Fatty acid, sterol and proximate compositions of peanut species

Los contenidos en aceite, proteína, ceniza e hidratos de carbono, índice de acidez, composiciones en ácidos grasos y esteroles fueron estudiadas en semillas de Arachis correntina, A. durannensis, A. Monticola, A. batizocoi, y A. cardenasii originaria de Bolivia y Argentina. El contenido en aceite fue mayor en A. batizocoi (valor medio 53,35%). El nivel de proteína fue más alto en A. monticoia (valor medio 29,40%) y A. durannensis (29,13%). El valor medio del ácido oleico varió entre 34,91% {A. durannensis y A. cardenasii) y 42,60% (Arachis correntina), y el ácido linoleíco osciló entre 40,23% (A. correntina) y 45,86% (A.durannensis). La mejor relación oleico a linoleico fue exhibida por A. correntina (1.06). El índice de iodo fue más bajo en A. batizocoi (106,0). La composición esterólica en las diferentes especies de cacahuetes mostró la más alta concentración de B-sitosterol (los valores medios oscilaron entre 55,70-58,70%) seguido por campesterol (15,18-16,47%), estigmasterol (10,67-12,27%) y A5-avenasterol (10,80-12,13%).


INTRODUCTION
At present there are 68 described wild species of Arachis native to South America (Krapovickas and Gregory, 1994).Substantial evidence suggests that the genetic base or gene pool of the cultivated peanut, A. hypogaea L., does not have the reserve gernnplasm needed to resist many of the new agricultural problems brought on by pollution, dwindling water supplies, and the necessity for biological control methods against insects and plant pathogens.Wild species of Arachis contain new sources of germplasm which can be used to increase variability in the genetic base of cultivated peanut (Stalker etaL, 1989).
Leguminous seeds make an important contribution to the diet in many tropical countries.They are a good source of protein, lipid, and fatty acids for human nutrition (Gaydou et al., 1983).The fatty acid composition of endogenous fats plays and important role in determining shelf life, nutrition, and flavor of food products.The lipids and proteins of cultivated peanut seeds have been widely studied (Ahmed and Young, 1982).The objective of this work was to characterize some chemical components of wild species of Arachis.I.A detailed description of the taxonomic classification of peanut was reported by Krapovickas and Gregory (1994).Maturity of seed was establish when they showed maxinnunn dry weight and nnaximunn viability (Crookston and Hill, 1978).

Determination of oil, ash, protein, moisture, and carbodydrate contents
Three samples each containing five seeds from each cultivar were examined for oil, protein, ash and moisture contents.These seeds were selected at random.The sampling size was previously calculated (Cochran, 1974) and it was proper to the statistical design.
Seeds were milled and oil was extracted for 16 h with petroleum ether (boiling range 30-60°G) in a Soxhiet apparatus.The extracted oils were dried over anhydrous sodium sulfate and the solvent removed under reduced pressure in a rotary film evaporator.Oil percentages was determined by weight difference.
Moisture, ash, nitrogen contents were determined according to AGAG (1980).Ash was performed by incineration in a muffle furnace at 525°G.The nitrogen content was estimated by the Kjeldahl method and converted to protein percentage by using the conversion factor 5,46 (Young and Mammons, 1973).Garbohydrate content was estimated by value difference of the other components (expressed on dry basis) using the formula: carbohydrate content = 100% -(% protein + % oil + % ash).

Fatty Acid Composition
Fatty acid methyl esters were prepared by transmethylation with a 3% solution of sulfuric acid in methanol, as previously described (Jellum and Worthington, 1966).The fatty acid methyl esters of total lipids were analyzed on a Shimadzu GC-R1A gas chromatograph equipped with flame ionization detector (FID).AT-WAX superox II capillary columm (30 m X 0.25 mm i.d.) was used.Column temperature was programmed from 180°C (held for 10 min) to 240°C (4°C/min).Injector temperature was 250°C.The carrier (nitrogen) had a flow rate of 1 mL/min.The separated fatty acid methyl esters were identified by comparing their retention times with those of authentic samples wich were purchased from SIGMA Chemical Co.Quantitative analysis of the fatty acids were performed using the heptadecanoic acid methyl ester as internal standard.Iodine values were calculated from fatty acid composition (Hashim et al., 1993) using the formula: I.V. =(% oleic x 0.8601) + (% linoleic x 1.7321) + (% eicosenoic x 0.7854).

Sterol Composition
Sterols of the unsaponifiable matter from 5 g of oil (after saponification with alcoholic 1 M potassium hydroxide) were purified by preparative thin-layer chromatography (TLC).TLC was performed on silica gel 60 G (20 x 20 cm, 0,5 mm layer thickness) plates using chloroform-diethyl ether (9:1 v/v) as the developing solvent.The approximate relative Revalues of the 4-desmethylsterols fraction was 0,27.The unsaponifiable matter was dissolved in chloroform (5%) and 150 |jl was deposited as a streak of 15 cm length on the plate.Cholesterol, used as standard, was spotted on the left and right hand sides of the plate.The corresponding band of 4-desmethylsterols was scraped off the plate and extracted with chloroform (Gaydou et al., 1983).Purified sterols were analyzed on a Shimadzu GC-R1A gas chromatograph equipped with FID.Shimadzu CBP1 capillary column (25 m x 0,25 mm i.d.) was used.Column temperature was programmed from 200 to 300 °C (4° C/min).Injector temperature was 320°C.The carrier (nitrogen) had a flow rate of 1 mL/min.Standard sterols (Sigma Chemical Co.) were run in order to use retention times in identifying sample peaks.The amount of sterols was determined from the weight of 5a-cholestane as internal standard.The data were calculated using a integrator of the chromatograph.

Statistical analysis
All analysis for each sample of the wild Arachis were done in triplicate.Mean and standard deviation for each species were determined and significant differences among mean values were evaluated using a t-test (Branch et al., 1990) due the data shown normal distribution according previously researchs performed in this material.Mean values of A. montícola were only two genotypes and was not included in the statistical analysis.

RESULTS AND DISCUSSION
Moisture, oil, protein, ash, and carbohydrate contents and iodine value are shown in Table II.Peanuts are characterized by high oil and protein contents and low carbohydrates and ash.Knowledge of these components is important in the end products of the industry (Ahmed and Young, 1982).The oil content was highest in A. batizocoi (mean value 53,35%) Arachis montícola and A. durannensis exhibited the highest protein levels.
Iodine value and 0/L ratio are both indicators of peanut oil stability and shelf-life (Ahmed and Young, 1982).Higher 0/L ratios and lower iodine values suggest better stability, longer shelf-life, and quality of the oils (Branch etal., 1990).All wild species of peanut had in relation to iodine value and 0/L ratio, lower stability and quality of their oils than the US peanut cultivars (Branch et al., 1990) and Argentina (Grosso et al., 1994).These variations could be due to differences in climatic conditions, soil moisture and air temperature during maturation of peanut seed.The best 0/L ratio and iodine values were found in A. correntina (mean value 1,06) and A. batizocoi (mean value 106), respectively (Table II).
The fatty acid composition is presented in Table III.Palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2), arachidic (20:0), eicosenoic (20:1), behenic (22:0) and lignoceric (24:0) acids were detected.Oleic and linoleic acids were the principal components (Table III).Oleic acid was predominant in the samples of A. correntina.The lowest oleic and highest linoleic concentrations were the principal differences with respect to the fatty acid composition of peanut cultivars previously published (Branch et al., 1990;Grosso and Guzman, 1995a) and with other species of Arachis (Stalker etal., 1989).The differences between species detected in this work are not probably due to climatic conditions, because all sample of especies were cultivated in the same year, growing season and locality.However, the differences observed with data previously reported could be due these causes.