C3. Starch Flashcards
3 common polysacs?
+ 4 types of gums
starch + cellulose + pectins
others:
- seed gums: guar gum, locust bean gum
- plant exudate gums: gum arabic, gum tragacanth
- seaweed gums: agar, carrageenan
- microbial gums: dextran, xantham gum
properties of polysacs are a function of (4)
- sugar/chemical makeup
- presence of unique functional groups
- molecular weight and degree of branching
- combination thereof
are proteins or carbs more complex?
proteins! bc less variation in base units of carbs that make up the polymer
2 most abundant form of stored carb available on earth?
starch and cellulose
- starch = energy store for seed germination
starch
- main source of ________ ________ (4 ex)
- large quantities isolated from ______ sources and used as functional ingredient in food industry due to 2 properties
- dietary carbohydrates (potatoes, rice, corn, wheat)
- plant sources
- thickening/texture-modifying agent + source of sugars and dextrins
starch
- made up exclusively of __-__________
- made up of 2 fractions + describe (shape + linkage)
- D-glucose
1. amylose: long chain linear polymer composed of D-glucose linked by a-1,4 glycosidic bonds
2. amylopectin: branched polymer composed of D-glucose linked by a-1,4 glucosidic bonds in its linear portion and a-1,6 bonds at its branch points
starch is found in most plant sources as ______ water- soluble/insoluble granules
hard water-insoluble granules
each starch source contains ________ and _________ in genetically predetermined ________
ie: corn starch? vs waxy maize
- amylose and amylopectin in genetically predetermined ratios
- corn starch: 28% amylose + 62% amylopectin
- waxy maize: 100% amylopectin
starch is readily isolated from plant source by ____ _______, then ________ out of solution and _______
- produces _____ _______ powder composed of ________ starch granules
- each granule is made up of _________ amylose and amylopectin tightly bound and held together by __________ between the polymers
- wet milling + centrifuged out + dried
- dry white powder
- anhydrous amylose and amylopectin
- H-bonds
what leads to starch gelatinization?
- process of gelatinization (5 steps)
heating in presence of moisture breaks the H-bonds = gelatinization
1. gradual swelling of the granule due to absorption of water facilitated by heat
2. increasing kinetic energy breaks H-bonds
3. granule slowly hydrates as water molecules access OH groups on the polymer
4. as kinetic energy is reduced, random H-bonds reform –> extensively H-bonded starch molecules trap water within matrix formed, producing a gel
at what temp does loss of granular integrity general occurs?
- gelatinization temperature is a function of the starch ________
- 60-70°C
- stach source
gelatinization:
- when granules have swollen extensively, viscosity of solution increases/decreases drastically, causing ________
- if solution is stirred, granules break up and ________ to form a ________ solution
- increases causing pasting
- disperse
- colloidal solution
gelatinization:
- if concentration of starch is high –> forms what? upon cooling
- if concentration of starch is low –> forms what upon cooling?
- high starch = gel upon cooling
- low starch = obtain a viscous solution that does not form a gel
gelatinization of starch occurs as function of 5 variables
- concentration of starch
- amylose:amylopctin ratio of starch
- average molecular weight (chain length) of starch
- rate of cooling
- presence of other components in solution (salt, sugar, fat)
amylose:
- linear polymer with what structure?
- solubility? why?
- functionally useful?
- long thin helical filament-like structure
- very low solubility bc readily hydrogen bonds to neighboring polymers to form large aggregates that precipitate out of solution
- on its own, useless functionally in food systems bc insoluble
amylopectin:
- structure?
- soluble in water?
- forms a _________ solution –> branching produces what in solution
- in presence of amylose which acts as a _____-_________ agent, a _____ can form, _________ water
- highly branched water dispersible macromolecule
- yes!
- forms a viscous solution
- branching produces a tangled net-like web in solution
- amylose = cross-linking agent –> 3D gel can form, entrapping water
what is retrogradation?
- happens when?
- when starch gels become firmer with time –> generally undesirable
- due to continued hydrogen bond formation between starch molecules
what 2 problems arise with retrogradation?
- texture of gel changes over time, which is generally undesirable
- syneresis (exudation of water) may occur, where gel tightens up so much that it starts to exude water
- retrogradation causes color change?
- retrogradation is one of key processes responsible for (2)
- causes starch gels to become more opaque with time
- staling of bread + stiffening of pie filling
(when you expose starch to air
normal corn starch vs waxy maize starch vs potato and tapioca starch
- which will form a gel?
- why?
- normal corn starch (28% amylose) –> set to a rigid opaque gel
- waxy maize starch (100% amylopectin) –> NOT form a gel
- potato and tapioca starch (23-28% amylose) form reasonable stable gels that tend to maintain their clarity and do not stiffen appreciably with time attributed to their high average molecular weight = less mobile –> H-bonds continue to develop btw them over time but at a much slower rate
sugars tend to accelerate/retard gel formation and retrogradation
why?
retard!
- they compete for available water and interfere with starch to starch hydrogen bonding
surface-active agents (surfactants and emulsifiers) tend to activate/inhibit retrogradation and gelation –> forming what?
inhibit
- form complexes with starch, most noticeable with high-amylose starch systems
pH of most foods lies in range of __-___
- affects behaviour of starches? why?
- vs highly acidic food + heat –> possibility of what?
4-7
- nope because natural starches are uncharged
- of reduction of viscosity due to hydrolysis of starch
does ionic strength have effect on most starches?
only minor effects bc most starches have no ionizable groups –> change in ionic strength has little influence relative to its effects on a macromolecules such as protein which may be extensively charged
effect of protein on starch in terms of ________ and _______
- effect of starch on ______ proteins is well known –> produce what?
proteins are capable of dramatically changing how a starch behaves in terms of gelation and viscosity (+ texture)
- milk –> pudding (smooth gel)
in North America, most of our starch is derived from what?
- starch used in native state? why or why not?
corn
- rarely used except in home applications –> modify starch to produce specific properties that are desired by food processor
pre-gelatinized starch:
- starch granules are heated above/below gelatinization temp –> causing them to _________, and are then ____-_____
- causes some _________ in starch granule, making it _________ in cold water + requires more/less heat or time to develop viscosity
- inconvenience?
- below –> swell –> drum-dried
- disorganization of starch granule –> dispersible in cold water
- requires less heat
- inconvenience: higher amount of pre-gelatinized starch is required to attain similar viscosity effects as with untreated starch
acid-modified starch
- produced by holding starch just below/above its gelatinization temp in an ________ medium –> then ____________ and ___________
- acid is used to partially and _________ly ____________ the ____________ linkages in starch granule without breaking up integrity of granule per say
- ______-________ starches are used in candy manufacture –> produce what?
- below in acidic medium
- then neutralized and dried
- randomly hydrolyzes glycosidic linkages in starch granules
- thin-boiling starches –> produce easy-to-handle low-viscosity fluids that can be poured into molds and will set into a firm gel upon cooling –> forming starch gum candies (jujubes)
cross-linked starches:
- used when what is desired?
- how is that reached? (2) –> however, they normally ______ down
- solution?
- typically what (3) are used as cross-linking agents? can do what?
- maximum viscosity or pasting of starch is desired
- max viscosity reached when granules are swollen extensively but still intact –> however, swollen granules normally break down with subsequent shear
- can hold granules together by cross-linking starch chemically so that swollen granule retains integrity
- acetic, citric or adipic anhydrides are used as cross-linking agents –> can bridge 2 hydroxyls from adjoining polymers and hold swollen granule structure together
result of cross-linked starch
- swell more/less?
- maintain integrity
- viscosity level?
- resistant to shear?
- susceptible to acid hydrolysis?
- swell somewhat less but maintain integrity
- maximum viscosity somewhat lowered BUT much more resistant to shear
- less susceptible to acid hydrolysis as cross-links make up for some of the bonds broken by actions of acid
starch derivatives are used to (3)
- Overall objective of starch derivatives?
- control retrogradation and staling
- improve freeze/thaw stability
- reduce textural and opacity changes
- reduce polymer-polymer hydrogen bonding via steric hindrance or introduction of charged groups (to keep polymers from reforming H-bonds)
what charged groups are added to starch?
- like-charged polymers ______ –> decreasing/increasing stability against retrogradation
orthophosphate groups (are charged except at low pH)
- repel!
- increasing stability against retrogradation
use of ______ _________ (NaOCl) to ____A______ a portion of hydroxyl groups to __________ groups (some __________ and __________ groups may also be introduced)
- sodium hypochlorite (NaOCl) to oxidize
- hydroxyl to carboxyl groups
- some aldehyde and ketone
oxidized starches become __________ –> thereby stabilized against __________ but more susceptible to variations in ____ and _______ __________ in food product
- solubility becomes _____ dependant
- charged
- against retrogradation
- pH and ionic strength
- pH dependant
2 basic types of starch conversions? name + produces what?
- dry pyroconversion: for production of cold-water soluble starches
- acid hydrolysis: wet conversion for production of dextrins or as a preliminary step in production of sugars from starch
pyroconversion:
- starch granules are sprayed with dilute ____ and subjected to relatively low/high temps
- 4 reactions may take place
- HCl + high temps
1. hydrolysis of a-1-4 and a-1-6 linkages
2. transglucosidation: breaking of 1-4 linkages and formation of other linkages, generally random in nature (1,3 or 1,5 linkages)
3. repolymerization: any free sugars produced can repolymerize
4. caramelization: free sugars can undergo caramelization reaction
3 basic products can be produced by pyroconversion
- name
- under what conditions (high/moderate/low moisture, acid, temp)
- color?
- soluble in water?
- white dextrins:
- high moisture, high acid, low temp
- hydrolysis predominates –> can be controlled to produce desired ranges of viscosity
- soluble in cold water - yellow dextrins:
- lower moisture, moderate acid, higher temp
- color due to non enzymatic browning reaction (caramelization)
- viscosity and degree of polymerization can be controlled - british gums:
- little to no acid used –> higher temps
- dark in color with little hydrolysis + much more transglucosidation
- product is much more viscous and capable of forming a stiff gel –> used extensively in candy manufacture
acid/enzymatic hydrolysis of starch:
- half of isolated starch produced is converted to __________ and __________
- old means vs today?
- starch is gelatinized in presence of __________ acid and __________ _____
- syrups and sugars
- old means: only acid hydrolysis VS today: acid hydrolysis = common preliminary step to make it lower MW (in conversion of starch to syrups/sugars)
- hydrochloric acid + pressurized steam
acid/enzymatic hydrolysis of starch (into syrups/sugars):
- hydrolysis is __________ and __________ –> producing (2)
- starch breakdown can be followed by measuring ________ ______ content of hydrolysate
- random and extensive
- produce dextrins (low molecular weight starch) and sugars
- reducing sugar content
as hydrolysis increases and mono, di and oligosacs are produced, reducing sugar content increases/decreases
increases!
reducing sugar content is expressed in terms of what (A)?
- (A) is the ratio of what?
- if hydrolysis of starch is 100% complete, then (A) = ?
- dextrose equivalents (DE)
- ratio of reducing power (RP/g of syrup) relative to (RP/g of dextrose (D-glucose)) * 100
- DE = 100
acid hydrolysis cannot yield dextrose equivalents much beyond _____ since equilibrium is reached btw __________ and __________
- 50
- hydrolysis and repolymerization
what is the best way to produce very sweet, high DE syrups? vs what?
enzymatic convertion of gelatinized starch!
- not acid hydrolysis (low DE)
4 enzymes that do enzymatic conversion + basic role
- together, convert what to what?
- net result: DE > ?
- a-amylase: reduce MW
- b-amylase: produces maltose
- glucoamylases: converts maltose to glucose
- pullalanase: takes care of branch points
- convert starch to predominantly D-glucose (dextrose)
- DE > 90
a-amylase
- endo/exo enzyme?
- attacks where?
- known as __________ enzyme. why?
- random _______ bond breakage increases/reduces overall MW of starch
- end products (4)
- endoenzyme
- attacks anywhere within starch molecule other than the a-1,6 branch point
- liquefying enzyme –> causes viscosity of starch solution to drop rapidly
- random internal bond breakage reduces MQ
- mainly oligosacs + some glucose, maltose and pannose (trisac containing a-1,6 linkage)
b-amylase:
- endo/exoenzyme?
- attacks where?
- produces what? how?
- known as ______ enzyme as it develops __________ rapidly but __________ does not change significantly
- exoenzyme
- attacks non-reducing end of the starch chain
- produces maltose by hydrolyzing a-1,4 glycosidic bond every 2 glucose units over
- saccharifying enzyme as it develops sweetness rapidly but viscosity doesn’t change
glucoamylases:
- capable of removing single __________ units from __________ end of starch and split __________ into what?
- combined action of a and b-amylase + glucoamylase leaves __________ __________ –> largely what?
- glucose units from non-reducing end of starch and split maltose into 2 glucose units
- limit dextrins –> largely the trisaccharide pannose
pullulanase
- hydrolyzes which linkages?
- attacks what?
- a-1,6 glycosidic linkages
- attacks branch points so that trisaccharide pannose can be broken down to glucose plus maltose
at DE > 90, glucose can be __________ out of solution and obtained in ______ form in manner very similar to __________ refining
- glucose is a useful sweetener but as sweet as sucrose?
- crystallized
- pure form
- sucrose refining
- glucose = not as sweet as sucrose
what enzyme converts D-glucose to D-fructose? how much %?
- D-fructose sweeter than glucose? vs sucrose?
- glucose isomerase, 45%
- D-fructose 1.3 times sweeter than glucose + somewhat sweeter than sucrose
high fructose corn syrups are similar to ________ _______ and are in direct competition with _________, which is commonly __________ (using enzyme ________) to stabilize its sweetness in acidic environments
- invert syrups
- sucrose
- inverted
- enzyme invertase
why is sucrose inverted?
glycosidic link in sucrose is acid labile and readily hydrolyzed by heat or acid –> for acidic beverages, sweetness can be stabilized by inversion
the term inverted refers to the change in specific rotation of plane-polarized light by a sucrose solution –> converted to a mixture of (3)
- in a fully converted solution, direction of rotation is inverted (from 66.5° to _____ in the solution of (3 mixture) generated by rxn of sucrose with invertase enzyme
- sucrose, glucose and fructose
- 66.5 to -39° in solution of sucrose, glucose and fructose
why does high-fructose syrup has great value? (3)
- intrinsically sweeter than glucose, becomes sweeter with decreasing temp
- more soluble
- does not crystallize readily bc it is a mixture of sugars (good for frozen products)
RECAP
- starch is mainly used for its __________-modifying properties and for production of __________/__________
- in acidic foods, _____-_____ starches are used to minimize __________ during heat treatment
- starches are extensively modified to prevent __________ in systems where action is detrimental (ie __________ foods)
- texture-modifying properties
- production of syrups/sweeteners
- cross-linked starches –> minimize hydrolysis
- retrogradation (frozen foods)
