Tools of the trade: understanding the Maillard and Strecker Degradation
The Maillard reaction is a very important flavour producing reaction, as illustrated when sugar forms brown nutty-flavoured caramel, in a process called non-enzymatic browning. It is also an important reaction in developing the flavour compounds in tea and coffee. Before coffee beans are roasted, they lack the aroma of roasted coffee beans, and the Maillard reaction is very important in providing the characteristic flavour of a brew.
The first step of the Maillard reaction is a sugar-amine condensation to form N-substituted glycosylamine. The carbonyl group on the glucose sugar (C=O in red) reacts with the amine group (NH2 in green) in a protein or amino acid in the food to form an imine bond (in pink) in the N-substituted glycosylamine, together with water. Notice that the sugar (glucose) reacts when it is in an open-chain form (you may remember that for most of the time, glucose exists in a cyclic form).
The mechanism for the first step of the Maillard reaction involves the nitrogen lone pair from the amine group attacking the electrophilic carbon atom on the aldehyde carbonyl (C=O). Electrons are pushed from the double bond onto the oxygen atom giving it a negative charge. A proton is then transferred from the positively charged nitrogen to the negatively charged oxygen atom forming a hemiaminal (in pink). The oxygen atom then donates its lone pair to a proton forming –OH2+, which is a very good leaving group. Finally, the nitrogen can donate its lone pair to form a new C=N bond of the imine in the N-substituted glycosylamine; in the process, it kicks out H2O as a leaving group.
The second step in the Maillard reaction is the Amadori rearrangement; a spontaneous reaction even at temperatures as low as 25 °C. The Amadori rearrangement is an isomerisation reaction (the N-substituted glycosylamine, 1,2-enaminol and the Amadori compound all have the same chemical formula, with the atoms arranged in different ways, and so they are isomers of each other) that results in the formation of a ketosamine called the Amadori compound – this contains both a ketose (a sugar bearing a ketone) and an amine.
The isomerisation reaction (called an imine-enamine tautomerism) that occurs between N-substituted glycosylamine and 1,2-enaminol is reversible. The nitrogen atom in N-substituted glycosylamine donates a lone pair of electrons to a proton, forming a new N–H covalent bond; following this, the two electrons in the C–H bond (in pink) are donated to form a new carbon-carbon double bond, pushing the electrons in the C=N double bond onto the positively charged nitrogen atom.
The second reversible isomerisation reaction occurs between the 1,2-enaminol and the Amadori compound, and is an example of keto-enol tautomerism. In the forward reaction, two electrons from the O–H oxygen (in the 1,2-enaminol) are donated to a proton, through the C=C double bond, and this is followed by deprotonation.
The ketosamine can then react to form several different products, including hydroxypropanone, all of which can react again to form even more products. The products that are formed depend on whether the reaction mixture is alkaline or acidic, so, this is a complicated process.
Melanoidins are a class of brown polymers with high molecular weights, and one of the potential end products of the Maillard reaction; melanoidins are the species that give cooked food its colour. The analysis of melanoidins is challenging, meaning that it is hard to assign the structures to the end products of the Maillard reaction; this difficulty stems from the vast numbers of Maillard reaction products and the difficulty in purifying and identifying individual products.
So, there is still a lot about the Maillard reaction that remains unclear, however, its responsibility for the flavour development of cooked or roasted foods is recognised as being very important.
As the Maillard reaction is such a complicated process, with a number of tricky reaction steps, we have summarised the key steps in the poster below.
Alongside the Maillard reaction, the Strecker degradation (established in the 1960’s) also plays an important role in the formation of flavour compounds - take a look at the information below.
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