Changes in chemical composition of catalytically hydrogenated orange oil ( Citrus sinensis)

Abstract

Orange oil obtained from fresh fruit peels by cold pressing was subjected to hydrogenation at 60, 70 and 80°C and 0.46 MPa (H 2 pressure) over Adam's catalyst (PtO 2 at 0.5 and 0.75%, w/w). The chemical composition of neat and hydrogenated oil was established by high-resolution gas chromatography (HRGC) with flame ionization (FID) and mass spectrometric (MS) detection. The principal components of orange oil were monoterpenes (limonene 94.00%, α-pinene 0.54%, sabinene 0.74%, ß-myrcene 1.18%), followed by oxygenated compounds such as alcohols (linalool 0.89% and α-terpineol 0.06%) and aldehydes (citral- Z 0.09%, citral- E 0.14%, citronellal 0.07%). HRGC-FID-MS analysis of the hydrogenated mixtures revealed the presence of over twenty components, mainly the products of hydrogenation of limonene, citral, linalool, ß-myrcene, sabinene and ß-ocimene. α-Pinene and aliphatic aldehydes did not react under these conditions. The appearance of α-terpinolene and the increase of γ-terpinene concentration from 0.05% to 0.15% in the modified oils indicated isomerization associated with hydrogenation. Citral ( E- and Z-) was converted into citronellal (0.06–0.12%) and dihydrocitronellal (0.06–0.16%). Linalool was transformed into 3,7-dimethyl-1-octen-3-ol (0.42–0.63%) and 3,7-dimethyloctan-3-ol (0.11–0.32%). Various isomers formed by H 2 addition to the endo and exo double bonds in limonene constituted the main products (up to 95%) in the hydrogenated orange oils. The concentration of hydrogenated products increased from 62.78 to 89.76% when the temperature dropped from 80 to 60°C at a catalyst concentration of 0.5% (w/w). A smaller change was observed with 0.75% (w/w) catalyst.