|Title: ||Unlocking the structural features and potential applications of V-type granular cold-water swelling starch|
|Other Titles: ||Inzicht in de structurele eigenschappen en potentiële toepassingen van V-type granulair koud-zwellend zetmeel|
|Authors: ||Dries, Dorien|
|Issue Date: ||3-Nov-2016 |
|Abstract: ||Starch 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.
|Publication status: ||accepted|
|KU Leuven publication type: ||TH|
|Appears in Collections:||Polymer Chemistry and Materials|
Centre for Food and Microbial Technology