University of Manitoba, Department of Foods and Nutrition, Winnipeg, Manitoba, R3T 2N2 Canada, E-mail: przybyl@ms.umanitoba.ca
a Federal University of Pelotas, Pelotas, Rio Grande do Sul, Brazil
ABSTRACT
Canola oils with modified fatty acid composition has been evaluated for changes in sterol fraction during accelerated storage and frying. Sterols were analysed by HPLC and GC for their composition and oxidation products formed. Changes of sterol components were dependent on the type of oil and contribution of polyunsaturated fatty acids. Oils with higher content of linoleic and linolenic acids showed faster disappearance in sterol content. Among sterols, brassicasterol was oxidized at the fastest rate followed by sitosterol. Phytosterols had higher rate of disappearance than cholesterol however similar oxidation products by chemical structure were identified. It has been observed that degradation of sterols was influenced by content of tocopherols. Rate of sterols disappearance was affected by type of sterol component and content of individual sterol(s).
KEYWORDS: Oxidation, changes, oxidized sterols,
INTRODUCTION
Phytosterols is a group of sterols, which are widely present in foods of plant origin. These compounds are closely related by chemical structure to cholesterol, animal origin sterol. Cholesterol is a subject to spontaneous oxidation during storage and food processing (Maerker 1987). There is accumulating evidence that cholesterol oxidation products have adverse effect on human health (Kubow 1993). Recent studies have demonstrated that similar products can be formed from phytosterols during frying and storage (Przybylski and Eskin 1991; Dutta and Appelqvist 1997).
MATERIALS AND METHODS
Oils: Regular canola oil (CAN); low linolenic canola oil (LLCAN); high oleic canola oil (HOCAN) and hydrogenated canola oils (HYCAN) were obtained from Canadian oil processors.
Standards of sterols and solvents used in this study were purchased from Sigma.
Methods: Standards of sterols were heated in glass beakers in an oven at 75, 95, 120, 155, 1800C for up to 12 hours.
Oils were heated in a glass beaker, 250 mL of each, at 1900C for 72 hours to simulate frying . For analysis oils were sampled after 0, 5, 12, 24, 48, 72 hours of heating.
Oils were place in glass beaker, to keep ratio between surface area to volume equal to one, and stored at 350C and 650C respectively with and without light exposure for 12 and 16 days.
Sterol analysis: Oil samples were saponified at room temperature (Przybylski and Eskin 1991) and analysed on HPLC with UV detector set at 205 nm and C18 column (25cm x 4.6 mm; 5 μm). Mobile phase contained 1% of water in methanol with the flow rate of 1 mL/min.
Fractions were collected from HPLC and analysed by GC-MS for composition and presence of oxidation products. Components were derivatized with SYLON-BTZ and separated on fused silica capillary column Rtx – 5 (30m x 0.25 mm; 0.1 μm). Column temperature was programmed from 900C to 2100C at the rate of 300C/min and further to 2800C at the rate of 50C/min. Upper and lower temperatures was held for 30 and 8 minutes, respectively.
RESULTS AND DISCUSSION
Simulated Frying. During thermal treatment of phytosterol standards changes at different rate were observed (Fig 1). Among analysed sterols, cholesterol showed better resistance to deterioration than phytosterols. Heating at temperature over 1200C caused changes in this component. While plant sterols at this temperature were disappearing at faster rate than cholesterol. Even at 950C significant changes for β-sitosterol and campesterol were observed. These two phytosterols showed the faster rate of deterioration far faster than cholesterol. This indicates that plant sterols can oxidise faster than cholesterol and can be a potential source of oxidation products. These products can have similar effect on cell lipid metabolism as established for cholesterol oxides. Similarity in chemical structure to cholesterol oxides was found. Phytosterol oxidation products can be of importance when restriction in cholesterol availability in our diet is applied, low fat and low animal product diet, body is replacing cholesterol with plant sterols. By this way phytosterol oxides can be implemented into our metabolic system.
During simulated frying, different rate of phytosterols was observed (Fig. 2). In all analysed oils about 50% of sterols were transferred from their natural form into other products. As we observed for standards, phytosterols are converted into oxidation and polymeric products. The same is probably happen during frying, sterol oxides were identified in oils after heating them at frying temperatures. The highest changes were observed in hydrogenated canola oil, where more than 60% of sterols disappeared after 72 hours of heating. Probably presence of metals such as nickel can stimulate decomposition or oxidation of sterol components however other explanation is possible. This unusual behaviour of phytosterols in hydrogenated oils need further attention to assess cause of accelerated oxidation. Among sterols, brassicasterol showed the lowest stability followed by β-sitosterol and campesterol, both of were also the least stable when evaluated in the pure form (Fig. 1).
Accelerated Storage. The same oils when stored under accelerated storage conditions and changes of sterols measured. In Table 1 kinetics of sterol changes is presented.
Table 1. Losses of sterols at the end of accelerated storage time (%).
Oil Brassicasterol Campesterol β-Sitosterol Total
Light Dark Light Dark Light Dark Light Dark
CAN 20.31 17.23 2.14 2.23 8.31 8.89 30.76 28.35
HOCAN 9.87 7.44 1.58 2.15 4.27 6.38 15.72 15.07
LLCAN 7.56 5.25 1.54 2.18 6.23 5.98 15.33 13.41
HYCAN 8.16 6.11 0.98 1.10 5.49 6.18 14.63 13.39
Light and Dark – accelerated storage with and without light presence.
The highest losses of sterols were observed in canola oil, almost one third of these components was transferred into other products at the end of storage period. Again brassicasterol showed the highest rate of oxidation/conversion, indicating that this component is the most susceptible to deterioration. Storage with light presence caused faster deterioration of sterol. Probably the same initiators work for both unsaturated fatty acids and sterols.
Conclusion
Phytosterols present in oils, as major component of minor compounds, can be a source of deterioration products such as oxides. Because their similarity in chemical structure to cholesterol and possible similar oxidation products their effect on lipid metabolism can be important. Due to changes in diet, lowering consumption of cholesterol, higher amounts of plant sterols can be digested and absorbed and with them oxidation products formed during processing and storage. These preliminary results require most extensive investigation on mechanism of sterol oxides formation and their metabolic effects.
References
1. Dutta, P.C., and Appelqvist, L.A. 1997. J. Am. Oil Chem. Soc. 74:647-657.
2. Kubow , S. 1993. Nutr. Rev. 51:33-40.
3. Maerker, G. 1987. J. Am. Oil Chem. Soc. 64:388-392.
4. Przybylski, R. and Eskin, N.A.M. 1991. Proceedings of the Eighth International Rapeseed Congress, Saskatoon, Canada, pp. 888-893.
