Annealing Process

Annealing is a process that involves heating granular starch in water with a concentration greater than 40% w/w at a temperature above the glass transition temperature (Tg), but below the gelatinization onset temperature. Annealing does not cause any visible external change to the starch granules, although there are a few reports of increased porosity, defects, and alteration of granule size. The most consistent effect of annealing is observed in the gelatinization characteristics of the starch, with annealed starches having an increased onset and peak gelatinization temperature and a narrower temperature range. The gelatinization enthalpy remains either unchanged or slightly increased with annealing. Most published studies have not observed any change in the native starch crystalline packing arrangement upon annealing, although recent reports have observed a partial transition from a mixed C polymorph to an A polymorph upon extended annealing.

Annealing does not create new double helices within normal or waxy starch at the molecular level, but it does increase the ordering and improves the registration of existing AP double helices in the crystalline regions of the granule. However, for high-AM maize starches, there is some evidence that annealing does create new double helices, which may lead to compartmentalization of like structures within the granule. Annealing also increases the Tg and rigidity of granule amorphous regions, which contributes to the observed changes in starch gelatinization behavior. Annealing tends to retard crystallite perfection in monophosphate moieties on AP molecules, resulting in a lesser ∆Tp for high P starches, but starches with high native monophosphate levels have greater potential for an increase in ∆H (relative to native starch) after annealing.

Annealing of starch granules leads to reduced solubility, swelling power, and AM leaching, but has little effect on existing AM-lipid complexes. Annealed starch gels have larger elastic modulus values compared to native starch gels, but the effect of annealing on starch pasting properties is inconsistent in the literature and varies depending on the starch source and rheological instrument used. Similarly, the susceptibility of annealed starches to acid- and enzyme-catalyzed hydrolysis is also starch-specific.

Annealing of starch can happen with as low as 20% moisture content and 15°C below the onset temperature (To), but it is more effective with excess moisture (60% w/w) and temperatures close to To. The greatest effect of annealing occurs in the first six hours, but crystalline polymorph transitions require more time. Annealing can be done in a single or multistep process, with temperature cycling resulting in higher gelatinization temperatures.

Leave a Reply

Your email address will not be published. Required fields are marked *