Comparison of fresh storage endurance and nuclear magnetic resonance study in four walnut varieties

Abstract:【Objective】Walnut (Juglans regia L.) is an important economic and oil crop. Its kernel is rich in polyunsaturated fatty acids, proteins, carbohydrates and omega-3 fats. It also contains higher amounts of antioxidants (such as carotene, vitamin E and so on) than most other foods. Since the fresh walnut has higher water content in general, the quality is prone to deterioration during storage. But low temperature can effectively prolong storage life. Nuclear magnetic resonance (NMR) is a rapid, accu- rate and non-invasive technology and widely used to detect the quality of food, particularly for fruits and vegetables, meat and aquatic products. The magnetic resonance imaging (MRI) technique is a pow- erful tool that allows non-destructive visualization of the dynamic changes of the interior structural in- formation of plants. At present, there is no report on the storage quality of fresh walnuts by NMR and also no report about storage life on 3 good varieties of Xinjiang Autonomous Region at low temperature (0±0.5 °C). This study aims to prove that NMR technology can be used to detect the quality of fresh walnuts and determine how long 4 walnut varieties can be stored at 0 °C【. Methods】In this experiment, fresh walnuts of Wen 185, Xinxin 2, Za 343 from Xinjiang and Xifu 1 from Shanxi were selected as the research samples. The changes in water content, oil content, acid value and T2 relaxation spectrum of walnut kernels under low temperature storage conditions, as well as browning and MRI T1-weighted im-aging of green walnut fruits were studied. Green husk browning was evaluated by measuring the total browned area on the surface of the green husk according to the following 5 classes: 0=no browning; 1= 0 to 20% browning; 2=20% to 40% browning; 3=40% to 60% browning; 4=60% to 80% browning; 5= 80% to 100% browning. Ten fruits were sampled from 3 replicate at 10 d intervals until the end of stor- age for browning index. The oil content was determined with the Soxhlet extractor method. Noninva- sive monitoring was conducted using a 23 MHz (0.5 T) permanent magnet MRI scanner (MiniMR-60, Niumag, China), equipped with an open-bored probe of 60 mm in diameter. Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences were used for the detection of NMR T2 relaxation spectra. MRI T1-weighted imaging was acquired by employing multiple-spin-echo (MSE) schemes. MSE parameters set were as follows: a repetition time (TR) of 1500 ms, echo-time (TE) of 0.4ms, number of replication (NS) of 4, a field of view (FOV) of 100 mm×100 mm, and 128×128 pixels. The scan location was set at the geomet- ric center of the walnut kernel during imaging. Data analysis was carried out using the SPSS 19.0. Mul- tiple comparison tests were used to statistically compare experimental results of different treatments at a 0.05 significance level.【Results】1. Compared to 0 d, all walnut samples showed a significant decline in oil content but increase in acid value and browning index of green husk browning (p<0.05) on 60 d. Since the storage tolerance of each cultivar can be evaluated according to the terminal browning index and acid value, Xifu 1 was selected as the storage-tolerant cultivar because of the smallest browning in- dex value and the lowest value in acid value at 60 d, while Wen 185 was the shortest one on storage pe- riod. 2. There was a significant linear relationship (R2=0.939) between the moisture content and the cor- responding NMR T2 relaxation spectrum amplitude. There was also a significant linear relationship (R2= 0.978 5) between the oil content and the corresponding NMR T2 relaxation spectrum amplitude. 3. The kernel was easily contaminated and deteriorated due to the structural damage caused by the browning of the green husk in the late storage period. However, Magnetic resonance imaging (MRI) can distinguish the tissue structure of walnuts, and can also observe the shrinkage and browning changes of the green skin, so the quality of fresh walnuts can be detected by MRI. 4. Since the storage quality of fresh wal- nuts can be evaluated by changes in oil content and acid value, and NMR method can be used to detect the oil content. Therefore, the quality of fresh walnut can be measured by NMR【. Conclusion】Compre- hensive evaluation was performed to rank the cold storage tolerance of the four cultivars as follows: Xifu 1>Xinxin 2 and Za 343>Wen 185. The experimental findings can provide a theoretical reference for storage of fresh walnut and application prospects in the study on quality traits of fresh walnuts and other oil crops by NMR.