WJEC Level 3 Certificate in Food Science and Nutrition

8 As with all micro-organisms, bacteria need optimum conditions in which to grow and multiply: i.e.: ⦁ A suitable temperature and pH (acidity /alkalinity) ⦁ A supply of moisture and nutrients (particularly protein, fats, minerals and sugar) ⦁ The right level of oxygen (aerobic bacteria need oxygen; anaerobic bacteria do not) ⦁ Sufficient time. The image on the left shows how different temperatures affect the growth rates of bacteria. The Danger Zone (5°C to 63°C) on the thermometer is the optimum range of temperatures for bacteria to grow and multiply, and they do so very rapidly, which is why it is important to store food at or below 5°C, heat it to at least 75°C (minimum 70°C) and keep it hot above 63°C. Most pathogenic bacteria will not grow at a pH below 4.0. High-risk foods are those that favour the growth and multiplication of bacteria and include meat, poultry, fish, seafood, eggs, milk, cream, and some cheeses. Low-risk foods provide less favourable conditions for microbial growth and multiplication and include foods with a low moisture content, e.g. biscuits, crackers, cereals, and those with a high concentration of salt, sugar or acid, e.g. salted fish, jams and syrups, and pickles and chutneys. Germinate – when a seed or spore starts to grow and develop High-risk foods – foods that favour the growth and multiplication of micro-organisms Low-risk foods – foods that do not provide all the conditions that micro- organisms need to grow and multiply Spore – a) a protective outer coating formed round a bacterial cell, in which the bacterium remains inactive until the right conditions for growth return b) a tiny reproductive unit produced by moulds to allow them to spread and multiply Toxins – another name for poisons Key terms Water boils 100°C Cook from raw to at least 75°C Reheat cooked food only once to at least 75°C Keep cooked food hot above 63°C Chill food to between 0°C and 5°C Freeze food to between minus18°C and minus 24°C Bacteria spores can survive very high temperatures Bacteria cells are dead 100 °C 90 80 70 60 50 40 30 20 10 0 −10 −20 Bacteria start to die Bacteria multiply slowly The danger zone Bacteria multiply rapidly from 5°C to 63°C Bacteria do not multiply,they are dormant – alive but inactive Starts to divide One bacteria in half Two bacteria cells forming Two bacteria cells separate How bacteria multiply In optimum conditions, most bacteria multiply about every 20 minutes – some can do so in 10 minutes. Each bacterium cell divides into two new bacterium cells by binary fission in this way: Therefore one bacterium can give rise to millions more in just a few hours. You can work this out on a calculator: there are three 20-minute periods in an hour. If you start with one bacterium, in 1 hour there will be 8 bacteria, in 2 hours 64, in 3 hours 512, in 4 hours 4096, and so on. There would never be only one bacterium cell to start with in some food – there are likely to be many hundreds of different types of bacteria present; so you can see just how many bacteria could be produced and why it would be so easy for food to become unsafe to eat. Some types of bacteria are really dangerous, because if the conditions are not right for them to multiply, they will produce spores . A spore is a protective outer coating to the Section 1: Food Safety (LO1) Chapter 1: Micro-organisms and food safety