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Abstract The contents of five kinds of fatty acids （palmitic acid， stearic acid， oleic acid， linoleic acid and linolenic acid） in 56 collected peanut （Arachis hypogaea Linn.） varieties were determined by gas chromatography. The results showed that in the 56 peanut varieties， oleic acid content was in the range of 36.859%-67.093%； linoleic acid content was in the range of 14.122%-61.025%； palmitic acid content was in the range of 8.583%-20.286%； stearic acid content was in the range of 2.442%-8.971%； and linolenic acid content was in the range of 0.028%-0.093%. Peanut samples No. 9304， 9355 and 9353 had higher oleic acid/linoleic acid （O/L） ratios， which were 4.751， 3.623 and 3.049， respectively， while peanut sample No. 9337 exhibited the lowest O/L value of 0.899.
Key words Arachis hypogaea Linn.； Fatty acid； Gas chromatography
Fatty acid components in peanut （Arachis hypogaea Linn.） seeds are important criterions for the evaluation of peanut quality， including unsaturated fatty acids， saturated fatty acids and a few special acids. Unsaturated fatty acids are the main components of fatty acids in peanut， and oleic acid and linoleic acid account for 80% of the total amount[1-3]. People， who take in a certain amount of unsaturated fatty acids every day， could obtain enhanced antioxidant and anti-free radical capacities， and the morbidity of some diseases could be reduced. For instance， linoleic acid could synthesize phospholipid and prostatic fluid in human body， and also plays an important role in preventing cardiovascular and cerebrovascular diseases and buildup of cholesterol[4-5]. Therefore， the improvement of the quality of peanut germplasm resources is a main technical innovation direction in future， and the determination of fatty acid content and composition in peanut germplasm resources could play a role of guiding planting and production of peanut[5，10].
In this study， with the 56 kinds of peanut seeds from the peanut experimental base of Hainan Tropical Ocean University as experimental materials， the contents of five kinds of fatty acids， i.e.， palmitic acid， stearic acid， oleic acid， linoleic acid and linolenic acid， were determined and analyzed by gas chromatography， so as to understand the differences in the contents of the five kinds of fatty acids in peanut samples. The study is of certain guiding significance to peanut cultivation， breeding and genetic modification and comprehensive development and utilization resources. Materials and Methods
Sources of peanut samples
Fifty six kinds of peanut seed samples were collected from the peanut experimental base of Hainan Tropical Ocean University.
GC conditions
Agilent 7890 gas chromatograph （equipped with FID detector）， constant-voltage mode； carrier gas： highly-pure N2； flow rate： 1 ml/min； flow rate of H2： 35 ml/min； air flow rate： 350 ml/min； sample size： 2 μl； split ratio： 10∶1； temperature at injection port： 230 ℃； detector temperature： 280 ℃； temperature of column oven： 180 ℃； temperature programming： 180 ℃， which was raised at a rate of 5 ℃/min to 230 ℃， which was kept for 12 min[6].
Extraction and identification of fatty acids
Peanut seeds were oven-dried， peeled and ground in a grinding miller for 5 min. Then， 0.2 g of each peanut sample was weighed into a test tube， and added with 2 ml of an extractant （petroleum ether∶diethyl ether=1∶1）. After 15 min of vibrating extraction， 1 ml of methanol-potassium hydroxide solution was added， and the test tube was sealed with thin film for 1 h of methyl esterification. The identification of fatty acids was performed according to standard samples and spectrum library， the peak area of each fatty acid was integrated by normalization method， and the percentage contents of various fatty acids were recorded and analyzed[8-9].
Results and Analysis
Chromatogram of fatty acid standards
The chromatogram of the mixture of five fatty acid standards is shown in Fig. 1.
Determination and analysis of five fatty acids in 56 peanut samples
In the 56 peanut samples， the average percentage contents of palmitic acid， stearic acid， oleic acid， linoleic acid and linolenic acid differed greatly. The average percentage content of oleic acid was as high as 42.717%， and the oleic acid content in peanut sample No. 9304 was the highest， reaching 67.093%. The average percentage content of linoleic acid was as high as 35.548%， and the linoleic acid content in peanut sample No. 9354 was the highest of 61.025%. The average percentage content of linolenic acid was the lowest （0.053%）， and peanut sample No. 9307 exhibited the lowest percentage content of linolenic acid， as low as 0.028%. Among the 56 peanut samples， sample No. 9354 had the highest percentage contents of palmitic acid， stearic acid， linoleic acid and linolenic acid， which also decided that the oleic acid content in sample No. 9354 was lower， and its oleic acid/linoleic acid （O/L） was also lower. Determination results of oleic acid in 56 peanut samples
The results （Fig. 2） showed that the percentage contents of oleic acid were distributed in the range of 35%-67.5%. The contents were mostly distributed in the range of 37.5%-42.5%， and rarely distributed in the range of 35.0%-37.5% and the range of 42.5%-67.5%. The average value of oleic acid was 42.717%， the highest content was 67.093% in peanut variety No. 9304， and the lowest content was 36.859% in peanut variety No. 9325.
Determination results of linoleic acid in 56 peanut samples
The results （Fig. 3） showed that the percentage contents of linoleic acid were distributed in the range of 10%-65%. The linoleic acid contents were mostly distributed in the range of 30%-40%， and less in the range of 10%-30% and the range of 40%-65%. The average percentage content of linoleic acid was 35.548%， the highest linoleic acid content was 61.025% in peanut variety No. 9354， and the lowest content was 14.122% in peanut variety No. 9304.
Determination results of palmitic acid in 56 peanut samples
The results （Fig. 4） showed that the percentage contents of palmitic acid were distributed in the range of 8%-21%. The palmitic acid contents were mostly distributed in the range of 11%-13%， and rarely in the range of 8%-11% and the range of 13%-21%. The average percentage content of palmitic acid was 12.085%， the highest palmitic acid was 20.286% in peanut variety No. 9354， and the lowest palmitic acid content was 8.583% in peanut variety No. 9355.
Determination results of stearic acid in 56 peanut samples
The results （Fig. 5） showed that the percentage contents of stearic acid were distributed in the range of 2%-9%. The stearic acid contents were mostly distributed in the range of 3%-4%， and rarely in the range of 2%-3% and the range of 4%-9%. The average percentage content of stearic acid was 3.818%， the highest stearic acid content was 8.971% in peanut variety No. 9354， and the lowest stearic acid content was 2.442% in peanut variety No. 9304.
Determination results of linolenic acid in 56 peanut samples
The results （Fig. 6） showed that the percentage contents of linolenic acid were distributed in the range of 0.025%-0.095%. The linolenic acid contents were mostly distributed in the range of 0.040%-0.060%， and rarely distributed in the range of 0.025%-0.040% and the range of 0.060%-0.095%. The average percentage content of linolenic acid was 0.053%， the highest linolenic acid content was 0.093% in peanut variety No. 9354， and the lowest linolenic acid content was 0.028% in peanut variety No. 9307. O/L ratio
O/L ratio is an important index for the stability of the quality of peanut oil， and therefore， for peanut oil， O/L is an important factor deciding its storage quality. Table 2 shows the oleic acid and linoleic acid contents and O/L ratios in 10 peanut samples which had higher O/L ratios. It could be seen from Table 2 that the O/L ratios of peanut samples No. 9304， 9355 and 9353 were， respectively， 4.751， 3.623 and 3.049， which were obviously higher than other varieties， and among all the varieties， the O/L ratio of peanut sample No. 9337 was only 0.899 9. This study will provide data support of the breeding of high-quality peanut， screening of seeds and improvement of peanut fatty acid genes.
Conclusion
In the 56 kinds of peanut samples from the peanut experimental base of Hainan Tropical Ocean University， the contents of the five kinds of fatty acids differed greatly. The average percentage content of oleic acid reached 42.717%， while the average percentage content of linolenic acid was as low as 0.052 7%. The values of oleic acid/linoleic acid （O/L） ratio in the peanut samples also differed greatly. Peanut samples， No. 9304， 9355 and 9353 had higher O/L values， which were 4.751， 3.623 and 3.049， respectively， while peanut sample No. 9337 showed the lowest O/L value of 0.899. The differences in the contents of the five kinds of fatty acids in the 56 kinds of peanut samples could provide data support for the screening of local peanut seeds in Hainan and genetic modification， playing a role of guiding the planting and comprehensive development and utilization of Hainan peanut theoretically.
References
[1] LU SJ， YANG YY， XU L， et al. Determination of 3 kinds of fatty acids in Arctium fructus by GC[J]. Chinese Journal of Experimental Traditional Medical Formulae， 2011， 17（20）： 56-60.
[2] LI YQ， ZHU YX， TAO NP， et al. Analysis of fatty acid profile in the meat of three kinds of Coilia ectenes by gas chromatography[J]. Science and Technology of Food Industry， 2014， 35（20）： 57-61， 65.
[3] YANG CD. Fatty acid analysis of peanut and differential expression of genes in 2 high oleic acid mutant[D]. Jilin： Jilin Agricultural University， 2012.
[4] HU R， LI BL. Action and mechanism of polyunsaturated fatty acids on cardiovascular system[J]. Northwest Pharmaceutical Journal， 2008， 23（2）： 118-120.
[5] WANG XY， ZHANG CY， JIA XY. Analysis of fatty acids composition and content in soybean varieties in Hebei Province[J]. Journal of Agricultural University of Hebei， 2007， 30（2）： 15-18. [6] WANG XX. Upgrade and application of gas chromatograph[J]. Technology Innovation and Application， 2014（6）： 292.
[7] CHU XL， LU WZ. Research and application progress of near infrared spectroscopy analytical technology in China in the past five years[J]. Spectroscopy and Spectral Analysis， 2014， 34（10）： 2495-2605.
[8] General Administration of Quality Supervision， Inspection and Quarantine of the People’s Republic of China. GB/T5512-2008， Inspect of grain and oilseeds—Determination of crude fat content in grain[S]. Beijing： China Standards Press， 2008.
[9] General Administration of Quality Supervision， Inspection and Quarantine of the People’s Republic of China. GB/T22507-2008， Animal and vegetable fats and oils—Determination of the content of trans fatty acid isomers of vegetable fats and oils—Gas chromatographic method[S]. Beijing： China Standards Press， 2008.
[10] SNAPP AR， KANG JL ， QI XL， et al. A fatty acid condensing enzyme from Physaria fendleri increases hydroxyl fatty acid accumulation in transgenic oilseeds of Camelina sativa[J]. Planta， 2014， 240（3）： 599-610.
Key words Arachis hypogaea Linn.； Fatty acid； Gas chromatography
Fatty acid components in peanut （Arachis hypogaea Linn.） seeds are important criterions for the evaluation of peanut quality， including unsaturated fatty acids， saturated fatty acids and a few special acids. Unsaturated fatty acids are the main components of fatty acids in peanut， and oleic acid and linoleic acid account for 80% of the total amount[1-3]. People， who take in a certain amount of unsaturated fatty acids every day， could obtain enhanced antioxidant and anti-free radical capacities， and the morbidity of some diseases could be reduced. For instance， linoleic acid could synthesize phospholipid and prostatic fluid in human body， and also plays an important role in preventing cardiovascular and cerebrovascular diseases and buildup of cholesterol[4-5]. Therefore， the improvement of the quality of peanut germplasm resources is a main technical innovation direction in future， and the determination of fatty acid content and composition in peanut germplasm resources could play a role of guiding planting and production of peanut[5，10].
In this study， with the 56 kinds of peanut seeds from the peanut experimental base of Hainan Tropical Ocean University as experimental materials， the contents of five kinds of fatty acids， i.e.， palmitic acid， stearic acid， oleic acid， linoleic acid and linolenic acid， were determined and analyzed by gas chromatography， so as to understand the differences in the contents of the five kinds of fatty acids in peanut samples. The study is of certain guiding significance to peanut cultivation， breeding and genetic modification and comprehensive development and utilization resources. Materials and Methods
Sources of peanut samples
Fifty six kinds of peanut seed samples were collected from the peanut experimental base of Hainan Tropical Ocean University.
GC conditions
Agilent 7890 gas chromatograph （equipped with FID detector）， constant-voltage mode； carrier gas： highly-pure N2； flow rate： 1 ml/min； flow rate of H2： 35 ml/min； air flow rate： 350 ml/min； sample size： 2 μl； split ratio： 10∶1； temperature at injection port： 230 ℃； detector temperature： 280 ℃； temperature of column oven： 180 ℃； temperature programming： 180 ℃， which was raised at a rate of 5 ℃/min to 230 ℃， which was kept for 12 min[6].
Extraction and identification of fatty acids
Peanut seeds were oven-dried， peeled and ground in a grinding miller for 5 min. Then， 0.2 g of each peanut sample was weighed into a test tube， and added with 2 ml of an extractant （petroleum ether∶diethyl ether=1∶1）. After 15 min of vibrating extraction， 1 ml of methanol-potassium hydroxide solution was added， and the test tube was sealed with thin film for 1 h of methyl esterification. The identification of fatty acids was performed according to standard samples and spectrum library， the peak area of each fatty acid was integrated by normalization method， and the percentage contents of various fatty acids were recorded and analyzed[8-9].
Results and Analysis
Chromatogram of fatty acid standards
The chromatogram of the mixture of five fatty acid standards is shown in Fig. 1.
Determination and analysis of five fatty acids in 56 peanut samples
In the 56 peanut samples， the average percentage contents of palmitic acid， stearic acid， oleic acid， linoleic acid and linolenic acid differed greatly. The average percentage content of oleic acid was as high as 42.717%， and the oleic acid content in peanut sample No. 9304 was the highest， reaching 67.093%. The average percentage content of linoleic acid was as high as 35.548%， and the linoleic acid content in peanut sample No. 9354 was the highest of 61.025%. The average percentage content of linolenic acid was the lowest （0.053%）， and peanut sample No. 9307 exhibited the lowest percentage content of linolenic acid， as low as 0.028%. Among the 56 peanut samples， sample No. 9354 had the highest percentage contents of palmitic acid， stearic acid， linoleic acid and linolenic acid， which also decided that the oleic acid content in sample No. 9354 was lower， and its oleic acid/linoleic acid （O/L） was also lower. Determination results of oleic acid in 56 peanut samples
The results （Fig. 2） showed that the percentage contents of oleic acid were distributed in the range of 35%-67.5%. The contents were mostly distributed in the range of 37.5%-42.5%， and rarely distributed in the range of 35.0%-37.5% and the range of 42.5%-67.5%. The average value of oleic acid was 42.717%， the highest content was 67.093% in peanut variety No. 9304， and the lowest content was 36.859% in peanut variety No. 9325.
Determination results of linoleic acid in 56 peanut samples
The results （Fig. 3） showed that the percentage contents of linoleic acid were distributed in the range of 10%-65%. The linoleic acid contents were mostly distributed in the range of 30%-40%， and less in the range of 10%-30% and the range of 40%-65%. The average percentage content of linoleic acid was 35.548%， the highest linoleic acid content was 61.025% in peanut variety No. 9354， and the lowest content was 14.122% in peanut variety No. 9304.
Determination results of palmitic acid in 56 peanut samples
The results （Fig. 4） showed that the percentage contents of palmitic acid were distributed in the range of 8%-21%. The palmitic acid contents were mostly distributed in the range of 11%-13%， and rarely in the range of 8%-11% and the range of 13%-21%. The average percentage content of palmitic acid was 12.085%， the highest palmitic acid was 20.286% in peanut variety No. 9354， and the lowest palmitic acid content was 8.583% in peanut variety No. 9355.
Determination results of stearic acid in 56 peanut samples
The results （Fig. 5） showed that the percentage contents of stearic acid were distributed in the range of 2%-9%. The stearic acid contents were mostly distributed in the range of 3%-4%， and rarely in the range of 2%-3% and the range of 4%-9%. The average percentage content of stearic acid was 3.818%， the highest stearic acid content was 8.971% in peanut variety No. 9354， and the lowest stearic acid content was 2.442% in peanut variety No. 9304.
Determination results of linolenic acid in 56 peanut samples
The results （Fig. 6） showed that the percentage contents of linolenic acid were distributed in the range of 0.025%-0.095%. The linolenic acid contents were mostly distributed in the range of 0.040%-0.060%， and rarely distributed in the range of 0.025%-0.040% and the range of 0.060%-0.095%. The average percentage content of linolenic acid was 0.053%， the highest linolenic acid content was 0.093% in peanut variety No. 9354， and the lowest linolenic acid content was 0.028% in peanut variety No. 9307. O/L ratio
O/L ratio is an important index for the stability of the quality of peanut oil， and therefore， for peanut oil， O/L is an important factor deciding its storage quality. Table 2 shows the oleic acid and linoleic acid contents and O/L ratios in 10 peanut samples which had higher O/L ratios. It could be seen from Table 2 that the O/L ratios of peanut samples No. 9304， 9355 and 9353 were， respectively， 4.751， 3.623 and 3.049， which were obviously higher than other varieties， and among all the varieties， the O/L ratio of peanut sample No. 9337 was only 0.899 9. This study will provide data support of the breeding of high-quality peanut， screening of seeds and improvement of peanut fatty acid genes.
Conclusion
In the 56 kinds of peanut samples from the peanut experimental base of Hainan Tropical Ocean University， the contents of the five kinds of fatty acids differed greatly. The average percentage content of oleic acid reached 42.717%， while the average percentage content of linolenic acid was as low as 0.052 7%. The values of oleic acid/linoleic acid （O/L） ratio in the peanut samples also differed greatly. Peanut samples， No. 9304， 9355 and 9353 had higher O/L values， which were 4.751， 3.623 and 3.049， respectively， while peanut sample No. 9337 showed the lowest O/L value of 0.899. The differences in the contents of the five kinds of fatty acids in the 56 kinds of peanut samples could provide data support for the screening of local peanut seeds in Hainan and genetic modification， playing a role of guiding the planting and comprehensive development and utilization of Hainan peanut theoretically.
References
[1] LU SJ， YANG YY， XU L， et al. Determination of 3 kinds of fatty acids in Arctium fructus by GC[J]. Chinese Journal of Experimental Traditional Medical Formulae， 2011， 17（20）： 56-60.
[2] LI YQ， ZHU YX， TAO NP， et al. Analysis of fatty acid profile in the meat of three kinds of Coilia ectenes by gas chromatography[J]. Science and Technology of Food Industry， 2014， 35（20）： 57-61， 65.
[3] YANG CD. Fatty acid analysis of peanut and differential expression of genes in 2 high oleic acid mutant[D]. Jilin： Jilin Agricultural University， 2012.
[4] HU R， LI BL. Action and mechanism of polyunsaturated fatty acids on cardiovascular system[J]. Northwest Pharmaceutical Journal， 2008， 23（2）： 118-120.
[5] WANG XY， ZHANG CY， JIA XY. Analysis of fatty acids composition and content in soybean varieties in Hebei Province[J]. Journal of Agricultural University of Hebei， 2007， 30（2）： 15-18. [6] WANG XX. Upgrade and application of gas chromatograph[J]. Technology Innovation and Application， 2014（6）： 292.
[7] CHU XL， LU WZ. Research and application progress of near infrared spectroscopy analytical technology in China in the past five years[J]. Spectroscopy and Spectral Analysis， 2014， 34（10）： 2495-2605.
[8] General Administration of Quality Supervision， Inspection and Quarantine of the People’s Republic of China. GB/T5512-2008， Inspect of grain and oilseeds—Determination of crude fat content in grain[S]. Beijing： China Standards Press， 2008.
[9] General Administration of Quality Supervision， Inspection and Quarantine of the People’s Republic of China. GB/T22507-2008， Animal and vegetable fats and oils—Determination of the content of trans fatty acid isomers of vegetable fats and oils—Gas chromatographic method[S]. Beijing： China Standards Press， 2008.
[10] SNAPP AR， KANG JL ， QI XL， et al. A fatty acid condensing enzyme from Physaria fendleri increases hydroxyl fatty acid accumulation in transgenic oilseeds of Camelina sativa[J]. Planta， 2014， 240（3）： 599-610.