Tutorial Exam Flashcards
What is the interaction taking place between the solute and solvent?
Hydrogen bonding (e.g., sucrose and water)
What is hydrophilicity?
What is hydrophobicity?
Hydrophilicity: water-liking; tends to interact with water
Hydrophobicity: water-fearing; tends to avoid water
How does hydrogen bonding lead to the solubilization of compounds?
- Force between sucrose molecules: Van der Waals force
- Dissolve in water: hydrophilicity of sugar
- Hydrogen bonding leads to hydration shell of solute
Viscosity: resistance to flow; ‘thickness’
What are the differences between sucrose and corn syrup that affect viscosity?
Sucrose viscosity is lower comparably, less H2O is bound as there is WBC only (i.e., water is bound through hydrogen bonding).
Corn syrup viscosity is higher comparably, more H2O is bound since corn syrup both entraps water (WHC) and binds water through hydrogen bonding (WBC). As such, there is less free H2O in corn syrup solutions and higher viscosity.
What is water binding capacity?
Tendency of water to associate with hydrophilic substances, mainly through hydrogen bonding.
What is water holding capacity?
Ability of a matrix of molecules to entrap large amounts of water in a manner such that exudation is prevented.
Try to explain the graph by discussing how water binds with the sucrose versus the corn syrup.
Sucrose → linear pattern; viscosity moderately increases with more solute due to binding with water through hydrogen bonds; this will increase until maximum solubility
Corn syrup → logarithmic pattern; viscosity first exponentially increases with more solute, then plateaus in viscosity when maximum solubility is reached
What does solubility refer to?
The rate a solute dissolves in water, which is influenced by the amount of the solute being dissolved and the temperature in which it is being dissolved.
When a soft drink is heated, what happens with the solubility of:
Sucrose
NaCl
CO2
Sucrose → ↑↑ Solubility
- Intermolecular forces broken: energy required
- Hydrogen bond formed: energy released
- Endothermic reaction: energy required > energy released
- Heating provides energy, causes faster molecular motion and greater chance of interactions
NaCl → ↑ Solubility
- Faster molecular motion at higher temperature
- Endothermic reaction
- Ionic bonding in salt > VdW force in sucrose: greater energy required: lower solubility than sucrose.
CO2 → ↑ Solubility
- Weak bonding between carbon dioxide and water: easily dissociated
- Increased temperature increases entropy (= more disorder) = gas state
How does temperature affect water binding capacity of cellulose compared to soy protein?
What about the viscosity of the two solutions?
- Cellulose
- Linear chain polymer of D-glucose
- Crystalline region of cellulose: intermolecular hydrogen bonding
- Increased temperature = increased energy to break hydrogen bonds
- Chains separate → water enters and forms hydrogen bonds with OH-group of cellulose = ↑ water binding capacity
- less free water = increased viscosity
- Soy protein
- Increased temperature increases protein denaturation = loss of intra and inter-molecular bonding and increased hydrogen bonding with water = ↑ water binding capacity
- Aggregation of denatured protein = entraps water = ↑ water holding capacity
- increased WBC + WHC = ↑↑ viscosity
What type of phospholipid is abundant in soy beans and egg yolks?
Phosphatidylcholine/PC (a.k.a. lecithin)
R = choline
What is browning?
The process of food turning brown due to the chemical reactions that take place within
What is enzymatic browning?
No sugars involved!
Browning reaction between oxygen and phenolic substrate catalyzed by an enzyme.
What are the two types of non-enzymatic browning?
Maillard reaction
Caramelization
What is the Maillard reaction?
Reaction between alpha-amino acid and reducing sugar
What is caramelization?
Dehydration and isomerization of sugar leading to the formation of enediols and dicarbonyls (caramel flavours and pigments)
Explain the reaction taking place through the results in Table I, i.e., the effect of pH, heating and sodium bisulfite on the browning process.
- Glucose → not much browning at any pH
- Glucose + lysine → reaction goes to completion at pH 9
- Sodium bisulfite effect → bisulfite forms adduct with aldehyde group of sugar = inhibits the browning reaction
- Heating → browning increased with temperature
- pH → greater degree of browning results in lower pH due to acidity of secondary Schiff bases as well as the product melanoidins.
What are conditions to think about for the Maillard reaciton?
Initial pH (must be alkaline)
Moisture content
aw
Temperature
Explain the results.
Water soaked → increased water content = inhibition of reducing sugar and amino acids to participate in MR
Glucose soaked → glucose: reducing sugar = increased reactants available to participate in MR → very brown
Sucrose-soaked → less brown than glucose; hydrolysis of glycosidic linkages yields glucose (aldose) & fructose (ketose): both reducing sugars, however fructose must isomerize to glucose
What is the frying process (with oil) and how does it lead to browning? What about frying butter?
- Heating evaporates water and leads to water loss from food
- Water leaves and oil enters
- Browning occurs because fat can act as a reducing agent and participate in MR → produces brown pigments
- Caramelization also occurs due to starch
- Butter contains milk solids (protein and lactose as well as fat): amino acids from proteins, and fats can participate in MR
Explain the results.
-
Sodium sulfite (no change in colour)
- Reducing agent: reducing hallachrome back to DOPA
- Oxygen scavenger (no oxygen for oxidation reaction)
- Enzyme inhibitor
-
EDTA (Faster red: slower black)
- Copper is required in the active site of tyrosinase
- EDTA: copper chelator → makes copper unavailable to enzyme → slows reaction
-
Tyrosine (faster red; slower black)
- More substrate
- Greater dopachrome formation (faster red colour)
-
Ascorbic acid (no change)
- Reducing agent
- Acidulant → lower pH deactivates enzyme
- Possible copper chelator; reduces tyrosinase activity
-
Citric acid (no change)
- Acidulant → decrease pH, inhibits enzyme
- Possible copper chelator
-
DOPA (very fast red, similar time black)
- More intermediate substrate
Describe reagent effects on enzymatic browning.
- Added reagents influenced enzyme activity, substrate availability, pH
- Other ways to influence enzymatic browning:
- Temperature
- oxygen availability to participate in reaction
- enzymatic activity (temperature, pH, copper chelation)
- Tyrosine (substate)
How may enzymatic browning be prevented?
- Eliminate enzymatic activity (heat treatments, sulphur dioxide, sulphites, acidulants)
- Reduce or remove oxygen exposure (vacuum packaging, brine immersion, or syrup immersion)
- Sequester Cu2+ ions (metal sequestering agents, acidulants)
Discuss how the apple treatments suppressed the enzymatic browning in apples and why certain treatments were more effective than others.
-
HCl or NaOH
- Low or high pH → denature enzyme = decreased activity
-
Water treatments
- Dilutes reactants
- Less oxygen dissolved in water than direct exposure to air
Discuss how the apple treatments suppressed the enzymatic browning in apples and why certain treatments were more effective than others.
-
HCl or NaOH
- Low or high pH → denature enzyme = decreased activity
-
Water treatments
- Dilutes reactants
- Less oxygen dissolved in water than direct exposure to air
Is there another way for the food industry to suppress enzymatic browning of fruits besides soaking the fruits in acids or base?
- Blanching → denature enzyme
- Addition of sulphites as reducing agents (common with dried fruits)
- Vacuum pack to reduce oxygen
- Modified atmosphere packaging to reduce oxygen
What are the reactants for MR?
Reducing sugar + amino group
What are the reactants for enzymatic browning?
Phenol compound + oxygen
What are the reactants for caramelization?
Sugar + heat
What are the factors that affect MR? [5]
Reactants, moisture content, water activity, temperature, pH
What are the factors that affect enzymatic browning?
Reactants, oxygen, enzyme activity → moisture content, pH, heat, copper chelating agents, acidulants
What happens when iodine is added to starch solution (1:10 solution)?
What happens upon heating?
What happens upon subsequent cooling?
What happens with addition of alkaline?
What happens with further addition of acid?
A blue colour appears when iodine is added to starch.
Heating the solution causes the colour to disappear
Cooling the solution returns the blue colour.
Adding alkaline causes a pale blue (near disappearance), and adding acid back brings back the blue colour
What is the mechanism behind the iodine test?
Mixing iodine (I2) into a mixture of potassium iodide (KI: dissociates) → formation of triiodide complex (I3-) → complex slips into amylase → transfer of electronic charge between amylose coil & iodine complex → alters absorption spectrum for visible light = blue colour
Why does heating a mixture of starch solution and iodine solution cause brown colour?
What does allowing the mixture to cool return the colour to blue?
Heating causes unfolding of amylose helix → dissociation of iodine-starch coordinate complex → can’t retain as much poly triiodide → brown colour
Cooling allows re-folding of right hand helix = blue colour
Why does NaOH cause a mixture of iodine solution and starch solution to become colourless?
Why does acetic acid return the blue colour?
Base can reach with iodine molecule and disrupt the iodide complex = colourless
Acid neutralizes base = releases iodide complex = triiodide complex and starch interaction returns = blue colour
What does the iodine test tell you about starch?
Iodine test tells the presence of amylose, the linear helix structure formed by glucose through alpha 1,4-glycosidic linkage.
How is the hydrophilic sol (starch and water) changed in the presence of alcohol?
Inducing flocculation?
Before introducing alcohol to the starch and water system: starch granule dissolves in water
The presence of alcohol will dehydrate starch, causing the flocculation of starch.
What does the Benedict’s Test do?
Identifies reducing sugars (monosaccharides and some disaccharides) which have a free ketone or aldehyde group
Test for all monosaccharides (aldose and alpha-hydroxy-ketose) and reducing sugars
Red precipitate mixed with blue solution
How does Benedict’s Test work?
CuSO4 (copper sulfate) → Cu2+ (cupric ion) + SO4-
2Cu2+ + Heat + Reducing sugar → 2Cu+ (cuprous ion)(electron donor)
2Cu+ + O2- → Cu2O (red precipitate mixed with blue solution)
Explain the results of the Benedict’s Test.
Glucose → aldohexose = reducing
Fructose → Ketohexose = reducing
Xylose → aldopentose = reducing
Sucrose → disaccharide = not reducing
Lactose → disaccharide = reducing
All ketoses are reducing sugars.
True or False?
False.
Ketoses are not reducing unless they can undergo keto-enol tautomerization (ketone → aldehyde formation)
e.g., fructose → glucose isomerization
What is a reducing sugar?
A sugar that contains an aldehyde or ketone group.
Aldoses have an aldehyde group that readily oxidizes to carboxylate in Benedict’s test.
Why is sucrose a reducing agent according to Benedict’s Test after adding acid?
Acid + heat → acid hydrolysis
Breakage of glycosidic linkage
Sucrose → glucose & fructose (both reducing sugars)
What is inulin?
A fructose polymer that typically has a terminal glucose
Units linked by beta-1,2 glycosidic bond
Explain the results of the Benedict’s Test.
Acid + heat → acid hydrolysis
Breakage of glycosidic bonds releases fructose monomers of inulin
Fructose isomerizes to glucose ( = reducing sugars)
What is glycogen?
Analogue of starch with alpha-1,4 and alpha-1,6 linkages
Made up of glucose units
After acid hydrolysis → free glucose: reducing sugar
Explain the results of the Benedict’s Test.
Glycogen is a branched chain polysaccharide. Its glucose monomers are attached by α-1,4 glycosidic linkages, with branches connected by α-1,6 glycosidic linkages. Glycogen has a multitude of terminal glucose units; however, there are many non-reducing ends and only one non-reducing end per glycogen molecule. The end of the glycogen molecule with the free anomeric carbon is reducing, and this end can reduce the copper (II) ions (i.e., cupric) in Benedict’s solution to copper (I) ions (i.e., cuprous). This helps explain why the colour of the solution was pale blue, and the colour of the precipitate was green, since the glycogen molecules have weak reducing power prior to glycosidic linkage hydrolysis by acid. Because only some copper (II) ions in the Benedict’s solution will be reduced to copper (I) ions, only some red copper oxide precipitate forms, which results in a solution that appears pale blue with green precipitate.
What is gelatinization?
Starch gelatinization is a process of breaking down the intermolecular bonds of starch molecules in the presence of water and heat, allowing the hydrogen bonding sites to engage more water. Three main processes happen to the starch granule: granule swelling, crystal melting, and amylose leaching.
What is transition temperature?
Temperature at which birefringence first disappears
~70 degrees C in this example
What are the structures of amylose and amylopectin?
Amylose = alpha 1,4 linkage
Amylose = alpha 1,4 and alpha 1,6 linkages
Why does the suspension of starch become transparent?
- Solubilization by water at higher temperture
- Increased temperature = decreased hydrogen bonding between amylose and amylopectin
- Forms hydrogen bonding with water = transparent
Why does the suspension of starch become transparent?
- Solubilization by water at higher temperture
- Increased temperature = decreased hydrogen bonding between amylose and amylopectin
- Forms hydrogen bonding with water = transparent
Describe how amylose is released from starch granules during gelatinization.
- Water first absorbed in the amorphous space of starch
- Heat causes these regions to become diffuse → amylose chain dissolves → separates into amorphous form
- Penetration of water causes swelling of starch granule → soluble amylose molecules leach into water → granule structure disintegrates (loss of birefringence)
Describe the chemical properties which account for the difference in colour in these results.
- Amylose → linear alpha-1,4 linkages form right hand helix
- Inclusion of I2 inside amylose helix forms poly (I3-) chain = blue colour
- Increased temperature denatures helix structure = brown colour
- Amylopectin → branched alpha-1,4 and alpha-1,6 linkages
- Shorter chain than amylose
- Not long enough to be blue = brown reddish purple colour
Discuss the viscosity trend of gelatinization.
- Viscosity increases to a peak, then falls to a value slightly higher than original.
- Less free water = increased viscosity
- Granule rupture = more free water
- Hydrogen bonds between amylose and amylopectin chains means there is less free water than compared to the start
Describe the structure of casein micelles.
A: submicelle (alpha-s1, alpha-s2, beta)
B: negatively charged protruding chain of kappa casein
C: calcium phosphate bridges
D: kappa casein (soluble)
E: phosphate group
Electrostatic repulsion between casein micelles
Explain the reason for the different solubility characteristics of casein in the various solutions in Table 1.
- Casein is not soluble in water because alpha and beta casein are hydrophobic; the kappa-casein was partially neutralized by the acid in the precipitation step, so its charge is no longer enough to solubilize the casein micelle
- Sodium chloride causes a ‘salting out’ effect; that is, when salt concentration is increased more water molecules interact with salt ions which decreases the number of water molecules available to interact with the charged part of casein protein, further decreasing its solubility
- Below pI, protein is positively charged
- Above pI, protein is negatively charged
- Casein is slightly negatively charged in water.
- HCl neutralizes charge, casein insoluble; enough acid addition will cause a positive charge and casein would become soluble again
- Protein becomes soluble at high pH, casein will have a net negative charge due to ionization of its side chain, so it is soluble in sodium hydroxide solution
What is occurring in the reduced sulfur test?
Aim → to determine the presence of sulphur-containing amino acids in milk proteins.
Lead acetate reacts with sulfide/sulfur from cysteine → quantify cysteine proportion in a protein
Alkaline sodium hydroxide + heat → hydrolysis of protein and cysteine residues releasing sulfide and ion
Lead cation from lead acetate reacts with sulfide from cysteine and forms lead sulfide precipitate.
Methionine also contains sulphur; but it cannot be hydrolyzed into sulfide and so cannot react with lead acetate
Explain these results of the reduced sulfur test with milk proteins.
There is a greater proportion of cysteine residues in casein protein compared to whey protein = grayish dark, cloudy solution
What are the principles of the Biuret test?
- Peptide bond is site of reaction
- Cupric sulphate (Cu2+) complexes with the peptide (co-ordinate covalent bond) producing a violet/purple colour
- Qualitative measurement → presence of colour = presence of peptide bond
- Quantitative measurement (spectrophotometry) → intensity of colour is correlated to number of peptide bonds / amount of protein
Describe the coagulation process in cheese.
Phase 1 → rennin cleaves negatively charged tail of k-casein
Phase 2 → phosphate-calcium bridges result in coagulation of micelles.
In the presence of rennin, why does coagulation not occur with the addition of sodium citrate?
Citrate ions from sodium citrate react with calcium ions to form calcium citrate.
Interrupts phase 2 of coagulation process (i.e., no calcium available to form calcium-phosphate bridges)
Why does the addition of calcium chloride cause coagulation in the presence of rennen?
Facilitates coagulation of milk
Does coagulation of milk occur faster with warmer temperatures?
Only to a point.
Once the enzyme becomes denatured by heat, the rate of action decreases.
Why is there a difference in % transmittance?
The difference in % transmittance is a result of the different levels of coagulation of egg albumen.
What effect did sucrose have on coagulation? (based on table)?
At the same temperature, % transmittance higher in (albumen + sucrose) than albumen alone → sucrose reduces albumen coagulation
Formation of native protein structure (usually folded) is due to hydrophobic interactions between protein molecules, the strength of which is determined by solvent structure
Solute effect: sucrose changes solvent structure → increases surface tension of water
Unfolding of protein requires energy to overcome surface tension = unfavourable; therefore, folded proteins are more favourable
Therefore: Hydrophobic interaction between pairs of hydrophobic groups in protein are stronger in sucrose solution than pure water; OR sucrose stabilizes hydrophobic interactions in protein
What effect did sodium carbonate have on coagulation? (based on table)?
No coagulation at all experimental temperatures (100% transmittance) → sodium carbonate retards albumen coagulation
In alkaline conditions, the net charge of proteins is negative = increased electrostatic repulsion = decreased coagulation
Overall: increased pH = increased coagulation temperature
What is the thiobarbituric acid (TBA) test?
Widely used test for measuring the extent of lipid peroxidation in foods.
Malonaldehyde reacts with TBA under acidic and heated conditions to form chromagen which has a pink-red colour that gives absorbance at 532 nm.
What are the steps of lipid peroxidation?
Initiation → a free radical will react with a lipid molecule, taking one proton and one electron from it, turning the lipid molecule into a lipid radical
Propagation → lipid radical will be oxidized into peroxyradical which can act as a free radical to initiate a new lipid peroxidation reaction and stabilizes itself into lipid peroxide; the new lipid molecule that has just been initiated will then enter the propagation stage
Termination → triggered by presence of an antioxidant which can donate electron to radicals (vitamin E can stabilize 2 radicals per vitamin E molecule)
Why was there a difference in the lipid peroxidation between the bulk oil and the oil-in-water emulsion stored under the same temperature and time?
Bulk oil = less oxygen exposure (oxygen can only access the surface)
Oil-in-water = oxygen in the water is homogenously distributed; thus, a greater surface area of the oil is exposed to oxygen.
Will a lipid-soluble antioxidant be effective in the bulk oil system or will a water-soluble antioxidant be more effective?
POLAR PARADOX
Hydrophilic antioxidants like ascorbic acid are more effective in a non-polar system (e.g., bulk oil, water-in-oil)
Hydrophobic antioxidants like tocopherol are more effective in polar systems (e.g., water, oil-in-water)
The type of antioxidant that is able to stay concentrated at the contacting surface will be more effective against lipid peroxidation (where oil and oxygen are available for the reaction)
What type of antioxidant works best in a non-polar liquid system (e.g., bulk oil, water-in-oil)?
Hydrophilic antioxidants like ascorbic acid
To prevent lipid oxidation in a new mayonnaise formulated with fish oil, which of these two antioxidants is best and why?
- Oil-in-water emulsion
- Site of oxidation → interface of oil droplet
- Ascorbyl palmitate more effective (hydrophobic component) → concentrated on the surface/interface of droplet
- Ascorbic acid (Water soluble) → evenly distributed throughout water/continuous phase
What is bronsted-lowry theory?
Defines acids and bases according to their abilities to transfer and accept protons.
Acid (HA) → proton donor
Base (A-) → proton acceptor
What is a conjugate base?
A base formed by the removal of a proton
Weakest acids make the strongest conjugate bases
What is Ka?
Acid dissociation constant = the equilibrium constant of the dissociation reaction, a quantitative measure of the strength of an acid in a solution
More complete dissociation = stronger acid
What is acetic acid?
Principal acid in white vinegar
Mostly used as an acidulant and flavouring agent
Sour taste and pungent smell
What is malic acid?
Commonly used in non-alcoholic beverages and hard candy
Responsible for: sourness of green apple, and tartness in wine
What is tannic acid?
A special form of tannin (astringent polyphenolic compounds)
Used as: an aroma additive in juices and pops; a flavour enhancer in wine; a chelator
Tannic acid is not found in tea (tannins are)
What is citric acid?
Naturally occurs in citrus fruits
Highly soluble in water and widely used in the food industry
Added as a flavour enhancer
Gives food a burst of tartness
Chelates transition metal ions = antioxidant
Can prevent enzymatic reactions
What is EDTA?
Ethylenediaminetetraacetic acid
Mainly used as a soluble chelator: chelates Fe3+ → EDTA-Fe(III) complex (very soluble complex because complex has 7 coordination sites, with one occupied by water)
How do sequestering agents/chelators work?
Form a metal-ligand complex (ligand = chelator)
Why are sequestering agents/chelators important?
Transition metals (e.g., Fe, Cu, Al) catalyze oxidation reactions leading to many undesireable effects
chelators/sequestering agents prevent such reactions by sequestering metals and making them unavailable for reaction
Discuss chelation with phosphates.
- In the form of salts or esters of phosphoric acid
- Form stable, insoluble complexes with heavy metal ions and alkaline earth metal ions (undesirable precipitates, off flavours, and discolouration)
- Used in the prevention of clouding or hazing in beer (occurs when heavy metals or alkaline earth metal ions react with organic compounds normally present in filtered beverage)
What is the chemical reaction in step 1?
What reactions are taking place in each step?
How do these results apply to the food system?
Phosphate → insoluble Fe chelator, causes precipitate
EDTA, tannic acid → soluble Fe chelators, prevent undesirable precipitation in food
Malic acid, acetic acid → no chelating properites
Discuss the ionic forms & dissociation constants of citrate.
citric acid = fully protonated
citrate-1 = one proton lost
citrate-2 = two protons lost
citrate-3 = fully deprotonated
What is the Henderson-Hasselbach equation?
citric acid = fully protonated
citrate-1 = one proton lost
citrate-2 = two protons lost
citrate-3 = fully deprotonated
What are the Henderson-Hasselbach equations for citric acid dissocation?
Given the ionization constants, use the Henderson-Hasselbach equation to calculate the ratios.
Calculate the ratios.
What is the structure of citrate?
Based on these ratios, determine which form of citrate is the predominant form in foods.
OJ, CTJ, GJ, CTS: citrate-1
LJ: citric acid
