Enzymatic Browning in Food: Principles and Control Methods

Protein shape is determined by amino acid sequence, hydrogen bonds, and R group interactions. Denaturation is a permanent loss of protein shape due to environmental changes. High temperatures break hydrogen bonds holding secondary structures, causing unfolding. pH affects proteins by disrupting hydrogen bonds—hydrogen ions from acids or hydroxide ions from bases interfere with electrostatic attractions. Enzyme activity follows a bell curve with temperature and pH, with optimal conditions at the peak and denaturation causing sharp activity drops beyond optimal values.

Enzyme activity is highly sensitive to environmental conditions; as temperature increases, reaction rate initially rises due to increased particle kinetic energy and collision frequency until reaching an optimum temperature (typically 37°C for human enzymes), beyond which high temperatures cause permanent denaturation by breaking bonds and altering the active site shape. Similarly, pH affects enzyme function by changing the active site's shape—extreme pH values denature enzymes, while each enzyme has a specific optimal pH (around pH 2 for stomach enzymes, around pH 7 for most body enzymes) where it functions most efficiently.

Proteins exist in four structural levels: primary, secondary, tertiary, and quaternary. Only tertiary and quaternary structures are functional. Denaturation disrupts protein structure through heat, pH changes, or chemicals, while renaturation can restore structure under certain conditions. Enzymes are classified as simple (protein only) or complex (protein plus cofactor). Complex enzymes consist of an apoenzyme (protein portion) and cofactor (non-protein portion). One apoenzyme can work with one cofactor, but one cofactor can work with multiple apoenzymes.

Low temperatures cause reversible enzyme inactivation - enzymes can regain their activity when temperature is raised. High temperatures cause irreversible enzyme inactivation (denaturation) - the enzyme's tertiary structure is permanently damaged and it cannot regain its function. For example, when milk is refrigerated, bacterial enzymes become temporarily inactive, preventing spoilage. When heated, these enzymes are permanently denatured.

Enzyme denaturation involves loss of secondary and tertiary structure, causing loss of biological activity. Denaturation is caused by high temperature, UV radiation, and extreme pH. The primary structure (amino acid sequence) remains intact. The helical structure of proteins is stabilized by hydrogen bonds. Understanding protein structure and denaturation is fundamental to biochemistry.

This section establishes the fundamental principles of redox (oxidation-reduction) reactions that underlie all metabolic processes. Oxidation is defined as the loss of electrons, loss of hydrogen atoms, or gain of oxygen atoms. Reduction is defined as the gain of electrons, gain of hydrogen atoms, or loss of oxygen atoms. These reactions always occur in pairs because electrons cannot exist freely in biological systems. Oxygen plays a crucial role as an oxidizing agent—when oxygen is added to a molecule, that molecule is oxidized. Free radicals, which result from incomplete redox reactions, are highly reactive molecules with unpaired electrons that can cause cellular damage. They are involved in cellular aging, skin aging, and can damage DNA, potentially leading to cancer. Understanding redox reactions is essential for understanding how cells protect themselves from oxidative damage.

Redox (oxidation-reduction) reactions are fundamental to biochemistry, particularly metabolism. Oxidation is loss of electrons (OIL RIG mnemonic), while reduction is gain of electrons. In biochemistry, redox is understood through electron density shifts rather than whole electron transfer. Electronegativity determines electron distribution: oxygen is highly electronegative and typically causes oxidation when bonded to, while hydrogen is less electronegative and typically causes reduction. The substance that gets reduced is called an oxidizing agent, while the substance that gets oxidized is called a reducing agent. Common biological reducing agents include glutathione, DTT, and beta-mercaptoethanol.

Oxidation-reduction (redox) reactions are fundamental biochemical processes where electrons are transferred between molecules: oxidation involves the removal of electrons from a chemical group, while reduction involves the addition of electrons to a molecule. These reactions always involve a reducing agent (electron donor) and an oxidizing agent (electron acceptor) working in complementary pairs. In biological systems, redox reactions are essential for energy production, including glycolysis (glucose oxidation), fatty acid oxidation, and oxidative phosphorylation through the electron transport chain, enabling cells to generate ATP and power metabolic processes.

Biological oxidation encompasses oxidation-reduction reactions in the body, mediated by oxidoreductase enzymes. Oxidation involves electron loss, while reduction involves electron gain, and these reactions are always coupled. Reduced molecules are represented with electrons and protons, while oxidized molecules lack them. Redox potential (measured at pH 7.0) indicates electron-donating or accepting tendency: Hydrogen (-0.42 V), NAD+ (-0.32 V), and Oxygen/Water (+0.82 V). The electron flow from NADH to oxygen has a potential of 1.1 volts.

Oxidation and reduction reactions always occur simultaneously as paired processes involving electron transfer. Oxidation involves the loss of electrons, while reduction involves the gain of electrons. These reactions require an electron donor and an electron acceptor. The term 'oxidation' relates to oxygen's strong ability to accept electrons. The mnemonic OIL RIG helps remember: Oxidation Involves Loss, Reduction Involves Gain. In biological systems, electron transfer often occurs alongside hydrogen transfer, which is crucial for understanding cellular respiration and energy production.

Plant cells differ significantly from animal cells in three key structural features. First, plant cells possess a rigid cell wall composed primarily of cellulose (a polysaccharide made of hundreds to thousands of glucose molecules) and pectin, which provides structural rigidity and distinguishes plant cells from the flexible animal cells. Second, plant cells contain large central vacuoles that store water, waste materials, and various molecules; this water storage creates turgor pressure against the cell wall, giving plant cells their characteristic rigidity. Third, plant cells contain chloroplasts, complex organelles with three membrane layers (outer membrane, inner membrane, and thylakoid membranes) that form stacked structures called grana. These thylakoid membranes house chlorophyll molecules essential for capturing light energy during photosynthesis.

Membrane-bound organelles provide evolutionary advantages by creating distinct microenvironments within the cell, enabling simultaneous occurrence of different chemical reactions. Different organelles contain different enzymes, preventing harmful reactions like lysosomal enzymes digesting other organelles. This compartmentalization enables cellular self-regulation through autophagy (destroying damaged organelles) and apoptosis (programmed cell suicide). Plant cells (eukaryotic) have distinguishing features: cell wall made of cellulose, large central vacuole occupying approximately 70% of cytoplasmic space containing water, proteins, carbohydrates, and toxic compounds, chloroplasts for photosynthesis, and nucleus displaced by the vacuole. The vacuole maintains turgor pressure for structural support, stores nutrients, and serves as a defense mechanism by storing toxins that deter herbivores.

The cell wall is the outermost layer of plant cells, primarily composed of cellulose. It has three layers: primary cell wall (cellulose, hemicellulose, pectin), secondary cell wall (cellulose, hemicellulose, pectin, lignin, cutin), and tertiary cell wall (cellulose, hemicellulose, lignin). Plasmodesmata are channels in the cell wall allowing communication between adjacent cells. Vacuoles are membrane-bound organelles surrounded by tonoplast containing cell sap (water, minerals, vitamins, proteins). They maintain turgor pressure for cell rigidity and support plant structure. In adverse conditions, vacuoles help absorb and retain water. Xerophytes have large vacuoles for water storage, while hydrophytes have small vacuoles. Animal cells typically lack large vacuoles. There are three types of plastids: chloroplasts (green, found in leaves and young stems), chromoplasts (colored, found in flowers and fruits), and leucoplasts (colorless, found in roots, seeds, and underground stems). Leucoplasts store nutrients: amyloplasts store carbohydrates, elaioplasts store fats, and aleuroplasts store proteins. Chloroplasts contain thylakoids (coin-shaped structures) that stack to form grana, where light-dependent reactions produce ATP and NADPH2. The stroma contains enzymes for light-independent reactions producing glucose. Chlorophyll (containing magnesium) gives leaves their green color. Chloroplasts are called the kitchen of plants and the center of photosynthesis. Photosynthesis includes both light-dependent and light-independent reactions that convert carbon dioxide and water into glucose using light energy, releasing oxygen as a byproduct.

Plant cell walls are rigid structures outside the plasma membrane, primarily composed of cellulose. Functions include mechanical protection, structural support, and maintaining turgor pressure. Plasmodesmata are channels through the cell wall connecting adjacent cells for communication. Vacuoles store water, ions, nutrients, and waste. In mature plant cells, a large central vacuole occupies most of the cell volume. Functions include maintaining turgor pressure (keeping plants upright), storing pigments (giving fruits and flowers colors), and regulating cellular pH and ion balance. Plastids are membrane-bound organelles found only in plants: chloroplasts (green, with grana containing chlorophyll for photosynthesis), leucoplasts (colorless, storing starch), and chromoplasts (colored, containing carotenoids for fruit and flower coloration). Photosynthesis equation: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2.

The cell wall and cell membrane are different structures. The cell membrane is thinner and found in all cells, located beneath the cell wall. The cell wall is more massive and provides greater protection, primarily composed of cellulose. Plant cells contain three types of plastids: chloroplasts (containing chlorophyll for photosynthesis), chromoplasts (colored plastids in shades other than green), and leucoplasts (colorless plastids for storing substances). Vacuoles can indicate cell age—young cells have many small vacuoles, while older cells have one large central vacuole. Plants store energy as starch, while animals and fungi store as glycogen.