3. Food Science

• Why is food cooked?

  • To make it safe to eat (destroys microorganisms)
  • To make it palatable - develops flavours, texture, colour
  • Extends shelf life
  • Makes it easier to digest
• What are the three cooking methods?

  Wet (moist), dry, or fat-based
• Examples of wet methods:

  • Boiling - softens - vegetables, rice, soup
  • Steaming - tenderises - vegetables, rice
  • Stewing
  • Poaching - tenderises - fish, chicken, eggs
  • Casseroling
  • Braising - tenderises, softens - meats, fish, vegetables
  • Simmering - tenderises, evaporates - stews, vegetables, curries
• Examples of dry methods:

  • Baking - browns, crisps - cakes, pastries, potatoes
  • Roasting - browns, crisps - roast chicken, potatoes
  • Grilling/BBQ - chars, browns, crisps - bacon, toast, cheese
  • Dry frying - browns, crisps - bacon, lardons, chorizo, nuts
• Examples of fat-based methods:

  • Deep frying - browns, crisps - battered fish, doughnuts
  • Shallow frying - sets, browns - eggs, onions, fish
  • Stir-frying - softens, reduces bulk - bean sprouts, sliced vegetables
• Microwave cooking:

  • Uses a type of radiation called microwaves
  • These travel in straight lines and penetrate the food
  • The microwaves make water molecules vibrate, which creates friction, and therefore heat
  • Hot spots could occur, so food should be mixed
• Three methods of heat transfer:

  • Conduction - solids
  • Convection - liquids (in air too, convection currents are made)
  • Radiation - air
• Protein denaturation - the structure of amino acids are altered; they change shape or unfold because chemical bonds are broken.
• Denaturation can be caused by heat, decreasing pH (more acidic), adding enzymes, or by mechanical actions.
• Protein coagulation - a type of protein denaturation.

  • Starts at 60 degrees Celsius and finishes at 70 degrees Celsius
  • Examples are egg custard and quiches
  • Causes change in texture; runny eggs become coagulated (set)
• Gluten formation occurs when water is added to a wheat flour to form a dough.
• • Wheat flour contains two proteins: glutenin and gliadin, which combine to form gluten.
• Gluten in bread: makes the dough stretchy and elastic, kneading helps strengthen gluten, gives the bread structure
• Gluten in pastry: Rubbing fat into the flour makes short gluten strands (shortening) which gives the pastry a rigid structure
• Gluten in pasta: helps the pasta hold its shape, also makes the pasta dough flexible.
• Enzymic browning - oxidation of the surface of cut fruit and vegetables. Cell enzymes react with the air, like in apples.
• Enzymic browning can be prevented by:

  • Blanching cut fruits/veg in boiling water
  • Blanching veg before freezing
  • Dipping fruit/veg in acid, like lemon juice
  • Submerging in water
  • Cooking
• Oxidation - causes discolouration. Also causes vitamins to be lost, and enables enzyme activity e.g. browning.
• Gelatinisation - when starches swell to thicken liquids. Requires heat and stirring.
• Gelatinisation in sauce-making:

  • 60 degrees Celsius - starch granules begin to swell and absorb the liquid
  • 80 degrees Celsius - starch grains burst and thicken the sauce
  • 100 degrees Celsius - starches complete thickening, the sauce is ready
  • Must be constantly stirred to prevent lumps and help heat transfer
• High viscosity - very thick

  Depends on the ratio of thickening agent to liquid; more starch will result in a thicker sauce, and changes the consistency
• Dextrinization - occurs when starch is toasted or cooked by dry heat
• Starch is broken down by dry heat to form dextrins (a brown sugar)
• Dextrinization is known as non-enzymic browning
• Dextrinization gives the products a golden, brown look, with a sweeter taste and makes them crisp
• Caramelisation - causes sugar to change colour due to dry or moist heat
• Caramelisation causes surfaces to go brown on baked goods that have sugar in them
• Caramelisation is also called non-enzymic browning
• Caramelisation results in a golden colour, browning gloss, sweetness, and stickiness
• Examples of caramelisation:

  • Frying onions
  • Making a crème brulee
• Types of fat:

  • Fat from animal sources - butter, lard. These are saturated.
  • Fat from vegetable sources - margarine. These are unsaturated and are suitable for vegetarians and vegans.
• Shortening - rubbing in fat to flour to give the pastry a crumbly texture. This prevents gluten formation.
• Plasticity - the ability of a fat to change properties over different temperatures. Cold fats are solid and firm, room temperature fats are spreadable and soft, and warm fats melt and become runny.
• Aeration - incorporating air into a mixture, which increases the volume of the product
• Aeration helps products have a light and open texture; methods that help aeration are beating, whipping, creaming, and whisking.
• When creaming fat and sugar together, aeration occurs, which makes the mixture become paler, and a foam is made.

  When baked, the trapped air expands, and the cake rises.
• Emulsions - mixtures of liquids that don't normally mix, like oil and water
• Emulsifiers have two ends:

  • Hydrophilic - water-loving - forms chemical bonds with water
  • Hydrophobic - water-hating - forms chemical bonds with oil
• Hollandaise - a hot emulsion

  Mayonnaise - a cold emulsion
• How to make mayonnaise:

  • Whisk lemon juice and egg yolks together
  • Pour some oil in gradually, keep whisking
  • If the emulsion looks too thick, add some water