Food spoilage and contamination

Overview

This study guide focuses on a cornerstone of the AQA GCSE Food Preparation and Nutrition specification: food spoilage and contamination. Mastery of this topic is essential, not only for achieving high marks but for understanding the fundamental principles of safe food handling. Examiners expect candidates to demonstrate a clear, scientific understanding of how and why food becomes unsafe. This involves distinguishing between sensory spoilage and the invisible threat of pathogenic contamination, applying the FAT TOM principles to control bacterial growth, and detailing practical methods for preventing food poisoning. This guide will equip you with the precise terminology, specific knowledge, and exam techniques required to confidently tackle questions on this vital subject, moving beyond common sense to a level of detail that is rewarded with maximum credit.

Key Concepts: Spoilage vs. Contamination

Food Spoilage

What it is: Food spoilage refers to the deterioration in the sensory qualities of food. It is a noticeable change in taste, smell, texture, or appearance. This is the process that makes a banana go brown and mushy, bread grow mould, or milk smell sour.

Why it happens: Spoilage is primarily caused by:

• Enzymes: These are natural proteins in food that speed up chemical reactions, like the enzymic browning of an apple once it's cut.
• Oxidation: This is a chemical reaction with oxygen in the air that can cause fats to go rancid or vitamins to be lost.
• Microorganisms: Non-pathogenic moulds and yeasts are common culprits. While they can spoil food, they are not typically the cause of acute food poisoning.

Exam Relevance: Candidates are given credit for distinguishing this from contamination. While spoiled food is undesirable, it is not always dangerous. The key is that its spoilage is detectable by our senses.

Food Contamination

What it is: Food contamination is the presence of harmful substances in food, particularly pathogenic bacteria. This is the invisible danger. A piece of chicken contaminated with Salmonella or Campylobacter will often look, smell, and taste perfectly normal.

Why it matters: This is the primary cause of food poisoning. Because there are no sensory clues, we must rely on scientific principles and safe practices to prevent it. This is a major focus for examiners, and they award marks for understanding the gravity of this distinction.

The Science of Bacterial Growth

Bacteria are single-celled microorganisms that reproduce through a process called binary fission, where one cell divides into two. Under ideal conditions, this can happen every 10-20 minutes, leading to exponential growth. Understanding the conditions that bacteria need to thrive is the key to controlling them.

FAT TOM: The Six Conditions for Growth

Examiners frequently reward candidates who can accurately recall and apply the FAT TOM mnemonic. It is a powerful tool for structuring answers on controlling bacterial growth.

• F - Food: Bacteria require nutrients to grow, particularly protein and carbohydrates. Foods high in these are known as high-risk foods. Examples include cooked meat, poultry, dairy products, eggs, and cooked rice.

• A - Acidity: Most bacteria thrive in neutral or slightly acidic conditions, with a pH between 4.6 and 7.5. Highly acidic foods like pickles (preserved in vinegar) or citrus fruits inhibit bacterial growth.

• T - Time: Given the right conditions, bacteria need time to multiply to dangerous levels. The critical window is often cited as 2-4 hours in the Danger Zone.

• T - Temperature: This is the most critical control point. Bacteria multiply most rapidly in the Temperature Danger Zone, which is between 5°C and 63°C.

• O - Oxygen: Most food poisoning bacteria are aerobic, meaning they require oxygen to grow. However, some are anaerobic (like Clostridium botulinum, which can cause botulism in improperly canned foods) and thrive in oxygen-free environments.

• M - Moisture: Bacteria need water to grow. This is measured by water activity (aw). Foods with high water activity, like fresh meat and vegetables, support bacterial growth. Dried foods like pasta, crackers, and dried fruit have low moisture and are shelf-stable.

Key Pathogens and Their Sources

Marks are awarded for linking specific pathogens to their sources and control measures.

Preventing Contamination: The Four Cs

Cross-Contamination

This is a major area of focus for AQA. It is the transfer of bacteria from a contaminated source (usually raw food) to a ready-to-eat food. This can happen directly or indirectly.

Prevention:

• Separate Equipment: Use colour-coded chopping boards (e.g., red for raw meat, green for vegetables).
• Handwashing: Wash hands thoroughly with soap and warm water after handling raw food.
• Fridge Storage: Store raw meat and poultry on the bottom shelf of the refrigerator in a covered container to prevent drips onto ready-to-eat foods below.

Cleaning

Effective cleaning removes food debris and bacteria, while disinfection kills any remaining bacteria. Use hot, soapy water for cleaning and a food-safe sanitiser for disinfection, following the manufacturer's instructions for contact time.

Cooking

Thorough cooking is one of the most effective ways to kill pathogenic bacteria. The critical core temperature that must be reached is 75°C. A digital food probe is the only reliable way to check this. For larger items like a whole chicken, the probe should be inserted into the thickest part of the meat, avoiding bone.

Chilling

Proper chilling slows down the multiplication of bacteria. Refrigerators should be set between 0°C and 5°C. Hot food should be cooled down as quickly as possible (ideally within 90 minutes) before being refrigerated. Freezing at -18°C stops bacterial growth altogether, making them dormant.