Shea butter is a high–value shea nut fat used as an edible oil, antimicrobial and moisturiser in the food, pharmaceutical and cosmetic industries, respectively. The annual worldwide export of shea nut from Africa is 350,000 MT of kernels with a market value of approximately $120 million to producing countries. The multifunctional properties of the shea butter depend strictly on its compositional properties: the peroxide value, moisture content, free fatty acid level and the insoluble impurities. Standard extraction technologies: the traditional, mechanized, enzymatic and chemical methods were used for shea butter extraction. Current extraction technologies which rely on different extraction parameters for shea butter extraction are yet to yield the desired qualities and efficiencies of butter. Application of hydrolysing enzymes during enzyme extraction however eliminates the laborious, tedious and labour–intensive extraction processes creating alternative, selective and mild extraction conditions. The current review gives an overview of shea butter extraction technologies, the efficiencies, qualities and a perspective into the shea butter industry.
Shea butter is the oleaginous material obtained from the kernel of the shea nut tree Vitellaria paradoxa. Research has consistently described the shea butter as a vegetable fat extracted from the kernels of the fruit of V. paradoxa, Sapotaceae (Hall et al., 1996; Pontillon, 1996; Kengue and Ndo, 2003; Elias and Carney, 2004; Schreckenberg, 2004). Others described the shea butter as a yellowish–grey solid material (Abdul–Mumeen et al., 2013) or yellowish white in colour with a strong smell (Tessy, 1992) extracted as fat from the kernels of the shea nut fruit. Shea butter extracted from shea kernels is raw and can be refined.
Shea butter is as good as table oil because of its high nutritive value and low cholesterol levels; widely used locally for curing leprosy and other ailments and has various industrial uses that include soap making, cosmetics, lubricants and paints (Olaniyan and Oje, 2007b). Shea butter is ideal for use as raw materials for cooking oil, margarine, cosmetics, soap, detergents and candles due to the presence of solid fat (stearin) and liquid oil (olein) (Chevalier, 1943; Boffa et al., 1996; Russo and Ethrington, 2001). Shea butter is used extensively in the food, pharmaceutical, cosmetic industries and often as cocoa butter substitute by chocolate manufacturers and for margarine and baking purposes (Martin et al., 1987; Williams and Bolton, 1950; Hall et al., 1996).
The American Shea Butter Institute (ASBI, 2004) reports that 100% pure natural shea butter is an all–natural vitamin A cream which has shown to be a superb moisturizer, with exceptional skin healing properties. ASBI (2004) has also asserted that shea butter has proved to be effective against skin and other skin related conditions such as dry skin, skin rash, skin peeling after tanning, blemishes and wrinkles, itching skin, sunburn, shaving cream for a smooth silky shave, small skin wounds, skin cracks and tough or rough skin, cold weather, frost bites, stretch mark prevention during pregnancy, insect bites, health skin, muscle fatigue, aches and tension, skin allergies such as poison ivy or poison oaks, eczema, dermatitis and skin damage from heat.
Africa produces about 1,760,000 tons of raw shea nuts annually from its wild trees (Mohammed et al., 2013). In Ghana, there are estimated 94 million shea nut trees which were projected to produce at least 60,000 metric tonnes of shea nuts per annum for the production of all shea butter processed locally (Ofosu, 2009). This yields about 150 tonnes of shea butter, 60% of which is used locally with 25% exported. Over 80% of the woody vegetation in Northern Ghana is Vitellaria (Lovett and Haq, 2000).
In the new global economy, shea butter also known as ‘kpakahili’ (raw oil) has become a central commodity for most industries and thus, in addition to the kernels, plays a significant role in poverty alleviation in Northern Ghana (Moore, 2008; Abdul–Mumeen, 2013). The shea nut market in Ghana is well established, being sold both locally and internationally.
In 1996, Ghana exported 21,467 tons of shea nuts worth $4,484,600 (Chaffin, 2004). It is reported that shea butter export estimates from Ghana for 2002/2003 reached 2,000 mt, although demand existed for over 6,000 mt per annum (p.a.) (Lovette, 2004), while the export of 4,969 mt of shea nuts earned the country US$1,339,000.00 (FAO, 2002) the same year. Shea kernel exports reached the highest of close to 180,000 mt in 2007 (Figure 1) between the period of 1996 and 2013 (GEPA, 2014). That cannot be said of shea butter export which was only 40,000 mt in 2013, the highest ever within the same period (GEPA, 2014). Carette et al. (2009) observed that records on overland exports, records on export quantities of shea butter and finished shea products were not documented consistently until when Lovett (2005) made export estimations.
Shea is mainly exported in the form of shea kernels, rather than shea butter or finished shea products. Recent reports however show that annual exports of shea kernels are still high but shea butter exports have also increased from 12,561.37mt (US$19,010,304) in 2009 to 32,782.61mt (US$24, 764,995) in 2010 representing an increase of 61.7% (GEPA, 2014).
Raw shea butter is obtained primarily by the traditional method of extraction (Abdul–Mumeen et al., 2013), from the shea fruit kernel, but can also be obtained by mechanical (Olaniyan and Oje, 2007a), enzymatic (Didia et al., 2018) and chemical methods (Apea and Larbi, 2013). It can be consumed raw without any further physical or chemical treatments or refinement.
However, much uncertainty still exists about the standard method of extraction of shea butter which will meet the standards declared by the various certification and standard organizations for shea butter quality. Thus, there still remains huge information regarding the reasons for differing approach to the extraction of shea butter and their efficiencies that are yet to be collected. This sort of information would extremely benefit not only the communities and industries within Ghana but all the countries within the West African sub–region. Additionally, once the shea nuts have been harvested, a huge amount of time and effort is spent on the processing and extraction methods currently employed. To date, no extensive qualitative review of these methods has been carried out and as a result there is limited expert advice available to the local communities and women groups that would benefit most from it. This research aimed to fill some of these information gaps through carrying out a comprehensive review of the shea butter processing for the local women groups in particular, researchers, policy directors and other actors in Ghana who are committed to the development of the shea butter industry. Therefore, the current review gives an overview of shea butter extraction technologies, the efficiency and quality of the shea butter produced and the perspective into the shea butter industry.
General treatment of shea fruits prior to shea butter extraction
The processing of shea butter is seasonal. The fruiting and gathering of the nuts occur between the months of May to August every year (Moore, 2008) during which the shea nuts are processed into kernel (Owoo and Lambon–Quayefio, 2017). The raw and ripe fruits are green but the ripe fruit is occasionally yellowish (Moore, 2008) and soft when felt. The fruit is composed of four layers: the epicarp, mesocarp, shell and the kernel. The epicarp together with the mesocarp is called the pulp; the shell and the kernel compose the nut. The kernel is the oil–bearing material that can be obtained by processing the shea nut fruit. The fruit primarily undergoes several processes for example, de-pulping, boiling, drying, de-shelling, winnowing and sorting to obtain the kernel from which the shea butter is extracted.
The fresh mature fruit of the shea tree is covered externally by the pulp (Figure 2) consisting of an epicarp (greenish) and a mesocarp (yellowish). De–pulping is the removal of the pulp (the epicarp and the mesocarp) when the shea fruit is ripe. The pulp which is mostly green becomes soft when the fruit ripens (Gyedu-Akoto et al., 2017). It has been well documented that the fruits are collected by African women from the ground and the pulp is removed by fermentation or manual peeling (Chaffin, 2004; Moharram et al., 2006). Fruit storage before de pulping, especially after three days, negatively affected the quality (Aculey et al, 2012) and quantity of the resulting butter because of the sugar rich pulp which assists fungal growth and thereby reduces oil content of the kernel (Carette et al., 2009). Ojo and Adebayo (2013) confirmed this when, during the bio–deterioration of the shea nut fruit pulp, they isolated eight fungi species (Aspergillus flavus, Aspergillus niger, Botrydiplodia theombromae, Botryosphaeria spp., Colletotrichum gleosphoriedes, Lisidiplodia spp., Pseudofasicocum spp. and Trichoderma viridae) from the fruit natural environment and from parboiled kernels (Aculey et al., 2012).
Boiling shea nuts for butter production
The shea nut (Figure 2), comprising the shell and the kernel, is obtained after the pulp has been removed. The shea kernel sticks to the shell wall and to separate them, the nuts are immersed in boiling water or on rare occasions smoked (Honfo et al., 2013) although Kpelly (2014, unpublished) hinted that smoking raised the FFAs and PAHs levels and could be carcinogenic. Smoking the nuts is specific for the Otamari socio–cultural group (Honfo et al., 2012). The nuts are usually boiled for about 30 – 45 min (Honfo et al., 2013) to temperatures ranging between 100 and 105ºC to deactivate all biological and enzymatic activities in the nut (Abdul–Mumeen, 2013). Boiling increases the fat output of the kernel and a possible explanation is that boiling softens the nuts leading to cell disruption and a better release of the oil (Honfo et al., 2013; Moore, 2008). Thus, to allow efficient extraction of the fat, research (Womeni et al., 2006; Lovette, 2004) stresses that boiling of shea nuts was necessary. Boiling also clean the surface of the nut of any remaining fruit pulp (Moore, 2008) that has the tendency to promote microbial growth.
Drying the shea nuts
After boiling, shea nuts are allowed to dry either via sunlight for 5-10 days or by using oven for 2-3 days (Moore, 2008). Sun drying is a widespread practice to reduce the moisture content of the nuts and to facilitate the shelling operation (Honfo et al., 2013). The nuts sun–dried after boiling can lead to mold contamination during the rainy season and this affects the quality of the shea butter and shea butter products (Moharram et al., 2006; Senyo, 2014). On rare occasions the nuts are solar dried. The advantage of solar dryers is that it checks the activity of Aspergillus fungi and Euphenestia caufella larvae, even during long–term storage (CRIG, 2002) of the nuts. Both parboiling duration and drying method significantly affect shea butter yield and quality and the free fatty acids levels especially (Aculey et al., 2012).
De–shelling or de–husking
Removal of shells from the nut after cracking and winnowing is a process described as de–husking or de–shelling (AOS, 2011). During the drying period, the kernels become detached from the shell wall. De-shelling is carried out using stone, hammers and pistles (Alonge and Olaniyan, 2007). Winnowing is achieved by holding basket filled with a mixture of the shells and kernel at arm’s length and allowing a gradual pour–out (Alonge and Olaniyan, 2007). If there is a strong wind, the pieces of shell will be blown away, if not, then the process is repeated many times (Fleury, 1981).
Sorting and further drying
Sorting is the removal of the remains of the shell pieces from the shea kernels after winnowing (Mohammed et al., 2013). At this stage, shea kernels that are broken, infected by mould or are black in colour are also removed to obtain clean unbroken shea kernels. The shea kernels can now be stored for several months without deterioration or processed into shea butter.
The pre–treatment and storage of the shea kernels before the butter extraction process is a critical stage that affect the quality of shea butter produced. The first adverse effects are seen in the decrease in oil phenols and in the reduction of volatile compounds responsible for the various properties of shea butter (Hee, 2011). Angerosa et al. (2004), notes that in several operative conditions involving long–term storage of seeds and high relative humidity, mould contamination increases the free acidity due to the production of fungal enzyme lipase, and simultaneously forms the characteristic sensory defect of "mould". This condition can affect the fatty acid (arachidic, linoleic oleic, palmitic, and stearic) composition and the free fatty acid content in particular thereby dictating the quality parameters of the butter and hence the international standards as set by the West African Regional Standards in 2006 (Table 1). There are several factors including the moisture content, pre–treatment ofthe shea kernel and the kneading session that affect the quality of shea butter (Abdulai et al., 2015). The research further reveals that the sandy soil, higher soil nitrogen levels, higher soil carbon levels and lower soil cation exchange capacity in general had significant positive impact on the quantity of fat produced in the kernels. Low levels of phosphorus in soils require higher levels of nitrogen for optimum shea seedling development and thus shea regeneration in shea parklands could still benefit from nitrogen supplementation Abubakari et al. (2012). Kapseu et al. (2001) and Womeni et al. (2006) showed that the drying time and roasting time of shea nut kernels affected the physico–chemical quality of shea butters. Overall, the target for the various pre–treatment processes is to extract all the 60% fat present in the kernel (Axtell et al., 1993). Figure 3 is a flow diagram of the various pre–treatment processes.
Shea butter extraction technologies
Addaquaye (2004) classified the processing technologies of shea butter into three methods: the traditional manual method, traditional semi–mechanized method and the fully mechanized method. However, recent studies have suggested that the traditional butter extraction encompass the traditional manual and the traditional semi–mechanized methods (Mohammed et al., 2013; Abdul–Mumeen, 2013) and that shea butter can be extracted by chemical and enzymatic processes (Didia et al., 2018; Otu et al., 2015; Apea and Larbie, 2013). Whether the various extraction technologies have answered the numerous challenges bedevilling the shea butter industry is discussed below.
The traditional extraction technology
In Africa and Ghana, shea butter is mainly produced by women by the traditional method (Abdul–Mumeen, 2013) also known as wet extraction process (Olaniyan and Oje, 2011) and this has become the most accessible income generating activity for most women up north of Ghana (Rammohan, 2010). This process is the main method of processing oils in most West African countries, including Ghana (Addaquaye, 2004). Indeed, about 80 per cent of Ghana’s shea butter is produced through traditional processing techniques (Mensah, 2001). For production of substantial amounts of oil the process takes 20–30 h (Hall et al. 1996) since the total time needed to process the shea butter (for one cycle) is between 5–6 h (Boffa, 1999) and that kneading alone, for one session, takes about 30 min (Abdul–Mumeen, 2013).
Kernel size reduction and dehydration
The traditional extraction technology begins with kernel size reduction by pounding in the mortar using the pestle and further dehydration by roasting (Abdul–Mumeen, 2013; Moore, 2008; Olaniyan and Oje, 2007b) to aid oil extraction. The roasted grits of kernel are ground to paste by either the use of stones in the pure traditional extraction process or by the use of grinding mill in the semi–mechanized traditional method. The size reduction and further milling increases the surface area (Abdul–Mumeen, 2013) for effective hydrolysis during kneading. Other studies determined the factors influencing the quality of fats during their preparation (Louppe et al., 1995; Hall et al., 1996; Kapseu et al., 2001; Womeni et al., 2006) and noted that blanching shea nuts improved shea butter quality. Hall et al. (1996), Semmelroch and Grosch (1998) and Sanz et al. (2001), underlined that the sensorial characteristics of shea and cocoa butters were linked to the kernel roasting time.
A kneading process takes place to break up oil cells and ease oil extraction and women take an average time of 30 min to complete one kneading session. Abdul–Mumeen (2013) explains that a kneading session involves taking a reasonable quantity of shea paste, adding an initial amount of about 3 litres of cold water, stirring slowly and then vigorously later, with the hand until the butter begins to rise in crude milky–white form. Some researchers suggest that traditional extractors boil water and skim off the released oil from the kernel (Alander, 2004) or by kneading and hand beating (Moharram et al., 2006). At this stage 10– 20 kg of finely pulverized paste is mixed with three litres of water and kneaded until a white bloom appears which marks an important enzymatic step and followed addition of hot water (ASBI, 2004; Abdul–Mumeen, 2013; Mohammed et al., 2013). Kneading is successful depending on the individual’s recognition of changes in temperature, consistency and appearance and this can only be assessed correctly with experience (CRIG, 2002).
Heating, oil separation and cooling
Once kneading is over the oily layer is harvested from the surface of the water layer leaving behind the water layer and particulate matter in the bottom of the pan (Tano–Debrah and Ohta, 1994). The oily layer or fat emulsion is washed with water, boiled to evaporate the water and the crude fat is obtained by decanting or gentle pouring. Finally, the decanted oil is allowed to cool to solidify taking 6–12 h and the product is Shea Butter (ASBI, 2004; Abdul–Mumeen, 2013).
Shea butter from traditional extraction technology is increasingly required abroad by cosmetic and pharmaceutical industries, to the detriment of solvent extracted shea butter (Elias and Carney, 2004). That not notwithstanding, the traditional extraction method is still considered low yielding and has several uncontrolled processes which account for the wide variability of shea butter quality in the market (Louppe et al., 1995; Hall et al., 1996; Kapseu et al., 2001; Womeni et al., 2006). About 23% of fat still remain in the shea nut cake after a successful extraction (Abdul–Mumeen, 2013) and it is considered grossly inefficient, yielding not more than 35% of the oil with the product quality often low (Ata, 1978; Olaniyan and Oje 2007b; Niess, 1983). But Ofosu (2009) asserts that shea butter extraction by the traditional method has reached 35–40% extraction efficiency.
Several studies have been undertaken in order to increase shea butter extraction rate but also to improve the shea butter quality using the traditional processing conditions (Louppe et al., 1995; Hall et al., 1996). Attempts were made to incorporate appropriate technology into a number of the processing stages, both to improve efficiency and to reduce the amount and drudgery of the labour, as well as impact on the environment (Hall et al., 1996; Elias and Carney, 2004; Schreckenberg, 2004).
Mensah (2010) discusses an attempt by GRATIS Foundation and Technology Consultancy Center (TCC) to remove the production bottlenecks in the traditional method of shea butter extraction. A successfully developed manually operated bridge press for the extraction of shea butter used the Intermediate Moisture Content (IMC) method to produce an average of 67 % extraction efficiency. The method was first tested in Gbimsi in the Northern Region and later used by some women processors in the region. The principle of operation involves grinding dry kernels of moisture content (4–6%) into paste using a local plate mill. The paste obtained at a temperature of 70°C is used in the extraction process without any heat treatment. The moisture content is raised to 12% by kneading with a predetermined amount of hot water. The moisturized paste at 60°C is put in small bags, and pressed in a “bridge press” to release the oil.
The IMC method comes with some advantages over the normal traditional processing such that it increases the extraction efficiency by 5% of the normal extraction rate; increases in daily production capacity by 200%; drastic decrease in firewood consumption: for every 85 kg of kernels processed about 8 kg of firewood is used instead of 72 kg; drastic decrease in water use, that is about 8 l of water used in place of 160 litres for 85 kg of kernels processed; reduction in the extraction operations stages from 7 to 5 by the removal of roasting and cream boiling; milder shea smell; environmentally friendly since few fuel wood and less waste water are involved.
With the pounding, roasting, milling, kneading, heating, decanting and cooling to solidify the shea butter, the process has been described as cumbersome, tedious, time–consuming and energy sapping (Olaniyan and Oje 2007b; Coulibaly et al., 2009). The returns do not commensurate the energy, material and financial input by women in shea extraction and besides their incomes are unstable due to low extraction efficiencies, inconsistent butter production, and supposedly low quality of the butter.
These challenges among others called for semi– mechanization technologies which later got developed further there was equipment designs designed to perform specific operations including oil digestion and oil pressing and eventually machines that combine several operations in the process (FAO, 2002).
The mechanical processing technology
The mechanical processing technology is usually referred to as the Cold Press Extraction method (Sekaf, 2008), so called because it does not involve the various different heating stages of the traditional procedure (Figure 4).
The mechanical press method of shea butter extraction has been reported by FAO and CFC (2002) but one of the earliest researches works on the use of the mechanical press was Marchand (1988). His research revealed that equipped with a jack that exerts 30 tonnes of force, a shea butter press could crush more than 3 kg of shea kernels within 20 min. The press could extract up to 85% of the fat contained in the kernel in a simplified process (Marchand, 1988) through a reduction in the various heating stages of the kernel and subsequently saves fuel wood.
The emergence and proliferation of processing shea butter by this method in the shea producing zones of Ghana was mainly due to the collaborative work between women groups and some development partners and Non–Governmental Organizations. The United Nations Fund for Women’s Development, Technoserve Ghana and the Netherlands Development Organisation (SNV) introduced these innovations in the form of mechanized technologies such as hydraulics and mechanical presses, which were locally designed and manufactured. The CRIG (2002) however notes that the Dagomba women of Ghana were the first to initiate the mechanization of the butter extraction process. These have reduced processing times and enhance water use.
The processing of shea butter by this technique is carried out in a plant comprising of a boiler, mechanical press system and a filter press system. The mechanical press applies a great deal of pressure to the pulverized seed (Sekaf, 2008) to turn out more shea butter from the process (Yonas, 2014). Other inventions targeted single unit operations among which were a kneading machine, grinders, a hydraulic hand press, solar dryers, a heater and mixer. These inventions collectively achieved extraction efficiencies of 60 to 85% (CRIG, 2002; Marchand, 1988). Others have reported lower (35.9 to 45%) fat output at 82.28°C for the press (Alonge and Olaniyan 2007; Olaniyan and Oje, 2007b). About 30–33% of shea butter is extracted from the shea nuts with the mechanical expeller (Abdul–Mumeen, 2013) although combination of the mechanical with chemical methods has achieved 98% extraction efficiencies (Abdul–Mumeen, 2013).
In extraction, the dry kernels are fed into boiler or heating chamber where they are first heated to temperatures of between 15–20ºC and then directed into a crushing unit where they are reduced in size to increase the surface area for effective butter yield. The oil is pressed out from the pulverized nuts with some traces of the residue which are filtered out through the filter press to obtain clear oil. The cake remaining as a result of the first extraction is directed into another expeller where it is pressed the second time to produce more butter which is then allowed to cool and solidify (Abdul–Mumeen, 2013) or directed into another chamber for further refinement.
Unfortunately, the shea butter press still leaves huge problems for the village woman into shea butter production. The affordability and availability of the butter press in addition to its operation remains a problem for the local woman and for the local industries manned by these women.
It is a method recommended for the large production of commercial quantity of shea butter. The method was not only developed to increase productivity and save time, but to reduce stress on the processors since traditional boiling method was found to be labour intensive and time consuming (Masters and Puga, 1994).
The advantages of the mechanical press method notwithstanding, the equipment are expensive, scarce and unaffordable by most local industries (Alonge and Olaniyan, 2007) which predominates developing countries including Ghana. Another shortcoming of the mechanical separation process using the press machine is that it does not completely remove all the oil from the mass of the paste (Apea and Larbi, 2013), that is about 19% fat remains in the cake (Abdul–Mumeen et al., 2013).
The centrifuge method
The centrifuge method is one of the mechanical extraction technologies and Coulibaly et al. (2009), focused on shea butter extraction with a centrifuge machine (Figure 5) but the extraction efficiency was found to be barely higher than 30% on average. This was similar to the mean efficiency values with traditional methods. The procedure adopted during the said research is outlined in the diagram below. The process was compared to traditional and the mechanical press.
The extraction principle involved separating the oil, water and shea nut cake from a pre–prepared water–paste emulsion. The extraction machine encompassed a movable unit driven by a motor/engine. A shaft driven by the motor at one end was equipped with a rotating drum at the other end.
The drum built with 10 kg kernel loading capacity had an angular frequency of 1,000 rpm. The separation entities (oil, water and cake) occurred in layers and according to their mass. The oil which was light and floating was discharged into 2 bailing devices fitted in the drum. The process was repeated until clear oil was obtained. However, the extraction efficiency was not different from the traditional and screw press methods.
The chemical extraction processes
With this method, the dried kernels are first crushed into paste and fed into the Soxhlet extractor. Afterward an organic solvent such as n–hexane or ether is added. The mixture is allowed to stand for some number of h for the oil to be separated which is decanted and allowed to solidify.
The types of the solvents used in the extraction have some influence on the quality characteristics of shea butter especially the peroxide value of the butter. In a study conducted by Kar et al. (1981), on the best solvent for shea butter extraction, petroleum ether, n–hexane, chloroform, benzene, and water were employed. Hexane extraction gave the highest amount of fat from the kernel.
The principle with hexane extraction is that, the pulverized kernel is mixed with hexane which then unlocks the polymeric mass allowing all the oily and fatty constituents of the kernel to dissolve in it. The resulting oil–hexane mixture is later separated from the seed residue by filtration. The oil–hexane mixture is then heated to 68ºC to vaporise and recover the hexane to obtain the crude Shea Butter (Abdul–Mumeen, 2013; ASBI, 2004). The choice of hexane over other solvents for shea butter extraction is also informed by several factors: the physical properties of the solvent, the commercial economics of the butter and the edibility of shea oil from the extraction (Abdul–Mumeen, 2013).
Solvent extraction method has been reported to yield 47.5% of SB (Ikya et al., 2013), which is 32.9% greater than that of mechanical extraction (Olaniyan and Oje, 2007b) but have reached 98% when combined with the mechanical methods (Abdul–Mumeen, 2013). The use of organic solvent extraction has been acknowledged to showed low or no detectable level of peroxides.
Although the use of organic solvent for shea butter extraction gives a high yield, it is considered not wholesome for consumption due to some traces of the solvent that may remain in the butter (Apea and Larbi, 2013). According to FAO and CFC (2002), solvent extraction method is not usually used in domestic and commercial shea butter extraction in developing countries due to the high costs involved, environmental problems and the lack of technical skills associated with it. The natural integrity of unrefined shea is interrupted or changed during the chemical extraction process. For safety reasons, shea butter prepared by the chemical method, must be refined before it is permitted on the market. Additionally, hexane extraction removes most if not all of the healing properties from shea butter. Hexane is a liquid alkane whose chemical properties are similar to that of gasoline. Its use for shea butter extraction therefore requires a well–trained chemist and a well–equipped chemical laboratory (ASBI, 2004). Thus, the general view is that the use of chemicals in the extraction process take shea butter away from natural (Apea and Larbi, 2013).
Enzyme assisted extraction technology
Enzymatic extraction of vegetable oil with water–soluble enzymes involves the degradation of the cell wall and then the release of the oil (Perez et al., 2013). Aqueous enzymatic extractions are potentially used in the oil industries due to their high specificity and low operating temperatures (Ahmadi and Karimi, 2013). Several enzymes such as: amylase, glucanase, protease, pectinase, cellulase and hemicellulase have been used for the extraction of various vegetable oils from their kernels (Dominguez et al., 1995). Cellulase and hemicellulase have been reported to be the most suitable enzymes for cell wall degradation while pectinase has been identified as an effective enzyme for vegetable oils extraction (Dominguez et al., 1994; Perez et al., 2013).
Lipases can be produced by animals, plants, and microorganisms. Microbial lipases however primarily catalyze the hydrolysis of triacylglycerols and have been extensively studied due to their interesting characteristics of stability in organic solvents, action under mild conditions, and high substrate specificity (Gandhi, 1997; Sharma et al., 2001; Kempka et al., 2008).
Enzymatic extraction offers numerous advantages such as: increased yield, improved and high-quality vegetable oil, improved quality of the residual meal; reduced fibre content, preserved protein properties of defatted meal, low–peroxide and free fatty acid values (Soto et al., 2007; Perez et al., 2013). These are the reasons which make enzyme processes more economical for oil extraction processes (Rosenthal et al., 1996).
Potential of shea kernel for enzymatic extraction
The kernel, according to Axtell et al. (1993), contains about 60% edible fat and the residual product, from which the butter is extracted, is an excellent ingredient for livestock feed production. The enzymes break down the cell structure of plants and the cell wall of plants consists mainly of pectic substances. Hydrolytic enzymes like pectinase break down the cell wall of plants, while proteases permeabilize the liposome membrane and facilitate oil release from the oil body (Rosenthal et al., 1996; Fullbrook, 1983). Table 1 shows the characteristic potential of the shea nut kernel for enzymatic extraction of its oil.
Recent studies by Didia et al. (2018) suggest that enzymes technology is the way for shea butter extraction in Ghana. An aliquot equivalent to 50 g of shea nut biomass in 600 mL beaker was stirred with water in the ratio of 1:4 wt/vol and treated with commercial enzymes at optimized conditions. Three percentage (3%) enzyme–substrate concentrations for combined enzyme effect at 60°C for 2 h in a medium of pH 5 for three different substrates used: ‘raw kernels’, ‘roasted kernels’ and ‘roasted kernels ground to paste’ was subjected to the traditional procedure and this yielded 50, 54 and 70% butter respectively. The reaction was terminated after adding boiling water (100°C). The emulsion which formed the top layer was collected into another beaker and gently boiled until clear oil was obtained. It was then decanted into a weighed aluminium dish, cooled and weighed to estimate the percentage yield.
Otu et al. (2015), extracted shea butter using locally produced pectinases from S. cerevisiae ATCC 52712 via corn cobs substrate in a solid-state fermentation and compared its efficacy with commercial enzymes. The crude pectinase whose estimated optimal protein concentration was 7.00 mg/mL with 0.86 u/mg activity used against Viscozyme L (beta endo–1,3 (4)–glucanase (100 FBG/g) and Pectinex 5XL (4500 PECTU/mL). At 1.20% enzyme–substrate concentration the crude pectinase gave an optimum oil recovery of 44.00% while the commercial Pectinex and Viscozyme gave 58.60% and 72.00% at enzyme–substrate concentration of 0.80%.
Previously Tano–Debrah and Ohta (1994) carried out preliminary analysis of crude enzyme extraction of shea butter using: amylases, proteases, hemicellulases, cellulases and pectinases obtained from Shin Nihon Chemicals Co. (Anjoh, Japan). Although the traditional extraction method was adopted, it was done under strict laboratory principles. To deactivate the supposedly inherent lipases, shea kernel paste was weighed mixed with water in 1:4 ratio and autoclaved at 100~ for 5 min. The crude enzymes were added to the paste–water colloid singly and in combination each at the same level of activity (500 u/g). A uniform meal weight of 20 g was used in each treatment. The enzyme–paste uniform mixtures were incubated in a water–bath–shaker at 50°C and shaked at 100 revolutions per min for 4 h. The mixtures were autoclaved at 100°C for 3 min and centrifuged at 12,300 rpm for 20 min. The supernatants were extracted with petroleum ether in separatory funnels, and the ether phase was washed several times with warm water until the washings were clean. The ether phases were collected in weighed dishes, evaporated on a water–bath and dried in an air–oven at 100°C for 2 h. Weights of oil extracted were determined after cooling the dishes and were expressed as a percentage of the value obtained by the Soxhlet method. For all enzyme single dosage and combinations, the extraction efficiency ranged from 47–74%. Table 2 summarises the various methods for their extraction conditions, merits and demerits.
Quality and efficiency of shea butter generated
Quality of shea butter generated
The general quality of shea butter, according to most international standards, is pinned on four shea nut quality parameters: peroxide value, moisture content, free fatty acid levels, and the insoluble impurities.
Peroxide value (PV) is an indicator for stability and level of deterioration of shea butter and it measures the milli–equivalents of oxygen or hydro–peroxides in 1 g of fat or oil (Ikya et al. 2013). It is a valuable measure of oil quality as it serves as an indicator of degradation of the long fatty acid chains through auto–oxidation into peroxides that can later break down into other chemicals including foul–smelling ketones and aldehydes. The parameter deprives the butter of its stability and thus butter with high peroxide value (PV>10 meq kg–1) is associated with the development of rancidity, which eventually limits their use in the food industry (Shahidi, 2005). Peroxide value is the most common determinant of lipid oxidation (Shahidi, 2005). Hydro–peroxides under normal condition is remarked to have no flavour or odour of their own, they are however unstable and usually break down rapidly to other products such as aldehydes and thereby developing strong, disagreeable flavour and scent.
Free Fatty Acids (FFA) by definition are the fatty acids present in oil or fat which has not been neutralized (Guy, 2009) or just unattached fatty acids present in a fat (Sapna and Nirmali, 2009). FFAs are related to their acid values. The acid value is a parameter expressed as the number of milligrams of potassium hydroxide required to neutralize the free fatty acids contained in one gram of fat or oil (Kardash and Turâ€Ÿyan, 2004) and it is twice the FFA of a fat and therefore acid value is directly proportional to free fatty acids (Roger et al., 2010) and thus the lower the acid value of oil, the fewer FFA it contains.
Moisture is a chemical contaminant usually mixed with oil. Presence of moisture in oil affects the quality of the oil and significant amount of moisture in oil support microbial growth (Alirezalu et al., 2011; Hee, 2011) and lipid oxidation leading to rancidity (Hee, 2011) thereby reducing the shelf life of the fat and its corresponding products.
Conversely, low moisture content of shea butter is indicative of good quality (Olaniyan and Oje, 2007b). Difference in moisture content of shea butter can be attributed to shea vegetation (Quainoo et al., 2013), although the minimum moisture content of shea butter is 0.05% but can go as high as 2.0% (West African Regional Standards, 2006).
Insoluble impurities refer to dirt and other foreign materials in shea butter (Hamilton et al., 1986; Hee, 2011). It has been reported that some of these materials are bonded to the butter via the machinery employed in the extraction of the butter. Insoluble impurities may also make their way into the butter through physical contact of the butter with the soil, water, ground as well as packaging materials. The amount of insoluble impurities is an important quality parameter which determines shea butter deterioration since metals can catalyse the oxidation of shea butter and thus decreases its market value (Hee, 2011). Table 3 summarises the general range of values of the different quality parameters by different researchers with their respective efficiencies.