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Biofuels Webquest: Yo! gurt

Yo! gurt



Yogurt is delicious and full of beneficial protein and calcium but did you also know it contains bacteria!


What are these bacteria doing in my yogurt and how can I use them to make more yogurt from milk?


How to make home made yogurt:



Beneficial Bacteria and Yogurt


Despite our longstanding association of bacteria with disease, most bacteria are essentially harmless. In fact, many bacteria are beneficial. Some types of bacteria are necessary for the manufacture of certain food products, such as cheese, sour cream, pickles, and yogurt.


The life of anaerobic bacteria!  Yogurt forming bacteria are anaerobes. Yogurt forming bacteria are anaerobes and break down milk sugar (lactose) into pyruvic acid and then into lactic acid using enzymes.  Lactic acid is the by-product or waste product made by lactic acid bacteria. Lactic acid also lowers the pH of milk making it acidic. The acidic conditions cause casein (a common protein in milk) to denature (or curdle) and become more solid.  In addition the acidic conditions inhibit the growth of other microorganisms that might spoil the yogurt. Thus lactic acid causes the yogurt to stay fresh, while at the same time remaining digestible by people who can break lactic acid down for additional energy.


Constructing a Concept Map of Yogurt.


Before you move on to the lab section of this lesson:  Construct a concept map of the important terms that you have been introduced to in this lesson.  These terms and key concepts should appear in your concept map.  A Concept Map is a technique for visualizing the relationships between different concepts. A concept map is a diagram showing the relationships in between concepts. Concepts are connected with labelled arrows, in a downward-branching hierarchical structure. The relationship between concepts is articulated in linking phrases, e.g., "gives rise to", "results in", "is required by," or "contributes to".


Use any app you would like to complete your mind map:

Here are some suggestions:

or you could use any iPad app such as Mindnode, Idea Sketch, Popplet, Maptini, Mindmeister or Inspiration Maps for iPads.




Yogurt is made by adding specific strains of bacteria into milk, which is then fermented under controlled temperatures and environmental conditions. The bacteria ingest natural milk sugars and release lactic acid as a waste product thus making the milk acidic. The increased acidity causes casein (the most common milk protein) to tangle into a solid mass (called curd) in a process called denaturation. 


A small amount of live yogurt can be used to inoculate a new batch of yogurt, as the bacteria reproduce and multiply during fermentation. Pasteurized products, which have no living bacteria, are called fermented milk.


Yogurt has nutritional benefits beyond those of milk—people who are lactose intolerant often enjoy yogurt without ill effects, apparently because live yogurt cultures contain enzymes which help break down lactose inside the intestine. Yogurt also has medical uses, in particular for a variety of gastrointestinal conditions, such as preventing antibiotic-associated diarrhea.


In this lab you will isolate the bacterial strains found in yogurt then use those same strains to inoculate fresh milk to find out if they can reproduce the same yogurt.


You should be able to conclude that the acidic, solidified nature of yogurt is caused by bacteria acting upon milk.


Once you have the basic method down, there are all sorts of tweaks and changes you can make. Some people like to add dry milk powder for extra thickness, others like to strain off the liquid whey for a dense Greek style yogurt.


Using different brands of commercial yogurt to culture the milk can also give you subtly different flavors and nutritional benefits.




• 1/2 gallon milk - whole or 2% are best, but skim can also be used

• 1/2 cup commercial yogurt - be sure that the yogurt contains active cultures




• a sauce pan with a lid - large enough to hold 1/2 gallon of milk with a few inches of head room

• spatula

• thermometer - that clips on the side of the pan is best

• small bowl

• whisk

• an incubator - this can be anything from the dutch oven used to heat the milk to a commercial yogurt machine




1. Heat the Milk - In your saucepan, heat the milk to right below boiling, 200°F. Stir the milk gently as it heats to make sure the bottom doesn't scorch and the milk doesn't boil over.  This heating step is necessary to change the protein structure in the milk so it sets as a solid instead of separating.


2. Cool the Milk - Let the milk cool until it is just hot to the touch, 112°F - 115°F. This goes faster if you set the pan over an ice water and gently stir the milk.


3. Inoculate the Milk - Pour about a cup of the warm milk into a small bowl and whisk it with the yogurt. Once it's smooth, whisk this back into the pan of milk.


4. Incubate the Yogurt - Now comes the long wait period where the milk actually transforms into yogurt. The trick is keeping the milk around 110°F until it has set, usually 4-6 hours. Commercial incubating equipment is handy for maintaining a consistent temperature, but not necessary.


First, warm the oven to about 115°.  Put the lid on the dutch oven or saucepan with your inoculated milk and wrap the whole pot in a few layers of towels. These will insulate the pot and keep it warm. Set this bundle in the warmed (but turned off!) oven and set the timer. It's important not to jostle the milk too much as it's incubating so that it sets properly - the temptation to peek is so hard to resist!


The longer the yogurt sits, the thicker and more tangy it will become. Check it around the 4-hour mark and give it a taste. The texture should be creamy, like a barely-set custard, and the flavor will be tart yet milky. If you like it, pull it out. If you'd like it tangier, leave it for another hour or two.


5. Cool the Yogurt - If we cool the yogurt in the same container we incubated it in, we end up with a smoother end result. Once it's completely chilled, we transfer it to air tight containers for easier storage. Sometimes there will be a film of watery whey on top of the yogurt. You can strain this off or just stir it back into the yogurt. Yogurt lasts about two weeks in the refrigerator.


Making a iMovie Presentation:


Make an iMovie and upload it to You Tube.  Show the world how much you have learned and how easy it is to understand how enzymes help us make yogurt.  Pretend that you are the host of a TV cooking show. 

Subject Guide

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Jennifer Jourdain
Montachusett Regional Voc Tech School
1050 Westminster Street
Fitchburg, MA 01420
(978) 345-9200 x5125