Yogurt is a fermented dairy product created by introducing specific bacterial cultures to milk. The food product itself is nonliving, but it is populated with billions of individual, living microorganisms. These microscopic inhabitants are responsible for the distinct texture, flavor, and acidity that define yogurt.
The Biological Status of Yogurt
Yogurt, as a whole, is categorized as a nonliving substance, similar to a loaf of bread or cheese. Its structure is an inert food matrix composed primarily of milk solids, proteins, and water. However, the unique properties of yogurt come from the bacterial organisms that reside within this matrix. These starter cultures are individually living entities that exhibit all the characteristics of life.
These microorganisms consume nutrients, excrete waste, and reproduce under favorable conditions. They metabolize lactose, the sugar found in milk, converting it into lactic acid. The presence of these viable cells gives the food its “living” reputation. The life within yogurt refers only to the microscopic bacteria, not the milk gel itself.
How Yogurt Is Made
The conversion of milk into yogurt begins with heating the milk to around 180°F (82°C) to eliminate competing bacteria and alter the milk proteins. The milk is then cooled to an ideal incubation temperature, typically between 109°F and 115°F (43°C and 46°C). This range is necessary for the starter cultures to thrive.
The two required bacterial species, Streptococcus thermophilus and Lactobacillus bulgaricus, are then added to the cooled milk. These bacteria work synergistically to ferment the milk’s lactose. S. thermophilus begins the process by producing acids that lower the pH, stimulating the growth of L. bulgaricus. As the bacteria consume the lactose, they generate lactic acid, causing the milk proteins to coagulate into the familiar gel texture of yogurt.
This drop in pH, usually to around 4.6, gives yogurt its characteristic tart flavor and helps preserve the product by inhibiting the growth of spoilage organisms. The incubation process is stopped by cooling the yogurt, which slows the bacteria’s metabolic activity. This process balances temperature and time to maximize bacterial activity before lactic acid inhibits their own growth.
Live Cultures and Product Labeling
Not all commercial yogurts contain the viable microorganisms present at the end of fermentation. Some manufacturers pasteurize the yogurt after fermentation to extend shelf life or achieve a milder flavor profile. This post-fermentation heat treatment kills the bacterial cultures.
For consumers seeking beneficial bacteria, it is important to look for labeling that confirms their presence. The phrase “Contains Live and Active Cultures” (LAC) indicates the product has not been heat-treated after fermentation and contains a significant number of viable organisms. To qualify for this seal, refrigerated yogurt must contain at least 100 million Colony Forming Units (CFU) per gram at the time of manufacture. Checking for the LAC seal is the most reliable way to ensure the yogurt retains the biological activity from the fermentation process.