Moisture and pH
Measuring moisture content and pH in A. hypogea seeds is a quality indicator commonly used to assess moisture levels and provide information on the product’s ability to resist the risk of deterioration during storage.
The pH of the different peanuts ranged from 6.3 ± 0.06 to 6.48 ± 0.02 for samples P3 and P5-3 respectively. The results show that the pH of the different samples was significantly different (p˃0.05). Moisture levels varied between 4.21 and 3.88% (Fig. 2). There was no significant difference between moisture levels.
Moisture content and pH of peanut cultivar seeds. % H: moisture content.
Moisture and pH influence the development of fungi on a substrate. A very high moisture content and a low acid pH favor the growth of fungi, whose presence will lead to the deterioration of peanut seeds37. As far as pH is concerned, fungi are much more tolerant than bacteria. Bacteria often require pH levels between 7 and 8, while most fungi can grow between pH 3 and 8. Their optimum growth is generally in the pH range between 5 and 6. Because they tend to be acidic (with a pH < 6), foods such as fruit, vegetables, and meat are more susceptible to spoilage caused by fungi than by bacteria31. Our results corroborate previous studies that found respectively on 37 peanut samples a pH ranging from 5.73 ± 0.19 to 6.36 ± 0.03 and in non-germinated peanut seeds a pH of 6.2737. The pH of peanut seeds stored before marketing or for later use must be between 3 and 8 respectively31,38.
Moisture content is an important factor in preservation and deterioration. If it is kept below 10%, this guarantees good preservation of plant products. A water content below 10% ensures good preservation for plant products1. Our values are lower than those found in a previous study on A.hypogaea seeds, which contained 5.43% moisture39. However, moisture favors mold growth and According to UNECE STANDARD, 2022, the moisture content of raw peanut seeds must not exceed 10.0%31. This shows that our peanut seeds can be stored for a long time with little risk of microbial contamination40.
Aflatoxin contamination
The limits of detection and quantification of 0.01 µg/kg and 0.02 µg/kg respectively showed the high sensitivity of the technique used for determination. Calibration curves were made based on standards of different concentrations 5, 10, 20, 40, and 80 (Fig. 3). Aflatoxin G2 levels in seeds of the different peanut varieties were below the quantification threshold (LOQ) for all samples. As for aflatoxin G1, several samples were below the LOQ. The highest aflatoxin content was recorded in sample P5-4, which showed the highest aflatoxin G1 content at 1.08 ± 0.97 µg/Kg (Table 6). Aflatoxins B1 and B2 were quantified in all samples. Aflatoxin B2 levels ranged from 0.02 ± 0.03 (P4) to 0.34 ± 0.19 (P6-2). Aflatoxin B1 concentrations ranged from 0.01 ± 0.01 (P1) to 0.44 ± 0.07 µg/Kg (P5-4). Total aflatoxin content ranged from 0.04 ± 0.04 to 1.86 ± 0.42 µg/Kg in samples P5-3 and P5-4 respectively. For the different types of aflatoxins (AFG1, AFG2, AFB1, AFB2) there was no statistically significant difference (95% confidence interval). However, analysis of variance shows a significant difference for total aflatoxin content. (Table 6). The chromatograms of sample P5-4 were shown as example. An example of the chromatogram obtained for each type of aflatoxin is shown in Fig. 4. Along with the equation used to calculate aflatoxin concentrations and R2 (Fig. 4).
Chromatogram of a reference at 5 µg/kg of AFB1, AFB2, AFG1 and AFG2. AFG1: aflatoxin G1; AFG2: aflatoxin G2; AFB2: aflatoxinB2; AFB1: aflatoxin B1; AFT: total aflatoxin.
chromatogram of sample P5-4 1st and 2nd injection. AFG1: aflatoxin G1; AFG2: aflatoxin G2; AFB2: aflatoxinB2 ; AFB1: aflatoxin B1; AFT: total aflatoxin.
Aflatoxin B1 is the most common aflatoxin contaminant of plant material (USAID, 2012). The data obtained on aflatoxin contamination of peanut seeds are comparable to those found in a study conducted in Côte d’Ivoire, a country bordering Burkina Faso5. This study also showed the presence of aflatoxin B in all peanut samples compared with aflatoxin G, which was not detected in all samples. Other previous studies have shown that aflatoxin B1 is predominant in contaminated products and is also the most toxic aflatoxin23. Differences in aflatoxin levels between samples could be explained by sample storage conditions, and could also be linked to genetic factors.
Aspergillus species such as Aspergillus flavus and Aspergillus parasiticus produce aflatoxin B, but aflatoxin type G is produced by Aspergillus parasiticus. Indeed, previous work has shown that the double bond present on the dihydrofuran ring of AFB1 and AFG1 could be the origin of the difference in toxicity with their respective AFB2 and AFG2 homologs. On the other hand, the difference in toxicity between group B and G aflatoxins is thought to be due to the substitution of the cyclopentane ring by a lactone ring40. The toxic effect of aflatoxins is mainly based on aflatoxin B1, considered the main genotoxic metabolite and with the highest carcinogenic potential of all aflatoxins. Aflatoxin B1 stands out among these various mycotoxins for its exceptionally high toxicity and highly carcinogenic potential. It is frequently present in many food products, particularly peanuts and peanut derivatives13,14,37.
The European Commission has set the limits of acceptable aflatoxin B1 content at 2 μg/kg and 4 μg/kg for total aflatoxin content (AFB1 + AFB2 + AFG1 + AFG2) in nuts, dried fruit, ready-to-sell cereals. Codex Alimentarius sets a maximum limit of 12 µg/kg for total aflatoxins in peanut samples41. A comparison of the aflatoxin content of our samples with these international standards shows that the various peanut varieties are of good sanitary quality in terms of aflatoxin contamination. Although there are traces of aflatoxins in peanuts of local varieties collected in the towns of Dedougou, Bobo Dioulasso and Ouagadougou, the state of this contamination does not present a health risk for consumers, as the levels are below the threshold of international standards. It should be noted that the moisture content of around 4% and the slightly acidic pH of the samples help to limit aflatoxin contamination. It should be noted that high levels of aflatoxin ingestion can lead to acute aflatoxicosis, which is often fatal. Recent assessments of past aflatoxicosis outbreaks have estimated that toxic and potentially fatal doses of toxic and potentially fatal doses of AFB1 in humans range from 20 to 120 μg/kg body weight per day when consumed throughout 1 to 3 weeks. In addition, consumption of staple foods containing aflatoxin concentrations of 1 mg/kg or more has also been suspected of causing acute aflatoxicosis41. This information is essential from a nutritional point of view, as peanut seeds are widely used in technology to produce oil for consumption, pastes for cooking, tarts for bread, and so on. It should also be noted that no discriminatory differences were observed between samples from INERA selections and those from local crops. Another study conducted in Burkina Faso showed total aflatoxin levels of 14.19 μg/kg, which exceeds the threshold in force in Burkina Faso, which is the same as the Codex Alimentarius threshold42. This difference could be explained by post-harvest peanut storage conditions. Indeed, our samples come from a research center and have certainly benefited from better storage conditions, in contrast to the samples in the above-mentioned study, which were collected from market vendors.
Phenolic compound contents of peanut seeds
Total phenolics, flavonoids and flavonols
Quantification of phenolic compounds revealed significant variations from one variety to another. Total phenolics, flavonols, and flavonoids were determined. Total phenolic content ranged from 5.64 ± 1.35 to 14.94 ± 2.79 mg GAE/g for samples P1 and P5-2 respectively. Flavonoid content ranged from 1.23 ± 0.11 to 2.24 ± 0.15 mg QE/g for samples P4 and P3 respectively (Fig. 5). Analysis of the means showed that the values differed significantly. Seeds also contained flavonols in small quantities, with levels ranging from 0.14 ± 0.09 mg QE/g for sample P5-2 to 0.85 ± 0.36 mg QE/g for sample P1. Statistical analysis showed statistically significant differences between samples for quantified compounds. Phenolic compound composition showed no discriminatory statistical differences between varietal selection samples and locally grown varieties.
Total phenolics, flavonoids, and flavonols contents. Poly: polyphenols (mg GAE/g), Flav: flavonoids (mg QE/g), Flavon: flavonols (mg QE/g).
The phytochemical screening test realized indicates the presence of polyphenols in all extracts. These phenolic compounds are generally concentrated in the skin covering the cotyledons43. Flavonoids and phenolic compounds are the main components responsible for the antioxidant activity of extracts. Indeed, previous studies have reported a direct relationship between flavonoid and phenolic content and antioxidant activity44,45,46.
The phenolic content is similar to that reported in a previous study on the phenolic profiles and antioxidant activity of sprouted peanuts, which revealed an average phenolic content of 18.21 mg GAE/g in peanuts47. Other previous studies have found contents of 129.56 ± 1.61 to 160.17 ± 1.17 mg/g and 85.58 ± 0.59 to 209.18 ± 2.53 mg/g, higher than those recorded in the present study48,49, while lower average contents of 2.1 mg GAE/g DW and 3.28 mg GAE/g have also been reported50. The variation in phenolic compound content could be explained by the influence of environmental factors on the plant. Indeed, phenolic compounds are produced by the plant in response to physical aggression and environmental stress, notably edaphic and climatic factors50. This variability may also be linked to genetic factors, i.e. to the variety or cultivar.
Most of the phenolic compounds present in peanut seeds are concentrated in the tegument51. Indeed, a previous study reported that the peanut tegument contains phenolic compound contents of 160 mg GAE/g versus 13 mg GAE/g in the cotyledon43. Another study conducted in Thailand showed higher levels than those found in this study. These were 21.36 ± 1.90 µg GAE/mg, 8.65 ± 0.16 µg QE/mg, 0.249 ± 0.004 µg QE/mg respectively for total polyphenols, total flavonoïds and flavonols52. The authors stated that phenolic compounds confer a functional role on peanuts. The fact that peanut seeds contain phenolic compounds gives them antioxidant, anti-inflammatory, and antimicrobial properties. These compounds can scavenge free radicals, which are generated by the body or formed in reaction to environmental aggressions (tobacco, pollutants, etc.)11. They help fight certain metabolic diseases such as cancer, as well as degenerative diseases. Previous studies have demonstrated the beneficial role of polyphenols and flavonoids on human health. Indeed, these compounds play a preventive and protective role against certain chronic diseases, notably cardiovascular diseases, metabolic diseases, and certain cancers53. The action of phenolic compounds on human cancer cell lines reduces the number and growth of tumors54. These compounds act through several mechanisms, including estrogenic or anti-estrogenic activity, antiproliferative effects, induction of cell cycle arrest or apoptosis, prevention of oxidative stress, anti-inflammatory activity, and modifications of cell signaling55. Several studies have demonstrated the positive effect of polyphenol consumption on health and disease prevention, thanks to its antioxidant activity. They also help to prevent lipid peroxidation, an undesirable reaction that can occur in food products, thus ensuring good food preservation56.
The presence of phenolic compounds in the seeds of different peanut varieties is therefore an important factor to take into account, because of all the properties they confer on the materials containing them.
Hydrolysable tannin and condensed tannin contents
Comparison of means showed statistically significant differences. Peanut seeds had very low concentrations of condensed tannins, ranging from 0.14 ± 0.02 to 0.26 ± 0.02 mg/g (Fig. 6). Hydrolysable tannins are found in trace amounts in the samples. Maximum hydrolyzable tannin levels were 0.03 ± 0.01, while minimum levels were 0.16 ± 0.09 mg/g (Fig. 6).
Content of condensed tannins and hydrolysable tannins.
Tannins are polyphenols with molecular weights ranging from 500 to 3000 D. Tannins belong to the polyphenol family and are produced by certain plants to combat micro-organisms. Previous studies have found average levels of 35.38 ± 0.58 mg TAE/g in raw peanut seed samples57. Other studies showed lower tannin contents of 88 ± 0.19 and 4.73 ± 0.43 mg WT/g for cotyledons and 29.7 to 84.7 mg WT/g on 6 peanut varieties58. The results of this research clearly show that both tannins and polyphenols are more concentrated in the seed coat than in the seed. Tannins are phenolic compounds found in many plant-based foods such as fruits, vegetables, tea, wine, cereals, and certain herbs. Their nutritional importance is diverse and they can have beneficial effects on health and may also inhibit hydrolytic enzymes36. Tannins play an important role in immune system defense for organisms exposed to oxidative damage47,50. Tannins stop bleeding and fight infections. Tannin-rich plants are used to tighten soft tissues as in varicose veins to drain excessive secretions as in diarrhea. and to repair tissues damaged by eczema or burns57. It’s important to note that the effects of tannins can vary depending on their type concentration and food source. A balanced and varied consumption of tannin-rich foods as part of an overall healthy diet can help to benefit from their properties while minimizing any adverse effects. However high tannin levels can be detrimental to human health. For this reason, the low tannin content of our peanut samples may be an advantage given their nutritional importance.
Correlation between water content, aflatoxin, and phenolic contents
Principal component analysis revealed the links between physico-chemical parameters, aflatoxin levels and secondary metabolites in the different peanut varieties studied. Aflatoxin levels AFB1, B2, G1, and G2 are carried by the F1 axis (30.65%), while pH and metabolites are also carried by the F2 axis (22.83%) (Fig. 7). Furthermore, samples with high levels of phytochemicals show a negative correlation with aflatoxins levels (i.e. low contamination). The dendrogram shows two clusters and six groups (Fig. 8). The cluster C1 regroups {P1, P2, P4, P5-1, P6-4}; and the cluster C2 regroups{P5-4, P6-2, QH, P5-2, P6-1, P6-3, P5-3, SH}. The varieties included in each cluster show a similar profile in terms of phenolic compound composition and total aflatoxin content. There was also a positive correlation between polyphenol, hydrolyzable tannin, flavonoid and flavonol content, and Aflatoxin B1, B2, G1, and G2 levels, and between condensed tannin and dry matter content (Table 7).
Principal component analysis of correlations between parameters and varieties.
Dendrogram showing the genetic similarities of varieties according to the parameters studied.
The sum of F1 and F2 gives 53.48%. The correlation graph shows that polyphenols, hydrolysable tannins and pH are strongly positively correlated. On the other hand, with the exception of aflatoxin G2, the other forms of aflatoxin showed positive correlations. This shows that there are no major statistical differences in aflatoxin content between the different samples. It should also be noted that there was little correlation between phenolic compound levels and aflatoxin levels. These compounds are thought to be factors limiting the proliferation of the Aspergillus genus. their antimicrobial properties mean that they create a more or less hostile environment for microorganisms. It seems logical that samples containing high levels of phytochemicals showed a negative correlation with aflatoxin content because phenolic compounds are produced by plants to fight against their physical or chemical attackers. Phytochemicals have been proven to have antifungal power24. The presence of several groupings on the dendrogram indicates a genetic similarity between the varieties making up the different groups. The varieties have not been grown on the same soil types, nor have they been subjected to the same climate. It has been shown that genetic factors are largely responsible for the traits expressed by plants58. Even if these genetic factors are often influenced by the environment59. This information can be very important if you want to use different varieties to fortify foods for particular groups, especially to prevent certain metabolic diseases. The fact that peanuts are not produced under the same environmental conditions (soil types, fertilizers, same locations) may be a limitation for this study and would partly explain some of the differences observed.