JIA-2019-11

2645 WANG Hao et al. Journal of Integrative Agriculture 2019, 18(11): 2644–2651 can be adopted to reduce moisture loss, it will improve the storability and edible quality of fresh corn. However, the development of preserving and processing measures for fresh corn has lagged behind the industry need due to the lack of systematic research on the moisture changes of fresh corn during storage. All these have restrained the expansion of production and consumption of fresh corn. Plants contain water in three different states, free water, semi-bound water, and bound water. Free water refers to water that can flow relatively freely within plants. It has high fluidity and freezes at the freezing temperature. Due to its high fluidity, free water moves outward from plants as vapor via capillary action and osmosis due to the vapor potential difference between the inside and outside of plants. Therefore, free water is easy to be removed during the plant drying process. Semi-bound water, also called adsorbed water, is the moisture absorbed on the surface of colloids in plants. When most of the free water has evaporated, semi-bound water in plants begins to be removed. Bound water refers to water that is in a state of being combined with material molecules in plants. Water is an important component of most foods. It affects the rheological properties, shelf-life, and stability of foods (Zeng et al . 1996). The magnitude and nature of water combined with other food ingredients have direct effects on the quality and appearance of foods. Therefore, appropriate content, distribution, fluidity, and coupling state of water are extremely important for food quality control. As for other foods, water is an important ingredient in fresh corn. The loss of water from fresh corn during storage results in the loss of its weight and freshness. The quality including flavor of fresh corn has a positive relationship with moisture content within a certain range of moisture level. Therefore, understanding of moisture changes in fresh corn during storage is imperative in order to prevent water loss, and thus maintain the quality of fresh corn for a longer time. Nuclear magnetic resonance (NMR) technology is being increasingly used in food research. The major advantages of using NMR lie in the non-destructive and non-invasive nature, high speed, and simplicity of the method. Early in the 1980s, NMR was used to measure the moisture content in grains of corn (Ratkovic 1987) and wheat (Brusewitz and Stone 1987). The transient moisture pro le of corn grains during drying was examined by Song et al . (1990). The moisture losses in different parts of corn kernels with cobs were determined from cross sections of the cobs. During storage, the rate of moisture loss in fresh corn was influenced by the temperature, storage time, and initial moisture content (Song et al . 1990). Song et al . (1992) found that the decrease in signal intensity of corn kernels under NMR was mainly linked to the moisture loss. Mannina and Segre (2002) observed two different phases of change in moisture content of the cereal grains: The diffusion of moisture from the internal parts to the periphery of grain and the evaporation of moisture from the grain surface. Transient moisture transfer in solid food particles during drying is important information for the evaluation of existing drying theories and optimization of the drying process (Song et al . 1992). Nuclear magnetic resonance imaging (MRI) is a non-destructive and non-invasive technique that helps determine the moisture distribution inside intact kernels. Several pioneer studies on grain moisture with MRI were reported in the late 1980s. For instance, Jenner et al . (1998) used MRI microscopy to examine the movement of water in developing wheat grain. The fundamentals and applications of MRI in food sciences were reviewed in detail by McCarthy (1994). According to McCarthy (1994), studies on the measurements of moisture movement with MRI did not lead to any dramatic improvement in the understanding of food materials. However, MRI is a useful non-destructive method for tomographic imaging of sections of food grains. It allows intuitive real-time observation of water changes during food processing and storage (Massimo et al . 2013), which can provide useful information for improvements in food processing and storage conditions (Alexandru et al . 2013). The NMR and MRI technologies have been extensively used in food research. However, moisture changes, particularly moisture migration among the kernels, cobs, and shucks, have not been adequately investigated in fresh ear corn under different temperatures and time during storage. We examined the distribution and migration of moisture in harvested fresh ear corn under different storage conditions. The objective of this research was to generate in-depth understanding of the mechanisms controlling moisture distribution and migration in fresh ear corn during storage, and thus provide a scientific basis for the development of preserving and processing technologies for fresh ear corn. 2. Materials and methods 2.1. Preparation of fresh ear corn samples Ears of fresh corn ( cv . Kenru 1st) with the cob and shuck were picked at the milk stage from an experimental field of Changchun Tianjin Corn Co., Ltd., in Changchun, Jilin Province, China during August to September in 2016. Corn ears were randomly divided into two groups (three ears per group): a, ears were sliced into approximately 1-cm long pieces, wrapped with gauze, and then attached at the ends to two driers to keep the environment arid. The samples selected were stored for 0, 0.5, 1.0, 1.5, 2.0, and 4.0 h, and water distribution and migration in these samples were examined with NMR and MRI at each storage time. b, ears