What you'll need:
As always, be sure to ask your mom, dad or another adult to help- they'll want to taste your result. All utensils and ingredients should be clean and food-safe if you want to taste your results.
Mother's milk is an amazing substance, which really isn't surprising considering that it contains all of the nutrients a baby needs to survive and grow. The plain white liquid you see is actually a uniform (homogenous) mixture consisting of fats or oils (composed of molecules called fatty acids), sugars, proteins, vitamins and some other chemicals, but mostly water molecules. This is called an emulsion or suspension- a mixture of liquids and solids, most of which are not truly dissolved. Normally these various components would separate (and in raw or non-homogenized milk some, notably the fats, still do), but the protein and fat molecules have both hydrophilic (water-loving) and hydrophobic (water-hating) regions, which causes these long molecules to twist around and group together into more stable arrangements, making it possible for them to stay suspended in the water that makes up most of milk by volume. The proteins form micelles, sort of like little water balloons (or in this case, protein balloons), with their hydrophobic ends on the inside and their hydrophilic ends on the outside facing the water molecules. The fat molecules form similar but much larger balloon-like structures called globules.
Our "I can't believe It IS Butter!" experiment dealt with the fat globules in milk, while in this experiment we focus instead on the protein micelles. There are dozens of different proteins in milk, but they can be grouped into two main categories: the so-called curds and whey proteins from the "Little Miss Muffet" nursery rhyme. In cow's milk the curd proteins, called caseins, outnumber whey proteins about 4 to 1 by weight. One important difference is that caseins will coagulate or form solid clumps under acidic conditions while the whey proteins will remain suspended in the liquid, and that is what is happening in this experiment. Milk solid curds are used to make cheeses, or in our case cottage cheese. Most cottage cheese you buy in the grocery store is salted, which is why the cottage cheese curds you made may have tasted a little different (you may have also tasted the lemon or lime if you didn't rinse them well).
Lemons and limes have an acid called citric acid in their juice, as do other citrus fruits like oranges and grapefruit. Normally the casein micelles have negative electric charge on their outer surfaces so that they all repel each other and remain suspended in milk. When you added lemon or lime juice to your milk the citric acid neutralized these negative charges and allowed micelles to begin sticking together in larger and larger clumps, which also trap much of the fat in the milk as they form. We can also say that the milk has curdled. It was these casein curds that you caught in your strainer, while the liquid that passed through contained the remaining whey proteins. Heating the milk also strongly enhances this coagulation process. Special chemicals called enzymes can also be used to produce milk curds (described in some of the links below).
In this experiment you produced a simple cottage cheese, which is essentially the first step in making any kind of cheese, though most types require many more steps (see links below for more on cheese making). Sometimes when milk spoils it can curdle (see link below) due to bacteria converting the primary sugar found in milk- lactose- into lactic acid which causes casein to coagulate just as you did with citric acid. The process of converting sugar to acid with bacteria is a type of fermentation, and this is how yogurt is made (although the fermentation is carefully controlled so the milk doesn't actually spoil).
Variations and Related Activities:
Try other citrus fruits in place of the lemon or lime, or different acids such as vinegar (acetic acid). Another interesting experiment uses fresh pineapple juice, which is too weak an acid to be very effective but also contains an enzyme that can produce curds by a different mechanism (see the Scientific American version below). You can also try different milks (low fat, skim, buttermilk, etc.)
A fun extension of this experiment is to make your own casein glue. For our cottage cheese we used acid to make the casein micelles coagulate, which allowed us to separate them from the other whey proteins. The opposite of an acid is a base, and if we add a base like baking soda (sodium bicarbonate) to the casein curds we can reverse the process and re-suspend the sticky casein molecules in liquid to make a simple type of glue (links below).
References and links to more information:
Others versions of this activity:
Make better cottage cheese:
Why milk spoils:
Milk Science Classroom Activities for older students:
Learn more about milk chemistry:
Making Casein Glue: