Rice paste viscosities and gel texture resulting from varying drying and tempering regimen

Abstract

Background and objectives Heated‐air drying followed by tempering (HAT) is effective for increasing rough rice drying rates without compromising head rice yield (HRY). However, relatively little is known about the specific nature of the effects of HAT regimens on end‐use properties of rice. This study determined the effects of drying air conditions (air temperature and relative humidity) and tempering durations on changes in rice paste viscosities and gel texture. Findings Rice paste viscosities and gel texture were not only dependent on drying air temperature but also on how long the rice was exposed to a given temperature during drying and tempering. Air relative humidity (rh) had an indirect effect on rice end‐use properties. Using different HAT regimens, peak viscosity was altered by 16%, final viscosity by 21%, breakdown by 24%, and setback by >500% compared with control samples. Gel strength was increased by approximately 40% with HAT regimens. Conclusions Heated‐air drying and tempering conditions must be carefully selected, not only to minimize HRY reductions but also to minimize variations in the functional properties of rice intended for specified end uses. Likewise, HAT regimens could be selected to produce desired properties for specified end uses. Significance and novelty Inconsistencies in rice flour functionality are a hindrance to an expansive use of rice in end‐use, gluten‐free product development. This study shows that drying, a critical postharvest operation, could introduce variations in rice flour quality, which also implies that HAT regimens could deliberately be utilized to produce flour for various end‐use applications.

Thermal exposure values for predicting changes in rice end‐use properties during drying

“…Again, the most practical changes (≥10%) in breakdown occurred when GATE values exceeded 40 min. Graham‐Acquaah and Siebenmorgen (2020a) observed increases in breakdown of rice samples dried and tempered at temperatures that did not exceed 60°C. Malumba, Massaux, Deroanne, Masimango, & Bera (2009) also observed that drying corn at 60℃ increased peak and breakdown viscosities and concluded that flour samples that swell to a high degree are also usually less resistant to breakdown.…”

“…Tempering helps to reduce moisture and material state gradients that occur during the actual drying process, which is also referred to as active drying in this manuscript. However, tempering durations affect rice end‐use to different extents depending on the moisture content of the rice after active drying (Graham‐Acquaah & Siebenmorgen, 2020a); this suggests that any form of tempering could mask the effects of active drying on rice properties. Therefore, in this study, dried samples were not tempered to permit the evaluation of changes in rice properties (HRY, paste viscosities, and gel texture) without the confounding effect of tempering.…”

“…Rice paste generated from each RVA run was formed into a 20‐mm diameter × 20‐mm‐high gel and the hardness (strength) of the gel measured using a texture analyzer (TA‐XT2i, Texture Technologies, Scarsdale, NY) as described by Graham‐Acquaah and Siebenmorgen (2020a).…”

“…These inconsistencies are due to the notable differences in cultivars’ properties and postharvest handling practices. For instance, varying drying regimens cause significant differences in rice paste viscosities and gel texture, two critical determinants of cereal grains’ suitability for food applications (Bhattacharya et al., 1999; Graham‐Acquaah & Siebenmorgen, 2020a; Malumba et al., 2009).…”

“…Research on rice drying has focused mostly on accurate prediction of moisture content (MC) and has yielded several thin‐layer and deep‐bed drying models. There are also reports on the effects of drying conditions on fissuring, HRY reductions, and end‐use properties (Chen et al., 1997; Dillahunty et al., 2001; Graham‐Acquaah & Siebenmorgen, 2020a; Ondier et al., 2013). These studies have shown that the extent of moisture removal and changes in rice physicochemical properties depend on interactions among drying conditions, particularly temperature and duration of exposure during drying and tempering.…”