Food science Flashcards
What are colloidal systems
Colloids are formed when one substance is dispersed through another, but does not form a solution. There are many types of colloidal systems, depending on the state of the two substances mixed together.
What are sols and gels
A sol is a colloid in which solid particles are dispersed through a liquid continuous phase. Sometimes the mixture needs to be heated and stirred to disperse the solid particles evenly. When this solution cools, the sol may form a gel, in which the solid particles form a network (continuous phase), trapping the liquid (disperse phase).Both proteins and starches are capable of this.
How do emulsions work?
An emulsifier is a molecule with two distinct parts, known as the ‘head’ and ‘tail’. The head is hydrophilic (water loving) and the tail is hydrophobic (water hating).This means that the tail will sit within the oil and the head will sit within the water. This keeps the small bubbles of oil or water from coalescing (joining together).The emulsifier holds the disperse phase within the continuous phase. This results in the emulsion becoming stable.
Give an example of an emulsion
Mayonnaise is an example of a stable emulsion of oil and vinegar, when egg yolk (lecithin) may be used as an emulsifier. Sometimes mustard is also used which contains another emulsifier (mucilage).
What is a foam?
Foams are composed of small bubbles of gas (usually air) dispersed in a liquid, e.g. egg white foam. As liquid egg white is whisked, air bubbles are incorporated.
The mechanical action causes proteins in the egg white to unfold and form a network, trapping the air.
If an egg white foam is heated, proteins coagulate and moisture is driven off. This forms a solid foam, e.g. a meringue.
Ice cream, bread and cakes are other examples of solid foams.
What are raising agents
Raising agents include anything that causes rising within foods, and are usually used in baked goods.
Raising agents are added to baked products during the preparation stage. They create gas, air or steam which expands when heated and causes the food to rise.
When heated, the gas that is trapped within the product expands. This is because heating causes the molecules in the air to move more rapidly and apply more force
What are Mechanical raising agents
Air is a commonly used and effective raising agent. It can be added to a mixture in a variety of ways:
sieving flour or lifting flour when rubbing in fat, e.g. pastry, scones, cakes;
creaming fat and sugar to incorporate air, e.g. creamed cake mixture;
whisking to trap air e.g. Eggs, whisked to create a foam for sponge cakes, meringues, soufflés;
beating ingredients together helps to trap air, e.g. beating eggs into a creamed mixture;
rolling and folding pastry (creating laminations) traps air, e.g. flaky pastry, rough puff pastry.
What are bioloigical raising agents?
Yeast is a biological raising agent. There are different types of yeast available that can be used to make bread and bread products. These are: fresh yeast, dried yeast and easy-blend yeast.
Given the right conditions (warmth, carbohydrates and moisture) yeast converts sugars to alcohol and carbon dioxide. This is called fermentation.
The carbon dioxide in the bread dough will expand when placed in a hot oven; steam is also produced to help raise the dough.
What are chemical raising agents?
Chemical raising agents are those that require a chemical reaction in order to function. Common examples of chemical raising agents include:
* baking soda (bicarbonate of soda);
* baking powder (bicarbonate of soda + cream of tartar);
* self-raising flour.
Baking soda is an alkali that reacts with an acid (in the presence of water) to produce carbon dioxide, which causes rising.
Baking powder is baking soda with an acid (e.g. cream of tartar) already present, so only water need be added.
Self-raising flour is flour that already contains baking powder, and therefore only requires water to be activated
What is the heat transfer radiation
Radiation is energy in the form of rays. The rays pass through the air until they come into contact with the food. Some are absorbed while others are reflected.
Grilling involves the use of infra-red heat rays created by gas flames, charcoal or glowing electric elements
What is the heat transfer convection
In this method of cooking, currents of hot gas or hot liquid transfer the heat energy to the food.When gases (such as air) or liquids (such as water) are heated, the molecules become more energetic. This makes them move around more quickly.Because of this, the molecules move further apart, making them less dense. This causes them to rise.Cooler liquid or gas falls to take its place. This in turn becomes heated and rises up, creating moving ‘convection currents’.
Boiling and fan assisted ovens
What is the heat transfer conduction?
Conduction is the direct transfer of heat energy from a hot surface.When a pan or baking tin is heated, the molecules in the metal begin to vibrate and pass the energy to neighbouring molecules.
These molecules can then directly transfer heat to the food.
Cooking by conduction depends on good contact between the:
source of heat;
cooking equipment;
food to be cooked.
frying
What is tenderisation
Mechanical tenderisation
– a meat cleaver or meat
hammer may be used to
beat the meat. Cutting
into small cubes or
mincing can also help.
* Chemical tenderisation
(marinating) –the
addition of any liquid to
flavour or soften meat
before cooking.
Explain gluten formation
Two proteins, gliadin and glutenin,
found in wheat flour, form gluten
when mixed with water. Gluten is
strong, elastic and forms a 3D
network in dough. In the production
of bread, kneading helps untangle
the gluten strands and align them.
Gluten helps give structure to the
bread and keeps in the gases that
expand during cooking.
What are the 3 methods of browning?
Maillard reaction
Foods which are baked, grilled or
roasted undergo colour, odour and
flavour changes. This is primarily due
to a group of reactions involving
amino acids (from protein) and
reducing sugars.
Dextrinisation
When foods containing starch are
heated they can also produce brown
compounds due to dextrinisation.
Dextrinisation occurs when the heat
breaks the large starch
polysaccharides into smaller
molecules known as dextrins which
produce a brown colour.
Caramelisation
When sucrose (table sugar) is
heated above its melting point it
undergoes physical and chemical
changes to produce caramel
