Food Science Flashcards
(45 cards)
1
Q
Why is food cooked?
A
- Food is cooked in order to: make it safe to eat (cooking destroys micro-organisms) change raw food to cooked food make it palatable: develop flavours; improve mouthfeel; improve texture; reduce bulk; improve colour help keep quality (extend shelf life) make it easier to digest give variety to diet.
2
Q
How is food cooked?
A
- Cooking uses heat to change texture, flavour and colour of food.
- Cooking methods are wet (moist), dry, or fat-based:
- Wet methods are boiling, steaming, stewing, poaching, casseroling and braising.
- Dry methods are baking, roasting and grilling.
- Fat-based methods are frying and stir-frying.
- Pre-cooking methods can improve tenderness and flavour, for example, a marinade for meat or fish.
The selection of a cooking method depends on: * the type of food being cooked the time available * the skill of the cook and the facilities available * the need to achieve desired characteristics, e.g. browning the need to conserve vitamins, e.g. steam rather than boil to prevent loss of vitamins * the desire to improve palatability, which affects the appearance, colour, flavour, texture and smell.
3
Q
What happens when you microwave something?
A
** Microwave cooking uses a type of radiation called microwaves, which travel in straight lines and penetrate the food.
**
1. The microwaves vibrate water molecules creating friction, which makes heat. two theal eating
2. Hot spots can occur, so food should be stirred and left to stand to allow the heat to be distributed evenly.
Microwaves alone do not brown or crisp foods.
4
Q
Cooking Methods
A
**Water - based
**
* boiling - softens - vegetables, rice
* simmering - tendriese + evaporates - stews,sauces, curry
* poaching - tenderises - fish,chicken, egg
* braising - tenderises,softens - meats, fish, vegetables
Oven
* moist or fat - tenderises - vegetables, joints of meat, potatoes
Dry
* grilling - chars, browns, crisps - bacon,toast, cheese
* baking - browns,crisps - cakes,pastries
* dry frying - browns,crisps - bacon,lardons,chorizo,nuts
* roasting -brown, crisps - roast chicken, potatoes
**Fat - based
**
* deep frying - browns, crisps - battered fish
* shallow frying - sets,browns - eggs,onions
* stir fry - soften, reduces bulk - bean sprouts
5
Q
How does convection work to transfer heat into food?
A
- Convection takes place in air (in ovens) or liquids.
- Currents occur as heated air or water. rises and cooler air. or water falls, e.g. boiling water.
6
Q
How does conduction work to transfer heat to food?
A
- Food molecules vibrate to transfer heat via conduction. ·
- Heat is transferred by contact of heat source to pan to food, e.g. frying.
7
Q
How does radiation work to transfer heat into food?
A
- Heat energy passing in direct lines to the food, que.g. grill to food.
- Energy from microwaves penetrate food to transfer energy.
8
Q
What is protein desaturation and what can cause protein to be denaturated?
A
- Denaturation occurs when the structure of amino acids found in protein are altered. They change shape or unfold because chemical bonds are broken.
- Protein in foods can be denatured (altered) by heat, reduction of pH level (more acid), enzymes and mechanical actions. See chart below.
9
Q
How does heat denature protein?
A
- Cooking denatures proteins.
- Denaturation occurs when the structure of amino acids found in protein change shape after cooking - the protein molecule uncoils when cooked.
10
Q
How does pH denature protein?
A
- pH is the level of acidity or alkalinity in a food.
- pH is measured from 1, which is very acid, to 14, which is very alkaline.
- Reducing the pH by using lemon juice or vinegar in a marinade denatures the protein in foods to make them tender, tasty and moist.
11
Q
How does enzymes denature proteins?
A
- Meat tenderisers cause protein denaturation, making meat more tender.
- Enzymic tenderisers are in the form of papain and bromelain. Papain can be found naturally in papaya and bromelain can be found in fresh pineapple.
- Acidic pH from yogurt, buttermilk, vinegar or citrus fruits helps to tenderise fish or meat (marinades).
12
Q
How does mechanical actions denature proteins?
A
Whisking egg white denatures the protein by uncoiling and unfolding, e.g. foam formation occurs (gas in liquid).
13
Q
What is protein coagulation?
A
- Protein coagulation is a type of protein denaturation.
- Examples of coagulated foods are egg custards and quiches. It causes a change in texture, for example, runny eggs become coagulated (set).
- It usually starts at 60°C and is completed by 70°C.
- It is irreversible and causes loss of solubility.
14
Q
When does gluten formation occur?
A
- 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.
- Strong wheat flour for bread making contains more gluten than plain flour.
15
Q
What is the function of gluten in bread?
A
- Gluten makes dough stretchy and elastic.
- Salt and kneading help strengthen gluten.
- Gluten forms the structure of a baked loaf of bread.
16
Q
What is the function of gluten in pastry?
A
- Rubbing fat into flour makes short gluten strands - the scientific term for this is shortening.
- Gluten forms the structure in baked pastry.
17
Q
What is the function of gluten in pasta making?
A
- Gluten in wheat flour helps pasta hold its shape, e.g. fusilli.
- Gluten also makes the pasta dough flexible and increases its ability to hold various shapes.
18
Q
How to get the right ratios in gelatinisation?
Define: Retrogradation + Synerisis
A
- A sauce is viscous, which means it can either be poured (e.g. parsley sauce) or used to coat an ingredient (e.g. cauliflower cheese) or to bind other dry ingredients together (e.g. rissoles).
The change in viscosity is due to the ratio of thickening agent to liquid; more starch gives a thicker sauce. The ratio selected changes the consistency of the sauce from runny to thick.
Retrogradation is the deterioration of a starch-based sauce on keeping - this results in shrinkage, drying and cracking.
Synerisis is the loss of fluid from a foam or set mixture, e.g. lemon meringue pie, and in cheese making.
18
Q
What is enzymic browning?
Why does it happen?
How can you prevent it?
A
- Enzymic browing occurs on the surface of cut fruits, such as apples, and on the surface of cut vegetables, such as potatoes.
- It happens due to cell enzymes reacting with air (oxidation).
- Enzymic browning can be prevented (inhibited) by:
-blanching cut fruits or vegetables in boiling water
-blanching vegetables before freezing, which inhibits the enzymic action that can discolour food dipping fruit or vegetables in acid, e.g. lemon juice
-removal of air by submerging in water cooking, e.g. stewing fruit, roasting parsnips.
18
Q
What is gelatinisation and when does it occur?
A
- Gelatinisation occurs when starches (wheatflour, cornflour or arrowroot) thicken liquids.
- The process needs heat and agitation (stirring), especially in sauce making. It occurs during the cooking of starchy food such as potatoes, rice or pasta.
18
Q
What is oxidisation and how can you prevent it?
A
- Oxidation causes discolouration, e.g, cut lettuce leaves turn pink-brown. Oxidation also causes vitamins to be lost, particularly vitamin C. Oxidation enables enzyme activity, e.g. browning, discolouration.
- Oxidation can be reduced during preparation and cooking by:
-cooking vegetables in small amounts of water
-using a quicker, shorter method of cooking, e.g.
-steaming, stir fry
-serving vegetables immediately after cooking keeping the -lid on when boiling vegetables
-using the cooking water in gravy.
19
Q
How does gelatinsiation occur in sauce making?
A
**When making a sauce gelatinisation occurs.
**
1. Starch grains absorb liquid, swell, burst (at 80°C) and finally thicken a sauce.
2. Starches complete thickening at just under 100°C, so it is important to cook to boiling point to avoid a sauce with a raw taste.
3. Sauces need stirring (agitating) to prevent lumps forming, to stop the sauce from sticking to the bottom of the pan and burning, and to help the process of gelatinisation.
4. Basic recipes use different methods and different thickening agents, as seen when using the roux method, the blending method and the all-in-one method.
20
Q
How are modified starch used to help gelatinisation (give examples)?
A
- Modified starches are used to help gelatinisation occur in different ways. Quick-cook pasta or rice are modified by pre-gelatinisation.
- Milkshakes use starch modified to allow cold liquid thickening.
- Instant thickening granules are modified starch that can be sprinkled into boiling liquids.
- Modified starches thicken cold desserts without the application of heat, and are used to thicken and stabilise salad dressings.
21
Q
What is dextrinisation?
A
- Dextrinisation occurs when starch is toasted or cooked by dry heat, e.g. toasted or charred bread.
- It is a result of starch breakdown by dry heat to form dextrins.
- It changes the properties of starch as a result of heat application.
- Dextrinisation is known as non-enzymic browning.
- Dextrins taste sweeter than starch and add flavour to toasted, charred or baked goods.
- Dextrins are hygroscopic, absorbing moisture from the air, e.g. toasted or baked products soften slightly on keeping.
- Baked produce is best stored in an airtight tin or container with lid.
- Characteristics of dextrinisation are golden colours, browning, sweeter taste and crispness.
22
What is caramelisation?
* Caramelisation causes sugar to change colour and flavour due to dry or moist heat.
* It causes surface browning on baked goods containing sugar. It changes the properties of sugar; solutions become syrups.
* It is known as non-enzymic browning.
* Characteristics of caramelisation are a golden colour, browning, gloss, sweetness and stickiness.
23
Examples of caramelisation in food
* Frying onions, or frying or roasting parsnips, potatoes or squash.
* Making a crème brulee or crème caramel.
* Spun sugar caramelisation is created through the application of dry heat to sugar.
* Fudge, toffee or halva are created through the application of wet heat to caramelise the sugar.
* Preparing glazed pork ribs or chicken wings.
24
What are the types of fats?
* Fat from animal sources include butter and lard - these are saturated fats.
* Fat from vegetable sources include margarine and vegetable shortening - these are unsaturated fats and are suitable for vegetarians, vegans and certain religions.
25
How does browning occur in terms of carbohydrates?
* Dry methods of cooking gradually turn starches and sugars golden brown.
* This aids the palatability of the products by improving flavour, texture and colour.
* Browning is often used as an indicator of adequate cooking e.g. cook until golden brown.
26
What is shortening?
**Shortening is a process using fat that creates a characteristic short, crumbly texture.
**
* Shortening is seen in shortcrust pastry, shortbread biscuits, cookies and rich pastries.
* The process in food preparation most likely to bring about shortening is the rubbing-in method.
27
How does a fat shorten a pastry mixture?
* Fats with plasticity are good shortening agents because they rub-in easily.
* Fat coats the flour grains, preventing gluten development.
* The cooked texture is short and crumbly.
28
What happens during the cooking of pastry?
* The flour grains absorb the fat.
* Pastry changes from pliable to rigid (the gluten sets).
* Pastry turns golden brown.
29
What is plasticity?
**Plasticity means the ability of a fat to change properties over a range of temperatures.
**
* Temperature is an important factor in be plasticity of fats.
* Cold fats are solid and firm.
* Fats at room temperature become spreadable and soft.
* Plastic fats such as butter or margarine can be used for spreading, rubbing-in, creaming, melting-method cooking or for muffins.
* Warmed fats melt and become runny.
30
What is aeration?
**Aeration helps products have a light and open texture.
**
* Fats aerate mixtures during beating or creaming with sugar.
* Aeration increases the volume of a product by incorporating air.
* Beating, whipping, creaming and whisking are methods that help aeration.
**During preparation of a creamed mixture:
**
-the fat and sugar are creamed together, trapping air (aeration)
-the mixture becomes paler
- an air-in-fat foam is formed.
**During baking:
**
-trapped air expands
-the cake rises.
31
What is emulsions?
**Emulsions are mixtures of liquids that do not normally mix (known as 'immiscible' liquids), e.g. oil and water.
* Emulsifiers have a hydrophilic end, which is oil-in-water water-loving and forms types of emulsions water-in-oil chemical bonds with water, and a hydrophobic end, which is water-hating and forms. chemical bonds with oil
* Fats and oils add texture, flavour and colour to emulsified sauces such as Hollandaise, which is a hot emulsion, and mayonnaise, which is a cold emulsion.
* Stabilisers keep emulsions mixed, preventing them separating.
32
Explain the process of emulsification?
* Emulsification requires agitation by whisking, by mixer or food processor.
* It requires slow addition of oil to prevent the emulsion splitting.
* Emulsification is helped by a natural emulsifier called lecithin, which is present in egg yolks.
* A good example of emulsification is seen when making mayonnaise, a smooth, stable emulsion for salad dressing.
* Vinaigrette dressing (oil and vinegar) is not a stable emulsion - it will separate out on standing.
33
What is physical methods of raising and aerating?
* Physical raising methods such as air, water vapour or steam help products to have a light, open texture.
* Recipes that need to be light have ingredients that function as raising agents such as water, milk or egg whites.
34
What is mechanical methods of raising and aerating?
* Food preparation metho such as sieving, whisking or beating can be used to trap air.
* Combinations of physical and mechanical methods work well in food preparation to make mixtures light, e.g. batters for Yorkshire puddings.
35
Why is air an effective raising agent?
* Air is a very effective raising agent because it expands when it is heated. Air pockets swell and volume increases.
* Food preparation techniques help prevent loss of air, e.g. folding in flour when making a whisked sponge cake.
36
Why is steam an effective raising agent?
* Steam is produced from water in a mixture; this is a physical change. Steam produces light, open and uneven textures and adds volume during cooking, e.g. profiteroles.
* Moist mixtures produce steam during cooking.
37
Why is foam an effective raising agent?
* Foams - whisking helps trap air, creating foam. * Ingredients containing protein form foams, e.g. milk froth, egg whites. .
* Egg whites stretch and unravel to trap air to form a gas-in- liquid foam. Sugar stabilises foam, e.g. old-set soufflé.
* Egg white foams set mousses.
* Cooking stabilises foam, e.g. roulade, meringue.
38
What are chemical raising agents?
* Chemical raising agents produce carbon dioxide when heated with a liquid.
* They cause effervescent fizzing and bubbles of gas.
* Chemical raising agents must be carefully measured.
39
What bicarbonate of soda?
* Bicarbonate of soda is an alkaline powder.
* It can leave a soapy aftertaste but strong flavours, e.g. gingerbread, will mask the aftertaste.
* It works more effectively with an acid ingredient such as buttermilk or cream of tartar, e.g. soda bread.
* The acid neutralises the alkali and prevents soapy aftertaste.
* Cream of tartar is an acid raising agent, which is frequently used alongside bicarbonate of soda, e.g. in scones.
40
What is baking powder?
Baking powder is a ready-to-use mixture of cream of tartar plus bicarbonate of soda and rice flour.
41
What is self raising flour?
* Self raising flour is plain flour and baking powder added together to create rise.
* Plain flour alone does not contain a raising agent.
* Self-raising flour can be brown or white.
* Self-raising flour contains a pre-sieved precisely measured amount of baking powder for ease and speed of use.
42
What are biological raising agents and give examples of when they are used?
* Yeast is a biological raising agent. It ferments to give off carbon dioxide gas.
* Fermentation in yeast is a biological (also known as biotechnological) raising agent.
* The conditions for yeast fermentation are warm temperature 25°C-35°C; moisture; food; time.
* Temperatures above 60°C during baking will inactivate and finally destroy yeast cells.
* Boiling liquids will inactivate yeast, preventing fermentation from taking place.
* Yeast is the raising agent in bread, bread rolls, buns and rich pastries (Danish pastries).
* Leavened bread contains raising agent in the form of yeast or bicarbonate of soda.
* Unleavened bread contains no raising agent and is flat in structure.
