IGCSE Food Technology

NUTRITIONAL PROPERTIES

Starch

• thickens a liquid by forming a suspension such as a sauce
• forms a gel when the suspension is heated eg. adding cornflour to a powder and milk mix

Sugar

• flavors by sweetening
• colors by caramelizing, when heated
• aerates when beaten with fat eg. in a cake mix

Proteins

• can coagulate (when a liquid becomes a solid) eg. when an egg is heated
• can aerate a mixture eg. whisking egg whites in a meringue mix

Fats

• shortens pastry (makes it more crumbly) by making it less stretchy
• can act as an emulsifying agent (a substance that helps two liquids stay mixed together  eg. oil and water)
• moistens a baked mixture such as a cake
• plasticity (property of fat)
  • fats that are solid at room temp. and contain both solid fat crystals and liquid oil
  • liquid held in a network of small crystals
  • due to this unique combination of liquid and solid, the fat can be molded or pressed into various shapes without breaking
  • this property of fat is called plasticity

GELATINIZATION

Gelatinization is the process where heated starch granules absorb liquid and swell, then burst to thicken a liquid, creating a gel. It is an irreversible reaction.

Examples: custard, gravy, white sauce

During gelatinization, the following occurs:

1. Starch particles suspended in the liquid (they do not dissolve)

2. Stirring the liquid keeps the starch particles suspended – if the suspension isn’t stirred they stick together and sink to the bottom – forming lumps. Will not cook correctly.

3. When the liquid reaches approx 60°C the starch grains begin to swell as they absorb the water.

4. As heating continues (approx 80°C) the particles break open and release starch. This makes the mixture thick and viscous - gelatinization.

5. Gelatinisation is completed when the liquid reaches 100°C. The liquid now forms a gel.

6. On cooling the gel solidifies – it will set to form a mold  e.g. Blancmange.

AERATION: RAISING AGENTS

Aeration: when air is trapped in a mixture  eg. whisking egg whites for meringues

Raising agents are added to baked products to make them rise and have an open and light texture - which consumers find desirable

Why are raising agents added to food?

Product cooked without raising agent will be heavy, moist and stodgy

How do raising agents work?

• gases introduced before baking
• when the product cooks, heat causes the gases to expand
• gas bubbles set in the product and provide a soft sponge-like texture

Successful raising agent:

• light texture and volume
• light and well risen
• clear air bubbles

3 gases used:

• air - egg whites, beating, creaming, rubbing-in
• steam - profiteroles, choux pastry, Yorkshire pudding
• carbon dioxide - yeast fermentation, baking powder, self-raising flour

3 types of raising agents:

Mechanical - manually adding air

• sieving, creaming, whisking rubbing-in
• folding and rolling (puff pastry)

Chemical - heated causes a chemical reaction with raising agent, produces carbon dioxide

• baking powder
• bicarbonate of soda
• self-raising flour (plain flour with baking powder)
• added as an ingredient

Biological - added to produce carbon dioxide

• air
• steam
• yeast

Fermentation process:

Yeast needs...

• sugar (food)
• warmth
• moisture
• time

Yeast activated with sugar, warmth, and moisture to produce carbon dioxide. The carbon dioxide expands when baked, creating air bubbles, causing the product to rise and gives structure to the product when the gas bubbles set.

COAGULATION: PROTEINS

When something thickens from a liquid to a solid, brought about by heat, mechanical action or acids. Enzymes may also cause protein coagulation eg. cheese making.

Irreversible reaction. Proteins cannot be turned back into their liquid form.

1. Proteins shaped like coils or springs. When exposed to heat, they denature (their coils unwind)

2. When proteins denature, they bond together, or coagulate, and form solid clumps

3. As proteins coagulate, they lose some of their capacity to hold water - gives off moisture as they cook, even if they are steamed or poached

Raw eggs - clear and runny, but becomes white and solid when heated

• Heat causes protein to denature - coagulates
• Egg whites coagulate at 60°C, egg yolks at 65°C
• Full coagulation occurs at 70°C

TENDERISING

Tenderising is breaking down or softening the connective tissue in meat by marinating acid or mechanical action.

Tenderising tough meat makes it easier to eat.

Marinade:

• lemon juice
• vinegar
• wine

Mechanical action:

• meat mallet
• slow cooking - stewing, pressure cooker

FINISHING

Finishing techniques are used to make the food look good. For example:

• Browning - uses fats, eggs, sugar, milk, flour or oil, which darkens a food when heated
• Glazing - adds a shiny coating  eg. pastry brushed with a beaten egg before cooking
• Icing - can add color and texture

Finishing can help improve palatability - the appeal of food - which includes:

• Taste
• Colour
• Smell

You should know and be able to describe the following, including examples of products that use them...

Binding - uses fats, eggs, cereals, and flour to bind ingredients  

eg. egg used to bind together a biscuit mixture

Bulking - forms the main structure of a food product 

eg. flour in biscuits and cakes

Enrobing - coating a food with another ingredient

eg. dipping fish in beaten egg and then breadcrumbs

Enriching - the addition of an ingredient to improve the quality 

eg. nutrients are sometimes added to increase the nutritional value

Emulsification - when two unblendable liquids are mixed together (eg. oil and water/vinegar), an emulsifier is needed to stop them from separating 

eg. lecithin in egg yolks holds mayonnaise together. Manufacturers use emulsifiers in salad dressing, mayonnaise, margarine, butter, cake mix etc.

Fermentation - uses yeast to convert carbohydrates into alcohol and carbon dioxide 

eg. in bread making, yeast is added to flour and water causing the dough to rise

Flavoring - can be savory (herbs and spices) or sweet (sugar or sweeteners)

eg. sugar helps to soften the sharp taste of grapefruit

Shortening - when fat coats flour particles, preventing the absorption of water and development of gluten in pastry, making the dough less stretchy, resulting in a crumbly texture.

eg. butter in shortcrust pastry - gives a crumbly texture

Stabilising - helps food keep its structure

eg. eggs and flour used for stabilizing

Setting - using ingredients to make foods firm

eg. using gelatine to set cold desserts (mousse, panna cotta, jelly)