Abstract: In order to investigate the quality changes in different parts of rice ball during storage at 4 ℃, rice ball was divided into outer, middle and inner parts from outside to inside, and low-field nuclear magic resonance (NMR) spectroscopy, a rice taste analyzer, an X-ray diffractometer, scanning electron microscopy (SEM) and gas chromatography-ion mobility spectrometry (GC-IMS) were used to analyze the physicochemical and microstructural changes of the three parts of rice ball during storage. The results showed that with the increase in storage time, the moisture content of rice ball changed little, but the transverse relaxation time (T2) of each part of rice ball exhibited an overall downward trend, and the bound water was partially transformed into free water. The appearance and taste decreased significantly (P < 0.05), and the overall scores of the outer, middle, and inner layers decreased from 66, 65, and 63 to 50, 49, and 50 points, respectively. The hardness increased significantly (P < 0.05), the viscosity decreased significantly (P < 0.05), while no significant change in elasticity was observed. The concentrations of three volatile components in each part of rice ball decreased, while the concentrations of eight other volatile components increased. Rice ball starch exhibited a B-type crystalline structure, and the relative crystallinity of starch in the outer, middle and inner layers increased from 6.21%, 5.14% and 5.24% to 10.62%, 9.99% and 9.23%, respectively. The reticular structure in the cross-section of rice ball collapsed and the pores became heterogeneous in size. At the same storage time, the difference between the outer and inner layers of rice ball was the greatest, and the outer layer was more seriously aged and deteriorated when compared with the inner layer. The results of the study can provide a theoretical basis for the maintenance and improvement of rice ball quality.

Key words: rice ball; storage; retrogradation; microstructure; volatile components