Why does food deteriorate, and how does the science of preservation slow it down?
Food deterioration and preservation: the causes of food spoilage (micro-organisms, enzymes, oxidation and physical damage); the conditions micro-organisms need to grow; and the scientific principles behind preservation methods (temperature control, dehydration, acidity, sugar and salt, vacuum and modified atmosphere, and heat treatment).
An SQA Advanced Higher Health and Food Technology answer on food deterioration and preservation, covering the causes of spoilage (micro-organisms, enzymes, oxidation and physical damage), the conditions micro-organisms need to grow, and the scientific principles behind preservation methods such as temperature control, dehydration, acidity, sugar, salt and heat treatment.
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What this key area is asking
The SQA wants you to explain why food deteriorates (the roles of micro-organisms, enzymes, oxidation and physical damage), the conditions micro-organisms need to grow, and the scientific principle behind each preservation method, so that you can say not just what a method is but how it stops or slows spoilage.
Why food deteriorates
The conditions micro-organisms need
The science of preservation
Common mistakes
Examples in context
Example 1. A vacuum-packed cooked ham. Removing the oxygen by vacuum packing slows the growth of oxygen-needing spoilage bacteria, and chilling keeps the product below 5 degrees, so the ham keeps far longer than an unwrapped slice. The two principles, oxygen removal and temperature control, work together.
Example 2. Pickled onions. Steeping onions in vinegar lowers the pH well below what most spoilage and food-poisoning bacteria tolerate, so they cannot grow. The acid principle preserves the onions without refrigeration until the jar is opened.
Try this
Q1. State the approximate temperature range of the bacterial "danger zone". [1 mark]
- Cue. About 5 to 63 degrees Celsius.
Q2. Explain why adding a high concentration of salt preserves fish. [2 marks]
- Cue. The concentrated salt draws water out of the food and out of microbial cells by osmosis, so there is too little available water for micro-organisms to grow.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH style5 marksExplain the conditions that micro-organisms need to grow, and explain how chilling and freezing use this knowledge to preserve food.Show worked answer →
A 5-mark answer needs the growth conditions, then how cold temperatures act on them.
Micro-organisms such as bacteria need warmth, moisture, food (nutrients) and time to grow; many also need a suitable pH and, for some, oxygen. In the danger zone of about 5 to 63 degrees Celsius they multiply rapidly.
Chilling keeps food below about 5 degrees Celsius. This does not kill micro-organisms but slows their growth and the action of enzymes greatly, so the food stays safe and fresh for longer, though only for days.
Freezing takes food below about minus 18 degrees Celsius. The very low temperature stops microbial growth almost completely and turns most of the available water to ice, so micro-organisms cannot use it; the food keeps for months. The organisms are not killed, so they can grow again once the food thaws.
Markers reward (1) warmth, moisture, food and time (and pH or oxygen) are needed, (2) chilling below 5 degrees slows growth without killing, (3) freezing below minus 18 stops growth, (4) freezing makes water unavailable as ice, and (5) thawing allows growth to resume.
SQA AH style4 marksExplain how dehydration and the addition of sugar or salt preserve food, referring to the water available to micro-organisms.Show worked answer →
A 4-mark answer should centre on water availability for all three methods.
Micro-organisms need available water to grow. Dehydration (drying) removes most of the water from the food, so there is too little for micro-organisms to grow and for spoilage enzymes to act, as in dried fruit and milk powder.
Adding a high concentration of sugar (jam) or salt (cured meat and fish) preserves by osmosis: the concentrated solution draws water out of any micro-organism cells and lowers the water available in the food. With little available water, micro-organisms cannot grow or are killed by dehydration.
Markers reward (1) micro-organisms need available water, (2) drying removes water so they cannot grow, (3) high sugar or salt lowers available water, and (4) osmosis draws water out of microbial cells.
Related dot points
- Functional properties of ingredients: the functional properties of proteins (denaturation, coagulation, gluten formation, foam formation), carbohydrates (gelatinisation, dextrinisation, caramelisation, crystallisation), and fats (shortening, aeration, plasticity, emulsification); how these properties are used and controlled in food preparation and manufacture.
An SQA Advanced Higher Health and Food Technology answer on the functional properties of ingredients, covering protein properties (denaturation, coagulation, gluten and foam formation), carbohydrate properties (gelatinisation, dextrinisation, caramelisation, crystallisation) and fat properties (shortening, aeration, plasticity, emulsification), and how each is controlled in cooking.
- Food additives and fortification: the functions of additives (preservatives, antioxidants, colours, flavourings, emulsifiers and stabilisers, sweeteners); the E-number system and the control of additives; and the fortification and enrichment of foods, including the reasons for adding nutrients and examples of fortified foods.
An SQA Advanced Higher Health and Food Technology answer on food additives and fortification, covering the functions of preservatives, antioxidants, colours, flavourings, emulsifiers, stabilisers and sweeteners, the E-number system and how additives are controlled, and the reasons for fortifying and enriching foods with examples.
- Manufacturing technology and quality: production systems (job, batch and continuous-flow production); the use of technology and automation in manufacturing; quality control and quality assurance; and food-safety management, including hazard analysis (HACCP) and critical control points.
An SQA Advanced Higher Health and Food Technology answer on manufacturing technology and quality, covering job, batch and continuous-flow production systems, the use of technology and automation, the difference between quality control and quality assurance, and food-safety management through hazard analysis (HACCP) and critical control points.
Sources & how we know this
- Advanced Higher Health and Food Technology Course Specification — SQA (2019)
- Advanced Higher Health and Food Technology (Course Code C836 77) — Planit (Skills Development Scotland) (2024)