Volatile Constituents of Mandarin (Citrus reticulata Blanco) Peel Oil from Burundi

Journal of Essential Oil Research: JEOR, Nov/Dec 2006 by Njoroge, Simon Muhoho, Mungai, Hellen Njoki, Koaze, Hiroshi, Phi, Nguyen Thi Lan, Sawamura, Masayoshi

Abstract

The essential oil constituents of mandarin (Citrus reticulata Blanco) grown in Burundi were extracted by cold-pressing method and analyzed by GC and GC/MS. Fifty-eight constituents, amounting to 97.2% of the total volatiles were identified. Monoterpene hydrocarbons accounted for the most abundant chemical group (94.7%). Limonene was the most prominent constituent (84.8%), followed by γ-terpinene (5.4%), myrcene (2.2%) and α-pinene (1.1%). Sesquiterpene hydrocarbons accounted for a minor quantity (0.2%), where germacrene D and valencene were the main constituents. Oxygenated compounds of various chemical groups constituted 2.3%. Aliphatic aldehydes (0.7%) and terpene alcohols (0.7%) were the major chemical groups. The main constituents were linalool (0.7%), octanal (0.5%) and decanal (0.2%). Ocryl acetate, α-sinensal, decanol and perillaldehyde occurred at 0.1% levels. Thymol, α-sinensal, methyl thymol, and the acetate esters, bornyl, α-terpinyl, geranyl, citronellyl and decyl acetates were detected, each at

Key Word Index

Citrus reticulata, Rutaceae, mandarin oil, essential oil composition, limonene.

Introduction

Mandarins, Citrus reticulata Blanco (Rutaceae), are some of the most highly regarded Citrus fruits for fresh consumption. They are sometimes referred to as 'tangerines,' although 'mandarin' is the more common name. There are several varieties and hybrids of the mandarin species (1). The popular cultivated species include C. unshiu Marcovitch (Japanese Satsuma mandarins, also called Unshiu mikan), C. nobilis Loureiro (king mandarins), C. deliciosa Tenore (Mediterranean mandarins) and C. reticulata Blanco (common mandarins) (1,2). Mandarins are among the major Citrus fruits cultivated in several countries. Burundi is a small country (25,650 Km^sup 2^) located on the northeastern shoreline of lake Tanganyika, in east-central Africa. It is in the tropical zone, between 2°-4°S and 29°- 31°E. It is surrounded by Congo, Rwanda and Tanzania. The altitude ranges between 772 m and 2,670 m above sea level. The temperature varies from 17°-23°C, and the average annual rainfall is about 150 cm. The two wet seasons are February to May and September to December, and the dry seasons are from June to August, and December to January. The main fruits in the country are bananas, papayas, mangoes, pineapples and Citrus. Mandarins are grown on a small scale for household consumption. They are of good eating quality, with pleasant taste, abundant juice and rich aroma. The surplus fruit is sold in the local markets and neighboring countries. Although many studies have been published on mandarin oils (2-6), the literature on Burundian Citrus fruits has been lacking. The volatile components of the Citrus genus have recently been reviewed (4). Limonene constitutes > 85% in most mandarin oils, while α-pinene, β-pinene, sabinene, α-phellandrene, β-phellandrene, (Z)-β-ocimene and terpinolene occur at

Experimental

The mandarin fruit at its best harvest maturity in early May, 2004 was obtained from Bujumbura, the capital of Burundi having been cultivated in the surrounding areas. The fruit peel had a smooth texture and orange yellow color. The sample was kept under refrigeration until preparation of the oil, which was completed within a few days of sample acquisition. The peel oils were prepared in the Department of Food Science and Technology, Kigali Institute of Science, Technology and Management, Kigali, Rwanda. Standard chemical compounds for identification of the oil constituents were purchased from Wako Pure Chemical Industries, Osaka, Japan, Fluka Fine Chemicals, Switzerland, and Aldrich Chemical Co., USA. The peel essential oils were isolated by cold-pressing method as described previously (5,10,11). The peel flavedo was pressed by hand to express the oil, which was collected in a brine solution on ice. The extract was centrifuged at 2000 g for 15 min at 4°C. The supernatant was dehydrated with anhydrous sodium sulfate at 5°C for 24 h, and then filtered. The cold-pressed oil was kept at -21°C until analyzed.

Gas chromatography was conducted using a Shimadzu GC-17A gas chromatograph, fitted with DB-Wax fused silica capillary column (60 m x 0.25 mm, 0.25 µm film thickness, J & WScientific, Folsom.CA, USA),andaflameionization detector (FID). The column temperature was programmed from 70°C (2 min) to 230°C (20 min) at an increasing rate of 2°C/min. The injector and detector temperatures were 250°C. The oil sample of 0.5 µL, was injected. The split ratio of the detector was 1:50. Nitrogen was the carrier gas at a flow rate of 2 mL/ min. The separated peaks were integrated using a Shimadzu C-R8A Chromatopack integrator (Shimadzu, Kyoto).

Gas chromatography/mass spectrometry (GC/MS) was conducted usinga Shimadzu GC-17A coupled with a Shimadzu QP-5050A MS (Shimadzu, Kyoto). The GC operating conditions were the same as those described above. The MS conditions were: ionization voltage, of 70 eV; ion source temperature, of 250°C; scanning range of 25-400 m/z. The oil sample of 0.2 µLwas injected, and the split ratio was 1:50. Identification of the constituents was achieved by comparing their mass spectra with those of compounds registered in the NIST 107 and Wiley 229 commercial spectral libraries of the GC/MS. In addition, the retention indices (RI) of the constituents were compared to those of standard compounds determined under similar conditions, in relation to a homologous series of n-alkanes (C^sub 9^-C^sub 27^) for confirmation of identity. Where standard compounds were not available, only MS data was used, and the identification was tentative. The constituents were quantified (% weight per weight) using internal standards (hexanol and nonadecane).

Results and Discussion

The average weight of the mandarin whole fruit was 91 g, out of which the flavedo constituted 22% (w/w). The yield of essential oil (weight of oil/weight of fruit) was 0.08%. The oil had a strong aroma characteristic of the fruit. The GC elution profile of the constituents is shown in Figure 1, and the identity of the constituents and their concentrations (%) are given in Table I. A total of 59 compounds amounting to 97.2% were separated, out of which 58 (97.1%) were identified. The components were categorized into chemical groups as summarized in the Table I.

Hydrocarbons: The oil contained 12 monoterpene hydrocarbons, amounting to 94.7%. The main constituents were limonene (84.8%), γ-terpinene (5.4%), myrcene (2.2%) and α-pinene (1.1%). The quantities of the major monoterpene hydrocarbons were closely similar to those reported for most mandarin oils (2,3,4). Limonene varied between 57 and 97% among different mandarin varieties (2,4,5). The present oil exhibited a more complex composition of monoterpene hydrocarbons than reported for most mandarin oils (3,4). Sesquiterpene hydrocarbons constituted a low amount (0.2%). Germacrene D and valencene were the main constituents, each at 0.1%. δ- and β-Elemene, (Z)-farnesene, β-cubebene, germacrene B, bicyclogermacrene and β-caryophyllene occurred at