Maillard Reactions in Foods

Gibbs Free Energy

 In the natural world, Gibbs free energy dictates many varieties of processes, ranging from photosynthesis to coal burning.

Definition: Gibbs Free Energy

The enthalpy of the system minus the product of the temperature times the entropy of the system.¹

 ΔG=ΔH-TΔS

 

Further expanding its definition, enthalpy represents change of heat in the system, while entropy is determined by its change in disorder.

Sign of ΔH	Sign of ΔS	Sign of ΔG	Resulting Reaction Spontaneity
+	+	- with High Temperature	Spontaneous with High Temperature
+	-	+	Nonspontaneous
-	+	-	Spontaneous
-	-	- with Low Temperature	Spontaneous with Low Temperature

 

For our purposes, it is important to understand that a positive ΔH and ΔS require a high temperature for ΔG to be negative and thus spontaneous.¹

Maillard Reactions

The Maillard reaction, a reaction between amino acids and sugars that results in nonenzymatic browning in foods, demonstrates the potential necessity of high temperatures in a Gibbs free energy equation.² 

The ΔH of the first step of a Maillard reaction is positive as demonstrated by the sample problem:

Example \(\PageIndex{1}\): Determining Change in Enthalpy

The Maillard Reaction only proceeds in an open-chain form of glucose, but glucose is naturally in a ring formation. Thus, the ring form must convert to an open chain. 

                                     

Calculate H of the formation of the open chain form of glucose from the closed chain of glucose. 

 ΔH C – C: 347 kJ/mol       ΔH C == O:  736 kJ/mol        ΔH C – O: 360 kJ/mol       ΔH O – H: 463 kJ/mol

 ΔH C – H: 413 kJ/mol

Solution

           Broken     Formed

C – C       5               5

C = O      0               1

C – O      7               5

O – H      5               5

C – H      7               7

Net: ΔH = 2(337 kJ/mol) - 1(736 kJ/mol) = 18 kJ/mol

Since the ΔH is a positive value, energy is consumed in this reaction. Endothermic reactions such as these are not enthalpically favored at room temperature, which is why heat is required to ensure the reaction proceeds.

Additionally, since the open-chain form of glucose has greater freedom to rotate (allowing for more microstates) than its ring form, it’s known that the first step of the reaction has a positive ΔS.²

If both ΔH and ΔS of the system are positive, the reaction must involve a high temperature in order to raise the value of entropy and allow its negative value to dominate the system so that ΔG is also negative. 

Thus, this explains why foods that undergo Maillard reactions must be cooked at very high temperatures (close to 200℃).

Example \(\PageIndex{2}\): Calculating Entropy Based on Temperature and Enthalpy

We just determined that ΔH=18 kJ/mol in a conversion of ring glucose to open-chain glucose. Additionally, we can estimate that the temperature needed for the reaction to proceed is about 200℃.

So, we can calculate the ΔS assuming that the reaction changes from nonspontaneous to spontaneous when the temperature hits 200℃.

ΔG=ΔH-TΔS

0= 18 kJ/mol- ((200+273)K)ΔS

(-18 kJ/mol)/(-473K)= ΔS

ΔS= 0.03805 kJ (1000 J/ 1 kJ)/(mol K) 

ΔS= 38.05 J/(mol K) 

Solution

The ΔS of a conversion from ring glucose to open-chain glucose is 38.05 J/(mol K).

Maillard Reactions in Cookies

For instance, cookies undergo Maillard reactions when baking at over 150℃. The reaction, taking place between dough’s sugar and the eggs’ protein, results in a toasted, nutty flavor as well as a browned exterior.⁹ Since white granulated sugar does not participate very well in Maillard reactions because it contains mostly sucrose, it is important to also include darker sugars, such as brown sugar, molasses, and honey, in cookies because they contain more glucose and fructose— these compounds react more readily in the Maillard reaction.⁹

The Reaction's Four Steps

The first step is the reaction between glucose or fructose from the sugar and the amino acid from the proteins. This produces a glycosylamine compound that isomerises and undergoes Amadori rearrangement in the second step to form a ketosamine. After this step, the reaction can form over one-hundred different compounds that each impact the flavor of the cookie differently.⁶ The final product molecules are influenced by pH and temperature among other factors.

The only product that is always formed is melanoidins, which are responsible for the browning on the surface of the cookies. It has not been determined how to completely control the final products of the reaction, thus resulting in a unique batch of cookies for every bake.⁶

 

Maillard Reactions in Other Foods

To more broadly expand into other realms of cooking, the Maillard reaction can produce many different products that impact the flavor and color of most cooked foods. As previously mentioned, many of the smaller details of the Maillard reaction are extremely complex and consequently unknown to scientists. From what is understood, its reaction mechanism plays a large role in the formation of thousands of flavor compounds. But, there are some defined elementary reactions that make up the overall reaction progress. 

                                                       

Figure 1 Steps of the General Maillard Reaction ⁷

The 1st step involves a reaction between the carbonyl group of a sugar and an amino acid, the second step undergoes Amadori rearrangement, and the third step reacts the Amadori compound through several different pathways to form multiple products.⁶ In this mechanism, the Amadori compound is a key intermediate which is necessary to proceed with the third step. Additionally, it is known that the second step producing the aforementioned compound is the rate determining step. It is apparent that the Amadori compound is perhaps the most prominent in a Maillard reaction. 

Potential Resulting Flavors

Among many products of the Maillard reaction are heterocyclic flavor compounds. Even at very low concentrations, these molecules possess such potent flavor properties that they are able to alter the taste of foods drastically. The figure below lays out many of the potential heterocyclic flavor compounds and their associated tastes:

                                                                      

Figure 2 Potential Flavor Compounds Produced by Maillard Reactions ⁶

For example, Thiophenes are associated with a strong umami flavor, so they are produced in all cooked meats. Furans, furanones, and pyranones are responsible for a strong, sweet caramel flavor which is present in most baked goods such as bread, cookies, and muffins. Alkypyridines are responsible for the nutty and roasted flavor in coffee. Cereal and crackers are also given their distinct flavors from pyrroles and Acylpyridines respectively.⁸

References

1. Chemical thermodynamics. https://chemed.chem.purdue.edu/gench...te%20functions. (accessed Dec 6, 2022).

2. Shakoor, X.; Yang, J.; Xie, C.; Zhang, A. Maillard reaction chemistry in formation of critical intermediates and flavour compounds and their antioxidant properties. https://pubmed.ncbi.nlm.nih.gov/35696950/ (accessed Dec 6, 2022).

3. Amadori rearrangement. https://www.sciencedirect.com/topics...-rearrangement (accessed Dec 6, 2022).

4. Ren GR;Zhao LJ;Sun Q;Xie HJ;Lei QF;Fang WJ; Explore the reaction mechanism of the Maillard reaction: A density functional theory study. https://pubmed.ncbi.nlm.nih.gov/2593...20respectively. (accessed Dec 6, 2022).

5. Welner, A.; Huettl, C.; Henle, T. ACS Publications: Chemistry journals, books, and references published ... https://pubs.acs.org/doi/10.1021/jf2013293 (accessed Dec 6, 2022).

6. Compound Interest. Food Chemistry – the Maillard reaction. https://www.compoundchem.com/2015/01...llardreaction/ (accessed Dec 6, 2022).

7. Maillard Reaction: Definition, Equation, and Products https://www.chemistrylearner.com/che...llard-reaction.

8. Boekel, M. V. van. [pdf] formation of flavour compounds in the Maillard reaction.: Semantic scholar. https://www.semanticscholar.org/pape...e3f6#extracted (accessed Dec 6, 2022).

9. Doucleff, M. Cookie-baking chemistry: How to engineer your perfect sweet treat. https://www.npr.org/sections/thesalt...ct-sweet-treat (accessed Dec 6, 2022).

 

‌Contributors and Attributions

• Sophia Park (Duke University), Natalie Sanders (Duke University), Dr. Justin Shorb (Duke University), and Dr. Charles Cox (Duke University)