Cooking Chemistry

• The chemistry of cooking lies at the heart of how raspberries curdle milk. The first step in avoiding this unhappy result is to understand the complex composition of milk. Any kind of milk, whether it's from cows, sheep, goats, buffaloes or another milk-producing creature, is a combination of fat (the "butter" globules), protein and water. Curdling involves a specific milk protein known as casein.

Proteins Repel

• Typically, groups of casein float around in the watery portion of milk without attaching, or bonding, to anything else. These microscopic groups of casein are known as micelles. They have a negative charge at the chemical level, which means that they repel other caseins. This repelling action is how the protein usually stays well dispersed throughout milk.

Acid Enables Bonding

• When raspberries are added to milk, the citric acid in the fruit adds positively charged hydrogen molecules. The positive molecules neutralize the negative charge in the casein, enabling it to bond with other similar groups of the protein. Hence, the clumping we call curdling occurs when large groups of proteins band together. Adding acid to milk is how cheese is made, but this kind of curdling isn't always welcome in other recipes. This action of citric acid on milk isn't limited to raspberries. For example, adding lemon juice to milk and heating the mixture is a traditional way to make the fresh cheese known as ricotta or paneer.

Limit Acid's Effect

• If the addition of raspberries to a recipe results in curdled milk, there are two options. First, cold tends to inhibit the dispersal of acid in milk; to test the effect of heat on a milk-acid mixture, add some lemon juice to milk while heating it and observe the reaction. To solve the curdling problem with raspberries, try adding the berries while the milk is very cold. Second, try using a smaller amount of raspberries, which reduces the amount of citric acid added to the milk. Raspberries have such a strong flavor that their taste should still come through even when the amount is reduced in the recipe.