Can conveniently reheated packaged rice still offer the aroma of freshly cooked rice? A 2020 study tested a method in which containers filled with rice and water were moved back and forth during hydration. Early heat transfer improved, but an important aroma compound in aromatic rice was lower than in rice cooked in an electric rice cooker.
The paper combined aroma analysis with heating-process research, starch and color quality measurements, and a survey of what consumers want from convenient rice products.
Hydrating and heating rice inside the container
The researchers compared three rice types: US aromatic Clearfield Jazzman, Thai Jasmine, and nonaromatic parboiled rice. Containers were filled with 128 g of dry rice and 141 g of water, sealed, and used for both hydration and heating. Rice cooked in an electric rice cooker at the same rice-to-water ratio served as a comparison.
The retort treatments included a static condition and reciprocal agitation at 45, 90, or 130 strokes per minute. Agitation was applied during initial heating and the hydration stage, which included a 10-minute hold at 60°C. Subsequent retort heating was static. The aim was to distribute water and heat uniformly before extensive starch gelatinization.
Each process was performed twice with 10 containers per run. Four containers were used for temperature measurements, and the remainder for composition and quality analyses.
Better heat transfer during hydration, but not a shorter overall process
With reciprocal agitation, the temperature inside the containers approached the equipment temperature faster during hydration. The difference between 90 and 130 strokes per minute was small, however, and heat transfer was similar after agitation stopped. Continuing agitation after rice starch had gelatinized did not improve heat transfer.
Calculated heating times at matched sterilization conditions did not differ significantly between static processing and 130 strokes per minute. The demonstrated advantage was therefore mainly in early hydration and heat transfer. The study did not establish a shorter manufacturing process overall.
Less aroma compound than in rice-cooker samples
In the two aromatic rice varieties, the researchers measured 2-acetyl-1-pyrroline (2-AP), a characteristic aromatic-rice compound associated with popcorn-like and buttery notes. Samples were freeze-dried and ground for analysis.
The original table reports 2-AP concentrations in Clearfield Jazzman of 0.776 ppm after rice-cooker preparation, 0.083 ppm after static retorting, and 0.030–0.065 ppm after retorting with reciprocal agitation. For Thai Jasmine, the corresponding values were 0.271 ppm, 0.030 ppm, and 0.006–0.007 ppm. Both contained less after retort processing; the results do not simply show that adding agitation preserves aroma.
In these samples, Clearfield Jazzman contained more 2-AP than Thai Jasmine even after processing. The authors draw attention to its retention of the compound. Because the rice-cooker controls already differed in concentration, however, a higher remaining concentration cannot automatically be interpreted as a higher percentage retained. The process and choice of rice need to be considered together.
The original paper describes reductions uniformly as “15-fold” or “10–100-fold” in some passages, but these descriptions do not align with the condition-specific table values. The measured table values are used here. Furthermore, 2-AP is one indicator of aroma, not a human assessment of the overall aroma or liking when eating the rice.
Starch and color were relatively resistant to change
Resistant starch, digestible starch, and their sum did not differ significantly from each rice type’s rice-cooker control. There were nevertheless individual differences, including higher amylose in Clearfield Jazzman at high agitation speeds and higher total starch in parboiled rice at the highest speed. These findings do not establish the process as a way to increase resistant starch.
Rice type, as well as processing speed, contributed substantially to color differences. In many conditions, the color difference between static and agitated retort samples was small, while the difference from rice-cooker samples tended to be larger. These were instrument-based color measurements. Neither instrumental texture measurements nor sensory assessments of hardness and stickiness during eating were performed; the authors identify them as future work toward practical application.
Consumers care about more than aroma and the rice itself
The other component was a survey of 199 consumers in the southeastern United States. Eligibility included geographic and household-income criteria, so the sample was not representative of the whole United States or Japan. Participants did not taste the rice. They read combinations of descriptions about cooking, meal use, convenience, and packaging, and rated whether they would want to buy and use the product.
The analysis identified roughly three clusters of interests. The first responded to packaging descriptions, including cans, plastic, pouches, reuse, and recycling. The second emphasized rice’s role as a convenient option on busy days, adaptable to different dishes and satisfying to eat. The third focused on cooking and meal preparation, such as heating only the amount needed or cooking in batches for storage.
The original paper’s subgroup counts sum to one person more than the overall sample, and its stated coefficient thresholds for strong responses also differ. It is therefore more appropriate to focus on the diversity of interests than on precise group proportions. Purchase intentions elicited by descriptions must also be distinguished from actual purchases or satisfaction after eating.
Summary
Moving containers back and forth improved heat transfer while rice hydrated inside them. Starch and color changes were relatively small, but 2-AP in aromatic rice declined substantially compared with rice-cooker preparation. In these samples, Clearfield Jazzman contained more after processing. Preserving aroma still requires attention to variety and heating conditions, not just the hydration stage.
The consumer survey also revealed different interests in packaging, convenient rice meals, and cooking and storage practices. Developing convenient rice products benefits both from techniques that preserve components and from understanding what convenience means to the people using them.
The study received support from the LSU AgCenter Hatch program and the Board of Regents, with rice supplied by a research station. At the time, the first author was affiliated with Louisiana State University and Nestlé Purina PetCare Company and participated in process design, analysis, writing, and other work. The coauthors also included researchers affiliated with Mind Genomics Associates. The authors declared no conflict of interest and stated that funders had no role in study design, analysis, writing, or the publication decision.
Reference
Dixon WR, Morales-Contreras BE, Kongchum M, et al. Aroma, Quality, and Consumer Mindsets for Shelf-Stable Rice Thermally Processed by Reciprocal Agitation. Foods. 2020;9(11):1559. doi:10.3390/foods9111559
