Analysis of volatile components of citrus oil causing oleocellosis in citrus fruits

Abstract: Citrus, consumed widely throughout the world since its appearance, flavor and nutritional value, is one of the most popular subtropical fruits in the world. However, oleocellosis that often occurs at harvest time and during postharvest storage has a negative effect on the external quality of citrus fruits due to the appearance defects formed on the surface of the pericarp, eventually causes extensive economic damage to citrus industry. Oleocellosis, a major physiology disorder in citrus, is characterized by the appearance of outstanding oil glands of blemish area, tissue necrosis occurred in the adjacent epidermis of oil glands, and formation of green, yellow even brown spots in the disorder area. It was reported that the occurrence of oleocellosis is highly correlated with the release of peel oil from the glands located in citrus pericarp; oleocellosis results from various injuries such as friction from hail and the mechanical damage, and the injuries will induce the breakdown of glands to form oleocellosis development. Those studies about oleocellosis development revealed that there was a certain correlation between oleocellosis and citrus oil during the oleocellosis occurrance. At present, little information was available on which citrus oil components could cause the occurrence of oleocellosis in citrus fruits. In generally, the citrus oil obtained by the cold compression method consists of non-volatile components (including two kinds of compounds: lipo-soluble and water-soluble) and volatile components. The induction of these two components in citrus oil on oleocellosis development of citrus fruit has not been reported. In terms of that, the fruits of Jincheng, Navel orange and Ponkan mandarin fruits were used as raw material, and the commercial citrus oil and extracted citrus oils from the three citrus varieties were used to treat the three types of citrus fruit to confirm whether the oleocellosis could be induced by the two types of oils, respectively. The results showed that the oleocellosis symptoms both occurred in the three citrus types after the treatment of commercial citrus oil and extracted citrus oils, respectively, which indicated that the citrus oil could induce the oleocellosis. Additionally, non-volatile components in citrus oil which were treated with the distilled water- and ethanol-soluble also were used to induce the three citrus types, respectively. The results showed that the treatment of dissolving substances (including liposoluble and water-soluble) of non-volatile compositions in citrus oil on citrus fruits did not induce the occurrence of oleocellosis. From those results above, it could be concluded that the volatile components in the citrus oil were the main reason for oleocellosis development. Therefore, the twenty-six kinds of volatile componends that that significantly changed in oleocellosis peels compared to the healthy ones were applied to induce the three citrus types of fruits to study the oleocellosis incidence and symptoms based on this conclusion, respectively. The results showed that twenty-one volatile components contained five terpene hydrocarbons (including d-limonene, β-myrcene, sabinese, γ-terpinolene and α-terpinolene), five terpene alcohols (including linalool, β-terpineol, α-terpineol, geraniol and citronellol), six aldehydes (including nonanal, decanal, undecanal, dodecanal, citronellal and geranial), two oxides (including (E)-limonene oxide and (Z)-limonene oxide), two esters (including geranyl acetate and neryl acetate) and one ketone (including (-)-carvone) compounds could induce oleocellosis development of Jincheng, Navel orange and Ponkan fruits, however, the volatile compositions of α-pinene, β-caryophyllene, valencene, β-farnesene and α-farnesene could not incuce the oleocellosis development of the three citrus types, respectively. In summary, the volatile components in citrus oil which could induce oleocellosis development of citrus fruit were identified by studying the induction of commercial citrus oil, extracted citrus oils and non-volatile compositions in citrus oil on oleocellosis development of citrus fruit. In addition, oleocellosis development in Jincheng, Navel orange and Ponkan induced by the twenty-one volatile components of citrus oil has their own characteristics, for example, the phenomena that parts of oil glands in disease areas collapsed were more obvious in Jincheng and Ponkan oleocellosis fruits induced by terpene hydrocarbons compared to Navel orange, and the color of disease areas showed pale brown in the two types of oleocellosis fruits, however, Navel orange oleocellosis fruits showed purple brown.. However, the symptoms of oleocellosis in three varieties of citrus fruit had slightly different, which might be due to different varieties of sensitivity in citrus fruit. Studying the effect of specific components in citrus oil on the oleocellosis of citrus peels will provide a theoretical basis for explaining the mechanism of oleocellosis in citrus peel and theoretical guidance for further exploration on the prevention and control measures.