Food Science Reactions Flashcards
Define gelatinisation
When starch is heated in the presence of water, the starch granules absorb water & swell. They eventually rupture upon further heating, resulting in a thick consistency.
Gelatinisation softens the hard texture of starchy food (Eg, rice)
Define caramelisation
Involves simple sugars
When sugar becomes brown after it is heated to high temperatures (160<)
Contributes to the caramel flavour that is sweet, bitter and nutty
Define dextrinization
When dry heat Is applied to starch at high temperatures, the long chain starch molecules are broken down into shorter chain dextrins, resulting in toasty and nutty flavours
Define shortening
Fat’s ability to shorten glucose strands formed when water is added to wheat flour.
When butter is added to wheat flour, the fat molecules coat the flour proteins (gliadin & glutenin).
This reduces the contact between the water & flour when water is added.
As a result, fewer gluten networks are formed. This gives a crumbly texture that is desired in some baked products (Eg. scones)
Define emulsion
A mixture formed by combining 2 liquids that do not normally mix with an emusifier. (Can but it’s temporary and unstable)
An emulsifier is a substance that consists of a fat-loving part & a water-loving part.
When added to water & oil mixtures, emulsifiers can bring the water & oil phases together without separation. (Eg. Lecithin in egg yolk)
2 types of emulsions:
- oil in water and emulsions (Eg. Mayonnaise/cream) in which oil droplets are dispersed throughout a water phase
- water in oil emulsions (Eg. Butter/margarine) in which tiny water droplets are dispersed throughout an oil phase
Define foaming
When denatured protein molecules trap air in a liquid/solid by wrapping around the air bubbles, producing foam.
Sugar is often used to stabilise egg white foam
Define coagulation
Te solidifying/curdling of protein rich food brought about by heat/acids.
Causes change in physical appearance of food
Normally happens after denaturation
Upon heating, uncoiled protein strands come together to form a network which changed liquid egg white to solid, egg yolk becomes hard and powdery
Define gluten development
A protein network developed when wheat flour is mixed with water and subsequently kneaded.
When wheat flour is mixed with water, the wheat proteins (gliadin & glutenin) join together to form gluten. Kneading or further mixing rearranges the gluten to form a network of dough.
This network can trap gases produced by yeast & baking powder, increasing the size of the dough during fermentation & baking. The gluten network coagulates upon heating, giving structure & volume to baked products, (Eg.bread)
Define Millard browning
Occurs as a result of reactions between sugars and proteins at high temperatures (140<)
Contribute to the desirable brown colour of roasted meat & distinctive roasted flavour
Define denaturation
When proteins are subjected to heat, agitation or extreme pH changes, it uncoils and loses its original structure - normally changes the physical appearance & texture of food
Define melting point (of a fat)
The temperature at which the fat turns from a solid to a liquid state.
Plant oils tend to have lower melting points - liquiid @ rtp
Animal fats usually solid @ rtp - higher melting point
Some fats have a range of melting points
Affects mouthfeel of some high-fat food
Define smoke point ( of a fat )
The temperature at which it begins to decompose & discolour, producing toxic smoke that has an unpleasant smell.
Under high heat, fats are broken down into glycerol & free fatty acids, producing smoke.
Heating hot oil further above smoke point may cause it to catch fire
Oils with lower smoke points (olive/canola) for salad making
Oils with higher smoke points (corn/soya bean) for deep frying
Define Enzymatic browning
A reaction observed on surface of certain fruit & vegetables when they are cut or bruised.
When the flesh of these fruit & vegetables are exposed to oxygen in the air for a period of time, a brown discolouration is formed because of an enzyme found in the plant tissues.
Although the brown discolouration is not toxic, it causes an unappealing appearance.
5 Measures that can be taken to reduce enzymatic browning
- immerse cut fruit pieces in lemon juice to reduce its pH, which inactivates the enzyme
- keep cut fruit in the chiller, in cling wrap. Low temperature reduces enzyme activity. Cling wrap reduces contact between cut surface & the oxygen in the air
- pack cut fruit in vacuum packs to reduce its contact with air, so oxygen is less available for enzymatic browning
- coat cut fruit with sugar syrup & salt water. They have the ability to reduces enzyme activity
- cut fruit & vegetables just before use / consumption
Flour in cake making
- 2 main components - protein & starch
- wheat flour with a low protein content (Eg, cake flour) is commonly used for making cakes as it forms less gluten.
- This gives cakes a soft & more tender texture, which is a desirable sensory quality
- protein & starch in the flour undergoes coagulation & gelatinisation respectively, & add structure to the cake.
- the golden crust of cakes is contributed by the dextrinisation of starch & Millard browning between the sugar & flour proteins
Raising agents in cake making
Used to increase the volume of cakes
chemical raising agents:
- when baking powder (a mixture of baking soda & acid) is mixed in a moist environment, the chemical reaction between the baking soda & acid produces carbon dioxide.
- the gas expands when heated & adds volume to the baked cake
- baking soda may also be used as a raising agent, but acidic ingredients (Eg. Lemon juice/cream of tartar) need to be added for the chemical reaction to take place
physical raising agents:
- in cream cakes, air is normally incorporated by creaming sugar & fats (Eg. Butter). Butter is able to trap air to form air bubbles, which expand during baking.
- in sponge cakes, air is normally incorporated by whisking eggs & sugar until a foam is formed. The air bubbles in the foam expand when heated, giving volume to sponge cakes. Steam converted from water at high temperatures also helps to increase the height of cakes during baking
Sugar in cake making
- provides sweetness to cakes
- contributes to the browning of the cake surface via caramelisation & Millard browning
- improves tenderness of cakes. As sugar has the ability to attract water, it competes with flour proteins & starch for water. This limits gluten formation & delays gelatinisation, making the cake tender.
- traps more air during creaming of fat when making cream cakes
- stabilised egg foam & increases the volume in whisked cakes
- retains moisture in cakes due to its ability to attract water from the surroundings. As a result, cakes can be kept for a longer period without drying out
Eggs in cake making
- as eggs are beaten into the cake mixture, air is trapped. This increases volume & contributes to the soft & fluffy texture of some cakes
- provide moisture to the cake & contribute flavour. Along with fats, eggs contribute to the golden brown colour of eggs through Millard browning
- lecithin, found in egg yolk, is a natural emulsifier. It helps to bind water & fats together, resulting in a finer & more even texture
Fats in cake making
- coat flour proteins & make cakes softer by shortening the gluten strands
- solid fats, together with sugar, can trap air during creaming, contributing to the fluffiness & volume of cakes.
- contributes to the golden yellow colour & distinctive aroma of cakes
Liquids in cake making
- function as an agent to bind the dry ingredients & a solvent to dissolve ingredients
- are essential for starch gelatinisation & the formation of a gluten network because flour must be hydrated before these 2 reactions can take place
- steam converted from the liquids found in the batter during baking also helps the cake to rise, increasing its volume
- used in many baked products (Eg, milk & eggs) if used in significant amts, additional water may not be required
Rubbing in method
Fats are rubbed into the flour to delay gluten formation
Use cold, hard fats as solid fats are able to coat flour proteins more thoroughly, thus delaying gluten formation
Chemical raising agents are commonly added to increase the volume of cakes
Creaming method
- produces cakes with rich flavour & tender texture
- creaming of fat & sugar introduces air into the creamed mixture. These air bubbles expand during baking, increasing the volume of the cake
Whisking method
- produces cakes with a light & airy texture
- produce egg foam by whisking egg & sugar until a soft peak is formed. This incorporates air into the mixture.
- sifted flour is then gently folded to prevent knocking out of air bubbles in the foam.
- during baking, the heat causes the air trapped in the egg foam to expand, giving volume to the cake.
- gelatinisation of wheat flour & coagulation of egg proteins during baking sets the cake, providing structure.
Flour in pastry making
All purpose/plain flour is commonly used in pastry making as it is able to form a strong gluten network to support structure yet does not create a tough texture like that of bread.
Dextrinisation of starch molecules contributes to golden brown colour of pastries
Undergo browning due to caramelisation of sugar & Millard browning between sugar and flour proteins
