Hi everybody! Welcome back to Synthetic Biology One. The first yogurt probably happened by accident. Yogurt-making bacteria like Lactobacillus live basically everywhere: on plants and animals, in the soil, and even in the human digestive system. All they need to do their thing is milk and a warm place to grow. When people discovered that yogurt was delicious, they started making it on purpose, long before they knew what a bacterium was. It was basically synthetic biology for cave people.
So what is the deal with yogurt? How does yogurt work? If we can’t explain that with all of our fancy modern science, we might as well just go home. Specifically, there are three things that I want to understand. Why does it taste sour? Why is it solid? And why how does it work to preserve milk?
Let’s talk about sour. Yogurt becomes sour during the process of fermentation, which is the way that yogurt bacteria make energy. It is not special to yogurt. The process will be similar for most microbes that you want to grow. So it is something every synthetic biologist should understand.
During fermentation, sugars are broken down into smaller molecules called fermentation products. The main sugar present in milk is lactose. A typical glass of whole milk is about 5% lactose, by weight. And we can break it down and get some energy.
First, Lactose is a disaccharide, meaning two simple sugars linked together. The enzyme lactase splits lactose into glucose and galactose. Simple sugars are imported into the cell where they enter the metabolic pathway of glycolysis.
Glycolysis the like the power house of the cell. More enzymes work to split the six carbon glucose or galactose molecules into three-carbon molecules of lactic acid. This conversion produces energy in the form of ATP and NADH, which the cell uses to grow.
Lactic acid is a waste product of glycolysis. It is the carbon left over after the bacteria have extracted energy from the sugar. It is also, like the name says, an acid. As the yogurt bacteria consume the sugar and produce lactic acid, the pH of the milk drops from about 6.8 down to about 4.0. The acid is what we taste as sour, and it is the main chemical ingredient that makes yogurt, yogurt.
So why is yogurt solid? The acid produced by fermentation is also indirectly responsible. Milk contains proteins, long chains of amino acids folded together into tight balls. The acid in the yogurt disrupts the hydrogen bonds and hydrophobic interactions that keep the proteins folded. As a result, the proteins become denatured: they lose their precise folded structure and become disorganized, tangled chains of amino acids. These denatured proteins scramble together like old spaghetti to make a gel which creates the semi-solid texture of yogurt.
The acid also acts to preserve the yogurt . Most microbes simply can not survive at low pH, so the lactic acid strictly limits what bacteria can grow. By the way this also works for industrial food production, where lactic acid is commonly added as a preservative. Even Lactobacillus itself can only tolerate so much acid. It is typically their own acid production that stops them from growing, even if there is still lactose sugar left in the milk.
The basic chemistry of fermentation is extremely common as a microbial strategy to produce energy. But the exact products of fermentation are not always the same. Yeast, for example and secrete ethanol as a waste product instead of lactic acid, making them very useful for producing beer or wine. And even though microbes use most of their sugar for glycolysis and energy metabolism, they produce many other chemical byproducts as they grow and divide. All these waste products give yogurt it’s unique flavor and nutritional value. When you eat it, you really are tasting the bacteria themselves.
So until next time – happy fermenting!