Analysis of the moisture content and migration changes of blueberries during storage by low-field nuclear magnetic resonance

Water content can directly dominate the edible quality of blueberries during the storage process. This study aims to explore the changes in water content and migration in the internal blueberry with the storage time at different temperatures. The Low-Field Nuclear Magnetic Resonance (LF-NMR) and imaging technology were used to collect the relaxation spectrum and proton density image of blueberries stored at 0℃, 8℃, and 23℃ for 0, 3, 6, 9, and 12 days. Then, the changes and migration of water content in the blueberries were analyzed under different storage conditions. The experimental results showed that the relaxation time and the peak area of the relaxation spectrum were used to effectively estimate the moisture content and migration changes of blueberries during storage. Among them, the temperature was sensitive to the moisture content. Both the vacuolar water content (A23) and the total water content (A2) showed an overall downward trend, as the storage time increased. The relaxation time was shifted to the right with the increase of storage time during the storage of blueberries at 0℃ and 8℃ for 12 days. Specifically, there was no significant change in the water content of the cell wall (A21), whereas, a slightly increasing trend was observed in the cytoplasmic water content (A22). Once the blueberries were stored at 23 ℃ for 9 days, the cell wall and cytoplasmic water increased rapidly. Especially, the vacuolar water decreased sharply after being stored for 12 days, and then transformed into cytoplasmic and cell wall water. There was a slower migration of internal water in the low-temperature storage, where the loss was less, compared with the storage at 23 ℃. In addition to the moisture, an analysis was made on the variations in the gray value, pseudo-color value, shape, and water loss rate of blueberries during storage. It was found that the blueberries at low temperature were stored for at least 12 days, and the shelf life of blueberries was about one week at room temperature. The gray value of blueberries decreased slowly at 0 ℃ and 8 ℃ until 12 d. The decline loss values were all within 17.00% of the gray value at 0 d, and then dropped the cliff-like curve on the 6th day at 23 ℃. There was the relatively intact shape of blueberries stored at 0 ℃ and 8 ℃ for 12 days, indicating the relatively uniform water distribution. However, the blueberries rotted after 9 days of storage at 23 ℃, resulting in the deformation of shape and irregular distribution of water. The water loss rates of blueberries stored at different temperatures for 12 days were 23.68% (0℃), 34.39% (8℃), and 36.74% (23℃), respectively. Among them, the water loss rates at 0 ℃ and 8 ℃ were continued to rise during this period, whereas, there was a decrease at 23 ℃ on the 9th day, due to the rotted already. Consequently, the 'Ruika' blueberries can be stored for more than 12 days at low temperatures, but only about one week at room temperature. The findings can provide theoretical support and data reference for the storage and preservation of blueberries at different temperatures.