The Chemistry of Wine Ageing | Fundamentals of the Maillard Reaction

A chemical reaction known as the Maillard reaction exists. This reaction is also called the amino-carbonyl reaction. The browning of meat when grilled, the appetizing golden color of baked cookies or toast, and the appealing color of fish teriyaki glazed with mirin—all of these are the result of this chemical reaction.

The Maillard reaction is also called a browning reaction and represents the primary cause of food turning brown in color. Meanwhile, it also occurs within living organisms, including the human body, where reaction products called AGEs (Advanced Glycation End-products) are said to be involved in human aging and the onset of age-related diseases.

This chemical reaction, which can be observed everywhere—in food and in the human body—can also be found in alcoholic beverages. This is aging.

The Maillard reaction is considered to be involved in the aging of fermented alcoholic beverages. Particularly in sake, the proportion of its involvement in the aging process is greater than in other fermented beverages, and it is discussed more frequently than in other alcoholic drinks. In contrast, it is rarely mentioned in the context of wine. However, the absence of discussion does not mean it is unrelated. The Maillard reaction is also partially involved in wine aging. Its presence becomes particularly significant in sparkling wines and fortified wines such as sherry.

This article explains what the Maillard reaction is and covers its fundamentals.

A Reaction Arising from Sugars and Amino Acids

The Maillard reaction can be described in one phrase as a condensation reaction between amino acids and reducing sugars. It was reported in 1912 by a French researcher named Louis Camille Maillard.

While this may sound simple when expressed in words, the content is extremely complex, with parts that remain incompletely understood even today. Moreover, since there are multiple types of both reducing sugars and amino acids, and the substances produced as a result of the reaction differ depending on their combinations, the types of compounds created by the Maillard reaction are said to reach hundreds to thousands of varieties. Another characteristic of this reaction is that it is a non-enzymatic reaction, meaning enzymes are not involved in the process.

What appears simple is, in reality, not simple at all—this is the Maillard reaction.

What Are Reducing Sugars?

The substances involved in the Maillard reaction are broadly just two: reducing sugars and amino acids. Strictly speaking, water is also necessary, but since the reaction occurs when water activity is 0.2 or higher, it does not need to be considered too strictly.

Reducing sugars are defined as "sugars that form aldehyde or ketone groups in basic solutions." Expressed very broadly, they are "sugars that act as reducing agents due to their easily oxidizable properties." They are called reducing sugars because of their characteristic ability to act as reducing agents. Additionally, the ability to form aldehyde or ketone groups means that all these sugars possess a chemical structure called a carbonyl group.

Setting aside the detailed chemical explanations, reducing sugars specifically include all monosaccharides such as glucose (grape sugar) and fructose (fruit sugar), as well as disaccharides such as maltose (malt sugar) and lactose (milk sugar). On the other hand, even among disaccharides, sucrose (table sugar) and trehalose are not included among reducing sugars.

The fact that monosaccharides are reducing sugars while sucrose (table sugar) is not a reducing sugar is an extremely important point for understanding the Maillard reaction in wine.

It should be noted that sucrose is a combination of glucose and fructose, both of which are reducing sugars. The reason why sucrose is not a reducing sugar despite being composed of reducing sugars is that the respective carbonyl groups of glucose and fructose bond together to form sucrose, preventing the carbonyl groups from forming additional aldehyde or ketone groups.

Amino Acids, Peptides, and Proteins

The other component of the Maillard reaction is amino acids. Amino acids are generally defined as organic compounds that possess both amino groups and carboxyl groups. The amino group structure holds important meaning in the Maillard reaction.

Amino acids are also the building blocks of proteins. Humans are composed of proteins made from 20 types of amino acids. In other words, wherever proteins exist, amino acids are always present, though their types may vary.

Peptides are also compounds formed by the bonding of amino acids. Compounds with two or more amino acids bonded together are called peptides, and long peptides with 50 or more amino acids connected are called proteins. Thus, proteins are peptides and are also amino acids.

Many substances produced within living organisms use proteins as their raw material. For example, enzymes are proteins with functional properties. Since proteins are also amino acids, it can be understood that enzymes are ultimately amino acids.

In alcoholic fermentation, which is indispensable for winemaking, the yeast that plays an active role uses more than 10 types of enzymes for alcohol metabolism. These enzymes used in alcoholic fermentation remain in the wine even after fermentation is complete. Both grapes and yeast contain proteins as components. In fact, wine contains many amino acids.

A Chemical Reaction Progressing in Three Stages

It is understood that the Maillard reaction progresses through three distinct stages. The first reaction to occur in this series is a condensation reaction between the carbonyl groups of reducing sugars and the amino groups of amino acids, also called glycation.

A condensation reaction is a chemical reaction in which two or more compounds react while simple compounds such as water are eliminated, generating new compounds.

In the initial reaction of the Maillard reaction, compounds called Amadori rearrangement products are generated from reducing sugars and amino acids. Subsequently, the intermediate reaction encompasses the generation of furfural, 5-HMF (5-(hydroxymethyl)furfural), and carbonyl compounds from these Amadori compounds. The final reaction involves the generation of imines and brown substances called melanoidins from the compounds produced in the intermediate reaction.

In cooking where coloring by melanoidins holds great significance, it is important for the Maillard reaction to progress to the final stage. However, regarding wine, it is considered that this final stage is rarely reached.

The Maillard reaction is accelerated by heating the target substance, but heating itself is not an essential element for the reaction. As long as reducing sugars and amino acids are present, the reaction will basically progress even at room temperature or in low-temperature environments, though it takes time. On the other hand, reactions that generate melanoidins are said to require high-temperature environments, and the environmental conditions under which wine is typically consumed or stored cannot meet these requirements.

In fact, indicators for sparkling wine aging typically use 5-HMF, which is generated in the intermediate reaction, rather than products from the final reaction.

Regarding color, since the Amadori compounds generated in the early stages of the Maillard reaction are colorless compounds, color changes in wine due to the Maillard reaction are not zero but are minimal. It should be noted that melanoidin coloring is mentioned in sake aging. This is thought to be due to differences in that sake contains far more amino acids and sugars compared to wine, in addition to differences in pH.

Intermediate Reactions Hold Great Significance for Aging

What is important in the aging of alcoholic beverages, including wine, is the change in aroma. The Maillard reaction is also accompanied by changes in aroma. These aroma changes are brought about by the generation of carbonyl compounds, pyrroles, pyridines, and α-dicarbonyl compounds during the intermediate reaction stage.

From foods that have undergone the Maillard reaction, one can often perceive diverse aromas such as the fragrant scents of bread and toast, as well as caramel, nuts, burning, roasting, and meat-like aromas. Most of these are created based on carbonyl compounds generated using Amadori compounds as raw materials. Carbonyl compounds serve as precursors to aroma components.

Along with the generation of carbonyl compounds, α-dicarbonyl compounds hold important significance. α-Dicarbonyl compounds are classified as non-sugar carbonyl compounds, but they are also produced through sugar dehydration reactions in addition to generation from Amadori compounds. It should be noted that diacetyl, which is often treated as an off-flavor in wine and sake, is also included in this α-dicarbonyl category.

These α-dicarbonyl compounds trigger a chemical reaction called Strecker degradation by bonding with amino acids, generating aldehydes with one fewer carbon atom than the original amino acids, along with carbon dioxide. This Strecker degradation also produces many volatile compounds.

Strecker Degradation Changes Aromas

Strecker degradation is positioned as a side reaction of the Maillard reaction. While it performs different reactions from the Maillard reaction pathway that generates melanoidins, it basically occurs in parallel with the Maillard reaction.

In Strecker degradation, the type of amino compound that bonds with α-dicarbonyl holds important significance. When bonding with methionine, it produces 3-propanal (beta-(Methylmercapto) propionaldehyde) with a soy sauce-like aroma, and when bonding with phenylalanine, it produces phenylacetaldehyde with a floral aroma—each generating different volatile compounds. Strecker degradation also generates volatile compounds considered to be defect odors, including sulfur odors, as well as volatile compounds created through further reactions of compounds generated by Strecker degradation, such as pyrazines with fragrant aromas.

Through the Maillard reaction and consequently Strecker degradation, the aromas of foods and wine change significantly.

It should be noted that Strecker aldehydes generated by Strecker degradation have colors ranging from colorless to yellow, also affecting the appearance of the target substance.

Related Factors

The Maillard reaction itself is basically a reaction that progresses when reducing sugars, amino acids, and a small amount of water are present. Meanwhile, numerous factors are involved regarding the speed and extent of the reaction.

Factors considered to be related to the Maillard reaction include the following:

• Temperature and time
• pH
• Sugar and amino acid composition
• Metal ions
• Pressure
• Water activity

The Maillard reaction is said to progress more easily in alkaline environments. For this reason, it has been reported that in environments like wine, where pH is low and storage temperature is also low, only about 1% or less of the contained reducing sugars undergo the Maillard reaction.

Regarding sugars, it is also known that reactivity differs by type, and even if a certain amount of reducing sugars is contained, variations in reaction progress are suggested to occur due to differences in their composition. In terms of reactivity, glucose is lower compared to other sugars, while regarding browning effects, fructose is said to have the lowest impact.

Metal ions are known to primarily affect browning. Magnesium ions have long been pointed out as promoting the Maillard reaction in beer brewing, causing browning.

In wine environments, it is also suggested that SO2 content may affect the progress of the Maillard reaction.

The Meaning and Significance of the Maillard Reaction

The elements that compose the Maillard reaction are reducing sugars, amino acids, and a small amount of water. These components are contained in a very large number of foods that exist in the world, making it extremely difficult to prevent the Maillard reaction itself. On the other hand, the fact that the taste and aroma of ingredients and dishes change through the Maillard reaction, making them feel more delicious, is one of the important significances of this chemical reaction.

The effects that the Maillard reaction brings to the human body have been debated for a long time. While the Maillard reaction generates compounds called antioxidants and antimutagenic substances that have beneficial effects on human health, it is also known that compounds with completely opposite effects may be generated. It is also pointed out that the reaction itself consumes amino acids, affecting the nutritional value of food.

Against this background, recent attempts have been made to control the Maillard reaction to obtain only desirable effects. While the verification itself is conducted on foods with high amino acid and protein content where this reaction has a greater impact, the results obtained can be meaningful for alcoholic beverages such as wine.

Unlike sake, wine has particularly low pH and relatively low amino acid and protein content, so the Maillard reaction is not considered particularly important. However, with sparkling wines that involve sugar addition and fortified wines that retain much sugar, as well as against the background of the recent natural wine boom, environments where the Maillard reaction is more likely to occur are emerging within wine bottles.

Winemakers may now need to reconsider the significance of the Maillard reaction.