Bacteria Culture Club

None needed.

Heat the milk over medium heat until it reaches 180°F (82°C). Stir the milk as it heats to make sure it doesn’t scald the bottom of the pot.

Take the milk off the heat, and let it cool to 110–115°F (43–46°C). You can do this in a sink full of ice water to speed things up.

Thoroughly mix in the prepared yogurt.

Make a hot water bath in a cooler or plastic bin that will keep the yogurt insulated at 110–115°F (43–46°C).

Portion the milk/yogurt mixture into small cups and place them in the bath for 8–12 hours. If you don’t want individual servings, you can place the whole amount in a large container and make a giant batch.

Refrigerate the yogurt cups for at least two hours.

Optional: Strain through cheesecloth if you like thicker, Greek-style yogurt.

Eat and repeat.

Yogurt is the product that results from the bacterial fermentation of milk. When you buy yogurt that contains “live active cultures,” it means there are still living bacteria present inside. By feeding these bacteria and keeping them at their optimal temperature, they will eat and divide and process the milk into yogurt. These cultures are made up of a general class of bacteria called lactic acid bacteria. The ones used in yogurt-making metabolize lactose, a form of sugar present almost exclusively in milk, for energy, and create lactic acid as a waste product. This acid helps give yogurt its texture and tangy flavor.

Milk is made of proteins, carbohydrates (in the form of lactose), fat, and water. When you heat it up in the first step, the proteins denature from their tightly coiled state into relaxed chains. The temperature needed for this is too high for many bacteria to survive, so this has the added benefit of sterilizing your milk. The milk is then cooled to the optimal temperature for the yogurt bacteria to undergo metabolism. Adding a lot of acid at once would cause the protein chains to precipitate out of the milk into clumps, but since the bacteria produce lactic acid gradually, proteins gradually turn into solids and gently coagulate into a network of chains. This network is able to trap liquid inside, and the end product is a smooth gel that gets firmer over time. Even though the bacteria divide fairly quickly, it takes several hours for them to work through the entire quantity of milk. Longer fermentation times will yield a tangier and stronger-flavored yogurt since more metabolism occurs. Straining the final gel results in an even denser yogurt as it physically separates the solid curds from the liquid whey. The amount of fat in the milk doesn’t really participate in this process, but will affect the overall texture and flavor of the final product.

If you have access to a compound microscope, it’s fun to look at yogurt bacteria at high magnification.

Make a wet mount with a tiny toothpick amount of yogurt and a drop of water. Under 400x, you can see individual bacteria swimming around. Store-bought yogurt often contains these species: Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus acidophilus, Bifidobacterium, and Lactobacillus casei.

The first two are the main yogurt-makers and work in synergy to digest lactose and produce lactic acid. They work so well together because they each make a waste product that the other uses for metabolism and are both thermophilic (survive at relatively high temperatures) bacteria that grow best at 105–113°F (40–45°C). The other three strains work better at temperatures closer to the human body temperature of 98.6°F(37°C). They are often added as probiotics since they are already present in different parts of your body. As the temperature and pH of the yogurt changes, different species are able to metabolize at better or worse rates, and the end result is a unique flavor profile of everyone’s waste.

In general, these bacteria come in two main shapes, with the Lactobacillus having an oblong rod shape and the Streptococcus, which is Greek for “twisted berry,” like little spheres. After they divide they can still be attached to each other; they sometimes look like beads in a chain.

Check out this episode of Hungry for Science to see what the yogurt looks like at different stages.