FACTORS MODIFYING FATTY ACID COMPOSITION IN RAPESEED (CULTIVAR, HARVEST TIME)

Izabella Jackowska¹, Jerzy Tys^2
1 Department of Chemistry, Agricultural University of Lublin, Poland
² Institute of Agrophysics, Polish Academy of Sciences, Lublin, Poland

ABSTRACT

A study was performed on rapeseed cultivars whose oil, depending on the purpose of their production, would be characterized by suitable fatty acid composition. The study, carried out in the years 2001-2003 on Lirajet, Lisek, Rasmus, Kaszub and Contakt rapeseed cultivars, showed that the fatty acid composition and its profile can be controlled through adjusting the harvest time.

Key words: rapeseed, cultivar, harvest time, fatty acids.

INTRODUCTION

The properties of fats depend primarily on the type of fatty acids included in their composition. The effect of fats consumed on human health results from the ratios between saturated, monounsaturated and polyunsaturated fatty acids [10]. An excessive amount of fats in the diet and their nutritionally unfavourable composition are conducive to the development of certain civilisation diseases [1, 3].

Current nutrition standards specify that the total amount of fats in the daily nutrition ration for an adult human should provide 30% of the energy requirement [2]. Optimum proportions of fatty acids are specified as follows: saturated fatty acids – 8% of the energy requirement, monounsaturated fatty acids – 13% of the energy, polyunsaturated fatty acids – 10% of the energy required. An additional factor increasing the risk involved in consuming excessive amounts of polyunsaturated fatty acids is the fact that those compounds are easily affected by oxidation processes. The products of such processes are hydroxides, endogenous peroxides, epoxy compounds and free radicals. Those products affect the stability of cell membranes, their permeability, functioning and the activity of enzymes occurring in the cell membranes. Until the physiological role of the necessary fatty acids and their derivatives has been thoroughly determined, one should take into account the possibility of harmful effects of excessive content of such nutrients in the diet [14]. In turn, the presence of unsaturated fatty acids in fuels reduces their oxidative stability. Therefore, the optimum composition of fatty acids in fuels is as follows: oleic acid – approx. 75%, linoleic and linolenic acids – not more than 18% [11].

Rapeseed is a rich source of mono- and poly-enic acids characterized by favourable, from the nutritional point of view, ratios between monounsaturated fatty acids – MUFA and polyunsaturated fatty acids – PUFA. Rapeseed content of oil and its composition are affected by the duration of the period of ripening [4].

In view of the above, a study was undertaken, with the objective of assessing the effect of the time of harvest of rapeseed of various cultivars on their fatty acid composition. The study was focused on acids whose presence is of primary significance in rapeseed use in fuel production – 16-, 18- and 20-carbon saturated and unsaturated acids. The study was carried out in the years 2001-2003, on Lirajet, Lisek, Rasmus, Kaszub and Contakt rapeseed cultivars.

MATERIAL AND METHODS

Seed of 4 rape cultivars in 2001 and of 5 cultivars in 2002 and 2003 was collected at various times using the single-stage harvest technology. Analysis of the rapeseed fatty acid composition – both qualitative and quantitative (acc. to AOAC – 969.33) – was performed by means of the method of gas chromatography using the CP 3800 Varian apparatus. Extracted oil was subjected to alkaline hydrolysis and released fatty acids were converted into methyl esters. Samples of the prepared methyl esters were placed on the column with the help of an autosampler:

• carrier gas: helium, rate of flow through the column – 1.4 cm³/min.;

• operating temperature of the column – 120°C with 2°C/min. increments up to 210°C;

• operating time – 127 minutes;

• injector temperature – 260°C;

• detector temperature – 260°C;

• auxiliary gases – hydrogen and air.

Fatty acids were determined on a CP WAX 52CB DF 0.25 UM capillary column with a length of 60 m.

RESULTS AND DISCUSSION

Only in 2001 the content of saturated fatty acids in rapeseed of the Lirajet cultivar was the highest at the delayed harvest time, while in 2002 and 2003 it was the highest at the earliest harvest times. In the successive years the total content of fatty acids was strongly differentiated and amounted to 7.63, 6.02 and 3.55% d.m., respectively. In none of the years the time of harvest affected the variability of the content of palmitoleic acid in rapeseed of that cultivar. In 2002 and 2003 rapeseed collected at the delayed times of harvest contained greater amounts of oleic acid and the lowest content of eicosanoic (arachidic) acid. Among the unsaturated 18-carbon acids, the highest content of oleic acid and at the same time the lowest content of linoleic and linolenic acids were recorded in 2001 for the earliest time of harvest. A similar relationship was observed in 2003 for the final two times of rapeseed harvest (tab. 1).

Rapeseed of the Lisek cultivar, collected at the earliest harvest times in all the years of the study, were characterized by the highest content of saturated fatty acids – palmitic and stearic acids. A similar relationship was observed for an unsaturated acid – palmitoleic acid. In 2001 and 2002 the content of that last acid in rapeseed collected at the delayed harvest time was less than a half of that for the earlier harvest time. With delay in the time of harvest in 2002 and 2003, the content of C_16:2 acid decreased. Rapeseed collected in 2002 and 2003 at the latest harvest times contained the greatest amounts of oleic acid and had the lowest content of linolenic acid (tab. 2).

Rapeseed of the Rasmus cultivar in 2003 had the least differentiation in the content of palmitic acid, ranging from 3.32 to 3.81% d.m. Saturated acids – palmitic and stearic – were determined in the greatest amounts at the earlier times of harvest (except for the C₁₈ acid in 2003). Rapeseed collected at earlier harvest times in all the years of the study contained the greatest amounts of palmitoleic acid with, at the same time, the lowest content of the C_16:2 acid. Among the unsaturated 18-carbon acids, the highest content of oleic acid was recorded in rapeseed collected in 2001 and 2002 at the latest harvest time. In 2003 the time of harvest only slightly differentiated the content of oleic acid (66.22-66.70% d.m.). The highest content of unsaturated acids – linoleic and linolenic – in 2001 and 2003 was observed for the most delayed times of harvest (tab. 3).

Rapeseed of the Kaszub cultivar collected at the earlier times of harvest was characterized by a higher content of saturated acids – palmitic, stearic and arachidic (with the exception of the C₂₀ acid in 2001). There was no clear relation between the time of harvest and the content of 16-carbon unsaturated fatty acids. In 2001, at the 2nd time of harvest (19th July), the highest content of oleic acid was observed in the rapeseed, with simultaneous lowest content of linoleic and linolenic acids. In 2002, also at the 2nd time of harvest (8th July), the rapeseed had the lowest content of linoleic and linolenic acids (tab. 4).

In rapeseed of the Contakt cultivar collected at the earlier times of harvest the total content of saturated acids was the highest in both the years and at a very similar level of 5.24-5.56% d.m. At all the harvest times, the content of unsaturated 16-carbon acids was very low. In both the years, irrespective of the time of harvest, the content of oleic acid varied within the range of 73.79-78.36% d.m., but the highest content was recorded or the later times of harvest. In 2002, rapeseed collected at the later harvest times had the lowest content of linoleic and linolenic acids (tab. 5).

Kotecki et al. [7] found that the content of particular fatty acids in rapeseed oil depended primarily on the weather and on the cultivar factor. Weather conditions during the vegetation period affected the content of particular fatty acids in a non-uniform manner. The ratios of specific fatty acids in the total oil content in rapeseed determine the possible applications of rapeseed oil. The elimination, by means of breeding methods, of erucic acid produced rapeseed oil with increased content of 18-carbon unsaturated acids, and especially of oleic, linoleic and linolenic acids [8].

Linoleic and linolenic acids constitute a highly valuable group of unsaturated fatty acids that are not produced by the human organism and that have to be supplied with food. Also important is the ratio of linoleic acid to linolenic acid; in contemporary doubly improved cultivars of rapeseed that ratio is 2:1, while studies on human nutrition indicate that it should fall within the range of from 6:1 to 3:1 [5, 9].

Plant organs producing the crop are subject, during their life cycle, to a series of growth and development processes that ultimately end with ageing and decay. Both the plant and animal tissues contain the enzyme of D9-desaturase which catalyses the creation of a dual bond between the 9^th and 10^th atoms of carbon in the molecule of a saturated acid. This transforms the palmitic acid into the palmoleic acid, and the stearic acid into the oleic acid. In plant tissues there are also enzymes that permit the insertion of a double bond between the existing double bond at C₉, and of the remaining carbons in the direction of the methyl end of the methyl chain. The linoleic and linolenic acids are transformed into polyunsaturated fatty acids through the processes of desaturation and elongation of the molecular chain [5].

Current breeding studies are concerned with the ratios of the particular fatty acids [11, 13], as there is a present market demand for rapeseed oil with different ratios of fatty acids, e.g. oil with better stability at high temperatures, for frying and for the production of biofuels [6, 10, 12].

CONCLUSIONS

1. The content of mono- and polyunsaturated fatty acids is a feature that can be controlled by adjusting the timing of rapeseed harvest.

2. The profile of fatty acids proved to be the feature that was the most closely related to the rapeseed cultivar. The ratio of unsaturated fatty acids to saturated ftty acids was the highest in the Lirajet cultivar and the lowest in Rasmus.

3. Rapeseed of the Contakt and Rasmus cultivars was characterized by highly favourable profiles of unsaturated fatty acids, which creates a chance for high oxidative stability of oil.

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Accepted for print: 12.12.2006

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