Shelf-life study of rice pudding: Differences in assorted rice varieties and how they change in texture over time

Abstract: This master thesis was performed in collaboration with Orkla Foods Sweden AB. The aim of the study was to investigate rice pudding and how storage time affects the texture properties of the rice grains inside the pudding, and if this could be related to some of the properties of the raw rice. Rice pudding is produced by Orkla Foods under the label Risifrutti and is a popular in-between meal snack in Sweden. Six different rice varieties with different origin or harvest date were selected for investigation and boiled into rice pudding according to a recipe mimicking the factory production process. The texture of the rice grains inside the rice puddings were analyzed on selected days during a 29-day storage period, with the result that all rice varieties increased in firmness over time to different extent. To investigate what could be the reason for the differences in firmness increase, differential scanning calorimetry (DSC) measurements was performed in parallel with the texture analysis. This was to determine if starch retrogradation could be the cause for the increased grain firmness. It was expected for the DSC to detect endothermic peaks due to starch gelatinization around the starch gelatinization temperature, it was expected for the peaks increase in size over the studied storage period. The starch gelatinization temperature was measured through DSC measurements of the raw rice samples, giving gelatinization temperatures between 66°C and 76°C. Unfortunately, the DSC measurements of the cooked rice pudding grains only detected endothermic reactions around the starch gelatinization temperature in very few of the samples, and then the peaks were very small in size generating rather inconclusive results. Starch could therefore not be said to be the cause for the increased firmness of the cooked rice grains. For the raw rice samples also water absorption, water content and a microscopy study were performed. The water absorption test gave the results that the rice varieties absorbed between 228%-264% of their initial dry weight, and that the rice variety with the highest increase of firmness in the cooked rice grains also had the greatest water absorption. The water content measurements gave that all varieties had between 11.5%-12.7% water content, and the rice variety with the highest water content also had the highest firmness increase in the cooked rice grains. In conclusion, a rice variety with good water absorption, high water content, and high gelatinization temperature showed the lowest firmness of the cooked rice grains. These are all factors that indicate that the rice variety has a rather low amylose content. To avoid high firmness of the cooked rice grains in the rice pudding, amylose content was deemed to be an important factor when choosing rice variety. However, the firmness of the cooked rice grains is not only dependent on the amylose content but on complex interactions between water, amylose, amylopectin, proteins, and lipids. Factors that all need to be considered to get the wanted rice pudding properties.