INSTRUMENTAL EVALUATION OF SELECTED PROPERTIES OF OIL EXTRACTED FROM WALNUTS BEFORE AND AFTER THE ROASTING PROCESS

Nowadays consumers are increasingly looking for alternatives to animal fats. Fats are a source of energy for the body, in addition to being a source of essential fatty acids (EFAs). The composition of the fatty acids determines the nutritional value of the fat.
Walnuts (Juglans regia) have a high fat content of approximately 73%. The oil contained in the nuts is rich in unsaturated fatty acids, including monounsaturated oleic acid and polyunsaturated fatty acids, both of the n-3 and n-6 family. One important thermal process for nuts is roasting, which significantly increases their palatability. Technically, roasting is a drying process at high temperatures. The purpose of roasting is to reveal new flavour and aroma properties of the raw material.
The aim of the current study was to determine and compare the fatty acid composition and oxidative and hydrolytic stability of oils extracted from both roasted and raw walnuts.
The walnuts, which were purchased from the Polish market, were roasted at 100 and 160°C for 9 and 60 minutes (4 sets of conditions in total). Roasting of whole walnuts in a thin layer was carried out in a laboratory oven and repeated twice at each condition. After roasting, the nut oil was extracted with hexane. The following parameters of the extracted oil were determined: fatty acid composition (by gas chromatography), and acid and peroxide number (by titration). The oxidative stability of the oil was determined using a DSC calorimeter equipped with a high-pressure cell (PDSC). Samples were placed in aluminium vessels filled with oxygen, pressurised in an isobaric module (1400 kPa) with the temperature set at 120°C. The oxidation induction time was read from the PDSC curves.
The results indicate that hydrolytic stability decreased after roasting, as a slight increase in acid number was recorded in the oil extracted from roasted walnuts. Oil extracted from walnuts roasted at 100°C had the highest acid number. The oxidative stability of the walnut oil after roasting did not change significantly. The low oxidative stability of walnut oil may be related to the high content of polyunsaturated fatty acids (about 70%).