Osaka Metropolitan University finds rice cooked with superheated steam has porous structure, maintains deliciousness during refrigerated storage

A group from Osaka Metropolitan University and other institutions revealed that cooking rice using "superheated steam" heated to temperatures above 100 degrees Celsius results in a porous structure with small cavities inside the rice grains. A sensory evaluation confirmed that compared with rice cooked in electric rice cookers, it becomes less hard during refrigerated storage and maintains its deliciousness. This could lead to the development of cooking methods for sushi and other dishes that need to be refrigerated until consumption.

Rice becomes hard and loses its stickiness when it cools down. Rice that is heated enters an 'alpha-starch' state, where starch and water are tightly bound, but as it cools, it loses moisture and the starch transforms into a 'beta-starch' form, making the rice dry and crumbly. While households can store rice by dividing it into portions, wrapping it in plastic wrap, and freezing it, when handling large amounts of white rice that cannot be reheated in a microwave, such as for school lunches, or when storing sushi until consumption, refrigeration is necessary from a safety standpoint.

Lecturer Chinami Ishibashi, who studies Culinary Science at the Graduate School of Human Life and Ecology at Osaka Metropolitan University thought that cooking rice with superheated steam, which Professor Shigeo Takenaka from the same department was researching, might produce delicious rice even when refrigerated. Superheated steam is water in a gaseous state created by heating saturated steam to above 100 degrees Celsius at normal pressure. It is used for drying, roasting, sterilization, and enzyme inactivation. When cooking the rice, a superheated steam cooker from Ace System in Izumi City, Osaka Prefecture was used.

The experiment used Akitakomachi rice. 300 grams (2 cups) of white rice were washed with water three times and gently rubbed by hand 20 times without breaking the grains, then 450 grams of water was added, and the rice was soaked for one hour. The soaked rice was cooked using both a superheated steam cooker that releases steam at 120-130 degrees Celsius and an electric rice cooker that heats the pot with a heater installed at the bottom.

After stirring and cooling the cooked rice, weighing showed that the superheated steam cooker averaged 737.2 grams while the electric rice cooker averaged 677.3 grams. The superheated steam cooking appeared to contain more moisture. The cooked rice was stored at 4 degrees Celsius for 24, 48, and 72 hours, and the force required to break the rice grains was measured for each storage time. While all the freshly cooked rice had the same hardness, the electric rice cooker rice became harder during 4-degree storage.

When observing the microstructure inside the rice grains with a scanning electron microscope, it was found that rice cooked with the superheated steam cooker had a more porous structure with smaller cavities than the rice cooked in the electric rice cooker. The time spent at 60-70 degrees Celsius, the temperature at which alpha-starch conversion occurs during cooking, is longer in electric rice cookers, and there is a possibility that air pockets destabilized by gelatinization stick together and become larger. It is believed that the fine distribution of moisture suppresses beta-starch conversion.

When 29 people aged 20-22 compared rice cooked with the superheated steam cooker and rice cooked with the electric rice cooker and refrigerated at 4 degrees for 24 hours, the superheated steam rice was found to be softer, more lustrous, and had higher overall satisfaction.

Ishibashi stated, "In the future, by investigating the mechanism by which superheated steam cooking maintains rice deliciousness and the optimal temperature and time for refrigerated storage, I want to connect this to the development of cooking techniques applicable to the refrigerated storage of seasoned cooked rice such as vinegared rice."

This research was conducted jointly with Ace System and was published in the electronic version of the food-related academic journal Food and Humanity on April 30.