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  • Heidi E Golden

Yogurt Ecology Lab

Recently, I brought ecological principles to 6th graders via a laboratory exercise centered on yogurt! At its core, the field of ecology examines interactions between organisms and their environment, including both biotic and abiotic relationships. Creating yogurt in the classroom provided practical, hands-on, experiential learning involving organisms and their environments including feedbacks between yogurt bacteria and their milky habitat. Using yogurt as an example, we explored ecological concepts including pH sensitivity, growth rates, competition, and thermal optima. Yogurt provided a perfect medium with which to inculcate ecology into the middle school curriculum.

Yogurt provides a hands-on ecology example.

Ecological example of organisms interacting with their environment and each other.

An organisms presence in the environment over time can change the environment. And changes in the environment can influence which organisms are able to live there. Imagine what happens when an elephant poops in the pond! What happens to the environmental conditions of the water? What might that mean for other organisms, like zebras and birds? What other ecological interactions can you envision here?

Example of an acidic environment in Yellowstone National Park containing acidophilic bacteria.

Life on Earth began over 3 billion years ago with organisms similar to the cyanobacteria found in acidic thermal pool, like this one in Yellowstone National Park. In this idyllic bacterial environment, metabolic byproducts accumulated as the bacteria reproduced and grew in number, which helped create the oxygen rich atmosphere we and other organisms rely on for life today. Ecological interactions, especially with regard to bacteria, are critical in many ways but often go unnoticed because bacteria are microscopic.

Nevertheless, bacteria are essential transformers, critical for many ecological interactions that people depend on. They "fix" nitrogen from the air into the soil for use by plants, they transform dead and dying material into soil which helps create new life, and transform toxic forms of nitrogen, like ammonia, into beneficial forms, like nitrates, that algae and plants use as food. Organisms like cows and sheep also depend on bacteria to help them digest rough fibrous material into digestible food for energy. Furthermore, bacteria transform cow and sheep poop into fertilizer, which people can use in organic gardens to grow food.

Good versus bad bacteria is a matter of perspective.

Certain bacteria help create, preserve, and transform foods we eat and provide us with beneficial bacteria that sustain our health via a healthy "microbiome." Other bacteria, can make us ill if we ingest them. Whether we consider bacteria to be good or bad is a matter of perspective. For example, those that make us sick if eaten, might help turn dead matter into new soil. Different environmental conditions, like pH and temperature, can help select for different types of bacteria to grow.

Examples of common liquids and their ranges on the pH scale.

pH is an important environmental factor that can influence habitat suitability for organisms, including bacteria. The pH scale was developed to help classify different substances with regard to their hydrogen atom concentration, with pure water (pH = 7) considered neutral. Substances, like lemon juice, are acidic (pH < 7) because they contain more hydrogen atoms than pure water and substances, like bleach, are alkaline or basic (pH > 7) because they contain fewer hydrogen atoms than pure water.

Some bacteria are acidophiles, acid loving, like those in the Yellowstone pool, above. Others are alkaliphiles, alkaline loving, growing best in alkaline environments like those living in soda lakes, like Mono Lake, CA. Most bacteria on Earth, however, are neutrophilic, neutral loving, preferring a pH range between 5.5 and 8.5. Neutrophilic bacteria will grow best in neutral environments.

Shift in pH caused by bacteria transforming lactose (milk sugar) into lactic acid, with growth curves for neutrophilic bacteria (green) and acidophilic bacteria (blue). pH range for milk (green line) and yogurt (blue line) are also show.

Similar to pure water, milk is a substance that falls within the neutral pH range, but also contains milk sugar, lactose, that enhances bacterial growth. Some bacterial that use lactose as a food source transform the sugar into lactic acid. This metabolic byproduct accumulates as the bacteria reproduce and grow in number, which alters the conditions of their milky environment from a neutral pH to acidic pH. This change in pH also transforms milk proteins, causing them to clump together and thicken, which creates a new substance we call yogurt.

Thermal performance curve showing maximum performance (Pmax) at the thermal optimum (Topt) and the extremes, where performance falls to zero: critical thermal minimum (CTmin) and maximum (CTmax).

Temperature is another important environmental factor related to bacterial growth. All organisms have an optimal temperature range within which they function best. For humans, our internal thermal optimum (Topt) is 98.6 degrees F. Outside this range we either shiver or sweat to bring our body back to its optimal temperature for maximum performance, i.e. metabolic processes. Performance is any function we choose to examine, such as bacterial growth rate, for example. Maximum bacterial growth rate (Pmax) occurs at the thermal optimum (Topt). Outside Topt, growth rate decreases until the extremes are reached at either the critical thermal minimum (CTmin) or the critical thermal maximum (CTmax), where growth rate decreases to zero, often causing bacteria to die.

Both pH and temperature are important environmental factors that help us create a healthy environment for beneficial bacterial growth.

For making yogurt, we chose bacteria that would (1) benefit our health ("Good" bacteria), (2) grow well in milk's neutral environment (neutrophilic), (3) metabolize lactose in milk into lactic acid (which alters milk proteins), and (4) have a thermal optimum close to our own.

Optimal ranges of pH and temperature for three bacteria strains, Streptococcus thermophilus (red), Lactobacillus bulgaricus (purple), and Lactobacillus acidophilus (blue).

We chose three different bacteria strains, Streptococcus thermophilus, Lactobacillus bulgaricus, and Lactobacillus acidophilus, to use in our yogurt lab because their ideal growth rates for pH and temperature matched the environmental conditions necessary to change milk into yogurt and they are beneficial bacteria that will also grow well at our own thermal optimum of 98.6 degrees F.

Our pH test using litmus paper to determine pH of two different
types of milk, soy milk and 2% cow milk.

We used litmus paper, that changes color depending on pH, to test two different types of milk, soy milk and 2% cow milk, to determine the initial pH of each liquid and to ensure that our milks' starting pH corresponds with optimal growth for our bacteria. Both milks' pH matched pH = 7 on the color chart (above).

Heating jars and milk past Tmax to kill potential bacterial competitors.

We used our knowledge of thermal performance curves by heating our jars, utensils, and milk past most bacteria's thermal maximum (Tmax). This process for milk is called pasteurization, which ensured sterile starting conditions and reduced potential competition with other bacteria that might also thrive in neutral, warm, sugar-rich environments. The jars and utensils were boiled (212 degrees F) for 10 minutes in water, well past Tmax. We slowly heated the milks to 180 degrees F, enough to kill most bacteria without curdling the milk.

Cooling the pasteurized milk to Topt.

Adding our bacteria to the milk while still at or close to Tmax would kill our selected strains, so we prepared a cool water bath to reduce the milk's temperature to 110 degrees F, just above Topt and still within a range for high bacterial growth.

Adding the bacteria and ladling the mixture into sterile jars.

When the milk was cooled to 110 degrees F, we added our bacteria then ladled the mixture into our sterilized jars. Each group of students identified their jars by writing their initials on the lids.

Initialed jars with milk-bacteria mixture inside the incubator.

We made an incubator by placing clean 1-gallon milk jugs filled with warm water inside a food cooler. The incubator maintained a range of thermal conditions around Topt long enough for the bacteria to grow, transforming lactose to lactic acid through metabolism, and altering the pH of their environment.

Bacteria changed the pH of milk from neutral (pH = 7) to acidic (pH = 4.5)!

The interaction between organisms (bacteria) and the environment (milk), with initial neutral pH and optimal thermal conditions for growth, allowed for rapid metabolism of lactose into lactic acid, which transformed the environment's pH from neutral to acidic. This further transformed the environment by altering the milk's protein structure, creating tangy, yummy yogurt.

Understanding ecological interactions and the relationships between organisms and the environment provides insight into may processes, not just the creation of yogurt, that are occurring all around us. Complex environmental issues like climate change, micro-plastic pollution, coral bleaching, and hazardous algal blooms, are all caused by humans interacting with the environment in ways that cause harm to ourselves and other organisms. However, as we learned in yogurt lab, we can use our understanding of ecology to create beneficial environments, as well (i.e by reducing use of fossil fuels, plastics, and inorganic fertilizers), leading to a healthier, more sustainable world for all.

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