The pellicles, which cover the walnut kernels, protect the lipids from the kernel’s
endosperm against the oxidative degradation. In previous studies, we showed that for the
formation of the bad brown colour and bitter taste of walnut kernels, the oxidized polyphenols
(naphthoquinones) are responsible. It is necessary to mention that the ratio between the
average content of total polyphenols in pellicles and in kernels can reach 12.7, and this
excellently correlated with mass fraction of pellicle in the kernels (10–11 %). The majority
(more than 90 %) of phenolic compounds are concentrated in the pellicle.
The purpose of this research was to found the optimal extraction’s conditions of phenolic
compounds from kernel pellicles and to evaluate the antioxidant capacity of these. The yield of
extractive components was determined in function of the concentration of food-grade ethanol
as solvent, temperature of extraction and previous defatting procedure. Defatting was realized
by hydraulic pressing at 20MPa. Results showed that the maximum amounts of phenolic
compounds were extracted with 50 % aqueous ethanol at 22–24 °C from the pellicles, obtained
from native not-defatted kernels. The total polyphenol content in extract from defatted kernels
was of 1.2–1.4 times higher than from the native kernels. But we observed, that the
contamination of kernel’s pellicle with the lipids, extracted from endosperm during pressing,
make much more difficult extraction of phenolic compounds from the pellicle with aqueous
ethanol solutions. The optimization of extraction conditions gives the opportunity to obtain the
extracts with total phenolic content increased by 1.5–2.0 times. Due to a rich phenolic content,
in dry residue consisted approximately 55–60 %, these extracts exhibited pronounced
biological activity. HPLC analysis of polyphenols was performed at "Shimadzu LC-2030C 3DPlus"
with PDA detector, by gradient elution with water: acetic acid (polar phase A) and
acetonitrile: acetic acid (non-polar phase B), using reversed-phased C18 column "Phenomenex"
(150mm*4.6mm*5μm*80nm). HPLC show the presence in extracts of a high quantitates of
biologically active compounds: gallic acid and its derivatives, ellagic acid, casuarictin, catechin,
epicatechin and partially identified derivatives of rutin. Antioxidative activities of the extracts,
obtained from kernel’s pellicle, were determined in vitro by evaluation of peroxyl radical
scavenging capacity. Antioxidant activities were calculated in gallic acid equivalent (GAE) in
mM per gram of the dry residue. Antioxidant activities varied from 2.10 to 3.17 mM GAE/g, in
function of harvesting year and defatting pre-treatment.
To conclude, the pellicle of walnut kernels is a valuable source of the biologic active
phenolic compounds with antioxidative properties, which could be used for development of the
food compositions with functional properties. Notable fact: extraction of phenolic compounds
from the pellicle prevent a kernel’s browning and bittering. Use of so-called dephenolised
walnuts seems to be a promisingly way for the development of smart food technologies.