3.3 Food Science

why do we cook food

• safe to eat→high temps kill bacteria

• longer shelf life → killed bacteria so preserved and stays fresh

• flavour→chemical reactions sweeten or intensify flavours

• texture→more pleasant, softer to chew

• variety→same food can be cooked many different ways

how do the 3 methods of heat transfer work

conduction: through vibrating particles

1. in a solid, particles are closer together so one vibrating moves the others nearby

2. heat from a hob goes to pan to food causing particles to vibrate, metal is a good conductor so used in pans

3. heat passes all the way through food cooking it

convection: through gas or liquid

1. warm liquid/air rises and cooler liquid takes its place

2. top is further from heat source so coolsa gain

3. causing a circulation of fluid/air called a convection current

4. this results in all of the fluid being heated

radiation: through waves

1. waves reach food and are absorbed heating up food in toasters and grills

2. microwaves can only heat the water, fat and sugar molecules in food, and then they transfer energy by conduction

what are the 6 water-based cooking methods: (method, for, + and -)

boiling: heat in a pan of boiling water
for tough meat, potatoes, pasta, rice, veg
- lose water soluble vitamins, too harsh for delicate foods

steaming: cook with steam from boiled water
for fish, rice, vegetables
+ no direct contact w water so no water soluble vitamins lost, gentle

blanching: partially cook in boiling water before putting in cold water
for fruits and vegetables
+ makes skins easier to remove, preserve vitamins, removes harsh flavour from onions

simmering: boiling at a lower temperature
for soups, curry
+ preserve nutrients

poaching: simmering at a lower temperature
for eggs, fruits, fish
+gentle so keeps food tender, can simmer in other liquids e.g sauce for flavour

braising: slow cook in a liquid w herbs and veg in a covered pot
+tenderises meat, liquid adds flavour -takes a long time

2 fat based cooking methods( method, for, + and -)

stir frying: uses a small amount of oil in a pan/wok
for noodles, vegetables and small pieces of meats
+ keeps nutrients, quick - has to be moved constantly or burns

shallow frying: frying pan w a medium amount of oil
for meat, fish, eggs
+crispy -unhealthy

name:

the 4 ways to change properties in proteins

the 3 ways to change properties in carbohydrates

the 4 ways to change properties with fats

proteins: denaturation, coagulation, aeration, gluten formation

carbohydrates: gelatinisation, dextrinisation, caramelisation

fat: aeration, shortening, plasticity, emulsification

denaturation

• proteins have complex structures

• cooking denaturises them which irreversibly breaks down the chemical bonds holding them together

• they unravel, changing shape

• this can happen by adding heat, an acid e.g lemon juice, marinade, or physical agitation e.g whisking, beating, or kneading

coagulation:

• once denatured, proteins can join together with other molecules(coagulate)

• water gets trapped and the texture and appearance changes e.g egg: see through→white, steak red→brown and firm

• overcooking means too much coagulation so water is pushed out and it becomes dry and chewy

aeration:

• foam forms when a gas is trapped in a liquid

• when agitated by whisking protein denatures and stretches trapping air

• coagulation traps it in forming a foam

• when overwhisked new protein bonds break and air escapes so the the foam collapses

• some foam becomes solid when cooked e.g egg whites→meringues

gluten formation:

• gluten is a protein in wheat flours

• gluten molecules are coiled so they can bend and stretch

• kneading dough works the gluten so it is longer, stronger and stretchier

• at high temperatures, gluten coagulates and dough stays stretched resulting in a light, airy texture

gelatinisation

• when starch granules are mixed with a liquid, they become suspended in it, or sink if not stirred

• heat breaks the bonds between the starch molecules, allowing water to enter

• this softens and swells the starch

• at 62-80 C, they burst open releasing starch into the liquid causing it to thicken, more starch→thicker liquid

• when cooled it solidifies forming a solid gel useful for et desserts e.g lemon pie filling

• this also happens in water boiling pasta or rice

dextrinisation

• dry heat e.g toast, bake

• this breaks down starch into smaller molecules called dextrins that make food crispy and brown

caramelisation

• at high temperatures sugar breaks down turning brown and sweeter

• runny and sweet → smooth caramel→hard and candy like when cool

• it can burn easily, turning black and bitter so water is added

• it gives cream brulee and apple pie sweetness and foods containing sugars e.g onions can caramelise

aeration

• creaming(beating butter and sugar) traps air into the mixture

• this makes it fluffier and lighter in colour and have a spongey and light texture after being cooked

• also in whisking egg whites, or beating mixtures

shortening

• rubbing fat into flour gives it a waterproof coating

• this prevents long gluten molecules forming when water/liquid is added so the dough can’t become stretchy

• this gives the dough a firm crumbly taste e.g shortbread

• useful for the base of filled pies not to rise

• some fats are 100% fat w no water so no steam or gluten formation to rise food

plasticity

• ability to spread/shape, more plasticity→easier to spread

• this is because fats have a mixture of triglycerides that melt at different temperatures so gradually soften

• unsaturated fats are liquid at room temp so have a higher plasticity and triglycerides with a lower melting point

• useful for buttercream to make cake, spreads on sandwiches, shortening

emulsification

• oil and water shaken together makes emulsions e.g milk, mayo, margarine

• oil and water don’t mix so they separate unless you add an emulsifier

• they contain a hydrophilic(attracted to water) and hydrophobic(repulsed by water) end

• so when water bonds to phillic end and oil to the phobic end, they are held together in a stable emulsion and can’t be separated

• eggs and soya contain lecithin-a natural emulsifier so are used in margarine

• oil-in-water emulsions: mayo, milk, salad dressing

• water-in-oil emulsions: butter, margarine

chemical raising agents:

• bicarbonate of soda breaks down to produce CO₂ bubbles that expand so mixture rises

• it has a soapy, unpleasant taste that needs a strong flavour e.g chocolate, gingerbread, to mask it

• baking powder=bicarbonate of soda(alkali) + cream of tartar(acid) so a neutralisation reaction occurs so no soapy taste

• self raising flour= plain flour + baking powder

biological raising agents:

• yeast is a microorganism that causes fermentation to release CO₂ and alcohol

• proving(allow to rest in a warm place) helps this happen

• the CO₂ gas is released and trapped in the dough helping it to rise

• during baking the heat kills the yeast but the CO₂ expands rising the food even more and the alcohol evapourates

steam as a raising agent:

• puff pastry, choux pastry and batters contain lots of liquid

• water leaves as steam that rises, raising up the mxitrue

• this makes the mixture become more solid, but it the oven is open during cooking, cold air causes it to sink

mechanical raising agents:

• manually add air to mixtures by beating(vigorously driving air in with a spoon or fork when mixing) or whisking(beating w a whisk) or creaming(beating butter and sugar)

• sieving traps air between the individual flour particles.

• air can also be folded into mixtures with a spoon/spatula to pour liquid mixtures over itself trapping air, or by folding pastry doughs into layers trapping air between each fold