Midterm 2 Review_answers

Midterm 2 Review
Be familiar with the equation sheet!! It’s
posted on the isites website under
midterm 2 review materials..,. no excuse
not to know it tomorrow!
Heat Transfer
What are the equations that go with heat
Give an estimate for the temperature at which the following events occur and describe what’s happening:
a. Bread solidifying
~63-65C  when egg solidifies
Maillaird reactions
120C, amino acids and carbs reacting
Caramelization reactions
150C, sugars breaking down
Calculate the time it will take for the heat to diffuse to the center of the bread.
L = sqrt(4Dt)  t = L^2/4D  (6.3cm)^2/4*(1.4 x 10^-3 cm^2/s) = 7200 seconds
*L is shortest distance to center, in this case I only gave you length and width, so assume half of width (2.5 inches) is the
shortest distance to the center
If the L triples, the time to diffuse will increase by a factor of what?
L^2 = 4Dt  L^2 is proportional to t so if L is tripled, (3L)^2 = 9L^2 is proportional to t so t increases by a factor of
Draw the temperature distribution within the cake at the following time points: 1) before entering the oven, 2) 30 minutes
into baking, and 3) 15 minutes being taken out of the oven and cooling
Before entering the oven = straight line at room temp (temperature is evenly distributed)
30 minutes in = exponential decline from edge @ 175C
15 minutes after taken out of oven = straight line ~60C –temperature gradient has evened out
Why does the bread keep cooking when removed from the oven?
Air is a poor thermal conductor and thus acts like an insulator so the extra heat on the edges continues to diffuse toward
the center of the bread
Which equation(s) apply to viscosity?
Name Myhrvld’s 4 ways of increasing the viscosity of fluids and identify which ones are at work in the banana bread
In recipe batter = starches and possibly emulsions, Not in recipe = modernist thickeners & reduction
Calculate the volume fraction of flour. Does the flour affect the viscosity significantly?
2 cups/ (2 c + 2.33 c + .75c + .5c + .25c) = 2c/5.75c = .347
No, to affect viscosity, volume fraction must be above 0.5
10. The protein network formed by glutenin also helps to thicken the dough. Glutenin is a polymer made up of many
monomers. What are the monomers that make up glutenin?
Amino acids
How is a polymer naturally arranged in space?
Curled/bunched up, ie not a straight line
If glutenin is made up of 300 monomers, each with a radius of 4nm, what is the radius of a glutenin molecule?
Rp = Rmsqrt(Nm) = 4nm * sqrt (300) =69.28 nm
Calculate the volume of one glutenin molecule then.
V = (4/3)*pi* r3 = 1.4 x 106 nm2
Emulsions & Foams
11. Which equation(s) apply to emulsions and foams?
12. When combining the eggs and butter, you are able to create bubbles with a radius of 10 micrometers. Given the surface
tension of water-oil interfaces is 24mN/m and the volume fraction is 0.70, calculate the elasticity of the emulsion and the
Laplace pressure of each bubble.
E = surface tension/R (volume fraction – critical volume fraction)
E = (24 x 10^-3 N/m)/(10x10^-6 m) (.7-.64)
E= 144Pa
P = 2*surface tension/ R  from equation sheet!
P = 2(24 x 10^-3 N/m)/(10x10^-6 m) = 4800Pa
13. Name four ways emulsions and foams can fail. How can you prevent failure? Which ingredient in this recipe helps stabilize
the emulsion?
a. Four ways of failure: creaming/sedimentation, Oswald ripening, coalescence, inversion/breaking
How to prevent failure:
Stabilizing ingredient in this recipe: egg yolk/lecithin
14. How could you increase the elasticity of the emulsion?
Increase volume fraction by adding more of whichever ingredient is in the dispersed phase
Decrease radius of the bubbles by whisking harder
15. What equations apply to the baking week?
16. What is acting as the leavening agent in this recipe? Baking soda!
a. What’s its chemical formula? NaHCO3
b. What is its molecular weight? 84g/mol
c. How does it compare to the other leaveners?
Compared to baking powder, baking soda needs an acidic ingredient in the recipe to react with (unlike baking
soda which is comprised of 1/3 baking soda and 2/3 tartaric acid)
Compared to yeast, baking soda is a chemical level (not biological) and produces CO2 through a chemical reaction
versus yeast, which produces CO2 through metabolizing sugars. Yeast needs time to activate and digest the sugars
to produce CO2 (which is why you have such a long rising time in yeast recipes)
17. How does the all-purpose flour in this recipe compare to bread flour?
Bread flour has more glutenin
18. How do each of the following affect the glutenin network and subsequent texture of the bread:
a. Kneading
If you don’t knead enough, the elasticity of the bread is lower; too much and its higher
Salt increases the elasticity by providing a shielding effect on the glutenin proteins so they can be closer to each
other and form bonds
19. The recipe calls for “overripe” bananas; when a banana becomes overripe it has brown spots. What causes these brown
spots to occur? (Name the enzyme and compounds involved)
Polyphenoloxidase is an enzyme that promotes the a reaction between phenolic compounds in plant/fruit cells and
oxygen to create a large, brown compound
20. Many things can affect the rate of the formation of these brown spots. Name whether the following would increase or
decrease the rate of browning:
a. Refrigeration  decrease, lower temperature slows rate of reaction (look at equation of week!)
Having it out on the counter a hot summer day (~35C) without AC  increase, reaction rate increases with
temperature until the critical temperature when the enzymes are denatured
Flash-heating in an oven above 90C  inactivate the enzyme thus reducing rate
Storing the banana submerged in oil or wrapping in saran wrap decrease  prohibits oxygen from reaching
phenolic compounds
Coating the banana with an acid such as lemon juice  decrease (denatures/inactivates some of the enzymes)
21. Microbial growth. If initially only one E. coli bacterium (1×10−15 kg) was living on a banana, how long would it take for the
amount of bacteria to weigh the same amount as the banana (~300g) if it reproduced every 5 minutes?
240 min or ~ 4 hours
.3kg/(10^-15 kg) = number of bacterium at time t (N(t))
No = 1
K = ln2/tau = ln2/5 min
N(t) = No*ekt
.3kg/(10^-15 kg) = 1*e(ln2/5)t, solve for t