Effect of Gamma-Irradiation on Some Chemical Characteristics and Volatile Content of Linseed

Abstract

The effect of irradiation (2.5, 4.0, 5.5, and 7.0 kGy) on chemical properties and volatile contents of linseed was investigated. Consistent decreases were observed in both protein and oil content of the irradiated linseed samples with increasing irradiation doses. The ash content of the irradiated linseed samples increased significantly (P<.05) with increasing irradiation doses except for 5.5 kGy. Irradiation treatment caused irregular changes in palmitic and stearic acid content. Although styrene and p-xylene content decreased as a result of irradiation, 1-hexanol content only decreased at 7.0 kGy. Benzaldehyde, p-cymene, and nonanol were not determined at irradiation doses above 4.0 kGy

Introduction

D uring processing, storage, transportation, and marketing, microbial contamination of agricultural products is likely to occur, resulting in quality deterioration and economic loss. Radiation treatment has been widely accepted as an effective method for destroying or reducing insects or pathogens, thereby preserving the processing quality and nutritional value of products. The extension of shelf life is the main aim of radiation processing of foods. The requirement of minimal sample preparation, insignificant temperature rise in the product, fastness, and catalyst independence are among the advantages of the radiation treatment.¹ Radiation processing is also a nonthermal method (cold pasteurization) for postharvest preservation of food products.²

The World Health Organization encourages the appropriate use of food irradiation to preserve food products from diseases and losses.³ Consumer attitudes and market studies demonstrated consumers' acceptance of irradiated foods.⁴ Severe rules enforced by the European Union and other countries on the quality and safety of imported food and agricultural products, such as ban on the use of chemical fumigants, have resulted in the possibility of using food irradiation as a reliable physical method of preservation.⁵

Linseed is a valued crop and an important agricultural product both in industry as a food ingredient and in human nutrition and health because of its medicinal properties. Linseed is also one of the most important oilseeds in terms of its role as a functional food: It is a good source of omega-3 fatty acid, alpha-linolenic acid (52% of total fatty acids), and lignans (phenolic compounds).⁶ Consumption of linseed either raw or defatted reduces total and low-density lipoprotein cholesterol in humans.⁷ Linseed oil in high quantities reduces triacylglycerol levels⁸ and is a potent inhibitor of proinflammatory mediators, even when used in domestic food preparation.⁹ Alpha-linolenic acid from linseed oil protects against cardiovascular disease.¹⁰ Because the physiologic effects of linseed and its components are well known, the U.S. National Cancer Institute proposed linseed as 1 of the 6 plant materials for study as a cancer-preventive food.¹¹

Information on how various processes affect the nutritional characteristics of a valuable oilseed is a basic requirement. Although irradiation was proven to be useful in extending the shelf life of many agricultural products, to the best of our knowledge, the effect of irradiation on the chemical composition of linseed has not been investigated. Therefore, the major aim of the present study was to ascertain the effect of irradiation on the chemical composition, fatty acids, and volatile profile of linseed samples irradiated at various doses. The results of the present study suggest an alterative method for preserving linseed.

Materials And Methods

Materials

Linseed was obtained from plants cultivated at the Experimental Horticulture area of Cumra Agricultural Vocational School, Konya, Turkey.

Irradiation procedures

The linseed samples were placed in plastic bottles and irradiated under ambient conditions (20°C) at 4 different target doses (2.5, 4.0, 5.5, and 7.0 kGy) at the Gammapak Co., Cerkezkoy, Tekirdag, Turkey. The irradiation process was carried out in a cobalt-60 gamma irradiator (MDS, Nordion, Ottawa, Canada). Harwell Amber Perspex^® Dosimeters (batch R, type 3042; range, 1–30 kGy; Harwell, Oxfordshire, United Kingdom) were used as reference dosimeters to measure the irradiation doses that were absorbed by irradiated linseed samples (ILS). The control linseed sample (CLS) and irradiated samples were analyzed immediately after irradiation.

Proximate analysis

The CLS and ILS were analyzed for their dry matter, protein, ash, and oil contents according to the methods of AOAC International.¹² The results were expressed as percentage of wet weight.

Fatty acid composition

Fatty acid compositions of the samples were determined according to the method described by Agilent.¹³ For the analyses, the samples were extracted with ether and obtained extracts were stored in Eppendorf tubes; the air was then removed and replaced with nitrogen. The extracts were stored at −60°C until the analysis. The oil (100 mg) was saponified with 100 μL 2N KOH, and 3 mL hexane was added to the mixture. The mixture was vigorously shaken with a vortex for 1 minute and then subjected to centrifuge at 5000 rpm for 5 minutes. Fatty acid compositions were analyzed by gas chromatography (Agilent 6890, Santa Clara, CA, USA) equipped with a flame ionization detector and a 100 m×0.25 mm internal diameter HP-88 column. Injector temperature was 250°C. The oven temperature was kept at 103°C for 1 minute and then programmed from 103 to 170°C at 6.5°C/min, from 170 to 215°C for 12 minutes at 2.75°C/min, and, finally, 230°C for 5 minutes. The carrier gas was helium with a flow rate of 2 mL/min; split rate was 1/50. Fatty acid was identified by comparison of retention times of known standards. The results were expressed as g fatty acid/100 g total fatty acids (%).

Volatile component analysis

Analysis was performed according to the procedure described by Krist et al.¹⁴ by gas chromatography mass spectrometry (Agilent 7890A gas chromatography system) using a mass selective detector (Agilent Technologies) and HP-5MS column (60 m×0.250 mm internal diameter; film thickness, 0.25 μm). The oven temperature was held at 40°C for 10 minutes, heated to 95°C at 3°C/min, heated from 95 to 210°C at 10°C/min, and finally increased to 210°C/min and held for 10 minutes. The carrier gas was helium with a flow rate 0.5 mL/min. The voltage of electron ionization detector was 70 eV. The compounds adsorbed by the fibers were desorbed from the injection port for 15 minutes at 50°C in the splitless mode. The compounds were identified by comparison with spectra from the libraries Flavor 2, NIST 05a, and Wiley7n and by using internal standards.

Statistical analysis

SAS statistical software (SAS Institute, Inc., Cary, NC, USA) was used for data analysis. Data were subjected to analysis of variance, and the comparative analyses between means were conducted by using the Duncan multiple range test. A P value of less than .05 was considered to represent a statistically significant difference.

Results And Discussion

Proximate composition

Table 1 shows the proximate composition of the CLS and ILS. The extent of any change resulting from irradiation depends on several factors, including product properties, irradiation dose, and type of irradiation.¹⁵ External factors, such as temperature, the presence or absence of oxygen, and subsequent storage conditions, also influence the effectiveness of radiation.¹⁶

Table 1.

Proximate Compositions of Control and Irradiated Linseed Samples

Sample	Dry matter (%)	Protein (%)	Oil (%)	Ash (%)
Control sample	95.64^a±0.18	24.89^a±0.38	33.75^a±0.99	5.20^c±0.05
Irradiated linseed samples
2.5 kGy	95.44^a±0.09	24.46^ab±0.32	33.67^a±0.63	5.98^b±0.04
4.0 kGy	95.48^a±0.05	24.25^b±0.12	31.87^ab±1.36	6.37^a±0.07
5.5 kGy	95.39^a±0.06	24.13^b±0.22	31.24^b±0.31	5.21^c±0.01
7.0 kGy	95.61^a±0.01	23.29^c±0.32	33.69^a±0.24	6.11^b±0.15

Values are expressed as mean±standard deviation

abc

Different lowercase letters in the same column indicate the statistical difference (P<.05).

Consistent decreases (P<.05) in the protein content of ILS were noted with increasing irradiation dose. Hameed et al.¹⁷ irradiated 2 chickpea seed varieties at different radiation doses and reported a substantial loss in protein contents of only 1 type of chickpea after 0.5 and 0.8 kGy of gamma-irradiation doses. Bhat and Sridhar¹⁸ reported that the crude protein content of lotus seeds rich in unsaturated fatty acids nonsignificantly decreased with irradiation doses of 2.5, 5, 7.5, 10, 15, and 30 kGy. Gamma-irradiation of Mucuna pruriens seeds, a tropical legume, at various doses (2.5, 5, 7.5, 10, 15, and 30 kGy) resulted in a significant increase in crude protein at all doses.¹⁹ The disruption of protein molecules caused by irradiation can disrupt only the ordered structure of protein molecules, and it cannot affect the amount of protein.²⁰ Literature on the protein content of the irradiated oilseeds is lacking. The tendency of reduction in oil content was seen at all ILS, with the exception of ILS at 7.0 kGy. Generally, an increase in the oil content results from a decrease in the protein content during growth of the oilseeds. The reductions in both oil and protein content observed in this study might be considered meaningful because linseed samples fulfilled the growth period before the irradiation treatment. Hence, it can be said that irradiation treatment at studied doses caused significant decreases in both oil and protein content of the linseed. Niyas et al.²¹ examined the effect of gamma-irradiation on the lipid constituents of nutmeg (Myristica fragrans) at radiation doses of 2.5–10 kGy. They reported a dose-dependent decrease in the oil content and an increase in free fatty acids. Arici et al.²² reported that the oil contents of nonirradiated black cumin seeds and those irradiated with 10 kGy were 36.1% and 31.6%, respectively. They also reported an inverse relationship between oil content and irradiation dose. The results for oil content of ILS obtained in the present study confirm previous findings. On the other hand, ash content of samples significantly (P<.05) increased with increasing irradiation dose; the only exception was observed for the samples irradiated at 5.5 kGy, which have the same ash content with the CLS. This behavior could be attributed to a decrease in oil and protein contents of ILS. To our knowledge, no studies have investigated the ash and oil content of the oilseeds. Dry matter content was not affected by the irradiation treatment.

Fatty acid content

Table 2 shows the fatty acid composition of the CLS and ILS. The main fatty acid of the linseeds was alpha-linolenic acid. The second and third most abundant fatty acids were oleic and linoleic acids, respectively.

Table 2.

Fatty Acid Compositions of Control and Irradiated Linseed Samples

		Irradiated linseed samples
Fatty acid	Control sample	2.5 kGy	4.0 kGy	5.5 kGy	7.0 kGy
C16:0	5.32^bc±0.01	5.34^b±0.01	5.32^bc±0.01	5.30^c±0.01	5.37^a±0.01
C16:1	0.09^a±0.01	0.09^a±0.01	0.09^a±0.01	0.09^a±0.01	0.09^a±0.01
C18:0	5.31^bc±0.01	5.29^c±0.02	5.35^a±0.01	5.33^ab±0.01	5.29^c±0.01
C18:1 n-9	20.70^ab±0.01	20.70^ab±0.01	20.73^a±0.01	20.70^ab±0.01	20.73^a±0.02
C18:2 n-6	11.31^d±0.01	11.39^b±0.01	11.36^c±0.01	11.31^d±0.01	11.41^a±0.01
C18:3 n-6	0.18^a±0.01	0.18^a±0.01	0.18^a±0.01	0.18^a±0.01	0.18^a±0.01
C20:0	0.25^b±0.01	0.27^a±0.01	0.26^ab±0.01	0.27^a±0.01	0.27^a±0.01
C18:3 n-3	56.55^a±0.01	56.44^b±0.01	56.42^b±0.01	56.55^a±0.01	56.37^c±0.01
C20:1 n-9	0.14^a±0.01	0.14^a±0.01	0.14^a±0.01	0.13^b±0.01	0.13^b±0.01
C22:0	0.16^a±0.01	0.16^a±0.01	0.16^a±0.01	0.15^b±0.01	0.16^a±0.01
Total SAFA	11.24^a±0.02	11.06^c±0.01	11,09^b±0.01	11.05^c±0.01	11.09^b±0.01
Total MUFA	20.93^b±0.01	20.93^b±0.01	20.96^a±0.01	20.92^b±0.01	20.95^a±0.02
Total PUFA	68.04^a±0.01	67.96^b±0.01	67.96^b±0.01	68.04^a±0.01	67.96^b±0.01

Values are expressed as mean±standard deviation weight percentage of total fatty acid methyl esters.

abcd

Different lowercase letters in the same row indicate the statistical difference (P<.05).

MUFA, monounsaturated fatty acid; PUFA, polyunsaturated fatty acid; SAFA, total saturated fatty acid.

Radiation processing has been known to cause oxidative changes in lipid constituents.²³ It is well documented that irradiation generates hydroxyl radicals,²⁴ which are very reactive oxygen species initiating lipid oxidation.²⁵ This is particularly true for the irradiation of nuts, which contain high levels of unsaturated fatty acids prone to oxidation, resulting in the development of an off-flavor. The changes in fatty acid composition and concentration of irradiated products are related to the irradiation dose, temperature and storage conditions, and the initial fatty acid composition of the food.²⁶ A dose-dependent formation of hydroperoxides and of secondary oxidation products after irradiation was detected in various plant oils (e.g., sunflower and black cumin [Nigella sativa] oil).^22,27

In linseed, most abundant fatty acids are unsaturated fatty acids, which make up approximately 89% of the fatty acid composition of the linseed oil. This is a typical feature of the seed oils in the Linaceae family. Irradiation treatment decreased the alpha-linolenic and increased the linoleic acid content at all doses (P<.05), with the exception of 5.5 kGy. Reduction and increase in these 2 fatty acids are likely to be affected by each other. Bhat et al.²⁸ reported that amounts of unsaturated fatty acids in Mucuna seeds significantly decreased after irradiation. However, linoleic acid was not present in raw seeds but rather was detected after irradiation; it increased to high levels at 30 kGy. Irradiation of raw Mucuna pruriens seeds rich in unsaturated fatty acids resulted in decreases in some of them while linoleic acid steadily increased.²⁸ Oleic acid content of linseeds stayed almost constant. Other unsaturated fatty acids were found at minor levels. On the other hand, saturated fatty acid content was lower than that of unsaturated fatty acids. Palmitic and stearic acids were the most abundant saturated fatty acids, but the total amount of these fatty acids was approximately 10%. Irradiation treatment caused irregular changes in the contents of both fatty acids.

Another study reported that irradiation of hazelnuts at a dose of 7 kGy resulted in an increase in saturated fatty acids with a parallel decrease in unsaturated fatty acids.²⁹ We found no study in the literature on the effect of irradiation on fatty acid composition of the linseed. Irradiation of pine nuts (Pinus pinae) or fenugreek and turmeric had no effect on the fatty acid composition.^30,31

Arici et al.²² investigated the effect of radiation on fatty acid composition of black cumin and reported that irradiation affected stearic and oleic acid contents of samples; they found no effect for palmitic and linoleic acid contents. The effect of irradiation on the lipid constituents of nutmeg (Myristica fragrans) was examined at radiation doses of 2.5–10 kGy. A dose-dependent decrease was determined in the triacylglycerol content, whereas free fatty acids increased in the irradiated spice, suggesting a breakdown of acylglycerols during radiation processing. The changes were found to be significant at doses above 5 kGy.²¹

Volatile profile

Table 3 shows the volatile profile of CLS and ILS. The dominant volatile compound of CLS was limonene (8.04%), which has a monoterpene structure and was mostly found in spice essential oils. Other major volatile compounds of CLS were 1-hexanol, styrene (aromatic hydrocarbon), and p-xylene (aromatic hydrocarbon) at 7.54%, 6.98%, and 6.67%, respectively. Irradiation treatment resulted in decreases in the amounts of these volatiles but a slight increase in limonene and 1-hexanol content of ILS at 5.5 kGy. It has been known that irradiation generates hydroxyl radicals,²⁴ thereby initiating lipid oxidation.²⁵ In case of oilseeds containing high levels of unsaturated fatty acids such as linseed, development of an off-flavor is likely. Oxidation of fatty acid³² and degradation of carbohydrates³³ or amino acids³⁴ have been reported to be the main sources of volatile compounds formed as a result of irradiation treatment.

Table 3.

Major Volatile Components of Control and Irradiated Linseed Samples

		Irradiated linseed samples (mg/kg)
	Control samples (mg/kg)	2.5 kGy	4.0 kGy	5.5 kGy	7.0 kGy
p-Xylene	6.67^a±0.01	3.76^c±0.01	4.32^bc±0.01	6.52^ab±0.01	4.66^b±0.01
Limonene	8.04^ab±0.02	7.35^d±0.01	7.79^c±0.01	8.14^a±0.02	7.94^b±0.01
Styrene	6.98^a±0.02	5.54^b±0.01	5.40^c±0.01	4.42^d±0.01	5.39^c±0.01
p-Cymene	1.26^a±0.01	1.11^b±0.01	ND	ND	ND
1-Hexanol	7.54^c±0.02	7.72^b±0.01	7.53^c±0.01	8.36^a±0.02	6.99^d±0.01
Benzaldehyde	2.45^c±0.01	3.35^a±0.01	3.03^b±0.01	ND	ND
Nonanol	1.21^b±0.01	2.09^a±0.01	ND	ND	ND

Values are expressed as mean±standard deviation.

abcd

Different lowercase letters in the same row indicate the statistical difference (P<.05).

1-Hexanol content of ILS did not decrease at 2.5, 4.0, or 5.5 kGy but did so at 7.0 kGy (P<.05). Benzaldehyde (aromatic aldehyde), p-cymene (aromatic hydrocarbon), and nonanol were seen in small amounts, and these volatile compounds were not detected in any irradiated sample. Irradiation above 4.0 kGy resulted in the disappearance of such volatile compounds as benzaldehyde, p-cymene, and nonanol. An earlier study reported a progressive increase in 2-pentanone, 1-hexanol, hexanal, and nonanol contents of hazelnuts with increasing irradiation dose.²⁹ Gyawali et al.³⁵ reported that the response of compounds to irradiation varied. The content of some volatile compounds increased after gamma-irradiation, whereas the content of a few major compounds, such as 4-terpineol, myrtenal, tetramethyl-pyrazine, hexanoic acid, azulene, and p-cymene, and of hydrocarbon compounds, such as a-thujene, 2-methyl nonane, and 3,5-dimethyl octane, decreased after irradiation because of the high sensitivity of hydrocarbon compounds to irradiation treatment.³⁶ Dose-dependent behavior was observed for volatile compounds of licorice. The relative content of alcohols increases after irradiation up to 10 kGy, but decreases were seen at doses over 10 kGy. Similarly, other investigators reported that the relative content of such chemical classes as aldehyde, ester, and furan increased after irradiation, but acid and nitrogen-containing compounds decreased.³⁵ High-dose irradiation may break the chemical bonds in the molecules and form free radicals, and the combination of free radicals may produce variations in the kinds and amounts of molecules.³⁷

The results of the present study show that application of irradiation, especially at higher doses, causes significant decreases (P<.05) in the content of the volatile compounds of linseed. Many studies on the effect of irradiation on antioxidant activity of spices have been performed, but none have analyzed the effect of irradiation on volatile compounds of oilseeds.

Conclusions

During processing and postharvest storage of linseed, microbial contamination is likely to occur. Therefore, effective storage techniques are necessary to preserve the quality, quantity, and safety of linseed. The use of irradiation at recommended medium doses (2.5–7.0 kGy) may provide an effective nonthermal alternative to extending the shelf life of linseed. However, our results show that application of irradiation, especially at higher doses, significantly decreased volatile compounds of linseed. In addition, the tendency for a decrease in the protein and oil content of ILS and increase in the ash content suggested that with irradiation for linseed, the alterations in protein, oil, and volatile compounds must be considered.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

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