Changes in chemical composition of pumpkin seeds during the roasting process for production of pumpkin seed oil (Part 1: non-volatile compounds)
Introduction
Pumpkin (Cucurbita pepo L.) seed oil is a common salad oil in Austria. Due to its colour, the foaming and the strong aroma, it has only limited application for cooking. The seeds themselves are eaten and show good results in curing several prostate diseases (Nitsch-Fitz, Egger, Wutzel, & Maruna, 1979).
The oil is produced in the southern parts of Austria (Styria) by traditional very labour intensive methods in small mills that process lots of 50–100 kg of pumpkin seeds. After crushing, the seeds are roasted and pressed at elevated temperatures. The main characteristic of the Styrian Oil Pumpkin is the dark green colour of the thin-coated seeds (Teppner, 2000). The colour of the oil, which is pressed from the seeds, is dark green to red ochre and has a strong red fluorescence. The oil content of the pumpkin seed varies from 42–54% and the composition of fatty acids is dependent on several factors (variety, area in which the plants are grown, climate, state of ripeness). The dominant fatty acids comprise palmitic acid (C16:0, 9.5–14.5%), stearic acid (C18:0, 3.1–7.4%), oleic acid (C18:1, 21.0–46.9%) and linoleic acid (C18:2, 35.6–60.8%). These four fatty acids make up 98±0.13% of the total. Other fatty acids are well below 0.5% (Murkovic, Hillebrand, Winkler, Leitner, & Pfannhauser, 1996).
The content of vitamin E in pumpkin seeds is very high (Murkovic, Hillebrand, Winkler, & Pfannhauser, 1996). The main vitamin E isomers occurring in pumpkin seeds are α- and γ-tocopherols with concentration of n.d.–91 mg/kg and 41–620 mg/kg, respectively. The other two tocopherols occur at very low concentrations. Additionally, α- and γ-tocotrienol are found in significant amounts but have not yet been quantified.
Pumpkin seeds are also rich in plant sterols which have recently become of great interest because of their serum cholesterol-lowering effect (e.g. Miettinen et al., 1995, Jones et al., 2000). They may also be beneficial against colon cancer (Awad et al., 1998, Rao & Janezic, 1992). Plant sterols of pumpkin seeds are of special interest because of their highly characteristic composition (Breinhölder, 2001, Mandl, et al., 1999). In general, the distribution of sterols in the family Cucurbitaceae constitutes one of the most complex patterns known in the plant kingdom (Garg & Nes, 1986). Some of their sterols are characteristic of organisms much lower on the evolutionary scale. In the case of Cucurbita maxima, it was shown that marked changes in sterol composition occur during germination and seedling developments (Garg & Nes, 1984, Garg & Nes, 1985). Pumpkin seeds mainly contain various Δ-7-sterols, rare in other plant seeds (Akihisa et al., 1986, Garg & Nes, 1986, Mandl, et al., 1999). Therefore, the plant sterol profile of pumpkin seed oil can be used to detect adulteration (Mandl et al., 1999). On the other hand, properties of sterols typical of pumpkin seeds and oils are much less known than those of common sterols in other oils. Stability of sterols during food processing and storage is one of the questions which has created much recent research interest (Piironen, Lindsay, Miettinen, Toivo, & Lampi, 2000). However, no studies on stability of pumpkin seed sterols are available.
Polyphenols of the isoflavone type, such as daidzein and genistein (Adlercreutz & Mazur, 1977) and lignan-type like secoisolariciresinol (Kraushofer, 2002), occur in pumpkin seeds. The content of these isoflavones in pumpkin seeds is in the low ppb range (5.6–15.3 ng/g) in contrast to the higher concentrations (210 μg/g) found for the lignan secoisolariciresinol. These naturally occurring compounds are reported to possess estrogenic, antiestrogenic, antioxidative, antiviral, antibacterial, insecticidal or fungistatic properties and have been shown to be proliferative in relation to many types of tumors in cell culture (Mazur & Adlercreutz, 1998). Secoisolariciresinol diglucoside is known for an antitumor effect when provided in the early promotion stage of tumorigenesis (Thompson, Seidl, Rickard, Orcheson, & Fong, 1996). Therefore the levels of human exposure and the concentration responsible for these effects have to be investigated further. From that point of view, the stability of secoisolariciresinol during the roasting process of pumpkin seeds is of interest.
This work describes the chemical changes that occur in the pumpkin seeds during the roasting process. The changes in fatty acids and the micronutrients vitamin E, phytosterols and secosisolariciresinol are investigated from a typical production lot.
Section snippets
Materials
Secoisolariciresinol was bought from K. Wähälä (Department of Chemistry, University of Helsinki), cellulase 1.5 U/mg (Trichoderma viride) and other chemicals were from Merck (Darmstadt, Germany). Dihydrocholesterol (>99%) and the other plant sterols (sitosterol 95%, campesterol 98% and stigmasterol 95%) were purchased from Sigma Chem. Co., St. Louis, MO, USA. Ethanol and glacial acetic acid, were of analytical grade; methanol, n-hexane, dioxane, and acetonitrile were of LiChrosolv quality from
Fatty acids
The composition of the oil fraction of the pumpkin seeds before the roasting process was as follows: stearic acid 12.4%, palmitic acid 5.43%, oleic acid 27.6%, linoleic acid 54.6%. No change was found during the roasting process for three of the main fatty acids. The contents of palmitic acid, stearic acid, and oleic acid remained stable. Only linoleic acid decreased, from 54.6 to 54.2% (Fig. 2), a small but significant change. This small reduction led to oxidation products that were found by
Acknowledgments
The authors whish to thank Franz Seidl from the Ölmühle Wollsdorf/Austria for assistance with the technical roasting process.
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