The harvested fruits were transported to the laboratory, cleaned, and then left to ferment for five days to facilitate the detachment of the pulp adhering to the seed,and thus allow the isolation of the seed containing the kernel [10][11]. The hard shells of the stones were then broken to extract the kernels, which were subsequently prepared for drying.

Two drying methods were applied to Irvingiagabonensis kernels. The first method, carried out under ambient conditions at an average laboratory temperature of 25˚C, involved spreading the kernels on a clean tablecloth and exposing them to air drying for four consecutive days in June 2024. The second method was performed over the same period using a ventilated oven set at 60˚C in the laboratory. The air-dried and oven-dried kernels were designated as ADIgK (air-dried Irvingiagabonensis kernels) and ODIgK (oven-dried Irvingiagabonensis kernels), respectively. After drying, the kernels were ground using a porcelain mortar, and the resulting powder was used for subsequent analyses.

The determination of the residual moisture content of the dried Irvingiagabonensis kernels was carried out according to the oven-drying method described by the AOAC [12]. The powdered sample was placed in a ventilated oven at 105˚C for 24 hours. The moisture content was calculated using the following formula:

W₀: Weight of the sample before oven-drying (initial weight).

W₁: Weight of the sample after oven-drying (final weight).

Extraction of Irvingiagabonensis kernel oil was performed using the Soxhlet method. A Whatman thimble was filled with 30 g of crushed kernel sample and placed in a Soxhlet-type extractor connected to a condenser. A previously weighed flask containing 200 mL of hexane was placed on the Soxhlet heating block. The extraction was carried out continuously for 3 hours. After extraction, the solvent was evaporated, and the oil was recovered in the same flask. The percentage of oil yield determination was carried out in accordance with the AOAC method [13], using the following equation:

Saponification value

The Saponification value was determined according to the standard method described by [14]. This method is based on alkaline saponification of the oil sample followed by acid titration of the excess alkali. Two grams (2 g) of oil were weighed into a round-bottom flask, and 25 mL of a 0.5 mol/L alcoholic potassium hydroxide (KOH) solution was added. The mixture was refluxed for one hour, then cooled under running tap water. After cooling, 2 to 3 drops of phenolphthalein indicator were added, and the solution was titrated with 0.5 N Hydrochloric acid (HCl) until the pink color disappeared and the original color of the solution was restored. A blank test was prepared under the same conditions, replacing the oil with 2 mL of distilled water. The Saponification Value (SV) was calculated using the following formula:

V₀ = volume of HCl used for the blank (mL)

V = volume of HCl used for the sample (mL)

N = normality of the HCl solution

W = weight of the oil sample (g)

56.1 = molecular weight of KOH

Acid value

The acid value was determined by titrating free fatty acids with an alcoholic solution of potassium hydroxide, following the method described by [14]. One gram (1 g) of oil was placed in an Erlenmeyer flask, and 25 mL of 95% ethanol, along with 3 drops of 0.2% Phenolphthalein (pp) indicator, were added. After vigorous shaking, the free fatty acids were titrated with a 0.1 mol/L ethanolic potassium hydroxide (KOH) solution until a persistent pink color appeared. The Acid Value (AV) was calculated using the following formula:

V = volume of KOH used for titration (mL)

N = normality of the KOH solution

m = mass of the oil sample (g)

56.1 = molecular weight of KOH

Peroxidevalue

The peroxide value was determined according to the standard iodometric method, which is based on the reaction of oil dissolved in acetic acid and chloroform with potassium iodide (KI), followed by titration of the liberated iodine with a standardized sodium thiosulfate (Na₂S₂O₃) solution. For this procedure, 1 g of oil was placed in an Erlenmeyer flask, and 15 mL of acetic acid and 10 mL of chloroform were added simultaneously. After homogenization, 1 mL of a saturated potassium iodide (KI) solution was added. The flask was vigorously shaken for one minute and kept in the dark for 5 minutes to allow the reaction to proceed. Subsequently, 75 mL of distilled water was added, and the liberated iodine was immediately titrated with a standardized sodium thiosulfate (Na₂S₂O₃) solution under continuous shaking. A few drops of freshly prepared starch indicator were used to detect the completion of the titration, as indicated by the disappearance of the blue coloration. A blank sample was prepared under the same conditions, replacing the oil sample with distilled water. The Peroxide Value (PV) was calculated using the following equation:

V = volume of sodium thiosulfate used for the sample (mL)

V₀= volume used for the blank (mL)

N = normality of the sodium thiosulfate solution

m = mass of the oil sample (g)

3.6.1. Determination of Protein Content

Protein content was determined by measuring the total nitrogen content in the dry matter, using the Kjeldahl method AOAC [13]. Nitrogen was released from the dry matter after mineralization with sulfuric acid in the presence of a selenium-based catalyst. The resulting ammonia was distilled in the presence of excess sodium hydroxide and collected in boric acid, using bromocresol green and methyl red as indicators. The nitrogen content was then determined by titration with sulfuric acid until the indicator color changed from green to pink. The percentage of nitrogen obtained was converted to protein content by applying a conversion factor of 6.25.

3.6.2. Determination of Ash Content

Ash content was determined according to the AOAC procedure [13], which involves incinerating the dry matter in a muffle furnace at 550˚C for 6 hours. The ash content was expressed on a dry matter basis and reported as a percentage of the dry matter.

3.6.3. Determination of Mineral Content

After mineralization of the different samples in a muffle furnace at 450˚C, the ash was recovered, moistened with distilled water and concentrated hydrochloric acid, and then the mineral elements (P, Fe, Ca, K, Na, and Mg) were determined using the methods described by [13].

Determination of phosphorus content

Content Phosphorus was assayed by the cold colorimetric method using Murphy and Riley’s reagent [15]. Murphy and Riley’s reagent was obtained by combining solution A and solution B. Solution A was prepared by mixing 50 mL of water with 10 mL of concentrated sulfuric acid in a 100 mL flask. After cooling, 0.6 g of ammonium molybdate and 0.014 g of potassium antimonyl tartrate were added, and the flask was then filled up to the mark. For solution B, 2 g of ascorbic acid, 50 mL of distilled water, and 5 mL of concentrated hydrochloric acid were mixed, and the mixture was brought to volume. In a 200 mL flask, solutions A and B were mixed to obtain Murphy and Riley’s reagent. In a plastic vial, 0.5 mL of mineralized sample, 10 mL of distilled water, and 3 mL of Riley reagent were mixed. After 30 minutes of incubation, the absorbance of the mixture was read at the wavelength (λ) of 660 nm. The phosphorus content of the sample was obtained by calculation using a phosphorus calibration curve.

Determination of iron content

The iron content was determined using a colorimetric Spectro colorimeter [15]. Iron content was determined after mixing in a plastic pillbox, 5 mL of mineralized sample, 5 mL of hydroxylamine chloride, 2 mL of sodium citrate, 2 mL of sodium acetate buffer solution, pH 3.5, and 2 mL of orthophenantroline. Under the same conditions, an iron standard was prepared. After 30 minutes of incubation at room temperature, a red coloration developed. The absorbance of the reaction medium was measured using the spectrophotometer at 490 nm.

Determination of calcium and magnesium content

The calcium and magnesium content were determined using the complexometric titration method described by [15]. Calcium and magnesium form soluble Ca-EDTA and Mg-EDTA complexes with EDTA (Ethylene Diamine Tetraacetic Acid). A volume of 5 mL of mineralized sample was introduced into a 200 mL Erlenmeyer flask. Then, 45 mL of distilled water and 4 mL of buffer solution at pH 10 were added to the mixture. The titration was carried out with EDTA, using NET as color indicators, until the solution changed from pink to blue. At this stage, total calcium and magnesium (Ca + Mg) were determined simultaneously. For calcium determination, 5 mL of mineralized sample was placed in a 200 mL Erlenmeyer flask, to which 45 mL of distilled water, 1 mL of KCN, 5 mL of triethanolamine hydrochloride, and 5 mL of 2.5N NaOH were added. The pH of the solution was raised to 12 to precipitate the magnesium. Calcon was used as the color indicator during titration. The magnesium content was determined by subtracting the Ca concentration from the total (Ca + Mg) concentration.

Determination of potassium content

A volume of 0.5 mL of the mineralized sample was transferred into a 100 mL Erlenmeyer flask, and then a solution of lanthanum oxide was added, up to 25 mL. The dosage was performed using a flame spectrometer, using a range of standard solutions that enabled a calibration curve to be plotted. From this curve, the potassium concentration was determined.

Determination ofsodium content

A 15 mL aliquot of the mineralized sample was transferred into a 100 mL volumetric flask. A lanthanum oxide solution was added to bring the volume up to the calibration mark. The resulting solution was analyzed using a flame photometer. A series of sodium standard solutions was used to establish a calibration curve, from which the sodium concentration in the sample was determined by interpolation.

The data were processed using Microsoft Excel 2013 and analyzed with SPSS Statistics version 22.0. A two-sample t-test was performed to compare the means between groups.

The drying process, whether by air-drying or oven-drying, resulted in kernels with very low residual moisture content. The air-dried kernels exhibited a residual moisture content of 2.44 ± 0.01%, whereas the oven-dried kernels had a significantly lower value of 0.58 ± 0.01%.

Figure 1Figure 1illustrates the oil extraction yield from the dry matter of ADIgK and ODIgK. The results show that Irvingiagabonensis kernels air-dried and oven-dried have a high average oil yield. The oil yield of ODIgK is higher than that of ADIgK, with respective values of 74.68 ± 9.60% and 62.71 ± 7.65%. However, statistical analysis showed that this difference is not significant (P > 0.05).

ADIgK: Air-Dried Irvingiagabonensis Kernels, ODIgK: Oven-Dried Irvingiagabonensis Kernels.

Figure 1. Oil yield of two types of Irvingiagabonensis kernels.

Table 1 presents the chemical properties of the oils extracted from ADIgK and ODIgK.

Table 1. Chemical properties of the oil extracted from two categories of kernels.

O.ADIgK: Oil extracted from air-dried Irvingiagabonensis kernels; O.ODIgK: Oil extracted from oven-dried Irvingiagabonensis kernels. SV: Saponification Value; AV: Acid Value; PV: Peroxide Value. Means followed by the same superscript letter within a line are not significantly different at the 5% probability level (p > 0.05).

This table shows a saponification value of 179.55 mg KOH/g oil for ADIgK oil and 178.15 mg KOH/g oil for ODIgK oil. Regarding the acid value, the results were 0.79 mg KOH/g oil for ADIgK oil and 0.98 mg KOH/g oil for ODIgK oil. Finally, the peroxide value was 0.13 ± 0.03 meq O₂/kg for ADIgK oil and 0.14 ± 0.01 meq O₂/kg for ODIgK oil.

Figure 2Figure 2 presents the protein and ash contents of oil cakes from air-ADIgOC) and oven-dried (ODIgOC) Irvingiagabonensis kernels.

ADIgOC: Air-Dried Irvingiagabonensis Oil Cakes; ODIgOC: Oven-Dried Irvingiagabonensis Oil Cakes.

Figure 2. Protein and ash contents of two types of Irvingiagabonensis oil cakes.

The results show that Irvingiagabonensis oil cakes have relatively high average protein content, with values of 19.06 ± 0.36% for ADIgOC and 18.97 ± 0.62% for ODIgOC. However, the difference between the two values is only 0.1%. Regarding ash content, Figure 2Figure 2 shows values of 8.58 ± 0.41% for ADIgOC and 7.68 ± 0.12% for ODIgOC.

The results presented in Table 2 indicate that potassium is the predominant macro-element in both types of oil cake, with values of 2.30 ± 0.18 g/100 g for ADIgOC and 1.95 ± 0.15 g/100 g for ODIgOC. It is followed by calcium (0.79 ± 0.05 g/100 g and 0.82 ± 0.03 g/100 g), sodium (0.73 ± 0.18 g/100 g and 0.64 ± 0.17 g/100 g), and phosphorus (0.68 ± 0.02 g/100 g and 0.62 ± 0.01 g/100 g). Magnesium was the macro-element present in the lowest amounts in both types of oil cake, with values of 0.40 ± 0.06 g/100 g for ODIgOC and 0.33 ± 0.03 g/100g for ADIgOC. Both types of oil cake also showed identical iron contents of 10 ± 1 mg/100 g.

Table 2. Minerals content of both types of oil cakes.

ADIgOC: Air-Dried Irvingiagabonensis Oil Cake; ODIgOC: Oven-Dried Irvingiagabonensis Oil Cakes. Means followed by the same superscript letter within a line are not significantly different at the 5% probability level (p > 0.05); means followed by different letters differ significantly at this level (p < 0.05).

The results show that the residual moisture content of Irvingiagabonensis kernel varies significantly depending on the drying method used. Air-dried kernels have a higher residual moisture content than oven-dried kernels. This difference can be explained by the fact that, during oven drying, the temperature and ventilation are carefully controlled to accelerate the evaporation of water. This allows for a rapid achievement of low water content, thereby reducing the residual moisture in the kernels. Conversely, open-air drying relies on prolonged exposure to ambient air and is generally slower, resulting in higher water retention due to slower and less uniform evaporation. Similar results were reported in a study comparing oven and solar drying of mangoes [16].

These results confirm that the two drying methods used have a significant influence on the residual moisture content of Irvingiagabonensis kernels. Although oven drying results in a lower residual moisture content than air drying, both methods produce moisture levels low enough to effectively store Irvingiagabonensis kernels at room temperature. Moisture content is a crucial factor in maintaining the stability of oil cake over extended periods [17]. A level below 12% is considered safe for storage, as it prevents the rapid growth of mould [18].

The oil yield of the Irvingiagabonensis kernels is very high and varies slightly depending on the drying methods used. The high oil yield obtained from both types of kernels can be attributed to their low moisture content. These results were similar to those reported by an earlier study, which showed that an increase in the moisture content of dehulled sunflower seeds resulted in a progressive decrease in oil yield, from 91.01% at 6.2% moisture content to 85.45% at 8%, and further to 83.53% at 9.5% [19]. They also confirm the findings of a previous study which demonstrated that a low water content in dried Dacryodesedulis pulp yields higher oil [7].

The results show that oven-dried kernels have a slightly higher oil yield than those dried in the open air. This difference can be attributed to the lower residual moisture content of the oven-dried kernels. However, statistical analysis indicates that although oven drying results in a slightly higher oil yield, the difference observed between the two drying methods is not significant.

The saponification value provides an indication of the molecular weight and chain length of the fatty acids [20]. It also serves as a crucial parameter for evaluating the purity and overall quality of edible oils [21]. It has been reported that the shorter the fatty acid chain length, the higher the saponification value and the lower the molecular weight of the fatty acids [22]. Similarly, it was reported that a high saponification value indicates triacylglycerols with shorter fatty acyl chains [23]. Consequently, the Saponification Value (SV) provides a simple and reliable approach to assessing the average chain length of fatty acids in specific fats or oils [23]. The oil extracted from air-dried kernels had a slightly higher saponification index (179.55 mg KOH/g oil) than that obtained from oven-dried almonds (178.15 mg KOH/g oil). However, this difference was not statistically significant (p > 0.05). These values obtained this study remain lower than those reported for coconut oil (248.5 mg KOH/g) and palm oil (236.5 mg KOH/g), whose composition is rich in short-chain fatty acids, particularly lauric acid (C12:0), myristic acid (C14:0) and myristoleic acid (C14:1). Furthermore, these saponification values are also lower than the recommended range of 188-198 mg KOH/g for edible oils (FAO/WHO) [24]. Therefore, these results suggest that the oil extracted from the dried kernels of Irvingiagabonensis, regardless of the drying method used, is predominantly composed of long-chain fatty acids.

The acid value of oils extracted from oven-dried and air-dried kernels is very low, with no significant difference between the two drying methods. The acid value reflects the amount of free fatty acids present in the oil and serves as an indicator of its quality and of changes resulting from oxidation processes [25]. An increase in free fatty acid content indicates triglyceride hydrolysis and a greater susceptibility of the oil to oxidation [26]. This study demonstrates that both drying methods yield a very low acid value, below the recommended limit of 2.5% free fatty acids for virgin oils [27]. The low acid value suggests that both types of oil contain low amounts of free fatty acids. This result can be attributed to the low water content of the kernels used for oil extraction. Indeed, it has been reported that dehydration has a reducing effect on the acid value by limiting the availability of water required for lipase hydrolysis or by causing denaturation of the enzyme responsible for this reaction [20].

As regards the peroxide value, the results show that the oils extracted from oven-dried and air-dried kernels exhibit very low values, with no significant difference. This indicates that the drying method has no significant influence on this parameter. The peroxide value is an indicator of oil deterioration that leads to rancidity [28]. It reflects the early stages of oxidative deterioration in oils and is a parameter used to assess the quality of edible oils [7][29]. A high peroxide value is attributed to the formation of hydroperoxides and serves as an indicator of the initial stage of oxidation [30]. Therefore, the low peroxide value obtained in this study indicates a low level of oxidative rancidity in the oil and also suggests the presence of high levels of antioxidants [31]. The values obtained are below the limit of 10 meq O₂/kg, which is characteristic of good quality refined edible oils such as soybean, corn, and sunflower oils, according to the Codex Alimentarius, as well as the limit of 20 meq O₂/kg set by the International Olive Conseil Oléicole International for olive oil [32][33]. These results are also within the range of 0.50 to 2.67 meq O₂/kg reported for oils extracted from the same kernels [34]. Consequently, the oils extracted from Irvingiagabonensis kernels, dried using both methods, can be considered of good quality and are likely to exhibit better stability during storage.

The results obtained show that the protein and ash contents of the oil cakes from air-dried kernels are slightly higher than those of the oil cake from oven-dried kernels. However, statistical analysis revealed no significant difference between these values, suggesting that the drying method does not significantly affect the protein or ash content of the kernels. Oven drying, which involves the use of heat, did not lead to any significant protein loss, which can be attributed to the use of moderate temperatures during the drying process. Indeed, it has been shown that high temperatures can cause protein denaturation by breaking internal molecular bonds, and that the rate of denaturation increases with temperature [35][36].

The values obtained in this study are higher than those reported for kernel Irvingiagabonensis oil cake collected in Libreville, which had a protein content of 13.0 ± 0.2% and an ash content of 2.3 ± 0.0% [37].

The high protein and ash contents of Irvingiagabonensis kernel oil cake obtained in this study, regardless of the drying method used, indicate that it is an undeniable source of proteins and minerals (K, P, Mg, and Ca). Therefore, this oil cake can be utilized in human nutrition and in the formulation of functional foods, given the well-documented health benefits of plant proteins and minerals. It has been reported that a diet rich in plant proteins is associated with numerous health benefits, including reductions in body weight, blood cholesterol, and blood pressure [38]. This is the main reason why plant-based proteins are generally preferred over animal-based proteins [39].

The mineral content in food refers to the amount of specific inorganic components present in that food. Minerals act as cofactors in enzymatic reactions [40].

The results of this study also indicate that, although the oil cakes have high macro-element contents, no significant difference (p > 0.05) was found between the two drying methods for most of these elements (Ca, K, Na, and Mg), except for phosphorus (p < 0.05). Regarding the only microelement analyzed, iron, both types of oil cake had similar contents.

Although the differences in mineral content in the oil cake obtained by the two methods used are not statistically significant, this study showed that the oil cake obtained from air-dried kernels had a slightly higher mineral content. This suggests that air drying promotes better preservation of most of the macro elements studied, particularly potassium, calcium, magnesium, and sodium, compared to oven drying. Furthermore, the results also show that, regardless of the drying method used, potassium is the predominant macro element in the oil cake obtained from dried kernels, followed by calcium. Magnesium, on the other hand, is the least abundant macro element. The other macro elements are also present in appreciable amounts.

The high macro-element content obtained in this study is indicative of the good quality of the oil cakes derived from Irvingiagabonensis. Therefore, they are considered a source of minerals necessary for the normal functioning of the body system. Potassium, the predominant macronutrient, is the main cation of the intracellular compartment. It plays a key role in maintaining acid-base balance, regulating osmotic pressure, transmitting nerve impulses, enabling muscle contraction, ensuring proper cell membrane function, and supporting the activity of the Na⁺/K⁺-ATPase [41]. Moreover, calcium, which is also present in significant amounts in the cakes, is an essential macro-element. It plays a crucial role in regulating nerve and muscle functions [41].

The results of this study suggest that air drying, which better preserves most chemical parameters, is the best option for pre-treating Irvingiagabonensis kernels for oil processing in rural areas, as it requires no energy expenditure and is therefore economically accessible. However, although oven drying at 60˚C involves higher energy costs, the results showed that it produces a slightly higher oil yield and better preserves phosphorus in the oil cake. Therefore, processors must strike a balance between energy costs and the quality of the final products, as both pretreatments offer distinct advantages.