Note 31: Volatile Organic Composition in Several Cultivars of Peaches

INTRODUCTION

Flavor/fragrance qualities of peaches are greatly dependent on the volatile and semi-volatile organic compounds present both in the sample matrix and in the headspace aroma. There is a concern in the food industry over the quality of peaches due to plant origin and plant maturity. Several investigations have reported that the concentrations of volatile constituents increased with the maturation of peaches. Lactones, particularly delta-lactones, have also been implicated in peach aroma. (2,4) It was recently discovered that benzaldehyde, linalool, and the C10 lactones increased in the final period of peach maturation, while the C6 aldehydes decreased.(1)

The major volatiles of peach appear to be useful indices for determining maturity, and recently, the determination of flavor precursors and intermediates have become the target of flavor studies. To date, headspace GC analysis, cryofocusing techniques, and high-resolution GC have been used for the analysis of peach volatiles from cultivars under development.

However, static headspace techniques are limited in their detection and identification of many organic volatiles and especially the semi-volatiles organics. Other analytical techniques are needed to profile a wider range of volatile and semi-volatile organics in peach and to identify the flavors, fragrances, off-flavors, off-odors and potential contaminants that may be present. The purpose of this investigation was to develop an analytical technique that could detect and identify a wide range of volatile and semi-volatile organic compounds in peach. For this study, volatile organic compounds were purged from samples of peaches followed by trapping on Tenax® TA adsorbent resin using a dynamic purge and trap technique (P&T). The adsorbent traps were subsequently analyzed by thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS). The P&T technique permits the analysis of a wider range of both volatile and semi-volatile organic compounds and is more sensitive as compared to the static headspace technique.

Instrumentation

Samples were collected using a Scientific Instrument Services Purge and Trap System. This apparatus (Fig. 1) consists of a sparge gas inlet connected to a stainless steel purging needle that is inserted through an adapter fitting into a 10 ml test tube. A dry purge gas inlet is located at a right angle to the sparge gas inlet at the top of the apparatus. This can be left in the closed or open position. The purpose of the dry purge is to reduce the water vapor condensation on the adsorbent trap. Opposite the dry purge inlet is the connector for the glass-lined stainless steel (GLT) desorption tube containing the adsorbent resin. The Purge and Trap System also contains two ball rotameters with adjustable needle valves mounted on a stationary base and permits the visual indication and independent adjustment of the carrier gas flow to each of the gas inlets.

Figure 1 - Purge & Trap Apparatus

All experiments were conducted using a Scientific Instrument Services model TD-2 Short Path Thermal Desorption System accessory connected to the injection port of an HP 5890 Series II GC interfaced to an HP 5971 Mass Selective Detector. (3) The mass spectrometer was operated in the electron impact mode (EI) at 70 eV and scanned from 35 to 400 daltons during the GC run for the total ion chromatogram.

A short 0.5 meter by 0.53 mm diameter fused silica precolumn was attached to the injection port end of a 60 meter x 0.25 mm i.d. DB-5MS capillary column containing a 0.25 µm film thickness. The GC injection port was set to 250 degrees C and direct splitless analysis was used. The head of the column was maintained at -70 degrees C using an S.I.S. Cryo-Trap model 951 during the desorption and extraction process and then ballistically heated to 200 degrees C after which the oven was temperature programmed from 35 degrees C (hold for 5 minutes) to 80 degrees C at a rate of 10 degrees /min and to 280 degrees C at 4 degrees /min.

Experimental

The skin and fruit of four fresh cultivars of peach (White Hale, Jersey Peach, Raritan Rose & Loring) and the skin of one spoiled cultivar (White Hale) were analyzed to compare the volatile organics present. Sample sizes of 2-5 g of peach skin or fruit were transferred into a 10 ml test tube and heated to 60 degrees C for 30 minutes. Samples were sparged with high purity helium at 20 ml/min with an additional 25 ml/min dry purge using an S.I.S. Purge and Trap System. Volatile analytes were gas extracted and carried to a preconditioned 4.0 mm i.d. glass-lined stainless steel desorption tube packed with 100 mg of Tenax TA. The desorption tube with sample was then attached to the Short Path Thermal Desorption System and a syringe needle attached. The desorption tube was injected into the GC injection port at desorption block temperatures of 220 degrees C for 5 minutes.

Results and Discussion

Both the skin and fruit of four fresh peach cultivars (White Hale, Jersey Peach, Raritan Rose & Loring) (Figs. 2-5, 7-10) and the skin of one spoiled cultivar (White Hale) (Fig. 6) were analyzed to compare the volatile organics present. The peach skins were found to contain numerous straight and branched chain hydrocarbons, aldehydes, alcohols, ketones and esters (Figs. 2-6). Although they possess many common compounds, each peach had its own distinct fingerprint chromatograph. The flavor compounds benzaldehyde and the aliphatic compounds hexanal, nonanal and decanal were also identified in the peach skins. It has been assumed for a long time that unsaturated fatty acids, primarily linoleic and linolenic acids, are the precursors of these aliphatic compounds and may contribute to the development of rancid flavor. Other contributors to the ripe peach aroma detected in the peach skins included linalool, dihydro-beta-ionone and beta-ionone. Lactones, particularly delta-lactones, have been implicated in peach aroma. (2,4) It has been previously reported that benzaldehyde, linalool and the C10 lactones increased in the final stage of peach maturation, while the C6 aldehydes decreased. The esters found in the peach skins were mainly acetates with hexyl acetate the major one. Methyl octanoate which has a sweet fruit odor was detected only in the skin of the White Hale cultivar (Fig. 2). Octane concentrations were found in the White Hale, Jersey Peach and Loring cultivars and have previously been determined to increased with time during storage (Figs. 2, 3 & 5).

Figure 2 - White Hale Skin - 2.861g. Collected For 30 minutes At 60 Degrees C At 20 ml/min With 25 ml/min Dry Purge and Thermally Desorbed At 220 Degrees C For 5 minutes.

Figure 3 - Jersey Peach Skin - 3.448g.Collected For 30 minutes At 60 Degrees C At 20 ml/min With 25 ml/min Dry Purge and Thermally Desorbed At 220 Degrees C For 5 minutes.

Figure 4 - Raritan Rose Skin - 4.632g Collected For 30 minutes At 60 Degrees C At 20 ml/min With 25 ml/min Dry Purge and Thermally Desorbed At 220 Degrees C For 5 minutes.

Ethanol and a high concentration of the aliphatic compound 1-octen-3-ol were identified in the skin of the spoiled cultivar, White Hale (Fig. 6). The production of ethanol is characteristic of fruit during maturation, especially in an over-ripened fruit, whereas the presence of 1-octen-3-ol suggests that the activity of lipoxygenase and hydroperoxide lyase producing C8 compounds from linoleic acid was occurring in the spoiled peach.

Figure 5 - Loring Peach Skin - 2.462g Collected For 30 minutes At 60 Degrees C At 20 ml/min With 25 ml/min Dry Purge and Thermally Desorbed At 220 Degrees C For 5 minutes.

Figure 6 - White Hale Spoiled Skin - 2.717g Collected For 30 minutes At 60 Degrees C At 20 ml/min With 25 ml/min Dry Purge and Thermally Desorbed At 220 Degrees C For 5 minutes.

Figure 7 - White Hale Fruit - 2.745g Collected For 30 minutes At 60 degrees C At 20 ml/min With 25 ml/min Dry Purge and Thermally Desorbed At 220 Degrees C For 5 minutes.

Although present in peach skin, benzaldehyde, linalool, dihydro-beta-ionone and beta-ionone were not detected in the fruit. However, the alcohols (E)-2-hexenol and (E)-2-hexen-1-ol were detected in each of the fruits (Figs. 7-10). These alcohols probably were produced by the enzyme-induced oxidation of unsaturated fatty acids, primarily linoleic acid and linolenic acid. The major esters found in the fruit were hexyl acetate, 2-hexenyl acetate and (Z)-3-hexen-1-ol, acetate. It is generally considered that esters primarily contribute to the fruity and floral notes and lactones to the peachy background. (5)

Figure 8 - Jersey Peach Fruit - 3.580g Collected For 30 minutes At 60 degrees C At 20 ml/min With 25 ml/min Dry Purge and Thermally Desorbed At 220 Degrees C For 5 minutes.

Figure 9 - Raritan Rose Fruit - 2.541g Collected For 30 minutes At 60 degrees C At 20 ml/min With 25 ml/min Dry Purge and Thermally Desorbed At 220 Degrees C For 5 minutes.

Figure 10 - Loring Fruit - 2.044g Collected For 30 minutes At 60 Degrees C At 20 ml/min With 25 ml/min Dry Purge and Thermally Desorbed At 220 Degrees C For 5 minutes.

Conclusion

Many kinds of flavors are used in the food industry, and there is a demand for new and improved ones. Today, there is a concern in the food industry as to what constitutes the optimum conditions for harvesting peaches, in order to provide a consistent quality, flavor and aroma of peach to the consumer. To date headspace analysis, cryofocusing techniques, and high resolution GC have been used for the analysis of peach volatiles from promising cultivars under development.

The Short Path Thermal Desorption System used in conjunction with the Purge and Trap System permits the detection and identification of trace levels of volatile as well as semi-volatile organics responsible for flavors in peaches. These techniques present a tremendous improvement over the time-consuming solvent extraction techniques routinely used in the laboratory, and can be easily incorporated in flavor studies and general QA/QC testing.

References

1. Horvat RJ, Chapman GW. Comparison of volatile compounds from peach fruit and leaves (cv. Monroe) during maturation. J. Agric. Food Chem. 1990; 38:1442-1444.

2. Jennings WG, Sevenants MR. Volatile components of peach. J. Food Sci. 1964; 29: 796-801.

3. Manura JJ, Overton SV, Baker CW, Manos JN. Short path thermal desorption-design and theory. The Mass Spec Source 1990; XIII (4):22-8.

4. Sevenants MR, Jennings WG. Volatile components of peach. II. J. Food Sci. 1966; 31: 81-86.

5. Sumitani H, Suekane S, Nakatani A, Tatsuka K. Changes in composition of volatile compounds in high pressure treated peach. J. Agric. Food Chem. 1994; 42: 785-790.