Yogurt! It’s simply fermented milk. Yum. Milk, as a review, is a white liquid produced by the mammary glands of mammals. Yum, again. Intended to be a source of nutrition for baby mammals, milk is a colloid of butterfat globules within a water-based fluid that contains casein (milk protein), salts, minerals, vitamins and carbohydrates. A carb of particular importance is lactose, a disaccharide (composed of two monosaccharides fructose and galactose joined together minus one water molecule) that gives milk its sweet flavor.
Yogurt was probably first created by Central Asians in the Neolithic Age (10,200 BC – 3,000 BC). Bacteria that had originated on plants may have been transferred to the udders of cows via grazing or who knows what. Perhaps nomadic herdsmen may have kept milk in carrying containers like animal stomachs, which contained bacteria and turned the milk into yogurt. Since then, yogurt has been widely consumed throughout Asia, India, and Europe, and the Middle East for millennia. Other forms of fermentation, like alcohol-making, dates back to 7000 BC in China, and have also been practiced throughout history all around the world.
Louis Pasteur made the connection of yeast to fermentation in 1856, establishing that fermentation was like respiration (another cellular process) without air (or oxygen, hence, anaerobic).
The bacteria used to make yogurt are called bacterial cultures. I highlight this to myself because once I tried to explain them to a friend as “bacterial creatures”. Lactobacillus thermophilus and Streptococcus bulgaricus are their precise names. They ferment lactose into lactic acid, which then acts upon casein to give yogurt its texture and tanginess. Because it has much less lactose, yogurt can be consumed by many of the lactose-intolerant persuasion.
Back up though, what is fermentation? It’s a metabolic conversion that cells undergo all the time, to create ATP, which transports energy between cells. First, via the exothermic glycolysis, sugar breaks into pyruvate (an acid) and free energy, which is used to create ATP. Pyruvate converts into lactate, or lactic acid. This reduces the pH of the milk, killing harmful bacteria, making food tangy. Also, the hydrophobic protein begins to denature*, then reassembles with hydrophobic molecules like fat which creates a web-like structure that traps water and creates a semi-solid structure. Other forms of fermentation turn carbohydrates into carbon dioxide and ethanol.
Now that you understand zymology it’s time for some fun facts.
Buy some whole milk and some full-fat “starter” yogurt. This results in a creamier, fattier, tastier yogurt.
In a sterilized pot, heat your milk to about 178 degrees Fahrenheit, or to the point that it’s beginning to form bubbles. This kills bad bacteria. It will also denature some milk proteins and create a finer consistency.
Let the milk cool down to 115 degrees. For each quart of milk, add 2 tablespoons of active yogurt. The bacteria in it are thermophilic and prefer a warm temperature for growth. Put your milk in a warm container, cover it, and keep it warm for about four hours for it to set. My mom would put it in the oven she had used earlier in the day.
Refrigerate the container for several hours.
*What is denaturing again? It’s why boiled eggs become hard and why cooked meat becomes tough. When proteins are formed, they fold into themselves, as the hydrophobic elements curve inward and the hydrophilic elements stay on the outside. When a protein is denatured, these structures are broken and altered. Denaturation is caused by acids, bases, solvents, and heat.
I’m no scientist. Wikipedia helped me a lot with this one. Also, this link: