Chemical Food Preservation Methods: Preservatives & Additives

The pH scale (0-14) measures solution acidity/basicity: pH 7 is neutral, below 7 is acidic (high H+), above 7 is basic (high OH-). There is an inverse relationship between H+ concentration and pH, and a direct relationship between OH- concentration and pH. Hydrolysis is the reverse of neutralization, where salts dissociate in water to produce acids and bases. Strong acids/bases completely dissociate into ions, while weak ones remain as molecules. The acid-base nature of a salt can be predicted by examining hydrolysis products: if H+ appears, the salt is acidic; if OH- appears, it's basic; if neither appears, it's neutral. This principle helps predict solution pH and understand salt behavior in water.

Acids produce hydrogen ions (H+) when dissolved in water, while bases produce hydroxide ions (OH-). Acids are classified as strong (completely dissociate, like HCl, H2SO4) or weak (partially dissociate, like H2CO3). Bases are similarly classified as strong (like NaOH, KOH) or weak (like NH4OH). The pH scale measures solution acidity/alkalinity from 0-14, with pH 7 being neutral. pH below 7 indicates acidic solutions (high H+ concentration), while pH above 7 indicates basic solutions (low H+ concentration). Hydrolysis of salts produces acidic, basic, or neutral solutions depending on the parent acid and base. Acidic salts (from strong acid + weak base, like NH4Cl) increase H+ concentration. Basic salts (from strong base + weak acid, like NaHCO3) increase OH- concentration. Neutral salts (from strong acid + strong base, like NH4HCO3) produce neutral solutions.

Acids produce H+ ions, have sour taste, and pH < 7. Bases produce OH- ions, have bitter taste and slippery feel, and pH > 7. The pH scale (0-14) measures acidity/alkalinity logarithmically. Strong acids/bases completely dissociate; weak ones partially dissociate. Neutralization reactions produce salt and water. Salt hydrolysis determines solution pH: acidic salts produce H+, basic salts produce OH-, neutral salts produce neither. pH meters and indicator strips measure pH. Enzyme activity depends on pH, and blood pH must be maintained at 7.35-7.45 for proper physiological function.

When salts dissolve in water, they dissociate into ions. The resulting solution pH depends on the parent acid and base: (1) Acidic salts (from strong acid + weak base, e.g., NH4Cl) produce pH < 7, (2) Alkaline salts (from weak acid + strong base, e.g., Na2CO3) produce pH > 7, (3) Neutral salts (from strong acid + strong base, e.g., NaCl) produce pH ≈ 7.

The acid dissociation constant (Ka) measures the strength of an acid in solution. For the reaction HF ⇌ H3O+ + F-, Ka equals [H3O+][F-]/[HF], where concentrations of liquids and solids are excluded from the equilibrium expression. pKa is defined as the negative logarithm of Ka (-log(Ka)). Strong acids have large Ka values and small pKa values (often negative), while weak acids have smaller Ka values and larger pKa values. As acid strength increases, Ka increases and pKa decreases.

Enzymatic spoilage occurs when natural enzymes in living organisms continue working after death, breaking down tissues through autolysis or self-destruction. This causes changes in texture, color, and flavor. For example, tomatoes change color and become soft as enzymes break down their tissues. Enzyme browning in fruits and vegetables like apples and potatoes occurs when cut surfaces are exposed to air, converting colorless compounds to brown compounds. Cooking destroys these enzymes, preventing browning. Oxidation spoilage happens when food components react with oxygen, particularly after physical damage. This causes rancidity in fats and changes in color and flavor. Prevention requires air-tight packaging to minimize oxygen exposure.

Physical spoilage results from damage during harvesting, handling, transporting, and processing. High heat destroys nutrients, denatures proteins, and removes moisture; extreme cold degrades color, alters texture, and cracks surfaces permitting contamination. Pressure disintegrates structure. Foreign substances (glass, wood, excreta) pose hazards. Insect/pest activity accelerates decay through webbing and holes. Rats/mice carry pathogens on feet and feces. Chemical/enzymatic spoilage occurs through natural enzyme reactions: pectin breakdown causes fruit softening; enzymatic browning (apple exposure to air) produces brown pigments; autolysis damages skin and promotes mold growth; hydrolytic rancidity forms free fatty acids; lipid oxidation creates fishy odors. These processes reduce food quality and safety.

Food spoilage occurs through three main mechanisms: physical, microbial, and chemical. Chemical spoilage includes rancidity (oxidation of fats producing aldehydes and ketones causing off-flavors), enzymic browning (enzyme-catalyzed oxidation in fruits like apples), non-enzymic browning (caramelization), and freezer burn. Microbial spoilage involves microorganisms producing toxins such as patulin (from Penicillium in apple juice), aflatoxin (from Aspergillus in ground nuts), and ochratoxin (from Aspergillus in coffee beans and grapes).

Food alterations include mechanical damage, chemical changes (hydrolysis, oxidation), and microbial spoilage. Lipid hydrolysis releases free fatty acids causing off-flavors. Lipid oxidation requires oxygen and produces free radicals, catalyzed by heat and light. Ascorbic acid is destroyed by heat and oxygen. Mechanical damage accelerates spoilage. Microbial spoilage occurs mainly by yeasts and molds due to low pH, with yeast fermentation producing alcohol and CO2. Bacteria like Salmonella and E. coli are introduced through handling and irrigation. Clostridium botulinum poses serious health risks in low-acidity, anaerobic environments, requiring careful thermal processing.

Food spoilage occurs when food becomes unfit for consumption due to various factors: physical factors (temperature, moisture, mechanical damage, time), chemical factors (pH value), and biological factors (enzymes, microorganisms, macroorganisms). Enzymatic browning occurs when polyphenolic compounds react with oxygen via polyphenol oxidase enzyme, while non-enzymatic browning includes caramelization (sugar heating) and Maillard reaction (sugar-amino acid reaction). Rancidity in fats occurs through autoxidation (oxygen reaction) or enzymatic oxidation (water reaction with lipase enzymes).

Facilitated diffusion enables charged or polar solutes to cross the membrane via protein assistance. Channel-mediated diffusion uses water-filled channels specific to certain ions, with leak channels remaining open continuously and gated channels opening only in response to stimuli. Carrier-mediated diffusion involves proteins changing shape to transport polar molecules like glucose. Osmosis is the passive water movement through selectively permeable membranes via aquaporins or between phospholipids, driven by water concentration differences. Active transport requires energy, including primary active transport (ion pumps using ATP directly, such as the sodium-potassium pump) and secondary active transport (using energy from another substance's gradient movement).

The cell membrane is a thin, elastic, flexible structure (7.5-10 nm thick) primarily composed of proteins and lipids, with cholesterol and carbohydrates. Its primary function is separating the extracellular from intracellular environments through the lipid bilayer. The lipid bilayer consists of two layers of phospholipid molecules, each with a hydrophilic phosphate head and hydrophobic fatty acid tail. The hydrophobic tails attract each other to form the membrane core, while hydrophilic heads face aqueous environments. Membrane permeability depends on substance structure and cholesterol concentration.

Membrane transport mechanisms include passive processes (simple diffusion for small non-polar molecules like O2, CO2, steroid hormones, and lipid-soluble drugs moving down concentration gradients without energy, and facilitated diffusion using channels/carriers for charged/large molecules like glucose and ions) and active processes (primary active transport directly using ATP via ATPases like the sodium-potassium pump, and secondary active transport using gradients established by primary transporters for cotransport like sodium-glucose symporters); vesicular transport encompasses endocytosis (pinocytosis, phagocytosis, receptor-mediated endocytosis for LDL uptake) and exocytosis for secreting neurotransmitters, hormones, and other proteins.

The cell membrane is the structure delimiting all cells, organized according to the fluid mosaic model (Singer and Nicholson, 1972). Its main components are: phospholipids forming the bilayer matrix, proteins (integral and peripheral) forming channels and receptors, cholesterol (in animal cells) providing rigidity, and glycans on the external surface for cell recognition. The membrane is a lipoprotein structure that is fluid and selectively permeable. Its functions include controlling substance passage, maintaining chemical differences between cell interior and extracellular fluid (essential for nerve impulse transmission), and regulating internal cellular content. Cellular transport is classified into passive transport (along concentration gradient, no energy) and active transport (against gradient, requires ATP). Passive transport includes diffusion (simple through lipid bilayer for small lipophilic substances like O2, CO2, alcohol; facilitated through proteins for ions and glucose) and osmosis (water movement). Solutions are classified relative to cell interior: hypotonic (lower solute outside), isotonic (equal), hypertonic (higher solute outside). Active transport uses carrier proteins (pumps like sodium-potassium pump moving 3 Na+ out and 2 K+ in per ATP) and bulk transport via vesicles. Endocytosis brings materials in: phagocytosis (cell eating) engulfs solid particles like bacteria; pinocytosis (cell drinking) takes in dissolved substances. Exocytosis expels materials like waste or secretory products (insulin).