Effect of Boiling and roasting on lipid quality, proximate composition, and mineral content of walnut seeds (Tetracarpidium conophorum) produced and commercialized in Kumba, South‐West Region Cameroon

Abstract The effect of boiling and roasting on the lipid quality, proximate composition, and mineral content of African walnut seeds (Tetracarpidium conophorum) was assessed. Results indicated that the quality of walnut oil significantly (p < .05) reduces with the treatments. Oils extracted from DBWN 60 min (Dried and boiled walnuts 60 min) and FBWN 60 min (Boiled fresh walnuts 60 min) were the most altered. The proximate composition and mineral content of walnut seeds was also significantly affected (p < .05) by the treatments. This study reveals that, thermal processing has significant effects on the nutrients and quality of lipids of walnut oil. DTRWN 60 min (Dried and traditionally roasted walnuts 60 min), DORWN 60 min (Dried and oven roasted walnuts 60 min), and TRFWN 30 min (traditionally roasted fresh nuts 30 min) are the best methods for cooking walnut because they preserve the quality of its lipids and some of the nutrients.


| INTRODUCTION
African walnut (Tetracarpidium conophorum) is a popular Central and Western Africa plant where it is planted mainly for its nuts, which are usually eaten as snacks when the unshelled nuts are boiled and cracked (Babalola, 2011). The proximate composition of T. conophorum revealed that it is rich in protein (29.14%), fat (54.14%), carbohydrate (4.17%), ash (3.32%), and several vitamins (Arinola & Adesina, 2014). A bitter taste is usually observed upon drinking water immediately after eating the nuts, due to the presence of antinutrients. The leaves, barks, and nuts of T. conophorum have also been demonstrated as good sources of phenolic antioxidants with various biological properties. Food materials are usually processed in order to improve palatability and reduce toxicity, and as a means of preservation (Kanu, Kalu, & Okorie, 2015).
Walnuts seeds are generally consumed after several processing techniques. General method of processing of walnut prior to consumption involves prolonged cooking of the seeds by boiling and roasting with or without the shell. These processing methods usually improve the organoleptic properties of the nuts, reduce their antinutrients content, and prolong their shelf life (Ayankunbi, Keshinro, & Egele, 1991).
Thermal or heat processing is one of the most important methods developed by humans. During thermal processing, eventhough antinutritional components are reduced or eliminated; heat has a detrimental effect on the nutritional and functional properties of foods (Kanu et al., 2015). Then, the thermal treatment of walnuts can lead to chemical changes that can affect its nutritional value and the quality of its lipids. During boiling and roasting of the seeds, high temperatures can facilitate lipid oxidation and nonenzymatic browning reactions, which can reduce the nutritional value of foods, causing the loss of essential fatty acids, essential aminoacids and carbohydrates.
The amount of vitamins can also be reduced as well as the proteins digestibility (Cuvelier & Maillard, 2012). Additionally, these chemical alteration reactions may generate toxic compounds in edible seeds and the derived products, which can be harmful for the consumers .

Tetracarpidium conophorum is cultivated in littoral and western
Cameroon, where it is, respectively, known as "kaso" or "ngak." The seeds are boiled and roasted for commercialization, consumption, and biscuit-like snack food production (Babalola, 2011). Many studies have reported the effect of processing temperature on the nutritional, antinutritional values, and antioxidant properties, and African walnut seeds in some countries. These include: effect of boiling and traditional roasting on the nutritional, antinutritional, and antioxidant properties of African walnuts seeds (Arinola & Adesina, 2014); impact of processing on the nutrient content, vitamin, and mineral composition of African walnuts (Okonkwo & Ozoude, 2014); effect of cooking on phenolic content and antioxidant properties of African walnuts seeds Ademiluyi, Oboh, Aragbaiye, Oyeleye, and Ogunsuyi (2015). Though considerable attention had been given to the study of African walnut seeds, there is, however, very limited reports on the effects of processing on the walnut seeds grown in Cameroon, and especially on the quality of their lipids. Therefore, the objective of this study was to evaluate the effect of boiling and different roasting methods on lipid quality and proximate composition of walnut seeds.

| Material
The fresh walnut seeds (Tetracarpidium cornophorum) ( Figure 1) were harvested in the forest at Kumba, South-West region, Cameroon in July 2017.
All the chemicals and reagents used were of analytical reagent grade.

| Sample preparation and processing
The walnut seeds (all with the shells) were first cleaned and divided into 02 groups. The first group (G1) was processed fresh, and the second (G2) was dried to constant weight in an electric air-dried oven at 50°C for 7 days before processing.

| Maceration method
This method was used for oil extraction from dried samples (DTRWN 60 min, DORWN 60 min, and DWN) as described by Womeni et al. (2016). The nuts were separately grinded to pass 1 mm sieve. A quantity of 80 g of each power was separately macerated in 400 ml of hexane at room temperature for 24 hr with constant shaking. After that, the mixture was filtered using the whatman paper N1, and the filtrate was concentrated on a rotatory evaporator at 40°C. The extracted oils were stored in the refrigerator at 4°C for further analysis.

| Bligh and dyer method
Oils were extracted from samples containing water (FBWN 60 min, TRFWN 30 min, FWN, and DBWN 60 min) using the method described by Bligh and Dyer (1959). About 80 g of nuts were introduced in a grinding machine (Moulinex) to which 100 ml of chloroform and 200 ml of methanol were subsequently added. The mixture was grinded for 3 min; follow by the addition of 100 ml of chloroform and 100 ml of water. The mixture was again grinded for 1 min, and filtered. The final extraction was ensured by the addition of chloroform, this in order to respect the following proportion: 2:2:1.8 for chloroform, methanol, and F I G U R E 1 (a) Fresh walnut fruit; (b) Dried and unshelled walnut seeds water, respectively. After separating the different phases in a funnel, the organic phase was collected and dried using sodium anhydrous.
The organic solvent was then eliminated by evaporation on a rotatory evaporator at 45°C under reduced pressure. The extracted oils were stored in the refrigerator at 4°C for further analysis.

| Effect of processing on the quality of walnut oil
The determination of the peroxide value of walnut oil samples was made following the spectrophotometrical IDF standard method, 74A: 199174A: (IDF, 1991. Its iodine and acid values were determined according to the procedure of AOCS Official Method CD 1-25 and CD 3d-63 respectively (AOCS, 2003). Finally, its thiobarbituric acid value was evaluated as described by Draper and Hadley (1990).

| Effect of different processing methods on the proximate composition of walnut
Moisture, fat, ash, and protein content of all the samples were determined using standard analytical methods described by AOAC procedures (AOAC, 1990 1990). The total percentage carbohydrate content was determined by the difference method as reported by Onyeike et al. (2015). This method involved adding the total values of crude protein, crude fat, moisture and ash constituents of the sample and subtracting it from 100. All samples were analyzed in triplicate.

| Effect of processing on the mineral composition of walnut
For the determination of minerals, walnut seeds were ashed at 550°C and the ash boiled with 10 ml of 20% HCl in a beaker and then filtered into a 100 ml standard flask to determine the mineral content.
Calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), and iron (Fe) were determined by atomic absorption spectrometer (Varian 220FS Spectra AA, Les Ulis, France). Phosphorus (P) was determined colorimetrically using the vanado molybdate, according to AOAC procedure 965.17 (AOAC, 1999). Mineral contents of the samples were determined from calibration curves of standards minerals. All samples were analyzed in triplicate.

| Statistical analysis
Results obtained in this study were subjected to one-way analysis of variance (ANOVA) with Student-Newman-Keuls tests using Graphpad-InStat version 3.05, to evaluate the statistical significance of the data. A probability value at p < .05 was considered statistically significant.

| Peroxide value
Peroxide value (PV) is commonly used to determine the magnitude of primary oxidation products (mainly hydroperoxides) in oils (Shahidi & Wanasundara, 2008 (Nyam, Wong, Long, & Tan, 2013;Womeni et al., 2016). These results are in accordance with those of Tenyang et al. (2017) who demonstrated that, the peroxide values of sesame oil increases with the thermal treatment of sesame seeds.

| TBA value
Thiobarbituric acid value (TBA) value measures secondary oxidation production mainly malonaldehyde, which may contribute off-flavor to oxidized oil . The effect of different processing methods on the TBA value of walnut oil is shown in cates that, the rate of primary and secondary oxidation in this sample was significantly higher (p < .05). This is the confirmation that, drying + boiling 60 min (DBWN 60 min) has significantly alters the quality of walnut oil compared to other treatments. The increase in TBA value registered in the cooked samples is the consequence of the formation of malonaldehyde, which is a secondary oxidation product obtained from the decomposition of hydroperoxides (Womeni et al., 2016). The fact that the cooking processes can increase the rate of production of secondary oxidation products in edible seeds has already been demonstrated. Tenyang et al. (2017) showed that the p-anisidine value of sesame oil significantly increase with roasting and boiling temperatures and times.

| Iodine value
The iodine value (IV) generally determines the degree of unsaturation of edible oils and fats. A decrease in this parameter is generally attributed to the destruction of the double bonds of polyunsaturated fatty acids by free radicals (Tynek, Hazuka, Pawlowicz, & Dudek, 2001). The changes in IV of walnut oil samples during processing are presented in Table 1. Generally, the treatments have significantly decreased (p < .05) the iodine value of walnut oil compared to the con-

| Acid value
The increase in acid value (AV) of oils might be an important measure of rancidity of foods. Free fatty acids are formed due to hydrolysis of triglycerides and may get promoted by reaction of oil with moisture (Frega, Mozzon, & Lercker, 1999). The changes in acidity of walnut oil during processing are shown in Table 1

| Effect of processing on the proximate composition of walnut seeds
The effect of boiling and roasting on the proximate composition of walnut seeds is presented in Table 2. Results showed that, the mois- than that of DWN. It has been demonstrated that, low moisture content of food samples increase their shelf-lives by reducing the microbial and enzymatic activities (Oyenga, 2013). Concerning the changes in ash content, it was ranged between 7.30 and 9.03%. No significant difference (p > .05) was observed between the amount of ash of DWN (control) and those of DBWN 60 min, FBWN 60 min, DORWN 60 min, and TRFWN 60 min. However, a significantly higher (p < .05) ash content (9.03%) was registered with DTRWN 60 min, but that amount of ash was similar (p > .05) to that of DBWN 60 min (7.65%). This result suggests that drying + traditional roasting increases the amount of ash in walnut. The concentration of ash detected in this study is higher than those reported by Arinola and Adesina (2014). These authors reported that the amount of ash in raw, boiled and roasted walnut was ranged between 2 and 4%, and was decreasing with the treatments. However, our findings were not far from those of Onyeike, Anyalogbu, and Monanu (2015) who demonstrated that ash content of cooked and raw walnut seeds of 5.45%-6.05%. They also find no significant difference between the amount of ash in cooked and raw walnut.
This composition is closed to that previously reported by Arinola and Adesina (2014) with the same nuts. These authors showed that the total lipids, proteins and carbohydrates content of cooked and raw walnut varied between 54.14 and 62.65%, 22.47 and 29.14%, and 11.41 and 13.4%, respectively. It is also observed in Table 2 that the amounts of lipid of FBWN 60 min, DTRWN 60 min, DORWN 60 min, and TRFWN 30 min were significantly higher (p < .05) compared to that of DWN (control) and DBWN 60 min. This might be due to the loss of water, which facilitate lipids extraction. However, no significant difference (p > .05) was registered between the lipid content of DWN and DBWN 60 min. Similar trend has been obtained by Onyeike et al. (2015) when evaluating the effect of heat processing on the proximate composition and energy value of African walnut seeds. Talking about the total protein content, a significant decrease (p < .05) in its concentration was registered with DBWN 60 min, DTRWN 60 min, and DORWN 60 min compared to the control (DWN). This reduction can be attributed to the Maillard reaction, as proteins are substrates of nonenzymatic browning (Tenyang et al., 2017). Similar result has been reported by Arinola and Adesina (2014), who showed that the amount of protein of walnut seed was decreasing during boiling and roasting. However, no difference was registered between the amount of protein of FBWN 60 min, TRFWN 30 min, and DWN (Control). The total carbohydrate content of processed and raw walnut sample also presented in Table 2 showed that, the amount of sugar varies significantly (p < .05) with the processing method.  (Tenyang et al., 2017). These results are in accordance with those of Onyeike et al. (2015) who demonstrated that the total carbohydrate content of walnut seeds was decreasing during processing.

| Effect of processing on the mineral content of walnut seeds
The changes in mineral composition of walnut seeds during processing are presented in Table 3. It can be observed that the nuts contain important amount of macrominerals, among which calcium, magnesium, and potassium are the most represented. Sodium and phosphorus were also present, but at low concentration compared to the previous minerals. The importance of these mineral elements in human health has already been proven. They are implicated in several body functions such as enzymatic reactions, energy production, transmission of nerve impulses, and multiple biological reactions (Steinberg, Bearden, & Keen, 2003). The data of calcium shows that its amount has significantly decreased (p < .05) during cooking compared to the control (DWN). The lowest amounts of calcium were found in DBWN 60 min and FBWN 60 min, and were 569.00 and 671.50 mg/100 g, respectively. Globally, the concentration of calcium obtained in this study is higher than 433.5 mg/100 g as reported by Ayoola, Onawumi, and Faboya (2011). This mineral, together with the phosphorus are very essential for bone metabolism (Nwaoguikpe, Ujowundu, & Wesley, 2012 during processing through the diffusion process. However, its elevation in DTRWN 60 min and DORWN 60 min can be due to the fact that, the antinutritional present in the nuts and which were complexed to the mineral were significantly destroyed by heat, leading to the increase in its concentration (Makinde & Akinoso, 2013).
The amount of magnesium obtained in this study was higher than 171.12 mg/100 g as previously reported by Ayoola et al. (2011) in the same nuts.
From Table 3, we can also see that the amount of potassium and sodium has significantly decreased (p < .05) during cooking compared to the control (DWN). However, their concentrations were significantly higher than 625 mg/100 g and 26 mg/100 g as reported by Ayoola et al. (2011) and Nwaoguikpe et al. (2012), respectively. The presence of these minerals in African walnut is also beneficial, due to their direct relationship with hypertension in humans. This may be the reason why the plant is used to prevent and control high blood pressure (James, 2000). The phosphorus content of walnut was also reducing in almost all the cooked samples. Its concentration was ranged between 16.86 and 22.38 mg/100 g, which is lower than 35 mg/100 g as previously reported by (Nwaoguikpe et al., 2012).
Concerning the single micronutrient analyzed, (iron) its concentration has significantly increased (p < .05) in DBWN 60 min, FBWN 60 min, and DORWN 60 min compared to the control. However the amount of iron in DTRWN 60 min and TRFWN 30 min were significantly lower than those previously mentioned. This indicates that traditional roasting significantly decreases the amount of iron of walnut. The concentrations of iron obtained in this study vary between 14.64 and 43.32 mg/100 g, which is higher than 11.0 mg/100 g as previously reported by Oyoola et al. (2011). The amount of iron provided by walnut can helps in the prevention of iron-nutritional anemia.

| CONCLUSION
The objective of this study was to evaluate the effect of boiling and roasting on lipid quality and nutritional value of walnut seeds.
Results show that the quality of walnut oil was significantly affected by boiling and roasting. However DBWN 60 min and FBWN 60 min were the most altered samples. These treatments are not suitable for the preservation of walnut oil quality. The proximate composition of walnut seeds was also significantly affected by the treatments. Almost all the cooking methods have significantly increased the lipid content of the nuts, while their protein and carbohydrate concentrations decreased. Traditional roasting for 60 min increases the ash content of walnut seeds. The minerals of the nuts were also significantly modified during cooking. Both boiling and roasting have significantly decreased the K, Ca, Na, and P content of walnut.
Traditional and oven roasting for 60 min considerably increase the amount of magnesium, while the other treatments decrease its concentration. All processing technique including boiling leads to a significant loss of calcium and magnesium. Traditional roasting reduces the concentration of iron of walnut seeds.