Browning

BROWNING OF FOOD

• Browning occurs during processing and storage of:

  • Meat

  • Fish

  • Vegetable products

  • Fresh fruits and vegetables (especially when subjected to mechanical injury)

• Consequences of browning:

  • Impairs properties of products:

  • Color

  • Flavor

  • Nutritional properties

  • Loss of nutritional quality due to:

  • Destruction of essential amino acids

  • Decrease in digestibility

  • Inhibition of proteolytic and glycolytic enzymes

  • Production of anti-nutritional and toxic compounds that may further reduce the nutritional value and safety of foods

• Major groups of reactions leading to browning:

  • Enzymatic browning

  • Non-enzymatic browning

  • Favored by heat treatment

  • Includes:

    • Maillard reaction

    • Caramelization

    • Chemical oxidation of phenols

ENZYMATIC BROWNING

• Food color influenced by various compounds:

  • Naturally occurring pigments (chlorophylls, carotenoids, anthocyanins)

  • Colors formed through enzymatic and non-enzymatic reactions

• Enzymatic browning:

  • Involves oxidation of endogenous phenolic compounds by Polyphenoloxidase (PPO)

• Polyphenol Oxidase (PPO):

  • A cupric enzyme with active site harboring two copper atoms liganded to three histidine residues

  • Interacts with molecular oxygen and phenolic substrates

  • Also known as:

  • Polyphenolase

  • Phenolase

  • Tyrosinase

  • Cresolase

  • Catalyzes hydroxylation of monophenols to diphenols and oxidation of diphenols to quinones

• Quinones:

  • Reactive compounds formed that can create dark pigments (melanins), altering food's structural and nutritional properties

  • Visible as melanosis in processed food

FACTORS AFFECTING PPO ACTIVITY

• Temperature:

  • Activation by heat is important for food processing quality

  • Temperature/time profiles determined to prevent melanosis

• pH:

  • Optimal pH varies based on physiological conditions

• Activators/Inhibitors:

  • PPO exists in inactive/latent state, activated by agents such as:

  • Ammonium sulphate

  • Anionic detergents

  • Urea

  • Guanidine salts

  • Short-time acid/base treatment

  • Alcohol (methanol)

  • Control agents include:

  • Ascorbic acid

  • Cysteine

  • Glutathione

  • Thiourea

  • Hexylresocinol

• Substrate Specificity:

  • Activity varies based on substrate

CONTROL OF ENZYMATIC BROWNING

• PPO catalyzes oxidation of phenols to quinones; the pigmentation can be controlled by removing essential components:

  • Oxygen

  • Enzyme

  • Copper

  • Substrate

METHODOLOGIES TO PREVENT PPO ACTION

• Use of Chemicals:

  • Reducing Agents:

  • Prevent browning by reducing quinones to colorless diphenols

  • Sulfiting Agents:

  • Multifunctional, used to prevent browning, control microorganisms, act as antioxidants

  • Cost-effective but may affect texture and nutrient content (e.g., Vitamin B1)

  • Potential adverse health effects for sensitive individuals

  • Cysteine:

  • Reduces PPO activity, forms colorless cysteine-quinone adduct; may negatively impact taste

  • Acidulants:

  • Citric, malic, phosphoric acids inhibit PPO by lowering pH below optimal activity

  • Chela copper and affect enzyme's structural integrity

  • Chelators:

  • Agents like EDTA that bind to metal ions at the active sites rendering them inactive

  • Proteases:

  • Found effective in browning inhibition

Examples of Contributors to Enzymatic Browning

• Maillard Reaction Products (MRPs):

  • Prevent oxidation through various mechanisms (scavenging, metal ion chelation)

• Appropriate Technology:

  • Techniques such as:

  • Heating

  • Refrigeration

  • Freezing

  • Irradiation

  • High-pressure treatment

  • High-pressure Treatment:

  • Inactivate PPO and extend shelf-life

  • Modified Atmosphere:

  • Modified CO2 atmospheres can impact PPO activity

  • Irradiation:

  • Ionizing radiation to improve shelf life by reducing microbial activity

• Chilling Storage:

  • Reduces enzyme activity by maintaining low temperatures

NON-ENZYMATIC BROWNING

• Non-enzymatic reactions include:

  1. Degradation of ascorbic acid

  2. Lipid peroxidation

  3. Sugar-sugar caramelization

  4. Maillard reaction

MAILLARD REACTION

• Non-enzymatic browning between amino acids and reducing sugars

• Formation of Maillard Reaction Products (MRPs)

• Phases of Maillard Reaction:

  1. Early Phase:

  • Involves condensation of reducing sugar with amino groups to form glycosylamine

  1. Advanced Phase:

  • Degradation of glycosylamine products (pH-dependent)

  1. Final Phase:

  • Formation of melanoidins and brown nitrogenous polymers

• Factors Affecting Maillard Reaction:

  • Temperature

  • Reactant concentration

  • pH

  • Water activity (aw)

CARAMELIZATION

• A result of thermally induced reactions involving sugars

• Forms varying colors and flavors based on conditions (time, temperature)

• Involves an initial isomerization and then complex degradation processes that lead to color production and caramel aroma.