The Effect of milk curdling (other ideas)

Abstract

Has a milk-based soup, sauce, or gravy ever curdled on you (formed lumps) as you were preparing it? Curdling is the process of coagulation that occurs where the proteins in the milk clump together. Sometimes curdling is desirable—for example, if you want to make a delicious cheese or yogurt—but if you are trying to make a milk-based soup or gratin, or if you're adding milk to a hot drink, curdling is very unwanted because you lose the smooth, creamy texture. Nobody likes clumps and lumps in their hot chocolate, unless they're marshmallows!

What influences the curdling of milk proteins? The primary factors are acids, such as those found in juices and vegetables; tannins, such as those found in potatoes, coffee, or tea; and bacteria, which are sometimes deliberately added (if you are making cheese or yogurt), but may also develop if the milk is no longer fresh and starts to sour. As the bacteria grow and multiply, they produce lactic acid, which causes the milk proteins to clump together.

In this cooking and food science fair project, you will investigate cow milks that have different percentages of milk fat, and discover which ones are less prone to curdling, and are therefore more desirable for milk-based soups, gravies, and gratins. You will first need to read about the structure of milk: the milk proteins (the caseins and the whey), themicelles, the calcium phosphate, and the fat globules. You will then test and compare the curdling properties of cow milks that have different percentages of milk fat, from skim milk on up to cream. For each trial, choose one type of milk and slowly warm and whisk it in a saucepan until it comes to a simmer. Add a small amount of an acid, such as lemon juice or vinegar, and then remove the milk from the heat. Allow the milk to cool and then strain it. Measure the contents of the strainer (the curds) by weight or by volume. Be sure your milks are very fresh, use the same amount of milk for each trial, and cook each of the milks in the same fashion. As you compare your curd measurements, think about the relative protein content of each type of milk. Be sure to record all your data, amounts, and settings for your trials in your lab notebook.

When you finish this science fair project, you will be a milk-cooking master, and know how to avoid those dreadful lumps!

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Variations

• At what pH do different cow milks curdle? In this variation, you will choose cow milks that have varying degrees of milk fat, and add different amounts of an acid to determine the acidity at which each of the milks first curdles. You will need a way to measure the acidity of the milk-acid mixture, and you will need to keep the temperature of the milks constant for each trial. Hint: Bring a large saucepan of milk up to a certain temperature, and then add equal amounts of the warmed milk to small bowls with differing amounts of an acid inside.
• Mammal milks contain different ratios of proteins, fat, and carbohydrates, depending on the needs of the infant mammal. Compare the curdling properties of different mammal milks, such as cow, goat, sheep, and buffalo. Do your results make sense when you think about the different amounts of protein that each contains?

• For more science project ideas in this area of science, see Cooking & Food Science Project Ideas.