FOOD2150 Set 5 Flashcards
(83 cards)
1
Q
What are fats?
A
- typically exist as mono-, di-, triesters of glycerol with fatty acid
- mono,di,tri
- recall: SN2 does not cleave, remains with triglyceride
2
Q
How do lipids exist?
A
- liquids (oils) or solids (fats)
- non-polar organic compounds (soluble in organic solvents)
- includes fats and oils, sterols, waxes, fat-soluble vitamins (ADEK)
3
Q
What do fatty acids look like?
A
- straight-chain aliphatic carboxylic acids
- Fatty acids vary based on length, saturation, and type of saturation (all even numbered)
Three main types of fatty acids - saturated fats (solids)
- cis unsaturated fats
- trans unsaturated fats
4
Q
What are short- medium- and long-chain lipids?
A
short: 4-8
medium: 10-12
long: 16-20
5
Q
What is the lipid nomenclature?
A
- Start numbering from the carboxylic acid
- Number in front indicates double bond position
- Replace the alkane with alkene
- For the alkanoic acid it becomes alkenoic acid
- Two double bonds alkdienoic acid
- Three double bonds alktrienoic aci
6
Q
Compare Cis and Trans isomers
A
CIS:
- same side, naturally occuring
TRANS:
- different side, industrially occuruing
6
Q
What is the only food with naturally occuring trans fats?
A
dairy: biohydrogenation in animals but
- not the same as the ones we regularly consume in our diet
7
Q
What is partial hydrogenation?
A
- double bonds changed to hydrogens to make more saturated
- trans fat has similar MP to saturated fat
8
Q
What are other types of distribution?
A
even/widest:
Fatty acids are as broadly distributed as possible
random:
- fatty acids are distributed randomly on within the triglycerides
- no preference on where in molecule FA located
9
Q
What is fatty acid (restricted random) distribution?
A
- not completely random or ordered
- restricted random: ex cocoa butter (2/3 saturated)
- preference is given to the position on the triglyceride
*ex: one fatty acid may be more apt to reside at position 2 while it will be in lower quantities at position 1 and 3
10
Q
What does melting point depend on for triglycerides?
A
- degree of saturation and carbon length
11
Q
What are semi-crystalline solids?
A
- not true crystals (arranging in orderly and repeating manner that extend in 3 dimensions)
- polycrystalline (composed of pieces of crystals).
- semi-crystalline (not 100 % solid — contains
amorphous oil phases)
12
Q
What are the 2 stages of crystallization?
A
- nucleation (formation of first crystals)
- crystal growth (enlargement of nuclei)
13
Q
Describe different liquid forms of lipid crystallization
A
alpha: lowest density and stability
beta prime: intermediate density and stability
beta triclinic: stable
14
Q
What is supersaturation?
A
- Lipids crystallize at temperatures lower than they melt
- The temperature difference between the melting
temperature and crystallization temperature is the
supersaturation temperature - This undercooling is the thermodynamic driving force
- defect: gravy fats
15
Q
What are the 2 types of nucleation?
A
Homogeneous nucleation
- Large undercooling
- Nuclei appear at once
Heterogeneous nucleation (don’t like in food industry)
- Low undercooling
- Nuclei form in the presence of other nuclei (larger, different polymorphic in system)
16
Q
What is crystal size and distribution?
A
- final crystal size is a balance between the growth rate and the nucleation rate
- small crystals are good: can’t detect, form larger network: liquid oil gets trapped, no phase separation (more nuclei= larger # small crystals)
- Very important for final properties of fat (small have higher SA/V ratio)
17
Q
How does packing affect alignment?
A
- The α-form crystallizes initially under high supercooling
- The more perfect the packing, the more
18
Q
What is the tempering process?
A
- Melt all TAGs no crystals remain to “template” nucleation
- Cool the chocolate to get Form β-V nuclei
- Heat the chocolate to melt β’ polymorphs melt but not β-V
- Let chocolate “mature” so Form β-V nuclei grow
18
Q
Describe polymorphism
A
- identical TAGs can exhibit different crystal packing
- affects many of the properties of fat materials
- transformations are monotropic and occurs toward more stable species
- Melt-mediated polymorphic transformations
- Solid-state polymorphic transformations
19
Q
What is bloom?
A
- Is a physical imperfection in chocolate
- Poor color & not glossy
- Occurs due to uncontrolled recrystallization
of the β-form V to β-form VI - accelerate by adding inclusions, and bad temperature handling
19
Q
What is hydrolytic rancidity?
A
breakdown of triglycerides into glycerol and individual fatty acids
- MP, BP, smoke point drop
- free fatty acids breaking off triglyceride
- very carcinogenic when cooked with !
20
Q
What is hydrolytic rancidity accelerated by?
A
water, lipases, agitation
20
Q
Where is hydrolytic rancidity a problem?
A
- Frying oils
- Milk fats (Goat and cow especially)
- Lauric oils (Coconut and palm kerne
21
How does lipase affect milk?
- milk naturally rich in lipase (mechanism by which it cleaves)
* Milk TAGs are highly asymmetric
* SCFA always in the sn-3 position
* Milk lipase is specific only for the sn-3
* Lipases work at the water-oil interface
* Pasteurize, then homogenize
* Homogenization “exposes” w/o interface
* Pasteurization deactivates lipases first (always pasteurize before homogenize)
22
What does hydrolytic rancidity require?
- water
- heat (or lipase)
23
What is lipid oxidation?
- oxidative rancidity
- addition of oxygen to unsaturated lipids
- linoleic acid 10/40x more likely to oxidize than oleic
- oxidation rate doubles with addition of double bond
- most difficult reaction to control in foods and is main chemical reaction limiting shelf life of foods
24
What is the rate of oxidation?
* thermodynamically non-spontaneous (requires initiator to make first L radical)
- rate of oxidation is dependent on how quickly an electron is extracted
* e easily extracted with DB, and if they're methylene interpreted
25
Describe the initiation of lipid oxidation
* The removal of a hydrogen atom from a fatty acid to form a fatty acid radical or “alkyl radical”
* Resonance stabilization = stabilization of structure by movement of electrons.
* Resonant stabilized structures form more easily.
- ease of initiation characterized by dissociation energy: high diss. energy: more stable, low diss. energy: less stable
26
Describe the propagation and chain branching of lipid oxidation
* Transfer of free radicals from one lipid to another propagates the reaction
* Peroxyl radicals (LOO*) can abstract (pull off) a hydrogen from a neighboring molecule
* Unstable lipid hydroperoxides (LOOH) are formed in the process.
27
Describe the hydroperoxide breakdown of lipid oxidation
* The end-product of primary oxidation, hydroperoxides, ROOH, are very unstable
* ROOH can decompose into: alkoxy radical (RO·) and hydroxyl radical (·OH)
* The alkoxy radical (RO·) is even more reactive than the alkyl or peroxy radicals
27
Describe the termination-beta-scission of lipid oxidation
- Alkoxy radical “steals” an electron from a neighboring bond
* Breaks apart the alkyl chain:
* Aldehydes and more alkyl radicals form
* Repeated β-scission can generate a mind-boggling array of products
* The more double bonds, the smaller the secondary oxidation products
27
Describe the termination of lipid oxidation
R· + R · → R2
R· + ROO· → ROOR
ROO· + ROO· → ROOR +O2
- Termination concurs with oxidation slowing and stable
products accumulating
* At this stage, rancidity detectable
* Lipid free radicals form non-radical products by two major mechanisms: radical recombination, scission reactions when proton sources (water) are present to stabilize products
27
Describe the termination of lipid oxidation
* Free radicals cause the break down to small compounds such as ketones and aldehydes
* These react with other components in foods producing
amines
* Cause off flavours and odours
* The aldehydes and ketones small compounds can
polymerize increasing the viscosity of oil
28
What is a primary antioxidant?
- chain-breaking
* Reacts with alkyl, peroxy and alkoxy radicals
* Prevents propagation by breaking the chain
reaction
* Usually contains a phenol group
- major structural requirements: must donate a H and stabilize free radical (conjugation, stearic hindrance, new covalent bond)
29
What is a secondary antioxidant?
- preventive
* Do not react with radicals
* Deactivate pro-oxidants
* Quench UV light
29
How do primary antioxidants stabilize by resonance?
- double bond conjugation
* Linear or cyclic alternating single and double bonds
* Allows dislocation of the unpaired electron
30
How do primary antioxidants stabilize stearically?
- bulky electron clouds shield free radical: BHT
* Once the antioxidant donates a hydrogen and electron to the radical, the antioxidant becomes a radical
* Antioxidant radicals must be stable
* “Resonance stabilized” delocalization of unpaired electron around a phenol ring
* Antioxidant radicals undergo termination
reactions with other radicals readily (free
radical scavenging)
31
What are the goals of hydrogenation?
* Makes oil more stable by removing unsaturated double bonds (frying oils)
* Converts cis-unsaturated fatty acids to either saturated or trans-unsaturated fatty acids
* Changes the physical properties of an oil from a liquid to a solid (margarine)
31
How did hydrogenation first start?
- adapted from petrochemical industry
- first food oils: on whale and fish oils
* Marine oils are highly unstable due to highly unsaturated fatty acids
* Marine oils contain few natural antioxidants
32
How can we hydrogenate an oil?
* H2 gas (dangerous)
* Pressurized reaction (414 kPa typical, max < 1000 kPa)
* High temperatures: 250oC to 300oC
* Catalyst (usually nickel).
* Reaction Time: 40 to 60 minutes
32
Describe steps 1-2 of oil hydrogenation with nickel
- The double bond interacts with the metal catalyst by virtue of its π electrons.
- Hydrogen is adsorbed (or may already have been prior to #1) to the surface of the metal catalyst.
33
Describe steps 3-4 of oil hydrogenation with nickel
- An adsorbed hydrogen atom is transferred to a carbon
participating in the double bond while the other carbon is σ-bonded to the catalyst
- Transfer of a second hydrogen atom to the σ-bonded liberates the fatty acid from the catalyst.
- No H present: lipids come away from catalysis, remains in saturated state
34
How does the fatty acid % affect the time for hydrogenation?
more double bonds: less time for fatty acid %
lineolenic < linoleic < oleic < stearic
35
What is partial hydrogenation?
* Isomerization always occurs (undesirable)
* Geometrical: cis- double bonds become trans- double bonds and vice versa
* Positional: The location of the double bond shifts over by one carbon up or down the chain
- not used in canada, little importance in food industry
- we dont fully hydrogenate fish oil, or all of its use would be gone
36
What is full hydrogenation?
* Run to completion (solid fat)
* No unsaturation remains
* All fat is saturated at this point
- still important in food industry
37
How do we avoid partial hydrogenation?
- we do not partially hydrogenate, we get more transalatic acid (18:1 in trans form)
- so we pump a lot more hydrogen in, so we know it is saturated with hydrogen
38
What is the tristearin-vast majority?
- very difficult to work with
* If you mix it with oil, it is very grainy
* Good for frying oils
39
What is interesterification vs intraesterification?
inter: substitutes fatty acids on different glycerol molecules; moves position and glycerol
intra: intra: randomizes it on one fatty acid, not glycerol
39
What is atherosclerosis (arterial hardening)?
- plaque deposited on the inside walls of blood vessels, specifically arteries (presence of high shear
– promotes injury; and free radicals
– promotes oxidation of LDL and generation of foam cells)
40
What is plaque (atheroma)?
- made up of foam cells: lipids (especially oxidized LDLs) + white blood cells (macrophages) + calcium (hardening)
* Plaque is very insoluble (need special enzymes to dissolve them)
* Plaques are started by LDLs binding to blood vessel walls, perhaps for repair
41
Describe interesterification and what it is used for
- a process that rearranges the fatty acids in a fat or oil, typically a mixture of triglycerides
- remove partially hydrogenated fat
42
Describe intraesterification and what it is used for
- the process of shuffling fatty acids within a triacylglycerol (TAG) molecule
- create soft fats
43
What are the aliphatic side-chains of AA?
- to not ionize, only have H and C
G: H-
A: CH3-
V: (CH3)2CH-
L: (CH3)2CHCH2
I: CH3Ch2Ch(CH3)
P: cyclic; -Ch2CH2CH2-
44
What are the polar neutral side chains AA?
- side chains do not ionize under biological conditions
S: HOCH2-
T: CH3CHOH-
N: NH2COCH2-
Q: NH2COCH2CH2-
45
What are the sulfur-containing side chains AA?
C: HSCH2-
M: CH2SCH2CH2-
46
What are the aromatic side-chain AA?
- side chains do not ionize
F: C9H11NO2
Y: C9H11NO3
W: C11H12N2O2
47
What are the cationic side-chains AA?
H: C6H9N3O2
K: C6H12N2O2
R: C6H14N4O2
48
What are the anionic side-chains AA?
D: -O2CCH2-
E: -O2CCH2CH2-
48
What is a protein-zwitterion
* Molecule having separate positively and negatively charged groups
* Molecule has NO NET charge
49
What is a protein-peptide bond?
- upstream AA with downstrea AA: dehydration synthesis forms amide with a peptide bond
- R-groups alternate in trans conformation
50
Describe the protein-peptide bond
- covalent bound via peptide bond (loss of water)
- pi-pi bond delocalizes between C=O
- C-N occur as C=N and both behave as DB
- bond is rigid and planar, rotation from alpha C
51
Describe key points of the primary structure of a protein
= controlled at genetic level
- not modified by physical changes; through hydrolysis or post-translational chemical modifications
1. length of AA dictates molecular weight
2. similarity in composition doesn't = sequence similarity
51
Describe key points of the secondary structure of a protein
* The local conformation of the polypeptide backbone (molten globule structure)
* Sections of the polypeptide strand “self-organize” into β-sheets and α-helices
* The rigid planar amide bond is crucial for secondary structure as it limits the
conformations that amino acids can assume
52
Describe the a helix of a protein secondary structure
- 3.6 AA per helical turn
- 13 atoms H-bonded in ring
- N-H H bonds with C=O 4 residues earlier
- M, A, L, E, K, R, Q, H
- defined as i+4-> i
52
Describe the alpha-helix
* most common conformation
* stabilized by H-bonding of all the carboxylic acid and amino groups on the 4AA apart on the polypeptide backbone
* Each amino acid has a rise of 1.5A and a 100o rotation which leads to 3.6 amino acids per twist of the -helix
* With a 100o rotation per amino acid two adjacent amino acids are on opposite sides of the helix
53
Describe the b sheet of a protein secondary structure
- basic unit is continuous sequence of amino acids (β-strand)
* 5 - 15 AA
* β-strands interact via hydrogen bonding to form a β-pleated sheet
* Unlike the α-helix, the hydrogen bonds are not intrasegment but are intersegmental
- Y, F, W, Y, I, V, C
53
Compare anti-parallel and parallel B-sheets
anti: opposite biochemical direction
- H-bonds formed without any angle
parallel: same biochemical direction
- H bonds formed at angle
54
Describe B-turns of proline
- Molecular configuration does not allow formation of secondary structure
* No hydrogen atom at the α-NH2 for H-bonding
* Proline disrupts α-helices and β-sheets
* Proline is found as the first residue of an
α-helix & at the edge strands of β-sheets
* Proline is common in loop sequences
54
Describe B-turns of glycine
* No side chain carbons to prevent water from hydrogen bonding
* No bulky sidechain to drive β-sheets
* Glycine breaks α-helices
* Glycine is common in loop sequences
55
What is the tertiary structure of protein stabilized by?
* Disulfide Bridges (Covalent)
* Salt Bridges (Electrostatic)
* Hydrogen Bonds (Permanent Dipoles)
* Van der Waals Forces (Transient Dipoles)
56
What is protein folding driven by in the tertiary protein structure?
- hydrophobic effect
* For water to interact with hydrophobic residues water must form clathrates
* Clathrates are cage-like structures of water arranged to cancel waters dipole moment
57
What is the protein function determined from?
- its 3 structure
58
What are most structural vs functional proteins?
structural:
- fibrous (insoluble in water)
- collagen, hair and wool, myosin, fibroin
functional:
- globular (soluble in water)
- most enzymes
- ovalbumin, whey proteins
59
Describe myoglobin as a quaternary structure
- water soluble animal pigment
- hemoglobin is 4 myoglobin with Fe2+, making a quaternary structure
60
What causes structure loss in proteins?
DENATURING AGENTS
- heat
- pH
- ionic strength
- solvent polarity
- shear
61
What denatures?
- secondary, tertiary, quaternary protein structures
- native structure for most proteins only marginally stable; function of protein's environment
62
How can denaturation be desirable or undesirable in food?
Desirable: Fried egg, trypsin inhibitors, foaming, emulsification
Undesirable: Pale, Soft, Exudative meat (pork)
63
What are some examples of protein functionality?
* Texture of meat
* Thickness of yogurt
* Stability salad dressings
* Gelation of Jell-O & fried egg
* Texture of bread products
64
What happens to a protein when it undergoes denaturation?
- exposes hydrophobic groups to water decreasing solubility
* Proteins interact via hydrophobic interactions
* Heat can cause formation of di-sulfide bridges
65
What are the physical properties of denatured proteins?
* Decreased solubility (aggregation and gelation)
* Decreased biological/enzymatic activity
* Increased digestibility (exposure to enzymes)
* Increased viscosity (higher hydrodynamic
volume)
* Increased water-binding
66
What are the two proteins essential to wheat's texture in baked wheat products?
Glutenin (high molecular mass): responsible for the strength and elasticity of dough
Gliadin (monomeric (low molecular mass)): responsible for the extensibility of dough
66
What is a zymogen?
- inactive precursor of an enzyme requiring a biochemical change for it to become an active enzyme
* Essential to prevent self-digestion
