Lab Activity 12: Maillard reaction (nucleophilic addition)

Baked bread, seared meat, dark beer, roasted coffee. One way to brown food and produce wonderful flavors and tastes is to use the Maillard reaction (“The Maillard Reaction Turns 100” http://cen.acs.org/articles/90/i40/Maillard-Reaction-Turns- 100.html). This reaction is the most practiced reaction in the world although it is done mainly in kitchens rather than in the lab. (Pictures credit: Shutterstock)

An overview of this reaction and reactions that produce flavor and aromas is shown in Figure 1.

Figure 1. Overview of Maillard reaction and products (from http://sciencegeist.net/the-maillard-reaction/). 1. The description below is from “Favorite Chemical Reactions” http://cen.acs.org/articles/89/i47/Maillard-Reaction.html and http://sciencegeist.net/the-maillard-reaction/ and accompanies Figure 1.

“The set of reactions that take place under the general description of the Maillard reaction can be generalized as follows. (Please refer to the figure for more detail.) A sugar (1) combines with an amine (in this case, NHn-AminoAcid) to form 2. 2 rearranges into a glycosylamine (3), which is unstable in these conditions. The glycosylamine rearranges into an aminoketose (5) through an aminoenol (4) intermediate. The aminoketose is one of the main products of the Maillard reaction. It is called the Amadori component because, well, Amadori isolated these compounds from Maillard reaction products in the 1930s. And, while this is a primary component, it really isn’t very interesting. The tasty parts of the Maillard reaction come about when 4 is converted into a deoxy-hexosulose (7) or the Amadori product rearranges into an enediol (6), which is further converted into a deoxy-hexodiulose (8). 7 and 8 are the intermediates that ultimately lead to the small- molecule aroma, flavor, and color compounds that our senses recognize as the products of the Maillard reaction. The exact mechanism by which 7 and 8 are converted into these small molecules is still not fully understood. I imagine that ANY number of reasonable or creative electron pushing descriptions have been used.

Before the reaction starts producing 7 and 8 (up to the point when the sugar is attached to a protein through an amino acid), the Maillard reaction isn’t making any molecules beneficial for humans. The protein is actually less nutritious than before the reaction. While our bodies recycle the amino acids that we consume, modified amino acids, like the Amadori product, contain little nutritional value. Because the Amadori product (5) is produced in higher amounts than other molecules, evolutionary arguments would suggest that humans should shy away from foods that have undergone the Maillard reaction. But our personal observations tell us that this is not the case. We recognize and hunger for the aroma/flavor/color molecules that the Maillard reaction produces in relatively low amounts. The simplistic argument is that we have developed the ability to sense these molecules in cooked food because cooking kills bacteria. And food with less bacteria is less likely to make us ill.” a. (i) Are lysine and proline sugars or amino acids? (ii) Is Compound 1 a sugar or amino acid? b. Compound 3 is called a glycosylamine. What is this compound called, e.g., diol, acetal, eneamine, according to CHM 12B lecture? (Hint: see the carbon-nitrogen bond.) c. We looked how a nucleophile reacts with a carbonyl carbon in an aldehyde or ketone to form a tetrahedral intermediate. In the first step of the Maillard reaction, the amine group in a protein or amino acid reacts with an aldehyde group in a sugar to form an imine (Compound 3). (i) Which compound is the tetrahedral intermediate? Use curved arrows to show how the tetrahedral intermediate forms. (ii) Use curved arrows to show how the tetrahedral intermediate forms Compound 3. What leaving group is involved? d. (i) How does high pH, e.g., a base, affect the formation of Compound 3? In other words, how does a base react with Compound 1? Will Compound 3 form? Relate your answer to your experimental observations. (ii) How does low pH, e.g., an acid, affect the formation of Compound 3? In other words, how does an acid react with Compound 1? Will Compound 3 form? e. How is the formation of Compound 3 from Compound 1 related to the Grignard reaction? f. We looked at enols in CHM 12. See Compounds 1-8. Which of these compound(s) is/are an enol? The enol forms (tautomerizes) into a _____ because the enol is ____ stable than the ______. g. Glucose, C6H12O6, exists as a ring or chain. Which form of glucose, the chain or ring, reacts with an amino acid in the Maillard reaction? Give reasons. Draw the structures of the chain and ring forms of glucose to support your answer.

2. You will test Question 1d by making toast. An imine is produced by reacting a ___ with a ____. Take a slice of bread. Add (sprinkle, spread) a small amount of base to a small area of the bread. What base in your kitchen can (did) you use? To another area of the bread, add (sprinkle, spread) a small amount of acid. What acid in your kitchen can (did) you use? Toast your bread. Record your observations. Your toast is brown. Make a “brownness” scale from 1-5 (1 = light brown, 5 = dark brown). Draw a conclusion from your results.

Table 1. Maillard Reaction data and results on toast. Bread Brand = ______. Bread color = ______. Experiment Substance Toaster Time Color Scale Taste (Optional) Control Acid (what acid did you use?) Base (what base did you use?)

Waste Disposal: toast – in your mouth.