Study On Polyphenol Extration From Walnut Kernel Skin And Stability Of Walnut Oil And Walnut Protein Powder

Walnut has high nutrition value and unique flavor. Many kinds of walnut products are very popular among consumers, such as walnut milk, walnut edible oil, etc. But in its conventional processing, walnut kernel pellicle is often treated as waste and discarded. The reason is that it contains a large amount of phenolic substance, and they would bring out three consequences. Firstly, it would generate a strong convergence of unpleasant taste. Secondly, it would make walnut protein aggregation and precipitation. Thirdly, it would affect the color and sensory quality of walnut products. Meanwhile, numerous studies showed that the polyphenols in walnut fruits, especially its pellicle have very high antioxidant activity and related health promotion effect. Besides, with the improvement of living standards and more attention paying to their health. Low-fat food has become an irreversible trend, and it is better for middle aged and elderly people's health, and also bring the Gospel to the people deeply puzzled by obesity.This research aims at developing an innovative comprehensive method for walnut kernel utilization. With dehulled walnut kernel as material, a new pretreatment method was put forward, i.e., mild acidic ethanol ultrasound-assisted extraction, and dephenolized kernel was processed with aqueous medium processing for oil and protein powder recovery. This process not only reserved highly active walnut phenolic substance, but also provide high quality walnut oil and walnut protein powder. The main findings were as follows:(1) Acidic ethanol ultrasonic extraction was used to extract polyphenols from walnut kernel. Single factor experimental results shows that the optimum extraction process was hydrochloric acid concentration of 0.05 mol/L, extraction temperature 65?, extraction duration 10 min, repeated extraction times 4. Comparing with traditional alkaline peeling process, ultrasonic extraction reserved as high as 14.85 mg polyphenol /g walnut kernel and the remained polyphenol in the kernel reached as low as 0.52 mg/g walnuts kernel. While after alkaline incubation and peeling processing, most polyphenols in the extract had been destroyed with only 0.51 mg polyphenol /g walnuts kernel, and the remained polyphenol was about 0.65 mg/g walnut kernel.(2) The antioxidant property of walnut polyphenols was measured by using 1,1 diphenyl-2-trinitrobenzene hydrazine(DPPH), 2,2-azino-bis-(3-ethyl-benzothiazole-6-sulfonic acid) ammonium salt(ABTS), iron reduction method, beta-carotene/linoleic acid bleaching method. And the extract was compared with tertiary butyl hydroquinone(TBHQ), VC, and VE. The results show that in the first three systems, walnut polyphenols extract showed stronger in vitro radical scavenging ability or ferrous restore ability even at low concentrations. With UPLC-MS/MS analysis, 40 walnut polyphenols were detected, mainly as tannins hydrolysates, such as casuarinin and casuarictin, certain amount of phenolic acids, such as gallic acid, and flavonoids such as quercetin. Among these polyphenols, 35 kinds was identified.(3) The walnut oil was obtained with aqueous processing method and its storage stability was assessed after adding different antioxidants at temperatures of 4, 25 and 60oC. The peroxide value and acid value were analyzed at different intervals. Compared with the oil from peeled walnut and the squeezed walnut oil, the oil from dephenolized walnut remained the same acid value of about 0.3 mg KOH/g. During storage, the acid value of squeezed walnut oil increased at all temperatures irrespective of added antioxidants. Results also show that at 4?, all three kinds of walnut oil were relatively stable; while at 25?, oil oxidation began to accelerate after a month storage, especially the walnut oil added VE. This shows that VE has oxidation promoting effect in walnut oil system. When the storage temperature was set at 60?, only walnut oil added TBHQ was comparatively stable.(4) The storage stability of walnut protein powder at different temperature from both dephenolized and peeled walnut was studied. The evaluation indexes were the color value, wettability, nitrogen solubility index, protein dispersion index and carbonyl content of walnut powder. The results showed that the dephenolized walnut powder was much whiter than peeled walnut powder, also had a better wettability, a lower carbonyl content and a slower protein dispersion falling index.