of energy) in the milk.[6] We can attribute the "off" taste to those pesky bacteria too...the hydrolysis (i.e. splitting up of) or autooxidation of fats results in rancidity where fats are concerned.[9] The lactic acid messes with the proteins in solution. The proteins are denatured and that's what those clumps are. Your intuition might tell you that denatured proteins actually wouldn't clump together. Interestingly enough this isn't the case (at least in our situation). We also can see a solidification of proteins in egg protein (i.e. albumin) when we cook our eggs.[7] 

Coffee is indeed slightly basic due to its own denaturation of protein-like molecules called amino acids.[8] 

[1] https://en.wikipedia.org/wiki/Mixture 
[2] https://en.wikipedia.org/wiki/Denaturation_(biochemistry) 
[3] http://www.thefreedictionary.com/Kinetic+molecular+theory 
[4] https://en.wikipedia.org/wiki/PH 
[5] https://www.quora.com/How-does-protein-denature-at-a-low-pH 
[6] https://www.scienceworld.ca/blog/what-made-our-milk-go-bad 
[7] https://en.wikipedia.org/wiki/Egg_white 
[8] http://www.coffeeresearch.org/science/sourmain.htm 
[9] https://en.wikipedia.org/wiki/Rancidification 

So we see that the milk's own spoiling can be exacerbated by the coffee's slightly acidic pH and more clumps can form. However, it's most likely that the milk's clumps are a result of its own spoiling and not the coffee itself (as coffee is only slightly acidic given that its pH is about 6.5-6.7 and neutral pH is 7). 

It has nothing to do with fat separating, although clumps can form when you freeze milk if the milk itself is spoiling at all...even a little bit. This is because the concentration of water is being decreased in solution as it turns into crystals due to freezing and the concentration of H+ becomes greater relative to the water in solution (of course no new H+ is being made it's just the conditions change from the water being frozen). The same phenomenon occurs with the H+ denaturing the milk proteins as before. Bacteria causes the rancid/off taste.

Answer

Interesting spiel, Ilovescience_foodie, but that doesn't explain why I too have these lumps in a bottle of fresh milk that has not been opened yet. I am trying to work out if the problem is at my place or something the manufacturer is doing?

Answer

I greatly appreciate the info Ilovescience_foodie provided. Signed up for an account just to say thanks.

Answer

This can be avoided if you put your milk and sugar in your cup first, then pour the hot coffee in.

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Milk Not "Off" But Clumps in Coffee?

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