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2628900-11430000Midterm 2 ReviewBe 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 TransferWhat are the equations that go with heat transfer?Give an estimate for the temperature at which the following events occur and describe what’s happening: Bread solidifying~63-65C when egg solidifiesMaillaird reactions120C, amino acids and carbs reactingCaramelization reactions150C, sugars breaking downCalculate 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 centerIf 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 coolingBefore entering the oven = straight line at room temp (temperature is evenly distributed)30 minutes in = exponential decline from edge @ 175C15 minutes after taken out of oven = straight line ~60C –temperature gradient has evened outWhy 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 breadViscosityWhich 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 batter.In recipe batter = starches and possibly emulsions, Not in recipe = modernist thickeners & reductionCalculate 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 0.5The 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 acidsHow is a polymer naturally arranged in space?Curled/bunched up, ie not a straight lineIf 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 nmCalculate the volume of one glutenin molecule then.V = (4/3)*pi* r3 = 1.4 x 106 nm2Emulsions & FoamsWhich equation(s) apply to emulsions and foams?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= 144PaP = 2*surface tension/ R from equation sheet!P = 2(24 x 10^-3 N/m)/(10x10^-6 m) = 4800PaName four ways emulsions and foams can fail. How can you prevent failure? Which ingredient in this recipe helps stabilize the emulsion?Four ways of failure: creaming/sedimentation, Oswald ripening, coalescence, inversion/breakingHow to prevent failure:surfactants!Stabilizing ingredient in this recipe: egg yolk/lecithin How could you increase the elasticity of the emulsion?Increase volume fraction by adding more of whichever ingredient is in the dispersed phaseDecrease radius of the bubbles by whisking harderBakingWhat equations apply to the baking week?What is acting as the leavening agent in this recipe? Baking soda!What’s its chemical formula? NaHCO3What is its molecular weight? 84g/molHow 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)How does the all-purpose flour in this recipe compare to bread flour?Bread flour has more gluteninHow do each of the following affect the glutenin network and subsequent texture of the bread:KneadingIf you don’t knead enough, the elasticity of the bread is lower; too much and its higherSaltSalt increases the elasticity by providing a shielding effect on the glutenin proteins so they can be closer to each other and form bondsBrowningThe 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 compoundMany things can affect the rate of the formation of these brown spots. Name whether the following would increase or decrease the rate of browning: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 denaturedFlash-heating in an oven above 90C inactivate the enzyme thus reducing rateStoring the banana submerged in oil or wrapping in saran wrap decrease prohibits oxygen from reaching phenolic compoundsCoating the banana with an acid such as lemon juice decrease (denatures/inactivates some of the enzymes)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 = 1K = ln2/tau = ln2/5 minN(t) = No*ekt.3kg/(10^-15 kg) = 1*e(ln2/5)t, solve for t ................
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