4个鲜食核桃品种耐贮藏性比较及核磁共振研究

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

【目的】探究新疆主栽核桃品种的鲜食贮藏潜力及核磁共振(NMR)技术检测核桃品质的可行性。【方法】以新疆的核桃品种温185、新新2号、扎343和陕西的耐贮品种西扶1号为材料,研究了低温(0±0.5℃)贮藏过程中及核仁水量、含油量、酸价、青皮褐变指数及NMR弛豫谱特征的变化,并对核桃果实进行了核磁共振成像。【结果】核桃的品质随贮藏期的延长而下降,体现在贮藏末期含油量的显著下降(p<0.05)和酸价及褐变指数的显著上升(p<0.05)。核桃仁的T2弛豫谱特征呈现一小一大的双峰曲线,小峰为水峰而大峰为油峰。相关性分析表明,含水量与水峰面积、含油量与油峰面积呈极显著相关性(p<0.01)。核磁共振成像(MRI)能很好地区分核桃的组织结构,还能观察青皮皱缩及褐变变化。【结论】4个核桃品种的耐贮性:西扶1号>新新2号、扎343>温185,核磁共振T2弛豫谱的变化可以表征鲜核桃仁中的水和油脂组分的变化,NMR法可以表征核桃品质。

【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,accurate 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 powerful tool that allows non-destructive visualization of the dynamic changes of the interior structural information 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℃).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℃【.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 imaging 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 storage for browning index.The oil content was determined with the Soxhlet extractor method.Noninvasive 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 geometric center of the walnut kernel during imaging.Data analysis was carried out using the SPSS 19.0.Multiple 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 index value and the lowest value in acid value at 60 d,while Wen 185 was the shortest one on storage period.2.There was a significant linear relationship(R2=0.939)between the moisture content and the corresponding NMR T2 relaxation spectrum amplitude.There was also a significant linear relationship(R2=0.9785)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 walnuts 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】Comprehensive 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.