lec 9 eggs and egg products Flashcards
what does the egg consist of?
yolk: 28-29% wt/wt
shell and membranes: 9-11% wt/wt
air cell (blunt end)
egg white/albumen: 60-63% wt/wt
what is the protein content in the albumen?
almost 90%!
what is responsible for the ”slimy” texture of albumen?
High MW ovomucin
what plays an important role for gel formation in the albumin?
Ovotransferrin has a reltively low Td (denaturation temp) and plays an important role for gel formation
where do Albumen proteins adsorb and stabalize at?
the air-water interface
- Inter-molecular interaction between proteins play an important role – increase in protein film stiffness –> increased foam stability
when is there Improved foaming of albumen proteins?
when pH is lowered – decrease in protein
charge
* Divalent metal ions aid in foam stabilization – crosslinking of proteins
* Traditionally known from whipping in cupper bowls
* Unfolding of proteins: Exposure of hydrophobic groups and formation of S-S bridges
what impairs foaming?
Small amounts of yolk contaminants
what are the steps in Gelling of albumen proteins?
Native proteins
–>Heating
(Pre-)Denaturation – unfolding – exposure of hydrophobic groups
–>heating
Aggregate formation
–>heating
Intermolecular S-S bridges –>
Gel formation
–>Cooling
H-bond formation – rapid increase in firmness/elasticity
(Albumen powders are heat treated at 70-80 °C ->”pre-denaturation of proteins ->
increases gel strength)
what is the Highest gel strength in the albumen
around the pI of the main proteins
what is the role of S-ovalbumin in Albumen gel strength?
- Upon storage of eggs/albumen, ovalbumin converts to S-ovalbumin
- changes in secondary protein structure
- Fresh eggs: 95% ovalbumin, 5% S-ovalbumin
- Typical supermarket egg: 50% ovalbumin, 50% S-ovalbumin
- Egg after 6 months storage: 20% ovalbumin, 80% S-ovalbumin
S-ovalbumin has higher heat stability!
what does a High S-ovalbumin content lead to?
lower gel strength and water holding capacity
what is the composition of the yolk?
Yolk is a protein solution as well as a dispersion of supramolecular lipoprotein
structures
Serum proteins (in solution)
Low density lipoprotein LDL (natural emulsion droplet)
High density lipoprotein HDL (complex aggregates structure)
Phospholipid (PL) composition (approximate):
Phosphatidylcholine 76% (lecithin)
Phosphatidylethanolamine 22%
Other phospholipids 2%
explain the Emulsification with egg yolk
Surface active egg yolk components adsorb at the oil- water interface and provides repulsion between oil droplets
* Upon adsorption proteins and PLs spread at the interface
* Triglycerides are ”emptied” into the oil phase.
where does the Main emulsion-stabilzing effect come from?
lipoproteins!
what Factors influence emulsions with egg yolk
- Salt and pH
- Partial denaturation
- Enzymatic modification
how does the salt and pH influence emulsions with egg yolk?
- Lowering of pH -> lipoproteins highly charged -> repulsion between oil droplets -> stability
- HDL is linked to phosvitin through phosphocalcic bridges ->granule structure
- At increased salt (as in e.g. mayonnaisse) dissociation occurs
- At pH approx. < 4.5 (mayonnaisse pH ≈ 3.6 - 4.0) dissociation occurs
- Dissocation increases the solubility of HDL proteins -> positive effect on emulsification and emulsion stability
how does partial denaturation during spray drying influence emulsions with egg yolk?
high temp –> positive impact
how does enzymatic treatment with phospholipase A2 (PLA2) influence emulsions with egg yolk?
- PLA2 treatment yields improved emulsifying properties
- ↑ emulsification capacity
- ↑ increased viscosity
- ↑ heat stability of yolk emulsions
- Not directly related to the presence of lyso-PL
- Related to structural changes in HDL and LDL
- Solubilization of HDL and LDL proteins?
what does PLA2 treatment of LDL do?
*PLA2 treatment does not disintegrate the LDL structure.
*PLA2 prevents aggregation of LDL upon heating.
* Improved heat stability –> pasteurization possible
* Most yolk sold as an ingredient powder is PLA2 treated
