Carbohydrates 2 Flashcards
what are homopolysaccharides?
PS consisting of only one type of monosaccharide
what are the functions of polysaccharides in plants?
energy storage (starch), structure (cellulose)
what are the functions of polysaccharides in humans?
energy (glycogen), colon health (fibre)
what are the functions of polysaccharides in foods?
texture and stability (starch, gums, pectin)
what is DP, degre of polymerization?
the number of monosaccharide units in a polysaccharide
what is the general scientific term for polysaccharides?
glycans
what is non-freezable or plasticizing water?
water naturally hydrogen-bonded to polysaccharide molecules. it will not freeze. they can exchange freely and rapidly with bulk water molecules.
what happens when a polysaccharide solution is frozen?
a two-phase system of ice and a glass consisting of about 70% PS molecules and 30% nonfreezable water.
the viscosity of a polymer solution is a function of
the size and shape of its molecules and the conformations they adopt in the solvent
the shapes of polysaccharide molecules in solution are a function of
the allowable rotations about the bonds of glycosidic linkages
which polysaccharides produce the greatest viscosity?
longer, linear, more extended, more rigid
why does a highly branched PS give lower viscosity?
it occupies less volume and collides less often in solution
why can unbranched glycans with regular repeating unit structures precipitate or gel rapidly?
segments of the long molecules associate in a zipper-like fashion. more molecules associate and form a crystallite, which may precipitate. amylose retrogradation is and example of this
what are the two general kinds of rheological flow?
pseudoplastic and thixotropic
what is pseudoplastic flow?
the greater the applied force, the less viscous it becomes. upon removal of the applied force, the solution regains its initial viscosity instantaneously
what is thixotropic flow?
the viscosity reduction that results from an increase in the rate of flow does not occur instantaneously. the viscosity decreases under a constant rate of shear in a time-dependent manner, and regains the original viscosity after cessation of shear.
what is a gel?
a continuous, three-dimensional network of connected molecules or particles entrapping a large volume of a continuous liquid phase
in many food products, the gel network consists of…
PS or protein molecules or fibrils formed from polymer molecules joined in junction zones by hydrogen bonding, hydrophobic associations, ionic cross bridges, entanglements and/or covalent bonds over small segments of their lengths, while the liquid phase is an aqueous solution/dispersion of low-molecular weight solutes and segments of the polymer chain not involved in junction zones
a gel can be described as a viscoelastic…
semisolid, partly that of an elastic solid and partly viscous liquid
what is syneresis?
the expulsion of liquid from a gel
what is amylose?
a linear chain of alpha-1-4 linked glucose units
what is the shape of amylose?
the linkage gives the molecule a helical shape. the interior of the helix contains a predominance of hydrogen atoms and is hydrophobic
what is amylopectin?
alpha-1-4 linked glucose units with branching alpha-1-6 linked chains
how is the amylopectin molecule shaped?
longer, unbranched sections (C chain, only reducing end) that carry B chains, which in turn carry A chains of which two adjacent chains form a double helix. “backbone model”
what is a waxy starch?
one that contains only amylopectin
what catalyzes polysaccharide hydrolysis?
acids and/or enzymes
what are gums?
branched, soluble PS. bind water and give a high viscosity, stabilize emulsions, smooth texture, gelling agents.
what are gums physiological effects?
cholesterol drain, lower GI
how is the starch granule structured?
granules contain both semicrystalline and non-crystalline (so called amorphous) regions or shells in alternating layers, like onions. the semicrystalline or more dense layers contain greater amounts of crystalline structure. the crystalline framework is largely provided by amylopectin and is stabilized by hydrogen bonds among and within the chains
how do undamaged starch granules behave in cold water?
they swell slightly and return to their original state upon drying
what is gelatinization?
starch granules heated in water gelatinize. crystallite melting and unfolding of double helical structures due to the disruption of hydrogen bonds stabilizing starch chains within the native granules. amylose leaks out.
continued heating of gelatinized starch granules in excess water leads to…
further granule swelling, additional leaching of amylose and eventually, with application of force, total disruption of starch granules. this results in a starch paste
what is the effect of water in starch gelatinization?
when starch granules are in the presence of sufficient water (>60%) and a specific temperature Tg is reached, the plasticized amorphous regions of the granule undergo a phase transition from a glassy state to a rubbery state. water acts as a plasticizer. water molecules enter between chains, break interchain bonds and establish hydration layers around the separated molecules. these hydration layers plasticize (lubricate) the chains such that they become more fully separated and solvated.
what is Tg?
the glass transition temperature
what happens when a gelatinized starch is stirred?
highly swollen granules can easily disintegrate from stirring, resulting in a decrease in viscosity
enzymatic degradation of starch is used to produce…
glucose syrup, maltose syrup, corn syrup and high fructose corn syrup
why does HFC syrup fetch a higher price than glucose syrup?
since fructose is sweeter than glucose less substance is needed and it could be considered a sweetener of higher quality if the purpose is to maximize sweetness
what is the action of the pullalanase enzyme?
it cleaves alpha-1-6 branching linkages in starch
what are the steps of enzymatic degradation of starch(lecture)?
- heating of starch in acidic water with a thermostable alpha-amylase produces oligosaccharides. -temperature lowered to 60 degrees, pullalanase is added along with either glucoamylase to produce glucose syrup or beta-amylase to produce maltose syrup. - if glucose is produced and the desired end product is fructose, glucose isomerase is added. - pH and temp needs to be adjusted during the process to optimize enzyme action
what is the action of the beta-amylase enzyme?
it cleaves glucose chains at alpha-1-4 linkages, cleaving every other glycosidic bond to create maltose (disaccharide). it cannot cleave 1-6 branching points and leaves a “pruned” amylopectin residue named a limit dextrin
what is the action of the glucoamylase enzyme?
it sequentially releases single D-glucosyl units from non-reducing ends of amylose and amylopectin, even including 1-6 bonds (at a lower rate)
what is the action of the glucose isomerase enzyme?
it catalyzes isomerization of glucose to fructose
what is the biological function of pectin?
it is found in the primary cell wall and middle lamella and holds cells together
what happens with pectin in cooking and cooling?
it is solubilized during cooking and forms good gels upon cooling
why is a better pectin gel formed at low pH?
high concentration of H+ neutralizes COO- found in the pectin backbone of alpha-galacturonic acid. neutralized these do not repel each other. methyl groups have the same effect.
how are starches chemically modified?
the hydroxyl groups are used as handles for chemical modification, the 6th hydroxyl group on C6, which protrudes, is most easily accessed and most reactive and thus usually the one to be modified. citric acid can be used to crosslink amylose and amylopectin for a more stable network.
hydroxypropylated starch is commonly used to form
more stable solutions, it is pH-resistant and has better film forming properties.
unmodified potato starch is used in…
extruded cereal foods and snacks, and in dry mizes for soups and cakes
cooling a hot starch paste produces…
a firm viscoelastic gel, as junction zones are formed
what happens as plant tissue is heated?
- semicrystalline starch granules take up available water within cells and undergo swelling and gelatinization. - slightly higher temperature required than in isolated starch (due to the presence of solutes). - granule swelling is limited by the boundaries of the surrounding cell walls. - a high starch content and swelling capacity is generally more effective in binding up free moisture within the cooked tissue, producing a dry mouthfeel
why are amylose chains able to form complexes?
they are helical with hydrophobic interiors, in this space linear hydrophobic molecules may fit and form complexes.
starch molecules are depolymerized by
hot acids
incomplete hydrolysis of cooked starch with either acid or enzymes produces…
mixtures of maltooligosaccharides, called maltodextrins
what is DE, dextrose equivalency?
DE = 100/DP. DE is a products reducing power as a percentage of the reducing power of pure D-glucose (dextrose), DE is thus inversely related to molecular weight.
what are maltodextrins?
products with DE values measurable but below 20, that is average DP >5
how is glucose syrup/corn syrup made?
a slurry of starch in water is mixed with a thermally stable alpha-amylase and subjected to rapid gelatinization and enzyme-catalyzed hydrolysis (liquefaction) takes pleace. after cooling to 55-60 deg C, hydrolysis is continued with glucoamylase. then the syrup is clarified, concentrated, carbon-refined and ion-exchanged
why would you want to modify starches?
native starches generally produce weak-bodied, cohesive, rubbery pastes when cooked and undesirable gel properties and stability when the pastes are cooled
chemical modifications of starch include
cross-linked, stabilized, oxidized and depolymerized starch products
physical modifications of starch include
pregelatinized, cold-water swelling/cold water soluble, reduced or slowed digestibility starches
cross-linked starches have…
higher gelatinization and pasting temperatures, increased resistance to shear, improved stability to low pH conditions and generally produce psates with greater viscosity and temperature stability as compared to the base starch.
stabilized starch products have
lower gelatinization and pasting temperatures, are easier to redisperse when pregelatinized, and produce pastes and gels with a reduced tendency for retrogradation, that is, greater stability, improved freeze-thaw stability and greater clarity compared to the base starch
hypochlorite-oxidized starch products have
lower gelatinization and pasting temperatures, produce a lower maximum paste viscosity, result in softer and clearer gels and are whiter
thinned (slightly depolymerized) starch products have
lower gelatinization and pasting temperatures and produce hot pastes with less viscosity, but exhibit increased gel strength upon cooling
what is starch stabilization?
starch products esterified or etherified with monofunctional reagents resist interchain associations, which reduces the tendency for precipitation
what is cross-linking of starches?
occurs when starch granules are reacted with bifunctional reagents that react with hydroxyl groups on two different molecules or adjacent chains within the granule. this reduces rate and degree of granule swelling and subsequent disintegration since the granule structure is reinforced
what is pregelatinized or instant starch?
gelatinized and pasted starch that is dried into a powder. the resulting products are cold-water soluble and will produce viscous dispersions when stirred into room-temperature water
what is cellulose?
a high-molecular-weight, linear, insoluble homopolysaccharide of repeating beta-D-glucose units linked with 1-4 bonds
how is microcrystalline cellulose made?
by partial hydrolysis of purified wood pulp cellulose, with hydrolysis taking place in the amorphous regions, followed by separation of the released microcrystals.
what are amorphous regions of cellulose?
cellulose molecules are linear and fairly rigid, and can easily associate in long junction zones, however the chains do not align perfectly, which creates breaks in the crystalline structure. here cellulose chains diverge and arrange more randomly
what are guar and locust bean gums?
ground endosperm of seeds, thickening polysaccharides. both consist mainly of galactomannans, 1-4 joined beta-D-mannose with single unit alpha-D-galactose branches attached to the O6 oxygen. LBG is more irregular and less branched
how do guar gum and LBG differ practically?
guaran has more evenly distributed branches and is less prone to formation of junction zones. LBG has longer (~80 units) stretches of unbranched mannoses that can form junction zones and requires heating to 90 degrees for complete solubilization
how is xanthan produced?
xanthomonas campestris, a bacteria, produces a polysaccharide called xanthan. it is produced in large fermentation vats.
what is the structure of xanthan?
a backbone chain indentical with cellulose. every other glucose has a trisaccharide side chain on the O3. the trisaccharide consists of mannose, glucuronic acid and mannose. the trisaccharide side chains interact with the main backbone and make the xanthan chain rather stiff. it is structured as a double-stranded helix
how do xanthan and LBG interact?
the unbranched sections of LBG interact with xanthan and produce an increase in solution viscosity, which produces a heat-reversible gel
what are some of the characteristics of xanthan that makes it so widely used?
- soluble in both hot and cold water - high solution viscosity at low conc - no discernible change in viscosity between 0-100 deg C - strong pseudoplastic behaviour - soluble and stable in acidic systems - excellent compatibility with salt - stabilizing emulsions, gels, and freezing/thawing well - does not increase viscosity upon cooling which makes it irreplaceable for thickening and stabilizing ex salad dressing
what is carrageenan?
a group of sulfated galactans extracted from red seaweeds with dilute alkaline solutions. linear chains of D-galactose joined with alternating 1-3-alpha and 1-4-beta linkages, often esterified with sulfate half-ester groups at C2/C6. extracted carrageenans are mixtures of nonhomogenous polysaccharides. three main behavioural types: kappa, iota and lambda
how do carrageenans behave in water?
they dissolve to form highly viscous solutions. viscosity is quite stable over different pH due to constantly ionized half-ester sulfate groups. undergo depolymerization in hot acidic solutions.
how can carrageenans be made to gel?
by cooling a hot solution containing double-helical segments of k or i-type carrageenan and potassium or calcium ions. with potassium = stiff, brittle if kappa. calcium ions are less effective in causing gelation, but potassium+Ca together produce a high gel strength. K-type gels are the strongest. iota-type gels best with Ca. During cooling, gelation occurs because the linear molecules are unable to form continuous double helices due to the presence of structural irregularities. all salts of lambda-type are soluble and non-gelling
what are some uses of carrageenan?
stabilize milk-containing products like chocolate milk, ice cream (k-type complexes with casein micelles) - increasing brine uptake & slicing in cold hams (k-type) - water binding capacity in lean meats, sausages
what is agar?
another food gum also derived from red seaweeds, structurally and in properties closely related to carrageenans. agar is compatible with large amounts of sugar, products made with it don’t melt at high storage temp or stick to packaging materials
what is the difference between rapid-set and slow-set pectins?
the degree of esterification, 70 % and 60%.
what is a definition of dietary fibre?
dietary fibre means carbohydrate polymers with ten or more monomeric units, which are not hydrolyzed by endogenous enzymes in the small intestine of humans and are naturally occuring, extracted from food material or synthetic carbohydrate polymers
what are some examples of soluble dietary fibre? physiological effect?
beta-glucan and arabinoxylan. lower blood cholesterol, blood glucose, aid growth of beneficial bacteria
what are some examples of insoluble dietary fibre? physiological effect?
cellulose. counteracts constipation, binds h2o.
what is beta-glucan?
1-4 beta linked glucose units interrupted by 1-3 beta linkages. betaglucan that is dissolved is highly viscous which has physiological effects like slowing down the processing of food in our intestines, lowering cholesterol and blood glucose, increases nutrient uptake. broken down in bread produced with yeast or sourdough.
fructans are used by plants as
energy reserve
what is gum arabic?
an exudate of acacia trees. 70% PS chainsm 30% higher MW molecules with protein as integral part of their structures. these are highly branched acidic arabinogalactans attached to protein by linkage to hydroxyproline. it is highly soluble and produces low-viscosity solutions. very good for stabilizing emulsions. prevents sucrose crystallization, emulsifies and distributes fatty components, helps prevent bloom. used in caramels and candies.
what is inulin?
a naturally occuring storage carbohydrate composed of beta-D-fructofuranosyl units linked 2-1. undergoes acid hydrolysis quite easily. inulin preparations are mixtures of fructooligosaccharides and small polysaccharides. both inulin and fructo-OS are prebiotics. when hot solutions of inulin are cooled, thermo-reversible gels are formed. can be used as fat mimetic in reduced-fat products.
