Abstract

The objective of this study is to evaluate and compare the chemical properties of oils and seed cakes of the two peanuts varieties sold on the Brazzaville markets with a view to identify their appropriate uses in nutrition and food technology. The extraction and chemical characterization of the seed oils and cakes were carried out according to standard analysis methods. The proximal composition of the seeds of the two varieties has approximately identical moisture contents (g/100gDM) (4.77±0.011 for the Var-R peanut and 4.15±0.007 for the Var-B peanut). The protein content (g/100gDM) of Var-B peanut cakes (37.76±0.003) is slightly higher than that of Var-R peanut cakes (35.71±0.001). The ash content (g/100gDM) is 5.16±0.006 for Var-B peanut cakes and 4.93±0.033 for Var-R peanut cakes. The carbohydrate content (g/100gDM) is 10.72±0.001 for Var-R peanut cakes and 3.67±0.001 for Var-B peanut cakes. The energy value (Kcal/100g) of Var-B peanut (609.06 ± 0.023) is higher than that of Var-R peanut cake (580.55± 0.041). The profile of the mineral elements analyzed is: P˃ Ca ˃ Mg ˃ Fe with contents (mg/100gDM) of the most abundant 14.30±0.93 for Var-B peanut cakes and 13.75 ± 1.5 for Var-R peanut cakes and the least abundant 0.095±0.003 for Var-B peanut cakes and 0.11±0.13 for Var-R peanut cakes. The Ca/P ratios in the seed cakes of the two peanut varieties are close. The oil yields (g/100gDM) of Var-B peanut seeds (49.26 ± 0.002) are significantly higher than those of Var-R peanut seeds (43.87 ± 0.01). The peroxide index (meq of O₂.kg^-1) of Var-B peanut oil (5.9) is twice as high as that of Var-R peanut oil (2. 67). The saponification indices (mg KOH/g of oil) of the oils of the two varieties are almost identical (189.177 for Var-B peanut oil and 187.303 for Var-R peanut oil). The same goes for the ester index (mg KOH/g of oil) (188.255 for Var-B peanut oil and 186.381 for Var-R peanut oil). On the other hand, the acid indices (mg KOH/g of oil) of the oils of the two varieties are identical (0.922). The oils of the two peanut varieties contain (in %) the three classes of fatty acids: saturated (22.4 for Var-B and 18.60 for Var-R), monounsaturated (40.53 for Var-B and 48.02 for Var-R) and polyunsaturated (36.12 for Var-B and 33.10 for Var-R). Among the polyunsaturated fatty acids we have, in varying proportions, ⍵-3 and ⍵-6. Oleic and linoleic acids constitute on average 79.876% for Var-B and 80.20% for Var-R, of total fatty acids.

1. Introduction

The peanut (Arachis hypogaea L.) is a tropical plant native to South America ^{1, 2} and is divided into two subspecies and three varieties corresponding to the Virginia, Spanish and Valencia types ³ which are to in turn divided into different varietal subgroups according to the climatic and edaphic conditions specific to their development and cultivation ⁴. It is cultivated on all continents, in approximately 120 countries, over a total area of 24.6 million hectares for a production of 38.8 million tonnes ⁵. It is of great importance in the food and medicinal industries. It represents 10% of global oilseed production ⁶. Peanut production on the African continent has experienced significant growth since the beginning of the 1990s. With its 10 million tonnes, the African continent occupies second place ahead of the American continent ⁷. Peanuts are grown for their seeds which are used in many types of foods. It is consumed either as seeds (after shelling the pods), or in the form of oil (after industrial or artisanal crushing of the seeds), or in more or less elaborate forms (butter, paste, flour, confectionery, etc.). Protein-rich meals are used as human or animal food ⁸. Peanuts, like most grain leguminous plants, are a good source of lipids, proteins and mineral salts. The seeds contain approximately (45-50)% lipids, (25-30)% proteins, (5-12)% carbohydrates and 3% fiber. The nutritional values of peanuts have recently been taken advantage of in the composition of foods with high nutritional value used for the treatment of severe malnutrition and the reduction of the risks of cardiovascular diseases ⁹. In Congo, 16 varietal subgroups of peanuts are cultivated, namely: Manga; Mani-pintar; Mbéngui; Loudima red; Nguesson; Lady's heel; Lekana/Djambala; Kubelavé; Kindamba; JL-24; Blanche de Loudima; E-119; Israel; Otendé; Red beauty; Mâ-bouésso ¹⁰.

Among these different varietal sub-groups, we were interested in the Blanche de Loudima varieties, cultivated in the southern part of the country, more precisely in the Bouenza department, and the Lekana/Djambala variety, cultivated in the northern part of the country, more precisely in the plateau department. Nowadays, no study on the physicochemical characterization of oils and cakes of these two varieties has been carried out. This study therefore consists of carrying out a comparative study of the chemical and biochemical characteristics of these two varieties of peanuts in order to identify their possible uses in agri-food and cosmetics and thus add value to this product which is little valued locally.

2. Material and Methods

This study was carried out in various laboratories: the Pôle d'Excellence Régional en Alimentation et Nutrition of the Faculty of Science and Technology at Marien Ngouabi University, the analytical chemistry laboratory of the IRSEN research zone in Pointe-Noire, and at the Centre de Coopération Internationale en Recherche, Agronomique pour le Développement (CIRAD) in France.

The agricultural products used in this study consist of the seeds of two varieties of Arachis hypogaea L (the Blanche de Loudima variety noted Var-B and the Lékana/Djambala variety noted Var-R). Var-B comes from Loudima (13°3'30'' East and 4°6'45'' South) in the Bouenza department and Var-R comes from Lékana (2° 19' 49'' South, 14° 36′ 0″ East) in the Plateaux department. These seeds were transported to the laboratory where they were cleaned and dried in an oven at 35°C until a constant mass was obtained before being ground using a manual grinder to give a fine powder. This powder was introduced into a glass bottle, sealed hermetically and stored at 4°C away from light.

The moisture content was determined by drying the product at a temperature of 103°C, in an isothermal oven and at atmospheric pressure to a constant mass.

The residual water content (TEr) was determined according to the formula (1):

(1)

With: M₀: mass of the product before drying; M₁: mass of the product after drying.

Oil extraction was carried out using the Soxhlet method following the standard protocol ¹¹. A mass M₀ of peanut seed powder is placed in an extraction cartridge which is introduced into a Soxhlet extractor fixed on a round bottom flask filled 2/3 with hexane. After 8 hours of extraction at a temperature of 70°C, we obtain cakes and a mixture of oil and hexane. The oil and hexane were separated using a Büchi type rotary evaporator system and traces of hexane in the cakes were eliminated in the open air.

The oil extraction yield (Oil %) was calculated using the formula (2):

(2)

with: M_H = mass of the extracted oil; M₀ = mass of the seed powder used to make the extraction.

The oil was collected in a smoked glass bottle, labeled and placed in the refrigerator (4°C) and the cakes, freed from traces of solvent in the open air, were wrapped in aluminum foil before any analyses.

The Kjeldahl method ¹² was used to measure all the nitrogen; for this it was necessary to destroy the organic compounds in order to obtain all the nitrogen in the same mineral form. To do this, mineralization was carried out. The nitrogen was then dosed following an acid-base reaction.

The calculation of the percentage of nitrogen and protein was carried out by the following formulas ¹³:

(3)

(4)

With: A: Volume (ml) of HCl titrating the sample; B: Volume (ml) of HCl titrating the blank; 6.25: Kjaldahl factor; 100: Percentage factor.; N: Normality of hydrochloric acid solution used for titration; 14.01: Relative atomic mass of nitrogen; M = sample mass.

Raw ash is the residue obtained after incineration of an organic product. The ash content was determined according to the method ¹⁴. The principle is based on the incineration of a mass (M₀) of a sample at 550±15°C in an electrically heated muffle furnace until the mass becomes practically constant. Weighing (M₁) after cooling the ash obtained in a desiccator made it possible to determine the ash content according to the formula (5):

(5)

The total carbohydrate content was determined by the difference between the dry extract and the sum of proteins, lipids and Ash.

The energy value was calculated by applying the coefficients from ¹⁵, adopted by ¹⁶, namely: 4 for total proteins and carbohydrates and 9 for lipids.

(6)

The mineral elements (P, Ca, Mg and Fe) were measured by flame atomic absorption spectrophotometry after mineralization of the sample. The mineralization of the cakes was carried out by the wet method (acid attack). The Varian Vista brand spectrophotometer, equipped with the CCD (Coupled Charge Device) detector, was used for this analysis. The determination of the mineral element contents was carried out using a pre-established calibration curve.

The saponification indices (IS), acid (IA), peroxide (IP) were determined following the protocol described by standard NF T 60-206 ¹¹. The ester index (EI) was calculated by taking the difference between the saponification index and the acid index.

The fatty acids were analyzed by gas chromatography coupled with a flame ionization detector (GC/FID) according to standard NF T 60-233 ¹¹.

The device used was the FOCUS GC 800 chromatograph equipped with a CP-Sil 88 column with helium as carrier gas at 17 Psi and a temperature gradient from 140°C to 200°C, in a total of 40 min. the injection port was maintained at 250°C, with a split ratio of 1:100 and the detector set at 270°C. The fatty acids were identified by comparison to the standard.

The data collected at the end of this study were subjected to statistical analyses. For each parameter measured, three tests were carried out and the result is expressed as mean ± standard deviation. A multivariate analysis of variance was carried out to assess the existence of differences between the samples studied. For these statistical treatments, the XLSTAT software version 2016.02.28451 which is an EXCEL macro command was used.

3. Results

Chemical characterization of seeds shows that these two varieties have virtually identical moisture contents, with 4.77±0.01g/100gMS for Var-R and 4.15±0.01g/100gMS for var-B. On the other hand, the protein content of Var-B oilcake (37.760±0.003g/100gMS) is slightly higher than that of Var-R (35.710±0.001g/100gMS). Likewise for the ash content, these values correspond respectively to 5.16±0.01 g/100gMS for Var-B and 4.93±0.03 g/100gMS for Var-R. On the other hand, the carbohydrate contents for the two varieties are significantly different (P<0.05), i.e. 10.720 ± 0.001g/100gMS and 3.670±0.001g/100gMS respectively for Var-R and Var-B cakes. The energy values whose value is 609.06±0.02 Kcal/100g for Var-B, a value higher than that of Var-R (580.55± 0.04 Kcal/100g) (Table 1). The fat yields of Var-B seeds (49.260 ± 0.002 g/100gMS) are significantly higher than those of Var-R seeds (43.87± 0.01 g/100gMS).

The mineral element contents of the seed cakes of the two peanut varieties are presented in Table 2.

Among these elements, two are present in significant quantities and which the human body needs and called "macroelements". These are Calcium, the content of which is higher in Var-B cakes (7.5±0.07 mg/100gMS) than in Var-R cakes (5.90±0.07mg/100gMS) and phosphorus whose contents for the two varieties are almost identical (around 14 mg/100gMs). The other two which are in tiny quantities called “trace elements” are represented by magnesium, the content of which is lower in Var-B cakes (0.12±0.10 mg/100gMS) than in Var-R cakes. (0.95±0.17mg/100gMS) and iron, the contents of which for the two varieties are close (around 0.10 mg/100gMS). The Ca/P ratios in the seed cakes of the two peanut varieties are close.

The peroxide index of Var-B peanut oil (5.9±0.5 meq of O₂.kg^-1) is twice as high as that of Var-R peanut oil (2.67±0.13 meq of O₂.kg^-1). The saponification indices of the oils of the two varieties are almost identical (189.18±3.62 mg KOH/g of oil for Var-B peanut oil and 187.303±2.50 mg KOH/g of oil for that of peanut Var-R). Likewise for the ester index (188.26±3.63 mg KOH/g of oil for Var-B peanut oil and 186.38±2.51 mg KOH/g of oil for that of peanut Var-R). On the other hand, the acid indices of the oils of the two varieties are identical (0.92±0.01 mg KOH/g of oil) (Table 3).

The fatty acid (FA) content of the sheaths of the two peanut varieties grown in Congo is shown in Table 4.

This analysis revealed three classes of fatty acids in varying proportions in the oils of the two peanut varieties: saturated (22.4% for Var-B and 18.60% for Var-R), monounsaturated (40.53% for Var-B and 48.02% for Var-R) and polyunsaturated (36.12% for Var-B and 33.10% for Var-R). Among the polyunsaturated fatty acids there is the presence of ⍵-3 and ⍵-6, two essential fatty acids.

4. Discussion

The seeds of the two varieties of Congo peanut have a fat content (49.26 g/100g MS for Var-B and 43.87 g/100g MS for Var-R) which is much higher than that of the seeds of peanut from Daloa (Ivory Coast) (28.26 g/100g MS) studied by ¹⁷ and much lower than that of Irvingia gabonensis (60 to 80 g/100g MS) ¹⁸. The humidity level has a lot of influence on the conservation of seeds. A lower content allows storage for long periods at room temperature because it delays the growth of micro-organisms. The moisture levels of the seeds of the two peanut varieties studied comply with the standard ¹⁹ (less than 9%) which shows that they can be stored for a long time with little risk of microbial contamination. The protein contents obtained are lower than those of peanut seeds reported by ¹⁷ and ²⁰, which are 47.0 g/100gMS and 49.8 g/100gMS respectively.

Carbohydrate levels in the sheaths of the two peanut varieties studied are significantly higher than those found in the sheaths of Sesanum indicum L. (2.05 g/100gMS) ²¹. They are lower than those found by ¹⁷ in peanut seeds from Daloa (Ivory Coast) (15.48 g/100gMS). The oilcakes of these two peanut varieties can contribute to the fight against protein-energy diseases and can be used in livestock feed due to their high protein content.

The ash levels are of the order of 5 g/100gMS, a value higher than that found by ¹⁷ in peanut seeds from Daloa (Ivory Coast) (3.57 g/100gMS) thus indicating that these seeds are important sources of minerals for the populations who consume them.

For all the mineral elements analyzed, the most abundant in the seeds of the two peanut varieties is phosphorus while the least abundant is iron, with the following decreasing profile: P˃ Ca ˃ Mg ˃ Fe. The seeds of the two varieties of peanut are richer in phosphorus and poor in calcium than the kernels of Irvingia gabonensis ²². Calcium and phosphorus are necessary for bone growth.

Calcium deficiency causes growth retardation, rickets in children, spasmophilia in adults and osteoporosis in the elderly. The Ca/P ratio between 0.5 and 0.8 promotes intestinal absorption of calcium. The presence of magnesium in the seeds of both varieties of peanuts is an advantage because magnesium is vital for bones, muscles and the nervous system. To improve the biological availability of iron, it is necessary to supplement a food ration based on peanut meals with other sources of iron.

The peroxide values of the oils of the two peanut varieties are lower than those found by ¹⁷ for peanut oil from Daloa (Ivory Coast) (7.87 meq O₂.kg^-1). Moreover, these peroxide values are lower than those found by ²³ for Olive oil (11 to 19 meq O₂.kg^-1). These peroxide values are lower than the 10 meq O₂.kg^-1 found in most conventional oils ²⁴. In fact, peroxide values below 10 meq O₂.kg^-1 are generally considered to reflect an acceptable level of oxidation ²⁵. The saponification indices of the oils of two peanut varieties studied are higher than that found by ¹⁷ on peanut oil from Daloa (Ivory Coast) (173.98 mgKOH/100g) and lower than that obtained by ²⁶ on the oil of Irvingia gabonensis (195.02 mg KOH/100g). Which shows the cosmetic interest of these oils ²⁷. The acid value of the oils studied complies with standards ¹⁹ (less than 4 mg KOH/g oil). This value is lower than that of some common oils such as soybean (max = 6) and sunflower (max = 4) ²⁴. These low acid number values characterize the purity and stability of Congo peanut oils at room temperature, and their resistance to rancidity according to ²⁸. The ester index values for the two peanut varieties are slightly lower than that of Citrullus colocynthis seeds (192.32 mg KOH/g of oil) ¹³. The ester index makes it possible to determine the molar mass (therefore the structure) of the triglycerides in an oil. For the seed oil of the two varieties of Congo peanuts, the value of the calculated molar mass of triglycerides is equal to 898.40 g/mol.

Peanut oil is particularly rich in oleic and linoleic acids, which make up an average of 79.876% for Var-B and 80.20% for Var-R, of total fatty acids. This allows it to be classified as an oleic/linoleic oil like olive oil. It can be compared with baobab seed oil, which also has the same fatty acid profile ²⁹. Its oleic acid content makes this oil particularly useful for cholesterol regulation.

Taking into account the total contents of unsaturated fatty acids (76.65% for Var-B and 81.12% for Var-R) and saturated fatty acids (22.94% for Var-B and 18.6% for Var-R), peanut oil is close to Sesanum indicum L. oil ³⁰. This oil could be very interesting for food and fight against cardiovascular diseases and other diseases linked to the central nervous system. Thanks to its high linoleic acid content, peanut oil has revitalizing properties. The work of ³¹ confirms the work of ³² and [33] which shows that the deficiency in polyunsaturated fatty acids leads to neurochemical abnormalities explained by increased behavioral reactivity and learning difficulties.

5. Conclusion

This study made it possible to evaluate and compare the chemical properties of the oils seed and seeds of the two peanuts varieties sold on the Brazzaville markets. The oil yield of Var-B peanut (49.26 ± 0.002 g/100gMS) is higher than that of Var-R peanut (43.87± 0.01 g/100gMS). The peroxide value and acid value of the oils of both varieties comply with Codex Alimentarius standards. The saponification indices of the oils of the two varieties are almost identical. The same goes for the ester index. The proximal composition shows that these two varieties have approximately identical moisture contents.

On the other hand, the protein content of Var-B peanuts is slightly higher than that of Var-R peanuts. The same applies to ash content. Carbohydrate content and energy values are very different for the two varieties. The oilcakes of these two peanut varieties can contribute to the control of protein-energy diseases and can be used in livestock feed due to their high protein content. For the mineral elements analyzed, phosphorus and calcium are the predominant elements in both varieties. Oleic and linoleic acids, which make up an average 79.876% of total fatty acids for Var-B and 80.20% for Var-R, classify these oils as oleic/linoleic.

ACKNOWLEDGEMENTS

The authors would like to warmly thank Professor David Mampouya and Doctor Bob Wilfrid Loumouamou for their multifaceted assistance which enabled this work to be carried out.

References