Effect of boiling and oven roasting on some physicochemical properties of sunflower seeds produced in Far North, Cameroon

Abstract The effects of different processing methods on proximate composition, total phenolic content, antioxidant activity, lipid oxidation, and mineral contents of sunflower seeds produced in Far North Region of Cameroon were evaluated. Mean moisture, ash, lipid, protein, fiber, and carbohydrate contents of raw sunflower seeds were 6.60%, 2.55%, 44.65%, 20.17%, 4.08%, and 21.25%, respectively. The changes in moisture, ash (excepted in boiled samples), lipid, protein, fiber, and carbohydrate (excepted in roasted samples) were found to be significant for all cooking methods. Ash and lipid contents of samples roasted at 120°C were found to be significant when compared with other cooking methods. Antioxidant activity increased with treatment. After processing, the acid, peroxide, and thiobarbituric acid values increased significantly, whereas iodine value decreased. The roasting process improved the induction time, and samples roasted at 120°C were found to have the highest induction time (2.29 ± 0.09 hr). Raw sunflower seeds were good sources of potassium (K), magnesium (Mg), calcium (Ca), iron (Fe), zinc (Zn), and manganese (Mn). Increase in contents of Ca, Mg, Zn, Cu, and Fe was observed during processing. Roasting compared with boiling appeared to be the best cooking method of sunflower seeds concerning nutrient content, antioxidant stability, and lipid stability.


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TENYANG ET Al. 25% and 22% of the total world production, respectively, and in Africa, the largest sunflower seed producer was South Africa with 46.1% of total continent's production (FAO, 2007).
The importance of sunflower as source of edible oil and highquality protein is continuously increasing. The nutritional quality of H. annuus is comparable with soybean, and most other oilseed proteins including conventional legumes. Their potential as a dietary protein source in animal feeds is well recognized (Olvera-Novoa et al., 2002). Protein contents of the seeds ranged from 20% to 30% (Ezra et al., 2014). More importantly, the proteins of sunflower contain tryptophan, which is an important amino acid essential for growth, especially in children. Sunflower seeds contain about 48%-53% edible oil, and this oil is used for a variety of cooking purposes.
Due to their light yellow color, good flavor, high linoleic acid content, and high smoke point, sunflower oil is considered premium to other vegetable oils (Sharma et al., 2013). Research studies have shown that oleic and linoleic acids present in sunflower oil help to reduce the low density lipoprotein cholesterol and total cholesterol, decreasing the chance of coronary artery diseases (Chowdhury et al., 2007). Sunflower seeds are also a rich source of carbohydrate, ash, dietary fiber, vitamin E, B vitamin complex (especially vitamin B1 and B5), and minerals. Additionally, they are a good source of antioxidants, such as flavonoids and phenolic acids (Pasko et al., 2009). Nadeem et al. (2010) reported that the chemical composition of H. annuus seeds varies and depends on climatic conditions, genetics, varieties, soil type, and processing methods.
In Africa, seeds are mostly used in human nutrition and animal feeding. Before they have been used, they are submitted to different processing methods. The most popular methods of preparing sunflower seeds are boiling and roasting. The main goal of technological treatment is to improve the organoleptic characteristics, the antioxidant activity, and bioactive molecules and to enhance the nutritional quality of seeds. However, processing will also cause chemical changes that may negatively affect the nutritional value of seeds. Lipid oxidation, for example, strongly affects shelf life of oil. Oxidation reactions depend on many factors such as amount of unsaturated fatty acids in oil, enzymatic activity, and presence of natural antioxidants (Ozdenir & Devres, 2000).
Several studies have reported the effects of processing of sunflower seeds in some countries; these include chemical, nutritional, and antinutritional study of new varieties of oil seeds from sunflower, safflower, and groundnut (Satish & Shrivastava, 2011); effect of some processing techniques on the proximate and antinutrient composition of H. annuus seeds (Sharma et al., 2013); effect of different air-drying temperature on sunflower seed oil and ash content (Matin et al., 2017); and effect of processing on the proximate composition of sunflower seeds (Adesina, 2018).
Though considerable attention had been given to the study of sunflower seeds, there is however very limited information on chemical composition and the effects of processing on H. annuus grown in Cameroon. We can hypothesize that nutritional value of sunflower seeds from different locations is different, and boiling and ovendrying temperature affect differently the physicochemical properties of sunflower seeds cultivated in Far North, Cameroon. Therefore, the aim of the present work was to investigate the effect of boiling and ovendrying temperature on proximate composition, phenolic content, lipid oxidation, and mineral contents of sunflower seeds cultivated in Far North Region of Cameroon.

| Plant material and study site
The sunflower seeds used in the present study are commonly cultivated in Maroua (Far North Region of Cameroon). This area is located between latitude 10° and 13° North and between longitude 13° and 16° East. It has a tropical Sudano-Sahelian climate type with a long dry season of about 8 months and a short rainy season of 4 months (Tenyang et al., 2017). Four kilograms of dry raw sunflower seeds (Trt-9 variety) was collected from the Institute of Laboratory. In the IRAD, after harvesting and threshing, the seeds were dried in the shade for 20 days at mean room temperature of 30°C. After drying, only seeds that were not damaged were chosen for further analyses.
Hexane and standard solutions for atomic absorption spectrometry (Ca, Mg, Na, K, Fe, Zn, Mn, and Cu; purity 99.99%) were purchased from Courtage Analyses Services (Mont-Saint-Aignan, France). All reagents used in the analysis were of analytical grade.

| Heat treatments
The dried sunflower seeds were divided into three different groups.
The first group (200 g) was considered as control, the second group (800 g) was divided into four subgroups of 200 g each for roasting processing (60, 80, 100, and 120°C for 30 min each), and the third group (200 g) was used for boiling treatment.
For the roasting process, 800 g of whole sunflower seeds was arranged in a single layer in aluminum foil dishes (12.8 cm) and then placed in electric air-drying oven model G48 (Balay) on a stainless steel grill with the thermostat. The seeds were roasted at 60, 80, 100, and 120°C for 30 min after the set temperature was attained (60, 80, 100, and 120°C, respectively).
For the boiling process, 200 g of whole seeds was dipped into boiling water at ~97°C at ratio of 1:5 seed : water for 30 min.
The raw, roasted, and boiled sunflower seeds were allowed to cool down to ambient temperature and then grounded in an electric grinder (Panasonic), and the obtained powder was stored at 4°C for further analysis.

| Lipid extraction
Seventy grams of sunflower flour was soaked in 300 ml of hexane during 72 hr with shaking from time to time at room temperature (~25°C). The mixture was filtered on Whatman paper no. 4, and hexane was concentrated under vacuum using rotary vapour (40°C). After which, the supernatant was collected and evaporated to obtain the solvent-free oil. Then, the extracted oil was dried using anhydrous sodium sulfate.

| Extraction of sunflower polyphenols
The extraction of sunflower was performed following the method described by Womeni et al. (2016). Twenty grams of each processed sunflower powder was extracted with 400 ml of methanol for 48 hr at room temperature, with regular shaking during extraction. The extract was filtered with Whatman filter paper no. 1, and the residue was again extracted with 200 ml of methanol to ensure maximum extraction of phenol compounds. The combined filtrates were evaporated at 40°C under reduced pressure for removal of the solvent. The dried extract was used for further analysis.

| Proximate composition of sunflower seeds
Moisture, lipid, ash, protein, and crude fiber in the samples were determined using standard analytical methods described by AOAC (1990)

| Total polyphenol of sunflower seeds
The total phenolic content (TPC) of sunflower seed samples was determined using the Folin-Ciocalteu colorimetric method, as described by Gao et al. (2000). The seeds extracts (20 µl) were mixed in a test tube with 0.2 ml of Folin-Ciocalteu reagent and 2 ml of distilled water and incubated at room temperature for 3 min. Following this, 1 ml of 20% solution of Na 2 CO 3 was added to the mixture and re-incubated for 24 hr at room temperature.
The absorbance of the resulting blue color was measured using a quartz cuvette at 765 nm. The procedure was repeated to all standard gallic acid solutions, and standard curve was obtained.
The content of phenols in the test samples was determined from the standard curve, and the results were expressed as milligrams gallic acid equivalents per gram of extract.

| Antioxidant activity of sunflower seeds
The method described by Braca et al. (2002) was used to determine the ability of each extract to scavenge the DPPH radical.

| Chemical analysis of sunflower oil
The determination of free fatty acid (FFA) content and peroxide value (PV) of sunflower seed oils was performed according to the procedure of AFNOR (1981). Its iodine and TBA values were evalu-

| Mineral composition of sunflower seeds
For the determination of minerals, sunflower seeds were ashed at 550°C, and the ash was 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), iron (Fe), zinc (Zn), and copper (Cu) were determined by atomic absorption spectrometer (Varian 220FS SpectrAA). Reference sample from parts of the daily routine in the laboratory was used for quality control. Certified reference material 1570a was purchased from the National Institute of Standards and Technology. After initial standardization of techniques during a pilot study, the samples were treated identically. Mineral contents of the samples were determined from calibration curves of standard minerals. All samples were analyzed in triplicate.

| Statistical analysis
Data were analyzed by one-way analysis of variance using Statistical Package for the Social Sciences (SPSS version 16.0). Comparison was made between species and treatments. Significance level was set at p < .05.

| Effect of processing methods on proximate composition of sunflower seeds
The effect of common processing methods on proximate composition of sunflower seeds is presented in Table 1. The moisture content of samples was 6.60% for raw sample, 7.10% for boiled sample, and 4.4%, 3.38%, 2.76%, and 2.32%, respectively, for roasted samples at 60, 80, 100, and 120°C for 30 min. The raw sample was significantly (p <.05) higher in moisture content when compared with processed samples. The roasted sample at 120°C for 30 min had the lowest moisture content. Boiling process was found to slightly increase the moisture content of boiled sunflowers seeds. Increase of the moisture of boiled seeds may be due to the absorption of water by seeds during processing. During roasting process, decrease of moisture content may be linked to dehydration due to the increase of temperature in dry roasting conditions. These findings in the present study are similar to the results reported by Tenyang et al. (2017) when analyzing the effect of boiling and roasting on proximate composition of two sesame varieties consumed in Far North, Cameroon.
Some investigations have shown that low moisture content of food is a desirable phenomenon, because the microbial activity is reduced.
So as low moisture content in food samples increased the storage time of food product, this indicates that roasted sunflowers seeds and particularly those roasted at 120°C for 30 min can be stored for a long time and will be less prone to microbial attack during storage (Oyenga, 2013).
The ash content of raw sunflower seeds was 2.55%, which is low compared with the value reported by Adesina (2018) in the study of the proximate composition of sunflower seeds grown in Nigeria. Variation may be linked to species or geographical environment. It was observed that boiling process has no significant effect (p > .05) on ash content of sunflower seeds. However, seeds roasted at 60°C for 30 min have very close ash contents to boiled samples, whereas those roasted at more than 60°C (80, 100, and 120°C) led to an increase in ash content, and roasted samples at 120°C were found to have the highest ash content.
These results are different from the findings of Tenyang et al. (2017).
However, they are in agreement with the finding of Matin et al. (2017) who noted that increase of dry temperature between 60 and 100°C increases the ash content of sunflower seeds hybrid PR63D82. The increase in ash content noted in roasted samples in this study is explained by the reduction in moisture content as presented in Table 1.
The results show that the lipid contents of sunflower seeds ranged between 44.65% and 55.28%, with significant change in content between each treatment. A roasted sunflower seed at 120°C for 30 min was found to have the highest lipid content, whereas the raw sample was found to have the lowest lipid content. The amount of lipids in raw sample was higher to that reported by Satish and Shrivastava (2011)  Raw sunflower seeds were rich in protein (20.17%). This value was higher than those reported by Muesser et al. (2017), who obtained 10.91% of sunflower seeds cultivated in Turkey, but still lower than that noted by Ezra et al. (2014;29.6%) in Tanzania. Protein content of sunflower seeds decreased with boiling treatment, whereas it increased with roasting temperature, and roasted sample at 100°C for 30 min was found to have higher protein content (23.17%). At 120°C, the protein content of roasted sunflower seeds decreases TA B L E 1 Effect of processing methods on chemical composition (%) of sunflower seeds In this study, the results obtained also indicated significant variation (p < .05) on the crude fiber between raw, boiled, and roasted samples. The crude fiber of raw sunflower seeds (4.08%) was lower than 23%, values reported by Ezra et al. (2014) in Tanzania sunflower seeds.
The observed difference may be linked to species and geographic environment. The crude fiber content of sunflower seeds decreased with boiling process but increased with roasting temperature, and roasted sample at 120°C for 30 min contained relatively high amount of fiber.
These observations are in accordance with the finding of Tenyang et al. (2017) in roasted sesame seeds.
As shown in Table 1, the carbohydrate content of raw sunflower seeds was 21.25% and still lower than those noted by Satish and Shrivastava (2011) (2015) also reported the decrease of total carbohydrate content during roasting process of walnut seeds. These may be linked to the Maillard reaction occurring during processing, in which some carbohydrates reach with other compounds like amino acids to form sensory compounds, improving the taste, color, and odor of processed foods.

| Total phenolic content
The phenolic compounds present in fruits and seeds are the main antioxidants protecting the body from reactive oxygen species that cause cell damage. Figure 1 shows the TPC of raw, boiled, and roasted sunflower seeds. As seen in the figure, the TPC of raw sample was 2279 mg GAE/100 g. This result was slightly lower than the value obtained by Nadeem et al. (2010) in sunflower seeds in Pakistan (2965.90 mg GAE/100 g). The TPC of sunflower seeds is closely related to the analytical methodologies, the difference in sample material and origin. During boiling process, a significant decrease was found in TPC of sunflower seeds (p < .05). The loss of TPC observed during this process could be due to leaching out and denaturation of polyphenols (Sun et al., 2012). There was no significant difference (p > .05) in TPC of the roasted sunflower seeds at 60°C for 30 min when compared with raw sample. But at 80°C and more, the TPC of sunflower increased significantly (p < .05), and the highest content was found in samples roasted at 120°C for 30 min.
These changes were similar to those found by Behrooz et al. (2013) in roasted sesame seeds. Increases of TPC during roasting processes have been attributed to dehydration of food matrix and an improved extractability of phenolic compound from the seeds. Dewanto et al. (2002) also reported that increase in TPC in soybean during roasting treatment was primarily due to the increased release of phytochemicals as phenolic acids. The heat disrupts the cell membranes and cell walls, which releases the soluble phenolic contents from the insoluble ester bonds. The roasting temperature after 60°C is able to destroy the polyphenol oxidase enzymes, leading to the inhibition of polyphenolics degradation. Ozdemir et al. (2001) attributed the increase of TPC during roasting of hazelnuts to the increased generation of Maillard reaction product during processing.

| Acid value
Phospholipids, triglycerides, and diglycerides present in oils and fats are prone to thermal hydrolysis particularly in the presence of water, releasing fatty acids from their ester linkage, and increasing FFA.
FFA content is therefore frequently used to evaluate hydrolysis extension, a very important quality issue of oils and fats. The FFA values of raw, boiled, and roasted sunflower seeds are shown in Table 2.
The FFA of sunflower oil samples was in the range of 0.28%-1.65% oleic acid. The FFA of raw sunflower seed oil obtained in this study is higher than 0.10% oleic acid obtained by Issa et al. (2017) Khalid et al. (2017). The increase in FFA during processing may be attributed to heat, which is responsible for lipid hydrolysis and release of FFA. Decrease of FFA during boiling may be due to their destruction in other oxidative components during processing.
There is an inversely proportional relationship between the edibility of oil and the total amount of FFA. When oil has a low amount of FFA, there is a sign that the degree of their edibility is high, and the storage life is also long. The value of FFA obtained in all the processed sunflower seeds in this study revealed that all the oil samples maintained good qualities as their value did not exceed 2% oleic acid, the maximum limit of oil according to Codex Alimentarius (2013).

| Iodine value
Iodine value (IV) is a parameter frequently used to measure the degree of unsaturation in oil. It determines their stability to F I G U R E 1 Changes in total phenolic content of sunflower seeds during processing  oxidation after processing. Table 2 shows the IV of raw, boiled, and roasted sunflower seeds. It can be observed that IV of sunflower seed oil ranged between 90.27 and 102.06 g I 2 /100 g oil.
The values of IV noted in our sample are lower than 122 and 144 g I 2 /100 g oil obtained respectively by Khalid et al. (2017) in China and Ezra et al. (2014) in Tanzania in raw sunflower seed oils.
Variation may be linked to species and variety and their environment. The IV decreased significantly during processing, and the lower value (90.27 g I 2 /100 g oil) was found in oil from sunflower seeds boiled for 30 min. There was no significant variation between IV of roasted sunflower seed oil at 60°C and that roasted at 80°C. The oil of roasted sunflower seeds at 120°C compared with other roasted sunflower seeds was found to have a lower IV. Decreases in IV noted in roasted samples in this study are in agreement with the finding of Khalid et al. (2017). A decrease in IV is proportional to the reduction of unsaturated fatty acid present in oil. These characterize the oxidative changes in oils during processing. In fact, during processing, free radicals attack the double bonds of unsaturated fatty acids in sunflower seed oils, resulting in the reduce of IV of oil. Orthoefer et al. (1996) in their finding reported that heat treatment can cause the oxidative rancidity of oils and fats.

| Peroxide value
Peroxide value reflects the content of primary oxidation products that are formed during both initial and propagation phases of lipid oxidation. It is an important parameter used to assess the quality of lipid. In the present study, the PV of all treated sunflower seeds varied with process as shown in Table 2. Except roasted sample at 120°C for 30 min, there was noticeable increase in PV when the roasting temperatures increased. Boiled sunflower seed oil compared with all samples was found to present the high PV (13.65 meq/kg oil). The lowest PV (4.48 meq/kg oil) was recorded in raw sample. No significant difference (p > .05) was observed between the PV of raw sample and that of roasted sample at 120°C for 30 min. The increase of PV during processing indicates that exposition of sunflower seeds in water and at high temperature potentiates the formation of lipid peroxide molecules, as results of free radical attacking the unsaturated fatty acids (Nkpa et al., 1990). These results are in line with those noted by Khalid et al. (2017) who showed that the PV of oils extracted from sunflower seeds was increasing with the roasting time. A low level of PV in roasted sunflower seeds at 120°C compared with other treated samples may be due to their high level of TPC as presented in Figure 1. Apart from boiled sunflower seed oil that exhibited a PV higher than the recommended value, which is 10 meq O 2 /kg oil

| TBA value
To assess the secondary oxidation state of oils or fats, the TBA test is widely used. The secondary products occur in oils and fat when hydroperoxides are decomposed into nonvolatile carbonyls during the lipid oxidation. The change in TBA of sunflower seed oil samples is presented in Table 2. A general increase in TBA value was observed when processing change. Boiled sample, which has previously presented the high PV, has also showed highest TBA value (2.19 mg MDA/kg of oil). No significant differences (p > .05) were noted between roasted samples at 80, 100, and 120°C for 30 min.

| Racimat test
Oxidative stability is known as the resistance to oxidation processes established through well-defined conditions. The thermal oxidative stabilities of oils extracted from raw, boiled, and roasted sunflower seeds were evaluated using the Racimat method. This is the most common test used to assess the oxidative stability of edible oils, which is expressed as the oxidative induction time (Farhoosh & Kafrani, 2011). Induction time refers to a period of time before the chain reaction of oil oxidation begins to accelerate, and thus, a longer induction time indicates superior oxidative stability. Induction time of raw, boiled, and roasted sunflower seed oils is presented in Table 3. Raw sunflower seed oil exhibited a 0.75 hr induction time. A significant decrease (p < .05) in induction time was noted after 30 min of sunflower seed boiling. However, during roasting, significant increase (p < .05) was observed and when roasting temperature increases. The roasted sunflower seed oil at 120°C exhibited the highest induction time  Table 3 depicts the mineral composition of raw, boiled, and roasted sunflower seeds. As shown in this table, in sunflower seeds, macrominerals were the most important, with the highest being potassium (K), followed by magnesium (Mg), calcium (Ca), and sodium (Na).

| Effect of boiling and roasting temperature on minerals content of sunflower seeds
The other minerals were in lower concentrations. The K, Ca, and Na contents of sunflower seeds in this study were higher than the values reported by Bolanos et al. (2019) in Argentina sunflower seeds.
However, the Mg content obtained by the same authors was higher than the value determined in this study. Concerning microminerals, iron (Fe) was the most abundant (10.62 mg/100 g), followed by zinc (Zn: 7.37 mg/100 g) and manganese (Mn: 7.15 mg/100 g). Copper (Cu) was the lowest micromineral (2.71 mg/100 g). The Fe and Mn contents in this study are highest than those obtained in some sunflower hybrid seeds in Pakistan. However, the Zn content noted by these authors is higher than 7.37 mg/100 g, value obtained in this study. The difference noted may be due to species, the different environments, and analytical techniques used. Between raw, boiled, and roasted sunflower seeds, there was a significant difference (p < .05) in mineral contents. The K content of all the processed samples significantly (p < .05) increased, and the sample roasted at 120°C for 30 min exhibited the highest K content. Boiled sample compared with all roasted samples had the lower K content. All treatments also increased significantly (p < .05) the Ca, Mg, and Na contents of sunflower seeds, and the highest values were found in the samples roasted at 120°C for 30 min. There was no significant (p ˃ .05) difference between Ca and Mg contents of roasted sunflower seeds at 60 and 80°C. Boiled sunflower seeds compared with raw sample significantly decreased their Na content. The K proportion in all processed samples was the highest, followed by Mg, Ca, and Na. The reduction of Na in the case of boiling treatment may be linked to their leaching into boiling water. Increase in all the macromineral proportion in this study after roasting treatment may be justified by the increase in ash content noted in Table 1. The Mn proportion of raw sunflower seeds was found to be 7.15 mg/100 g. This value increases significantly (p < .05) after boiling, roasted at 60, 80, and 100°C for 30 min. After roasting at 120°C, the Mn content of sunflower seeds decreases significantly. The results also revealed that the highest Fe, Zn, and Cu contents after processing were found in roasted sunflower seeds at 120°C. There were no significant (p < .05) differences between the Cu and Zn contents of roasted sunflower seeds at 60 and 80°C for 30 min. Increase of mineral contents is the consequence of heat. Umoren et al. (2005) reported that high temperature increased the digestibility of seeds that initiated the release of some minerals. Seena et al. (2006) reported that Fe present in sunflower seeds is an essential mineral in human health and plays a role in immune function, cardiovascular health, and cognitive development. Tenyang et al. (2017) also mentioned that Mn and Zn abundant in processed sunflower seeds have vital role in bone mineralization, enzyme synthesis, taste, appetite, and growth. Consumption of processed sunflower seeds may help to prevent many disorders due to mineral deficiency in humans.

| CON CLUS ION
Sunflower seeds cultivated in the Far North Region of Cameroon due to their high content in lipid, protein, and minerals were considered having an important nutritional value. Their proximate composition, TPC, and antioxidant activity increased considerably after boiling and roasting. The major mineral contents also increased significantly, and the highest values were found in samples roasted at 120°C for 30 min. The oil analysis showed that all processing significantly affected the quality indexes: the acid value, PV, and TBA value significantly increased during processing. However, IV considerably decreased. Boiling process was found to be the most severe treatment, negatively affecting the index qualities. The roasting process improved the induction time of sunflower seeds, and samples roasted at 120°C were found to have the highest induction time. The processed sunflower seeds cultivated in this region of Cameroon can be adopted as supplement in human and animal diet due their high quality.

ACK N OWLED G M ENT
The authors would like to thank Dr. Teboukeu Gires for his assistance during the antioxidant activity analysis.

CO N FLI C T O F I NTE R E S T
There are no conflicts of interest to declare.

E TH I C A L A PPROVA L
This article does not contain any studies with participants or animals requiring ethical approval.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data are available upon request from the authors.