Physicochemical properties and fatty acid profile of eight peanut varieties grown in Mexico

Abstract

Eight peanut (Arachis hypogaea) cultivars that were grown in Mexico were analyzed for the physical and chemical characteristics of their seeds and for the physicochemical properties and fatty acid profiles of their oils to select the most promising candidate in terms of oil stability and nutrient composition. The results showed that the protein ranged from 28.5% to 32.9% and the oil varied from 37.9% to 56.3%. The major fatty acids found in the oil samples were palmitic (11.9–13.2%), oleic (45.2–53.8%) and linoleic (25.1–29.2%) acids. The oleic/linoleic ratio was between 1.8 and 2.1. The physicochemical characteristics under evaluation were as follows: the iodine value (88.6–105.4), saponification value (142.5–181.8) and acidity (1.1–2.5%). Ranferi Díaz was the variety that presented higher stability and greater health benefits. This variety would be a good choice for agronomic purposes and genetic breeding programs.

Se estudiaron las características físicas y químicas de 8 variedades de cacahuate (Arachis hypogaea) cultivadas en México, así como las propiedades fisicoquímicas y el perfil de ácidos grasos de su aceite, con el fin de seleccionar la más promisoria en términos de estabilidad del aceite y cualidades nutricionales. Los contenidos de proteína variaron entre 28,5 y 32,9% mientras que los de grasa entre 37,9 y 56,3%. Los ácidos grasos más abundantes fueron el palmítico (11,9–13,2%), el oleico (45,2–53,8%) y el linoleico (25,1–29,2%). La relación oleico/linoleico estuvo entre 1,8 y 2,1. Las características fisicoquímicas del aceite evaluadas fueron: el índice de yodo (88,6–105,4), el índice de saponificación (142,5–181,8) y la acidez (1,1–2,5%). La variedad Ranferi Díaz fue la que presentó mayor estabilidad y beneficios a la salud. Esta variedad sería una buena elección para fines agronómicos y programas de mejoramiento genético.

Keywords:

• Arachis hypogaea L
• fatty acids
• peanuts
• Mexican varieties
• O/L ratio
• physicochemical properties

Palabras claves:

• Arachis hypogaea L
• ácidos grasos
• cacahuates
• variedades mexicanas
• relación O/L
• propiedades fisicoquímicas

Introduction

The peanut (Arachis hypogaea) is the fifth most important oilseed crop in the world with production of approximately 40 million tons (USDA, 2014). This oilseed (which is also a legume) is part of the economy of many countries such as the United States, India, China, Russia and Mexico, among others. Most of the peanut crop is processed into peanut butter, salted peanuts, candies and snacks. The peanut grain is converted into oil and cake in other countries, such as some in Africa in addition to India and China, where most of the world’s peanut cultivation is performed (Ortega & Ochoa, 2003).

Peanuts contain a considerable amount of protein (approximately 30%), which is more than tree nuts, and they also have high oil contents (44–56%) (Hassan & Ahmed, 2012). In addition, peanuts are a rich source of other nutrients such as fiber, niacin, folate, vitamin E, magnesium, manganese and phosphorous, in addition to phytochemicals such as phytosterols and phenolic compounds, which have important beneficial effects on human health (Higgs, 2003). One tablespoon of peanut butter (15 g) is considered to be nutritionally equivalent to 1 oz (28 g) of lean meat, poultry or fish, and 1.5 oz (42 g) of peanut seeds provide >10% of the adult male recommended dietary allowance for protein, fiber, iron, magnesium, phosphorous, zinc, copper, vitamin E, thiamin, niacin, pantothenic acid, linoleic acid and folate (King, Blumberg, Ingwersen, Jenab, & Tuker, 2008).

Several studies have associated peanut consumption with a reduced risk of cardiovascular disease without causing significant weight gain (Rajaram & Sabaté, 2008). This finding has linked this health benefit to the high dietary fiber, folate, alpha tocopherol, copper, magnesium, arginine and unsaturated fatty acid contents of peanuts (Alper & Mattes, 2003; Higgs, 2002; Kris-Etherton, Hu, Ros, & Sabaté, 2008). Peanut oil is rich in oleic and linoleic acids, which represent approximately 75% of the oil (Campos-Mondragón et al., 2009). These fatty acids have been related to a reduced cardiovascular disease (CVD) risk. Several studies have demonstrated that oleic acid, a monounsaturated fatty acid (MUFA), decreases low-density lipoprotein (LDL) cholesterol levels without reducing good high-density lipoprotein (HDL) cholesterol. MUFAs may decrease platelet aggregation and increase fibrinolysis, thereby protecting against thrombogenesis (Kris-Etherton, 1999). In comparison with saturated fatty acids, MUFAs lower the total and LDL cholesterol levels, and they increase HDL cholesterol levels and decrease plasma triglyceride levels in comparison with carbohydrates (Kris-Etherton, 1999). However, studies have shown that linoleic acid (18 carbons, 2 double bonds, omega-6) is the most effective polyunsaturated fatty acid for lowering serum cholesterol and the best for preventing cardiovascular disease (Harris, 2008; Sacks & Campos, 2006).

The storage and nutritional quality (and flavor) of peanuts can be affected by the fatty acid composition of its oil. The oleic-to-linoleic acid ratio is considered to be an indicator of peanut oil stability, and it is a shelf-life index for industrial applications (Bolton & Sanders, 2002; Campos-Mondragón et al., 2009). The higher the oleic/linoleic ratio, the more stable the oil. A negative correlation between oleic and linoleic acids has been reported in peanuts (Hassan & Ahmed, 2012). Variations in climatic conditions affect the oleic/linoleic acid ratio. The oil composition is affected by the production location, cultivar and climate (primarily the soil humidity and temperature) (Bolton & Sanders, 2002; Grosso & Guzman, 1995; Grosso, Lamarque, Maestri, Zygadlo, & Guzman, 1994).

The upper Balsas River basin is the largest ecological area with peanut production under rainfall conditions in Mexico (Joaquín et al., 2005). The elevation above sea level, rainfall and temperature greatly vary along the basin (635–1600 m, 667–1086 mm and 22.1–27.8°C, respectively). Breeding programs are being conducted to improve yield or to increase resistance to diseases and drought in this area, but these programs are rarely directed at improving the oil composition. The aim of this work was to analyze the physicochemical properties and fatty acid composition of eight peanut varieties grown in the Upper Balsas River basin of Mexico to select the most promising candidate in terms of stability, human health and nutritional qualities.

Materials and methods

The eight peanut cultivars used in this study (Huitzuco 93, Río Balsas, Ocozocuautla, Tlaxmalac, Gerardo Uribe, Ranferi Díaz, A-18 and RF-214) were grown by the Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias in the city of Iguala, Guerrero, Mexico, at 635 m above sea level. The average monthly temperature was 27.8°C, and the total precipitation was 1086 mm.

Seed chemical composition

The moisture, proteins, lipids and ashes in peanut seeds were determined according to the Association of Official Analytical Chemists (AOAC, 1995) methodology: moisture (AOAC, 950.46); crude protein (AOAC, 928.08); crude fat (AOAC, 920.39c); and ashes (AOAC, 923.03). The total carbohydrate content was calculated by difference.

Oil extraction

Peanut kernels were ground, and their oil was extracted twice with hexane at 4°C for 18 h with agitation. After the extraction, the solvent was evaporated under reduced pressure, and the extracted oil from each sample was kept at 4°C in the dark until analysis.

Physicochemical properties of peanut oil

Standard AOAC (1995) methods were used to determine the acid, peroxide, iodine and saponification values.

Fatty acid profile

The fatty acid profile was determined according to the Ce-62 method by the American Oil Chemists' Society (AOCS, 1983). Previously saponified and methylated samples were analyzed in a Perkin Elmer Auto System XL with a flame ionization detector and a BPX 70 capillary column (60 m × 0.25 mm id × 0.25 μm film). Nitrogen was used as a carrier gas at a flow rate of 10 mL/min. The detector and injector temperatures were 250°C and 230°C, respectively. The temperature program ran from 130 to 230°C with a 4°C/min gradient. The methyl esters of the fatty acids were identified by comparing the retention times of the samples with those of reference standards and by internal standardization. The fatty acid composition was expressed as the percentage of total fatty acids.

Statistical analysis

The results were expressed as the mean ± standard deviation, and significance was determined by Tukey test. Analyses were performed in Excel (Microsoft Excel, 2007) and using methods suggested by Montgomery (2003). Differences in which p ˂ 0.05 were considered to be significant.

Results and discussion

Physical characteristics

Table 1 shows the physical characteristics of the eight peanut cultivars analyzed in this study. The weight of 1000 kernels varied from 812.7 for RF-214 (Spanish) to 1193.5 for Huitzuco (Virginia). These results are classified in accordance with the market type of the samples; for example, Virginia-type peanuts are known to be larger than Spanish-type ones (Ortega & Ochoa, 2003).

Table 1. Physical characteristics of Arachis hypogaea cultivars.

Tabla 1. Características físicas de los cultivares de Arachis hypogaea.

Cultivar	Growth habit	Market type	Weight of 1000 kernels (g)	Width/length
Huitzuco 93	Spreading	Virginia	1193.5 f	0.49 b
Río Balsas	Bunch	Virginia	936.4 c	0.49 b
Ocozocuautla	Bunch	Virginia	993.0 d	0.50 b
Tlaxmalac	Bunch	Virginia	1009.5 e	0.64 d
Gerardo Uribe	Bunch	Virginia	980.6 d	0.49 b
Ranferi Díaz	Bunch	Virginia	1008.1 e	0.43 a
A-18	Bunch	Virginia	901.61 b	0.56 c
RF-214	Spreading	Spanish	812.7 a	0.54 c

Note: Means with different letters in each column are significantly different (Tukey, 0.05).

Nota: Medias con diferente letra en cada columna son significativamente diferentes (Tukey, 0,05).

Chemical composition

The moisture contents of the peanut samples varied between 4.1% and 4.8% (Table 2). These values are lower than those reported in other studies (Campos-Mondragón et al., 2009; Misuna, Swatsitang, & Jogloy, 2008; Özcan & Seven, 2003), and they are similar to those found by Shokunbi, Fayomi, Sonuga, and Tayo (2012) for Nigerian peanuts. Low moisture values increase the self-life of the seeds. The average crude protein contents ranged from 27.54% (RF-214) to 32.9% (Huitzuco 93), the oil varied from 37.9% (Gerardo Uribe) to 56.3% (Ranferi Díaz) and the ashes ranged from 2.2% (Ocozocuautla) to 2.5% (Río Balsas). Although these values are within the ranges of those obtained in other studies (Asibuo et al., 2008; Grosso & Guzman, 1995; Misuna et al., 2008; Shad, Pervez, Nawaz, Khan, & Ullah, 2009; Shokunbi et al., 2012), there was great variability among cultivars, especially in the oil content. Because the eight varieties were cultivated under the same conditions, these differences can be attributed to the cultivar. Peanuts are a rich source of protein and oil. However, although peanuts are a high-fat, energy-dense food, clinical and epidemiological studies show that peanut consumption is not associated with weight gain; on the contrary, there is an inverse relation between the frequency of nut (or peanut) consumption and the body mass index (Mattes, Kriss-Etherton, & Foster, 2008).

Table 2. Chemical composition of eight peanut (Arachis hypogaea L.) cultivars (g/100 g).

Tabla 2. Composición química de ocho cultivares de cacahuate (Arachis hypogaea L.) (g/100 g).

Cultivar	Moisture	Protein	Oil	Ash
Huitzuco 93	4.75 e	32.85 b	46.32 d	2.49 c
Río Balsas	4.17 ab	28.50 ab	49.55 e	2.50 c
Ocozocuautla	4.52 de	31.37 b	43.85 c	2.22 a
Tlaxmalac	4.40 cd	31.37 b	53.27 ef	2.40 bc
Gerardo Uribe	4.27 b	29.24 ab	37.90 a	2.35 b
Ranferi Díaz	4.18 ab	31.45 b	56.31 f	2.35 b
A-18	4.12 a	29.88 b	49.99 e	2.29 ab
RF-214	4.42 cd	27.54 a	39.84 b	2.37 abc

Note: Means with different letters in each column are significantly different (Tukey, 0.05).

Nota: Medias con diferente letra en cada columna son significativamente diferentes (Tukey, 0,05).

Fatty acid profile

Table 3 shows the fatty acid profile and physicochemical characteristics of oil from the different peanut cultivars. The most abundant among the saturated fatty acids was palmitic acid, which varied from 11.9% for Río Balsas to 13.2% for Tlaxmalac. The major fatty acids found in the oil samples were the unsaturated oleic and linoleic acids, which ranged from 74% to 81% of the peanut fatty acid profiles. Oleic acid varied from 45.2% (Tlaxmalac) to 53.8% (Ocozocuautla) and linoleic acid from 25.1% (Ocozocuautla) to 29.2% (A-18). These results were similar to those obtained in different studies (Dean, Hendrix, Holbrook, & Sanders, 2009; Hassan & Ahmed, 2012; Özcan & Seven, 2003). The oleic acid percentage was higher and the linoleic acid percentage was lower than the percentages obtained by Campos-Mondragón et al. (2009) for the Ranferi Díaz peanut variety. This finding was most likely explained by different growing conditions. An inverse relation was observed between oleic and linoleic acids. This finding has been reported by other investigations for different vegetable oils including peanut oil (Andersen, Hill, Gorbet, & Brodbeck, 1998; Hassan & Ahmed, 2012).

Table 3. Fatty acid profile, physicochemical properties and oleic/linoleic (O/L) ratio of peanut oils and seeds.

Tabla 3. Perfil de ácidos grasos, propiedades fisicoquímicas y relación Oleico/Linoleico (O/L) del aceite y semillas de cacahuate.

	Cultivar
Fatty acid (%)	Huitz	Río B	Ocoz	Tlax	Ger U	Ranf D	A-18	RF-214
Oil
Palmitic (16:0)	13.1 c	11.9 a	12.2 b	13.2 c	12.6 bc	12.3 b	12.3 b	11.9 a
Stearic (18:0)	3.2 cd	3.0 c	3.6 d	2.5 a	3.8 d	3.2 cd	2.7 b	3.0 c
Oleic (18:1)	52.9 d	50.5 b	53.8 e	45.2 a	52.1 c	52.3 c	51.3 bc	50.8 b
Linoleic (18:2)	26.8 bc	27.5 c	25.1 a	29.1 d	26.1 b	26.8 bc	29.2 d	27.6 c
Arachidic (20:0)	0.6 a	0.8 b	1.1 c	1.0 bc	0.9 b	0.7 ab	0.5 a	0.7 ab
Gadoleic (20:1)	0.6 a	1.2 bc	1.0 b	2.1 d	1.0 b	1.0 b	1.0 b	1.4 c
Behenic (22:0)	1.1 b	0.8 a	1.5 c	2.2 d	0.9 a	0.9 a	1.0 ab	1.0 ab
Erucic (22:1)	0.9 b	2.3 d	1.2 bc	2.6 d	1.2 bc	1.1 b	0.6 a	1.5 c
Lignoceric (24:0)	0.6 a	1.4 c	0.5 a	0.6 a	0.7 ab	0.8 b	1.2 c	1.3 c
Nervonic (24:1)	0.0 a	0.8 c	0.0 a	1.0 d	0.6 c	0.9 cd	0.1 b	0.7 c
O/L	2.0 c	1.8 b	2.1 c	1.6 a	2.0 c	2.0 c	1.8 b	1.8 b
Iodine value	100.3 bc	105.0 d	97.1 b	88.6 a	105.4 d	101.9 bc	99.0 b	99.3 b
Saponification value	181.8 cd	167.2 c	177.3 cd	142.5 a	167.2 c	172.8 c	157.1 b	142.5 a
Acidity (% oleic)	1.3 b	1.1 a	2.4 d	1.1 a	2.5 e	1.8 c	1.3 b	1.3 b
Seeds
Oleic	24.5 e	25.0 f	23.6 c	24.1 d	19.7 a	29.4 b	25.6 g	20.2 b
Linoleic	12.4 c	13.7 d	11.0 b	15.5 f	9.9 a	15.1 e	14.6 b	11.0 b
Total saturated	8.7 c	8.8 c	8.3 b	10.4 e	7.1 a	10.0 d	8.8 c	7.1 a
Total unsaturated	37.6 bc	40.7 c	35.5 b	42.6 cd	30.7 a	46.2 d	41.1 c	32.7 b

Note: Means with different letters in each row are significantly different (Tukey, 0.05).

Nota: Medias con diferente letra en cada fila son significativamente diferentes (Tukey, 0,05).

The oleic/linoleic ratio (O/L) is an index of stability to oxidative damage during refining and storage and thus the shelf life of oils. Higher ratios are associated with a longer shelf life (Hassan & Ahmed, 2012; Shad, Pervez, Zafar, Nawaz, & Khan, 2012) because the linoleic acid (which has two double bonds) is more susceptible to oxidative rancidity than oleic acid (one double bond). Dean et al. (2009) studied the composition of a core from the US peanut germplasm collection that was composed of 108 lines. Among those peanut lines, the O/L ratio varied between 1.1 and 3.2. Misuna et al. (2008) reported O/L ratios between 1.2 and 3.8 in eight commercial peanut varieties in Thailand. The results obtained in our study (1.8–2.1) were higher than those obtained by Grosso et al. (1994) and Grosso and Guzman (1995) using varieties from Argentina (1.00–1.46) and Peru (0.77–1.19), respectively, and by Campos-Mondragón (2009) with varieties from Mexico in which the O/L ratio reported for the Ranferi Díaz cultivar was 1.3 (this variety had an O/L ratio of 2.0 in our study). It is well known that the O/L ratio is affected by the production location, cultivar, market grade, temperature, rain and soil type (Grosso & Guzman, 1995; Grosso et al., 1994). The soil and climatic conditions were likely more favorable in our study than in those mentioned above.

In considering the fatty acid contents of the whole peanut seeds, the Ranferi Díaz cultivar was the one that had the highest oleic acid content, a good linoleic acid content and a high O/L ratio, whereas the Gerardo Uribe had a minor percentage of these fatty acids (Table 3). The beneficial effect of peanut consumption in preventing coronary heart disease has been attributed to its high oleic and linoleic acid contents, among other bioactive components of the seed (Kriss-Etherton et al., 2008). These healthy fats help to reduce several CVD risk factors such as cholesterol levels, platelet aggregation and inflammatory markers (Harris, 2008; Kris-Etherton, 1999; Sacks & Campos, 2006). In addition, there is an inverse relation between the oleic/linoleic ratio of plasma LDL and biomarkers of oxidative stress according to a clinical trial by Cicero et al. (2008) after the ingestion of olive oil by 200 healthy European subjects. A higher ingestion of oleic acid produced larger O/L ratios in plasma LDL and decreased levels of oxidative stress biomarkers.

Physicochemical properties of peanut oil

The iodine values observed in the oil samples varied from 88.6 for Tlaxmalac to 105.4 for Gerardo Uribe. These results are similar to those reported in other studies on peanut oil; Grosso et al. (1994) obtained values from 101 to 108 for runner peanuts that were grown in different localities of Argentina; Grosso and Guzman (1995) reported values from 98 to 117 for different cultivars from Peru; and Dean et al. (2009) found iodine values from 84.1 to 100.3 in 108 lines from a germplasm collection in the United States. The saponification values fluctuated from 142.5 (Tlaxmalac and RF-214) to 181.8 (Huitzuco). These results were highly uneven, but the values reported in other works were also highly variable; Özcan and Seven (2003) obtained values of 190 and 166 for the COM and NC-7 cultivars, respectively; Shad et al. (2012) reported values from 226.4 to 242.5 for different varieties cultivated in the arid zones of Pakistan; and Anyasor, Ogunwenmo, Oyelana, and Dangana (2009) reported values between 112 and 140 for peanut oils from Nigeria. The saponification value is related to the average molecular weight, or chain length, of all the fatty acids present; therefore, it depends on the different fatty acid compositions of the diverse cultivars. With respect to the acidity percentages, the values obtained in this study (1.1–2.5%) were high relative to those permitted for refined oils but similar to those found for raw oil from peanut seeds in other studies (Anyasor et al., 2009; Özcan & Seven, 2003; Shad et al., 2012).

Conclusions

Among the studied cultivars, Ranferi Díaz had better characteristics in terms of its high yield, high protein and oil contents, high oleic acid content, good linoleic acid content and high O/L ratio, and it therefore had higher stability and greater health benefits. This variety would be a good choice for agronomic purposes and genetic breeding programs.