Polysaccharrides Flashcards
What is a homopolysaccharide?
These are composed entirely of the same sugar. Homopolysaccharides include:
-
Pentosans
- Arabans
- Xylans
-
Hexosans
-
Glucans
- Starch
- Dextrins
- Glycogen
- Cellulose
-
Fructans
- Inulin
- Levan
- Galactans
- Mannans
-
Glucans
What is a heteropolysaccharide?
These are composed of more than one sugar component. Heteropolysaccharides include:
- Hemicellulose
- Gums
- Mucilages
- Pectic substances
- Mucopolysaccharides
What is a polysaccharide?
Polymers of mono-/oligo-saccharides joined by glycosidic linkages, which are easily broken by acid or temperature application. May be linear or branched. Greater utility than simple sugars.
What is DP?
The # of monomers in a polysaccharide is referred to as the degree of polymerization (DP).
DP range: 200-3000
Describe the anomeric configuration of starch vs. cellulose.
Cellulose ⇢ glucose units are linked in beta form (cannot digest; lacking enzyme)
Starch ⇢ glucose units are linked in alpha form (alpha-amylase can digest)
Describe the possible anomeric configurations of polysaccharides.
- alpha ⇢ C1 hydroxyl in same direction as hydroxyl of C4
- beta⇢ C1 hydroxyl in different direction as hydroxyl of C4
What is starch?
Starch is the main component of cereals, roots, and tubers. Less crystalline than cellulose. Amylose and amylopectin DP > 60,000. Packed into granules consisting of a mixture of amylose and amylopectin. Polymer of glucose.
Starch is packed into granules consisting of a mixture of amylose and amylopectin. What is the ratio?
amylose: amylopectin
20: 80
or
30:70
Different ratio, different functionality. (e.g., sticky rice vs. basmati rice)
What is cellulose?
Insoluble, structural component of the primary cell wall of green plants. Polymer of glucose.
DP = 7000 - 15,000
What is amylose?
linear chain of alpha ⇢ 1,4 linked D-glucopyranosyl units; forms helical structure
Amylose and cellulose are both linear polymers of glucose linked with 1,4-bonds. The main difference is the anomeric configuration.
What is amylopectin?
large, branched; glucose units are linked in a linear way with alpha ⇢ 1,4 bonds; branching takes place with alpha ⇢ 1,6 bonds ever 24-30 glucose units
Describe the starch granule. [6]
- Contains amylose and amylopectin; ratio depends on type
- Crystalline and non-crystalline regions
- Crystallinity produced by ordering of pectin chains.
- Morphology can identify starches and prevent adulteration in the industry.
- When dry = crystalline
- Starch granule characteristics: arranged radially ⇢ gives maltese cross under plane-polarized light; centre of cross is at hilum (i.e., origin of growth)
Describe gelatinization. [3]
- Disruption of molecular order within granules results in loss of birefringence
- Starch granules undergo this process when heated in hot water.
- Amylose’s linear structure gelatinizes first
Why are undamaged starch granules insoluble in cold water?
Due to the strength of the hydrogen bonds that hold starch chains together, as well as hydrophobic interactions.
Describe characteristics of gelatinization.
- larger granules gelatinize first
- temperature depends on the starch (water ratio, granule type/size, heterogeneities within the granule, ratio of amylose:amylopectin)
- Occurs over a temperature range
What are the steps in the process of gelatinization? [4]
- Starch granule begins to hydrate: granule swells
- An increase in starch solubility and clarity occurs with a parallel rapid increase in viscosity.
- Dissolution of amylose and diffusion out from the granule.
- Irreversible swelling, loss of birefringence, loss of crystallinity, granule will burst: amorphous gel product
Describe retrogradation.
- Rupture of the granule, produces a paste (soluble amylose or amylopectin); retrogradation when cooling of hot paste = gel formation (polysaccharides interact with each other).
- Retrogradation is a realignment of amylose and amylopectin chains after gelatinization occurs.
- The linear components form junction zones; that is, hydrogen bonds at lower temperature = crystalline structure
Discern between initial and advanced retrogradation.
- Initial retrogradation causes liquid to turn to gel form
- Advanced retrogradation results in syneresis where water is expelled = amorphous structure (polysaccharides begin to recrystallize)
Amylose retrogradates at a much faster rate due to its linear structure. What else does the rate of retrogradation of starch depend on? [5]
Rate depends on:
- source of starch
- the ratio of amylose to amylopectin
- temperature
- starch concentration
- presence of other ingredients (e.g., salt, surfactants; they bind water and reduce rate of gelatinization)
Describe the effect of temperature and pH on gelatinization.
- Initial increase in viscosity due to solubilization of amylose and hydration of amylopectin
- pH results in acid hydrolysis of glycosidic bonds = low WHC = lower viscosity; dextrins produced by hydrolysis leave the granule; granule bursts
- During cooling, paste formation contributes to the increase in viscosity (optimal pH for maximum viscosity is around neutrality)
What are 6 factors that considerations for starch retrogradation?
- The water content and storage temperature (i.e., cooling rate) are major factors influencing the rate of retrogradation.
- Amylose content
- Retrogradation MT increases and enthalpy of retrogradation decreases after ultrasound processing (breaks bonds, decreases MW of starch, changing the size/structure of starch crystallinity complex)
- Extrusion (shear effect) weakens the interaction between chains (affects morphology and crystallinity)
- Effects of microwave do not destroy chemical bonds, but changes viscosity due to the mobility of the molecules when exposed to radiant energy.
- High pressure (non-thermal to avoid heat damage; maintain nutrient content) changes the morphology, rheology, and thermal properties of starch granules (destroys structure; reduces crystallinity, inhibits retrogradation)
What identifies the time when gelatinization begins?
- The Tp value; dependent on type of starch
- The Tp value is the temperature of plasticization associated with the solubilization of amylose
What is staling? What is it correlated to? [3]
Transition from amorphous to partially crystalline within intact granule (e.g., crust on bread - protects from mold & retrogradation)
Correlated to (1) starch recrystallization (retrogradation of amylose and some amylopectin), (2) the storage of the product, and (3) the storage temperature.
Discuss bread stored with crust versus bread stored without crust.
Moisture redistribution from crumb to crust plays a significant role in firming; bread stored with crust becomes significantly firmer than bread stored without crust and contained more recrystallized amylopectin.
Packaging stops water transfer from crust to the surrounding atmosphere so starch in the crumb remains moist.
What is DE?
The degree of hydrolysis measured in dextrose equivalency (DE).
DE = 100/DP
What is liquefaction?
Glycosidic bonds of starch can be hydrolyzed by acids or enzymatic action = liquefaction.
What are the steps of starch hydrolysis? [4]
- Commercial hydrolysis of starch starts with sprayed HCl gas.
- The mixture is heated until the desired degree of polymerization.
- The acid is neutralized.
- Product is recovered, washed, and dried.
Describe the production of hydrolysis products.
- Solubilize starch by adding water, adjusting the pH to 6.5 and stabilizing it with calcium salts = forms starch slurry.
- Heat slurry to 105C; amylose fraction dissolution and diffusion from granule; addition of alpha-amylase is added to hydrolyze amylose to dextrins.
- Liquefaction (=complete hydrolysis of amylose)
- Liquefied starch may be saccharified to glucose (97% D-glucose) or maltose syrup (56% maltose and 28% maltotriose).
- Converted to glucose syrup (99DE) by addition of glucoamylase, and pullulanase at 60C for 72h = almost complete hydrolysis.
- Converted to maltose syrup (44DE) by addition of fungal alpha-amylase, at 55C for 48 hours (shorter time) = only partial hydrolysis.
Describe substitution of starch hydroxyl groups.
- Strategy to control gelatinization; reduces capacity of amylose to solubilize (=more energy required for swelling)
- Substitution of starch hydroxyl group by acetylation or by introduction of any group which is less hydrophilic than hydroxyl group
- Increases water resistance of starch
- Common derivatives = hydroxypropyl ether, acetate ether, monostarch phosphate ether
Describe cross-linking of starches.
- Occurs when starch granules are reacted with bifunctional reagents that react with hydroxyl groups on two different molecules within the granule
- Connections to other molecules controls gelatinization
- used in canned soups, gravies, puddings, batter mixes, pie fillings
- Starch is reacted during gelatinization with phosphoryl chloride, sodium trimetaphosphate, or adipic acid in an alkaline slurry, then dried with heat
- Product is able to resist heat, shear, and acids.
Describe the functional properties of cross-linked starches.
- Increased resistance to swelling and gelatinization due to reduced capacity of amylose to solubilize
- Reduction of cohesive, rubbery, elastic characteristics
- Higher tensile strength
- Storage-stable thickening
- Heat and acid stable
High fibre bread takes longer to chew and longer to digest (=greater sensory specific satiety than low fibre bread).
Discuss the relationship between viscosity and glycemic index.
- Greater fibre content increases viscosity of gastric contents (= decreased accessibility for digestive enzymes)
- Inverse relationship between viscosity and glycemic index
Describe Beta-D-Glucan.
A water soluble, unbranched polymer; dietary fibre
B-D-glucopyranosyl units; 70% 1-4 linkages and 30% 1-3 linkages
Found in cereal fibre, wheat, oats, and barley
Increases fecal bulk, increases viscosity of intestinal contents.
Describe gums. (e.g., guar gum, seaweed gums)
- Dietary galactomannan, water soluble with 1,4 and 1,2 linkages present
- Form viscous gels within the intestine upon ingestion leading to reduced mineral bioavailability in the small intestine due to ability to sequester magnesium and calcium ions.
- However, fermentation releases trapped calcium in the large intestine.
- Free calcium enables paracellular calcium absorption to compensate for the reduced calcium absorption occurring in the small intestine.
Describe Inulin [7].
- Soluble fibres
- Fructose ‘cousin’ of glycogen (i.e., branched fructooligosaccharide)
- Has terminal glucose; less soluble than oligofructose
- Fat replacer; smooth, creamy texture; non-digestible; reduces caloric content of foods
- Prebiotic (i.e., can be fermented)
- Benefits vary based on duration of fermentation, age, and health status (i.e., no typical outcome)
- Competitive exclusion: prevents pathogenic bacteria from proliferating in the gut microbiome; pre-biotics facilitate the success of probiotics (e.g., bifidobacteria, lactobaccili)
- Present in many vegetables like onions, leek, garlic, green bananas, asparagus, etc.
Describe LM pectins.
- Lacks capacity to form junction zones; weaker than HM pectins
- Thermoreversible gels on cooling
- Addition of cation (e.g., Ca2+) associates with methoxy group and forms ‘eggbox cavity’ (= increased water holding capacity)
How is the gelation in LM-pectins controlled?
- Through interactions of unesterified, charged galacturonic acid residues (a.k.a. pectic acid) with divalent calcium cations.
- They interact with neighbouring molecules to form junction zones with ‘eggbox cavities’.
- LM pectins gel over a wide range of sugar content, as low as 10%, and within a wide pH range, pH > 5, making them very suitable for low-sugar jams and jellies.
- Form gels at higher pH than HM pectins; gels produced are weaker.
- Syneresis can occur; water-binders often added (e.g., gums, xanthan gum), to help hold the water in the LM gels.
Summarize LM vs HM pectins.
Hypercolloids; polysaccharides that form viscous solutions or dispersions when in cold or hot water.
Ability to form gels is associated more with the degree of esterification and not molecular weight.
HM pectins: gelling facilitated by reducing pH ~2.5-3 and adding soluble sugar content >55%. (= WHC of sugars and ionic forms of the galacturonic acid chains holding the water in a 3D gel form)
LM pectins: does not gel strongly; cation addition forms an ionic linkage that enables junction zones to associate and entrap water; susceptible to syneresis.
