Ethylene- and Acetylene-Induced Degreening on the Composition of Kagzi Lime (Citrus aurantifolia Swingle) Peel Oil

Journal of Essential Oil Research: JEOR, Nov/Dec 2004 by Selvaraj, Y, Venkateshwarlu, G, Shivashankara, K S, Roy, T K

Abstract

The peel oil from mature (dark green), ripe (full yellow) and degreened 'kagzi' lime (Citrus aurantifolia Swingle) fruit was analyzed by capillary gas chromatography for the aroma composition. Degreening did not affect the peel oil content to any great extent. However, it affected the oil composition. The relative amounts of the major compounds - namely limonene, β-pinene and γ-terpinene - were found to be very similar to those of dark green and degreened fruits. The carbonyl compounds, n-decanal, neral and geranial, in degreened fruits showed proximity with that of ripe fruit.

Key Word Index

Citrus aurantifolia, Rutaceae 'kagzi' lime, degreening, essential oil composition, limonene, β-pinene.

Introduction

Non-uniformity and delayed color development at ripening is a major constraint for consumer acceptance of citrus fruits. Removal of chlorophyll in citrus fruit peel with postharvest application of ethylene and its role in degreening the peel are well understood (1-3). Acetylene, the ethylene analogue, also induces degreening of chlorophyll in peel. Effects of ethylene application on the quality of fruits and vegetables have been reviewed (4). In addition, the effect of ethylene on flavor quality in fruits has been studied (5). Ethylene was reported to induce undesirable flavor in sweet potato (6) and bitter flavor in carrot (7). In the present study, the effect of ethylene and acetylene on 'kagzi' lime flavor during degreening was investigated. Since most of the components which contribute to the characteristic aroma of lime fruit originate in the peel (8), the differences in the yield and major composition of peel oil were compared between ethylene, acetylene treated fruits and mature (dark green) and ripe (full yellow) fruits.

Experimental

Materials: Fresh mature (dark green) and ripe (full yellow) lime fruits were harvested from experimental orchards of Indian Institute of Horticultural Research, Bangalore.

Degreening of lime fruits: Mature (dark green) lime fruits (900 g) were exposed to ethylene in a 5 1 plastic tub. Ethylene was generated from ethrel (2.5 mL) containing 30% actual ingredient (2-chloroethyl phosphonic acid) by adding a few drops of 40% NaOH (9). The experiment was replicated three times.

Mature (dark green) lime fruits (900 g) were exposed to acetylene in a 5 1 tub. Acetylene was generated from calcium carbide (1 g/kg fruit) (10). The experiment was replicated three times.

Sample preparation: The oils were obtained by hydrodistillation (2 h) of lime peel (200 g) of ethylene and acetylene treated as well as untreated mature green and yellow fruits using an all-glass Clevenger-type apparatus.

Gas chromatography: Hewlett Packard 5890 series-II gas Chromatograph equipped with FID and (25 m × 0.2 mm, 0.2 µm thickness) fused silica capillary column coated with Carbowax 20 M (non-polar) and (50 m × 0.32 mm) HP-101 (polar) was employed. Operational conditions: initial oven temperature, 60°C for 6 min; temperature rise rate was 5°C/min to 200°C, then held for 6 min (total run 40 min); injection temperature, 230°C; detector temperature, 250°C; carrier gas, nitrogen; flow rate, 1.0 mL/min; sample size, 0.1 µL; split ratio, 1:40.

The constituents were identified by co-chromatography with standards (limonene, γ-terpinene, linalool, citronellol, geraniol and decanal) and by comparison of their retention indices with literature values (11). The retention indices were determined using homologous series of n-alkanes (C^sub 8^-C^sub 91^) as standards (12). The relative percentages of the constituents were computed from GC (FTD) peak areas without using correction factors

Results and Discussion

Nineteen components representing 83.6-92.8% of lime peel oil collected from various treatments were identified (Table I). Though the ethylene, acetylene treated and dark green and full yellow lime fruits almost had similar oil yields (0.5-0.6%), the composition of degreened fruit peel oil was found to be different from that of normal dark green and full yellow stages. The constituents of lime oil mainly belong to two categories such as monoterpene hydrocarbons and the organoleptically important oxygenated compounds (13,14). Venkateshwarlu and Selvaraj (15) reported a decrease in oxygenated monoterpenes and an increase in monoterpene hydrocarbons during ripening of'kagzi' lime fruit. Contrary to this, the relative percentage of the monoterpene hydrocarbon content in ethylene treated yellow fruit was found to be similar to that of dark green rather than naturally ripened yellow fruits. The most abundant component of lime peel oil is Iimonene and its percentage was found to be 37.5% in ethylene treated yellow fruits and 35.3% and 46.1% in dark green and yellow fruits, respectively. The content of β-pinene was also found almost similar in dark green (15.1%) and ethylene treated yellow (15.5%) fruits. Among the oxygenated components, the content of linalool, terpinen-4-ol, α-terpineol and citronellol were found nearer to that of dark green stage. However, the relative percentage of carbonyl compounds namely decanal, neral and geranial, which are considered to be important flavor constituents of each citrus oil (16) showed proximity to that of yellow stage.