proteins Flashcards
what aa are polar?
serine, threonine, tyrosine, asparagine and glutamine(weakly polar)
what aa are non-polar?
gly, ala, cys, val, leu, isoleucine, pro, methionine, phenylalanine, tryptophan
what aa are acidic?
aspartic and glutamic acid
what aa are basic?
lysine, arginine, histidine
What are the two optical isomers of amino acids?
L- and D-isomers exist due to the asymmetric α-carbon (except glycine).
What are the 9 essential amino acids?
Histidine
Isoleucine
Leucine
Threonine
Lysine
Methionine
Proline
Tryptophan
Valine
How do amino acids contribute to food structure?
Nonpolar amino acids (valine, leucine, isoleucine) → Hydrophobic surfaces.
Polar amino acids (serine, threonine, cysteine) → Hydrogen bonding & disulfide bridges.
How do amino acids affect taste? what aa exhibit sweet, bitter, salty, sour and umami flavour?
Sweet: Glycine, Alanine, Glutamine, Threonine.
Bitter: Arginine, Histidine, Leucine, Methionine, Tyrosine.
Salty: Lysine, Glutamic acid, Aspartic acid.
Sour: Glutamic acid, Aspartic acid.
Umami (savory): Glutamate (MSG).
What is the Maillard reaction? what is involved? what is it called if the food does not have protein?
Reaction between amino acids & reducing sugars → Brown pigments & flavors
caramelization in foods that dont have protein
What is Strecker degradation? what is involved? what impact does this have on food?
it is a continuation of the maillard rxn in which the oxidation of amino acids with a dicarbonyl compound forms aldehydes, pyrazines, sugar, ammonia and CO2
produces distinct flavours/aromas based on the amino acid
what are amadori compounds? how do they relate to the maillard rxn?
amadori compounds are the first products produced from the maillard rxn
They play a large role in flavour developement in the maillard rxn as they act as precursors to dicarbonyl compounds
explain the relationship between heat and amino acid aroma?
different flavour and aroma profiles result from varying temperatures
what is tyrosine oxidation? what is involved?
It is a type of enzymatic browing
Tyrosine oxidation → Melanin formation.
Requires oxygen + polyphenol oxidase (tyrosinase) and Cu as a cofactor
how does bruising impact browning rxns?
bruising will physically damage cell structure, releasing enzymes to undergo browning rxn
what are conditions inhibit tyrosine oxidation?
Low pH
redcuing agents (ascorbic acid)
Reducing oxygen exposure
What are biogenic amines, and how are they formed? provide examples for biogenic amines and where they are found.
- Formed by decarboxylation (removal of COOH) of amino acids by bacteria and can be toxic/alter food safety and flavour
what is tyramine? where is it found? what implication does it have?
a biological amine fromed from bacteria action in cheese
- found in bananas/pinapples
-can causes hypertension
what is histamine? where is it found? what implication does it have?
biogenic amine formed from gram negative bacteria
-found in fish
-can be made by colonic bacteria
-causes headache, nausea and hypertension
-signs of inflammation due to release of histidine decarboxylase
how does sodium nitrite impact amino storage?
the use of sodium nitrite in cured meat is a cause of protien oxidation
what is the risk of using nitrites? what is the benefit?
benefit: used to protect against C. botulinum (preservation) and as a colour aid
risk: can be reduced to nitrous anhydride, which reacts with amines to form carginogenic nitrosamines
What Are Secondary Amines? explain the structure and their significance
Secondary amines (R₂NH) have two alkyl or aryl groups attached to the nitrogen instead of hydrogen.
They can react with various compounds, leading to the formation of N-nitrosamines, Maillard reaction products, and other biologically significant molecules.
how are nitrosamines formed? what conditions are they produced in?
Secondary amine + Nitrite (NO₂⁻) → N-nitrosamine (R₂N-N=O)
-low pH
-high T
which amino acids contain ionizable groups?
proline, tyrptophan, tyrosine, cysteine, arginine and histine
what are the 3 classifications of proteins based on solubility and structure?
1) simple
2) conjugated
3) derived proteins
what classifies an protein as simple? provide the 4 types
simple proteins yield amino acids after being hydrolyzed
Albumins: eggs, lactalbumin
Globulins: myosin, actin, serum globulin
Glutelins: glutenin and oryzenin
Prolamins: protein in cereals
what classifies a protien ad conjugated? provide examples
proteins that contain aa combined with a non-protein substituent
Phosphoproteins: phosphate + OH
- casein in milk
Lipoproteins: Lipid + protein
Nucleoproteins: nucleic acid + protein
Glycoproteins: carb + protein
Chromoproteins: prostetic group + protein (hemoglobin, flavoprotein, etc)
what is a by-product/derivative protein? what are the 2 kinds?
protein obtained by chemical/ enzymatic methods
1) primary: partial hydrolysis resulting in coagulated proteins (casein/cheese curd)
2) secondary: peptides; combination of 2 or more amino acids
What are the four levels of protein structure? what classifies each of them?
1) Primary: Amino acid sequence.
- linked by peptide bonds
2) Secondary: repeated strutcure with an amide group attached to C=O through H-bonds
-α-helices & β-sheets, beta turns and random coils stabilized by hydrogen bonds.
3) Tertiary: 3D folding of protein chains with a-helixes/b-sheets; stabilized by disulfide bonds, hydrophobic interactions, ionic bonds.
4) Quaternary: Multiple polypeptide chains forming a functional unit (e.g., hemoglobin).
what bonds make up the primary structure of proteins?
covalent peptide bonds
what bond makes up secondary protein structure?
H-bonds: stabilizes strutcure
is a beta sheet or alpha helix more stable?
beta sheet
-fibrous in nature and insoluble in (aq) solvents
what bonds make up tertiary structures?
H-bonds, hydrophobic, electrostatic, disulphide bridges, van der waals and hydrophobic bonds
what bonds make up quaternary structures?
same as tertiary:
H-bonds, hydrophobic, electrostatic, disulphide bridges, van der waals and hydrophobic bonds
what type of bond has the highest energy? what follows in terms of energy?
covalent>ionic> H-bond> Hydrophobic> Electrostatic repulsion» Van der waals
why do H-bonds and hydrophobic bonds play such a big role in protein structure if they have a low energy?
there are a large # of the bonds which increases the overall impact of these interactions
explain the importance of SH bonds?
disulfide bonds are one of the strongest covalent bonds and play a large role in stabilization of the folded structure
-occur between two cys groups
what is the importance of hydrophobic interactions?
they are major forces that drive protein folding
-interaction between non polar side chains is the major driver of unique tertiry folding
what is protein denaturation?
changes in secondary, tertiary and quaternary structure without cleavage of the peptide bonds (backbone)
what causes denaturation?
acids, heat, strong salt, alkaline solutions and radiation
provide an example of reversible vs irreversible denaturation process?
reversible: jello formation and melting
irreversible: cooking an egg
what occurs to proteins after they are denatured?
they lose their activity, resulting in:
1) decreased solubility
2) increased viscosity
3) altered water holding capacity
4) increased digestibillity
5) increased exposure to enzymes
6) improved foaming / emulsification
explain the changes in each type of protein structure following denaturation:
primary: no change from denaturation
secondary: loss of pattern and adoption of random coil configuration
tertiary: disruption of non-covalent interactions only (peptide/disulfide remain in tact)
quaternary: dissociation of protein sub units
what are functional properties of proteins in foods?
solubility, viscosity, water binding, gelation, cohesion-adhesion, elasticity, emulsification, foaming, fat and flavour binding
What is Water Binding Capacity (WBC)?
The ability of a protein to bind and retain water, measured as grams of water bound per gram of protein at 90-95% relative humidity.
What factors affect Water Binding Capacity (WBC)?
- pH & Isoelectric Point (pI)
- Salt Concentration
- Temperature
How does pH affect Water Binding Capacity (WBC)?
At pI, proteins aggregate, leading to low hydration; above and below pI, WBC increases due to electrostatic repulsion.
What is Water Holding Capacity (WHC)?
A protein’s ability to retain water against gravitational force, including bound water, hydrodynamic water, and physically entrapped water.
What factors influence Water Holding Capacity (WHC)?
- Protein structure
- pH and salts
- Cooking & heat
What is emulsification in food science?
A system where two immiscible liquids (oil & water) are stabilized by emulsifiers like proteins.
What are the mechanisms of protein emulsification?
- Hydrophobic regions surround oil droplets
- Hydrophilic regions interact with water
What defines foaming in food science?
A dispersion of gas bubbles in a continuous liquid phase, stabilized by proteins.
What factors affect foam stability?
- Protein concentration
- pH
- Salts
- Sugars
How do proteins bind flavors?
Proteins can bind flavors via noncovalent interactions, including hydrophobic interactions and hydrogen bonding.
What is gelation?
The transformation of proteins into a gel (semi-solid structure).
What are the two types of gels formed during gelation?
- Coagulum gels (opaque)
- Translucent gels
What is salting in?
Occurs when low concentrations of salt (≤ 0.2M) increase protein solubility.
What is salting out?
Occurs when high salt concentrations (> 0.5M) decrease protein solubility.
What is bound water in food systems?
Water tightly associated with food components via hydrogen bonds, ionic interactions, and van der Waals forces.
What is immobilized water?
Water physically trapped within cellular structures or protein matrices.
What is free water?
Water not bound to food components, easily lost through evaporation or freezing.
What is vicinal water?
Water tightly bound to polar groups on proteins, carbohydrates, and minerals.
What is constitutional water?
Water deeply embedded within molecular structures, acting as structural water.
What is multilayer water?
Water that forms multiple layers beyond vicinal water but is still immobilized.
What is the difference between water-in-oil (W/O) and oil-in-water (O/W) emulsions?
W/O emulsions have oil as the continuous phase, while O/W emulsions have water as the continuous phase.
What is the role of structural water in proteins and enzymes?
Acts as structural water, stabilizing macromolecules
Example: Water integrated within protein helices or enzyme active sites.
What defines multilayer water?
Forms multiple layers beyond the vicinal layer but is still immobilized
More loosely bound than vicinal water but less mobile than free water.
What are Oil-in-Water (O/W) emulsions?
Oil droplets dispersed in water, with water as the continuous phase
Stabilized by hydrophilic emulsifiers (e.g., proteins, lecithin). Examples: Milk, mayonnaise, salad dressings.
What are Water-in-Oil (W/O) emulsions?
Water droplets dispersed in oil, with oil as the continuous phase
Stabilized by lipophilic emulsifiers (e.g., monoglycerides, phospholipids). Examples: Butter, margarine.
What is the definition of humectants?
Hydrophilic compounds that attract and retain moisture from the surrounding environment
Prevent food from drying out.
How do humectants work?
Bind water molecules via hydrogen bonding, lower water activity, prevent crystallization
Examples: Glycerol, sorbitol, honey & fructose.
What factors affect protein gelation?
Protein concentration, pH, salt (ionic strength), temperature & heating rate, protein type & structure, water content, cross-linking agents
Each factor influences gel strength, structure, and stability.
What is the effect of protein concentration on gelation?
Higher protein concentration provides more network-forming units, leading to stronger gels.
What happens at the isoelectric point (pI) of proteins?
Protein solubility is low, leading to coarse, weak gels.
How does salt affect protein gelation?
Low salt enhances gelation; high salt may lead to salting-out, reducing gel-forming ability.
What temperature range promotes protein gelation?
Moderate heating (50-80°C) unfolds proteins gradually, promoting gelation.
What is the role of cross-linking agents in protein gelation?
Enzymes create stronger, irreversible protein cross-links; sugar & polyols stabilize gels.
What is casein?
Major milk protein (~80% of total protein in bovine milk) that exists in micelles.
What are the types of casein?
α-Casein, β-Casein, κ-Casein
κ-Casein stabilizes micelles and is calcium-insensitive.
What is the process of enzymatic coagulation in cheese formation?
Chymosin cleaves κ-casein, leading to aggregation into a hydrophobic gel (cheese curds).
How does acid coagulation occur?
Lowering pH to ~4.6 precipitates casein
Used in yogurt and acid cheeses.
What are whey proteins?
Proteins that remain soluble after casein precipitation, comprising 20% of milk protein.
What are the major components of whey proteins?
β-Lactoglobulin, α-Lactalbumin, serum albumin, immunoglobulins.
What is the cheese production process?
Milk is heated and acidified or renneted to coagulate casein, then curds are separated from whey.
What happens during the aging of cheese?
Bacterial & enzymatic reactions develop texture and flavor.
What is the difference between hard and soft cheeses?
Hard cheeses are aged longer and have a firmer texture; soft cheeses have higher moisture content and shorter aging.
How is butter formed?
Churning cream disrupts fat globules, allowing fat to coalesce and separate from buttermilk.
