| As the main by-product of citrus processing, citrus essential oil is one of the largest natural flavors and fragrances. It has a fresh and lively flavor, and widely used in food, cosmetics, chemical, pharmaceutical and other fields. The citrus essential oil consists of a large fraction of terpenes and a small fraction of oxygenated components responsible for its distinct smell. The terpenes fraction is composed mostly of limonene which is not stable and the oxygenated aroma compounds are composed mainly of linalool and decanal. So, the terpenes must be removed partly in order to increase concentrations of oxygenated aroma compounds, up-grade quality and prolong the storage time of citrus oil. To achieve high fold concentrated and a high valuable essential oil, in this work, we studied the extraction and concentration technologies of grapefruit essential oil, investigated the effects of different extraction methods on oil quality and characteristics, optimized the conditions of grapefruit essential oil by molecular distillation technology, and then compared the differences in physical and chemical properties between raw oil and concentrated oil. The results were as follows:(1) The grapefruit essential oils were extracted by steam distillation and cold-pressed methods from peels, respectively. The compositions of essential oil extracted by different methods were analyzed using gas chromatography-mass spectrometer (GC-MS). The results indicated that the yields of distillation and cold-pressed methods were2.13%and1.14%, respectively. However, color and aroma of the distilled oil was not so good as the cold-pressed one.39compounds have been identified in the essential oil extracted by distillation method, and37compounds identified in cold-pressed oil. Limonene is the main component in two essential oils, accounting for67.53%and62.95%of total oil, respectively. In addition, there were also hydrocarbons, alcohols, aldehydes and other compounds in essential oils. The contents of linalool in distilled and cold-pressed essential oil were0.8%and1.4%, respectively. And those of decanal in two oils were0.35%and0.4%, respectively. Overall, the quality of cold-pressed essential oil was better than that of the distilled oil.(2) Concentration of cold-pressed grapefruit essential oil was performed by molecular distillation technology, and the oil was separated into two fractions (distillated fraction and residual concentrated fraction). Contents of limonene in distillated and residual concentrated fractions and contents of linalool and decanal in residual concentrated fraction was taken for the response values, the effects of distillation conditions on concentration of grapefruit essential oil were investigated and optimized by using the response surface methodology (RSM). Essential oils were distillated under different temperatures (40-50℃) and pressures (500-700Pa), then the response values were measured. Four quadratic polynomial models were established, and the fitting degree were analyzed through RSM. The results indicated that the response and variables were fitted well to each other by multiple regressions. Meanwhile, the effects of temperature and pressure on the concentration condition of essential oil were evaluated by analyzing the surface of the model diagram, and the optimum conditions for concentration of grapefruit essential oil were determined as:temperature44.89℃, pressure500Pa, feed speed about2ml/min, and scraping speed150r/min. Under the optimal conditions, the contents of limonene (Yi) in distillated fraction, limonene (Y2), linalool (Y3) and decanal (Y4) in residual concentrated fraction were94.89%,49.72%,1.59%,2.34%, respectively. Comparatively, the concentrations of these three substances in raw oil were91.48%,0.198%and0.57%, respectively.(3) Chemical composition in the raw grapefruit oil, distillated fraction and residual concentrated fraction obtained by molecular distillation were characterized by GC-MS. In the distillated fraction, the terpenes accounted for more than98%, among with limonene accounted for93.86%, and the oxygenated compounds was scarcely. However, in the residual concentrated fraction, the content of limonene was decreased from87.22%(in raw oil) to63.33%, while the total content of oxygenated compounds increased greatly from3.41%in crude oil to29.31%, for example, the content of linalool increased from trace to1.82%, and the content of decanal rose from1.15%to4.14%. And the concentration of grapefruit essential oil was about8.6-fold.(4) Aroma-adding capacities of raw grapefruit oil and concentrated oil to grapefruit juice were compared by electronic nose technology and artificial description. Results showed that the additions of essential oils could both significantly improve the aromas of grapefruit juice; however the aromatizing capacity of concentrated oil was superior to that of raw oil. Raw grapefruit oil could endow the juice with grapefruit flavor but slightly nose-touching green flavor, and the concentrated oil could endow juice with rich fruit aroma and sweet fragrance. In addition, by adding concentrated oil, a desired performance of juice could be obtained with a lower additive amount. |
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