Pasting Properties of Chemically Modified Starch

The pasting properties reflect the structural changes in starches during heating and cooling. This can be measured by Brabender Viscoamylograph or the RVA and rheometer to obtain a plot of viscosity versus temperature. The peak viscosity (Pv) at a given concentration indicate the ability of the granules to swell freely before their physical breakdown (gelatinization). Starches that are capable of swelling to a high degree are less resistant to breakdown on cooking and hence exhibit significant viscosity decreases after the peak viscosity is reached. Modification method, reaction conditions, and starch source are the critical factors that govern the rheological/pasting behavior of starch pastes. Chemical modification leads to a considerable change in the pasting properties of starches that hydrophilic substituents such as hydroxypropyl, acetyl, and phosphate groups of the esterified starch usually increase pasting viscosity, whereas the hydrophobic substituents such as benzyl, allyl, and methyl of the etherified starch often result in more complicated changes. Starch paste viscosity can be increased or reduced by applying a suitable chemical modification (Table 1).

The Pasting Parameters for Different Chemically Modified Starches
Table 1. The Pasting Parameters for Different Chemically Modified Starches

Unit of pasting viscosity is in BU* and RVU**.

a Adebowale and Lawal (2003).

b Olayinka et al. (2015).

c Huijbrechts et al. (2008).

d Bao et al. (2003).

AEMS, amylose-enriched maize starch; AHP, 1-Allyloxy-2-hydroxypropyl; BV, (breakdown value) = Pv -Hv; Cv, cold-paste viscosity; DS, degree of substitution; Hv, hot paste viscosity; MS, maize starch; OSA, octenyl succinic anhydrate; Pv, peak viscosity; SB (set back) value = Cv- Pv; WMS, waxy maize starch.

The peak viscosity (Pv), hot paste viscosity (Hv), and cold-paste viscosity (Cv) of the oxidized macuna starch decreased because of the weakening of molecular and granular structure of starches during oxidation by increasing carbonyl and carboxyl group and depolymering of starch. These substituent groups restrict the tendency of the starch molecules to realign after cooling, thus facilitating lower setback values of the oxidized macuna starches. Also, the breakdown value (BV) of oxidized mucuna starch increased indicating its reduced resistance to shear and heat. Starch acetylation enhances the waterholding capacity of the mucuna starch resulting in a higher peak viscosity and the development of more organized structures, leading to a higher resistance to deformation leading to a reduction in BV (Adebowale and Lawal, 2003). The effects of hydroxypropylation on the interaction between starch polymers are either by preventing the close association of starch chains and restricting the formation of interchain hydrogen bonds or by changing the hydrophilicity of the starch molecules and thus altering bonding with water molecules. The overall observed effects of hydroxypropylation are consistent with a reduction in bonding between starch chains and consequently increase the hydration of the starch granule (Liu et al., 1999; Singh et al., 2007).

Hydroxypropylated yellow sorghum starch increased peak viscosity because the hydrophilic group caused a loosened swollen starch polymer network with additional water and resulted in higher viscosity in increasing hydration volume (Olayinka et al., 2015). Etherified WMS [1-allyloxy-2-hydroxypropyl (AHP)-WMS] and etherified maize starch (AHP-MS) showed a higher peak viscosity, breakdown, setback, and a higher final paste viscosity than the native starch indicating that the ability to form a viscous paste is increased when the WMS and MS are etherified. Amylose-enriched maize starch (AEMS) hardly showed any swelling because of a high amylose content, and thus displayed very low viscosity. The substituted AEMS also showed a low peak viscosity, low breakdown, and low setback, indicating less swelling of granules and the formation of gel network did not seem to occur in AHP-modified starch (Huijbrechts et al., 2008). Substitution of starch by octenyl succinic anhydrate introduces both hydrophilic and hydrophobic groups to starch. With the optimum DS (0.04), the PV of the wheat starch increased with an increase of DS. The hydrophobic alkenyl groups were introduced to hydrophilic native wheat starch, leading to starch granule aggregation. As a result, with an increase of DS, the more aggregated wheat granules break down resulting in a lower HV. Therefore, OSA-modified wheat starch with low DS level could be used as a thickener to replace unmodified starch (Bao et al., 2003).

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