| Walnut pellicle was a thin film surrounding the surface of walnut kernel. The pellicle of fresh walnutwas yellow or light yellow, and a certain degree of browning happened after drying process. Walnutpellicle was always treated as waste in processing industry. For example, in walnut milk production, toprevent protein precipitation, the first step was to remove the pellicle. However, many researches showedthat walnut pellicle was rich in polyphenols. Similar to tea polyphenols and grape seed polyphenols, walnutpellicle polyphenols were also having many nutrition and health care functions, including quenching thebody free radicals, anti-aging, inhibition of tumor proliferation, etc.Therefore, the development andutilization of walnut pellicle polyphenols were quite important to the total efficiency of walnut industrialchain.This study used eight cultivars of walnut which was mainly grown in northern China, which were‘Jing861’，‘Lipin2’，‘Baofeng’，‘Zhonglin1’，‘Zha343’，‘Luguang’，‘Liaoning1’，‘Xiangling’. Firstlywe measured and compared their contents of phenolic compounds, then the activities of antioxidant andenzyme-inhibition was evaluated, finally the basic constructions of chemical compounds existed in thepellicle were identified. The main results were as follows:(1) Comparing with water-immersing method and alkali-boiling method, liquid nitrogen freezingmethod possessed many advantages such as shorter operation time, better peeling effect, stable kernel qualityand few environmental pollution. After processing by these three pellicle-removing methods, the content ofphenolics remain in pellicles were28%,17%,0%, respectively, showing that the liquid nitrogen freezingmethod was better for the subsequent process of pellicles.(2) Through ultrasonic assisted solvent extraction, a response surface analysis was applied to optimisethe extraction conditions. The results indicated that the optimum ultrasonic extraction conditions for walnutpellicle phenolics were as follows: extraction temperature53℃，extraction time55min，liquid/solid ratio58mL/g. Under this optimum condition, the maxium extraction yield was30.789%.(3) According to the principle of colorimetry, we determined the total phenolic content, total flavonoidcontent and total condensed tannin content, varying from205.37±5.13to307.31±5.87mg GAE/g pellicle, 79.11±1.80to126.75±2.87mg RE/g pellicle,11.11±0.21to34.54±0.20mg CE/g pellicle, respectively.‘Jing861’was the highest in all the three contents, indicating it had longer shelf-life and greater potential inpellicle development.‘Xiangling’, which was widely cultivated in China, was almost the lowest in all thethree contents, suggesting slighter astringent and better flavor.(4) When using DPPH, ABTS and Ferric Reducing assays as the assessment systems, polyphenols fromwalnut pellicle showed great antioxidant activities and such activities were closely positively related to thetotal phenolic content, with a correlation coefficient of0.9092，0.9184，0.8973, respectively; however, whenusing β-Carotene/Linoleic Acid system, the antioxidant activity was not related the total phenolic content,nor the total flavonoid content and total condensed tannin content, and the value of activity fluctuated withthe reaction process. We suggested that to evaluate the antioxidant activity precisely, proper assessmentsystem should be used to give consistent and repeatable results.(5) Using a-Amylase as the targeting enzyme and soluble starch as the substrate, we evaluated theinhibition activity of polyphenols on this enzyme in vitro. The results showed the median inhibitoryconcentration IC50among eight walnut cultivars varied from100to250μg/mL, and such high inhhibitionactivity was only closely related to the content of condensed tannin, with a correlation coefficient of0.8129,suggesting that pellicles with high content of condensed tannin could be used to develop diabetes drug.(6) Twenty-six chemical compositions of walnut pellicle was identified by Py-GC-MS. These chemicalswere characterized to three types according to their structure features, which were small moleculealcohol/acid class, phenolic acid class and long-chain fatty acid class, occupying the total peak area with apercentage of0.548%,4.46%,7.635%, respectively. The retension time of phenolic acid class was between14to22min, and the specific chemicals included pyrogallic acid（0.182%）、1,2,4-benzenetriol（0.053%）、p-hydroxybenzoic acid（0.076%）、2，4，6-trihydroxybenzoic acid（0.028%）、3，4，5-trihydroxytoluene（0.035%）、1，2，3，4-benzenetetrol-tetracetic acid（0.026%）、benzene-1,2-dicarboxylic acid（0.023%）、3,4-dihydroxyphenylacetic acid（0.054%）、 p-hydroxy-cinnamic acid（0.029%）、gallic acid（3.843%）、3,4,5-trihydroxybenzhydrazide（0.076%）and caffeic acid （0.035%）. Among the identified26chemicalcomponents,14kinds ofchemicals were widely used raw materials, indicating that pellicles had industrialvalues. The above results showed that walnut pellicle was rich in polyphenols, and its activities of antioxidantand enzyme-inhibition were remarkable, thus it was suitable for the development of anti-aging health careproducts. Its chemical components had many specific applications, suggesting a high recycling value. |
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