Lecture Notes for Midterm 2.docx

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Hospitality and Tourism Management
HTM 2700
Val Allen

October 11 - 18 13/10/2010 08:26:00 Colloidal Dispersions Remember? Foods are mixtures of substances in various molecular states  Solid  Liquid  Gas Called dispersion systems Dispersion systems have  Dispersed phase  Continuous phase Dispersed phase is scattered throughout continuous phase Dispersion Systems Can be classified according to the  State of matter of the two phases  Size of the dispersed particles o True solutions  Very small molecules or particles dispersed in a liquid o Colloidal dispersions  Intermediate size particles, but still relatively small  Super unique o Suspensions  Comparatively large particles True Solutions Very small molecules or particles dispersed in a liquid Small molecules or ions  Sugars  Salts  Vitamins Molecules are in constant kinetic motion Homogeneous mixture (dispersed phase EVENLY distributed throughout continuous phase) Very stable (dispersed phase does not separate from continuous phase) Do NOT form a gel  GEL o 3 dimensional mixture which holds its shape when turned out of a container Colloidal Dispersions Dispersed particles are  Intermediate in size  Macromolecules OR groups of smaller molecules  Examples o Proteins o Pectic substances o Cooked starch Special properties due to size of dispersed particles Dispersed particles have less kinetic energy, do not move as rapidly Mixtures are not as homogeneous Moderately stable Dispersed particles do NOT separate QUICKLY from continuous phase BUT under certain circumstances particles will “settle out of colloidal dispersion” or away from the continuous phase  Occurs when dispersed particles bond together and form larger sized particles Have the unique ability to form a gel and take on shape of container Suspensions  Comparatively large dispersed particles OR large groups of molecules  Examples o Cornstarch granules in cold H2O o Groups of molecules like fat globules  Very unstable o Gravity causes dispersed particles to quickly separate from continuous phase upon standing  Do NOT form gels More About Colloidal Dispersions Moderately stable Dispersed particles stay scattered throughout continuous phase due to 3 stabilizing factors  Brownian movement  Like charges repelling  Water of hydration If dispersed particles bond together their size increases  Larger particles are less stable  Larger particles “settle out of colloidal dispersion” Brownian movement  Random movement of colloidally dispersed particles as they are constantly and unevenly bombarded by H2O molecules aka they’re constantly moving  Constant slow movement in all directions  Least effective/least important factor Like charges repelling Dispersed particles have the same electrical charge on the surface of every particle, therefore they repel each other Colloidally dispersed food particles have negative charges on surface Like charges repel each other Keeps colloidal particles separate Water of hydration Layer of H2O molecules attached to the surface of the colloidally dispersed particles by weak hydrogen bonds Moves with colloidal particles Forms protective shell Helps prevent contact and bonding between colloidal particles blue stuff = water molecules Ability to Form Gels Colloidal dispersions normally exist as a sol (liquid and pourable) Colloidal dispersions have unique ability to form gels  Take on a 3D container’s shape  Height, width & depth Under proper conditions sol can be transformed into gel (holds its shape when turned out of container) During gel formation  Colloidally dispersed particles loosely join to form a continuous, 3D network  Liquid from continuous phase is trapped in network SOL Dispersed colloidal particles in a continuous liquid medium Thickened mixture which can be poured from a container Molecules move randomly  Hot jam or jelly  Hot white/béchamel sauce GEL High degree of attraction between a continuous system of solid material that holds finely dispersed liquid 3 dimensional takes on AND KEEPS shape of container  cold starch thickened pudding  set fruit jelly Sol to Gel Conversion in Grape Jelly Used pectin (bran name Certo) to make Grape Jelly SOL Grape juice, sugar, acid and pectin heated together Hot mixture was thick but still pourable GEL Mixture allowed to cool Mixture set and took on shape of jars 3D Pectin Molecules  Negatively charged at the pH of fruit juices o Like charges repel each other o Therefore pectin molecules are stable colloidal dispersions in H2O  Hydrophilic (Water-loving) o Have a layer of water of hydration around outside of molecule o Therefore creating stable colloidal dispersion Gel Formation in Jams and Jellies  Add acid (H+ ions) o pH goes down but o positively charged hydrogen ions are added o neutralizes negative charges around pectin molecules  like charges no longer repel therefore pectin molecules form hydrogen bonds with each other  GEL forms  Add large quantities of sugar o Sugar is hydroscopic and readily absorbs H2O  Removes water of hydration around pectin molecules o Therefore pectin molecules form hydrogen bonds with each other  GEL forms Starch Raw starch + cold H2O forms a suspension Starch cooked in H2O forms a colloidal dispersion Sources of Starch  Roots o Potatoes o Arrowroot o Tapioca  Cereals o Wheat o Rice o Corn o Found in endosperm of the grain Starch granules composed of 2 types of starch molecules  Amylose  Amylopectin Amylose  Linear chain molecule  Polymer of glucose  Actual number of glucose molecules depends on source of starch  Glucose molecules joined together by alpha 1, 4 glycosidic linkages  Human body can break these linkages and use he glucose for energy  Therefore starch is a source of energy Amylopectin  Branched molecule (bushy, but compact)  Polymer of glucose  At the branching points, glucose molecules are joined by alpha 1, 6 glycosidic linkages Gelatinatization Irreversible swelling of starch granules when heated in the presence of water Occurs over 60 – 75 degrees C temperature range Temperature varies slightly for each type of starch During heating, kinetic energy of water increases Within starch granule, hydrogen bonds between starch molecules (amylose and amylopectin) are broken Starch granules expand in size as H2O moves into granules Some amylose molecules diffuse out of granules SOL is created  = thickened, but still pourable starch mixture  increase in thickness/viscosity  increase in translucency (clearness) Starch Granule at Gelatinization Temperature Range  H2O penetrates granule and breaks hydrogen bonds between starch molecules Retrogradation Setting of a cooled gelatinized starch sol Cooling means reduction in kinetic energy Amylose molecules, which diffused out of granules during gelatinization, form hydrogen bonds with Other amylose molecules Surface of swollen starch granules Branches of amylopectin molecules sticking out of granules Results in GEL (3-dimensional structure) H2O is trapped within the gel Primarily due to amylose  More amylose = firmer gel Cereal starches contain more amylose than root starches  Cereal starches = firmer gels Amylose Content of Common Starches Cornstarch  26% amylose  cereal starch Wheat Starch (includes flour)  25% amylose  cereal starch Potato Starch  22% amylose  root starch Tapioca  17% amylose  root starch Waxy cornstarch (synthetic)  0% amylose (100% amylopectin)  why bother developing? o Thickens mixtures (will not retrograde) o Good for making sauces that won’t turn into the gel in fridge o Good freeze-thaw stability  Same thickness before and after freezing  No syneresis (leakage of liquid) when thawed Syneresis = leakage of liquid from a gel  Caused by  Cutting  overheating of egg gels Summary of Starch Reactions  Gelatinization o Source of H2O + heat = SOL o Thicker but still pourable o More translucent  Retrogradation o Cooling = GEL o 3D with H2O trapped inside Factors Affecting Thickness of SOLs and Firmness of GELs  Concentration of starch first things to look at when looking at a recipe o More starch = thicker SOL and firmer GEL o Medium white sauce: 25 mL flour to 250 mL milk  Source of starch first things to look at when looking at a recipe o Cereal starches contain more amylose then root starches o Cereal starches produce thicker SOLs and firmer GELs  Cooking temperatures o Overheating ruptures/bursts swollen starch granules  Thinner SOL  Therefore use a double boiler or oven poaching  Prevents overheating/keeps temperature under 100 degrees C  Effect of acid o Lemon juice o Vinegar o On SOL  Ruptures swollen starch granules causing H2O to leak out  Acid and heat results in hydrolysis of starch molecules (amylose and amylopectin) into dextrin’s causing  Decrease in thickness/viscosity of mixture  Increase in translucency o On GEL  Decreased gel firmness or no gel formation  Therefore add acid after gelatinization, but before retrogradation (near end of recipe)  Effect of sugar o On SOL  Competes with starch for H2O therefore  Decrease in thickness  Increase in gelatinization temperature  Because sugar forms hydrogen bonds with starch granules  Protects swollen starch granules from rupturing due to overheating or acid o On GEL  Decrease in gel firmness  Stabilizes gel (less syneresis occurs) because sugar forms hydrogen bonds with starch granules and any extra H2O, holding water in a gel  Therefore add sugar after gelatinization, but before retrogradation (near end of recipe) White Sauce vs. Brown Sauce  Same ingredients in same amounts  Preparation method differs  White Sauce o Melt butter and mix in flour to form a roux = starch-fat mixture  Equal proportions o Add milk, then stir and cook until thickened = SOL o After cooling, gel forms  Brown Sauce o Heat flour in pot without moisture until flour browns o Add butter and mix to form roux o Add milk, then stir and cook until thickened = SOL but less thick than white sauce o After cooling, still a SOL  No GEL forms Why is a brown sauce:  Less thick than a white sauce?  Not capable of forming a GEL when cooled? Heating starch without moisture causes starch molecules (amylose and amylopectin) to hydrolyze into DEXTRINS DEXTRINS = short chain of glucose molecules Dextrins have less ability to thicken a SOL and cannot form a GEL Why is a brown sauce:  Brown in colour? Under dry heat conditions, dextrin’s react with each other to form brown pigments in a non-enzymatic browning reaction Course pack, last page of summary notes for starch Preventing lumping in starch thickened mixtures  Before heating the starch granules must be separated to allow each starch granule to gelatinize separately and completely by o Dry mixing starch granules with sugar OR o Suspending starch granules in cold liquid OR o Mixing starch granules with melted or liquid fat  Forming a roux  DURING HEATING, MUST ALSO STIR CONSTANTLY October 22 - 29 13/10/2010 08:26:00 Types of Proteins  Simple proteins o Globular  Somewhat rounded in shape  Ovalbumin  In egg whites  Lactalbumin  In milk  Lactoglobulin  In milk  Gliadin  In wheat  Glutenin  In wheat o Fibrous  Coiled or extended in shape  Kind of shaped like a slinky/DNA helix  Collagen  In the connective tissue of meat  Elastin  In the connective tissue of meat  Myosin  In the muscle of meat  Conjugated/Complex Proteins Complex compounds composed of globular protein & non- protein material Phosphoproteins (protein and phosphoric acid)  Caseins in milk  Glycoproteins (protein and carbohydrate)  Ovomucin in eggs  Lipoproteins (protein and fatty substance)  Lipoproteins in egg yolks and in whipping cream  Chromoproteins (protein and coloured material)  Myoglobin in the muscle of meat Amino Acids  22 different amino acids in food o 9 are “essential” because  human body cannot synthesize them  must be obtained from our diet  are  isoleucine  methionine  tryptophane  leucine  phenylalanine  valine  lysine  threaonine  histidine o glycine is the simplest amino acid o every amino acid has at least one carboxyl group (COOH) and one amino group (NH2) Peptide Bond linkage between 2 amino acids that connects the amino group of one amino acid to the carboxyl group of the other amino acid  within proteins, amino acids are linked together by peptide bonds  very strong bonds o unlike hydrogen bonds  not broken by normal cooking methods Protein Structure  Primary o Amino acids linked by peptide bonds to form polypeptide chains  Secondary o Spring-like coiling of polypeptide…  Tertiary o Helix folds back on itself to form a globular (rounded/ball- like) structure  Simple globular proteins  Quarternary o Globular proteins combine with each other or non-protein substance  Conjugated/complex proteins Isoelectric Point (IEP) Carboxyl group donates H+ ions Amino group accepts H+ ions pH at which a protein molecule has an overall charge of “0”  equal number of positive and negative charges always expressed as a pH value  for example IEP of casein proteins in milk is pH 4.6 each protein has its own IEP  usually pH value between pH 4.5 – 7.0 Why is IEP important? Like charges are NOT repelling Molecules are attracted to each other and larger molecules form Proteins quickly denature at their IEP Denatured proteins are no longer colloidally dispersed  Proteins change Food proteins exist at a pH different from their IEP Therefore, proteins in food exist as stable colloidal dispersions  For example, casein proteins in milk have an IEP of pH 4.6 but the pH of milk is 6.6 Proteins are sensitive to changes in: 1. pH  almost all proteins  least stable at their IEP 2. Temperature  almost all proteins  particularly increases due to cooking  also decreases (e.g. freezing) 3. Mechanical Action  beating of egg whites - unique any of the above result in  denaturation and POSSIBLY  coagulation Denaturation Change from the naturally ordered configuration (shape) of a protein molecule to a more randomly structured molecule  Hydrogen bonds are broken  Peptide bonds are NOT broken Coagulation The formation of NEW hydrogen bonds at NEW locations along polypeptide chain (within protein molecule)  Always occurs after denaturation  NOT reforming the original hydrogen bonds  Cause can be the same or different from the cause of denaturation o Most common cause is heat OVER-COAGULATION IS BAD BAD BAD BAD BAAAAAAAD Result of prolonged exposure to 1. pH change (normally a decrease in pH) 2. heat (too high or too long) 3. mechanical action (least likely) polypeptide chains compress together and squeeze out H2O because excess hydrogen bonds form = SYNERESIS in a GEL = CURDLING in a SOL to prevent over-coagulation need to prevent over-heating by using:  a double boiler (top of the stove) o can be created with a heat resistant bowl (ceramic or stainless steel) over top of a pot  oven poaching (in the oven)  keep temperatures < or = 100 degrees C Eggs Food Protein (Summary)  Macromolecules which are polymers of amino acids  Amino acids in proteins are joined by strong peptide bonds, which are not broken during normal coking  Colloidally dispersed  Undergo 2 reactions o Denaturation and POSSIBLY coagulation (see Course Pack) o Denaturation always occurs first  Can exist as SOLs or GELs  Least stable and most quickly denatured at their isoelectric point (IEP)  Denaturation and possibly coagulation cuased by o Increase
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