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Set 3 : The science of food
Q
No.
Question Answer
1. Why do we cook food?^ I.^ To aid digestion,
II. To improve the palatability (taste, texture and appearance), III. To avoid food contamination
2. What are the^3 types of heat transfer?^ I.^ Conduction
II. Convection III. Radiation (infra-red and microwave)
3. How is heat transferred to food by
conduction? Heat is transferred by direct contact with a hot surface
4. Give some examples of how food is
cooked using conduction? Dry-frying, griddling, searing, sautéing.
5. How is heat transferred to food by
convection? Heat is transferred when heated particles (liquid or gas) move into a cooler area.
6. Give some examples of how food is
cooked using convection? Baking, roasting, deep frying, boiling, simmering, poaching
7. How is heat transferred to food by^ infra-
red radiation? Heat is transferred using electromagnetic waves of heat or light.
8. Give some examples of how food is
cooked using infra-red radiation? Toasting, grilling, barbequing
9. How is heat transferred to food by
microwave radiation? The microwave oven converts electrical energy to microwaves which penetrate the food
10. Give some examples of how food is
cooked using microwave radiation? Defrosting frozen food, heating up leftovers, ready meals.
11. Give an example of a^ dish^ which uses
more than one method of heat transfer in its production. E.g. roast potatoes are cooked by convection in the air around them in the oven and conduction from the pan in which they sit.
12. Give an example of a^ choice of cooking
method which preserves nutritional value? E.g. steaming rather than boiling retains the water- soluble vitamin content of vegetables.
13. Describe how starch can be used to
thicken a sauce by gelatinization. When starch is heated in a liquid the starch granules soften and absorb water and the mixture thickens. Gelatinization (when a gel is made) happens at 66oC and above.
14. What happens when starchy foods are
cooked in a dry heat? Dextrinisation – the food turns brown and the flavour changes.
15. What happens when sugar^ is^ cooked?^ Caramelisation^ –^ the food turns brown and the
flavour changes.
16. Describe the two ways fats are used to
create texture in cakes and pastry?
- Shortening - fat coats the flour particles, making a waterproof (or hydrophobic) coating, so the flour doesn’t absorb water and gluten development is prevented.
- Aeration – when creamed with sugar, fats aerate the mixture (trap tiny air bubbles) which helps make it rise.
17. What happens to fat as they are cooked?^ They soften and liquify and add flavour and colour.
18. What do we mean by the plasticity of
fats? Plasticity is the ability of fats to be spread and shaped.
Set 3 : The science of food
19. What does the plasticity of different fats
depend on? Each fats plasticity depends upon the temperature at which it liquifies.
20. What is an emulsion?^ A fine mixture of droplets of one liquid in another.
21. What is a stable emulsion?^ A stable emulsion is one that won’t separate.
22. How can we make a stable emulsion?^ Using an emulsifier, like egg yolk.
23. What happens to proteins when they are
cooked, beaten or have acid added to them? The structure of the protein is irreversibly changed as the molecules denature (unravel) and coagulate.
24. What happens when protein rich foods,
like meat, are cooked in a dry heat? The Maillard reaction – the food turns brown and the flavour changes.
25. How is a foam created? Give some
examples of foams. A protein is aerated, by whisking or whipping, which denatures the proteins making them light and airy. E.g. beaten egg whites, whipped cream.
26. How is gluten formed?^ When flour is mixed with water two proteins in it
(gliadin and glutenin) combine to create gluten strands. These strands will get stronger as the dough is kneaded.
27. What is strong flour, what would you use
it for and why? Strong flour contains more of the proteins which form gluten, which makes the dough elastic. It is used to make breads, pasta and choux pastry, which need a chewy texture.
28. What is soft flour, what would you use it
for and why? Soft flour contains less protein so less gluten is formed and is used to make cakes, biscuits and scones which need a light texture.
29. What effect does oxygen have on fruit
and vegetables? Oxygen reacts with the enzymes in cut fruit and vegetables making them brown – enzymic browning.
30. How can enzymic browning be slowed
down? Put cut vegetables in cold water and toss sliced fruit in lemon juice (citric acid).
31. Whys do we use raising agents?^ They produce a risen, light airy texture in food.
32. How do mechanical raising agents work?^ They trap air, which expands when heated.
33. List 5 ways of incorporating air into
recipes and give an example of each. i. Sieving flour (cakes) ii. Whisking eggs (meringue) iii. Rubbing in fat into flour (pastry) iv. Creaming fat and sugar (cakes) v. Lamination traps air between layers (flaky pastry)
34. Describe how steam is a physical raising
agent. In products containing a lot of water (like Yorkshire pudding or choux pastry) the water turns into steam and pushes the batter upwards. The heat of the oven set the flour and egg mixture around the hollows created by the steam.
35. What products are used to add CO^2 to
create a light, airy baked texture? Bicarbonate of soda, baking powder and self-raising flour.
36. Why should food be fried in hot fats or
oils? If the fat is not hot the food will absorb the fat or oil, becoming greasy.
37. Why do cakes sink in the middle?^ If they are not cooked for long enough or the oven
door is opened during cooking.
38. Why might a cake^ surface crack?^ The oven was too hot or too much raising agent was
used.
Set 3 : The science of food
57. Name and explain the 3 ways food can
be contaminated? i. Physical contamination - objects falling into food ii. Chemical contamination – cleaning products and pesticides in food iii. Bacterial contamination – microorganisms (bacteria, viruses, moulds and fungi) in food
58. What type of bacteria cause disease?^ Pathogenic
59. How does^ the bacteria which
contaminates food come from? i. Poor hygiene ii. Raw meat and poultry iii. High risk foods iv. Pests v. Dust, dirty bins, waste food vi. Contaminated water
60. What is cross-contamination?^ When bacteria from one food is transferred to
another, as a result of poor hygiene.
61. List the high risk foods.^ i.^ All animal protein foods (dairy, eggs, meat,
poultry, fish, shellfish) ii. Gravies, soups and stocks iii. Cooked rice and pasta iv. Ready-made meals
62. What are the^ symptoms of food
poisoning? Tummy pain, diarrhoea, nausea, vomiting, fever or chills.
63. Name 4 pathogenic bacteria and say
where they are found. i. Campylobacter – raw poultry, meat, milk, sewage ii. Salmonella – human gut, raw poultry, meat, eggs, milk iii. Staphylococcus A – in human skin, hair, throat, nose, mouth, cuts, spots iv. E. coli - human and animal sewage, water, raw meat, muddy vegetables
64. What is the danger zone?^ Temperatures in between 8
oC and 63oC, at which bacteria will grow rapidly.
65. How can foods be preserved to keep
them safe for longer? i. Heating, e.g. pasteurisation ii. Freezing – bacteria will stop reproducing at low temperatures, but will not be killed iii. Drying e.g. dried noodles or pasta iv. Removing air, e.g. in cans and other packaging v. Chemical, e.g. preserving in vinegar, salt, sugar, smoke vi. Irradiation – low doses of radiation will kill all microorganisms
66. How can mishandling of food lead to
increased food waste? If food is mishandled it becomes spoiled and has to be thrown away.
67. What are the benefits of reducing food
waste? It helps the environment (less landfill) and saves us money.
