So, in the run up to my shows at the British Science Festival in September, I’m going to be pinging out exciting little facts all about the science of bread, cheese, alcohol and microbes, plus probably a few on the science and psychology of comfort food too.
To kick off, a post on acid coagulated cheeses. These include ones like cream cheese, paneer, many soft goats cheeses, and ricotta, which differ from cheeses like Cheddar, Parmesan and Roquefort in that they do not use rennet in their production. To explain the difference, let’s take a quick step back to look at milk, and why it curdles, and how acid and rennet do it differently.
Cows’ milk is around 3.5% protein by weight (most of it is water), which can be split into two types; caseins and whey proteins. Caseins make up roughly 80% of the protein in milk, and are found bound up with calcium ions in little hairy-looking balls called micelles (check out my expertly drawn picture). It’s these micelles that rennet acts on (but more on that later). Whey proteins are only about 20% of the protein content, but include types of proteins called immunoglobulins, which can cause allergic reactions. They’re very stable once heated, and are what stabilises the bubbles in the milk foam you get on your cappuccino in the morning.
Curdling milk to make cheese means allowing the proteins to clump together. Rennet does this because it contains an enzyme that strips the casein micelles of their negatively charged coatings, stopping them from repelling each other and making them able to get closer together and form curds. If just using rennet to make cheese, the whey proteins are left by the ‘whey’-side (sorry…), and are drained off. But they do get in on the act if acid is used to curdle the milk. Many of us have experienced that horrible moment when that slightly-old-but-probably-still-ok-to-make-tea milk curdles when you add it to the cup. This is because the tannins in tea act like acids, neutralising the negative charges on the micelles, plus dissolving the Calcium bridges holding them together, and also denaturing the whey proteins, making a horribly lumpy cuppa.
Using acid to curdle or coagulate milk allows all of the proteins to bind together into a much finer, closely linked structure than if rennet had been used. This means that when acid-coagulated cheeses are heated, the first thing to be lost is water, actually making the cheese harder. This is why paneer can be cooked in whole pieces in curries without falling apart, why cottage cheese doesn’t melt like cheddar on a baked potato, and why ricotta stays in tasty little blobs on pizzas or in lasagna, rather than spreading out like mozarella.