food science

• Why do we cook food?

  • to make it safe to eat: by killing bacteria and parasites, inactivating harmful enzymes and toxins
  • to develop flavour: water evaporation makes flavours more pronounced, sugar caramelisation and other reactions change the initial flavour of the food
  • to improve texture: cooking alters the texture of food products, making them easier to chew and more pleasurable to eat
  • to improve shelf life: kills microorganisms which could spoil food, so can be stored for longer
  • to increase variety: one products may be cooked in many different ways
• Conduction:

  • direct transfer of heat from saucepan to food inside. heat makes metal particles vibrate, vibrations transferred to particles of food, food particles vibrate and heat up
• Convection

  • indirect transfer of heat through water or air
  • convection current makes hot air/steam go up while colder air falls
• Radiation

  • indirect transfer of heat through heat waves
  • microwaves send electromagnetic waves, heating up water particles in food, water particles vibrate and heat up whole meal
• Microwave cooking

  • uses microwave radiation
  • microwaves vibrate water molecules creating friction, which makes heat
  • hot spots can occur so food must be stirred and left to stand to allow heat to distributed evenly
  • microwaves alone do not brown or crisp foods
• Maillard reaction

  • at high temperatures, sugar and protein react producing brown compounds which affect the colour, taste and smell of foods such as cocoa or coffee
• Water-based cooking methods

  • Steaming: helps preserve nutritional value of food. low in fat
  • Boiling: may cause vitamin loss. low in fat
  • Simmering: long time required. Causes vitamin loss
  • Blanching: prevent enzymic browning and oxidation, preserving nutritional value
  • Poaching: idea for preparing delicate ingredients
  • Braising: long time required. causes vitamin loss.
• Dry cooking methods

  • Baking: long time required. causes vitamin loss, palatability is improved
  • Roasting: helps to reduce the amount of fat in food. long time required, decreases vitamin content. helps to obtain a crispy skin on surface
  • Grilling: may create harmful substances, usually low in fat
  • Dry-frying: reduced amount of fat in food. Nutritional value preserved
• Oil-based cooking methods

  • deep-frying: foods become golden and crunchy but nutritional value is poor
  • Shallow-frying: seals surface of food to obtain crunchy top and juicy interior
  • Stir-frying: low-fat. helps to preserve nutritional value of food
• Protein Denaturation

  • occurs when structure of amino acids is altered, the change shape or unfold because chemical bonds are broken
  • Heat: cooking denatures proteins
  • pH: reducing pH by using lemon juice or vinegar is a marinade denatures proteins to make food tender, tasty and moist
  • Enzymes: enzymic tenderisers are papain (papaya) and bromelain (pineapple)
  • Mechanical actions: during whisking, protein uncoils and exposes hydrophobic areas which stick together and form a foam
• Protein Coagulation

  • aggregation of protein particles into larger lumps, causing it to set.
  • usually starts around 60C and completed by 70C
  • irreversible and causes a loss of solubility
• Gluten formation

  • occurs when water is added to wheat flour to form a dough. Wheat flour contains glutenin and gliadin, which combine to form gluten
  • two proteins cross-link with each other, creating a net which can hold air bubbles during proving and baking of bread and bakery products
• Enzymic browning

  • occurs on surface of cut fruits due to cell enzymes reacting with air (oxidation)
  • prevented by: blanching fruits or veg in boiling water
  • blanching vegetables before freezing
  • dipping fruit or veg in acid
  • removal of air by submerging in water
  • cooking
• Oxidation

  • causes discolouration
  • causes vitamin loss
  • enables enzyme activity
• Gelatinisation: happens when starch granules absorb water, swell and break during heating (80C) , causing mixture to thicken and form a gel when cooled

  • starches complete thickening at 100C so important to cook to boiling point to avoid a sauce with raw taste
  • need stirring to prevent lumps
  • the change in viscosity is due to the ratio of thickening agent to liquid; more starch = thicker sauce
  • Retrogradation is the deterioration of a starch-based sauce
  • Syneresis is the loss of fluid from foam or set-mixture
• Dextrinisation:

  • occurs when starch is cooked by dry heat
  • result of starch breakdown by dry heat to form dextrins
  • dextrins taste sweeter than starch and add flavour
  • dextrins are hygroscopic, absorbing moisture from the air
• Caramelisation

  • causes sugar to change colour and flavour due to heat
  • causes surface browning on baked goods containing sugar
  • changes the properties of sugar
  • golden colour, browning, gloss, sweetness, stickiness
• Shortening: when fat particles surround starch so that it cannot access water and therefore, prevent gluten formation

  • rubbing-in method
• Plasticity: ability of fat to be easily spreadable and melt at various temperatures, depending on the length of fatty acid chains in the fat particles
• Aeration: trapping air bubbles in a fat mixture to improve its texture
• Emulsions: mixture of liquids that do no normally mix

  Emulsifiers have a hydrophilic end and form chemical bonds with water and a hydrophobic end and forms chemical bonds with oil

  • process requires agitation by whisking.
  • requires slow addition of oil to prevent emulsion splitting
  • natural emulsifier - lecithin, present in egg yolks