Presentation on theme: "Megan Erickson Central Washington University"— Presentation transcript:
1Megan Erickson Central Washington University StarchMegan EricksonCentral Washington University
2What is it? Complex carbohydrate made up of two components Components: AmyloseAmylopectinProperties depend on amounts of the components
3Where is it found? Roots/Tubers Cereal Potato Arrowroot Tapioca Corn Waxy cornWheatRiceWaxy rice
4Amylose Linear component of starch Contains 1,4-alpha-glucosidic bonds Molecular weight: less than 0.5 millionCan form coils which will trap iodine and turn blueEvery 6 glucoses in the helix can bind one iodine. Test used to find percentage of amylose in starch
5Amylopectin Branched component of starch Contains 1,4-alpha-glucosidic as well as 1,6-alpha-glucosidic bondsMolecular weight: millionLimited coiling causes purplish-red color when iodine addedMuch larger in size than amylosePurplish-red due to lack of helix.
6Amylose vs. Amylopectin Starches usually contain more amylopectin than amyloseGenerally roots/tubers contain more amylopectin than cerealsRoots/Tubers: 80% amylopectinCereals: 75% amylopectinWaxy corn and rice contain virtually all amylopectin
8Starch Granule Made in the cytoplasm of plant cells Amylopectin forms in concentric circles with amyose dispersed in betweenHeld together by hydrogen bondsThe granule swells when heated in water
9Starch GranuleWhen looked at under polarized filters at right angles to each other, starch looks like birefringence pattern (maltese cross). When heated, pattern disappears.
10Functions Gelatinization Dextrinization Gelation Structure in baked productsThickener in sauces, soups, and dressingsDextrinizationGelationPie filling
11Gelatinization When starch is heated in water Hydrogen bonds break, allowing water to enter the granule and the granule swellsAmylose migrates out of the granuleH-bonding between water and amylopectin increasesReduced free water changes the viscosity of the starch mixture, thickening it
1270 degrees, some imploding and some still swelling
13Gelatinization and Temperature Gradually thicken with temperatureCan be heated to 100oC without much granule ruptureIf held at 95oC will implode and lose viscosityLoses viscosity with prolonged heating
14Gelatinization and Type of Starch Best thickening ability: potato starchWorst thickening ability: wheat starchMore amylopectin=more translucent=more stringyRoots generally better thickeners than cereals because they have more amylopectin. Waxy starches better thickeners because lots amylopectin.Flour is worse than wheat starch because of protein content.Stringy is undesirable, smooth is desirable. This is why process tapioca into pearl tapioca so less stringy. Corn starch is good b/c not stringy, waxy cornstarch can be cross-linked so not stringy and translucent.
16Gelatinization and Sugar Used together in pie fillings and puddingsSugar competes with the starch for water so less water available for gelatinizationDelays gelatinization and decreases viscosityIncreases gelatinization temperatureThe more sugar added, the longer the delayDisaccharides have a stronger effect than monosaccharidesSome cross-linking between starch and sugar also delays gelatinization.Sugar increases translucence.
17Gelatinization and Acid Used together in fruit pie fillings, specifically lemon fillingsAcid breaks down starch molecules so the paste is thinnerDecreases viscosityAcid effect can be minimized by adding after gelatinization or heating rapidlyHeat rapidly b/c lessens time the molecules can be hydrolyzed.If add lemon juice after gelatinization need to remember that it is part of the liquid component and not bound to starch granules: may decrease viscosity.
18Gelation As a starch paste cools, a gel is formed Free amylose molecules lose energy as the temperature decreases and form hydrogen bondsThe bonds create a network that holds the swelled granules in placeH-bonding can occur and break until gel is fully cooled.
19Gelation and Starch Source The more amylopectin (less amylose), the softer the gelPotato starch=high amylopectin=good thickening agent=soft gelCorn starch=less amylopectin=less effective thickening agent=strong gelWaxy starches=high amylopectin=very soft gel.Generally roots=good thickeners, cereals=strong gels=opaque gels due to protein content
20Gelation and Other Effects HeatingModerate temperature and rate of heatingEnough amylose needs to be released from the granule without the granule burstingAgitationAgitation during cooling disrupts amylose networkShould mix flavorings immediately after removing from heatIf temp. too high, will burst, if temp too low, limited amylose released.If stir while heating too much, will break up granules and get weakened gel.
21Gelation and Other Effects SugarDecreases gelatinization and amylose releaseSofter gelAcidDecreases gelatinization by hydrolysis of granules
22Aging Gels Syneresis Retrogradation Loss of water from a gel Amylose molecules pull together, squeezing water outRetrogradationRealignment of amylose moleculesHydrogen bonds break and reform into more orderly crystalsCan by reversed by gently heatingExamples: refrigerated pudding, stale breadWater is bonded to amylose and granules and free water is trapped, but not bonded, around the structure.Syneresis happens due to aging and when cut a gel and water leaks out.Retrogradation results in gritty texture. Heat breaks the H-bonds holding the amylose and lets them move within the gel.
23Dextrinization When starch is heated without water A higher temperature is reached than with waterBonds break throughout the starch forming dextrins
24Genetically-modified Starches Waxy starchHigh in amylopectinUsed in fruit pies because thickens well, but does not gel wellHave good freeze-thaw stabilityHigh amylose starchAmylose creates strong bonds to form strong gelsUsed in edible films to coat foodProblems with freezing and thawing are syneresis is pies. Not a problem in stews or other items that can be stirred so the water is recombined with the starch.Edible films used in cough drops, candy, coatings for deep fried potatoes, produce, baked products
25How to compare starches? Line spread test:Measures thickening powerPoor heated starch into cylinder, lift cylinder and measure spread after specified time using concentric circlesUniversal Texture Analyzer:Measures gel strengthPercent sag:Measure molded gel height and compare to unmolded gel heightStronger gel=small % sag, weaker gel=large % sag
26Modified Starches Physically or chemically modifying native starches Are used for specific applications in the food industry, Why?Native starches have undesirable qualities:Poor processing tolerance to heat, shear and acidPoor texturesDo not store, hold, and freeze/thaw wellPoor processing to heat/shear/acid: due to granules bursting which decreases viscosityFreeze/thaw: has to do with amylose binding
27Pre-gelatinized Starches Use:Instant puddingDehydrated gelatinized starchHeated so granule swells and then dehydratedSwells when water added, no heat necessaryDecreases preparation timePhysical change
28Thin-Boiling Starches Use:Pass freely through pipesAcid-hydrolyzed starchHydrolyzes 1,6-alpha-glucosidic bondsAmylopectin in smaller piecesDecreases thickening power, but makes a strong gel because hydrogen bonds form more readily
29Cross-linked Starch Use: Cross-linked starch molecules Increases storage time because of reduced retrogradationMore stable at high temperature, with agitation, and with acid additionSalad dressings, baby foods, pie fillingsCross-linked starch moleculesAlter hydroxyl ends under alkaline conditions by acetic anhydride, succinic anhydride, or ethylene oxideExample of retrogradation is in refrigerated puddings. Amylose doesn’t realign b/c altered hydroxyl group doesn’t line up well.Cross-links amylose
31Resistant StarchSmall intestine is unable to digest, limited digestion in large intestineClassificationsRS1: trapped in cells (seeds/legumes)RS2: native starch (raw potatoes, bananas, waxy maize)RS3: crystalline, non-granular starch (cooked potatoes)RS4: chemically modifiedCan contribute fiber to food without the fat that bran hasTakes up less water than other fiber, making dough less stickySmooth even textureLess than 3 cal/g