What Happens to Food When It Undergoes a Chemical Reaction?

January 11, 2026 •

4 min read

According to chemistry Nobel Prize winner Jean-Marie Lehn, the Maillard reaction is “by far, the most widely practiced chemical reaction in the world”. It's just one example of the myriad of processes that explain what happens to food when it undergoes a chemical reaction, affecting its flavor, texture, and appearance.

Quick Summary

Food's properties change through chemical reactions like Maillard, caramelization, and fermentation. These processes alter color, flavor, and texture, influencing everything from cooking to spoilage and digestion. Factors like heat, oxygen, and enzymes drive these transformations.

Key Points

Maillard Reaction: The browning and flavor development in cooked foods (like seared meat or toasted bread) is a chemical reaction between amino acids and sugars, not just heat.
Caramelization: This browning process only involves sugars and creates distinct, nutty flavors and colors in foods like onions or toffee.
Enzymatic Browning: The discoloration of cut fruits and vegetables, such as apples turning brown, is caused by enzymes reacting with oxygen.
Fermentation: Microorganisms like yeast and bacteria cause chemical changes that produce acids, gases, or alcohol, creating foods like yogurt, cheese, and sourdough bread.
Oxidation and Rancidity: Exposure to oxygen can cause fats to break down and become rancid, resulting in off-flavors and odors, which is a major cause of food spoilage.
Impact on Nutrition: Chemical reactions during cooking and storage can both increase (e.g., fermentation) and decrease (e.g., heat degradation) a food's nutritional value.
Acrylamide Formation: High-heat cooking methods can lead to the formation of potentially harmful compounds like acrylamide, a chemical concern addressed by food safety science.

The Diverse World of Food Chemistry

From the moment it is harvested or prepared, food is a complex and dynamic system of chemical compounds. The changes observed during cooking, storage, and digestion are not magic but are the direct result of various chemical reactions. Understanding these transformations is key to controlling the quality, safety, and sensory experience of what we eat. These reactions can be both beneficial, creating desirable aromas and textures, or detrimental, leading to spoilage or the formation of potentially harmful substances.

The Major Players: Key Chemical Reactions in Food

The Maillard Reaction

Named after chemist Louis-Camille Maillard, this reaction is a form of non-enzymatic browning that occurs when amino acids and reducing sugars react at high temperatures (typically above 284°F or 140°C). It is responsible for the signature browning and complex flavor profile of many foods. Examples include:

The dark, savory crust on a perfectly seared steak.
The rich, roasted aroma of coffee beans and baked bread.
The golden-brown crust of toast.

Caramelization

Similar to the Maillard reaction in its outcome but distinct in its mechanism, caramelization involves only the breakdown of sugars when heated to high temperatures (above 320°F or 160°C). This process creates a wide range of complex flavors and colors, from light golden to deep amber. Onions, for example, develop a rich, sweet flavor when sautéed slowly, a classic example of caramelization.

Enzymatic Browning

This reaction is caused by enzymes present in fruits and vegetables. When a fruit like an apple is cut, the enzyme polyphenol oxidase is exposed to oxygen, triggering a series of reactions that create brown pigments. This is why sliced apples and avocados turn brown over time. Many techniques, like adding lemon juice or blanching, are used to inhibit this enzymatic process.

Fermentation

An ancient preservation and flavor-enhancement technique, fermentation involves microorganisms like yeast or bacteria breaking down carbohydrates in an anaerobic (oxygen-free) environment. This produces acids, gasses, or alcohol that change the food's composition, flavor, and texture. Famous examples include the production of yogurt, cheese, and sauerkraut.

The Physical and Sensory Changes

When chemical reactions occur, the physical properties of food change dramatically. The effects are evident in:

Flavor and Aroma: The Maillard reaction and caramelization produce new volatile compounds that are responsible for the appealing smells and tastes of cooked foods. Off-flavors can also develop from oxidation.
Color: Browning is the most obvious visual change, whether from the Maillard reaction, caramelization, or enzymatic activity. Oxidation can also cause fading of pigments.
Texture: Protein denaturation, caused by heat or acid, alters texture significantly. An egg solidifies when cooked because the proteins unfold and re-bond. Gluten formation in bread dough is another example, where proteins form an elastic network that traps gas, creating a light texture.
Nutritional Value: Chemical changes can affect nutrient bioavailability. Heat can destroy certain vitamins, while fermentation can make some nutrients more accessible. The formation of new compounds, like beneficial antioxidants or potentially harmful carcinogens (e.g., acrylamide), is also a consequence.

Impact on Nutritional Value and Safety

While many reactions enhance food, others raise nutritional and safety concerns. For instance, high-temperature cooking can lead to the formation of harmful compounds, such as acrylamide, a potential carcinogen. Food scientists study these reactions to develop better processing and cooking methods to minimize these risks. The storage of food also involves chemical changes. Oxidation of fats, for example, can lead to rancidity, affecting flavor and causing a loss of fat-soluble vitamins. Understanding and controlling these reactions is crucial for extending shelf life and ensuring food remains safe and nutritious.

Maillard Reaction vs. Caramelization: A Comparison


Feature	Maillard Reaction	Caramelization
Reactants	Amino acids and reducing sugars	Sugars only
Temperature Range	Typically from 284°F (140°C)	Typically above 320°F (160°C)
Environment	Less dependent on atmospheric conditions; moisture present	Dry heat is required; low moisture
Resulting Flavors	Complex, savory, savory-sweet, toasted, and meaty	Rich, nutty, buttery, and often bitter with high heat
Foods Examples	Seared steak, toasted bread, roasted coffee, baked goods	Caramel sauce, caramelized onions, toffee, crème brûlée

Conclusion

What happens to food when it undergoes a chemical reaction is a fascinating intersection of chemistry and cooking. From the browning of toast to the tang of yogurt, chemical changes are at the heart of our culinary experiences. They can create mouthwatering aromas, transform textures, and influence a food's nutritional profile. While some reactions are intentionally harnessed for flavor, others must be controlled to prevent spoilage and ensure safety. By understanding these fundamental processes, both home cooks and professional food scientists can better appreciate and manipulate the food we consume every day. The study of food chemistry continues to evolve, shedding light on the intricate transformations that turn raw ingredients into delicious and safe meals.

Further Reading: Food Processing and Maillard Reaction Products: Effect on Health

Frequently Asked Questions

A physical change, like freezing water or chopping a carrot, alters the form but not the chemical composition. A chemical reaction, like cooking an egg or baking bread, creates new substances with different properties (e.g., color, flavor, texture) at the molecular level.

Baking bread involves multiple chemical reactions. Yeast in the dough ferments carbohydrates, releasing carbon dioxide gas. This gas gets trapped by the elastic gluten network (a result of another chemical reaction), causing the dough to rise and giving the bread its airy texture.

Cooking uses heat to cause chemical reactions that can kill harmful bacteria and deactivate naturally occurring toxins. For example, pasteurization uses heat to kill microorganisms, making milk safer to consume.

Yes, fermentation can improve health by producing beneficial probiotics and making nutrients more bioavailable. For example, fermenting legumes can reduce anti-nutritional factors and increase mineral absorption.

Lipid oxidation is a chemical reaction involving the breakdown of fats when exposed to oxygen, resulting in rancid odors and flavors. It can be prevented by using antioxidants (like vitamin E), proper packaging to limit oxygen exposure, and by storing food in cool, dark places.

Adding an acidic substance like lemon juice prevents enzymatic browning because the enzyme responsible for the reaction (polyphenol oxidase) is less active in acidic conditions. It effectively slows down or halts the chemical process.

Yes, under certain conditions, especially high-heat cooking, chemical reactions can produce potentially harmful compounds. A well-known example is the formation of acrylamide in starchy foods like potatoes when fried or baked at high temperatures.