Food Technology and Chemistry

The Maillard Reaction : 

                               Something different from Caramelization

The Maillard reaction is a Chemical Reaction between an Amino Acid and a Reducing Sugar, usually requiring the addition of heat. Like Caramelization, it is a form of non-enzymatic browning but, however, it shouldn't confused with it.

 The reactive carbonyl group of the sugar interacts with the nucleophilic amino group of the amino acid, and interesting but poorly characterized odor and flavor molecules result. This process accelerates in an alkaline environment because the amino groups do not neutralize. This reaction is the basis of the flavoring industry, since the type of amino acid determines the resulting flavor.

In the process, hundreds of different flavor compounds are created. These compounds in turn break down to form yet more new flavor compounds, and so on. Each type of food has a very distinctive set of flavor compounds that are formed during the Maillard reaction. It is these same compounds that flavor scientists have used over the years to create artificial flavors.

The Maillard reaction should not be confused with Caramelization which occurs with sugars. 

Chemical and Phisical products with Maillard reactions

The Maillard reaction is responsible for many colors and flavors in foodstuffs:

• Caramel made from milk and sugar
• The browning of bread into toast
• The color of beer, chocolate, coffee, and maple syrup
• Self-tanning products
• The flavor of roast meat
• The color of dried or condensed milk

6-acetyl-1,2,3,4-tetrahydropyridine (1) is responsible for the biscuit or cracker-like odor present in baked goods like bread, popcorn, tortilla products. 2-acetyl-1-pyrroline (2) flavours aromatic varieties of cooked rice. Both compounds have odor thresholds below 0.06 ng/l.

Hystory and His Discover

The Maillard reaction takes its name from French chemist Louis-Camille Maillard, who originally described the reaction between amino acids and sugars in 1912. His study did not offer much in the way of analysis on the reaction’s impact on flavour and aroma in cooking, however; it was not until the 1950s that its mechanisms and culinary contributions would become more clearly understood.

In 1973, American chemist John E Hodge published a mechanism for the different steps of the reaction, categorising its stages and identifying a range of the different products produced as a result of these. 

He identified the first stage as being the reaction between the sugar and the amino acid; this produced a Glycosylamine compound, which in the second step rearranged to produce a ketosamine. 

The final stage consists of this compound reacting in a number of ways to produce several different compounds, which can themselves react to produce further products.

The Process 

The Melanoidins are one of the potential end products. These are long, polymeric compounds, which act as brown pigments, giving the cooked food its brown colouration. The Maillard reaction is referred to a non-enzymatic browning reaction, as these melanoidins are produced without the aid of enzymes; this differs from enzymatic browning, which is what turns fruits such as avocados brown.

Hundreds of other organic compounds are formed. A subset of these can contribute to the food’s flavour and aroma, and some of the different families of these compounds are detailed in the graphic. As a consequence of the complexity of the Maillard reaction, different amounts of different compounds can be formed in different foodstuffs, giving the wide variety of potential flavours. 

Cooking conditions can also influence the flavours produced; temperature and pH, amongst other factors, can have an influence.

The Chemistry of Process

1.      The Carbonyl group of the sugar reacts with the Amino group of the Amino Acid, producing N-substituted Glycosylamine and Water

2.      The unstable Glycosylamine undergoes Amadori rearrangement, forming Ketosamines

3.      There are several ways for the Ketosamines to react further:

o    Produce 2 Water and Reductones

o    Diacetyl, Aspirin, Pyruvaldehyde and other short-chain hydrolytic fission products can be formed

o    Produce brown Nitrogenous Polymers and Melanoidins

Key Factors

Pentose sugars react more than hexoses, which react more than disaccharides.

Different amino acids produce different amounts of browning.

Since the Maillard reaction produces water, having a high water activity environment inhibits the reaction.

Conclusion

It’s not just in your kitchen that the Maillard reaction is taking place. It also occurs at a much slower rate in our bodies, and researchers have suggested that it may have a role in the formation of some types of cataracts. It’s also been linked as a contributor to other medical conditions.

The products of the Maillard reaction aren’t all good news, however. The carcinogenic compound, acrylamide, can also be produced as a result of the reaction, and the levels of it rise as food is heated for a longer period of time.

A 2002 study found that fast food can contain particularly high levels of acrylamide, though measures have since been taken to try and reduce these levels. This gives some perspective to the discussion of carcinogens in food products; whilst, of course, we’d prefer to limit our exposure to these types of chemicals, in many cases carcinogenic compounds are already present as a natural consequence of cooking.

The Maillard Reaction in Cuisine

The Maillard reaction occurs in cooking of almost all kinds of foods, although the simple sugars and amino acids present produce distinctly different aromas. 

This is why baking bread doesn’t smell like roasting meat or frying fish, even though all these foods depend on Maillard reactions for flavor. The Maillard reaction, or its absence, distinguishes the flavors of boiled, poached, or steamed foods from the flavors of the same foods that have been grilled, roasted, or otherwise cooked at temperatures high enough to dehydrate the surface rapidly—in other words, at temperatures above the boiling point of water. 

These two factors, dryness and temperature, are the key controls for the rate of the Maillard reaction.

High-temperature cooking speeds up the Maillard reaction because heat both increases the rate of chemical reactions and accelerates the evaporation of water. As the food dries, the concentration of reactant compounds increases and the temperature climbs more rapidly.

Temperatures need to be high to bring about the Maillard reaction, but as long as the food is very wet, its temperature won’t climb above the boiling point of water.

 At atmospheric pressure, only high-heat cooking techniques can dry out the food enough to raise the temperature sufficiently. It’s not the water that stops the reaction, but rather the low boiling point at normal, sea-level pressure. In the sealed environment of a pressure cooker, the Maillard reaction can, and does, occur. 

This is something we exploit when making soups, like in ourCaramelized Carrot Soup, or purees, like the broccoli puree in our Brassicas recipe.