There are many thousands of bacterial species. And those are just the ones scientists know about —there are more being discovered every year. Several kinds of bacteria help to turn milk into foods we eat every day.
This picture shows Streptococcus thermophilus (orange beads) and Lactobacillus bulgaricus (blue rods) bacteria in yogurt. Both these bacteria can fight bad bacteria that make us sick.
Making yogurt
Two bacteria often used to make yogurt are Streptococcus thermophilus and Lactobacillus bulgaricus. First the milk is heated to condense it (reduce its liquid content to make it thicker), and then the bacteria are added. With the milk sugars to feed on, the bacteria multiply quickly. They go to work on the milk, thickening it and giving it a yogurty taste.
Good bacteria in food can help us digest food. They also compete with bad bacteria in our bodies.
Ripening cheese
Cheese is made from the protein in milk. First, bacteria produce the acid that separates the protein from the liquid part of the milk. The protein forms soft, solid stuff called curd. Curd makes unripened cheeses, such as cottage cheese.
The curd can then be ripened with the help of other microbes that harden cheese and give it flavor. Different microbes—either bacteria or fungi—produce different kinds of cheese. The bacteria that make Swiss cheese, for example, are called propionibacteria. They release gas bubbles, and when the cheese hardens, there are holes where the bubbles were.
Blue cheese
Blue cheese ripens with the help of fungi. We learned earlier how fungi spread through bread. When the mold Penicillium roqueforti is put into cheese, it also starts to spread and form blue veins in the cheese.
The veins you see in blue cheese are the hyphae and spores of the fungus Penicillium roqueforti.
