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-enzymicbrowning
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 - rubbingin 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