role of protein in food

 

6.3: The Role of Proteins in Foods - Cooking and Denaturation

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Learning Objectives

• Explain the process of 
  denaturation
   and its consequences. Give an example.

In addition to having many vital functions within the body, 

proteins

 perform different roles in our foods by adding certain functional qualities to them. Protein provides food with structure and texture and enables water retention. For example, 

proteins

 foam when agitated. (Picture whisking egg whites to make angel food cake. The foam bubbles are what give the angel food cake its airy texture.) Yogurt is another good example of 

proteins

 providing texture. Milk 

proteins

 called caseins coagulate, increasing yogurt’s thickness. Cooked 

proteins

 add some color to foods as the amino group binds with 

carbohydrates

 and produces a brown pigment. Eggs are between 10 and 15 percent protein by weight. Most cake recipes use eggs because the egg 

proteins

 help bind all the other ingredients together into a uniform cake batter. The 

proteins

 aggregate into a network during mixing and baking that gives cake structure.

Protein gives structure and texture to cakes. A layered pound cake, with alternating interstitial spaces filled with raspberry jam and lemon curd, finished with buttercream frosting. Image use with permission (Public domain; Scheinwerfermann).

Protein 

Denaturation

: Unraveling the Fold

When a cake is baked, the 

proteins

 are denatured. 

Denaturation

 refers to the physical changes that take place in a protein exposed to abnormal conditions in the environment. Heat, acid, high salt concentrations, alcohol, and mechanical agitation can cause 

proteins

 to denature. When a protein denatures, its complicated folded structure unravels, and it becomes just a long strand of 

amino acids

 again. Weak chemical forces that hold tertiary and secondary protein structures together are broken when a protein is exposed to unnatural conditions. Because 

proteins

’ function is dependent on their shape, denatured 

proteins

 are no longer functional. During cooking the applied heat causes 

proteins

 to vibrate. This destroys the weak bonds holding 

proteins

 in their complex shape (though this does not happen to the stronger peptide bonds). The unraveled protein strands then stick together, forming an aggregate (or network).

When a protein is exposed to a different environment, such as increased temperature, it unfolds into a single strand of 

amino acids

.

Interactive 6.3.1$\mathrm{6.3.}1$: Denaturing of Egg 

Proteins

View this animation of egg 

proteins

 denaturing. http://www.sumanasinc.com/webcontent/animations/content/proteinstructure.html

Key Takeaways

• Proteins
   provide food not only with nutrition, but also with structure and texture.
• When a protein denatures, its complicated structure unfolds into a strand of 
  amino acids
  .

Discussion Starters

1. Gelatin is a mixture of 
   collagen
    
   proteins
   . How does gelatin add texture and structure to foods? Conduct an experiment by making some Jell-O™ using different methods of 
   denaturation
    and discuss what happens to the texture and structure.
   1. Make the first batch of Jell-O™ following the directions. Does it set?
   2. Next, make up the Jell-O™, but replace hot water with cold. Does it set?
   3. Instead of adding hot water, add 2 teaspoons of salt and only cold water. Does it set?
   4. Take the first batch (that is now set) and either whisk it by hand or use an electric mixer. What happens?
2. If pouring salt on a live slug produces a puddle of goo, how is it possible to cook snails with salt and get a delicious dish?