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Polysaccharides 1.

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Presentation on theme: "Polysaccharides 1."— Presentation transcript:

1 Polysaccharides 1

2 Introduction Those carbohydrates that consist of more than 10 (sometimes defined as >20) monosaccharide units linked via a glycosidic bond Glycosidic bond

3 Introduction Those carbohydrates that consist of more than 10 (sometimes defined as >20) monosaccharide units linked via a glycosidic bond Also called glycans They are classified as: Homo-polysaccharides Glycans composed of a single monosaccharide unit with one or more type of glycosidic linkages Example: Starch Hetero-polysaccharides Glycans composed of more than one type of monosaccharides Examples: Gums, dietary fibers, pectinic substances, hemicellulose

4 Starch Second largest biomass on earth, after cellulose
The predominant food reserve substance in plants where it occurs as starch granules 1. Seeds (e.g. peas, grains, beans) ? 2. Roots, tubers , stems (e.g. potato, tapioca, pineapple) ? 3. Fruits (e.g. plantain, bananas) ? 4. Leaves (e.g. tobacco) ? One of our main carbohydrate source (Two more?) Primary use in foods Vast number of uses and functionalities Naturally present in plant derived foods E.g. contributes to the texture of bread, cooked rice, pasta and potatoes, etc. Added to foods for viscosity, gelling, moisture retaining, stabilizing, texturizing, thickening, anti-staling, film-forming, binding, foaming applications, etc. etc.

5 Uses of Starch Flavors and Beverage Clouds Canning Cereals and Snacks
encapsulation of flavors, fats, spray dried flavors for dry beverage beverage emulsions liquid and powdered non-dairy creamers Canning filling viscosity aid suspension aid for particulates opacity agent body for soups, sauces, puddings and gravies aseptically canned products beverages such as coffee, teas or chocolate Cereals and Snacks hot extruded snacks chips, pretzels, etc. extruded and fried foods ready-to-eat cereals Cooked Meat Binder water binder for formed meat smoked meats, low-fat meats pet foods (dried and canned)

6 Uses of Starch Frozen Foods Bakery Dressings, Soups and Sauces
fruit fillings meat pies dairy products soups, sauces entrees cream-based products Bakery pies, tarts fillings, glazes custards and icings cakes, donuts Dressings, Soups and Sauces mayonnaise-type pourable salad dressings (high shear) spoonable dressings instant dry salad dressing mixes low-fat dressing canned gravies and sauces frozen gravies and sauces soups and chowders Confectionery dusting powder, licorice jelly gums, hard gums panned candies

7 Polymer of D-glucose units Two major forms:
CHEMICAL STRUCTURE Polymer of D-glucose units Two major forms: AMYLOSE : Large linear molecule with  1-4 glycosidic bonds Some may have a few branches 4 1

8 AMYLOSE : Large linear molecule with  1-4 glycosidic bonds
Some may have a few branches Linear, helical structure Strong helical complex Glucose units Importance of structure: Determines its packing ability e.g. Retrogradation

9 Polymer of D-glucose units Two major forms:
CHEMICAL STRUCTURE Polymer of D-glucose units Two major forms: AMYLOPECTIN: Highly branched and very large molecule Branches linked to the main chain via  1-6 bonds Branches very close giving amylopectin a cluster structure a (1-6) linkage 1 6

10 Polymer of D-glucose units Two major forms:
CHEMICAL STRUCTURE Polymer of D-glucose units Two major forms: AMYLOPECTIN: Highly branched and very large molecule Branches linked to the main chain via  1-6 bonds Branches very close giving amylopectin a cluster structure

11 AMYLOPECTIN: Highly branched and very large molecule
Branches linked to the main chain via  1-6 bonds Branches very close giving amylopectin a cluster structure Helical branches

12 Starch Summary of the different properties of amylose and amylopectin

13 Starch The structure and ratio of amylose/amylopectin varies with starch variety and age of plant Starch granule morphology also varies with plant source

14 Starch

15 Starch Different sizes and shapes of starch granules from different plant sources Polarized light micrographs of potato starch “Maltese cross” 10 μm Amylose and amylopectin inside the granules are tightly packed via extensive H-bonding -Crystalline property Granules are insoluble in cold water Corn starch (B) Wheat starch (C) Potato starch (D) Rice starch

16 Starch Different starch granules have also different x-ray diffraction patterns: A, B and C (combination of A+B) X-ray diffraction studies show that the starch granule is a mixture of crystalline and amorphous regions The crystalline region consists of starch in the form of a double helix – very stable Hydrophobic and H-bonds Insoluble in water at room T; need energy to break the bonds Starch double helix

17 Crystalline and Amorphous

18                                                                                                                                        <> Maltese cross

19 Pea starch granules Mutations can induce dramatic changes in starch granule morphology and thus modified function

20 Starch The aforementioned variations lead to different functional properties, e.g.: More H-bonds and packing  higher gelatinization temps. Need more energy to disrupt the double helix More phosphate groups  lower gelatinization T, higher viscosity and clarity of starch paste Electrostatic repulsion due to charged P-group Amylose/ Amylopectin ratio Great effect on gelation and texture of starch pastes Waxy starch: 0-8% amylose Disperses more easily, produces a clear viscous paste rather than gels Normal starch: 20-30% amylose High tendency to form gels High amylose starch: >50% amylose Very high tendency to form gels

21 Gelatinization of Starch
Room Temp (27oC) Heating (40oC) Raw Starch Swelling (50oC) Swelling (60oC) Rupturing (65oC) Gelatinization and pasting (70-90oC) Imploding (90oC)

When starch is heated sufficiently (to break the hydrophobic and H- bonds) in the presence of water the granule begins to swell (loses order): Granule imbibes water Viscosity is increased Clarity is increased Some amylose may leach out Get loss of birefringence (“Maltese cross” disappears) Starch goes from “glassy” to rubbery state (Tg) E.g. uncooked vs. cooked spaghetti The Tg varies depending on e.g. %H2O and starch type Starch Gelatinization T Corn 62-70ºC Waxy corn ºC Sorghum 68-75ºC Potato ºC Tapioca ºC

23 Starch PASTING Second stage swelling occurs when starch is heated above the gelatinization T This is needed to obtain maximum viscosity More amylose is leached out and it eventually gels (retrogradation – on cooling) Eventually the starch granule will completely disrupt The end point T needed to obtain the maximum viscosity is starch dependent Starch End Point T Corn 95ºC Waxy corn 75ºC Sorghum Potato 70ºC Tapioca 85ºC

24 Amylose (linear) Amylopectin (branched) A cereal starch gel showing how water is being trapped by the gel network

25 Starch granule Corn starch granules Wheat starch granules Amylose Amylose Starch was heated at 80°C for 30 min

26 Starch Changes in starch viscosity are evaluated using a Brabender amylograph Starch slurry is heated (and cooled) at a constant rate with constant stirring and viscosity measured Can get a lot of useful information from the amylographs E.g if starch has high peak viscosity then you know they need more energy to stir and manipulate By comparing different starches you can pick the one just right for your application

27 Peak viscosity

28 Each type of starch has a characteristic gelatinization/pasting profile

Gelatinization only occurs with heat and water (Dry heating = Dextrinization)

When heating begins, water absorbed on granule surface; granules still clumped together (30oC)

At 40oC, more water absorbed and granules start separating

At 50oC, more water absorbed and granules start swelling; H-bonds within granule broken; Amylose leak out

At 65oC, more water absorbed and granules start rupturing;

At 70oC, more rupturing and leakage, Gelatinization: Loss of birefringence (Maltese cross disappears), more viscous, translucent, enzyme action; more clarity

At 90oC, optimum gelatinization; granules may disintegrate or implode, Overcooking could decrease viscosity !!

36 Starch On cooling, gelatinized starch retains some of its crystal structure –Retrogradation Starch paste on cooling form intermolecular bonds – gel Gel structure mostly from Amylose bound to each other and from slight interaction with Amylopectin molecules. This phenomenon is called retrogradation Critical for ultimate rigidity of final product

37 Starch Retrogradation and staling
On cooling, amylose line-up and form H-bonds. They form crystals- Retrogradation Starch granule After gelatinization, amylose is removed from the granule

38 Viscosity Gelation

39 Starch Ratio of amylose to amylopectin and their size is important for the different behaviors seen:  Amylose swelling greater loss of peak viscosity Amylose  swelling stronger gels Larger amylose/amylopectin molecules More viscosity Weaker gels Chemical groups and interactions with other ingredients also play an important role E.g. potato starch and phosphate groups Cereal starches and phospholipids

40 Starch Uses of High Amylose starch Stronger gels
High gelling capacity useful for making candies – decreases hardening times from 72 h to 24 h – reduces cost Firm, crisp, crunchy films – coating battered products such as French fries, frozen fish, poultry, vegetables

41 Starch Uses of low Amylose starch Viscous pastes with stringy texture
Stabilizers and all-purpose thickeners for food products – particularly those which undergo large temperature changes during processing Waxy starch do not lose water during freezing and thawing – frozen food products Emulsifier for salad dressings

42 Starch Examples of starch properties and function
Cereal starches (corn, wheat, rice etc.) A type (low mol. weight amylose and amylopectin with short branches) High degree of packing/crystallinity Higher gelatinization/pasting T Low swelling power (10-20 g H2O/g dry starch) Viscous, short bodied paste Opaque gels on cooling

43 Starch Examples of starch properties and function
Root and tuber starches B type (high mol. weight amylose and amylopectin with long branches) Low degree of packing/crystallinity Lower gelatinization/pasting T High swelling power ( g H2O/g dry starch) Highly viscous, long bodied paste Weak clear gels

44 Starch Examples of starch properties and function
Waxy starches (waxy corn) Genetically modified (bred) to contain only amylopectin Less order and less packing (why? AP?) Higher swelling power than cereal starches Very heavy bodied and stringy No or poor gelation

45 Starch Retrogradation and staling
When a hot starch is cooled it generally forms a viscoelastic, firm and rigid gel or precipitate starch molecules are realigning and forming double helixes that then will aggregate  starch becomes progressively less soluble Applies to the linear portion of the starch molecules This phenomena is called RETROGRADATION Happens far more rapidly with amylose vs. amylopectin During heating amylose leeches out of the starch granule and gels on cooling High mol. weight amylose less prone to escape High conc. of amylopectin, e.g. in waxy maize, leads to less/slower retrogradation

46 Starch Retrogradation and staling
Retrogradation is the cause behind firming of bread as it cools after baking Causes bread to become STALE on storage First event: during baking amylose leeches out and retrogrades almost fully on cooling  positively affects texture by giving an elastic and tender crumb Second event: On storage amylopectin branches slowly associates  crumb and bread hardens = STALING Happens faster during refrigeration but is inhibited (slowed down during freezing)

47 Starch Some factors affecting retrogradation
Size of linear starch molecule Smaller amyloses have more tendency than large to closely associate into “crystal precipitates” Amylose/amylopectin ratio More amylose = more and faster retrogradation (Why?) The branches of amylopectin participate in retrogradation at later stages Rate of cooling Fast  Gel Slow  Crystal precipitates (retrogradation)

48 Starch Some factors affecting retrogradation 4. Temperature
Cooling accelerates Freezing stops Heating reverses (e.g. bread) 5. Interfering molecules Fatty acids Form insoluble complex with starch and interfere with retrogradation Emulsifiers/surfactants (both hydrophilic and hydrophobic nature) Inhibit crystallization by complexing with helical amylose Commonly used in dough for bread and other baked goods to increase shelf life

49 Starch Interactions of starch with other molecules Iodine
I3- complexes with both amylose (blue color) and amylopectin (red-purple color – branches are too short) helixes Used to determine amount of amylose in starch Has to be linear Over 45 units Under 12 units – no color

50 Starch Interactions of starch with other molecules
Fatty acids and polar lipids (e.g. emulsifiers) Bind tightly into the hydrophobic region of the helix Interfere with gelatinization and pasting Entangle starch molecules and lead to less swelling, more opacity and “short” viscosity Inhibit crystallization (retrogradation) Surface active lipids can coat the starch granule and make it “waterproof” Water If too low  no gelatinization (need >60%) Sugars Compete for water  retard/delay gelatinization (Tg) pH Acid can cleave the glycosidic bonds  less swelling/viscosity

51 Starch Starch modification Necessary for many food applications
Native starch is very sensitive to harsh processing and storage conditions: Low pH High and low Temperature Pumping Storage and transportation Modifications can be chemical, physical and enzymatic Limited number of modified starches approved by FDA Have to be labeled as “modified food starch”

52 Starch Starch modification 1) Hydrolysis (Hydro - lysis)
Use acid to hydrolyze starch into smaller units Need high heat (hot acid sprayed over starch) Limited to non-crystalline region Dark pigments and off-flavors can develop Changes induced by decreased chain length 1. Sweetness increases (Why?) 2. Increased reactivity as reducing sugar (Why?) 3. Decreased viscosity and low peak viscosity (Why?) 4. Increased solubility (Why?) 5. Increased fermentability 6. Less swelling on heating Get an increase in Dextrose Equivalence (DE) with increased hydrolysis DE=100/DP (Measure of reducing group) DP = Degree of Polymerization

53 Starch Starch modification (Hydrolysis cont.)
Products of acid hydrolysis Thin boiled starch Starch heated and sprayed with hot acid (or treated with hot HCl gas)  Starch neutralized  Starch washed and dried Starch granule remains “intact” Granule breaks easily when heated in water Clearer and stronger gels than native starch Gum candy (jelly beans, jujubes) Films and adhesives (candy and nut coatings) Processed cheese

54 Starch Starch modification (Hydrolysis cont.)
Can also use enzymes to hydrolyze starch a – amylase Endo-enzymes = cleave the glycosidic bond within starch in the non-crystalline regions (cleaves a- 1-4 linkage) Products are DEXTRINS (small starch units), MALTOSE and MALTOTRIOSE a -amylase

55 Starch Starch modification (Hydrolysis cont.)
Can also use enzymes to hydrolyze starch -Amylase Exo-enzyme = hydrolyzes starch at the non-reducing end Product is MALTOSE (cleaves a- 1-4 linkage) b-amylase

56 Starch Starch modification (Hydrolysis cont.) Amyloglucosidase
Exo-enzyme = hydrolyzes starch at the non-reducing end at -1-4 and -1-6 linkages Product is D-GLUCOSE (low calorie beer) Amyloglucosidase

57 Starch Starch modification (Hydrolysis cont.)
Iso-amylase & Pullulanase -> hydrolyze -1-6 bonds (remove branches) Iso-amylase

58 Starch Starch modification (Hydrolysis cont.)
Products of enzymatic hydrolysis Dextrins Heated starch treated with enzymes (can also use acid) Can produce a range of partially hydrolyzed starch polymers Maltodextrin <20 DE Bland flavor, no sweetness Good bulking agents Good humectants/water absorbers at ~20 DE Fat-like properties at ~5 DE 1-4 kcal/g

59 Starch Starch modification ( Hydrolysis cont.)
Products of enzymatic hydrolysis Corn syrups A combination of enzymes used to produce corn syrups Corn syrup solids Hot starch hydrolyzed with enzymes and then dried 20-60 DE High Fructose Corn syrup (liquid) Gelatinized starch + enzyme  dextrose (liquid glucose)  glucose isomerase  Fructose (42%) + Glucose (58%)  Cation exchange column  high fructose corn syrup (>55% fructose) e.g. Colas

60 Starch modification Cross linked starch
Derivatized starch or substituted starch Anionic (negative charges) Cationic (positive charge) Non-ionic (hydroxy alkyl starch)

61 Starch Starch modification 2) Cross-linked starches
Most common chemical modification method Can react starch with a variety of bi-functional cross-linking reagents E.g. sodium trimetaphosphate at alkaline pH followed by drying Get phosphate diester bond (only need a few) Results resistance to granule rupture and degradation Firmer texture and viscosity Can tolerate harsh processing conditions Acid, shear and thermal treatment (e.g. canning) Very stable on processing and storage

62 Degree of cross linking
Application Lightly cross linked Neutral and slightly acidic foods Medium cross linked High acid foods;delay gelation for some canned foods; foods stored at low temperature; Highly cross linked High shear, high temperature

63 Starch Starch modification 3) Derivatized or substituted starches
Can react OH groups of starch with various compounds to change function A) Oxidized starches React starch with hypochlorite at pH 9-9.5 Increased whiteness and partial hydrolysis More electrostatic repulsion  clearer pastes and less retrogradation and more stability Cake flour and frying batter

64 Starch Starch modification (Derivatized starches cont.)
B) Phosphorylated starches Treat starch with phosphates or polyphosphates Starch gets a negative charge and thus there is more electrostatic repulsion between the starch molecules Lower Tg, viscosity + clarity + stability (low retrogradation) Can control viscosity by salt addition Add salt and viscosity drops (“screens” electrostatic repulsion) This starch is useful in products that are to be frozen and thawed and it is an excellent thickening agent

65 Starch Starch modification (Derivatized starches cont.)
C) Hydroxypropylated starches Treat starch with propylene oxide (Starch-O-CH2-CHOH-CH3) at alkaline pH Great cold-storage and freeze-thaw stability = clear paste with no retrogradation  used in frozen foods and desserts as thickening agent Also smooth texture and often used to improve viscosity under acidic conditions

66 Starch Starch modification (Derivatized starches cont.)
D) Acetylated starches Starch treated with acetic anhydride at alkaline pH Can only be used at very low acetylation levels in foods Low Tg, improved freeze-thaw stability, low retrogradation and good clarity E) Succinated starches (Butanedioic acid ) Starch treated with succinic anhydride at alkaline pH Get a negative charge = repulsion (similar to phosphorylated starch) solubility and viscosity Can make by treating with octenyl succinic anhydride (has a hydrophobic part) Now have a hydrophilic and hydrophobic starch Good emulsifier (stabilize both water and lipid phase)

67 Starch Starch modification 4) Physically modified starches
A) Pre-gelatinized starch Starch paste heated in a drum dryer where granules almost instantaneously gelatinizes and pastes  paste dries off and is made into a powder The powder is now soluble in cold water! Provides instant viscosity upon addition (no heat needed) Applications Instant soups/puddings Pizza toppings Instant cakes etc.

68 Starch Starch modification (Physically modified starches cont.)
B) Cold water soluble/swelling starch Starch (corn) heated in 70-90% ethanol (alcohol) Swells extensively in cold water Dispersible in sugar solutions (when stirred rapidly) Pour in molds and you get gum candy on setting Used in muffin batters to hold particles (e.g. berries)

69 Starch Starch modification (Physically modified starches cont.)
C) Resistant starch Highly crystalline starch formed by repeated freezing and thawing (retrogradation) of high amylose starch Non-crystalline regions removed by enzymes and the rest is resistant starch Used as dietary fiber (non-digestive)

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