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Lab Activity 1

IUG, Fall 2012Dr. Tarek M Zaida

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Nonenzymatic Browning

• BackgroundBrown colors often develop during: 1. processing, 2. storage, and 3. preparation of foods and food ingredients.

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Types of Browning

• 1. EnzymaticEnzymes (e.g. Oxidases) responsible for reactions that produce brown colors in food.

Example:The enzyme polyphenoloxidase acts on phenols and oxygen in plant foods to form brown colors. It only occurs in fresh cut plants where the enzyme is still active and has access to its substrates.Note:Polyphenoloxidase requires Cu as a cofactor, so copper chelators can limit enzymatic browning.

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2. Nonenzymatic

In such browning cases, enzymes are not required for the production of brown colors in food.

There three types of nonenzymatic browning:1. Caramelization2. Lipid Browning3. Maillard reaction

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1. Caramelization

• The process in which brown colors and pleasing aromas are resulted during the heating of sugars.

Caramel colors are widely used to color cola beverages and other foods.

• Sugars in solution are quite stable to heat in the pH range of 3-7.

• Melting dry sugar or heating its solutions in the presence of acidic or basic catalysts, causes the sugar to caramelize.

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2. Lipid Browning

• The polymerization of lipids after extensive heating (e.g., in old frying oils).

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3. Maillard Reaction

• Also known as the browning reaction,• Is the phenomenon responsible for • Turning meat brown, • Converting bread to toast, etc.,• The Maillard reaction is named for Louis-

Camille Maillard, a French chemist who studied the science of browning during the early 1900s.

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• Certain foods contain carbohydrates in the form of sugars, while others contain amino acids in the form of proteins.

• Sugars and amino acids often exist side-by-side, as in the case of raw meats.

• They may also be blended together, as in the case of bread dough.

As long as there is no outside catalyst, or cause for change, the meat remains red and the bread dough remains white.

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• The Maillard reaction is the catalyst for change, primarily by the addition of heat.

• When bread dough or meat is introduced to a hot oven, a complex chemical reaction occurs on the surface.

• The carbon molecules contained in the sugars, or carbohydrates, combine with the amino acids of the proteins.

• This combination cannot occur without the additional heat source. The end result of this chemical recombination is the Maillard

reaction. The surface of the heated bread dough is now brown, as is the outer layer of the roasted meat.

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Everyday-life applications of maillard reaction

1. Maillard reaction responsible for the brown color as well as for the change of flavor (meat & bread).2. Recipes containing both eggs, which contain protein, and flour, which contains carbohydrates, benefit from the Maillard reaction to achieve a pleasing browned appearance.3. Self-tanning products also rely on the reaction between amino acids and sugars to create a brown skin tone.4. The Maillard reaction is also used to create artificial flavorings, based on the hundreds of complex amino acid/sugar combinations formed after the process.

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The Maillard reaction occurs in three main steps:

1. The initial step: formation N glycoside

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2. After formation of N glycoside the immonium ion is formed and then isomerize, this reaction is called Amadori rearrangement and forms a compound called ketosamine:

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3. The ketosamine products then either dehydrates into reductones and dehydro reductones, which are caramel, or products short chain hydrolytic fission products such as diacetyl, acetol or pyruvaldehyde which then undergo the Strecker degradation.

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Inhibition of Maillard Reaction

The Maillard reaction can be inhibited by:1. keeping the temperature low, 2. reducing the accessibility of reagents or 3. adding sulfite ions.

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• Sulfite ions are strong nucleophiles that can add to aldehyde groups to form an intermediate that does not take part in browning.

The mechanism of action of a sulfite is believed to be as follows:

• the browning intermediate DH reversibly binds a sulfite ion to form a sulfonated intermediate (DSH) which is not capable of browning.

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DH= 3-deoxyhexosulose

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Because sulfite ion is readily volatile as sulfur dioxide gas and sulfur dioxide gas can cause attacks in a proportion of asthmatics, the additive has been prohibited by the FDA in foods that are meant to be eaten raw.

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• Sulfites are allowed to a defined limit in some cooked foods because the additive escapes as gas during the cooking process.

• There is no widely accepted replacement for sulfites in foods although deprotonated thiols (R-S-) are capable of acting as nucleophiles in a similar manner to sulfite and, if they react suitably with DH, may form the basis for an anti-Maillard browning additive if flavor problems can be overcome.

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Experiment

• Materials5% w/v solutions of the following: Sugar Amino AcidFructose Lysine Galactose Methionine Glucose Serine Maltose Valine Sucrose

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Procedure

• 1. Combine 3 mL of each sugar solution with 3 mL of each amino acid solution, in screw cap test tubes. 2. Cap the tubes loosely. 3. Autoclave at 121º C for 60 min (or prepare a boiling water bath by bringing a beaker of water to a boil on a hotplate), and heat the tubes for 60 min.4. Remove tubes and allow cooling before handling. 5. Describe the aromas produced and the degree of browning for each solution.

Record the color as 0 = none, 1 = light yellow, 2 = deep yellow, and 3 = brown.