Midterm 2 Flashcards
1
Q
What are true solutions?
A
- Very small particles/ions dispersed in a liquid
- Constant kinetic motion
- Homogenous because evenly distributed
- Very stable, does not form a gel
2
Q
What are colloidal dispersions? What are the stabilizing factors? What happens under certain conditions?
A
- Intermediate sized macromolecules (proteins, cooked starch, pectic substances)
- Less kinetic motion
- If dispersed particles H bond with each other, will precipitate out of colloidal dispersion (GEL)
Stabilized by:
- Brownian movement: random movement of particles as they are constantly and unevenly bombarded by water molecules
- Repelling: dispersed molecules are negative and scatter
- Water of hydration: layer of water molecules attached to surface of particles by weak hydrogen bonds prevent contact and bonding between colloidal particles
3
Q
What are suspensions?
A
- Large dispersed particles or groups of molecules
- Include starch in cold water or fat globules or temporary emulsions
- Very unstable as gravity causes dispersed particles to separate upon standing
- Do not form GELs
4
Q
What is the composition of starch?
A
- Contains glucose units joined up
- Starch molecules: amylose and amylopectin
5
Q
What are the sources of starch?
A
Roots
- Eg. potatoes, arrowroot, tapioca
- Found in plastids in cytoplasm
Cereal
- Eg. wheat, rice, corn
- Found in endosperm of grain
6
Q
Talk about the formation of a SOL for starch. Mention suspensions too.
A
- When starch is placed in cold water, it forms a suspension
- During heating, the suspension is transformed into colloidal dispersion
- Gelatinization: irreversible swelling of starch granules when heated in water
- Kinetic energy of water molecules increases, breaking H bonds holding amylose and amylopectin together
- Water moves into starch granule and they expand (membrane breaks)
- Some amylose molecules diffuse out while amylopectin stays in
- Membrane only allows linear molecules through
- This creates a thick but pourable SOL (thickened mixture which can be poured from container, molecules move randomly, distributed evenly)
- Increase in viscosity and translucency
7
Q
Talk about the formation of a GEL for starch.
A
- Retrogradation: setting of cooled gelatinized starch SOL to form a GEL
- Reduction in kinetic energy
- Amylose molecules which diffused out form H bonds with other amylose molecules, surface of starch granules and branches of amylopectin molecules sticking out of granules
8
Q
Talk about syneresis for starch.
A
Leakage of liquid from gel
- Aging during prolonged storage or continuing to cool causes shrinkage of GEL because additional bonds are formed, which squeezes out water
- Freezing then thawing a gel (expansion breaks H bonds formed by amylose, letting water trapped to leak out)
- Cutting a gel (breaking starch molecules that were full of water)
9
Q
Discuss waxy cornstarch.
A
- 100% amylopectin
- Forms SOL but not a GEL because no H bonds
- Same thickness before and after going into freezer (no H bonds)
10
Q
What are the factors affecting SOLs and GELs for starch? What should you do?
A
- Concentration of starch
- More starch/amylose = thicker SOL, firmer GEL - Source of starch
- Cereal starches = more amylose = thicker SOL, firmer GEL - Cooking temperature
- Overcooking = bursts swollen granules = gummy, thinner SOL
- Use double boiler or oven poaching to keep temperatures under 100 C - Acid
- Ruptures swollen starch granules, causing water to leave
- With heat = hydrolysis of SOME amylose and amylopectin into dextrins (short chain glucose molecules) which can’t form GELs
- Decrease in thickness but still same translucency of SOL
- Decrease in GEL firmness
- Add acid after gelatinization but before retrogradation - Sugar
- Add sugar after gelatinization but before retrogradation unless you want less thick/firm SOL/GEL
SOL:
- Sugar competes with starch for water = decrease in thickness
- Sugar forms H bonds with starch = gelatinization temperature goes up
- Protects swollen starch granules from rupturing due to overheating or acid
GEL:
- Decrease in GEL firmness
- Forms H bonds with starch granules and excess water = stabilizes gel (less syneresis)
11
Q
What is lumping for starch and how do you prevent it?
A
- Gelatinized starch granules outside with raw, ungelatinized granules inside
To prevent, granules must be separated completely:
- Dry mixing starch granules with sugar (when added to liquid, sugar dissolves and allows liquid to surround each granule completely)
- Suspending starch granules in cold liquid (then heated)
- Mixing starch granules with melted or liquid fat (in a roux, fat coasts granules)
- AND stir constantly to allow water to move into all starch granules to create colloidal dispersion
12
Q
What is the difference between the white and brown sauce?
A
- In brown sauce, flour heated without moisture (dry heat)
- Amylose and amylopectin hydrolyzed into dextrins
- Cannot form a GEL, only a SOL
- React with each other to form non-brown pigments in a non-enzymatic browning reaction
13
Q
What are the types of proteins?
A
Simple
- Globular: rounded in shape
- Ovalbumin in egg whites
- Lactalbumin and lactoglobulin in milk
- Gliadin and glutenin in wheat - Fibrous: coiled/extended in shape
- Collagen and elastin in connective tissue of meat
- Myosin in muscle of meat
Conjugated/complex proteins: composed of simple protein and nonprotein material
- Phosphoproteins (protein + phosphoric acid)
- Casein in milk - Glycoproteins (protein + carbohydrate)
- Ovomucin in eggs - Lipoproteins (protein + fatty substance)
- Lipoproteins in egg yolks + whipping cream - Chromoproteins (protein + coloured material)
- Myoglobin in muscle of meat
14
Q
What is the structure of proteins?
A
- Primary: amino acids linked by peptide (covalent) bonds to form polypeptide chains
- Secondary: spring-like coiling of polypeptide chain (simple fibrous proteins)
- Tertiary: helix folds back on itself to form globular structure (simple globular proteins(
- Quaternary: globular proteins combine with each other or non-protein substance (conjugated/complex proteins)
15
Q
Discuss isoelectric point of proteins.
A
MEASURED IN PH.
- Carboxyl group donates H, amino group accepts H
- The point where amino acid no longer has electric charge
- Like charges of protein no longer repelling each other
- Least stable = denatured
- Protein molecules are attached and form H bonds with each other to create larger molecules
- Cannot stay in colloidal dispersion
16
Q
Discuss the factors of denaturation/coagulation.
A
- Change in pH to IEP
- Increase or decrease in temperature (cooking/freezing)
- Mechanical action (whipping, beating)
17
Q
What is denaturation, coagulation, and over-coagulation?
A
Denaturation: change from naturally ordered configuration of protein molecule to more randomly structured molecule (H bonds break)
Coagulation: new H bonds form at new locations among polypeptide chain within protein molecule (most common cause is heat)
Over-coagulation: polypeptide chains compress together and squeeze out water due to excessive H bond formation
- Result due to prolonged exposure to pH (usually decrease), heat (too high or too long), mechanical action (unlikely)
- To prevent, use double boiler or oven poaching to keep temperatures under 100 C
18
Q
What is the composition of eggs?
A
White: more water, 80% protein (albumins), no fat/carbohydrate
Yolk: ½ water, ½ solid (⅓ protein and ⅔ fat), lecithin, cholesterol, no carbohydrate, more minerals/vitamins
19
Q
How are eggs stored?
A
- Fresh eggs broken onto a flat plate stand up in rounded force due to viscosity of thick portion of egg white
- Lose moisture = air cell between two membranes to enlarge
- Lose CO2 = egg white becomes alkaline, thins out
- Egg yolk absorbs water from egg white and yolk membrane stretches
- When broken onto a plate, deteriorated egg yolk flattens and egg spreads
- Chalazae has disintegrated and can’t hold yolk in centre
- To prevent this, keep in original carton to prevent moisture loss and absorption of odours due to porous shell
- Can be refrigerated for 2 weeks
20
Q
What are the functional properties of eggs?
A
Thickeners (SOL)
Gelling agents (GEL)
Foaming agents (egg whites)
Emulsifiers (lecithin in egg yolks)
21
Q
Discuss the SOL/GEL formation for eggs.
A
- With heat, the protein becomes denatured
- Coagulation will occur in which unfolded polypeptide chains will form new H bonds at new sites along polypeptide chains
- Will result in formation of GEL if egg mixture is not stirred because formation of H bonds is not disrupted
- Will form SOL if egg mixture is stirred during coagulation because disrupts formation of new H bonds along unfolded polypeptide chain
22
Q
What are the factors affecting SOL/GEL for eggs?
A
- Temperature
- Egg white proteins coagulate easier than egg yolk proteins - Amount of egg proteins
- Diluting egg solution keep egg proteins further apart = less thick/firm SOL/GEL
- Higher temperatures needed for denaturation and coagulation - Sugar
- Forms H bonds; reducing bonding between actual egg proteins
- Slower rate of denaturation and coagulation
Increase in coagulation temperature
- Protect against over-coagulation
- Less chance of curdling/syneresis
- Decrease SOL/GEL thickness/firmness - Acid
- Contributes H ions which neutralize negative charges around egg protein molecules
- Egg proteins reach IEP and denature/coagulate faster
- Decrease in denaturation and coagulation temperatures
23
Q
What is an egg white foam? Talk about baking.
A
Colloidal dispersion in which air bubbles (dispersed phase) are trapped in a liquid (continuous phase)
Beating egg whites:
- Incorporates air bubbles into liquid portion of egg whites
- Denatures egg white proteins which collect on surface of egg white bubbles
- Stabilizes foam (longer time before foam collapses)
Baking egg whites causes egg proteins to coagulate to make foam permanent (never collapse)
24
Q
What are the factors for egg foam quality?
A
- Age of eggs (maximum volume from 2-3 days old)
- Temperature (maximum volume from room temperature)
- Length of beating
- Under beating = too few egg white proteins denatured = less stable
- Overbeating = denatured proteins are less flexible = air cells break = lower foam volume, less stable - Sugar
- Form H bonds with egg white proteins, interfering with denaturation
Increases time required to create foam and reduces foam volume if added at start of beating
- More stable/stiff foam, preventing overcoagulation, if add at soft peak stage - Acid
- H+ neutralize negative charges, allow proteins to reach IEP and denature faster
- The more denatured proteins, the more stable/larger volume/stiffer the foam
- Always added at beginning so foam forms faster - Fat
- Delays foam formation
- Lipoproteins in egg yolks form complex with egg white proteins preventing denaturation
- Use glass/metal bowl instead of plastic/silicone to beat egg whites as fat adheres to plastic
25
What is the grey-green colour for eggs? How do you prevent it?
- Hydrogen sulfide in egg whites combines with iron in egg yolk
- Forms ferrous sulfide which has grey green colour
- The longer the egg proteins are heated after coagulation, the more grey-green ring
Hard boiled:
- Slow cooling after cooking = pressure of heat allows contact between egg white and yolk
- SOLUTION: cool rapidly in cold water to reduce pressure on outside of egg drawing hydrogen sulfide in egg white away from yolk + crack eggs to remove the pressure from shell
Scrambled:
- Keeps eggs hot for too long after cooking/coagulation, allowing compounds to move around
- Iron and hydrogen sulfide come into contact while still warm for long period of time
- SOLUTION: prepare in small batches + limit heating time + minimize heat
26
Can eggs freeze/thaw?
Whites:
- Raw: yes
- Cooked: no (tough and rubbery)
Yolks:
- Raw: no (proteins denature = thick and gummy) unless you protect proteins by adding sugar/salt
- Cooked: yes
27
What are the constituents of milk in solution?
- Water soluble vitamins (good source of riboflavin, thiamin/niacin/ascorbic acid in small amounts)
- Minerals: ⅓ of Ca, K, Mg, Na
- Sugar: lactose which participates in Maillard browning reaction
- Salts
28
What are the constituents of milk in colloidal dispersion?
- ⅔ of Ca, phosphorus
- Proteins: whey and casein
- 80% is casein (alpha, beta, kappa) which exist with calcium and phosphate as calcium phosphocaseinate/casein micelle
- 20% is whey proteins (lactalbumin, lactoglobulin)
29
What are the constituents of milk in emulsion?
1. Milk fat
- Fat-in-water emulsion (fat is dispersed, water is continuous)
- Exists as globules kept in emulsion by emulsifying layer around fat globules composed of lipoproteins
2. Fat soluble vitamins: A & D
30
What is the difference between homogenization and pasteurization of milk?
- Homogenization: milk is pumped under extreme pressure through very small holes to reduce size of fat globules; fine emulsion; easier to digest/opaque.white
- Pasteurization: heating to kill pathogens
31
What is yogurt?
- Inoculated milk with nonpathogenic bacterial culture which ferments lactose into lactic acid
- pH decreases to IEP of casein (4.6) which denature and coagulate (curdles)
32
How does whey denature?
- Heat only
- Settle out of colloidal dispersion, settle to bottom of container
- If you continue to overheat, causes whey proteins to scorch bottom of pot (white film)
33
How does casein denature?
1. Acid
- Casein micelles are colloidally dispersed at pH of 6.7 (milk)
- Negative charges around casein micelle repel and keep stable
- Adding acid lowers pH, neutralizing negative charges (0) to bring casein to IEP
- Micelles are denatured and adhere together, forming larger unstable molecules (precipitate out of colloidal dispersion)
- Heat accelerates this reaction
2. Polyphenolic compounds
- Found in light coloured, low acid fruits and vegetables (potatoes, apples, asparagus, mushrooms, green peas, strong tea and coffee)
- Remove water of hydration around casein micelles, allowing them to denature/adhere together to form large/unstable molecules
- Precipitate out of colloidal dispersion
- Heat accelerates reaction
3. Enzyme rennin from stomach of calves
- Synthetically produced as chymosin
- Optimal temperature is 40 C
- Used to make cheese to denature/coagulate proteins
- Removes kappa casein from casein micelle
- Alpha and beta casein readily denature, but calcium remains in micelle
- Weak, unstable calcium caseinate GEL forms
34
How to avoid denaturation of milk proteins?
1. Minimize heating
- Use double boiler (< 100 C)
- Heat only to serving temperature (do not boil)
2. Minimize time that acid ingredient and casein in milk are together
- Add milk just before serving
- Last step before “serving immediately”
3. Thicken water phase around casein micelles to physically prevent denatured casein micelles from adhering together and precipitating out of colloidal dispersion
- Starch if product involves heat (gelatinized)
- Gelatin if conditions are cold
35
What does beating cream do?
- Incorporates air bubbles
- Disrupts emulsifying layer around fat globules, allowing them to clump together = more stable foam (longer time before bubbles burst and collapses)
- Denatures lipoproteins in emulsifying layer which collect on surface of air bubbles = even more stable foam
36
How do you make sure whipped cream foam is of good quality?
1. Use cream with > 32% fat because you need lipoproteins
2. Use cold temperatures (cream, beaters, bowl at 4 C or lower)
- Fat globules become more solid which clump together more easily on surface of air bubbles (more stable foam)
- Cream is thicker, making it easier to incorporate air bubbles (higher volume)
3. Addition of sugar near the end of beating
- Decreases foam volume + increase time to create foam if added too soon
- Add after partial foam is created
Increase foam stability (longer time before foam collapses)
- Decreases foam stiffness (softer peaks)
- Decrease chance of over-beating (butter) + increase foam stability
37
What happens when you age cheese?
1. Changes in texture (changes to protein)
- Firm tough and rubbery to softer and crumby
- Cheese blends better
2. Changes in flavour (changes to fat)
- Mild and bland to mellow and tangy
38
What are the properties of cheese?
- Melt: medium/high in moisture AND fat
| - Blend: melt well + be aged/ripened (changes in proteins)
39
What happens when you overcoagulate cheese?
- Too high a temperature or moderate temperature for extended period of time
- Results in tough and stringy cheese due to excessive H bond formation
- Proteins shrink and squeeze out water = water on top of cheese
- Fat emulsion breaks = oily layer on top of cheese
40
What does overcoagulation do for eggs?
- Overcoagulation causes excessive H bond formation resulting in syneresis in a GEL or curdling in a SOL
- Tough texture in egg proteins
