Egg Flashcards
What are the types of proteins?
- Simple
2. Conjugated/complex proteins: composed of simple protein and non-protein material
What are the types of simple proteins?
- Globular = somewhat rounded in shape
- Ovalbumin in egg whites
- Lactalbumin and lactoglobulin in milk
- Gliadin and glutenin in wheat - Fibrous = coiled or extended in shape
- Collagen and elastin in connective tissue of meat
- Myosin in the muscle of meat
What are the types of complex proteins?
- Phosphoproteins (protein + phosphoric acid)
- Casein proteins in milk - Glycoproteins (protein + carbohydrate)
- Ovomucin in eggs - Lipoproteins (protein + fatty substance)
- Lipoproteins in egg yolks and in whipping cream - Chromoproteins (protein + coloured material)
- Myoglobin in the muscle of meat
What is the protein structure?
- Primary: amino acids linked by peptide bonds (linkage/covalent bond) to form polypeptide chains
- Very strong bonds and cannot be broken by normal cooking methods - Secondary: spring-like coiling of polypeptide chain, usually into an alpha helix
- Eg. simple fibrous proteins - Tertiary: helix folds back on itself to form globular structure
- Eg. simple globular proteins - Quaternary: globular proteins combine with each other or non-protein substance
- Eg. conjugated/complex proteins
What is the isoelectric point?
- Carboxyl group donates H+
- Amino group accepts H+ ions
- The point where amino acid (a protein molecule) no longer has electrical charge
- The like charges of protein are no longer repelling each other, which makes them unstable
- Protein molecules are then attracted to each other and form hydrogen bonds with each other to create larger molecules
- Proteins cannot stay in colloidal dispersion because like charges no longer repel as the proteins grow in size
What are the factors affecting proteins?
- pH - least stable at IEP
- Temperature
- Partially increases due to cooking
- Decreases (freezing) - Mechanical action
- Eg. beating of egg white
What do the factors affecting proteins result in?
Denaturation
Possibly coagulation, or over-coagulation
What is denaturation?
Change from the naturally ordered configuration (shape) of a protein molecule to a more randomly structured molecule
Hydrogen bonds are broken but NOT peptide bonds
What is coagulation?
- New hydrogen bonds at new locations among polypeptide chain (within protein molecule)
- Not reforming the original hydrogen bonds
- Cause is the same or different from cause of denaturation
- Most common cause is heat (continuing heat, initially would cause denaturation)
What is over-coagulation?
Result due to prolonged exposure to:
- pH change (normally a decrease in pH)
- Heat (too high or too long)
- Mechanical action (least likely)
- Polypeptide chains compress together and squeeze out water because excess hydrogen bonds form
- Syneresis occurs in a GEL
- Curdling occurs in a SOL (cannot happen in starch because proteins are having excessive hydrogen bonds)
What are the functional properties of eggs?
- Thickeners - SOL forms
- Gelling agents - GEL forms
- Foaming agents - egg whites only
- Emulsifiers - lecithin in egg yolks only
What are the factors affecting coagulation of egg proteins?
- Temperature
- Egg white proteins: coagulation is 62-65 C, overcoagulation is 70 C
- Egg yolk proteins: coagulation is 65-70 C, overcoagulation is 75 C - Amount (quantity) of egg proteins
- Diluting egg solution with water, milk, or fruit juices in recipes keep egg proteins further apart
- Higher temperature needed for denaturation and coagulation because they are further apart
- A dilute solution results in a less thick SOL or a less firm GEL - Sugar
- Forms hydrogen bonds with egg proteins resulting in a slower rate of denaturation and coagulation
- Results in an increase in coagulation
- Decrease SOL thickness or GEL firmness
- Sugar reduces bonding between the actual egg proteins
- Helps protect against over-coagulation of
egg proteins
- Less chance of curdling in a SOL
- Less chance of syneresis in a GEL - Acids
- Contributes H+ ions which neutralize negative charges around egg protein molecules
- Stablizing factor in eggs is the negative charges repeling
- Egg proteins reach IEP and denature and coagulate faster
- Results in decrease in denaturation and coagulation temperatures (unstable)
How do you prevent over-coagulation?
Generally preventing over-heating is a good method
- Use double boiler (top of stove)
- Use oven poaching (in the oven)
- Keep temperatures higher or equal to boiling temperature (100 C)
What occurs during the storage of eggs?
- Fresh egg broken onto flat plate stands up in rounded form due to viscosity of
thick portion of egg white - As eggs age/deteriorate, amount of thin egg white increases
- Lose moisture which cause air cell between two membranes to enlarge
- Lose carbon dioxide which causes egg white to become alkaline, and thin out
- Egg yolk absorbs water from egg white and yolk membrane stretches
- When broken onto plate, deteriorated egg yolk flattens and whole egg
spreads - Chalazae disintegrates and doesn’t hold yolk in place at the center
- Eggs should be refrigerated to retain quality for 2 weeks
- Kept in original carton to prevent moisture loss and absorption of
odours - Porous shell allows absorption of strong odours (onions, apples, citrus
fruits, cabbage, cheese)
What is the composition of egg yolk?
- ½ water and ½ solid
- Solids are 1/3 protein and 2/3 fat
- Yolk contains lecithin for emulsifying properties
- Contains cholesterol
- No carbohydrate
What is the composition of egg white?
- Contain more water and are less concentrated than yolk
- Protein is main constituent
- Proteins called albumins account for 80% of protein
- Egg white can be called egg albumin
- Readily coagulated by heat
- Almost no fat or carbohydrate
- Mineral and vitamin content is lower than yolk
What is egg white foam?
Colloidal dispersion in which air bubbles (dispersed phase) are trapped in a liquid (continuous phase)
What does beating egg whites do?
- Incorporates air bubbles into the liquid portion of egg whites
- Denatures egg white proteins which collect on surface of egg white bubble
- Stabilizes foam (longer time before foam collapses)
What does baking egg white foam do?
Causes egg proteins to coagulate to make form permanent (will never collapse over time)
What are the factors affecting egg white foam quality (volume and stability)?
- Age of eggs
- Temperature
- Length of beating
- Sugar
- Acid
- Fat
How does age of eggs affect egg white foam quality?
- Fresh eggs produce higher foam
- Eggs greater than 10 days old produce lower volume
- Maximum volume comes from eggs that are 2-3 days old
How does temperature of eggs affect egg white foam quality?
Room temperature (15-25 C) produces largest volume of egg white foam
How does length of beating affect egg white foam quality?
- Under beating means too few egg white proteins are denatured = less stable foam which collapses faster
- Overbeating causes the denatured proteins to be less flexible and the air cells begin to break = lower foam volume, less stable
How does sugar affect egg white foam quality?
- Forms hydrogen bonds with egg white proteins, interfering with denaturation
- If sugar is added at the start of beating, the time required to create foam increases and foam volume will be reduced
- If sugar added at soft peak stage, you will get a more stable foam (stiff) to prevent over-coagulation
How does acid affect egg white foam quality?
- Eg. lemon juice, vinegar, or dry acid salt like cream of tartar
- Creates more stable, larger volume and stiffer foams
- H+ neutralize negative charges around egg white protein molecules
- Egg white proteins reach IEP and denature faster
- The more denatured proteins, the more stable the foam
- Always added at beginning of beating so foam forms faster
How does fat affect egg white foam quality? What should you do?
- Delays or prevents foam formation
- 0.1% of egg yolk prevents foam formation
- Egg yolk solids contain 67% fat
- Lipoproteins in egg yolks form complex with egg white proteins preventing denaturation
- Use glass or metal bowl, not plastic/silicone to beat egg whites (fat adheres to plastic so if you’ve baked with it before it stays)
What is the emulsifier in egg?
- Lecithin in egg yolks is an emulsifier
- Lecithin has hydrophilic and hydrophobic ends to bond to water/fat
- Keeps immiscible liquids from separation
Explain the cause of the grey-green colour in hard-boiled and scrambled eggs.
- Hydrogen sulfide (H₂S) in egg white combines with iron in egg yolk
- Forms ferrous sulfide (FeS₂) which has a grey-green colour
Hard boiled:
- Slow cooling after cooking, pressure of heat allows contact between egg white and yolk (allows hydrogen sulfide and iron to react)
Scrambled:
- Keep eggs hot for too long after cooking/after proteins have coagulated allows compounds to move around
- Iron in yolk and hydrogen sulfide in white come into contact while still warm for long period of time
Explain the solution to prevent the grey-green colour in hard-boiled and scrambled eggs.
Hard boiled:
- Cool rapidly in cold water
- Reduces pressure on outside of egg drawing hydrogen sulfide in egg white from yolk
- Crack eggs to remove shell to remove pressure
Scrambled:
- Prepare in small batches, limiting heating time, minimize heat
The longer the egg proteins are heated after coagulation, the more grey-green ring and also over-coagulation will occur (tough texture due to over-coagulation of proteins)
Can you freeze and thaw egg whites/egg yolks?
Whites:
- Raw, yes
- Cooked, no (tough and rubbery)
Yolks:
- Raw, no (proteins denature = thick and gummy)
- Cooked, yes
- If you add sugar or salt to raw yolk before freezing, it is fine because proteins are protected
