Problem X: Alfredo sauce (Greg)

30 Pages·2013·0.88 MB·English

2013SPU27Midterm2PracticeProblemKey
Problem X: Alfredo sauce (Greg)
ArecipeforAlfredosauceinvolvescombining2cupsofmilkwiththefollowingingredients:
• 2cupsmilk
• 1⁄ cupbutter
4
• 2tsp. flour(equivalentto1⁄ cup)
24
• 1tsp. salt
• 1⁄ tsp. pepper
2
• 1⁄ cupgratedParmesancheese
2
1. Whichingredient(s)contributetoviscosity?
cheese,butter,flour
2. Ifweremovedtheflour,wouldthesaucebeviscousstill? Why?
No,1⁄ cupcheese(mostpossibleviscosityit’sfatcouldadd)and1⁄ cupbutterdoesn’taddup
2 4
totherequired50%volumefractionrequiredforviscosity.
3. Your buddy is lactose intolerant, so you want to make a version with 2 cups of soymilk in-
steadofmilk,completelyomittingthebutterandcheese,andpossiblyaddingextrastarchfor
viscosity. Mustyouaddflourtomakethesauceviscous? Ifso,howmuch?
None, it will already be viscous because after the starch molecules expand to 30 times their
originalsize.
Problem X: Multiple choice and short answer (Greg)
1. Whataretheunitsofviscosity(inkg,mands)?
kg m kg
η = ρ⋅speedofmolecules⋅moleculesize∼ ⋅ ⋅m=
m3 s m⋅s
2. Whatare3waystocontrolviscosityincooking? (like,ifyouhadredwineandyouwantitto
begrossandviscous)
Reduction,starchbasedthickeners,polymerbasedthickeners(polymers),oremulsions(adding
fat)
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2013SPU27Midterm2PracticeProblemKey
3. Youaremakingasauceanditisalmosttothebrimofthepan. Ifyouwanttoaddsomething
toyourdishtomakeitmoreviscous,butyoudon’twantittooverflow,shouldyouaddfator
starch? Why?
Starch,becauseitsopsuptheliquidthatisalreadythere,whereasfatwouldaddmorevolume.
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2013SPU27Midterm2PracticeProblemKey
Problem X: The Perfect Steak (Emily)
Inspired by Carme Ruscalleda and Heston Blumenthal, you decide to embark on your own
culinary adventure to cook the perfect steak. You remember learning that the outside of a steak
often gets overcooked using traditional methods, so you decide to cook your steak in a sous-vide.
Youlikeyoursteakmedium,soyousetthesous-videto60○C.
1. Yoursteakis30mmthick. Howmanyminuteswillittakefortheheattoreachthecenterof
yoursteak? Assumeithasapproximatelythesameheatdiffusioncoefficientaswater.
√ L2 (1.5cm)2
L= 4Dt (cid:212)⇒ t= = =402sec=6.7min
4D 4⋅1.4x10−3cm2/s
2. Drawasketchofthefinaltemperaturedistributioninsideyoursteak.
Allthesametemperature(60○C)
3. Whatwouldbedifferentaboutthisdistributionifyoucookedyoursteakintheoveninstead?
Youwillhaveatemperaturegradient.Thecenterwouldstillbeat60○C,butitwouldgethotter
(toabout120○C)asyoumovedoutwardinthesteak.
4. Howwilltheelasticmodulusofyoursteakcomparetoonecookedintheoven?
Your steak will have a lower elastic modulus (be less tough/more tender). U will be ap-
int
proximately the same in both cases, but the oven cooked steak will have a smaller (cid:96). More
moistureisremovedintheoven(sinceyouactuallygoabove100○C),so(cid:96)decreasesgivinga
largerelasticmodulus.
5. You cut into your steak and it looks beautiful! You take a celebratory bite and are immedi-
atelydisappointed. Eventhoughit’saperfectmedium,itdoesn’ttastenearlyasgoodasyou’d
hoped. Whatareyoumissinginthisrecipe?
Youdon’thavethebrowningreactionsontheoutsideofyoursteak. Sincetheseoccuraround
120○C,theywon’toccurinasous-videsetto60○C.Thesebrowningreactionsareessentialto
getagoodtastingsteak.
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2013SPU27Midterm2PracticeProblemKey
Problem X: Cre`me Brule´e (Emily)
You decide to learn how to cook your TF’s favorite dessert in hopes of scoring some extra
credit.Therecipelooksalittlecomplicated,butyou’recertainyoucanmasteritonceyouunderstand
thesciencebehindit.
1. Whatishappeningwhenthecre`mebrule´ecooks?
Asthemixtureheatsup,proteinsintheeggyolksdenatureandcoagulatecreatingthegelled
custardstructure.
2. Theramekinshaveadiameterof12cmandare6cmdeep–youfillthem3/4oftheway. How
manyminuteswillittakefortheheattoreachthecenterofyourcremebrulee? Assumethe
batterhasapproximatelythesameheatdiffusioncoefficientaswater.
Thediffusiondistanceisshorterfromthetop/bottomthanfromthesides.
√ L2 (1 ⋅ 3 ⋅6cm)2
L= 4Dt (cid:212)⇒ t= = 2 4 =904sec=15min
4D 4⋅1.4x10−3cm2/s
3. Isthismeasurementconsistentwiththerecipe? Explain
Therecipeseemstocookthecre`mebruleeformuchlonger(40-45mins)thanourestimated
cook time (15 mins). However, a target temperature must be reached in order for the cre`me
bruleetosolidify,soitisnotnecessarilyfalsethatheatisreachingthecenterafteronlyfifteen
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2013SPU27Midterm2PracticeProblemKey
minutes of cooking. Also there is likely loss of heat to the environment, leading to a longer
cooktime.
4. Explainthepurposeofthewaterbathin2-3sentences.
Eventhoughtheovenissetto325○F,thewaterbathwon’tgoabovetheboilingpointofwater
(100○Cor212○F).Thisallowsforslow,gentle,andevencookingofthecre`mebrule´e.
5. Whyshouldyouuseatorchtocreatethesugarbrule´eratherthansomethinglikeanoven?
Wewanttocaramelizethesugarontopwithoutoverheatingtherestofthecustard. Thetorch
isextremelyhotandallowsyoutoquicklyburnathinlayerofsugarwithoutgivingtheheat
enoughtimetodiffusethroughtherestofthecre`mebrule´e(overcookingit).
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2013SPU27Midterm2PracticeProblemKey
Problem X: Gluten-free bread (Denise)
You’re an avid baker and your friend asks you to make some gluten-free bread. Your friend
saysit’stypicallyhardtofindgoodgluten-freebreadsoyousetouttoproveyourfriendwrong.
1. Whatkindsofproblemsdoestheabsenceofglutenprovoke? Howwouldthisaffectthefinal
textureofthebread?
YourelyonthetrustyInternetandfindthefollowingrecipethatseemedtohavesomepositivere-
views. Younoticetheyuseamodernistthickener-xanthangum!
WhiteRiceFlourYeastBread
1. Mix together the white rice flour, potato starch,
(Gluten-freeandlow-fructose)
tapioca starch, sugar, yeast, salt and xanthan gum
inalargebowl.
2cups whitericeflour
2. Slowlypourinthewarmmilkwhilemixingonlow.
2⁄ cup potatostarch
3
Addthebutterandbeatuntilthoroughlyblended.
1⁄ cup tapiocastarch/flour
3
3. Add the eggs one at a time, beating after each ad-
3tbsp. sugar
dition. Scrape the bottom and sides of bowl. Beat
2tsp. instant(quick-riseorrapid-
thebatterfor3minutesonhigh.
rise)yeast
4. Coverthebowlandletthebatterriseforonehour
11⁄ tsp. salt
4
inawarmplace.
11⁄ tsp. xanthangum
4
5. Scrapedownsidesofbowlonceagaintogentlyde-
1cup warm milk (or non-dairy
flatethebatter.Greasean8′′x4′′loafpanandpour
alternative)
batter into pan. Cover (with plastic wrap sprayed
1⁄ cup unsalted butter, softened
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with cooking spray) and let rise in a warm place
(ornon-dairyalternative)
just until loaf barely crowns above the rim of the
3 largeeggs
pan,about20to25minutes.
6. Preheatovento350○F.Bakethebreaduntilgolden
brown,about38to40minutes.
2. Whatroledoesthexanthangumplay? Canyoumaketheglutenfreebreadwithoutthexan-
thangum? Whatwouldhappenifyouomittedthisingredient?
Xanthan gum increases the viscosity of the bread, which helps to trap air bubbles in place
duringleaveningandcooking.Withoutthexanthangum,it’spossiblethatthecarbondioxide
bubbleswouldleavethebreaddoughduringcooking.
3. Therecipealsocallsforsugarto“feed”theyeast. Whyisthisnecessary?
Yeast help leaven the bread through a process called fermentation: they convert sugar into
ethanolandcarbondioxide. Yeastcanwaitforthestarchfromflourtobeconvertedtosugar
(seebelow),butthisprocessistime-consuming:forafastrise,itisabetterideatojustprovide
theyeastwithsimplesugars.
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4. Rice flour might not contain gluten, but it is rich in starches – a kind of carbohydrate com-
posedoflongsugarpolymers. Consequentlythismakesitdifficultforyeasttogetitsfavorite
food,glucose,fromtherice. Ifthisistrue,howdoestheyeastfeed?
Inthisrecipe,theyeastmayprimarilybeeatingthesimplesugarsaddedtotherecipe. More
generally,though,yeastrelyontheenzymesalreadypresentintheflourtodegradethestarch
intomonosaccharides.
5. Doyoupredictthatthisbreadwouldrisemoreorlessthanregularwheatglutenbread? Ex-
plain–considerstructuralaswellaschemicalfactors.
Itislikelythattheyeastwillproduceasmuchcarbondioxideasinnormalbread. Innormal
bread, the resulting carbon dioxide bubbles would not be able to fully expand due to the
rigidityoftheglutennetworkthatsurroundthem. However,sincenoglutenispresentinthe
recipe, the bubbles will likely be able to expand more. (One might predict that, similar to a
souffle´, this bread may also fall substantially on cooling since there is no gluten network to
“lockin”thebread’sfinalshapeandthexanthangum/eggisaninferiorsubstitute.
6. Thisrecipealsocallsfordairyproductssuchasmilkandbutter. However,younoticethatthe
dairyproductscouldbeswappedoutfornon-dairysubstitutessuchassoymilkormargarine.
Proposeareasonfortheadditionofthedairyproducts. Howdoesthischange,ifatall,ifthey
arereplacedbysoymilkormargarine?
Milkandbutterarebothemulsions:itispossiblethatthey,too,contributetothetotalvolume
fractionofthedispersedphaseinthebatterandthusmakethebattermoreviscous. However,
soymilkandmargarinecontaincomparableamountsoffat.
7. Thebasicbreadrecipecallsforwater,flour,yeast,andsalt. Whatisthepurposeofthe3eggs?
The egg proteins will denature and coagulate during baking, forming a network that con-
tributestotheelasticityofthebread.
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2013SPU27Midterm2PracticeProblemKey
Problem X: Potatoes (Sami)
Chefs have found that depending on how they boil their potatoes, they get firmer or softer
potatoes. You know that pectin in the plant cell can be either high methoxy (neutral) or low-
methoxy(negativelycharged),andyouthinkitmayhavesomethingtodowiththedifferentfirm-
ness. Yousetouttofindoutwhat.
1. Yourfirstexperimentistoboilthepotatoesinregularwaterorhardwater(waterwithahigh
concentrationofCa2+). Youfindthatpotatoesboiledinhardwaterarefirmer. Basedonthis,
whatcanyouconcludeaboutthetypeofpectinthatiscontainedinthepotatoes?
Pectin must be crosslinking with the positively charged calcium content. Therefore low-
methoxypectinshouldbepresentinthepotatoes.
2. Youtakesomerawpotatoesandimmersetheminhardwatertoseeifthatmakesthemfirmer.
Strangely, you find no change. What kind of pectin must be the major component in raw
potatoes? Inlightofthepreviousanswer,whatcanyouconcludeaboutthechangesinpectin
duringthecookingprocess?
High-methoxypectininrawpotatoesmustbeconvertingtolow-methoxypectinduringthe
cookingprocess.
3. Whataretwowaysyoucanincreasetherateofachemicalreaction?
Increasethetemperature,increasetheconcentrationofthereactants,oraddacatalyst.
Twomethodsofboilingpotatoesaretodumptheminhotwaterdirectlyortoputtheminto
cold water first and thenbring up the temperature. Bothboil for the same amount of time. When
youmeasurethemethoxycontent,youfindthatbarelyanyofthepectininthefirstmethodislow-
methoxy, while 50% of the pectin in the second method is low-methoxy. You know that heating
pectinupdoesnotchangetheratiobetweenlowmethoxyandhighmethoxypectin.
4. Explaintheseobservations. Ifyouwantedtomakethepotatoesevenfirmer,whatcouldyou
do?
• Sincetemperatureisincreasingtherateofreaction,anenzymemustbedoingthework.
• Puttingtheenzymeintohotwaterimmediatedenaturesit,soitcannotconvertHMtoLM
pectin
• Heating the potatoes slowly activates the enzyme, allowing it to work before being dena-
tured.
• To make potatoes even firmer, you should hold the potatoes at a higher temperature for
longer,toallowmorereactiontotakeplace.
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2013SPU27Midterm2PracticeProblemKey
Problem X: Alliinase (Sami)
Welearnedthatingarlicandonion,alliinasecatalyzestheconversionofalliinintoallicin.The
reactionactuallyoccursintwosteps. Inthefirststep,alliinasesplitsalliinintoanintermediateand
alanine. Twointermediatesthenreacttogetherspontaneouslytoproduceallicin. Thereactionsare
drawnbelow.
1. Whatgeneralpropertyofenzymesisillustratedinthefirstlineofthereaction?
Enzymesarenotusedupinachemicalreaction.
2. Inspace,nobodycanhearyoucry. Ifyouchoppedanonioninspace,wherethereisnooxy-
gen,coulditstillmakeyoutearup? Usethereactionsabovetheexplainyouranswer.
Yes, because oxygen is not part of the conversion of alliin into allicin. (If anything, the first
reactionisareductivecleavage.)
3. Supposealliinaselowerstheactivationenergybarrierbyhalffromitsvalueatroomtemper-
ature. Ifyoufrozetheonioninliquidnitrogen(77K)beforechoppingit,wouldyoucrywhile
choppingit? Explainyourreasoning.
Iftheactivationbarrierishalfashigh,thereactionwouldgojustasfastaswithoutenzymeat
halfofroomtemperature,whichis149K.Since(cid:96)N isbelowthis,youwouldnotcry.
2
Agoodamountofpartialcreditshouldbegivenforsayingno,becausethereactionratehalves
for every 10○C decrease in temperature (this is a rule of thumb, but we don’t know what the
rateinthepresenceoftheenzymeis).
4. Doyouexpecttogetthesamesortofflavorenhancementfrommincingpickledgarlicasyou
dofrommincingrawgarlic?
No,sincethelowerpHshouldhavedenaturedtheenzymealready.
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2013SPU27Midterm2PracticeProblemKey
Problem X: Anniversary cake (Miriam)
For Harvard’s 375th anniversary in 2011, renowned pastry chef and Harvard alum Joanne
ChangbakedahugeredvelvetcakeintheshapeofanHthatfedthousandsofpeople(recipeabove).
Joanne baked the full cake by first making smaller cakes and then stacking them in layers to form
theH.WewillexaminedifferentwaystocookoneofthearmsoftheH.
1. TheabbreviatedrecipeintheimageabovewasshownintheHarvardCrimsoninanticipation
of the anniversary celebration. This recipe, without any additions, will clearly fail to create
cake. Whyisthat,andwhatyoucouldaddtherecipetomakesureyoucreateagoodcake?
Norisingagent. Canaddbakingsodatogowiththebuttermilk,orbakingpowder.
Joannecreatedthecakebyfirstcookingsmallercakesandthenstackingthemtogether. The
armofthecakehasdimensions15feetby4.5feetby4.5feet:
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