In chapter 8 Nelson describes how to curdle milk. Milk is simplistically made from water, fat and the protein casein. The casein is a phosphoprotein that assembles into micelles. Curdling occurs when coagulation of the casein micelles (by overcoming the electrostatic repulsion between them) turns the milk into a gel. By adding acid (H+) the effective charge on the micelles is reduced, in turn decreasing the repulsion allowing the micelles to aggregate and curdle the milk.
But milk can also curdle over time when left on the bench instead of the fridge.
Previously Nelson stated that the value of the critical micelle concentration typically decreases at higher temperature. Therefore when not in the fridge the number of micelles in milk will increase. Would this decrease in separation space be enough to induce the aggregation of the micelles?
The problem is, milk can also be heated to higher temperatures (in the microwave, on the stove, using a coffee machine) and this coagulation will not occur. Furthermore when curdled by acid the milk does not give off a pungent aroma. The smell of bench-top milk points to the idea that the proteins are actually denaturing within the mixture. But we know that heating proteins to high temperatures can also denature them. So what exactly is going on? How can milk curdle with acid, curdle over time yet not with extreme heat?
Well naturally we understand that there are certain ranges of degrees within which proteins can exist without denaturing, yet when we as you say microwave it, or boil it in a kettle, we are obviously providing enough energy in the system to break down these proteins bonds, thereby preventing curdling and coagulation. When we leave milk on the bench, it is obviously at a temperature at which it doesn't denature. I should restate that as that it denatures (if we go by the fact that the smell is a result of denaturing), but not in the same process as what is happening when we heat it up
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