unit 2 food science DOES NOT HAVE DETAIL ON ROASTING OR DRY FRYING OR FAT BASED COOKING METHODS

fn aqa gcse

Why is food cooked?

To make it safe to eat, develop flavours, improve texture, appearance, and shelf life and to produce a variety of foods

How does cooking food make it safe to eat?

Destroys bacteria and toxins

How does cooking food improve taste, texture and appearance?

Causes chemical reactions (e.g. sugar caramelises), concentrates and intensifies flavour (e.g. by evaporating water), tenderises meat, gelatinises starch, melts fats, softens cell structure in vegetables, helps raw dough rises and become light/airy, helps some food develop a crunchy outside

How does cooking food improve shelf life?

Destroys harmful micro-organisms (e.g. bacteria, mould) which preserves food

How does cooking food give a variety of food?

Different ways a food product can be cooked, e.g. potatoes can be boiled, baked, fried, cut into wedges or chips, mashed and more

What are the 3 methods of heat transference?

Convection, conduction and radiation

How does conduction work?

Particles gain kinetic energy and begin to vibrate as they heat up, as they’re tightly packed in a lattice structure in metals (e.g. saucepans, baking trays) they transfer the heat energy to nearby particles when they knock into them, heat spreads evenly throughout item and transfers to food - poor quality saucepans (etc) may warp when heated, meaning the heat won’t spread evenly

Give 3 examples of conduction in cooking

Baking, roasting and dry frying

How does convection work?

Heat energy transfers from pan/kettle etc to fluid (gas or liquid) and heated molecules move to where it’s cooler, meaning the cold molecules move to where it’s warm and become heated, this pattern repeats causing convection currents, when there’s enough heat energy throughout a liquid it begins to boil, means the liquid is evenly heated and heat transfers to food in the fluid or evaporates as hot steam and cooks food through steaming, occurs in ovens as air forms convection currents as well

Give three examples of convection in cooking

Boiling, poaching and simmering

How does radiation work?

The food has to be directly in line with the light source as radiation uses infrared rays which hit the food, are absorbed and heat the food up to cook the food however if the food is too close, the outside might become overcooked while the inside is undercooked

Give 3 examples of radiation in cooking

Grilling, barbequing and toasting

What are the 6 main water based cooking methods?

Steaming, boiling, simmering, blanching, poaching and braising

What are the 4 main dry cooking methods?

Baking, roasting, grilling and dry frying

What are the 2 main fat based cooking methods?

Shallow frying and stir frying

Describe how food is boiled and give a pro and a con of boiling food

Boiling involves cooking food in water at around 100^oC and is healthy as no fat is needed, however, boiling vegetables for too long can result in loss of water-soluble vitamins, colour and flavour

Give 3 examples of food that would be boiled

Pasta, potatoes and vegetables

Describe how food is steamed and give a pro and a con of steaming food

Steaming food involves cooking it using the steam produced by a boiling liquid, there’s no fat or water involved, so less fat or water-soluble vitamins are lost, but there might not be as much flavour in steamed food compared to roasted/fried etc food

Give 3 examples of food that would be steamed

Fish, rice, vegetables

Describe how food is simmered and give a pro and a con of simmering food

Simmering is similar to boiling just at a lower temperature, it preserves more nutrients than boiling but may take longer

Give 3 examples of food that would be simmered

Vegetables, soups, curries

Describe how food is blanched and give a pro and a con of blanching food

Blanching is when food is partially cooked in boiling water for a very short time then put in cold or iced water which stops the cooking process, this helps preserve colour, flavour and nutrients and prepares food for freezing, but some nutrients are still lost during the boiling

Give 3 examples of food that would be blanched

Vegetables, fruit and almonds

Describe how food is poached and give a pro and a con of poaching food

Poaching involves cooking food in a shallow pan of water or wine just under its boiling point, as food can be poached in flavoured sauces they can absorb the flavour and bring variety, however nutrients, flavour and colour can be lost in the water

Give 3 examples of food that would be poached

Fish, eggs and fruit

Describe how food is braised and give a pro and a con of braising food

Braising involves slowly cooking food in a sealed dish with little liquid (wine, stock, water etc) - when meat is braised, it’s usually fried before hand to brown and seal the meat, keeping the juices inside the meat - any meat in the pan will absorb the flavours of the liquid, herbs, other ingredients etc which will give it more flavour, but some nutrients may be lost in the liquid

Give 3 examples of food that would be braised

Meat, poultry, vegetables

Describe how food is baked and give two pros and cons of baking food

Baking food involves cooking it in a hot oven, is healthy as no fat is added and can cook a variety of foods, however can take a long time and uses a lot of energy

Give 3 examples of food that would be baked

Pastries, potatoes, pizzas

Describe how food is grilled and give two pros and cons of grilling food

Grilling food involves cooking food using intense radiant heat under a heated grill element in an oven or above charcoal/flames in a barbeque - grilling cooks food quickly and produces a crispy texture and smoky flavour, but it can be hard to cook food evenly and can lead to cross-contamination when having both cooked and raw food on a barbeque

Give 3 examples of food that would be grilled

Sausages, burgers, halloumi cheese

What happens when proteins denature?

The chemical bonds holding them together break and they unravel, leading to an irreversible change in shape - this can be caused by physical agitation (e.g. whisking, beating), change in temperature (e.g. heat) or acids (e.g. lemon juice, marinades)

What happens when proteins coagulate?

The denatured protein molecules collide with other protein molecules and they join together, trapping water molecules between them, which changes the taste and texture of the food (e.g. egg whites change from translucent to white, steak becomes firmer as it’s cooked) - if food is overcooked, the proteins coagulate too much and the protein molecules tighten, forcing the water out and drying out the food

How are foams formed and what can go wrong? (6)

Foams are formed when gas becomes trapped (aeration) inside liquid - when liquids containing proteins are agitated (e.g. egg whites are whisked), the proteins denature and then coagulate, trapping the air in the proteins and creating a foam, some of which form a solid structure when cooked - if the liquid is agitated too much (e.g. overwhisked), the new proteins break and the air is released, causing the foam to collapse

How does gluten allow doughs to stretch and rise? (7)

Gluten is a protein found in wheat flours and is formed when water is mixed with the flour to make a dough, gives all doughs elasticity due to their coiled shape and ability to stretch and bend. When doughs are kneaded, the gluten strands become stronger, stretchier and longer, and when they reach a high temperature (e.g. when bread doughs are baked) they coagulate and the dough stays stretched, giving the food a light and airy texture

Why do some recipes tell you to leave a dough to rest before cooking it?

This allows the gluten strands to relax and return to their original shape - if they were cooked while still stretched, the heat would cause the stretched gluten to shrink rapidly (e.g. a pastry would become too small and filling would spill)

How does gelatinisation work? (8)

When starch granules are mixed with liquid they become suspended in the liquid and sink to the bottom if not stirred, and when they’re heated, the bonds between starch molecules begin to break, allowing water to enter, causing the granules to swell and soften. Between 62-80^oC, the granules begin to burst, releasing starch into the liquid and thickening it (the higher the starch concentration the thicker the liquid becomes), until gelatinisation is complete at 100^oC. When it’s left to cool, the liquid solidifies and forms a solid gel

What can go wrong with gelatinsation?

If the sauce isn’t stirred while gelatinising, the starch granules sink to the bottom and swell up, stick together and create a lumpy texture

What happens during dextrinisation? (4)

When starchy foods are cooked using dry heat, the starch molecules break down into smaller molecules called dextrins, giving food a crispier texture, different taste and browner colour, all of which become more intense the longer the food is cooked as more starch molecules break down into dextrins

What happens when sugar caramelises? (8)

When sugar is heated, the sugar molecules break down and forrm a runny, sweet liquid, then a smooth caramel, and eventually hardens and becomes similar to candy as it cools. Caramelised sugar changes flavour (usually becomes sweeter) and turns brown but can burn and blacken very quickly, creating a bitter taste, and to avoid this water is usually added during the early stages of heating (caramelisation doesn’t happen until the water has evaporated but this allows the temperature to rise before the sugar caramelises)

What is aeration and how does it work? (6)

Aeration means incorporating air, and when fats (e.g. butter) are beaten with sugar (creaming), air is trapped in the mixture, making it fluffier and lighter in colour - this is what gives cakes a spongy and light texture when they’re cooked - foods can be aerated in many different ways, e.g. whisking egg whites to form a meringue

How does shortening work and what does it do? (5)

When fat is rubbed into flour, the flour particles are covered with fat which gives them a waterproof coating, preventing long gluten strands from forming when water is added - this means the dough can’t become stretchy and instead has a hard, crumbly texture when cooked (e.g. shortbread) - this is used when making tarts/pies as the base doesn’t rise and forms a solid case

What are shortening fats?

Fats with an 100% fat content - there’s no water, which helps stop gluten formation and prevents steam from raising the food

What is plasticity and why do fats have it? (4)

The ability that fats have to be spread - it’s caused by the different triglyceride structures in fats which melt at different temperatures, meaning fats gradually soften at different temperatures rather than melting at once - unsaturated fats and fats with triglycerides that have low melting points tend to have more plasticity

Why is plasticity useful? (4)

Decorating cakes with buttercream icing, rubbing fat into flour to make a shortened dough, spreading butter on toast/sandwiches etc, putting cream cheese on bagels and much more

What happens when fats are emulsified? (6)

Emulsions are formed when oily and watery liquids are shaken together - usually oil and water repel eachother so emulsions instantly separate, however, emulsifiers, which have both a hydrophilic (attracted to water) and hydrophobic (repelled by water) ends, can be added to hold the water and oil together in a stable emulsion - the water binds to the hydrophilic end, and the oil binds to the hydrophobic end. Emulsions can either be oil-in-water (e.g. milk, mayonnaise) or water-in-oil (e.g. margarine, butter)

Give an example of a natural emulsifier

Lecithin in egg yolks

What are the four types of raising agents?

Chemical, biological, steam, mechanical

How do chemical and biological raising agents work?

They react and produce a gas (such as carbon dioxide) which expands when heated and makes the mixture rise

Give 2 examples of chemical raising agents

Bicarbonate of soda and baking powder (mixture of bicarbonate of soda and cream of tartar)

Why is bicarbonate of soda mixed with cream of tartar to make baking powder?

Bicarbonate of soda is an alkaline substance which produces a soapy taste and sometimes a green colour, so recipes using it need to have a strong taste to mask this - when mixed with cream of tartar (an acid) it’s neutralised, so it doesn’t produce a soapy taste

How does yeast (a biological raising agent) work?

Yeast is a microorganism that causes fermentation, which produces alcohol and carbon dioxide. Doughs using yeast are left to rest in a warm place so the yeast can ferment, as this is when the carbon dioxide expands to help the dough rise - when cooked, the alcohol evaporates, the yeast is killed, and the carbon dioxide expands even more

How does steam work as a raising agent and what could go wrong?

When a mixture that contains a lot of liquid is heated (to a very high temperature), steam is produced and causes the mixture to expand and rise - if the oven door is opened while the mixture is cooking, cold air will enter and cause the mixture to sink

How can mixtures be raised mechanically?

Air can be folded into mixtures/doughs in two different ways - repeatedly folding a mixture over itself using a spoon or folding pastry doughs and trapping air between the layers. Beating (or whisking) involves using a spoon or fork (or whisk) to mix ingredients and quickly drive air into the mixture. Sieving flour traps air between individual flour particles and air is added when creaming mixtures or rubbing fat into flour

What are the ingredients in a cake?

Flour, sugar, butter, eggs, baking powder

What is the function of flour in a cake?

It’s the bulk of the mixture and provides a gluten network - this traps bubbles of air and creates a light and fluffy texture

What is the function of sugar in a cake?

Makes it sweeter and helps hold air in the mixture when creamed with butter, helps create a soft texture, caramelises and helps makes the cake a golden brown colour

What is the function of butter in a cake?

Improves the shelf life, enhances texture and flavour, coats the flour and traps air while being creamed with sugar, helps cake rise and have a fluffy texture

What is the function of eggs in a cake?

Provides protein which denatures then coagulates, trapping moisture and providing a spongy texture

What is the function of baking powder in a cake?

It’s a chemical raising agent which helps the cake rise

Why might a cake be dense?

The butter and sugar weren’t creamed together enough, the flour wasn’t folded in gently enough and the air bubbles collapsed, the cake was overbaked causing the air bubbles inside to burst, there was no raising agent used

Why might a cake be dry and crumbly?

It was overbaked and the proteins coagulated too much, so all the moisture either evaporated or was squeezed out, not enough butter was added, strong flour was used and the gluten strands formed were too long and gave the cake a bread-like hard texture

Why might a cake sink in the middle?

The mixture was mixed wrong or overmixed and the air bubbles collapsed, the oven cooled while cooking and the cold air caused the mixture to sink, too much raising agent was added

Why might a cake go stale quickly?

It wasn’t stored correctly or the wrong amount of fat was added