Food Science

• Why is food cooked?

  • To make it safe to eat
  • Improve shelf life
  • To develop flavour
  • To improve texture
  • To give variety in diet
• Heat Transfer:

  • Conduction
  • Convection
  • Radiation
• Conduction-

  • Transfer of heat energy through the vibration of particles
  • Particles collide and pass on their heat energy
  • Processes continues until heat has passed all the way through
• Convection-

  • Transfer of heat energy through gases or liquids
  • Warmer liquid rises above its colder surroundings and takes it place
  • Colder liquid heated and when warm starts to rise and colder liquid takes its place
  • Process continues - ends up with a circulation of fluid (convection currents)
  • (in ovens- same way but hot air rises and cooler air fools)
• Radiation-

  • Transfer of heat energy through waves of radiation
  • No direct contact between heat source and food
  • Example- grills and toasters
  • emit waves of radiation- when waves hit food, they are absorbed and heat food up
• Water Based Cooking Methods:

  • Boiling (heating pan in boiling water)
  • Steaming (cooking food with steam from boiling water)
  • Blanching (part-cooking food for period of time in boiling water before putting it in cold or iced water)
  • Simmering (like boiling but more gently as temperature is slightly lower)
  • Poaching (cooking food in pan of liquid below boiling point around 80C)
  • Braising (slowly cooking food in ovenproof pot that has lid on, which contains liquid)
• Fat-Based cooking methods-

  • Stir Frying (done in wok, coated in small amount of oil)
  • Shallow frying (cooking food in frying pan, coated in medium amount of fat or oil)
• Dry Cooking methods-

  • Baking (dry heat in oven- wide variety of food can be cooked but takes long time)
  • Grilling (high temperature- fat drips off the food and outside become golden and crispy- quick to cook food but easy to burn)
  • Roasting (higher temperature- fat added to outside of food- not always healthy but can be more tasty)
  • Dry-Frying (in pan without fat or oil- healthier but takes longer to fry)
• Proteins-

  • Have a complex structure
  • When foods cook- proteins denature (chemical bonds holding structure together break down)
  • Proteins unravel and shape changes
  • Proteins denature in different ways: physical agitation (kneading, whisking), changes in temperature, acids
• Denatured proteins- COAGULATION

  • once they have denatured, protein molecules collide with one another and coagulate (join together)
  • During this process, water becomes trapped between protein molecules
  • Coagulation changes appearance, texture to food
  • Example- egg whites turn from see-through liquid into white solid
  • If over cooked- proteins tighten which forces water out of the molecules- making it dry and chewy
• Aeration-

  • Eg. chocolate mousse, whipped cream forms gas when gas becomes trapped in liquid
  • When liquid contains proteins are agitated (eg. egg whites are whisked)- proteins inside liquid denature which causes them to stretch and air becomes trapped inside the liquid
  • When proteins coagulate- air becomes trapped creating foam
  • Over whisking causes new protein bonds to break- air escapes and foam collapses
• Gluten-

  • protein found in wheat flours (made from wheat,barley,rye)
  • Formed when water is mixed with flour to make dough
  • Molecules of gluten are coiled- means they are able to stretch and bend- giving the dough elasticity
  • Dough needs to be kneaded to work the gluten- causing the strands to get longer, stronger and stretchier
  • When it reaches a high temperature, gluten coagulates and dough stays streched
• Carbohydrates- GELATINISATION

  • Helps thicken foods that contain starch (sauces, custards...)
  • When starch granules are first mixed with liquid, they become suspended in it
  • When granules are heated with water, the bonds between starch molecules start to break, allowing water molecules to enter
  • As water is absorbed, starch granules swell in size and soften
  • Between 62C and 80C, starch granules burst open and release their starch into the liquid
  • When it cools, liquid solidifies and a solid gel is formed
• Dextrinisation: (carbohydrates)

  • When starchy foods like bread and biscuits are cooked with dry heat eg.toasting
  • Starch molecules in food break down into smaller molecules called dextrins (called dextrinisation)
  • Gives food a browner colour and crispier texture and different taste
  • Longer the food is cooked, the more starch is converted to dextrins and darker and crispier the food becomes
• Caramelisation (carbohydrates)

  • Sugar molecules break down when they reach a high temperature- causes sugar to turn brown and change flavour (caramelisation)
  • First, liquid is runny and has a very sweet taste
  • Then, the sugar becomes more smooth like caramel
  • Then, eventually turns harder as it cools, becoming more like candy
  • Caramelised sugar can burn quickly, turning black, brittle and bitter to taste- to avoid this, add water in early stages of heating
• Changing properties- proteins:

  • Denaturation
  • Coagulation
  • Foams
  • Gluten
• Changing properties- carbohydrates:

  • Gelatinsation
  • Dextrinisation
  • Caramelisation
• Changing properties- fats and oils:

  • Aeration
  • Shortening
  • Plasticity
  • Emulsification
• Aeration (fats and oils)

  • when fats like butter are beaten with sugar (process is called creaming), air becomes trapped in the mixture. The air makes the mixture fluffier and lighter in colour
  • aeration gives cakes spongy and light texture when cooked
• Shortening (fat and oils):

  • When you rub fat into flour, you cover flour particles with fat - to give flour particles a waterproof coating
  • Coating prevents long gluten molecules forming when water is added to flour
  • Which means that dough can't become stretchy and have a firm texture (eg. shortbread)
  • Also used when making filled pies and tarts as the base then doesn't rise and forms a solid case
• Plasticity (fats and oil):

  • Fats have plasticity so able to spread and manipulate them
  • This is possible as fats contain mixture of different mixture of triglycerides.
  • The more plasticity a fat has, the easier it is to spread
  • Fats gradually soften over a range of temperatures
  • Useful for: decorating cakes, rubbing fat into flour, putting cream cheese on crackers
• Emulsification (fats and oils)

  Keep oil and water in stable emulsion
• Emulsions formation

  Oily and watery liquids are shaken together
• Emulsion examples

  • milk
  • margarine
  • mayonnaise
• Oils and water


  Don't mix together
• Emulsifier

  Molecules have two different ends: one is hydrophilic (attracted to water) and hydrophobic (repulsed to water)
• How emulsifier works

  1. Water molecules bond to the hydrophilic side and the oil molecules bond to hydrophobic side

  2. This holds the oil and water together in a stable emulsion, preventing them from separating
• Types of emulsions

  • Oil-in-water (eg.milk, mayonaise, salad dressing)
  • Water-in-oil (eg. margarine and butter)
• Mayonnaise is a stable emulsion of egg yolk, oil and vinegar
• Raising agents:

  • Chemical
  • Biological
  • Mechanical
• Chemical raising agents:

  • When it's heated, bicarbonate of soda breaks down to produce carbon dioxide bubbles that expand to make mixture rise
  • Has unpleasant alkaline (soapy) taste, so need to be used with a strong flavour to mask it
  • Baking powder is a mixture of bicarbonate of soda (alkali) and bicarbonate of soda (an acid). A neutralisation reaction takes place when baking powder is heated which gets rid of the soapy taste
  • Self-raising flours contain mixture of plain flour and baking powder