1,721,156 research outputs found
Hard X-ray techniques suitable for polymer experiments
Full text linkPolymers have been studied since 1979 with 8-12 keV synchrotron radiation X-ray scattering methods and the number and sophistication of the experiments have rapidly grown ever since. More recently, new experimental techniques have been developed that use softer or harder X rays in less conventional ways. This article provides a brief overview of the possibilities of hard X-ray techniques and indicates some areas that might gain from further developments
Conformational influence of fluorinated building blocks on the physical properties of polyesters
No full textThe interplay of the relative configuration of diastereomeric vicinal difluoride groups on the conformational properties of polyesters has been investigated and compared to their non-fluorinated and per-fluorinated counterparts. The incorporation of syn and meso vicinal difluoride units in the polymer backbone was expected to influence the rigidity and stacking behaviour of the polymers in a significant way, and therefore to result in different thermal properties, such as melting point and melting enthalpy. Both syn and meso-2,3-difluoro butanediol have been reacted with the diester dimethyl succinate, leading to the formation of polyesters that have been characterized with H-1, C-13 and F-19 NMR. The polyesters showed molar masses up to 20 kg/mol (SEC). Surprisingly, the syn and meso-polymers displayed identical crystallization and melting behaviour. In contrast, differences were observed in the crystallization kinetics and melting points of syn and meso oligomers. Relying on time-resolved synchrotron SAXS and WAXD experiments, the complex multiple melting behaviour of these oligomers was explained in terms of crystal size and surface effects. The slower crystallization kinetics for the meso oligomers was tentatively associated with a stronger tendency to adopt gauche configurations. Apparently, such effects no longer affect the crystallization kinetics when larger polymers crystallize. It was also found that the syn and meso-polymers have identical equilibrium melting points and melting enthalpies notwithstanding molecular and crystallographic differences
Combining Fast Scanning Chip Calorimetry with Structural and Morphological Characterization Techniques
No full textThanks to the development of fast-scanning (chip-based) calorimeters (FSC) it is nowadays possible to achieve very high cooling rates, which enabled the study of polymer crystallization at large supercoolings, in conditions similar to what is experienced in real industrial processes. In such extreme conditions formation of structures very different from those commonly obtained under relatively slow cooling can occur. Albeit important, the information about thermal events gained by FSC might not be sufficient for a proper understanding of polymer structuring. In this chapter, new exciting developments on the coupling between FSC/fast cooling devices and structural or morphological probes are reported. For example, atomic force and polarized optical microscopy can be applied ex situ to polymer samples which have been submitted to a chosen FSC thermal protocol. Moreover, ballasting cooling and FSC-based devices have been realized, to allow fast Wide Angle X-ray Diffraction measurements at synchrotron facilities. Some recent examples of real-time detection of polymer structuring during fast cooling are presented
State Diagrams of PBTTT Derivatives Mixed with PC61BM and Their Relevance for Organic Electronics: Influence of Incongruently Melting Co-crystals and Homocoupling Defects
No full textDouble binary state diagrams of the benchmark semi-crystalline conjugated polymer PBTTT and its alkoxy derivatives PBTTT-OR-R and PBTTT-(OR)(2), mixed with PC61BM, are constructed using Rapid Heat-Cool Differential Scanning Calorimetry and T-resolved synchrotron X-Ray Diffraction. The polymerization method is adapted to ensure the absence of homocouplings and obtain reliable state diagrams, supported by Flory-Huggins calculations. Co-crystallization always occurs at a 45:55 w/w% polymer:PC61BM mixing ratio. All co-crystals remain stable up to 260-280 degrees C in a wide composition range, and for the first time, it is proven that they show incongruent (peritectic) melting. The three state diagrams show one eutectic, i.e., between polymer and co-crystal in the case of PBTTT and PBTTT-OR-R, and between polymer and PC61BM for PBTTT-(OR)(2). The latter eutectic beyond 280 degrees C leads to a reversible "solid-solid" transformation with the corresponding co-crystal. Isothermal treatments, at the onset temperature of co-crystallization during non-isothermal cooling, show no loss of co-crystal quality in combination with PC61BM perfectioning for fullerene-rich PBTTT:PC61BM and PBTTT-OR-R:PC61BM mixtures, whereas PBTTT-(OR)(2):PC61BM, as well as Stille-polymerized PBTTT-OR-R:PC61BM with homocouplings, show suppression of co-crystallization with the formation of separate crystals of polymer and PC61BM. These opposing effects are explained by the state diagrams, showcasing their substantial value in selecting efficient annealing conditions.The authors thank the Research Foundation – Flanders (FWO Vlaanderen) for financial support (Ph.D. project 1S50822N, project G0B2718N, and DUBBLE project I001919N)
Nano ontwikkeling in polymeer-staal hybrides: chemische en fysische compatibilisatie
No full textThe
exploitation of the toughness of steel in steel-polymer hybrids in structural
applications is limited due to the huge difference in stiffness of the
reinforcement (200 GPa for steel) compared to the polymer matrix (between 1 and
3 GPa only). This stiffness mismatch leadsto stress concentrations at the
steel-polymer interface, which give rise to either early interface fracture or
plastic yielding. The guiding hypothesis within this PhD research, which
focuses on the optimization of polymer-steel hybrids, is that it is necessary
to decrease the polymer-steel stiffness mismatch and that the steel-polymer
adhesion as well as the polymer toughness and/or yield stress in the
interphasial region need to be improved.status: Publishe
Waterstofbrugvorming in Wateroplosbare Polymeren en alsSupramoleculaire Interactie in Co-kamcopolymeer Materialen.
No full textIn dit werk worden waterstofbruggen bestudeerd met het oog op hun invloed op het fasegedrag van polymeeroplossingen (deel I) en met betrekking tot hun rol in de vorming van nanostructuren in mengsels van polymeren (deel II).De eerste doelstelling is om een beter inzicht te verwerven aangaande de moleculaire aspecten van waterige N-(isopropyl)propionamide (NiPPA) oplossingen, om alzo een beter moleculair begrip te verkrijgen omtrent het vloeistof-vloeistof fasegedrag van het systeem. In dit opzicht werden drie hoofdonderwerpen onderzocht in deel I:De vast-vast en vast-vloeistof transitie van de zuivere component.NiPPA heeft een smeltpunt (Tm) bij 51 °C. Een tweede endotherme transitie (Ts) werd geklassificeerd als zijnde een transitie van de kristallijne (C) naar de plastich kristallijne (PC) fase. Deze PC toestand is een mesofase, gekenmerkt door kristalachtige positionele orde en lokale rotationele wanorde en mobiliteit. De kristalstructuur van de lage temperatuur fase kon bepaald worden uit WAXD data afkomstig van poederverstrooiing met een monocliene eenheidscel (ruimtegroep P21/a).Het LCST fasegedrag van NiPPA interagerend met solvent moleculen via gerichte verzadigingsinteracties.NiPPA is een wateroplosbaar molecule met een vloeistof-vloeistof ontmenggebied. Vergeleken met de coëxistentiecurve van het waterige polymeermengsel, is zijn ontmenggebied veel symmetrischer en smaller. Bijgevolg zullen beide coëxisterende fasen aanzienlijke hoeveelheden van beide componenten bevatten bij het binnenkomen van het ontmenggebied. FTIR heeft aangetoond dat vloeistof-vloeistof ontmenging gepaard gaat met dehydratatie van de NiPPA moleculen.De invloed van druk op het LCST fasegedrag van NiPPA/H2O.De invloed van hydrostatische druk op de spinodale en kritische condities van een ontmenggebied hangt af van de samenstellingsafhankelijke kromming van het excess volume en de excess enthalpie van het mengen. Voorts wordt het teken van de kromming van de excess enthalpie bepaald door de aard van het ontmenggebied, zijnde LCST of UCST. Vanuit het bovenstaande kan geconcludeerd worden dat het drukeffect kan voorspeld worden op basis van de samenstellingsafhankelijke kromming van het excess volume. Een verder aanname wordt gemaakt door te stellen dat de coëxisterende en spinodale curves op een gelijkaardige manier wijzigen met de druk. Aangezien de kroming van het excess volume bij het mengen positief is in het concentratiegebied en bij temperaturen corresponderend met de LCST coëxistentiecurve van NiPPA/H2O, wordt verwacht dat de transitietemperatuur zal toenemen onder invloed van druk.Ten tweede, trachten we in deel II de sleutelbegrippen vast te leggen voor de vorming van supramoleculaire co-kamcopolymeren:Het regelen van het fasegedrag van oligomeren via het copolymeer repulsie-effect.Het fasegedrag van binaire mengsels van oligomeerzijketens kan geregeld worden via het copolymer repulsie effect. Dit effect kan geïntroduceerd worden door de partiële bromering van één van de zijketencomponenten, oligo(styreen) (OS), resulterend in een coplymeer, oligo(styreen-co-p-bromostyreen). De mengbaarheid van het homopolymeer, oligo(ethyleen oxide), en het copolymeer neemt af met een toenemende bromeringsgraad.De supramoleculaire interacties van deze oligomeren met het hoofdketenpolymeer.Eind-gefunctionaliseerde oligomeren die in staat zijn om waterstofbruggen te vormen met de monomeereenheden van het hoofdketenpolymeer, poly(4vinylpyridine) (P4VP) werden onderzocht. De oligomeer-polymeer associatie resulteert in kamvormige copolymeren. De complexvorming werd gevolgd met behulp van FTIR spectroscopie.status: Publishe
Isolatie, productie, semi-kristallijne aggregatie en potentiële farmaceutische toepassing van amylose
No full textAmylose (AM) is an almost linear glucose polymer. Together with amylopectin (AP), it makes up the bulk of granular starch. The AM content in regular starches ranges from 20 to 35%. It greatly influences the functionality of starch in aqueous systems. In fully gelatinized starch dispersions, AM crystallization plays a major role in gelation. In many food preparation protocols, AM network formation starts immediately after starch gelatinization. Connected cylindrical objects and ultimately fractal structures are formed by aggregation of AM and the outer branches of AP. These structural transitions are exploited to provide texture to different food products. AM crystallization has also been used to produce type III enzyme resistant starch (RS), a type of starch that resists digestion, is fermentable in the colon and has several potential health benefits.
AM crystallization depends on factors such as its average degree of polymerization (DP), its concentration in the suspension, the temperature and time. Its concentration determines the type of crystal arrangement which it adopts. While in diluted [typically 10.0% (w/v)] systems. These crystal architectures resist digestion by pancreatic α-amylase. However, they are fermentable by the microbiota in the human colon.
Despite the relevance of AM as a standalone polymer, the underlying mechanisms of its aggregation have mainly been investigated in starch systems, which by their own nature also contain AP. Against this background, this doctoral dissertation aimed to provide a better understanding of the production or isolation of AM and its physico-chemical properties in order to provide a basis for exploiting AM crystallinity in the development of potential applications in and beyond the food industry.
A method for producing pure AM on laboratory scale was needed for studying the mechanisms of AM aggregation. Thus, the first part of this dissertation focused on producing or isolating AM. Three main in vitro approaches were considered for obtaining AM: enzymatic synthesis, AM leaching, and AM complexation following starch dispersion. However, the production or isolation of AM is not a simple task. The properties (i.e., purity, average DP and polydispersity) of isolated AM are influenced by the experimental conditions in each methodology. Aqueous leaching allows isolating AM on large scale and involves heating a starch suspension above the starch gelatinization temperature. Different factors influence leaching of AM, including the leaching temperature (LT) and starch concentration. A response surface analysis with a face centered central composite design was implemented to study the effect of maize (Zea mays L.) starch concentration [3.0–7.0% (w/v)] and LT (70–90 ˚C) on aqueous leaching of AM as a way to optimize the conditions for obtaining the highest yield of long chain AM [number average DP (DPn) ranging from 860 to 930] and highest purity. Second order empirical models were fitted via the least squares approach. Negligible terms were removed using backwards model reduction. Negligible lack of fit terms were obtained for the responses total leached carbohydrate and DPn. The optimization was complemented with a desirability test using the purity of the extracts. As optimization targets, maximum leachate yields, DPn ≈ 900, and purity > 95% were set. Contour plots and prediction profilers were obtained and can be used by others for tailor made production of leachates. LT had the most significant effect as yields and DPn increased with temperature at the expense of purity. Purity was highly compromised when treatments were at temperatures exceeding 85 ˚C. This was reflected in the high DPn values (> 1,500) which suggested the presence of AP material. When using 3.0% (w/v) maize starch suspension at an LT of 81 °C, the largest yield (15.0%, starch basis) of high DPn AM chains (DPn ≈ 900) and less than 3.3% of non-AM material were obtained.
The effect of starch crystallinity on the aqueous leaching of AM was also studied. Starch crystal stability was altered via annealing. Leaching was studied in a 60-90 °C temperature range. The leachate yield, average DP and purity were related to the extent of melting of the starch crystals at the LT as determined via differential scanning calorimetry (DSC). Annealing increased the AP crystal stability and hence the remaining crystallinity at a given LT. Negligible AM leaching occurred at temperatures below those of the annealing dependent onset of melting. Leaching thus benefited from partial melting. Similar AM leachates were obtained when the extent of starch melting was below 80%. Loss of more than 95% of the melting enthalpy resulted in higher leachate average DP at the expense of purity. As the crystallinity of annealed starches at a given LT was higher than that of the native starches, the purity of leachates obtained from such starches was higher. Although no residual AP crystals remained at 90 °C, annealed starches subjected to leaching at such temperature still yielded AM extracts in higher yields and of higher purity than did native starch. More effective leaching in this case may be due to annealing-induced strengthened AP-AP interactions and AM disentanglement from AP.
The second part of this work focused on the semicrystalline aggregation of AM. While previous studies had elucidated the role of AM average DP on its aggregation in diluted aqueous systems, no reports have elaborated on the crystallization of AM in concentrated systems. Here, AM samples with different weight average DP (DPw) were produced and subjected to a heating-cooling-heating process. Since AM crystals only melt at temperatures exceeding 100 °C, high-pressure devices were used to analyze the hydrated samples. High (DPw = 830), mid (DPw = 340), and low (DPw = 60) DP AM aqueous dispersions [25.0% (w/v)] were first heated to 180 ˚C to produce fully dissolved aqueous solutions of AM. During subsequent cooling to ambient temperature and re-heating to 180 ˚C, their thermal and structural transitions were studied by DSC and X-ray diffraction at small (SAXS) and wide (WAXD) angles. During cooling, spherulitic crystal aggregates were formed the sizes of which decreased in the order mid DP > low DP > high DP AM. The crystallization events also depended on AM DP. Mid DP AM crystals were formed at high temperatures and its exothermic transition peaked at 74 ˚C. Those from low and high DP AM were formed below 60 °C with peaks at 37 and 42 °C respectively. A second (small) fraction was visible for low DP AM and it appeared close to the observed high temperature transition for mid DP. During the subsequent heating, mid DP AM crystals were the most stable and their melting signal peaked at 156 °C. Low DP AM crystals melted in two broad temperature ranges with peaks at 104 °C (large fraction) and 150 °C (small fraction). Those from high DP AM melted in a similar range as the low temperature signal for low DP AM.
Time-resolved SAXS and WAXD measurements were implemented for the first time to reveal the nanostructural transitions of AM during the formation and disappearing of semicrystalline spherulites from low and mid DP AM. WAXD measurements revealed B-type AM crystals besides amorphous material, regardless of the temperature. Changes in the crystallinity index occurred in the temperature ranges where DSC revealed exo or endothermic transitions. Inter-crystallite interference was found in SAXS for low DP AM while this was not the case for mid DP AM. Mid DP AM spherulites were classified as open (no interference) while those of low DP AM as compact (with interference). Open spherulites had a lower internal crystallinity (below 20 %) than the compact ones (up to 80 %) but were able to fill the space completely at the end of cooling. Open spherulites from mid DP AM started from the crystallization of AM within a homogeneous liquid phase. This forms AM depletion zones around the crystals at a high crystallization rate. At lower temperatures spherulites grow and new ones are created at a lower rate. Here the liquid phase can follow the pace and can homogenize concomitantly. In the case of low DP AM spherulites, open spherulites similar to those from mid DP AM are first formed at high temperatures. During further cooling, the system separates into AM-rich and –poor liquid phases. Spherulites migrate to the AM-rich zone and further AM crystallization occurs rapidly turning the aggregates into compact spherulites. For high DP AM it was proposed that chain entanglement might inhibit the formation of large and ordered aggregates at high temperatures. Instead, liquid-liquid phase separation is favored and the crystallization of AM into small, disordered spherulites takes place in the AM-rich zone.
In a third and final part, the crystallinity of AM was exploited in the development of a potential pharmaceutical application. Type III RS from debranched cassava (Manihot esculenta Crantz) starch was produced by favoring AM crystallization in a hydrothermal treatment. A thermostable crystal fraction was formed. Indeed, type III RS crystal melting was only observed at temperatures exceeding 120 °C. RS levels measured via in vitro digestion increased from 36.6 (in the starting material) to 95.1% (in the final type III RS product). RS levels were positively correlated to the degree of crystallinity. Type III RS was highly fermentable in vitro by human fecal microflora and resulted in a significant production of short chains fatty acids. Acetate levels were much more increased than those of propionate and butyrate when compared to their levels noted for a fecal blank. A granulate of type III RS [60 - 70 % (w/w)] and ethyl cellulose [40 - 30 % (w/w)] was used to coat tablets of 5-aminosalicylic acid (5-ASA) using a compression-coating approach. Coating protected 5-ASA during its transit through a model for the gastrointestinal tract. No release was observed in gastric medium and negligible release was observed after 2h in intestinal medium containing pancreatic amylase. This material may be of more interest than polymers such as cellulose due to a (potential) therapeutic effect exerted by increased short chain fatty acid levels as a result of fermentation. This study thus also provided the basis for applications of AM crystals in the pharmaceutical industry.status: Publishe
Inzicht in de structurele eigenschappen en potentiële toepassingen van V-type granulair koud-zwellend zetmeel
No full textStarch is an abundantly available and multifunctional plant polysaccharide. In its native form, it is present under the form of semi-crystalline, water insoluble granules. The main starch constituent is the highly branched glucose polymer amylopectin (AP). Its outer branches interact to form stiff double helices, which crystallize into a dense A-, more open B- or mixed (A and B) C-type polymorph, as evidenced by X-ray diffraction. The other and essentially unbranched glucose polymer present in starch is amylose (AM). In native starch, AM is mainly amorphous. However, when AM is dispersed in aqueous media in the presence of a suitable ligand (e.g. linear alcohols or fatty acids), its conformation changes. A left-handed, single helix is formed with a central hydrophobic canal that can accommodate the apolar (part of the) ligand molecule. When AM is in this state, it is referred to as V-type AM. When V-type AM helices stack parallel into crystals, they give rise to a V-type X-ray diffraction pattern.
In food applications, industrial starches function mainly as thickening and texture determining ingredients. The use of starch as a functional ingredient to tune the viscosity in food systems relies on its property to imbibe large amounts of water when heated above a characteristic temperature (i.e. the gelatinization temperature), allowing native crystal melting. Gelatinized starch provides viscosity to a vast array of food systems. For use in instant and ready-to-use-applications where heating is not desired, starches of enhanced cold-water swelling capacity are used. These are typically produced by gelatinizing starch and subsequently recovering it by drum drying. However, the resultant pregelatinized starch no longer has granular integrity and does not have the viscosity and texture building capacity which its native counterpart displays following gelatinization in situ.
Another way to provide starch with cold-water swelling capacity is by heating native starch to sufficiently high temperatures in the presence of water and ethanol. This treatment results in a changeover from granules with double helical A-, B- or C-type crystals to granules which have single helical V-type crystals. The latter can be dispersed in water at ambient temperature. Such V-type granular cold-water swelling starch (GCWSS) is the subject of the present doctoral dissertation.
Over the years, little research has been devoted to V-type GCWSS. The only conversion mechanism described dates from 1986 and contains hypotheses which are not compatible with subsequent research findings. The alleged contribution of AP to V-type crystal formation for instance seemed rather unlikely, since AM-free (waxy) starch disintegrates during high-temperature aqueous alcohol treatment or remains amorphous after alcoholic-alkaline treatment, a methodology that allows conversion of native starch into GCWSS at reduced temperatures. Also, no direct evidence has been delivered for the alleged necessity to remove the alcohol from the single helical cavities to confer upon the resulting starch cold-water swelling capacity. In addition to this, shortcomings associated with more recent studies prevented making clear statements on the influence of using different ethanol to water ratios on the thermal requirements to produce GCWSS. Further, GCWSS from different botanical origins display different degrees of V-type crystallinity, which has been attributed to dissimilarities in molecular structure and supramolecular organization of the corresponding native starch. However, no research reports ever went beyond this qualitative statement. Finally, basic research questions regarding the supramolecular arrangement of the newly created V-type crystals and the relative kinetics of native crystal melting and V-type crystal formation remain unresolved.
Against the above background, this work aimed at unraveling the conversion mechanism of native into V-type GCWSS.
This was in first instance achieved by studying regular and waxy (AM free) maize starch as model systems. Native maize starch was gradually converted into GCWSS by aqueous ethanol treatments at elevated temperatures. At a treatment temperature of 95 °C, decreasing ethanol concentrations from 68 to 48% (v/v) led to decreased post-treatment gelatinization enthalpies in excess water, reflecting remaining original A-type crystals. Concomitantly to native A-type crystal melting, V-type crystals appeared. At an ethanol concentration of 48%, a GCWSS was successfully produced. All crystals in its granules were of the V-type and appeared birefringent when studied in ethanol under polarized light. Removal of all residual solvent by high-temperature drying did not influence swelling power, proving that a high temperature drying step is not necessary to induce cold-water swelling capacity. Based on in situ calorimetric measurements, the thermal requirements to produce GCWSS from different ethanol:water mixtures were elucidated. The V-type crystallinity (18%) of the regular maize GCWSS did not exceed its AM content (24%). Moreover, it was impossible to produce a V-type GCWSS from waxy maize starch. Removing all residual solvent by high-temperature drying did not increase the swelling power of the GCWSS.
To further elaborate on the structural role of AM, rice, cassava, potato and pea starches were studied. These covered a broad range in AM content and degree of polymerization (DP). They also represented the different native starch crystal types (A-, B- and C-types). These starches were converted gradually to V-type GCWSS by aqueous ethanol treatments at 95 °C in a 68 to 48% ethanol (v/v) range. Microscopic, X-ray diffraction and calorimetric analyses showed that loss of native molecular order already occurred at the highest ethanol concentrations for starches containing the intrinsically less stable B-type crystals, whereas lower ethanol concentrations were necessary to induce native crystal melting in A-type starches. C-type starch, containing a mixture of A- and B-type crystals, exhibited features characteristic of both A- and B-type starches. No native crystals remained and granular products containing V-type crystals only were formed for all starches when using 48% (v/v) ethanol. Surprisingly, there was no relation between V-type crystallinity and AM content, although V-type crystallinity again never exceeded the starch AM content. However, the relative kinetics of V-type crystallization, which were studied by DSC, depended on the AM DP. For low DP AM starches (maize and rice starches), V-type crystals formed already during heating to 95 °C and thus while the native crystals were melting. V-type crystallization went up to completion when samples were kept isothermally at 95 °C. For mid (pea starch) and high DP AM (potato and cassava starches) starches, V-type crystallization was initiated during holding at 95 °C and progressed further during subsequent cooling. The resulting V-type crystallinity decreased with increasing AM DP.
Maize and potato starches which differed in native crystalline structure as well as in AM content and DP were heated to 160 °C, cooled to ambient temperature and reheated to
160 °C in 48% (v/v) ethanol with continuous recording of the X-ray scattering at small (SAXS) and wide angles (WAXD). Their gelatinization in pure water was assessed as well. In the latter case, at the low-temperature side of the DSC gelatinization interval, SAXS picked up nano-morphologies of 58 and 34 nm in diameter, for maize and potato starch respectively. These values fell within the range of dimensions reported for starch building blocks (i.e. ‘blocklets’). Upon further heating, the native lamellar order was irreversibly lost. When maize starch was gelatinized in 48% (v/v) ethanol, it developed layer like next to fractal like demixed features. This occurred prior to the loss of native lamellar order. Slightly beyond the DSC based onset of gelatinization, V-type crystals started to form. At this point, the layer like features started to homogenize. Time-resolved polarized optical microscopy (POM) revealed that the granular integrity in maize starch was lost during the temperature interval wherein WAXD and SAXS revealed V-type crystal melting. During gelatinization of potato starch in 48% (v/v) ethanol, lamellar order was lost and demixed features were also created, but the latter did not homogenize. Instead, their length scale increased (coarsened) such that it probably exceeded the experimentally accessible SAXS window. This hypothesis was ground in the observation that potato starch did not lose its granular integrity even at very high temperatures (i.e. 160 °C) and that its granules showed an internal phase coarsening that became more pronounced at increasingly high temperatures. During subsequent cooling, (melts of) maize and potato starches crystallized into isolated and (a small quantity of) stacked V-type crystalline layers. The associated SAXS revealed that V-type crystals with similar average layer thicknesses (~ 70 Å) were created in both starches. However, for potato starch, this thickness value most likely resulted from the crystalline thicknesses of roughly two different crystal populations averaging out, whereas in maize starch, a more uniform crystal population was created. SAXS patterns collected during reheating revealed that (a fraction of) V-type crystals in potato starch started to reorganize at fairly low temperatures (~ 60 °C). The nature of this reorganization was not completely clear, but it clearly involved the formation of crystalline layer stacks. In maize starch, stack formation was limited. In both starches, V-type crystals melted and recrystallized into thicker crystals at higher temperatures (at the DSC based high-temperature melting endotherm).
In this doctoral dissertation, valuable knowledge was acquired on the micro- and nano-structural transitions that occur when starch is converted into its V-type granular cold-water swelling counterpart. Research questions that remained unresolved so far, like the relative contributions of AM and AP in V-type crystal formation, the supramolecular organization of the resulting V-type helices and the mechanism behind the preservation of granular integrity, were answered. The industrial relevance of this study stems from the increased importance of instant and convenience foods, in which current cold-water swelling starches are commonly used ingredients. Furthermore, GCWSS could have applications as an ingredient in gluten free foodstuffs or as fat replacer.status: Publishe
De moleculaire structuur van zetmeel: de sleutel tot het begrijpen van zijn lamellaire structuur en fysico-chemisch gedrag
No full textZetmeel is een belangrijk reservekoolhydraat in granen, wortels en knoll en. Het bestaat voornamelijk uit het nagenoeg lineaire amylose (AM, 18 33%), een a-(1,4)-glucosepolymeer, en het sterk vertakte amylopectine ( AP, 67 82%), eveneens een a-(1,4)-glucosepolymeer maar met ca. 5% a-(1 ,6)-bindingen. Verschillende organisatie-niveaus worden onderscheiden: g ranules (2 110 µm), groeiringen (120 500 nm), blocklets (2 0 500 nm), amorfe en kristallijne lagen (2 10 nm) en krist alrooster-afstanden (~0.1 2 nm). Deze complexe structuur is het r esultaat van de werking van allerlei enzymen tijdens de zetmeelbiosynthe se. Verwarming van zetmeel in water boven een karakteristieke temperatuur, g enaamd de verstijfselingstemperatuur, geeft aanleiding tot een onomkeerb aar verlies in moleculaire orde. Gelering treedt op tijdens koelen van v erstijfselde zetmeelsuspensies. Tijdens korte bewaarperiodes (enkele ure n of dagen) vindt AM kristallisatie plaats, terwijl langer bewaren (enke le dagen tot weken) aanleiding geeft tot AP retrogradatie. Zetmeel wordt gebruikt in voedingsmiddelen als stabiliseer-, verdikkings- of bindmidd el. De uiteenlopende functionele eigenschappen van zetmeel creëren diver se industriële toepassingsmogelijkheden. Toch voldoen natieve zetmelen n iet aan alle specifieke noden van de industrie. Genetische en chemische modificaties, en hydrothermale behandelingen (zoals temperen) kunnen geb ruikt worden om de gewenste functionaliteiten te bekomen. Het is echter nog onvoldoende begrepen hoe deze modificaties en behandelingen de zetme elstructuur wijzigen en aanleiding geven tot gewijzigde functionele eige nschappen. In dit doctoraat werd onderzoek verricht naar de invloed van de zetmeels tructuur op verschillende fysico-chemische eigenschappen van aardappel- en cassavezetmelen en hun mutanten. Tevens werden de veranderingen in la mellaire structuur en kristalliniteit bestudeerd tijdens het verwarmen v an zetmeelsuspensies. In een laatste deel werd de impact van temperen op zowel de structuur als op de fysico-chemische eigenschappen van aardapp elzetmelen uitgediept. Vijf wild type aardappelzetmelen (wtps), vijf amylosevrije aardappelz etmelen (amfps), vier hoog-amylose aardappelzetmelen (haps), één wild ty pe cassavezetmeel (wtcs) en één amylosevrij cassavezetmeel (amfcs) we rden in deze studie geanalyseerd. De substitutie met glucose-6-fosfaates ters op aardappel AP varieerde sterker tussen de onderzochte variëteiten dan tussen wtps en amfps. Voor haps werd een drie- tot zesvoudige toena me in het glucose-6-fosfaatgehalte waargenomen ten opzichte van de overe enkomstige wtps. De moleculaire grootteverdeling toonde aan dat amfps ho gere gehaltes aan hoog-moleculair AP heeft dan wtps. Het gehalte was ger educeerd voor haps, en hoog-moleculair AP was zelfs afwezig in het haps staal met het hoogste amylosegehalte. Wtcs and amfcs hadden de kortste A P zijketens terwijl de AP zijketens van haps het langst waren. Verstijfselingstemperaturen waren het hoogste voor haps, gevolgd door amfps, wtps, amfcs and wtcs, respectievelijk. Hogere gehaltes aan korte AP zijketens [polymerisatiegraad (DP) 6-9 en DP 10-14)] en lagere gehaltes aan langere AP zijketens (DP 18-25 and DP 25-80) leid den tot lagere verstijfselingstemperaturen. De verstijfselingsenthalpie was positief gecorreleerd met de kristalliniteit. Uit de studie van het zwelgedrag bleek dat wtps in twee fasen zwellen, terwijl amfps een ze er sterke zwelling vertoonden tot temperaturen tussen 70 en 90 °C. Bij hogere temperaturen werd geen gel gevormd voor amfps. Haps stalen ha dden een lager zwelvermogen dan wtps. Daarnaast was het zwelvermogen van de cassavezetmelen lager dan van de aardappelzetmelen maar de koolhydra atuitloging was gelijkaardig. Hogere zwelvermogens gaan gepaard met lage re gehaltes aan AP zijketens met DP18-25. Het zwel- en geleringsge drag, bestudeerd met Rapid Visco Analyser (RVA), werd sterk beïnvl oed door de zetmeelconcentratie. Bij 5.0% zetmeelsuspensies leidden hoge re gehaltes aan hoog-moleculair AP en lagere AM-gehaltes tot een toename in de piekviscositeit. Bij 8.0% zetmeelsuspensies nam de piekviscositei t toe met de granulegrootte. Bewaring van zetmeelsuspensies gaf aanleiding tot AP retrogradatie. H et gehalte aan geretrogradeerd AP nam toe tijdens bewaring gedurende vie r weken voor wtps en amfps, maar bleef steeds lager voor amfps. Na één d ag bewaren van haps werden hoge gehaltes geretrogradeerd zetmeel gemeten . Cassavezetmelen vertoonden geen retrogradatie gedurende de bewaarperio de van vier weken. Bovendien bleek retrogradatie positief gecorreleerd t e zijn met het gehalte aan AP zijketens met DP 18-25. Tevens werden hoge retrogradatietemperaturen waargenomen voor haps wat waarschijnlijk het gevolg is van een gedeeltelijke co-kristallisatie tussen AP en AM. Temperatuur/tijdsgeresolveerde grote hoek X-stralen diffractie (WAXD) en kleine hoek X-stralenverstrooiing (SAXS) metingen werden uitgevoerd tij dens verwarming van zetmeelsuspensies in overmaat water. Het kristallini teitsverlies gebeurde voornamelijk in het temperatuursgebied waarin de z etmeelverstijfseling plaatsvond, zoals gemeten met Differentiële Scannin g Calorimetrie (DSC). Er was reeds een kleine afname bij temperaturen la ger dan de begintemperatuur van verstijfseling en dit was het meest uitg esproken voor haps. SAXS metingen in combinatie met DSC toonden aan dat de oppervlakte-energie van de kristallen van amfps lager was dan van wtp s maar wtps had een hogere kristaldikte. Haps bevatten bovendien ook een fractie ongeordende gestapelde structuren die het gevolg was van een br ede distributie van de kristaldikte. Tijdens verwarmen van wtps en wtcs steeg het elektrondensiteitverschil tussen de kristallijne en amorfe laa g, terwijl dit niet werd waargenomen bij amfps en amfcs. Tijdens temperen nam de stabiliteit van de kristallen toe. Dit was voorn amelijk te wijten aan een toename van de oppervlaktekwaliteit van de wtp s en amfps kristallen, terwijl dit voor haps stalen werd toegeschreven a an een toename in kristaldikte. De getemperde zetmelen hadden hogere ver stijfselingstemperaturen en nauwere verstijfselingsintervallen dan de ov ereenkomstige niet-getemperde zetmelen. Dit effect was groter voor haps dan voor wtps en amfps. Temperen had eveneens een invloed op het zwel- e n geleringsgedrag van de aardappelzetmelen. De veranderingen in piekvisc ositeit boven de close packing concentratie waren voornamelijk te wijten aan grotere granulerigiditeit en/of de vorming van aggregaten van uitge loogd AM. De viscositeit bij 95 °C en de eindviscositeit namen toe door temperen voor vrijwel alle zetmelen en deze toename was groter naar mate de structurele veranderingen ten gevolge van temperen meer uitgespr oken waren.status: Publishe
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