We have a little milk frothing device that is like this one but a different brand. It’s got a little coiled wire ring that spins and aerates the milk as it heats to a nice foam for coffee.
We’ve noticed that as milk ages, it eventually stops frothing. This happens generally 2-4 days before there’s any detectable sour odor/taste. The milk is fine to drink or cook with. When the milk stops frothing, we use it as a cue to finish it quickly or freeze it for later recipes.
We’re using normal pasteurized whole milk.
What’s going on here? What is changing chemically about the milk that makes it not foam up when air is whipped into it?
Seriously fascinating question, OP. IANA chemist or lactologist (?), but I have read that what makes milk foam happen is the milk protein molecules encompassing the injected air bubbles.
And the souring process in milk involves the formation of lactic acid, which lowers the pH of the milk, which changes the structure of the milk proteins.
So my scientifically unconfimred guess is that the beginning of the souring process is producing enough lactic acid to start changing the structure of the milk proteins even before you can detect it by taste.
I am glad I’m not the only one fascinated by this.
I am going to come up with a set of experiments to try here. Milk is water with protein, fat, and sugar in it. The frother is clearly calibrated to a fairly narrow range of milkiness. It’ll froth whole milk and half and half, but not heavy cream (I think it doesn’t run long enough). I haven’t tried lower-fat milk because we don’t buy it.
The dominant lactobacillus reaction is (I think) turning lactose into lactic acid. So I should try some milk that will froth with a bit of extra acid in it to see if the acid is changing things, and I should try milk that won’t froth with a bit of extra sugar in it to see if the sugar is changing things.
Or maybe I should get something to measure sugar/pH/fat content? I know there are readily available pH measuring things. Are they sensitive enough? How would I measure sugar or fat content?
I have that exact frother and it works great for heavy cream. Better than milk actually. Though so far I’ve only used it at the minimum fill level.
I haven’t noticed your findings but I only rarely use it with milk. Maybe I’ll run some experiments as well though, like trying fresh milk with a few drops of lemon juice or vinegar.
Beware of serious clabbering impact from lemon juice even in small quantities, though. The standard recipe for curdling milk immediately is about one tablespoon of lemon juice or vinegar to one cup of milk. I’d say start out with highly diluted lemon juice, or preferably even diluted yogurt whey to do a very mild acidification to start with.
Also, I concur that the sugar or fat levels aren’t likely to be affecting this bc they don’t change as the milk ages? Also, I’ve read that changing the fat level doesn’t really affect frothability, although it affects whether the resulting froth is stiff (low fat) or supple (high fat).
I don’t know the answer, but I used to work at a coffeeshop and we observed a similar phenomenon there. My guess is something to do with the pH of the milk. We also noticed skim milk would foam stiffer than whole milk (and stay foamed in the milk pot for longer than whole milk.) It sure seemed to froth more easily, as well, but perhaps that’s just because it didn’t collapse as easily. And also what temp the milk was at affected the frothibility. We didn’t use thermometers, just learned through experience about how hot the milk should be for it to properly foam. (I mostly did it by sound. There was just a certain sound that indicated the milk was at the right temp, which I’d guess – but I don’t know for sure – was somewhere around 140F.)
I’d have thought that the proteins in milk have a role in frothing, and ‘spoilage’ bacteria like proteins, so at the point where the milk is close to going bad, I expect such bacteria might have consumed or altered the proteins.
No conclusion, just another data point.
I get a gallon of fresh milk every week from the Amish. Steam frother for lattes.
Two previous cows, some weeks frothed fine, other weeks were flat.
Current cow always froths fine.
When a cow goes dry on them they just replace it with another one as they don’t have time to breed and cycle it.
Numerous online articles talk about proteins, but I’ve never been able to find anything resembling actual scientific research to back this up. Here’s what I can tell you from my own experience:
Non-fat milk froths as well or better than full-fat milk. That implicates proteins rather than fats, or at least implies that a high ratio of protein:fat is favored.
Milk in opaque containers usually does better than milk in transparent plastic containers. Perhaps this again implicates proteins, because proteins are UV-labile, fats not so much. But it’s usually the cheapest milk in the plastic containers, so maybe the starting quality and freshness isn’t the same.
I don’t have that exact one. I have a Nespresso brand one, but I didn’t remember what brand it was at the time I asked, so I just looked for one that looked similar. We are usually making two coffees at a time and we like milk/cream, so we usually are pretty close to the max fill line.
Still an amusing coincidence. I had to do a double take that I hadn’t left my order page open in the background or something. Amazon still shows the “last purchased” popup.
At any rate, there are plenty of possible differences such as the temperature, heating rate, frothing time, etc. So all I can say is that heavy cream froths well on this model. I keep my fridge cold enough that the cream barely pours, but it powers through it.
The min fill line is still quite a lot. Enough for two normal cups of coffee. My Aeropress produces ~11 oz, and the min amount on this frother fills my ~16 oz mug the rest of the way.
In my experience, pasteurized american milk goes bad without getting sour. I was shocked, visiting Ireland, to attempt to drink some milk that was actually sour, like vinegar, without actually tasting like “old milk” as i am used to it.
I’m going to guess the issue is milk proteins denaturing over time (perhaps from bacteria, perhaps just from age, light, oxidation…) rather than specifically the pH.
That is a good point, but I assumed the milk “going bad” also changed its pH. It’s definitely not the same as unpasteurized milk going sour, which gives it more of a buttermilk/lactic-acid tang, and is pleasant enough to still drink, as opposed to spoiled milk, which I won’t even cook with, despite that everyone seems to.
You protein denaturing hypothesis does sound pretty good, though. But lowered pH does denature proteins, too.