Posted Mon Feb 4, 2013, 3:59pm Subject: Re: Milk curdling in coffee
I believe it has to do with the acidity of the coffee. I use only fresh soy milk in my coffees. Some coffees curdle the soy milk, others do not. It is definitely unappetizing when it curdles! I have used many, many different coffees, and some of them curdle, some don't. It's the acidity, not the temperature or the "freshness" of the milk, whatever it is.
I beg to differ on freshness. Tried several different batches of half n half on the same batch of coffee -- only the old one curdled. These were two different brands of half n half, bought from different stores.
Lee_M Senior Member Joined: 2 Dec 2012 Posts: 45 Location: Los Angeles Expertise: I live coffee
Grinder: Baratza Encore Drip: V60 Roaster: Popper
Posted Mon Feb 4, 2013, 4:56pm Subject: Re: Milk curdling in coffee
Milk curdles by acidification when its pH drops below 4.6.
Tannins can also curdle milk by binding together casein proteins.
Fresh milk has a pH of ~6.6, but its pH drops over time as lactic acid is produced.
Coffee generally has a pH of ~5.0.
Since coffee contains tannins, the curdling pH of milk in coffee will be somewhat higher than 4.6.
If the coffee is only slightly below this pH, the pH of a milk-coffee solution may exceed the curdling pH with fresh milk, but fall below it with old milk.
If the coffee is acidic enough, it will curdle even fresh milk.
RoyceRuiz is right on the money. Kenya coffee frequently curdles the cream (we use soy creamer, by the way) in this household. If it is roasted a little darker than normal, it may tend to moderate the acidity and, in those cases, it may not curdle. The curdling of the cream is confirmation that you are getting the high acid brightness for which many Kenyans are known. It doesn't really affect the flavor. Give the cup a stir before each sip to redistribute the cream. With your eyes closed you won't be able to tell the difference.
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