Combining social science and chemistry to uncover the potential of the traditional pit method for shea processing

doi:10.21203/rs.3.rs-4704167/v1

Download PDF

Article

Combining social science and chemistry to uncover the potential of the traditional pit method for shea processing

https://doi.org/10.21203/rs.3.rs-4704167/v1

This work is licensed under a CC BY 4.0 License

You are reading this latest preprint version

Shea oil, a widely consumed commodity globally, is intricately linked to the labor of women in Sub-Saharan Africa. Employing a traditional yet underexplored method, shea nuts are fermented in subterranean pits, presenting significant opportunities for local processors, the industry, and environmental sustainability. Through interdisciplinary inquiry, we investigate the adoption of this method within rural West African communities, considering its chemical and industrial implications. Local processors favor the pit method for its convenience and efficiency. Traditional practices of burying shea nuts for three to six months result in an optimal chemical profile, characterized by lower free fatty acid and polar lipid content compared to boiled kernels, enhancing quality and mechanical processability, both criteria desirable for industrial applications. This method has the potential to reduce the use of firewood and water in producing communities. Nevertheless, encouraging widespread adoption by new processors will likely require increased nut prices based on seasonal factors.

Biological sciences/Biochemistry/Lipids/Fatty acids

Biological sciences/Biochemistry/Lipids/Glycerides

Biological sciences/Biochemistry/Lipids/Phospholipids

Biological sciences/Biochemistry/Lipids/Sterols

Earth and environmental sciences/Environmental social sciences

Burkina Faso

fosse

Vitellaria paradoxa

lipid profiling

vegetable fat quality

indigenous and local knowledge

Shea oil has experienced a significant rise in global demand since being recognized as a Cocoa Butter Equivalent in 1990 [1][2]. The oil is derived from shea fruit harvested in the Sudano-Sahelian Savannah of Africa [3][4], for which the primary sourcing countries include Burkina Faso, Ghana, Mali, Nigeria, and the Ivory Coast [5]. Although obtaining precise data on shea kernel production and export is challenging, estimates indicate that annual kernel production in West Africa has increased from 217,000 tons in 2004 [5] to 1.63 million tons in 2015 [6], with 43% being exported [7]. In 2019, Burkina Faso, the main producing country, had an estimated production potential of 0.85 million tons. Major importing regions and countries include Western Europe, particularly Denmark, France, and the Netherlands, as well as the United States, Singapore, and West African countries such as Ghana and Togo. In importing countries, kernels are processed to extract shea oil for use in confectionary and the cosmetic industry [8][5]. It is estimated that 2.4 million women collect shea for the export value chain in West Africa, and an additional 4.8 million women collect shea for self-consumption without market linkage [7].

Shea nut collectors, referred to as processors in this paper, are crucial in preparing shea kernels for trade. They utilize various methods like boiling, parboiling, smoking, and sun drying to depulp, treat, dry, and deshell the nuts to extract the kernels [9][10]. Processing techniques aim to prevent germination and kernel degradation, ensuring high-quality kernels without bitter butter [11]. Another traditional method used in Burkina Faso and other West African countries involves using excavated pits in the ground (called “fosse” in French) to process shea nuts. While this method has received limited attention from the scientific community, it presents a presumably positive, but as yet unknown, potential for local shea processors, the shea oil industry, and the environment, and warrants further attention.

Descriptions of the pit method and its purpose are scarce, variable, and fragmented. The earliest documented mention of this method in literature dates back to 1956 (Servant et al. (1956) as cited in [12]). However, it was not extensively detailed until 1982 [11]. In the pit method, local processors deposit their shea fruits into pits ranging from 300 to 500 kg in capacity. They then gently stomp the fruits, cover the pits with soil or other materials, and allow them to ferment for a variable duration. Studies have reported that the optimal fermentation period ranges from a few days to several months [13][14][11]. After emptying the pit, the nuts are either boiled or smoked [13][12][11] and then sun-dried and deshelled, or directly sun-dried without further heat treatment [9][13]. Scholars have portrayed the pit method as both a storage practice [15] for depulped nuts, resulting in low-quality butter [14] and as a fermentation process that depulps the nuts, prevents germination, and yields both high [11] and low-quality [9][12] butter. However, none of those studies analyzed the chemical composition of the extracted butter, and the characterization of low- or high-quality butter was largely subjective. Thus, despite the pit method apparently being well described in the literature, reports on the method are typically brief and contradictory.

The pit method offers a promising alternative to traditional shea processing methods, such as boiling, parboiling, or roasting, as it eliminates the need for water and/or firewood. In contrast, the parboiling method requires approximately 48 kg of firewood to process an 80 kg bag of kernels [16], while boiling the nuts over open fires consumes about 114 kg of firewood to obtain the same amount of dried kernels [17]. Additionally, roasting the nuts, although not requiring water, consumes even more firewood than the boiling method [18]. While literature on the exact amounts of water and wood needed for these processes is scarce, they are consistently higher than those required for the pit method, which uses neither water nor wood. Consequently, the pit method holds significant potential in arid regions where firewood and water resources are limited. Despite its promising potential, understanding local preferences and experiences are crucial to assess its potential adoption among the local population.

Existing chemistry literature covers how storage, boiling, parboiling, roasting and smoking affect the physicochemical properties of shea kernels and butter [19][20][10][21][22]. Notably, reports on pit kernels are lacking. Preventing nut germination and degradation is crucial to avoid undesirable changes in triglyceride content and free fatty acid (FFA) levels. Such changes are detrimental to local butter processors and the shea oil industry [23].

In 2005, the West African Trade Hub of the United States Agency for International Development set FFA content limits for shea butter: < 1% for cosmetics and < 3% for food [24]. Industrial processing often requires FFA removal via neutralization, but this leads to triglyceride loss due to deprotonated FFA acting as soaps. Roughly, for every 1% FFA removed, 1% triglyceride is lost [25]. This unavoidable loss [26] lowers economic returns.

The shea industry prefers kernels with high triglyceride yield, minimal undesirable co-extractives e.g. [10][27], and easy mechanical processability. Brittle, porous kernels are preferred to soft, rubbery kernels, for efficient extraction. Total extractables (also 'total fat', ‘crude fat’ or ‘fat content’) serve as a quality indicator for mechanical processability e.g. [28][26]. This measure is the fraction of mass extracted from kernels by an apolar solvent using Soxhlet extraction. However, while total extractables may be high (67%), corresponding FFA content can reach 23% of that fraction [27]. Thus, while it only indirectly measures desirable fat-content, it directly assesses mechanical processability by quantifying apolar extractable solids.

Alongside total extractables, other endpoints like acid value, FFA content, peroxide value, iodine value, unsaponifiable matter, saponification value, and refractive index are used to assess butter from differently pretreated kernels [22]. Detailed chemical analyses have explored compounds like fatty acids (e.g. [10][21]), triglycerides (e.g. [29][30]), triterpene alcohols (e.g. [31]), -acetates, -cinnamates (e.g. [29][30][32]), tocopherols (e.g. [33][10][34]), phytosterols (e.g. [35][36]), phenolics (e.g. [37]), and volatiles (e.g. [38][22]). However, these analyses are seldom linked to different kernel pretreatment methods. Surprisingly, polar lipids in shea kernels remain poorly characterized, except for total lecithin measures [36]. Despite the diversity of methods for chemical analysis of shea kernels, research on the pit method’s impact on shea kernel chemistry and quality is scarce.

This study combines social science and chemistry to examine the use and chemical impact of the pit method. The study focuses on Burkina Faso due to its significance as a shea sourcing country and the traditional use of the pit method in the region. The study was guided by the following research questions: 1) How and why is the traditional pit method used in Burkina Faso? 2) What are the pros and cons of the pit method for the local processors, and what drives the adoption among new processors? 3) How does the pit method affect the chemical profile of the kernels? And 4) What are the pros and cons of the pit method for the shea industry?

During the investigation 130 shea processors were interviewed, and 28 shea kernel samples (fresh, boiled, and pit-processed) were chemically characterized (Figure 1).

Traditionally, the pit method is implemented diversely

All 130 processors interviewed were women. Most used a single round pit (1.8 m³ ± 0.7 SD, n=34), but some used up to four pits. Pits, owned individually (75%, n=24) or shared (25%, n=8), were used for several years. Filling a pit took around 1.5 months on average (± 0.8 SD, n=33), sometimes extending to two or four months. Processors deposited whole fruits in the pits, occasionally adding plastic, sacks, or leaves at the pit's bottom while filling it. Pits were gradually filled and compacted with no added water. Materials used to cover the pits varied: soil and leaves (34%, n=23), only leaves (30%, n=20), tarpaulin (25%, n=17), a combination of those (6%, n=4), or other materials (3%, n=3). Nuts were typically not stirred in the pit (94%, n=32). Pit location was influenced by proximity to homes or farmhouses for theft deterrence and ease of nut transportation. Most processors stated their method mirrored practices of their elders (82%, n=30), others were unsure about historical practices.

Burial periods for nuts varied: some for less than two weeks, others for one to four months, and a few for up to six months (Figure 2a). When immediate cash was needed, kernels were retrieved and sold (73% of processors, n=27), while a third kept a reserve at home to boil for quick sales (30%, n=11). Pits were used only when processors had sufficient nuts and were not constrained financially.

After the pit process, 73% of processors boiled the nuts (n=64), considering it crucial for high-quality butter and marketability. Another 23% sun-dried and deshelled the nuts without further treatment (n=20), due to factors like wood scarcity, time constraints, or because the nuts were deemed market-ready post pit-extraction (after sun-drying and deshelling). A minority roasted the nuts for butter (4%, n=4).

Despite its benefits for shea processors, new adoption of the pit method relies on financial considerations

Experienced processors highlighted benefits of the pit method (Figure 2b): 1) Prevents nut germination, crucial for quality. 2) Speeds up pulp putrefaction, aiding depulping. 3) Offers flexibility in processing times, e.g. the nuts are safe during the rainy season and can be dried during the dry season. 4) Allows processing close to gathering sites. 5) Protects nuts from animals. 6) Produces quality kernels, heavy and bright, for butter-making or selling. 7) Provides storage space, especially for processors lacking warehouses.

New processors valued the pit method differently from experienced ones (Figure 2b): 1) Required less effort. 2) Convenient for multitasking and choosing processing times. 3) Used less wood and water. They also appreciated 4) resulting kernel weight and quality, 5) avoiding fruit transportation, requiring less strength, and 6) no need to sort nuts. Some mentioned 7) storing nuts for future use. New processors, having not engaged in pit digging and emptying, were unable to assess the overall effort involved compared to other methods. With this caveat in mind, the majority of both experienced and new pit-processors did not mention any disadvantages with the pit method. A few experienced processors mentioned the unpleasant odor emitted by the kernels as the sole drawback, but most did not mind it, noting it dissipates after sun exposure. Some new processors cited challenges such as emptying the pits coinciding with animals roaming freely and difficulties monitoring the condition of the pits, including concerns about rain washing away the covering soil.

Most of the processors who started using the pit method in 2022 also continued their usual methods with other nuts, primarily smoking (65%, n=24) or boiling (51%, n=19). They did this to meet buyer demands, make butter, and address financial needs. Smoking or boiling the nuts post-extraction was deemed crucial for transforming them into shea butter for consumption and sale. Some respondents (24%, n=9) did not smoke or boil nuts because they were all in the pit experiment.

Encouraging future use of this method requires considering the following points: 1) Price incentive: Higher prices for pit-processed kernels motivate processors to continue using this method and compensates for the time during which kernels are fermenting in pits and cannot be sold. 2) Savings and financial planning: Fermenting kernels in pits allows effective work and financial management, acting as a form of savings. 3) Positive experience and ease of practice: Processors find the pit method simpler and less cumbersome, excluding considerations of pit digging and emptying.

Pit kernels exhibit a distinct chemical composition compared to fresh and boiled kernels

Chemical compositions of boiled, fresh and pit kernels were analyzed to determine the characteristic chemical profiles, and to assess variability. The investigated compound classes included FFAs, mono-, di- and triglycerides (MGs, DGs, TGs), fatty acid methyl esters (FAMEs) and polar lipids, as they were all expected to impact kernel and butter quality, with implications for the shea industry. Quality control plots for individual compounds are given in Supplementary Information, Figure SI-1.

High FFA levels indicate poor quality in locally produced butter and the industrially extracted TG fraction. A significantly systematic difference between boiled and pit kernels is observed for FFA, which are primarily oleic and stearic acids. FFA levels in pit kernels are at least three times lower and less variable than those in boiled kernels (Table 1).

Table 1: Total peak heights and RSDs of FFAs measured by LC-HRMS in boiled kernels and pit kernels stored for three and six months.

		Boiled kernels (n=12)	3- & 6-month pit kernels (n=11)	Ratio	p-value
Oleic acid, stearic acid, linoleic acid, icosanoic acid measured by LC-HRMS	Average total peak height	6.7 × 10⁴	1.9 × 10⁴	3.5	2.1 × 10^-3
	RSD	69%	30%	2.3

Total peak heights correlated well with measured FFA levels (in oleic acid equivalents) in the industrial crude extract. FFA levels in pit kernels were typically less than 3.5 %, except for one 6-month sample reaching 6.4 %. Some boiled samples, like one from site 3, had FFA levels as high as 10.9 % (Figure SI-2).

The relative abundances of chemical components in boiled kernels, fresh kernels, and 3- and 6-month pit kernels are summarized in a heatmap (Figure 3).

For statistical testing, samples were grouped into fresh kernels (n=5) and pit-kernels (n=11), as well as well-preserved boiled kernels (Boiled_2-1, 3 and 4 and Boiled_3-2, n=4) and poorly preserved boiled kernels (Boiled_1-1 to 4, Boiled_2-2, Boiled_3-1, 3 and 4, n=8) based on their FFA content.

FFA levels increase as kernels are stored in pits, with levels in pit kernels comparable to well-preserved boiled kernels, but significantly lower than levels in poorly-preserved boiled kernels (p-value < 10^-9 from Table SI-1; Figure 3a and Figure 4a).

Formation of MGs and DGs indicates early degradation and is not desirable, although they do not decrease the quality of the extracted TG fraction as much as FFAs. Figure 3c and Figure 4c-d show that MG and DG levels are low in fresh and well-preserved boiled kernels (p-values < 0.023, Tables SI-2 and SI-3). The levels are not significantly different for the pit kernels and poorly preserved boiled kernels, but are higher than for the fresh and well-preserved kernels, and exhibit higher variation. Thus, early degradation of TGs to MGs and DGs occurs to the same extent in poorly preserved boiled kernels and in pit kernels.

Despite increasing levels of degradation products, TG levels did not significantly vary between processing methods. However, well-preserved boiled kernels had slightly higher mean TG levels (9-16% higher) compared to fresh and poorly preserved kernels (Figure 3d, Table SI-4).

FAMEs are transformation products derived from e.g. FFAs. Their impact on kernel quality remains uncertain. FAMEs are more abundant in pit kernels, with the lowest levels observed in well-preserved boiled kernels (p-values < 2.0×10^-4, Table SI-5, Figure 3b and 4b). This suggests that FAME abundance does not necessarily reflect FFA levels; in boiled kernels FFAs and FAMEs are positively correlated. In contrast, fresh and pit kernels show low FFA but relatively high FAME levels. Thus, the proportion of FAMEs is significantly lower in boiled kernels compared to fresh and pit kernels (approximately 8 times lower).

Polar lipids are integral to plant membranes, and their composition likely varies with shea-processing methods. However, their influence on kernel properties remains poorly understood. Relative peak heights of 38 polar lipids [39] representing various classes were examined (Figure 3e-g). Pit kernels consistently showed lower levels of most polar lipid classes compared to boiled and fresh kernels (p-values from 0.039 to < 10^-9). Poorly-preserved boiled kernels also exhibited lower levels compared to well-preserved boiled kernels and fresh kernels (Figure 3g, Table SI-6 to Table SI-12). However, glycosphingolipids (HexCer) lipids showed consistent abundance across all samples (Figure 3f, Table SI-13). While sulfoquinovosyldiacylglycerols (SQDGs) followed a similar trend as other polar lipids, the difference magnitude was lesser (Figure 3e, Table SI-14).

In summary, the various processing methods yielded kernels that fell into four distinct categories based on their chemical characteristics: 1) fresh kernels; 2) poorly-preserved boiled kernels; 3) well-preserved boiled kernels; and 4) pit kernels (Figure 4).

Cluster analysis (see sample-clustered heatmap in Figure SI-3) and principal component analysis (Figure SI-5) further validate the distribution of kernel samples into the described categories. An exception is the second boiled sample from site 3 (Boiled_3-2), classified as well-preserved due to its low FFA content but which also exhibits low levels of polar lipids. Additionally, two fresh samples (Fresh_4-2 and 4) show slightly more polar lipid degradation compared to the other fresh kernel samples.

The pit method increases total extractables compared to boiling

The total extractables, indicative of industrial processability of the kernels, increases with the pit method compared to boiling. In Figure 5, polar lipid levels for selected fresh, boiled and pit kernels are plotted against total extractables. Polar lipid levels are typically higher in fresh kernels but decrease to below 20% of the maximum level for the 3- and 6-month pit kernels. Fresh kernels have less than 56% total extractables (average 52 ± 2% SD, n=5), while 3- and 6-month have more than 53% total extractables (3-month average 59 ± 4% SD, maximum 63%, n=5 and 6-month average 63 ± 5% SD, maximum 68%, n=5). Boiled samples have total extractables average of 56 ± 2% (SD, n=4). Despite large variability within each class and a low number of boiled kernel samples, the p-value of 0.16 suggests a potential positive effect of pit processes on total extractable (Table SI-15). For two pits (denoted by ● and ■ symbols), total extractables increase dramatically (by 12-13 %) between 3-month and 6-month pit kernels. For the other three pits, total extractables are highest for 3-month pit kernels but decrease slightly (by 0-4%) for 6-month pit kernels despite a small decrease in polar lipid levels.

The traditional pit method may lessen reliance on, and consumption of, natural resources such as firewood and water

The pit method has received limited academic attention in recent years. Through an interdisciplinary approach, we have explored its utilization by rural West African processors and its implications from chemical and industrial perspectives. Our study reveals diverse practices among processors, who use various materials to cover nuts and bury them for varying durations. While some keep nuts underground for less than two weeks (in line with e.g. [12] citing Servant et al. (1956)), others extend this period to months, influenced by convenience and time-efficiency, consistent with [15] and [14]. These findings challenge earlier accounts, such as [11] who recommended a specific burial duration of five to eight days. The method's portrayal in literature varies due to its adaptable nature, reflecting the diverse practices of female processors.

Our findings revealed that the majority of processors use the pit method as a pretreatment step before boiling or smoking the nuts, believing it necessary for producing high-quality butter. Nevertheless, processors also boil/smoke the kernels sold at markets. However, our chemical analysis showed that post-excavation processing is not essential to achieve low FFA levels. This suggests that omitting heat treatment could be beneficial and should be promoted to shea processors, highlighting the potential reduction in water and firewood usage associated with the pit method. To truly minimize resource consumption and labor, kernels should be sold directly after excavation and sun-drying, without further heat treatment.

The pit method offers convenience and time-efficiency for new and experienced processors, with few recognizable drawbacks

Experienced processors traditionally use the pit method as a depulping technique, safeguarding nuts from germination and from animals and theft until ready for sale or processing into shea butter. Situating pits near collection areas eliminates the need for transportation, allowing processors to engage in other activities while nuts ferment. This is especially beneficial during the busy shea collection season between April and August [40]. Overall, the method is seen as convenient and time-efficient, with few drawbacks for experienced and new processors.

New processors found the pit method less tedious because kernels were market-ready post-excavation and after sun drying, eliminating the need for boiling or smoking. They appreciated its convenience, allowing them to engage in other tasks, while ensuring nut safety. Practical benefits included avoiding water and firewood collection, fruit transportation, and nut sorting, while still obtaining high-quality kernels. Unlike traditional users, they did not prioritize germination prevention, likely because they were new to the method. Also, new processors were spared from digging pits themselves, as this was done by project officers. Future research should explore perceptions of labor dynamics in the full process, including family involvement in pit digging and emptying.

New processors expressed concern about the selling price when considering whether to adopt and continue using the pit method. The price of the kernels in the early season from April to June is significantly lower than in the later season from July to January [41], prompting processors to use the method as a form of savings, but only for those who can afford it. However, retrieval of nuts for immediate sale to meet financial needs is common, as depulping typically occurs within five to 12 days [11], [12].

Pit kernels show lower FFA and polar lipid content than boiled kernels but higher FAME levels

FFAs, a key indicator of poor quality, were found to be significantly lower in kernels buried in pits for six months, compared to those buried for three months, and with up to 5-7 times lower levels compared to boiled kernels. The most degraded boiled kernel samples, exhibited FFA content 20 times higher than the most well-preserved pit kernels. However, the most well-preserved boiled kernel samples, showed FFA levels comparable to the most well-preserved pit kernels, with only 9% absolute deviation. This aligns with previous research by [42], which found FFA levels in shea products to vary by a factor of 10 (range 1-11% in shea butter), with reported levels of up to 24% in the total extractables fraction e.g. [27].

Fresh kernels exhibited low levels of FFA, indicating good quality. However, FFA levels increased in pit kernels the longer they were buried. While traditional methods suggests kernels may be kept buried until commercialization or use [13], it is unlikely they can maintain low FFA levels over extended periods. In our study, burial times of three months appeared optimal for maintaining low FFA levels.

The variability in FFA levels was notably higher in boiled kernels, even from the same site, compared to pit kernels. While the FFA levels in the best-preserved boiled kernels were similar to those in the best pit kernels, the quality of boiled kernels was much less consistent and sometimes very poor.

MGs and DGs, early degradation products of TGs, increase as kernels are stored in pits and in poorly-preserved boiled kernels. However, this does not seem to affect TG content significantly. This suggests that degradation has commenced in pit kernels, particularly compared to well-preserved boiled kernels. Levels are low in fresh kernels and in well-preserved boiled kernels.

FAMEs may be formed from a condensation reaction between methanol and FFAs known as esterification. It was observed that the fraction of FAs occurring as FAMEs was low for boiled kernels, and up to eight times higher for fresh and pit kernels. Fermentation of biological material may result in the formation of methanol [43]. Therefore, a lower conversion of FFAs to FAMEs is expected for boiled and aerated kernels, where less methanol formation occurs. Nonetheless, it is surprising that levels of FAMEs in pit kernels are so high, when the FFA levels are low.

However, FAMEs can also be formed through the direct reaction between alcohols and acylglycerols, i.e. MGs, DGs and TGs, in a process known as alcoholysis or trans-esterification. In alcoholysis the FFAs are not the precursors for the formation of FAMEs. In biological systems, alcoholysis is catalysed by lipase enzymes [44], and it is the major mechanism of ester biosynthesis in dairy lactic acid bacteria and yeast [45]. In pit fermentation, the precursors acylglycerols and methanol, as well as lipases, are present. The differences in patterns of FFAs, FAMEs, MGs and DGs could originate from different degradation pathways dominating between boiled kernels (hydrolysis from TGs to DGs and MGs and to FFAs, with little conversion to FAMEs through esterification) and pit kernels (alcoholysis of acylglycerols in the presence of methanol to produce DGs, MGs and FAMEs, but not FFAs). In the case that FFAs are produced in pits, they will be quickly transformed to FAMEs by esterification in the presence of methanol [44].

The implication of FAME levels on the kernel quality is unknown. However, it is presumed FAMEs can be easily removed by the deodorization process e.g. [46] in the industrial refining of the extracted TG fraction. Thus, it is possible that it is an industrial advantage to have FAs converted to the methylated form to a larger degree, as found in the pit kernels. Nonetheless, this hypothesis warrants further exploration.

Polar lipids are membrane lipids and partly responsible for plant cell integrity [47][48][49]. Well-preserved boiled kernels often exhibit a rubbery texture due to the presence of these polar lipids. Industrial processing typically requires degumming to ease the mechanical processing of these rubbery kernels and to reduce downtime of mechanical equipment. By degumming, the phospholipids are solidified as a gum, and separated from the TG fraction by centrifugation [26]. In contrast, poorly-preserved boiled kernels have a more porous and brittle structure, making them easier to process mechanically. Our findings indicate that polar lipids undergo efficient degradation in poorly-preserved boiled kernels and pit kernels, suggesting breakdown of cell structures. These results suggest that the good mechanical properties (porous brittle structure and a lack of gum-like constituents) are also characteristic of pit kernels. This hypothesis is further supported by the fact that pit kernels exhibited higher, and up to 68%, total extractables.

The chemical profile of pit kernels indicates high quality with low FFA content and good mechanical processability, both crucial in an industrial context

The pit method yields kernels with low FFA levels akin to well-preserved boiled kernels as well as good mechanical processability and high total extractables, similar to poorly-preserved boiled kernels. This method thus combines the advantageous properties of both types of boiled kernels, resulting in kernels of more consistent quality compared to boiled kernels, which exhibited significant variations in chemical composition and processability.

The pit method may offer an advantage over other processing methods by producing FAMEs instead of FFAs as a primary degradation product. FAMEs are potentially easier to remove during the industrial processing due to their higher volatility, potentially eliminating the need for additional neutralization steps that decrease TG yield [26]. However, further research is needed to validate this hypothesis and fully understand the implications of higher FAME levels on kernel quality and industrial processability.

The findings from testing the 3- and 6-month pit process align with the traditional burial times of experienced processors, highlighting the optimal chemical profile achieved after three months. The pit method yields kernels with low free fatty acid (FFA) levels and high mechanical processability due to reduced polar lipid content, making them suitable for industrial processing. Notably, pit kernels contain higher levels of fatty acid methyl esters (FAMEs) compared to boiled kernels, the implications of which require further investigation. For processors intending to trade kernels, skipping post-pit boiling or smoking conserves resources and labor. However, local variations in burying times, post-pit processing and materials used should be respected to preserve biocultural diversity and traditional knowledge.

Interviews

The social science data on the use of the pit method is derived from 130 interviews conducted as follows:

Sample A: 56 shea processors were randomly selected from nine communities across Burkina Faso in April 2021, with follow-up interviews in April 2022.

Sample B: 37 shea processors were randomly chosen from a specific community in Burkina Faso known for its frequent use of this method. Each respondent was interviewed twice, in November 2021 and February 2022.

Sample C: 37 interviews were conducted in a community where the pit method was introduced for the purpose of this research. Each respondent received training to perform the anaerobic pit kernel experiment with six months of storage and was interviewed twice: three months after filling the pits in November 2021 and immediately after excavating the pits in February 2022. Five of these processors provided kernel samples for the chemical analyses. All participants obtained help to dig a pit.

All methods were performed in accordance with the relevant guidelines and regulations: informed consent was obtained from all subjects and/or their legal guardian(s), following Free Prior Informed Consent rules, GDPR rules, and ethical considerations for human research, as approved by the Ethical Committee and the Faculty Secretariat of the University of Copenhagen.

The interviews collected information about respondents' background, including gender, age, ethnicity, level of education, the number of collected bags, and years of experience in collecting and processing shea nuts. Additionally, the data covered during these interviews included:

Sample A: Data related to the traditional pit-method process, such as burying times (in days or months), the number of pits, covering materials, filling process, perceptions of the advantages and disadvantages of the pit method, and post-pit processing (boiling, smoking, and/or direct sun-drying).

Sample B: In addition to the data collected in Sample A), information was gathered about the size of the pits (width, length, and depth in meters), time required to fill the pits (in days or months), the pit's location, ownership of the pits, the use of other shea nut processing methods, benefits and disadvantages of other traditional processing methods, decision-making concerning the method(s) used, and how respondents manage the early market (between April and July).

Sample C: This sample included data on respondents' prior knowledge of the method, perceived advantages and disadvantages of the pit method and other traditional methods, decision-making regarding the method(s) used, and incentives for adopting and continuing each practiced method.

Chemical analysis

The sample preparation and chemical analyses of the shea kernel samples are briefly described below.

Shea kernels were collected from Burkina Faso and transported to the University of Copenhagen by courier. On arrival, the kernels were oven-dried at 60 °C for 48 h to improve stability and stored in a dehumidified room with app. 30% humidity until further analysis.

Boiled kernels originating from sites 1, 2 and 3 (Figure 1) were retrieved at time of evacuation in 2020 and represent the variability of samples at the point of entry to an industrial supply chain. Kernels from pits which had been sun-dried, but not boiled or smoked, were retrieved from five pits at site 4 at time 0 (n=5; fresh kernels), and after 3 months (n=5; 3-month pit kernels) and 6 months (n=5; 6-month pit kernels) of pit-treatment trailing the season of 2021. An additional kernel sample from site 5 obtained after 6 months of pit treatment trailing the season of 2020 was also included for comparison.

The chemical fingerprints of the shea samples were assessed using both liquid chromatography with electrospray quadrupole-time-of-flight high resolution mass spectrometry detection (LC-HRMS) and gas chromatography with electron ionization quadrupole mass spectrometry detection (GC-MS). Lipid analysis was carried out using an Acquity UPLC BEH C18 column (2.1 mm × 100 mm, 1.7 µm, 130 Å, Waters) maintained at 40 °C and four different LC-HRMS methods that targeted different lipid classes (FFAs, DGs, TGs and various classes of polar lipids. Mobile phases (A) acetonitrile: water (3:2 (v/v)) with 10 mM ammonium acetate and (B) isopropanol: acetonitrile (9:1 (v/v)) with 10 mM ammonium acetate were used for FFAs, and (A) acetonitrile: water (3:2 (v/v)) with 10 mM ammonium formate and (B) isopropanol: acetonitrile (9:1 (v/v)) with 10 mM ammonium formate for DGs, TGs and polar lipids. The HRMS was operated in all-ion-fragmentation mode, to yield information on both precursor and product ions. GC-MS analysis was carried out using a ZB-5HT Inferno™ column (15 m × 0.25 mm × 0.10 µm, Phenomenex) which allowed the temperature gradient to range from 60 °C to 370 °C. Samples were introduced by a 0.2 µL splitless injection. The MS was operated in selected ion monitoring (SIM) mode with 20 SIM ions ranging from m/z 28 to m/z 609 monitored in each of 29 different retention time windows. Data from the GC-MS analysis were used to evaluate levels of FAMEs and MGs.

Before chemical analysis the shea kernels were crushed and milled to a fine powder, and depending on the analysis methods, a variety of sample extraction and clean-up steps were used. All sample preparation was carried out with ice-cold solvents and stored at < 4 °C until analysis, to maintain sample integrity.

For lipid analysis, samples were initially extracted with a liquid-liquid extraction procedure using methanol: methyl tert-butyl ether: chloroform in a 1.33:1:1 (v/v/v) ratio, evaporated to dryness and reconstituted in a 2:1 (v/v) mixture of chloroform: methanol. The reconstituted extract was fractionated using solid phase extraction (SPE) with an NH₂-stationary phase and elution solvents (hexane, chloroform, ethyl acetate, methanol, 2 % acetic acid in diethyl ether, 0.2 M ammonium acetate in methanol) in various combinations and ratios of increasing polarity, to yield two purified fractions: “neutral lipids” and “polar lipids”. The two fractions were subsequently evaporated to dryness and reconstituted in chloroform: methanol in a 2:1 (v/v) ratio before analysis. The headspace was always replaced by nitrogen to prevent lipid oxidation.

The sample preparation for semi-volatiles analysis involved extraction with 2,6-di-tert-butyl-4-methylphenol (BHT; 5 mg / mL) in hexane. The hexane-extract was cleaned by liquid-liquid partitioning using 0.9 % NaCl in water, and residual water removed prior to GC-MS analysis.

Industrial measures were total extractables and FFA content. The total extractables were measured by Soxhlet extraction of kernel homogenate for 18 h using diethylether. The total free fatty acid content was quantified as weight percent of free fatty acids in oleic acid equivalents and the analysis was performed according to the AOCS Ca 5a-40 procedure ‘Free fatty acids in crude and refined fats and oils’. Instead of hot ethanol, a 1:1 (v/v) solution of 95% ethanol and diethylether was used according to the IUPAC 2.201 procedure ‘Determination of the acid value (a.v.) and acidity’. The quantities of FFAs were correlated to FFA peak heights obtained by MS-Dial (Figure SI-2).

Data analysis and statistics

The qualitative data from the interviews was coded and transformed into quantitative data, and descriptive statistical analyses were conducted using Excel.

Lipid peak heights were extracted using MS-Dial v4.9.2 [50]. Peak areas of FAMEs and MGs were extracted using MassHunter Quantitative Analysis v10.2 (Agilent). The p-values quoted indicate the significance assessed by one-way ANOVA with Tukey post hoc testing (OriginPro 2020, OriginLab Corporation). The polar lipids measure from Figure 5 represents the combined data for a total of 32 compounds or groups of isomers. It was calculated by first normalising each polar lipid to its maximum value, summing the polar lipid levels across each sample, and finally normalising the sum of polar lipids to the maximum value across samples. In that way, each polar lipid influences the sum equally, regardless of its concentration and ionisation efficiency.

The manuscript text was shortened using AI tools.

Author Contribution

N.T.G., N.J.N., J.H.C. and M.P. carried out conceptualization (equal). N.T.G., and N.J.N. co-led data acquisition design, analysis and writing (N.T.G. social science and N.J.N. chemistry). D.I.P. supported data analysis and writing, produced Figures 2-6 and supplementary materials (chemistry). N.T.G. produced Figure 1, and supplementary materials (interviews). E.W. and Y.D. supported data acquisition. E.M. supported data analysis (chemistry). All authors reviewed and edited the text. J.H.C. and M.P. led the funding acquisition.

Acknowledgement

The research was funded by Innovation Fund Denmark through a Grand Solutions Grant (project no. 9067-00030B). AAK Denmark is acknowledged for sharing expertise on industrial processing and for providing the industrial measures. We would like to express our gratitude to François Questiaux and Mustapha Traore for their assistance with data collection, to Peter Christensen and Jette Petersen for their technical assistance, to Eleni Pappa for the contribution to the literature review, and to Romina Forte Nerán for curating references.

Data Availability

Data is provided within the manuscript andsupplementary information files, and the data that support the findings of this study have been deposited in Figshare (10.6084/m9.figshare.26202266)

Chalfin, B. Shea butter republic: state power, global markets, and the making of an indigenous commodity (Routledge, 2004).
Wardell, A. & Fold, N. Globalization in a Nutshell: historical perspectives on the changing governance of the shea commodity chain in northern Ghana. International Journal of the Commons 7(2), 367-405 (2013).
Boffa, J. M. Agroforestry parklands in sub-Saharan Africa. (The Food and Agriculture Organization of the United Nations Conservation Guide, Rome, 1999)
Hall, J. B., Aebischer, D. P., Tomlinson, H. F., Osei-Amaning, E., Hindle, J. R. Vitellaria paradoxa. A monograph. (School of Agricultural and Forest Sciences Publications. University of Wales, Bangor, UK, 1996).
Lovett, P. N. The shea butter value chain: production, transformation and marketing in West Africa (West Africa Trade Hub, Technical Report No. 2, Accra, Ghana, 2004).
Naughton, C. C., Lovett, P. N. & Mihelcic, J. R. Land suitability modeling of shea (Vitellaria paradoxa) distribution across sub-Saharan Africa. Applied Geography 58, 217-227 (2015).
Bockel, L., Veyrier, M., Gopal, P., Adu, A. & Ouedraogo, A. Shea value chain as a key pro-poor carbon fixing engine in West Africa (The Food and Agriculture Organization of the United Nations and Global Shea Alliance, 2020).
ITC. Stratégie nationale de dévelopment durable de la filière karité du Burkina Faso 2015-2019. Report from Le Centre du Commerce International. [Accessed on 6^th March 2024 at https://faolex.fao.org/docs/pdf/Bkf188967.pdf]
Elias, M. & Carney, J. African Shea Butter: A Feminized Subsidy from Nature. Africa 77(1), 37–62 (2007).
Honfo, F. G., Linnemann, A. R., Akissoe, N., Soumanou, M. M. & van Boekel, M. A. J. S. Characteristics of traditionally processed shea kernels and butter. International Journal of Food Science and Technology 48(8), 1714–1721 (2013).
Terpend, M. N. La filière Karité: Produit de cueillette, Produit de luxe (Grapp, 1982).
Hyman, E. L. A comparison of labor-saving technologies for processing shea nut butter in Mali. World Development 19(9), 1247-1268 (1991).
François, M., Niculescu, N., Badini, Z. & Diarra, M. Le beurre de karité au Burkina Faso: Entre marché domestique et filiè res d’exportation. Cahiers Agricultures 18(4), 369–375 (2009).
Sidibé, A., Vellema, S., Dembélé, F., Témé, B., Yossi, H., Traoré, M. & Kuyper, T. W. Women, shea, and finance: how institutional practices in a Malian cooperative create development impact. International Journal of Agricultural Sustainability 12(3), 263–275 (2014).
Crèlerot, F. M. Importance of shea nuts for women’s activities and young child nutrition in Burkina Faso: the case of the Lobi [PhD dissertation]. The University of Wisconsin-Madison (1995).
Jasaw, G. S., Saito, O., Gasparatos, A., Shoyama, K., Boafo, Y. A. & Takeuchi, K. Ecosystem services trade-offs from high fuelwood use for traditional shea butter processing in semi-arid Ghana. Ecosystem Services 27(A), 127-138 (2017).
Noumi, E.S., Dabat, M. H. & Blin, J. Energy efficiency and waste reuse: a solution for sustainability in poor West African countries? Case study of the shea butter supply chain in Burkina Faso. Journal of Renewable and Sustainable Energy 5(5), 053134:1-15 (2013).
Naughton, C. C., Zhang, Q. & Mihelcic, J. R. Modelling energy and environmental impacts of traditional and improved shea butter production in West Africa for food security. Science of the Total Environment 576, 284-291 (2017).
Aculey, P. C., Lowor, S. T., Kumi, W. O. & Assuah, M. K. The effect of traditional primary processing of the Shea fruit on the kernel butter yield and quality. American Journal of Food Technology 2(7), 73–81 (2012).
Agene, V. N. Effect of different processing methods and periods of storage in different containers on some quality characteristics of kernel and extracted butter of shea nut. (Master Thesis) (2015) [Accessed on 5^th February 2022 at https://ir.knust.edu.gh/handle/123456789/8186]
Honfo, F. G., Linnemann, A. R., Soumanou, M. M., Akissoe, N. & van Boekel, M. A. J. S. Effect of storage period and boiling time of fresh shea nuts on physico-chemical characteristics of kernel and butter. African Journal of Food Science 16(6), 160-171 (2022).
Honfo, F. G., Linneman, A. R., Soumanou, M. M., Akissoe, N. & van Boekel, M. A. J. S. Quality Characteristics and Volatile Compounds of Shea Butter Under Different Storage Conditions. Agricultural and Food Science Journal of Ghana 15, 1507-1525 (2022).
Obibuzor, J. U., Abigor, R. D., Omoriyekemwen, V., Okogbenin, E. A. & Okunwaye, T. Effect of processing germinated shea kernels on the quality parameters of shea (Vitellaria paradoxa) butter. Journal of Cereals and Oilseeds 4(2), 26-31 (2013).
USAID/WATH (2005): Guide à l’exportation du beurre de karité. West Africa Trade Hub, Accra, Ghana. [Accessed on 30^th January 2024 at http://hubrural.org/IMG/pdf/wath_shea_butter_value_chain_study_synthesis_fr.pdf]
Chumsantea, S., Aryusuk, K., Lilitchan, S., Jeyashoke, N. & Krisnangkura, K. Reducing Oil Loses in Alkali Refining. Journal of the American Oil Chemists’ Society 89(10), 1913-1919 (2012).
Gunstone, F. D., Oils and fats in the food industry Ch.3 (Wiley-Blackwell, 2008).
Ajala, E. O., Aberuagba, F., Olaniyan, A. M. & Onifade, K. R. Optimization of solvent extraction of shea butter (Vitellaria paradoxa) using response surface methodology and its characterization. Journal of Food Science and Technology 53(1), 730–738 (2016).
Abdul-Hammed, M., Jaji, A. O. & Adegboyega, S. A. Comparative studies of thermophysical and physicochemical properties of shea butter prepared from cold press and solvent extraction methods. Journal of King Saud University – Science 32(4), 2343–2348 (2020).
Akihisa, T., Kojima, N., Katoh, N., Kikuchi, T., Fukatsu, M., Shimizu, N. & Masters, E. T. Triacylglycerol and Triterpene Ester Composition of Shea Nuts from Seven African Countries. Journal of Oleo Science 60(8), 385-391 (2011).
di Vincenzo, D., Maranz, S., Serraiocco, A., Vito, R., Wiesman, Z. & Bianchi, G. Regional variation in shea butter lipid and triterpene composition in four African countries. Journal of Agricultural and Food Chemistry 53(19), 7473–7479 (2005).
Akihisa, T., Kojima, N., Katoh, N., Ichimura, Y., Suzuki, H., Fukatsu, M., Maranz, S. & Masters, E. T. Triterpene Alcohol and Fatty Acid Composition of Shea Nuts from Seven African Countries. Journal of Oleo Science 59(7), 351-360 (2010).
Akihisa, T., Kojima, N., Kikuchi, T., Yasukawa, K., Tokuda, H., Masters, E. T., Manosroi, A. & Manosroi, J. Anti-Inflammatory and Chemopreventive Effects of Triterpene Cinnamates and Acetates from Shea Fat. Journal of Oleo Science 59(6), 273-280 (2010).
[33] Allal, F., Piombo, G., Kelly, B. A., Okullo, J. B. L., Thiam, M., Diallo, O. B., Nyarko, G., Davrieux, F., Lovett, P. N. & Bouvet, J. M. Fatty acid and tocopherol patterns of variation within the natural range of the shea tree (Vitellaria paradoxa). Agroforestry Systems 87(5), 1065–1082 (2013).
Maranz, S. & Wiesman, Z. Influence of Climate on the Tocopherol Content of Shea Butter. Journal of Agricultural and Food Chemistry 52(10), 2934–2937 (2004).
Segman, O., Wiesman, Z. & Yarmolinsky, L. Shea butter in chocolate and related products Ch.17 (AOCS Press, 2012).
[36] Njoku, O. U., Eneh, F. U., Ononogbu, I. C. & Adikwu, M. U. Compositional and toxicological studies on shea butter. Journal of Nutraceuticals, Functional and Medical Foods 2(3), 33–39 (2000).
Maranz, S., Wiesman, Z. & Garti, N. Phenolic constituents of shea (Vitellaria paradoxa) kernels. Journal of Agricultural and Food Chemistry 51(21), 6268–6273 (2003).
Honfo, F., Linnemann, A., Guo, M., Akissoe, N., Soumanou, M.M. & van Boekel, M. A. J. S. Influence of roasting of shea kernels on their fat content and some quality characteristics of shea butter. Journal of Food Studies 6(1), 66-80 (2017).
Liebisch, G., Fahy, E., Aoki, J., Dennis, E. A., Durand, T., Ejsing, C. S., Fedorova, M., Feussner, I., Griffiths, W. J., Köfeler, H., Merrill, A. H., Murphy, R. C., O’Donnell, V. B., Oskolkova, O., Subramaniam, S., Wakelam, M. J. O. & Spener, F. Update on LIPID MAPS classification, nomenclature, and shorthand notation for MS-derived lipid structures. Journal of Lipid Research, 61(12), 1539–1555 (2020).
Carney, J. & Elias, M. Revealing Gendered Landscapes: Indigenous Female Knowledge and Agroforestry of African Shea. Canadian Journal of African Studies / Revue Canadienne des Études Africaines 40(2) (2006).
Questiaux, F. Social differentiation and household dynamics associated with early season shea nut collection and trading in Burkina Faso. Forum for Development Studies 51(1), 101-120 (2024).
Honfo, F. G., Akissoe, N., Linnemann, A. R., Soumanou, M. & van Boekel, M. A. J. S. Nutritional Composition of Shea Products and Chemical Properties of Shea Butter: A Review. Critical Reviews in Food Science and Nutrition 54(5), 673–686 (2014).
Ohimain, E. I. Methanol contamination in traditionally fermented alcoholic beverages: the microbial dimension. SpringerPlus 5(1), 2193-1801 (2016).
Canet, A., Bonet-Ragel, K., Benaiges, M. D., Valero, F. Lipase-catalysed transesterification: Viewpoint of the mechanism and influence of free fatty acids. Biomass and Bioenergy 85, 94-99 (2016).
Liu, S. Q., Holland, R. & Crow, V. L. Esters and their biosynthesis in fermented dairy products: A review. International Dairy Journal 14 (11), 923–945 (2004).
Gupta, M. K. Practical Guide to Vegetable Oil Processing Ch. 8 (AOCS, 2017)
Yu, L., Zhou, C., Fan, J., Shanklin, J. & Xu, C. Mechanisms and functions of membrane lipid remodeling in plants. The Plant Journal 107(1), 37-53 (2021).
Reszczyńska, E.& Hanaka, A. Lipid composition in plant membranes. Cell Biochemistry and Biophysics 78, 401-414 (2020).
Fujii, S., Wada, H. & Kobayashi, K. Role of Galactolipids in Plastid Differentiation Before and after light exposure. Plants 8(10), 357 (2019).
Tsugawa, H., Ikeda, K. & Takahashi, M. A lipidome atlas in MS-DIAL 4. Natural Biotechinology 38, 1159-1163 (2020).

No competing interests reported.

SupplementaryInformationblind.pdf

Download PDF

Editor invited by journal
22 Jul, 2024
Submission checks completed at journal
18 Jul, 2024
First submitted to journal
08 Jul, 2024

You are reading this latest preprint version

Combining social science and chemistry to uncover the potential of the traditional pit method for shea processing

Status:

Version 1

Abstract

Figures

Introduction

Results

Traditionally, the pit method is implemented diversely

Despite its benefits for shea processors, new adoption of the pit method relies on financial considerations

Pit kernels exhibit a distinct chemical composition compared to fresh and boiled kernels

The pit method increases total extractables compared to boiling

Discussion

The traditional pit method may lessen reliance on, and consumption of, natural resources such as firewood and water

The pit method offers convenience and time-efficiency for new and experienced processors, with few recognizable drawbacks

Pit kernels show lower FFA and polar lipid content than boiled kernels but higher FAME levels

The chemical profile of pit kernels indicates high quality with low FFA content and good mechanical processability, both crucial in an industrial context

Conclusion

Methods

Interviews

Chemical analysis

Data analysis and statistics

Declarations

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Supplementary Files

Status:

Version 1