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Lab Activity 1_ Nonenzymatic Browning

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BackgroundBrown colors often developduring:1. processing,2. storage, and3. preparationoffoods and food ingredients.
1. EnzymaticEnzymes (e.g.Oxidases) responsible for reactions that produce brown colors in food.Example:Theenzymepolyphenoloxidaseacts onphenols and oxygen in plant foods to form browncolors.It only occurs in fresh cut plants where the enzyme is still active and has access to its substrates.Note:PolyphenoloxidaserequiresCuas a cofactor, so copperchelatorscan limit enzymatic browning.
In such browning cases, enzymes are not required for the production of brown colors in food.There three types ofnonenzymaticbrowning:CaramelizationLipid BrowningMaillardreaction
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.Sugarsin solution are quite stable to heat in the pH range of 3-7.Meltingdry sugar or heating its solutions in the presence of acidic or basic catalysts,causesthe sugar to caramelize.
2.Lipid Browning
Thepolymerization of lipids after extensive heating (e.g., in old frying oils).
Also knownas thebrowningreaction,Isthe phenomenon responsible forTurningmeat brown,Convertingbread to toast,etc.,TheMaillardreaction is named for Louis-CamilleMaillard, a French chemist who studied the science of browning during the early 1900s.
Certainfoods containcarbohydratesin the form of sugars, while others contain amino acids in the form of proteins.Sugarsandaminoacids often exist side-by-side, as in the case of raw meats.Theymay also be blended together, as in the case of bread dough.Aslong as there is no outsidecatalyst, or cause for change, the meat remains red and the bread dough remains white.
TheMaillardreaction 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.Thecarbon molecules contained in the sugars, or carbohydrates, combine with the amino acids of the proteins.Thiscombination cannot occur without the additional heat source.Theend result of this chemical recombination is theMaillardreaction. The surface of the heated bread dough is now brown, as is the outer layer of the roasted meat.
1.Maillardreaction responsible for the brown color as well as for the change of flavor (meat & bread).2. Recipescontaining both eggs, which contain protein, and flour, which contains carbohydrates, benefit from theMaillardreaction to achieve a pleasing browned appearance.3. Self-tanningproducts also rely on the reaction between amino acids and sugars to create a brown skin tone.4. TheMaillardreaction is also used to create artificial flavorings, based on the hundreds of complex amino acid/sugar combinations formed after the process.
TheMaillardreaction occurs in three mainsteps:
1. The initialstep:formationN glycoside
2. Afterformation of N glycoside theimmoniumion is formed and thenisomerize, this reaction is calledAmadorirearrangement and forms a compound calledketosamine:
3. Theketosamineproducts then either dehydrates intoreductonesanddehydroreductones, which are caramel, or products short chain hydrolytic fission products such asdiacetyl,acetolorpyruvaldehydewhich then undergo theStreckerdegradation.
TheMaillardreaction can be inhibitedby:1.keeping the temperature low,2. reducingthe accessibility of reagents or3. addingsulfite ions.
Sulfite ions are strongnucleophilesthat can add toaldehydegroups to form anintermediatethat does not takepart inbrowning.Themechanism of action of a sulfiteis believedto be asfollows:the browning intermediate DH reversibly binds a sulfite ionto form asulfonatedintermediate (DSH) which isnotcapable ofbrowning.
Becausesulfite ion is readily volatile as sulfur dioxide gasand sulfurdioxide gas cancauseattacks in a proportionof asthmatics, the additive has been prohibited by theFDA infoods that are meant to be eaten raw.
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 althoughdeprotonatedthiols(R-S-) are capable of acting asnucleophilesin a similar manner to sulfite and, if they react suitably with DH, may form the basis for an anti-Maillardbrowning additive if flavor problems can be overcome.
Materials5% w/v solutions of the following:SugarAminoAcidFructoseLysineGalactoseMethionineGlucoseSerineMaltoseValineSucrose
1. Combine 3mLof each sugar solution with 3mLof each amino acid solution,inscrew cap test tubes.2. Cap the tubes loosely.3. Autoclave at 121º C for 60 min (orpreparea 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.Recordthe color as 0 = none, 1 = light yellow, 2 = deep yellow, and 3 = brown.





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Lab Activity 1_ Nonenzymatic Browning