Megan Erickson Central Washington University Starch Megan Erickson Central Washington University

What is it? Complex carbohydrate made up of two components Components: Amylose Amylopectin Properties depend on amounts of the components

Where is it found? Roots/Tubers Cereal Potato Arrowroot Tapioca Corn Waxy corn Wheat Rice Waxy rice

Amylose Linear component of starch Contains 1,4-alpha-glucosidic bonds Molecular weight: less than 0.5 million Can form coils which will trap iodine and turn blue Every 6 glucoses in the helix can bind one iodine. Test used to find percentage of amylose in starch

Amylopectin Branched component of starch Contains 1,4-alpha-glucosidic as well as 1,6-alpha-glucosidic bonds Molecular weight: 50-500 million Limited coiling causes purplish-red color when iodine added Much larger in size than amylose Purplish-red due to lack of helix.

Amylose vs. Amylopectin Starches usually contain more amylopectin than amylose Generally roots/tubers contain more amylopectin than cereals Roots/Tubers: 80% amylopectin Cereals: 75% amylopectin Waxy corn and rice contain virtually all amylopectin

Starch Composition

Starch Granule Made in the cytoplasm of plant cells Amylopectin forms in concentric circles with amyose dispersed in between Held together by hydrogen bonds The granule swells when heated in water

Starch Granule When looked at under polarized filters at right angles to each other, starch looks like birefringence pattern (maltese cross). When heated, pattern disappears.

Functions Gelatinization Dextrinization Gelation Structure in baked products Thickener in sauces, soups, and dressings Dextrinization Gelation Pie filling

Gelatinization When starch is heated in water Hydrogen bonds break, allowing water to enter the granule and the granule swells Amylose migrates out of the granule H-bonding between water and amylopectin increases Reduced free water changes the viscosity of the starch mixture, thickening it

70 degrees, some imploding and some still swelling

Gelatinization and Temperature Gradually thicken with temperature Can be heated to 100oC without much granule rupture If held at 95oC will implode and lose viscosity Loses viscosity with prolonged heating

Gelatinization and Type of Starch Best thickening ability: potato starch Worst thickening ability: wheat starch More amylopectin=more translucent=more stringy Roots 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.

Viscosity and Type of Starch

Gelatinization and Sugar Used together in pie fillings and puddings Sugar competes with the starch for water so less water available for gelatinization Delays gelatinization and decreases viscosity Increases gelatinization temperature The more sugar added, the longer the delay Disaccharides have a stronger effect than monosaccharides Some cross-linking between starch and sugar also delays gelatinization. Sugar increases translucence.

Gelatinization and Acid Used together in fruit pie fillings, specifically lemon fillings Acid breaks down starch molecules so the paste is thinner Decreases viscosity Acid effect can be minimized by adding after gelatinization or heating rapidly Heat 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.

Gelation As a starch paste cools, a gel is formed Free amylose molecules lose energy as the temperature decreases and form hydrogen bonds The bonds create a network that holds the swelled granules in place H-bonding can occur and break until gel is fully cooled.

Gelation and Starch Source The more amylopectin (less amylose), the softer the gel Potato starch=high amylopectin=good thickening agent=soft gel Corn starch=less amylopectin=less effective thickening agent=strong gel Waxy starches=high amylopectin=very soft gel. Generally roots=good thickeners, cereals=strong gels=opaque gels due to protein content

Gelation and Other Effects Heating Moderate temperature and rate of heating Enough amylose needs to be released from the granule without the granule bursting Agitation Agitation during cooling disrupts amylose network Should mix flavorings immediately after removing from heat If 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.

Gelation and Other Effects Sugar Decreases gelatinization and amylose release Softer gel Acid Decreases gelatinization by hydrolysis of granules

Aging Gels Syneresis Retrogradation Loss of water from a gel Amylose molecules pull together, squeezing water out Retrogradation Realignment of amylose molecules Hydrogen bonds break and reform into more orderly crystals Can by reversed by gently heating Examples: refrigerated pudding, stale bread Water 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.

Dextrinization When starch is heated without water A higher temperature is reached than with water Bonds break throughout the starch forming dextrins

Genetically-modified Starches Waxy starch High in amylopectin Used in fruit pies because thickens well, but does not gel well Have good freeze-thaw stability High amylose starch Amylose creates strong bonds to form strong gels Used in edible films to coat food Problems 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

How to compare starches? Line spread test: Measures thickening power Poor heated starch into cylinder, lift cylinder and measure spread after specified time using concentric circles Universal Texture Analyzer: Measures gel strength Percent sag: Measure molded gel height and compare to unmolded gel height Stronger gel=small % sag, weaker gel=large % sag

Modified 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 acid Poor textures Do not store, hold, and freeze/thaw well Poor processing to heat/shear/acid: due to granules bursting which decreases viscosity Freeze/thaw: has to do with amylose binding

Pre-gelatinized Starches Use: Instant pudding Dehydrated gelatinized starch Heated so granule swells and then dehydrated Swells when water added, no heat necessary Decreases preparation time Physical change

Thin-Boiling Starches Use: Pass freely through pipes Acid-hydrolyzed starch Hydrolyzes 1,6-alpha-glucosidic bonds Amylopectin in smaller pieces Decreases thickening power, but makes a strong gel because hydrogen bonds form more readily

Cross-linked Starch Use: Cross-linked starch molecules Increases storage time because of reduced retrogradation More stable at high temperature, with agitation, and with acid addition Salad dressings, baby foods, pie fillings Cross-linked starch molecules Alter hydroxyl ends under alkaline conditions by acetic anhydride, succinic anhydride, or ethylene oxide Example of retrogradation is in refrigerated puddings. Amylose doesn’t realign b/c altered hydroxyl group doesn’t line up well. Cross-links amylose

Viscosity and Cross-linked Starches

Resistant Starch Small intestine is unable to digest, limited digestion in large intestine Classifications RS1: trapped in cells (seeds/legumes) RS2: native starch (raw potatoes, bananas, waxy maize) RS3: crystalline, non-granular starch (cooked potatoes) RS4: chemically modified Can contribute fiber to food without the fat that bran has Takes up less water than other fiber, making dough less sticky Smooth even texture Less than 3 cal/g