Objective: To prepare yogurt and cheddar cheese  

Theory: Fermentation is used in food processing to develop unique flavors and textures. In yogurt making a starter culture is used to acidify milk by producing lactic acid. The drop in pH causes casein to move closer to its isoelectric point resulting in coagulation. There are two methods for making yogurt, the set and the stirred method. In this lab, you will be following the set method. In this method, the yogurt is allowed to ferment and set the milk in the glass jars.

Cheddar cheese is also made with the use of bacteria. Cheese starter culture bacteria is first added to milk to produce lactic acid followed by the addition of rennin. The lactic acid produced by the starter culture provides an optimum environment for rennin to function. Rennin is a system of enzymes containing chymosin harvested from young calves. The purpose of rennin is to curdle the milk. In calves this is important since in the curdled form, milk is retained in the stomach longer, allowing maximum digestion. In making cheese, once curds are obtained the whey can be separated and the casein proteins solidified by pressing. Aging follows which may run from a month to years. During this time, enzymes in the cheese matrix continue to metabolize various components such as lipids, proteins and flavors to produce complex flavors.

Cheese can be made from pasteurized and homogenized milk from the store or from raw milk. The pasteurization process causes leaching of calcium from casein, thereby resulting in softer cheeses. To minimize this, calcium chloride may be added to replace lost calcium and produce a firmer cheese. Homogenization will result in lower whey-loss and retention of more lipids in the system which could contribute to lipid oxidation. Because of this, some cheese producers choose to produce their cheese from raw milk. If raw milk is used, it must be aged for at least 60 days before eating. This allows enough time to inhibit any pathogens that may be present.  

Procedure

Yogurt

  1. Pour 42 ounces of whole milk (5 ¼ cups) in a clean pot
  2. Heat gradually to 180oF
  3. Remove from heat and cool to 110oF to 113oF
  4. Place in pitcher and sprinkle 1 packet (5g) of yogurt starter and lightly whisk until dissolved
  5. Transfer 1-2 table spoons of jelly or jam into sterilized yogurt-maker jars
  6. Pour the lukewarm milk in the jelly/jam-based jars and fill to the neck
  7. Transfer (uncovered) to yogurt maker and incubate for 7 hours
  8. Remove from yogurt maker, seal and refrigerate for at least 6 hours before eating 

Cheddar Cheese

  1. In a large pot, heat pasteurized milk to 104°F, stirring frequently.
  2. As the milk is heating, add calcium chloride (1/8 tsp (or 20 drops) CaCl2 diluted in ¼ cup of purified water)
  3. When the milk gets to 104°F, add thermophilic culture, stir in with an up-and-down motion. Now cover, and ferment for 45 minutes.
  1. Stir to homogenize the milk, and slowly fold in diluted rennet (½ t-spoon diluted in ½ cup purified water). Using an up-and-down motion with your spoon will ensure that the rennet works its way through all the milk, so you can get the highest possible yield.
  2. Allow the cheese to set for 60 minutes, or until the whey begins to separate from the curd. You should see a layer of mostly clear whey floating on top of the curd, and the curd should be pulling away from the sides of the pot.
  3. Using the knife, carefully cut the curds into ¼-inch cubes
  4. Gently heat the curd up to 100oF and cook with gentle stirring for 30 minutes to help in removal of whey
  5. Using a cheese cloth, drain curds for 60 minutes 
  6. Once fully drained, remove from cheese cloth and thoroughly mix in 1 table spoon of non-iodized salt
  7. Wrap the salted curds in cheese cloth and place in the perforated aluminum container provided
  8. Place the aluminum container containing the cheese in the larger aluminum baking foil provided. Now fill your pot with water and balance it on top of the cheese to press out the whey for 10 minutes.
  9. Flip the cheese and repeat above for another 10 minutes
  10. Flip the cheese again and press overnight (12 hours)  
  11. Remove the cheese from the cheese cloth and place in a clean aluminum baking foil and cover with a clean cheese cloth to keep out dust and other similar contaminants. Leave for 3 days for a natural rind to develop on the surface. This rind will help to prevent drying out and mold development during aging
  12. Once the cheese has developed a natural rind, wrap it with grease paper, leaving one side open for it to breathe (Remember that the cheese is a living breathing organism). Note: Alternately cheese could be waxed, however we are not doing that in this lab
  13. Place inside a rigid plastic storage container about twice the size of the cheese to allow enough air to surround it. To create adequate humidity (about 70%) place a ball of paper towel soaked in water in the corner of the container. The towel should not be dripping wet and should not be touching the cheese. Note: prepare paper towel by soaking in dilute bleach to sanitize and then rinse with water to remove bleach.
  14. Age for 3 months or more by leaving it in the door of your refrigerator since this is the warmest place in your refrigerator. Note: Ideally, cheese should be aged at 55-65oF which is up 25 degrees warmer. Maintaining this ideal temperature may be difficult at home unless you have a basement that can maintain that temperature or if you have a wine refrigerator.
  15. During aging, the towel may dry out so check it weekly and replace it as it dries out. Opening the container weekly also allows fresh air to come in which will help the metabolic activity of bacteria in the system.

Lab Report Questions

  1. In more depth than have been outlined above, describe the biology of yogurt and cheese production
  2. Report on any challenges that you faced during the production of cheese and/yogurt and describe how you could improve the process
  3. Write a detailed description of the sensory attributes of the cheese and yogurt (include aroma, flavor, color, texture, tastes
Courtney Simons
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Courtney Simons is a food science writer. He holds a BS degree in food science and a PhD in cereal science from North Dakota State University.
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