CHEM 98T Lecture 3: Week 2 articles

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Effect of Potassium, Sodium, and Calcium on the Microstructure and Rheological Behavior of Kappa-Carrageenan Gels
Balance between fine/coarse gel strands was dependent on potassium concentration.
o In contrast to potassium form, sodium-kappa-carrageenan formed weak gels/showed no dependence on sodium ion concentration
Sodium form had network structure with flexible superstrands
o Calcium-kappa-carrageenan formed weak gels in limited Ca conc. range
Calcium induced gels had fine network without coarse superstrands seen in Na or K forms
Kappa-carrageenan: commonly used sulphated galactan.
o Has ability to form gel on cooling from hot solution/used to control texture, stability, diffusion, waterholding properties
o Gelation process: involves coil-helix transition followed by aggregation/network formation
o Most common counter-ions in gel prep: K, Na, Ca
Type of cations used impacts gel characteristics: monovalent cations give gels with decreasing shear/elastic moduli in order
Cs+>K+>Na+>Li+
o Coil-helix transition necessary for gels to form found that helixes of potassium-kappa-carrageenan formed only under conditions that
promoted aggregation
Meas there’s strog lik etwee heli foratio/gel foratio
o Microstructure/rheological behavior revealed transient state on gelation of K form of kappa-carrageenan
o In presence of K, sharp initial max during cooling/gel formation
When temp lowered, structure became unstable/helices associated into rigid rods
o Network structure changed from fine network where junction zones are double helices to destabilization on cooling and formation of
dimers of double helices
Transient fine network structure/rodlike structures formed on cooling. Further cooling assemblies of K-induced gels of
kappa-carrageenan.
Fine structure is more stable at lower cooling rates
o Results show that gelation of kappa-carrageenan is complex/involves several aggregation steps
Kinetics during cooling have influence on final gel properties
Fine structure is more stable at lower cooling rates, part of the rods strengthen fine structure whereas they to a large extent
merge into coarser supramolec. network at higher cooling rates
Strongest gel was composed of mixed structure, where fine network was intertwined with coarser network structure: mixed
structure can also be reason for stronger gels formed at lower cooling rates
Sodium induced weaker gels than potassium: similar to potassium-induced gelation of carrageenan, gelation in presence of
sodium is correlated with coil-helix transition
Gels formed in presence of sodium showed no dependence on ion conc. in contrast to potassium-induced gels
In presence of sodium, likely that domains of double helical structure formed, but structure cannot span whole
volume/form gel structure
Pure Ca-kappa-carrageenan gave rise to weak gels
No gel formed at high Na conc (where salting out effects)
Strong gels can form with mixtures of cations.
Intermediate forms indicate possibilities of controlling gel strength at given temp. by composition of counter-ions
Rheological Studies of Specific Cation Forms of Kappa Carrageenan Gels
Relative gelling efficiencies of cations attributed to their extent of hydration that controls solubility of salt form of polysaccharide
Gelation attributed to double helix formation involving regular sequences between kink points on 2 adjacent chains
o Evidence for double helix formation: x-ray diffraction studies on fibers, optical activity/nmr
o Rees’ odel: atio effets  proposig speifi atio idued aggregatio of doule helies
o 3 cations: Ca, K, Na. Investigate their roles on gelation of kappa carrageenan
Cs, Rb, K gel promoting cations; Na, Li are non-gelling cations
Cs and K are more strongly bound than Na or Li in gels
Binding occurs on gelation suggesting immobilization of cations at junction zones within gel
Gelation involves localized ordering/crystallization of polysaccharide salt forms on cooling
Solubility of polysaccharides is considered to be reduced on cooling promoting phase separation
Complete cryst. would be inhibited by kinks in chains/polymer entanglements
Relative efficiency of cations for gelling polymer linked to their ease of hydration
Efficiency in gelling biopolymers: ca> k> Na
Gelation is attributed to phase separation at low temp. leading to localized crystallization of short regions of aligned
polymer chains
Ease of gelation related to solubility of salt form of polysaccharide/extent of hydration of cations/polyanions
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