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) 1 2013SPU27Midterm2PracticeProblemKey 3. Youaremakingasauceanditisalmosttothebrimofthepan. Ifyouwanttoaddsomething toyourdishtomakeitmoreviscous,butyoudon’twantittooverflow,shouldyouaddfator starch? Why? Starch,becauseitsopsuptheliquidthatisalreadythere,whereasfatwouldaddmorevolume. 2 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. 3 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 4 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). 5 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 4 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. 6 2013SPU27Midterm2PracticeProblemKey 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. 7 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. 8 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. 9 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: 10