Volatile Constituents of the Peel Oils of Several Sweet Oranges in China

Abstract

The peel oil compositions of four kinds of sweet oranges in China, Citrus sinensis Osbeck var. Hongjian, C. sinensis Osbeck var. Anliu, C. sinensis Osbeck var. Sihui and C. sinensis Osbeck var. Washington navel, were investigated by GC and GC/MS. The essential oils were extracted by cold-pressing method. Forty-two to 53 compounds were quantitatively determined for each variety. Their percentages, respectively, were: > 97.3%, > 98.4%, > 97.5% and > 98.0% in hydrocarbons; > 1.5%, > 0.7%, > 0.8% and > 0.9% in total aldehydes; 0.8%, 0.5%, 0.5% and 0.5% in alcohols. Either cis-or trans-limonene oxide was detected in small amounts in each of the four samples, with Hongjiang containing both limonene oxides. δ-3-Carene was commonly quantified at a level of 0.1% in all the samples. The content of aliphatic aldehydes, including octanal, nonanal, decanal and dodecanal, exceeded that of terpene aldehydes, such as neral and geranial in Hongjiang (0.9%) and Washington navel (0.6%), whereas the aliphatic aldehydes in Anliu and Sihui were present to a lesser degree than the terpene aldehydes. Either α- or β-sinensal was detected in trace amounts in each of the four samples. Linalool was the major alcohol in all the samples. Nootkatone was not detected.

Key Word Index

Citrus sinensis, Rutaceae, sweet orange oil, essential oil composition, limonene, orange cultivars.

Introduction

It is said that the Citrus species originated around the district of Assam in India 30 to 40 million years ago. Citrus fruits subsequently spread to East Asia and the Mediterranean region, where a great number of varieties were born by numerous mutations. Today, it is said that there are thousands of citrus varieties on the earth. The most popular citrus fruit has been sweet orange, with production amounting to 66 million t in 2000. Brazil produces the greatest volume of this species, followed by the United States, China, Mexico and Spain. China is the origin of sweet orange whose botanical name smensis refers to China. It is believed that sweet orange was brought to Europe by the Arabs and by Venetian and Portuguese voyages. Subsequently, many varieties of sweet oranges such as Valencia, navel, blood and acidless oranges were bred. Today, China is also one of the countries that mass produces citrus fruits such as sweet oranges and mandarins. Citrus fruits are cultivated mainly in Sichuan, Guangdong and Jiangxi Provinces in China. The Chinese generally have a preference for local sweet oranges, which are mostly consumed in the domestic market. There have been many reports on citrus essential oils, and the studies have been reviewed (1-5). There are, however, few studies on the oil components of citrus fruits in China. Navel oranges are popular throughout the world, but other varieties of sweet orange from China are not well known worldwide. This study aimed to investigate the volatile constituents of cold-pressed oils of several sweet orange types from China.

Experimental

Four kinds of fresh sweet oranges were obtained in the same season, November 2000, in Guangzhou. Citrus sinensis var. Hongjiang (called 'hong jiang chen' in Chinese) and C. sinensis Osbeck var. Anliu (called 'luo gang chen') were obtained at an orchard in Luo gang in Guangzhou (25 km from the center of Guangzhou). Citrus sinensis var. Sihui (called 'sihui ju') was harvested at the Shigou Experimental Farm in Sihui City in Guangdong Province (75 km far away from Guangzhou). Citrus sinensis var. Washington navel (called 'qi chen') which was produced in Jiangxi Province (200 km from Guangzhou; bordering Guangdong Province), was purchased at the wholesale market in Guangzhou. All oranges were kept in a cold room until prepared a few days later.

Standard chemical compounds purchased from Wako Pure Chemical Industries, Osaka; Aldrich Chemical Co., USA, and Fluka Fine Chemicals, Switzerland, were used for identification of the oil components.

The peel oils were extracted by the cold-pressing method and the essential oils were collected in a brine solution on ice (6,7). The extracts were centrifuged at 4000 g for 15 min at 4°C. The supernatants were dehydrated with anhydrous sodium sulfate at 5°C for 24 h and then filtered. The oil samples were stored at -25°C until analyzed.

A Shimadzu gas Chromatograph GC-14A equipped with aflame ionization detector was used. Two capillary columns were used; the first was a DB-Wax column, 60 m × 0.25 mm with a film thickness of 0.25 µm (J&W Scientific, Folsom, CA) and the second was a DB-I column, 60 m × 0.25 mm with a film thickness of 0.25 µm (J&W Scientific, Folsom, CA). Peak areas were integrated with a Shimadzu C-R6A Chromatopackintegrator. The column temperature was programmed from 70°C (2 min) to 230°C (20 min) at a programmed rate of 2°C/min. The injector and detector temperatures were 250°C. Nitrogen was the carrier gas at a flow rate of 2 mL/min. An oil sample of 1 µL was injected, the split ratio of the injector being 1:50. A Shimadzu GC-17A coupled with a Shimadzu QP-5000 was used for GC and GC/MS. The GC conditions were the same as described above. The MS conditions were as follows: ionization voltage, 70 eV; ion source temperature, 250°C. Individual components were identified by comparison of both mass spectra and their GC retention times with those of authentic compounds previously analyzed and stored in a data system. The retention indices were also determined for all constituents by using a homologous series of n-alkanes (C^sub 7^-C^sub 27^).