Abstract
The effect of sugars and sugar replacers (i.e. plasticizers) on the gelatinization and pasting behaviour of wheat starch was studied. The intrinsic properties of the plasticizers, i.e. the molar volume density of effective hydroxyl groups NOH,s/vs, and the volumetric density of hydrogen bonds in the sugar solutions treated as a single solvent, i.e. Φw,eff, were proposed as factors controlling swelling (i.e. pasting) and gelatinization behaviour. Different classes of plasticizers were used including sugars, polyols, amino acids, soluble fibres such as oligofructoses, and mixtures thereof. The onset, peak and end temperature of starch gelatinization obtained by differential scanning calorimetry could be well described by Φw,eff for all solutions, following predictions from an adapted Flory-Huggins model for polymer melting. The multiple transitions involved in starch gelatinization could be well related to different ranges of Φw,eff following a side chain liquid crystalline model for starch. Deviations from the model predictions were observed mainly for Tonset in conditions of intermediate and excess solvent with high sugar concentrations (50% w/w). In such conditions phase separation likely occurs, increasing the effective starch concentration and consequently gelatinization temperatures. Pasting behaviour related to swelling, i.e. peak viscosity, was found to be a sigmoidal Fermi function of NOH,s/vs of the plasticizers. Plasticizers with high NOH,s/vs enhanced swelling compared to water while those with low NOH,s/vs had an inhibition effect. Overall, a comprehensive mechanism of starch plasticization, swelling and melting is proposed. Swelling associated with solvent ingress and helix-helix dissociation is affected by kinetic factors related to size and viscosity of the plasticizers (both described by NOH,s/vs) and by thermodynamic factors related to sugar partitioning and H-bonding ability (both related to Φw,eff). Melting of crystalline domains associated to helix-coil transition is controlled by thermodynamics, based on solvent H-bonding ability Φw,eff.
Highlights
Starch is a macro-constituent of many foods and its properties and interactions with other constituents during food processing are of interest to the food industry and for human nutrition
The palatability, structure and texture of cereal-based food is largely influenced by starch swelling and gelatinization behaviour
The The onset of starch gelatinization (Tonset), Tpeak and Tend of starch gelatinization was shown to be a function of the effective sol vent volume fraction Φw,eff, which expresses the volume density of hydrogen bonding sites available in solution for interactions with starch
Summary
Starch is a macro-constituent of many foods and its properties and interactions with other constituents (e.g. water, sugars and poly saccharides) during food processing are of interest to the food industry and for human nutrition. Starch occurs naturally as insoluble, semi-crystalline granules, made up of amylose and amylo pectin (Tester, Karkalas, & Qi, 2004; Copeland, Blazek, Salman, & Tang, 2009) These biopolymers are structurally organized in blocklets having a lamellar structure of alternating crystalline and amorphous layers (Donald, 2004; Donald, Kato, Perry, & Waigh, 2001; Perez, Baldwin, & Gallant, 2009). Under specific temperature and moisture conditions, starch un dergoes a phase transition named gelatinization, which is the irrevers ible disruption of the native, semi-crystalline organization of the starch granule into a polymer solution in the rubbery state This process involves multiple transitions, which can be described using the framework derived for synthetic side-chain liquid crystalline polymers (Donald, 2001; Waigh, Gidley, Komanshek, & Donald, 2000).
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