MAVO Year 4 · Nutrition and digestion · Section 12.1

Enzymes

Enzymes and their action

Metabolic processes, including photosynthesis, respiration and the building of organic substances, involve chemical reactions. Enzymes are biological catalysts, usually proteins, speeding up reactions without being consumed overall. They have specific active sites and usually catalyse a particular reaction or closely related group of reactions. An enzyme breaking substance A into B and C can be reused afterwards.

Temperature and pH

The rate of an enzyme-catalysed reaction is its activity. At low temperatures, reactions generally proceed slowly. Activity increases towards an optimum temperature. At excessive temperatures, the enzyme's structure changes: denaturation may make it permanently ineffective. A temperature-activity curve illustrates this pattern; there is no single universal temperature range for all enzymes.

pH also affects enzyme structure and activity. Different enzymes have different optimum pH values: some work near neutral pH, while stomach enzymes function in acid. Extreme conditions can reduce activity or denature an enzyme. Many cellular and digestive reactions take place in water-based surroundings.

Food spoilage and foodborne illness

Bacteria and fungi use enzymes to break down food. Suitable moisture and temperature can allow rapid growth and spoilage. Harmful contamination may be present even when food looks normal.

Foodborne infection occurs when pathogens in food infect the person eating it. Food intoxication occurs through toxins already present in food. Symptoms can include vomiting, abdominal pain, diarrhoea and sometimes fever. Heating does not destroy every toxin.

Preservation methods

Preservation extends food's usable life by killing microorganisms or limiting their growth.

• Chilling and freezing: low temperatures slow reactions and microbial growth. Refrigeration around 4°C and freezing at −18°C are common settings, but storage time depends on the food. Freezing does not reliably kill all pathogens.
• Pasteurisation: controlled heating, for example around 72°C for a specified short time in milk processing, kills many microorganisms but not all spores.
• Commercial sterilisation: stronger heat treatment, often above 100°C, makes sealed products stable under their specified storage conditions. UHT processes may use roughly 130–140°C or higher for a short time.
• Drying: removing available water restricts microbial growth and enzyme action.
• Packaging: suitable sealed packaging helps prevent recontamination. Vacuum packaging alone does not make food sterile; some microbes can grow without oxygen.
• Preservatives: acid, sufficiently concentrated salt or sugar, and approved additives such as nitrite inhibit particular microbes in different ways.