SAR HS 230 Lecture Notes - Lecture 3: Surface Tension, Isoelectric Point, Emulsion

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Lecture 3
Amino Acids
Three major categories of amino acid side chains
Non-polar hydrophobic
Polar
Charged
The pH at which the amino acid and side chain are neutral is called isoelectric point (pI)
To determine pI for an amino acid
pI (acidic side chain) = (pka1+pka3)/2
pI (basic side chain) = (pka2+pka3)/2
Isoelectric point indicates how the protein will interact
pI in foods are important because if pH is changed it will have different effects on the
properties and qualities of food
Proteins are made up of amino acids joined together via peptide bond
Proteins are very large containing hundreds to thousands of amino acids
They have 4 structures:
Primary
Linear sequence of proteins
No structures represented
Do not exist in real life
Used to determine the shape of the protein
Secondary
Start to get the structure of the protein
Does not exist in real life either
Puts into the protein some specific structures
Tertiary
Protein folded into 3D structure
Has interactions between the proteins to help fold
Hydrophobic interactions
Disulfide bonds: strongest covalent bonds inside proteins
Occurs between two cysteine (disulfide bonds)
Can occur intra or inter proteins
Can be reversible unless incinerated
More heat stable
Perms: chemically breaks bonds and cannot go back to normal
Straightener: temproarirly bends bonds to stay straight
Heat takes out water and when water goes back in, the bending is
disrupted and curles go back
Tertiary folding: hydrophobic residues want to stay away from water so they are
folded inwards into the core and the hydrophilic goes to the outside to interact
with the water
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It is impossible to get all residues where they have to go because steric strain
wo’t let so ay thigs together ad so ay beds before it breaks
The ratio of polar to non-polar surface residues influences its physiochemical
properties
Twi types of structure
Fibrous
High amount of ordered secondary structures
Pleated sheets and helices
Globular
Spherical or elliptical shape
Quaternary
No covalent interactions
The structure are mainly bonds between tertiary proteins
Protein denaturation
Denaturation is the change of second, tertiary or quaternary structure protein
unfolding
Never primary structure because peptide bond is too strong to break
Can result from
Temperature
pH
Does not cause chemical changes just structural
Effects
Negative:
Loss of solubility
Loss of functional properties
Postivie:
Food products
Cottage cheese lactic acid to produce curd
Meringes heat
Innactivates food spoilage enzymes
Improves digestibility of legume proteins inactivates trypsin inhibitors
Heat induced gelation
Improves a proteins foaming and emulsifying properties
Thermal processing
The most commonly used ethod of denaturation in food processing and
preservation
Proteins go through various degrees of denaturation affecting their
functional properties
Amino acid composition of the protein affects its thermal stability
Higher amoungs of hydrophobic amino acids increase thermal stability
Tightly packed proteins are more stables than those with cavities or void
spaces
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Sugars and low salt levels tend to increase thermal stability of proteins in
aqueous solutions
Sugar helps the protein be stronger and lock the structure and make
emulsion last longer
pH
Proteins are most stable against denaturation at t heir pI
The high charges associated with extreme pH results in the swelling and
unfolding of proteins
In some cases, extreme pH can hydrolyze peptide bonds
Denaturation by heat and pH can be reversible or irreversible depending on the
degree of denaturation
Use of proteins in food
Functional role of proteins in foods
Gels
Protein gels can be formed using:
Heat: set gels are formed via non-covalent interactions
Hydrophobic interactions
Hydrophilic interactions
Hydrogen bonding
Electrostatic interactions
Enzyme
Divalent cations
Steps for a protein gel formation
1. Protein sol is heated resulting in denaturation and the formation of a pro-
gel state
Sol to pro-gel step is irreversible due to denaturation
2. In the pro-gel state, the reactive residues are exposed and allow for non-
covalent interactions to occur
Forms soluble and insoluble complexes
3. Cooling the pro-gel results in gel formation
Gels formed with mostly hydrogen bonds and electrostatic interactions
can be melted back to the pro-gel state
Jello-O
Gels formed with mostly hydrophobic interactions cannot be melted back
Hydrophobic interactions are heat stable and increase in strength
with an increase in heating
Gels formed with proteins high in Cys and cysteine groups undergo
disulfide interchange
Are reversible
Coagulum (opaque) vs translucent gels
Opaque gels
Proteins with a large amount of non polar amino acid residues form opaque gels
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