Understanding Food Browning: Maillard Reaction and Enzymatic Processes
Food Browning: Types and Mechanisms
Overview of Food Changes
Food undergoes various changes that can affect its quality, safety, and appearance. These changes can be broadly categorized as physical, chemical, or biotic.
Physical Factors
- UV Light
- Oxygen
- Temperature
- pH
Chemical Factors
- Oxidation of lipids
- Browning reactions
- Toxic pollutants
- Natural additives
Biotic Factors
- Microbiological activity
- Parasitological contamination
The Maillard Reaction: Non-Enzymatic Browning
Definition and Characteristics
The Maillard reaction is a complex set of chemical reactions between proteins (or amines) and sugars, typically accelerated by heat in food or similar mixtures. It is responsible for many desirable brown colors and flavors in certain foods.
Benefits and Drawbacks of Maillard Reaction
Desirable Outcomes
- Color and aroma development
Nutritional Drawbacks
- Loss of nutritional value, especially when ascorbic acid and vitamin K are involved in the reactions.
- Decreased availability of lysine.
- Decreased protein digestibility.
Factors Favoring Non-Enzymatic Browning
Carbohydrate Nature
Pentoses are more reactive, followed by hexoses (such as glucose and fructose), and then reducing disaccharides (like lactose and maltose). Sucrose is not reactive unless it is first hydrolyzed into glucose and fructose, typically by acidic conditions.
Temperature
Low temperatures slow browning. Conversely, high temperatures, such as those used in cooking, frying, drying, and pasteurization, accelerate the browning process.
Water Activity (Aw)
The maximum browning rate occurs at a water activity (Aw) between 0.55 and 0.75.
pH Level
The reactions have optimum pH levels: Maillard condensation (pH 6 to 8), transpositions (close to pH 7), and degradation of ketosamines by enolization (pH 5.5).
Storage Conditions
Preventing Non-Enzymatic Browning
- Elimination of the substrate (reducing sugars); if sugars must be added, do so after heat treatment.
- Oxidation of carbohydrates (e.g., using glucose oxidase).
- Decrease pH in foods where permissible.
- Control temperature and humidity, especially for dehydrated foods, to mitigate browning risk.
- Use of inhibiting agents, such as sulfites and sulfur dioxide (gas).
Enzymatic Browning: Mechanisms and Control
Definition
Enzymatic browning is an alteration of food products resulting from oxidative enzyme-mediated reactions, leading to the formation of brown pigments called melanins.
Occurrence
It manifests in vegetables with high phenolic content, in the darkening of insect cuticles, and in the melanin pigment of animal skin (though not typically found in animal foods). It is produced in plants by mechanical tissue damage (e.g., knocks, cuts, trimming). Enzymatic browning is desirable in some products, such as during fermentation.
Key Substrates for Enzymatic Browning
- Pyrocatechol and its derivatives.
- 3,4-Dihydroxyphenylalanine (DOPA), e.g., in potatoes.
- 3,4-Dihydroxyphenylethylamine (dopamine), e.g., in bananas.
- Acids with aromatic rings: Gallic acid, chlorogenic acid (apples and pears), cinnamic acid.
- Flavonoids: Anthocyanins (blue, violet, red pigments).
Related Phenolic Compounds
- Flavones: Contribute a bitter taste in citrus.
- Tannins: Influence color, texture (astringency), and taste (e.g., in tea).
- Lignins: Provide rigidity to plant tissues.
Reaction Mechanism
Enzymes and substrates are typically found in cellular compartments separated by membranes. To trigger the reaction, enzymes and substrates must come into contact. Any tissue damage opens these compartments, allowing contact between the enzyme and substrate, which leads to browning.
Preventing Enzymatic Browning
- Avoid bruises and mechanical damage.
- Select varieties with lower phenolic substrate content.
- Use of heat treatment.
- Application of ascorbic acid solutions.
- For peeled or cut products: immersion in salt, sucrose, or glucose solutions.
- Lowering pH (e.g., with citric acid solution).
- Vacuum packaging.
- Active packaging.
- Use of sulfur dioxide and hydrogen sulfites.