Final Exam Material Flashcards
What are the fundamental building blocks for proteins?
Amino acids
All common amino acids have an asymmetric alpha-carbon.
True or False?
False.
Glycine does not.
At pH greater than the isoelectric point (pI) of an amino acids, the net charge is…
Negative
At pH lower than the isoelectric point (pI) of an amino acid, the net charge is…
Positive
In food systems, what do amino acids provide? [3]
Flavour
Colour
Bioactive compounds
What is/are the functional group(s) in proteins and amino acids that can react with reducing sugar in Maillard reaction?
Alpha-amino group
Epsilon-amino group
Enzymatic browning shares similarities with Maillard browning and caramelization.
True or False?
False.
What does enzymatic browning require? [5]
Oxygen
An enzyme
Copper
An aromatic compound as a substrate
A relatively neutral pH
What does decarboxylation of amino acids often lead to?
Potentially toxic compounds
Proteins consist only of amino acids.
True or False?
False.
They may have additional groups, e.g., carbs, lipids, etc.
What does the primary structure of a protein refer to?
The sequence of amino acids.
The alpha helix of proteins is stabilized by hydrophobic bonds.
True or False?
False.
The alpha helix of proteins is often the most stable secondary protein structure.
True or False?
True.
The beta sheet of secondary protein structure is comprised of beta strands.
True or False?
True.
The tertiary structure of a protein is stabilized by…
Numerous bonds/forces
Where does a hydrogen bond occur?
Between a hydrogen and an electronegative atom
How may disulfide bonds be broken?
By a reducing agent
Define: protein denaturation
Any change in the secondary, tertiary and/or quaternary structure without cleavage of the protein backbone.
Name 4 functional properties of proteins (not an exhaustive list)
Water solubility
Gelation
Emulsification
Viscosity
When is the water binding capacity of most proteins at its lowest?
At its isoelectric point
When a protein denatures, what happens to its emulsifying properties?
They increase
What happens to the emulsifying properties of a protein when its solubility increases (e.g., by pH changes, salt addition, etc.)?
They increase
Why are proteins good foaming agents?
They surround a gas phase
Presence of salts at low concentrations increases which properties of most proteins? [4]
Solubility
Water binding capacity
Water holding capacity
Emulsifying ability
When does a coagulum type gel result?
From hydrophobic bonds and possibly disulfide linkages
At extreme pH, protein gels are the strongest due to covalent linkages.
True or false?
False.
At extreme pH, protein gels are weakest due to charge properties.
What does curd formation in milk result from?
Enzyme action causing an increase in hydrophobicity of the micelle
What are amino acids? [3]
- Alpha-amino acids are the basic structural units of proteins
- Consist of an alpha-carbon atom covalently attached to a hydrogen, an amino group, a carboxyl group, and a side chain R group. (Contains C, H, O, & N, [S, P])
- Amino acids differ only in the chemical nature of the side chain of the R group
Describe the structure of an amino acid.
The alpha carbon is an asymmetric carbon atom in all amino acids (except for glycine), thus amino acids exist in two optically active forms: L- and D- isomers.
L-isomer amino acids are sweet and D-isomers are bitter.
True or False?
False.
Describe the solubility of amino acids. [2]
- Differences in solubility of properties of amino acids is due to the type of side chain.
- Dependent on the dissociation properties of the amino acid, known as dissociation constant (pKa)
What are the essential amino acids?
Essential amino acids cannot be synthesized de novo by the organism, and must be supplied in the diet.
PVT TIM HLL
Phenylalanine
Valine
Tryptophan
Threonine
Isoleucine
Methionine
Histidine
Arginine
Leucine
Lysine
What are limiting amino acids? [3]
- Quality of proteins depends on the richness of essential amino acids (limiting amino acids) and the digestibility of the protein.
- A limiting amino acid is the essential amino acid found in the smallest quantity in the food (e.g., gelatin lacks 4 eAA)
- Limiting amino acids affect the net protein utilization or biological value
Different amino acids provide different physicochemical properties to peptide chains or protein molecules.
Describe how valine, isoleucine, and leucine are similar?
These three amino acids have nonpolar side chains that are branched and thus restrict the internal flexibility and give rise to surface hydrophobicity.
Different amino acids provide different physicochemical properties to peptide chains or protein molecules.
Describe how serine and threonine are similar.
Polar side chains form hydrogen bonds.
Serine and threonine have hydroxyl groups that form hydrogen bonds.
Different amino acids provide different physicochemical properties to peptide chains or protein molecules.
Describe the uniqueness of cysteine.
Cysteine has an important role in forming sulfydryl (S-S) bridges between different parts of the peptide chain.
Describe the role of glutamate in foods. [2]
- L-glutamate suppresses certain ‘undesirable’ flavours, such as:
- Sharpness of onion flavour
- Rawness in many veggies
- Earthiness of potatoes
- Glutamate activity to modulate taste in foods is noticeable between pH 3.5 to 7.2, being more intense in lower pH.
Describe sweetness perception of amino acids. [6]
- D-amino acids generally evoke a sweet taste perception while L-amino acids are tasteless or bitter.
- Short side-chain of L-amino acids can also elicit a sweet taste
- As the side chain length of L-amino acids increases, the sweetness will change to bitter
- If the hydrophobicity increases on the L-amino acid, the sweetness will be lost and replaced either with bitter or no taste.
- Preference for D-amino acids tend to increase with molecular weight and hydrophobicity.
- The correlation between the hedonic score and D-amino acid molecular weight or bulk of the side chain reflects that the lower threshold score, the less required from the sample to elicit a positive preference.
What reaction has the greatest impact on protein sensory and nutritional properties?
Maillard reaction, which occurs in both food and biological systems.
Amino component → proteins and amino acids
Carbonyl component → reducing sugars, ascorbic acid (vitamin C) and carbonyl compounds
Describe Strecker degradation. [4]
- The 3rd pathway of the Maillard reaction; carbonyl-amine reaction
- The oxidation of amino acids by an alpha-dicarbonyl or other conjugated dicarbonyl compound that is produced on the breakdown of Amadori compounds
- Amino acids are degraded to aldehydes, pyrazines, sugar fragments, ammonia, and carbon dioxide.
- Strecker degradation of each amino acid produces a specific volatile compound and provide unique aromas and flavours
What are amino acids degraded to during Strecker degradation, the 3rd pathway of the Maillard reaction? [5]
Aldehydes, pyrazines, sugar fragments, ammonia, and carbon dioxide.
Describe tyrosine oxidation.
- The presence of oxygen and the enzyme tyrosinase are essential for the oxidation of tyrosine to dihydroxyphenylalanine (DOPA)
- Copper is an essential co-factor for tyrosinase enzymatic activity
- Optimal pH ranges from 6.7 - 7.2
- Sequestering copper agents (e.g., organic acids) and decreasing pH result in the inhibition of tyrosinase
- The reaction is also reduced by:
- Reduction of oxygen concentration (i.e., vacuum packaging)
- Use of reducing agents (e.g., ascorbic acid, sulfites)
What does tyrosine oxidation require?
Tyrosinase (and a copper cofactor catalyst)
pH 6.7 - 7.2
What reduces tyrosinase activity?
Sequestering copper agents (e.g., organic acids)
Reducing of oxygen concentration (e.g., vacuum packaging)
Use of reducing agents (e.g., ascorbic acid, sulfites)
How are biogenic amines formed? [3]
(e.g., vasoactive amines)
- Amino acid decarboxylation reactions result in their formation
- Tyrosine to tryamine
- Histidine to histamine
- Phenylalanine to phenylethylamine
- Arginine to putrescine
- Bioactive amines are the result of the action of the bacterial enzyme: L-amino acid decarboxylase
What is tyramine?
- Formed by decarboxylation reaction catalyzed by bacterial L-amino acid decarboxylase.
- Naturally present in small amounts in bananas, pineapples, and produced by streptococci action in cheddar cheese
- Poisoning can occur as low as 40mg
- Causes hypertension and intense headaches
What is histamine and how is it formed? [6]
- Decarboxylation reactions
- Formed from the enzymatic action of gram-negative bacteria mainly found in fish
- Poisoning symptoms include headache, nausea, hypertension, and vomiting
- Histamine production can also be synthesized by the normal flora of the colon.
- Intake of foods rich in histadine may favour endogenous production of histamine
- Histidine decarboxylase is also present in tissue mast cells and blood cell basophils, thus typical signs of inflammation can be due to the release of this enzyme.
Typical signs of inflammation may be due to…
Release of histidine decarboxylase, present in mast cells and blood cell basophils.
What is putrescine and how is it formed?
Decarboxylation reaction
What is tryptamine and how is it formed?
Decarboxylation reactions
Describe the cause of amino oxidation product formation, which amino acids are involved, and the consequences. [7]
- Protein in different food matrices like beef patties, chicken meat, raw pork, and different types of fish can be oxidized during frozen storage
- The usage of sodium nitrite, in cured meat, is another cause of protein oxidation.
- Specific amino acids are involved
- Lysine (alpha-aminoadipic acid)
- Tyrosine (kynurenine)
- Tryptophan (kynurenic acid)
- Consequences → protein functionality and human health
Which alpha-amino acids contain ionizable groups? [6]
Proline
Tryptophan
Tyrosine
Cysteine
Arginine
Histidine
What are nitrites used for?
Preservation against C. botulinum
What is nitrite reduced to? What does it react with? What does it form?
Nitrous anhydride, which reacts with amines containing ionizable groups, forming nitrosamines (risk of cancer)
Nitrosamides are derivatives of substituted amines
Under what conditions are nitrosamines produced?
Conditions of low pH (stomach environment) and at high temperatures (frying, roasting)
When are nitrosamines carcinogenic?
When metabolically activated by the hosts’ oxidative enzymes
What are proteins? [3]
Next to water, protein is the most plentiful substance in the body!
- Organic compounds formed by 100-500 amino acids linked together
- Necessary in the diet, since not all amino acids can be synthesized
- Built from a central carbon bonded to four different groups
- Hydrogen
- Amino group
- Carboxyl group
- Side chain
Simple proteins yield amino acids upon hydrolysis.
Name 4 types of simple proteins and describe their solubility.
- Albumins → soluble in water
- Globulins → soluble in neutral salt solutions
- Glutelins → soluble in dilute acid or alkali
- Prolamins → soluble in 70% alcohol
What are conjugated proteins?
Proteins that contain amino acids combined with a non-protein constituent.
What are phosphoproteins?
A conjugated protein
Phosphate groups linked to hydroxyl groups of serine or threonine
E.g., casein in milk and phosphoprotein in yolk
What are lipoproteins
Conjugated proteins
Combination of lipids and protein with excellent emulsifying properties
What are nucleoproteins?
Conjugated proteins
Combination of nucleic acid with protein
Found in cell nuclei
What are glycoproteins?
Conjugated proteins
With carbohydrate (8-20%)
Has strong allergenic properties
E.g., ovomucin in egg white
What are chromoproteins?
Conjugated proteins
Have prosthetic groups
E.g., hemoglobin, chlorophyll, myoglobin, flavoproteins
What are derived proteins?
Derived proteins are obtained by chemical or enzymatic methods.
They are soluble in water and not coagulated by heat.
What are primary-protein derivatives?
Limited hydrolyzed proteins - coagulated proteins
E.g., rennet coagulated casein
What are secondary protein derivatives?
Peptones and peptides: a combination of two or more amino acids
Examples are the breakdown products formed during processing of milk products: the peptides formed during the ripening of cheese
What is the primary structure of a protein?
Primary structural properties of proteins → sequence of amino acids
Associated with the properties of individual amino acids comprising the protein
Amino acids are linked by covalent bonds through amide bonds, also known as peptide bonds.
What is the secondary structure of a protein?
- Refers to the periodic spatial arrangement of amino acid residues at certain segments of the polypeptide chain
- The regularly repeating local structures are stabilized by hydrogen bonding; notable examples are the alpha helix, beta sheet, beta turns, and random coil
- Two forms of periodic secondary structures are found in proteins: helical and sheet structures.
- Because secondary structures are local, many regions of different secondary structure can be present in the same protein molecule.
Describe the alpha-helix secondary structure. [5]
- From the helical structures the major form in proteins is the alpha-helix, which is the most stable
- Each backbone N-H group is hydrogen bonded to the C=O group of the fourth preceding residue
- Can exist in either right or left-handed orientation → the right-handed helix is more stable
- The helix makes a complete turn every 3.6 amino acids.
- In the alpha-helix, the backbone atoms form a coil while the carbonyl groups form hydrogen bonds with the amide groups
The left-handed alpha-helix is the most stable.
True or False?
False.
The right-handed alpha-helix is more stable.
How many amino acids in a complete turn of an alpha-helix?
3.6
Describe the beta-sheet secondary structure of proteins. [8]
- The C=O and N-H groups are oriented perpendicular to the direction of the chain
- Hydrogen bonding is possible between segments but not within a segment
- Comprised of individual beta-strands usually 5-15 a.a. long
- Two beta-strands of the same molecule interact by hydrogen bonds, forming a beta-pleated sheet
- Beta-strands often have a hydrophilic and hydrophobic side
- Beta-sheet is generally more stable than the alpha-helix
- Fibrous in nature and insoluble in aqueous solvents
- E.g., soy globulin, beta-lactoglobulin
Which is more stable, alpha-helix or beta-sheet?
Beta-sheet
Beta-sheet structure is generally fibrous in nature and soluble in aqueous solvents.
True or False?
False.
They are INSOLUBLE in aqueous solvents (they are fibrous in nature).
Beta-strands in beta-pleated sheets run antiparallel.
True or False?
May be parallel or antiparallel.
Anti-parallel beta-sheets are stronger than parallel due to the hydrogen bonds lying on the same plane.
What is the tertiary structure of proteins?
- Refers to the spatial arrangement attained when a linear protein chain with defined (alpha-helix or beta-sheets) or non-defined (random coil) secondary structure segments folds further into a compact 3D form.
- Amino acid linkages fold over into compact structures stabilized by hydrogen bonds, disulphide bridges, and van der Waals forces.
What does quaternary structure of a protein refer to?
The spatial arrangement of a protein when it contains more than one polypeptide
Formation of oligomeric structures is the result of specific protein-protein interactions
What interactions determine the secondary and tertiary structure of proteins? [5]
Hydrogen bonds
van der Waals forces
Hydrophobic interactions
Disulfide linkages
Ionic interactions
Describe the following forces governing protein structure:
Covalent
Ionic
Hydrogen
Hydrophobic
Electrostatic repulsion
van der Waals repulsion
- Covalent → atoms bound by a common electron pair e.g., peptide bonds, disulfide bonds
- Ionic → attraction between opposite forces e.g., solvent interaction, salt working
- Hydrogen → hydrogen shared between two electronegative atoms; stabilizing/bridging
- Hydrophobic → apolarity; strand thickening, strengthening, stabilizing
- Electrostatic repulsion → coulombic repulsion between particles with like-charges e.g., polar groups of side chains
- van der Waals repulsion → repulsion of apolar groups that are close; steric hindrance between side chain groups
What is a hydrogen bond?
Interaction of a hydrogen atom that is covalently bonded to an electronegative atom (N, O, S) with another electronegative atom.
Describe how proteins fold into unique tertiary structures and what its stability depends on. [3]
- Protein stability depends on maintenance of an apolar environment
- Hydrophobic interactions are the major forces to drive protein folding and stability
- Hydrophobic interaction between nonpolar side chains of amino acid residues is the major reason by which proteins fold into unique tertiary structures.
What is the main reason by which proteins fold into unique tertiary structures?
Hydrophobic interactions
Describe disulfide bonds.
- Disulfide bonds occur between two cysteine (Cys) resides by oxidation of the sulfhydryl groups by molecular oxygen.
- Once formed, disulfide bonds help stabilize the folded structure of the protein.
What are causes of protein denaturation? [5]
- Acids
- Strong salt solutions
- Heat
- Alkaline conditions
- Radiation
Protein denaturation is irreversible.
True or False?
False.
Protein denaturation is usually reversible (in the absence of aggregation) when the denaturant is removed (except egg white).
Many biologically active proteins lose their activity upon denaturation.
Denaturation may result in: [6]
- Loss of solubility
- Increase intrinsic viscosity
- Altered water holding capacity
- Increased digestibility
- Increase susceptibility to protease attack
- Improves foaming and emulsifying properties
Many biologically active proteins lose their activity upon denaturation.
Denaturation may result in: [6]
- Loss of solubility
- Increase intrinsic viscosity
- Altered water holding capacity
- Increased digestibility
- Increase susceptibility to protease attack
- Improves foaming and emulsifying properties
Describe how a protein’s primary, secondary, tertiary, and quaternary structure change with denaturation.
- Primary structure is not disrupted.
- Secondary structure → proteins lose all regular repeating patterns like alpha helices and beta pleated sheets and adopt a random coil configuration.
- Tertiary structure → covalent interactions (like disulfide bonds), non covalent interactions between polar side chains and surrounding solvent, and v.dw. interactions between non polar amino acid side chains are all disrupted
- Quaternary → protein sub-units are dissociated and/or the spatial arrangement of protein subunits is disrupted
Functionality of food proteins is defined as “those physical and chemical properties which affect the behaviour of proteins in food systems during processing, storage, preparation, and consumption”. (Kinsella, 1976)
List the functional roles of proteins in food systems. [10]
- Solubility
- Viscosity
- Water binding
- Gelation
- Cohesion-adhesion
- Elasticity
- Emulsification
- Foaming
- Fat binding
- Flavour binding
What groups in proteins will water bind to? [5]
- Charged groups: ion-dipole interaction
- Backbone peptide groups
- Amide groups of Asn and Gln
- Hydroxyl groups of Ser, Thr, and Tyr residues
- Non-polar residues, dipole-induce dipole interaction and hydrophobic interaction
Define water binding capacity.
Grams of water per gram of protein when a dry protein is equilibrated with water vapour at 90-95% relative humidity.
Also known as hydration capacity.
What three factors influence water binding capacity?
pH and isoelectric point
Presence of salts
Temperature
How does pH and isoelectric point affect water binding capacity?
- At the isoelectric point, proteins exhibit the least hydration, but greatest protein-protein interaction
- Water binding capacity increases above and below the isoelectric pH
- The greatest is usually at pH 9-10
- Exposure of groups
Describe how presence of salts affects water binding capacity of proteins.
- At low concentrations (<0.2M), salts increase water binding capacity. Hydrated salt ions bind weakly to charged groups of proteins - “salting in”
- At high salt concentration, water is bound to salt ions, resulting in dehydration of the protein - “salting out”
Describe how temperature affects the water binding capacity of proteins.
- As temperature is raised, water binding capacity generally decreases because of decrease in hydrogen bonding and decrease in hydration of ionic groups
- If temperature leads to aggregation, water binding capacity decreases due to increase in protein-protein interactions
Define water holding capacity.
Refers to the ability of the protein to imbibe water and retain it against gravitational force within a protein matrix.
Refers to the (1) sum of bound water, (2) hydrodynamic water, and (3) the physically entrapped water.
The contribution of physically entrapped water is lesser than those of the bound and hydrodynamic water.
True or False?
False.
Its contribution is larger than those of the bound and hydrodynamic water.
What is the ability of protein to entrap water associated with?
Juiciness and tenderness of meat products
Desirable textural properties of bakery and other gel-type food products.
What is an emulsification?
A mixture of two immiscible substances, stabilized by emulsifiers.
Describe the emulsification properties of proteins.
- Proteins in native form act as emulsifiers by having hydrophobic amino acids hidden, or protected from the water phase, by hydrophilic amino acids.
- On denaturation, the protein unfolds and hydrophobic amino acids align with non-polar or oil phase and the hydrophilic amino acids align with polar or aqueous phase.
Describe how proteins are emulsifiers for oil in water (o/w) emulsions.
Because they are edible, surface active, and provide superior resistance to coalescence
They provide amphiphilic domains
With adsorption, native conformation is changed: side chains of hydrophobic amino acids → to surface of oil droplet; hydrophilic amino acid side chains remain in aqueous solution
Result = water-oil interface
Describe how proteins are the emulsifiers in water in oil (w/o) emulsions.
They cannot be used for (w/o) emulsions because of insolubility in oil. Reduces interfacial tension.
Describe five factors that affect protein emulsification.
- Solubility of protein → optimal at 25-80% solubility
- pH → proteins with high solubility at their isoelectric point have maximal emulsifying capacity.
- Salt → may increase protein solubility thereby increasing emulsification
- Temperature → small increase in temperature will influence the rate of diffusion and the rate of adsorption and protein unfolding thus increasing emulsification
- Denaturation → with partial denaturation, proteins start to unfold so that free sulfhydryl groups and disulphide bonds form interactions at the oil/water interface = increase flexibility and surface hydrophobicity = increase emulsification
Describe protein foaming.
- A foam consists of an aqueous continuous phase and a gaseous (air) phase
- The foaming property of a protein refers to its ability to form a thin tenacious film at gas-liquid interfaces so that large quantities of gas bubbles can be incorporated and stabilized
- Protein-stabilized foams are formed by bubbling, whipping, or shaking a protein solution
Describe flavour binding properties of proteins. [4]
- Proteins themselves are odorless.
- Flavours come mainly from aldehydes, ketones, and alcohols
- Noncovalent interactions are normally the mechanism of flavour binding to proteins; covalent bonding may also happen
- Only the noncovalently bound fraction can contribute to aroma and taste of the protein food
Describe 5 factors that affect flavour binding properties of proteins.
- Protein denaturation → thermally denatured proteins exhibit increased ability to bind flavours; ability to bind flavours is lower in denatured than in native proteins
- Salts → ‘salting in’ destabilize hydrophobic interactions, thus decreasing flavour binding; ‘salting out’ increases flavour binding
- pH → flavour binding is enhanced at alkaline pH
- Disulfide bonds → breakage of disulfide bonds causes unfolding of proteins thus increasing flavour binding
- Proteolysis → hydrolysis of protein causes disruption and decreases hydrophobic regions and thus decreases flavour binding; proteolysis usually produces peptides of bitter taste
What is gelation?
Protein gelation refers to the transformation of a protein from a solid state to a gel-like state.
Two types → coagulum and translucent
Describe coagulum gel (opaque gel).
Irreversible gel
Original protein have large amounts of non-polar residues that undergo hydrophobic aggregation, or insoluble aggregates that randomly associate
Describe translucent gel.
- Main linkages are H-bonds and electrostatic forces
- Formed by proteins with relatively small amounts of insoluble residues and high amounts of polar amino acid residues
- Reversible soluble complex after denaturation
- Ordered aggregation
What is a gel network mainly formed by?
Hydrogen bonding and hydrophobic and electrostatic interactions
Gels are highly hydrated systems, up to 98% water.
True or False?
True.
What type of gel is less prone to syneresis and why?
Translucent gels hold more water and are less prone to syneresis than coagulum-type gels, due to hydrogen bonding interactions.
Describe how the type of interaction affects protein gelation.
- Gels formed by noncovalent interactions, mainly hydrogen bonds, are thermally reversible
- Gels formed by hydrophobic interactions are irreversible (e.g., egg white gel)
- Proteins can undergo polymerization via sulfhydryl-disulfide interactions → usually thermally irreversible (e.g., whey gels)
Describe how protein concentration affects gelation.
Minimum point concentration, least concentration end point (LCE) is required
Soy protein = 8%
Egg albumin = 3%
Gelatin = 0.6%
Describe how humectants (sugars or salts) affect protein gelation.
Humectants bind to free water causing high protein-protein interaction and low protein-water interaction
This results in strong gels formation with greater heat stability.
Describe how pH affects protein gelation.
- At or near pI, proteins usually form a coagulum-type gel.
- Acid conditions increase the amount of free water, thus decreasing the negative charge and increasing aggregate formation
- Extreme pH → weak gels are formed because of strong electrostatic repulsion
- Optimum pH = 7-8 for most proteins
Describe how proteolysis affects protein gelation.
- Limited proteolysis facilitates gel formation
- Addition of chymosin (rennin) to casein in milk results in formation of coagulum-type gel
Describe how cross-linking affects protein gelation.
- Enzymatic cross-linking at room temperature can result in gelation
- Transglutaminase is used to form highly elastic and irreversible gels, even at low protein concentration
List 6 factors affecting protein gelation.
- Type of interaction
- Protein concentration
- Humectants (sugar or salt)
- pH
- Proteolysis
- Cross-linking
How much protein is in milk?
Bovine milk contains 30-36 g/L (%) of total protein
Milk proteins: casein (80%) and whey (20%)
Describe casein.
- Mainly alpha, beta, and kappa caseins
- Calcium sensitive alpha and beta (addition of calcium precipitates protein); kappa caseins are calcium insensitive (addition of protein will not affect protein, it remains soluble)
- Amphipathic
- Self-associate in solution to form micelles
Describe how the rennin enzyme affects casein micelles.
- It cleaves k-casein at Phe105 - Met106 into para-k-casein and a glycopeptide
- para-k-casein micelles form small aggregates, which then assemble into a gel
Compare the hydrophobicity, and calcium binding capacity of alpha beta and kappa casein.
(1) Hydrophobicity
- αs2-
(2) Calcium binding capacity
- The higher degree of phosphorylation, the higher binding capacity to calcium.
- αs2- > αs1- > β- > κ-
Describe the interactions of casein micelles.
Micelles are kept together by electrostatic interactions:
- Calcium or calcium phosphate bridges with phosphoserine and glutamic residues
- Hydrogen bonds and minimal hydrophobic bonding
Lipids are formed from structural units with a pronounced hydrophobicity.
True or False?
True
Lipids are generally insoluble in water but some are amphipathic molecules.
True or False?
True.
The nutritive/physiological importance of lipids is based on their role as energy molecules (e.g., 4 kcal/gm) and a source of essential fatty acids and vitamins.
True or False?
False.
9 kcal/gm
A refined lipid is available to consumers as an edible oil or fat.
True or False?
True.
The acyl residue has no influence on the hydrophobicity and reactivity of acyl lipids.
True or False?
False.
Fatty acids are not saponifiable.
True or False?
False
Phospholipids are saponifiable.
True or False?
True.
Waxes are saponifiable.
True or False?
False.
Acyl lipid hydrolysis releases aliphatic carboxylic acids which differ in chemical structure.
True or False?
True.
In general, it is true from the percentage data of fatty acid distribution that the unsaturated fatty acids predominate.
True or False?
True.
C18:2 Δ9,12 refers to linoleic acid.
True or False?
True.
Milk and coconut fats mainly consist of short chain, low molecular weight fatty acids (
True or False?
True.
Stearic acid is a long chain monosaturated fatty acid.
True or False?
False.
Eicosanoic acid is a HUFA found in lard.
True or False?
False
In the omega (ω) numbering system of unsaturated fatty acids, position of the first double bond (n) starts by counting from the methyl end of the hydrocarbon chain.
True or False?
True
cis-Oleic acid falls in the omega-9 family, where trans-Elaidic acid is an omega-6 fatty acid.
True or False?
False.
Melting properties of fats are determined by the arrangement of the acyl residues in the crystal.
True or False?
True.
The solubility of fatty acids increases with an increase in the number of cis-double bonds.
True or False?
True.
Lipases derived from the pancreas hydrolyze triacylglycerols at acyl residue positions 1 and 3.
True or False?
True.
Acyl lipid constituents, such as oleic, linoleic, and linolenic acids, have one or more allyl groups within the fatty acid molecule which makes them readily oxidized to hydroperoxides.
True or False?
True
Lipid peroxidation provides numerous volatile and nonvolatile compounds, some being pleasant while others objectionable.
True or False?
True.
What does the length of induction period and the rate of oxidation depend on? [3]
- Fatty acid composition of the lipid
- The number of allyl groups present
- The presence of antioxidants.
Briefly describe the oxidation of lipids. [4]
- Radical-induced chain reaction
- Divided into initiation, propagation, and termination phases
- Involves abstracting the H-atoms from activated methylene groups on unsaturated fatty acids
- Culminating into formation of non-hydroperoxide molecules
Which step of lipid oxidation are heavy metals involved in?
The initiation reactions
The peroxyl radical is slow reacting and therefore it selectively abstracts the most weakly bound H-atom from a fat molecule.
True or False?
True.
What does initiation of lipid oxidation require? [3]
Photosensitizers
Lipoxygenases
Transition metal ions
In the ground state oxygen is a triplet.
True or False?
True.
Oxygen goes from triplet state to singlet oxygen state by uptake of energy.
True or False?
True.
With oleic acid, the singlet state of oxygen attacks the 9-10 double bond generating 2 mono-hydroperoxides.
True or False?
True.
The primary products of autooxidation, mono-hydroperoxides, are odorless and tasteless.
True or False?
True.
Volatile carbonyl compounds are the main aroma carriers in lipid peroxidation.
True or False?
True.
Volatile carbonyls are formed by beta-scission of the mono-hydroperoxides.
True or False?
True
Products of beta-scission include short lived alkoxyl radicals.
True or False?
True.
Beta-scission can be catalyzed by heavy metal ions or heme compounds.
True or False?
True.
Describe steroid structure. [3]
Includes 4 condensed rings with the first 3 being in the chair configuration and the last one being planar.
Rings B and C, and C and D are fused by trans-configuration, rings A and B are fused in trans- or cis- configuration.
A characteristic of steroids is the presence of alcoholic OH-group in position C3.
Cholesterol is the main steroid in mammals and occurs in lipids in only free form.
True or False?
False.
Cholesterol in egg yolk is less than that in butter, when compared in the same weight of food (e.g., mg cholesterol per 100 g egg yolk or butter).
True or False?
False
Autooxidation of cholesterol can be accelerated by fatty acid oxidation products.
True or False?
True.
Cholesterol oxidation can be measured by TBARS assay.
True or False?
False.
TBARS assay aims to measure the primary product of lipid oxidation.
True or False?
False.
TBARS assay aims to measure the secondary product of lipid oxidation.
True or False?
True.
TBARS assay requires colorimetric or fluorometric measurement.
True or False?
True.
TBARS assay is useful for comparison of samples with significantly different compositions.
True or False?
False.
What does the TBA assay for quantitating lipid oxidation in pure oil require? [3]
- Butylated hydroxytoluene (BHT)
- A blank without lipid sample
- Construction of a standard curve
Quantification of peroxide value (PV) correlates well with off-flavours and off-aromas in oxidized lipid samples.
True or False?
False.
Peroxide value of herring oil during 50 degrees C storage was lower than that during 20 degrees C storage because higher temperature suppresses lipid oxidation.
True or False?
False.
Quality of lipid food is not affected until secondary volatile aldehydes and ketones are formed in lipid oxidation.
True or False?
True
Sensory evaluation is a direct method for evaluating off-flavours and off-aromas associated with lipid oxidation.
True or False?
True.
Lipids are fat-soluble (i.e., lipophilic molecules), and only soluble in organic solvents.
True or False?
True.
However, some lipids are surface active since some are amphipathic molecules. (e.g., phospholipids)
Oils are liquid while fats are solid at room temperature.
True or False?
True.
What are the main biological functions of lipids? [3]
Energy storage (9 kcal/g)
Structural component of cell membranes
Signalling molecules
What are some functions of lipids in foods?
Influence flavour and texture
Many fat soluble vitamins require lipids for bioavailability; low fat diets will reduce the absorption of lipid soluble vitamins (A, D, K, E)
Can be used as a cooking medium
What are the fat soluble vitamins?
A
D
E
K
What are the most abundant class of food lipids?
Acylglycerols [= Esters (glycerol + fatty acids)]
The acyl residues influence the hydrophobicity and reactivity of the acyl lipids.
How are fatty acids classified according to saturation?
Saturates
Monounsaturates (omega-9)
Polyunsaturates (omega-6, omega-3)
How is melting point affected by the length of the saturated fatty acid?
Generally as chain length increases melting point increases as well.
C12:0 Lauric acid
Saturated fatty acid
C16:0 Palmitic acid
Saturated fatty acid
C14:0 Myristic acid
Saturated fatty acid
C18:0 Stearic acid
Saturated fatty acid
C18:1 n9 Oleic acid
Unsaturated fatty acid
C18:3 n3 linolenic acid
Unsaturated fatty acid
C18:2 n6 linoleic acid
Unsaturated essential fatty acid
C20:4 n6 Arachidonic acid
Long-chain unsaturated fatty acid
C20:5 n3 Eicosapentaenoic acid
Long-chain unsaturated fatty acid
Found in fish oils
C22:5 n3 Docosapentaenoic acid
Long-chain unsaturated fatty acid
Found in fish oils
Describe the omega-9 elongation and desaturation pathway.
Oleic acid
- Desaturation*
- Elongation*
- Desaturation*
Palmitoleic or Eicosatrienoic acid
Describe the omega 6 elongation and desaturation pathway.
Linoleic acid (EFA)
- Desaturation*
- Elongation*
- Desaturation*
Arachidonic acid
Describe the omega-3 elongation and desaturation pathway.
Alpha-linolenic acid (EFA)
- Desaturation*
- Elongation*
- Desaturation*
- Elongation*
- Desaturation*
Docosahexaenoic acid
What is cis-conformation?
- Double bonds of unsaturated fatty acids an be either in the cis- or trans- conformation
- A cis configuration means that adjacent carbon atoms are on the same side of the double bond
- The cis isomer causes the chain to bend and restricts the conformational freedom of the fatty acid.
What is trans-conformation?
- A trans configuration means that the next two carbons are bound to opposite sides of the double bond
- Trans configuration does not cause the chain to bend much, and their shape is similar to straight saturated fatty acids
What are triacylglycerols?
The main constituents of vegetable oils and animal fats.
A.K.A. triglycerides
Fatty acids esterified to glycerol
What are phospholipids?
These molecules are made from glycerol, two fatty acids, and (in place of the 3rd fatty acid) a phosphate group, with another molecule attached to the other end.
The hydrocarbon tails of the fatty acids are hydrophobic, but the phosphate group end of the molecule is hydrophilic because of the oxygen constituents with all pairs of unshared electrons (= phospholipids are soluble in both water and oil)
What are the three primary polymorphic forms of triacylglyceride polymorphs?
There are three primary polymorphic forms: α, β, and β’
α → unstable due to high degree of disorder; random order chain axis; do not align efficiently or pack tightly; usually small and numerous
β → packed to a much greater extent compared to alpha with more aligned structures; allows formation of larger crystals; greater stability and can achieve longer lengths; more likely to accumulate in fats
β’ → parallel rows with the same orientation opposite to orientation of rows above and below; larger than α; smaller than β
Conversion of β to β’ is done in the production of margarine for example which allows a softer/spreadable fat.
Discuss the cooling parameters for formation of triglyceride polymorphs.
alpha → rapid cooling
beta-prime → slow cooling
beta → very slow cooling
Draw a schematic mechanism of the 3 phases of lipid oxidation resulting in lipid rancidity.
Initiation
Propagation
Termination
Describe the initiation phase of lipid oxidation.
- Characterized by the abstraction of a hydrogen atom from a methylene carbon that is located in between two double bonds of the unsaturated fatty acid.
- Has a fairly high energy of oxidation, which is why pro-oxidants are required to initiate the reaction
- Yields a carbon-centered radical
How does the degree of unsaturation affect the rate of lipid oxidation?
Concentration of the unsaturated fatty acid is in the rate law
Induction period is affected by the type of unsaturated fatty acid (e.g., stearic acid creates an extremely long induction period and the slowest oxidation rate; a single double bond would have an induction rate of 82 hours and the slowest rate of o; two double bonds lowers the induction bond further ~19 hours, with a much higher oxidation rate; three double bonds has an induction period of ~1.5 minutes with the highest oxidation rate)
What are the products of initiation and propagation?
Initiation = lipid radicals
Propagation = hydroperoxide
Secondary products = aldehydes, ketones, alcohols, small acids, alkanes
Discuss the secondary products of lipid oxidation.
Hydroperoxides undergo beta-scission
Malonaldehyde → dialdehyde preferentially formed by autooxidation of fatty acids with three or more double bonds; odorless; very reactive; through cross-linking can bind proteins and denature them or nucleic acids and inactivate them
Even at trace levels, malonaldehyde is an indicator of lipid oxidation (=TBA test)
What are lipoxygenases?
- A family of iron-containing enzymes that catalyze the dioxygenation of polyunsaturated fatty acids and associated esters or acylglycerols to produce active hydroperoxides. Causes oxidative damage and destruction of vitamin A and carotenes
- Activation: iron (ferrous - Fe+2) (oxidized) → iron (ferric - Fe+3)
- Has a copper prosthetic group
- May be prevented by blanching, pasteurization etc.
How does temperature influence lipid oxidation?
The higher the temperature, the faster the reaction.
Anisidine value is a measure of hydroperoxides (primary/initial product of lipid oxidation)
At 50 degrees C, the end of the line indicates that the hydroperoxides have begun to decompose into secondary lipid oxidation products
What is a free radical?
Molecules with one or unpaired electrons
Unstable, thus reactive
Reactive oxygen species
How does the control of free radicals maximize oxidative stability of food?
- Antioxidants bind to free radicals and form a more stable anti-oxidant radical thus blunting the reaction
- Free radical scavengers react mainly with the peroxyl radicals (LOO*)
- Carry-through characteristics (BHA & BHT - food additives/synthetic anti-oxidants)
- Regulations for use in foods (do not exceed 0.02%)
- Necessary because natural antioxidants cannot survive food thermal processing
What are sterols?
Sterols/steroid alcohols; synthesized endogenously from the precursor squalene.
High molecular weight alcohols which are present in the un-saponifiable fraction of lipids
Subgroup of steroids with a hydroxyl group in the 3-position of the A-ring.
Cholesterol
Sitosterol (corn)
Stigmasterol
What is the concern with cholesterol and health?
- An increase in cholesterol deposition within hepatic cells also results in an increase in the serum LDL-cholesterol, an atherogenic lipoprotein.
- Control of hypercholesterolemia is important for the prevention of atherosclerosis and coronary heart disease.
Discuss cholesterol oxidation.
- Initial products are 7α- and 7β-hydroperoxides.
- Decomposition of the hydroperoxides produce the isomeric 7α- and 7β-hydroxycholesterols, cholesterol α- and β-epoxides, and 7-ketocholesterol.
- Side-chain derivatization result in production of 20- and 25-hydroxycholesterols.
- Cholesterol 3,5-diene, cholesta-3,5-dien-7-one and other ketones arise from elimination reactions.
Discuss the mechanism of cholesterol oxidation.
- Cholesterol is initiated by a free radical or a trace metal ion to produce a allyl radical.
- Allyl radical reacts with oxygen to form a peroxyradical.
- The peroxy radical then further converts to a hydroperoxide.
- The hydroperoxide is transformed into either the more stable products (alcohol radicals) 7-hydroxycholesterol or 7-ketocholesterol OR it can become an even more cytotoxic isomer called 5,6-epoxide radical (of which we are particularly concerned about for cholesterol oxidation)
Discuss cholesterol oxides in egg products.
Fresh egg yolk has no cholesterol oxides.
Under storage conditions cholesterol oxides are generated.
Discuss the changes in cholesterol content in tallow heated to 155 and 190 degrees C.
Cholesterol disappears over time at these temperatures.
The higher the temperature the faster the disappearance.
The disappearance of cholesterol is correlated with the appearance of the oxidized products (not shown in graph).
What is peroxide value?
- Peroxides are the main initial products of autooxidation.
- Peroxides can be measured by their ability to liberate iodine from potassium iodine (iodimetry) or to oxidize ferrous to ferric ions (thiocynanate method).
- The amount of iodine liberated from KI by oxidative action of peroxides present in the oil is determined by titration in a biphasic system with Na2S2O3.
Describe the thiobarbituric test.
- Most frequently employed method; measures secondary products.
- Substrate is malondialdehyde (MDA), a major breakdown product of lipid peroxides
- MDA derives from beta-cleavage of endocyclization of fatty acids hydroperoxide and reacts with thiobarbituric acid (TBA) to give a pink and fluorescent chromogen that is measured by colorimetry or fluorimetry.
- This method lacks specificity for MDA since all compounds with reducing activity will react with TBA.
- The TBA test is often useful for comparing samples of a single material at different stages of oxidation.
Discuss deep fat frying.
Frying in hot oil under very high temperatures; thermal lipid oxidation is assured.
During frying, the sensory and nutritional quality of the frying oil change.
The increased temperature increases the solubility of oxygen; thus, making greater access between the oxygen and the lipid component.
In addition there are dynamic changes with water with release from the food sample and the dehydration associated with this; hydrolysis increases susceptibility to lipid oxidation
Processing vegetable oils are designed to remove undesirable substances.
What processes are included?
Settling and degumming
Neutralization
Bleaching
Deodorization
Describe the steps in fat and oil processing.
Crude oil (flax, sunflower, soybean etc.); oil systems with at least 8-10% oil content
→
degumming (involves removing components that would otherwise interfere with further processing such as lecithin or phytosterols)
→
neutralization (removal of free fatty acids associated with soap stock, a product of the previous reactions)
→
washing & drying
→
winterisation (specific lower temperatures to induce crystallization of the lipids in order to remove high molecular weight hydrocarbons like waxes; thus, providing a better textural oil)
→
bleaching (remove pigments that can bring about photooxidation)
→
deodorisation (bleaching step must be deodorised using steam; produces a fatty acid distillates; natural tocopherols are lost and must be added back, that is, fortified in order to provide some stability after the process)
→
refined oil
What is hydrogenation?
Involves the addition of hydrogen to double bonds in the fatty acid chains.
It is commercially used for the production of margarine, salad oils, shortening and cooking fats.
What are the requirements for hydrogenation processing?
- Oil, hydrogen gas and nickel catalyst are mixed together in a reactor.
- The starting oil must be refined, bleached, low in soap, and dry.
- The hydrogen gas must be dry and free of sulfur, CO2, or ammonia.
What is anisidine value (AV)?
Aka P-Anisidine value (PAV)
Measures aldehyde & ketone levels in oil or fat (especially sensitive to unsaturated)
Aldehyde/ketone + P-anisidine → schiff bases (yellow; absorb @350nm)
What are the effective buffering ranges for citric acid and sodium citrate (salt)?
2.1 - 4.7
Describe the use of acidulants for flavour enhancement.
Evoke tartness
Each acid has varying degrees of acidity.
Define: pH
Logarithm of the concentration of free protons, expressed with a positive sign.
Where are the common food sources of the following acids:
Citric
Malic
Oxalic
Acetic
Lactic
Carbonic
Citric → lemon, lime
Malic → apple
Oxalic → rhubarb
Acetic → vinegar
Lactic → sour milk
Carbonic → soft drink
Where are the common food sources of the following acids:
Citric
Malic
Oxalic
Acetic
Lactic
Carbonic
Citric → lemon, lime
Malic → apple
Oxalic → rhubarb
Acetic → vinegar
Lactic → sour milk
Carbonic → soft drink
Why is citric acid an especially useful acidulant?
Dual effect
Citric acid: pK1 = 3.09; pK2 = 4.74; pK3 = 5.41
- suppresses enzymatic browning
- Synergistic activity with antioxidants.
Citric acid: pK1 = 3.09; pK2 = 4.74; pK3 = 5.41
At pH = 2.2, what is the ionic form of citrate?
Citric acid predominates in the most protonated form (citric acid)
Citric acid: pK1 = 3.09; pK2 = 4.74; pK3 = 5.41
At pH = 7, what ionic form of citrate predominates?
The citric acid will lose all its protons and occur predominantly in the citrate-3 form.
How may a formation of metal complex be expressed?
M: metal ion
L: ligand
ML: metal-ligand complex
k = stability of complex
What is the Haber-Weiss cycle?
What are the resulting products?
Iron participates in the Haber-Weiss cycle in which ferrous ion reduces oxygen to superoxide anion (O-2).
The resulting products are H2O2 and O2 (these feed into the Fenton reaction)
This is a bidirectional reaction.
What is the Fenton reaction?
The combination of ferrous ion with hydrogen peroxide (H2O2).
The Fenton reaction produces highly reactive hydroxyl radical which oxidizes most food constituents.
What is EDTA?
Ethylenediamine tetraacetic acid
Very strong capacity to sequester iron.
It has 6 possible coordination interactions (3 on each side of the molecule)
This may be sufficient to sequester copper, but it’s not 100% effective for iron.
How do free radical terminators function?
Mitigate the free-radical chain reaction (e.g., vitamin E)
- Retard the free-radical chain reaction by donating hydrogen from their phenolic hydroxyl group.
- These antioxidants are:
- More easily oxidized than other substances
- Once oxidized they are relatively stable
Give examples of commonly used chelators with antioxidant function. [3]
Citric acid
EDTA
Phosphates
Citrate is more effective at sequestering copper than pyrophosphate.
True or False?
False.
How do chelating agents assist in antioxidant function?
The unshared pair of electrons in their molecular structure promotes the chelating/sequestering effect with the ions.
Phosphates are less effective than citrate at chelating copper.
True or False?
False
EDTA is more effective than both citrate and pyrophosphate at sequestering iron or copper.
True or False?
True
What is an oil in water emulsion?
Give examples.
When the outer phase is water (and inner is oil), we refer to it as an oil in water (o/w) type emulsion.
e.g., mayonnaise, ice cream (milk)
What is a water in oil emulsion?
Give examples.
When the outer phase is oil (and inner is water), we refer to it as a water in oil (w/o) type emulsion.
e.g., salad dressing, margarine
Emulsifiers act to prevent coalescence.
True or false?
True
What is Ostwalk Ripening?
Results in the breaking of an emulsion.
What factors influence the stability of O/W emulsions? [7]
- Chemical structure of lipids
- Oxygen concentration
- Antioxidants
- Interfacial characteristics
- Droplet characteristics
- Aqueous phase components
- Ingredient quality
What is the hydrophilic-lipophilic balance (HLB) system? [3]
- Indicates the kind of emulsion for which the emulsifier is best suited
- Emulsifiers with low HLB values tend to be oil soluble and are used to prepare w/o emulsions.
- High HLB values shows a water-soluble emulsifier and then will be effective in preparing o/w emulsions
Describe the application of emulsifiers in cake batters.
- Emulsifiers function as aerating agents, speeding up the whipping rate of batter, providing a finer distribution of air cells and improving the tenderness of the cake by retarding starch gelatinization.
Polar antioxidants are more effective where?
In less polar media (e.g., bulk oils)
Non-polar antioxidants are more effective where?
In polar media (e.g., O/W emulsions)
