Nowadays, it is important to reduce food waste and maximize the use of raw materials. However, oil industry by-products of the technological processes contain significant amounts of nutrients that can be used as valuable additives in functional food products. Microwave-assisted digestion employing a nitric acid and hydrogen peroxide mixture was conducted prior to the determination of elemental compositions in the analyzed oil-industry by-products using inductively coupled plasma mass spectrometry (ICP-MS). Elemental analysis of the studied rapeseed meal, rapeseed cake, milk thistle cake, flaxseed cake, and buckwheat husk indicates that these residues were rich sources of macro- (Na, K, Ca, Mg) and microelements (Mn, Zn, Fe, Cu), while heavy metal (Cd) was present only in trace amounts. Interrelationships between the concentrations of the analysed chemical elements and the various oil industry by-products were investigated using principal component analysis (PCA) and hierarchical cluster analysis (HCA). PCA revealed that the first two components accounted for 86.94% of the total variability in element content. Moreover, HCA classified the studied by-products into four main groups based on the determined inorganic analytes.
Kowaluk et al. (Sat,) studied this question.