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DETERMINATION OF PHYSICAL PROPERTIES AND CRYSTALLIZATION KINETICS OF OIL FROM ALLANBLACKIA SEEDS AND SHEA NUTS UNDER DIFFERENT THERMAL CONDITIONS

1Mercy Badu, 1Johannes Awudza, 2Peter M. Budd and 2Stephen Yeates

1Department of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi-Ghana.

2The School of Chemistry, University of Manchester, ManchesterM13 9PL -United Kingdom

Email addresses:

Mercy Badu: mbadu0@,

Tel: + 233244822969

Johannes Awudza: johannes_awudza@

Tel: +233244405662

Peter Budd: Peter.Budd@manchester.ac.uk

Tel: +44 (0)1612754711

Stephen Yeates: Stephen.Yeates@manchester.ac.uk

Tel:+44 (0)1612751421

Keywords Thermal behavior, Allanblackia seed oil, shea nut oil, crystallization, melting temperature, fatty acids.

List of Abbreviations

ASO – Allanblackia seed oil

SNO – shea nut oil

DSC – Differencal scanning calorimetry

GC/MS – Gas chromatography/ Mass spectrometry

TAG - Triacylglycerol

Abstract

In this study, the fatty acid content, melting and cooling profiles, crystallization properties at different cooling rates and kinetics of crystallization of Allanblackia seed oil (ASO) and shea nut oil (SNO) were investigated. The fatty acid content was determined using gas chromatography coupled with mass spectrometry (GC/MS). The melting and cooling profiles, crystallization and crystallization kinetics were investigated using differential scanning calorimetry (DSC) and the fat morphology was investigated using polarized light microscopy. The GC/MS results showed that ASO contains a high amount of saturated fats as compared with SNO. The DSC analysis revealed that ASO has a high melting temperature of 35.3(2.1 oC and crystallizes faster at 16.8(0.3 oC. The crystallization patterns of both oils were observed to be dependent on the cooling rate. Under isothermal conditions, it was observed that both ASO and SNO showed high and low melting peaks with the low melting peak disappearing as the crystallization progressed. The Avrami model was used to estimate the crystallization kinetics of the oils under the isothermal conditions and it was inferred that ASO has a faster nucleation and subsequent crystal growth as compared with SNO. At room temperature SNO formed overlapping fat crystal particles while ASO gave distinct crystallized fat fractions during the crystallization at room temperature, as shown by the polarized light microscopy.

Practical Application: The high melting and the crystallization mechanism of Allanblackia seed oil gives it good material functionality and desired plasticity, properties associated with possible industrial application. Additionally, the presence of high stearic acid content makes the Allanblackia seed oil a healthy choice for food, hence it can be used as a substitute for cocoa butter in the food industry.

Keywords: Allanblackia seed oil,shea nut oil, crystallization, melting profile, cooling profile, kinetics, physical properties.

1 INTRODUCTION

Cocoa butter is a naturally occurring fat obtained from cocoa beans [1,2]. It has been used extensively as a constituent in chocolate and other confectionary products [3,4]. Cocoa butter contains high levels of saturated fatty acids, such as palmitic and stearic acids, as well as monounsaturated oleic acid [5]. The fatty acid composition of the cocoa butter accounts for its advantageous properties and for its utilization in food production, pharmaceuticals and cosmetics, etc. However, the continuous demand on cocoa butter for use in food and other products has caused its competitive pricing and scarcity on the market. Lots of research is ongoing in the search for alternatives to cocoa butter [6]. Some of the common cocoa butter substitutes or alternatives are Lauric acid based formulations and hydrogenated vegetable oils [7], but these fats are not usually accepted in food-based products due to their tendency to cause an increase in blood cholesterol levels [8]. There is a need to search for more naturally occurring fats. Allanblackia and shea plants are plant species with good oil yield from their seeds and nuts, respectively [9]. The oils obtained from these plants are estimated to fall within 50 – 70 % of their total seed weight , and they solidify at room temperature and melt fast at temperatures above 30 to 35 oC.

Since 2002, the oil from Allanblackia seeds has been developed in Ghana, Nigeria, Cameroon and Tanzania as a rural based enterprise. Corporate entities such as Unilever public limited company (PLC) and other African agencies, farmers and non-governmental organizations under the “Novella Africa Project” are working to produce high quality consumer products for household use. It is reported that from this project, Unilever PLC has produced a stable market for the Allanblackia seed oil and this is expected to expand to a market value over $100 million [10]. Again, the oil from shea nut has been used extensively in food preparation in Africa as well as a substitute for cocoa butter in the chocolate industry, cosmetics and pharmaceuticals [11]. The presence of high stearic acid in both oils makes them a healthy choice in the food industry [10,12]. Considering the outlined importance of these oils, there is an important knowledge gap that needs to be addressed: How is the physical behavior of these oils affected by temperature variations? It is important to know how these oils behave with change in temperature since the physical properties of oils inform the functionality and texture of oil-based materials. However, no work has been done on the effect of temperature change on the physical behavior of the Allanblackia seed oil and shea nut oil. Data published so far has looked at the fatty acid composition, the triacylglycerol content and the physicochemical properties of these oils [10,13,14]. Therefore the main objective of this work was to investigate the thermal behavior of the oils obtained from the shea nuts and Allanblackia seeds. The focus of the study was to determine the fatty acid composition of the oils and investigate the effect of temperature change on their heating and cooling profiles, the crystallization pattern and the kinetics of crystallization.

2 MATERIALS AND METHODS

Ripe fruits of Allanblackia (Allanblackia paviflora) were collected from Dunkwa in the Western region of Ghana and shea (Vitellaria paradoxa) nuts were collected from Tamale in the Northern region of Ghana. The seeds and nuts of Allanblackia and shea, respectively, were removed by hand picking, after which the seeds and nuts were dried and cracked to obtain the kernel. The kernels were further dried in an oven at 50 oC for 24 hours to ensure complete dryness, then milled to powder. The oils were extracted as described by Adubofuor et. al, 2013 [15]. The extracted oils, namely Allanblackia seed oil (ASO) from A. paviflora and shea nut oil (SNO) from V. paradora, were put in glass vials, labeled and stored at room temperature until use. All experiments were performed at the Organic Materials Innovation Centre (OMIC) of the School of Chemistry at the University of Manchester, United Kingdom.

2.1 Fatty Acid Compositional Analysis using GC/MS

The fatty acid composition of ASO and SNO were determined by direct methylation. The fatty acid methyl esters (FAME) were prepared according to the method described by Lall et al (2009)[16] with slight modifications. The FAME were then analyzed on a GC/MS7890/15975C Chem-station fitted with an autosampler (Agilent Technologies, USA). The oven temperature was programmed as follows: 125 oC for 3 min, 125 oC to 206 oC at 3 oC/min, holding at 206 oC for 7 min, 206oC to 300 oC at 10 oC/min. The total runtime was 30 min. The MS used electron impact ionization mode and a full scan of m/z 40 – 560. The fatty acid components were identified by comparing their retention times with those of fatty acid standardmix and confirmed by their mass spectra. Quantification was based on relative peak area. All analyses were performed in triplicate.

2.2 Analyses of fat samples by DSC

The thermal behavior of both ASO and SNO were carried out under inert atmosphere (N2 gas) on a Diamond DSC hyperDSCTM (Perkin Elmer, Inc. US) with a refrigerated cooling system (Perkin Elmer, Inc. US). Nitrogen at a flow rate of 1 ml/min was used to purge the thermal analysis system. The DSC was calibrated with indium (Perkin Elmer, Inc. US), azobenzene (Sigma Aldrich) and undecane (Sigma Aldrich) before data acquisition. Data obtained were processed by instrument software (Pyris software, version 9.0.1.0174, Perkin Elmer, Inc. US.).

2.2.1 Heating and Cooling Profile

In order to determine heating and cooling behaviors, each of the ASO and SNO samples were weighed (5-6 mg) into an aluminum pan and sealed. The heating and cooling profiles of the oil were determined using the following time/temperature program: the samples were first rapidly heated from 30 oC to 70 oC at the rate of 30 oC/min and held at 70 oC for 10 min to ensure complete melting of the oil and destruction of any nuclei present. The melted oil was then cooled from 70 oC to -30 oC at a rate of 2oC/min and held for 10 min to ensure fully crystallized oil. The crystallized oil was then heated from -30 oCto 70 oC at a rate of 2oC/min and their cooling and melting profiles recorded.

2.2.2 Crystallization at Different Cooling Rates

The effect of change in the rate of cooling on the crystallization profile of the ASO and SNO were investigated. The samples were initially rapidly heated from room temperature to 70 oC at 30 oC/min and held at 70 oC for 10 min to erase any crystallization memory, after which the samples were cooled to -30 oC at rates of 5 oC/min, 10 oC/min, 15 oC/min and 20 oC/min, respectively, and the various cooling profiles recorded.

2.2.3 Isothermal Crystallization and the Crystallization Kinetics by the DSC Method

The isothermal crystallization curves of ASO and SNO were also obtained from the DSC instrument. The samples (5 – 10 mg) were sealed hermetically in aluminum pans and used for the determination. The following time/temperature program was applied: fat samples were heated rapidly from 30 oC to 70 oC and held for 10 min to ensure complete melting; the melted samples were cooled at 20 oC/min to the isothermal crystallization temperature (Tc) of -10 oC, 16oC and 20oC. The crystallization process was held for 60 minutes at each crystallization temperature. The 20 oC/min cooling rate was used to limit the amount of crystals formed during the cooling process.

From the isothermal crystallization curves, it was observed that some of the samples already began to crystalize during the cooling process, making it difficult to estimate the crystallization enthalpy. Therefore the DSC stop-and-return technique was used, as described by Foubert et al (2008) [17], for crystallization temperatures 16 and 20 oC based on the crystallization peaks obtained during the cooling process. The results obtained were fitted to the Avrami model [18] to estimate the kinetics of the crystallization process.

From the Avrami analysis, three kinetic parameters, namely the Avrami exponent (n), Avrami constant (k) and the half-time of crystallization (t1/2) were obtained from the linearization of the Avrami equation as shown below

(1(X) = exp(((ktn) ……(1)

Where X is the fraction of crystal transformed at the time t during the crystallization, k is the crystallization rate constant, which depends primarily on crystallization temperature, and n is the Avrami exponent, a constant relating the dimensionality of transformation.

The Avrami equation was expressed in a logarithmic form as

ln[(ln(1 – X)] = lnk + nlnt ……(2)

ln[(ln(1 – X)] was then plotted against lnt. The slope of the graph obtained represents the Avrami exponent (n). All measurements were taken in triplicate.

2.3 Microscopy Analysis

The crystal morphology of the samples at room temperature was studied with the Nikon OPTIPHOT-2 optical microscope fitted with a 5 megapixels DinoEye Eyepiece camera (Dino-Lite, Taiwan). The samples were allowed to crystallize at room temperature (25 oC). A small portion was then transferred onto a microscope glass slide and covered with a cover slide to spread uniformly. This was then placed on the microscope in a temperature-controlled cabinet to investigate the microstructure of the fat after crystallization (1 h). The visual field pictures were taken at magnification of 40(

3 RESULTS AND DISCUSSION

3.1 Fatty Acid Composition

Table 1 shows the fatty acid composition of ASO and SNO. The results show that ASO contains stearic and oleic acid with a small percentage (less than 1 %) representing palmitic acid and arachidic acid. SNO was found to contain high amounts of oleic and stearic acids and also a significant amount of linoleic acid. The presence of oleic and linoleic acids in the SNO accounts for its high level of unsaturated fatty acids. Comparing ASO and SNO, it was observed that ASO contains a high amount of stearic acid, which contributes significantly to the saturated fatty acid composition, and hence its characteristic solidified fat crystals at room temperature. The fatty acid composition of ASO and SNO were comparable to their iodine values of 36.86 (mg I2/100 g) and 52.06 (mg I2/100 g ), respectively. The iodine value of a fat or oil is defined as the number of grams of iodine consumed by 100 g of the fat. A high iodine value indicates a high degree of unsaturation in the fat. Therefore the high iodine value obtained for SNO is as a result of the presence of high amounts of unsaturated fatty acids. This study used the GC/MS to determine the fatty acid molecules found in ASO and SNO. The fatty acids exist in the oils as triglycerides. Each triglyceride molecule contains three fatty acids attached to the glycerol backbone [19,20]. Assuming, there are five representative triglyceride molecules having 15 fatty acid species, the distribution of each fatty acid in the 5 triglycerides is proportional to the percentages of the fatty acids present. Therefore the high percentage of stearic acid and oleic acid found in this study is likely to give high proportions of StOSt, StOO, OOO and StStSt. According to data reported in the literature, ASO contains 60-70 % stearic-oleic-stearic (SOS), 20-30 % stearic-oleic-oleic (SOO) and just about 10 % representing triacylglycerides (TAGs) with the minor fatty acids [10,13] which confirms the high content of stearic and oleic acids found in the ASO. Again, shea nut oil has also been reported to contain about 10 % trioleic (OOO), 35 % stearic-oleic-oleic (SOO), 40 % stearic-oleic-stearic (SOS) and 8 % palmitic-oleic-stearic (POS). It also contains other TAGs made up of linoleic acid and arachidic acid in minor quantities [14]. Again, according to Ray et al, (2013), The triglyceride composition of untreated shea stearin and silica treated shea stearin showed a high amount of StOSt (73.4 %) with TAGs such as POSt, StLSt, StOO, StStSt, AOSt etc showing levels below 10 % of the total TAG composition [21]. Data obtained from the study clearly shows that the content and composition of fatty acids found in the ASO and SNO are different. However, since the fatty acid distribution of the TAG structure significantly affects the physical properties of fats and oils, this study investigated the effect of temperature change on the physical properties of the selected fats.

3.2 Heating and Cooling Profiles of Allablackia Seed Oil and Shea Nut Seed Oil

Figure 1 gives the heating and cooling profiles of the ASO and SNO. ASO gave a three-peak profile for the melting and cooling process. One major peak occurred at a high temperature and two small peaks at low temperatures. Additionally, there was also the appearance of an exothermic peak during melting in the ASO (Figure 1A). The peaks observed in ASO were narrow and sharp and this may be attributed to the limited variety of TAG compounds as a result of the presence of a few fatty acid compounds in the ASO. In contrast, SNO gave a multiple peak profile during heating and a four-peak profile for the cooling (Figure. 1B). The melting (during heating) peaks were broad and overlapping. Diacylglycerides (DAGs), which arefound in natural fats and oils have been reported to significantly delay the crystallization of fats and oils [22]. Ray et al. reported the presence of 1-stearoyl-3-oleoyl-glycerol and 1-stearoyl-2-oleayl-glycerol DAGs in shea stearin [21], therefore the broadening of peaks may be attributed to the inhibitory effect on crystallization/recrystallization of DAGs present in the SNO. Again, shea butter has been reported to contain high levels of free fatty acids ranging from 1.47 mgKOH/g to 4.10 mgKOH/g [23,24]. On this basis, the multiple and overlapping peak profile may be as a result of the presence of free fatty acids in the shea nut oil.

|A |B |

|[pic] |[pic] |

Figure 1. Heating and cooling profiles of the oils obtained at temperatures between -30 oC to 70 oC at 2 oC per minute from the DSC: (A) Heating and cooling curves of ASO and (B) Heating and cooling curves of SNO

The physical behavior of fats and oils is largely dependent on the fatty acid distribution of their triglyceride structure [25–28]. The presence of high amounts of saturated fatty acids (57.78 %) in the ASO may account for the observed high melting temperature of a large portion of the fat. At about 22.50 oC, there was the appearance of an exothermic peak during the heating process of ASO. The presence of this exothermic peak in the ASO heating profile may be a result of recrystallization of some high melting fat fractions. It may also be due to the occurrence of a polymorphic transition as discussed by Che Man and Tan [9,27]. The multiple melting peaks observed for the SNO may be due to the presence of a wide variety of different fatty acids leading to the formation of a large number of TAG compounds. Comparing the peak profiles from the melting and cooling process, there were significant differences between the two oil samples.

3.3 The Effect of Change in the Rate of Cooling on the Crystallization Pattern

Figure 2 shows the crystallization curves obtained for ASO and SNO at different rates of cooling. Three crystallization peaks were observed for both oils at all cooling rates. Peaks for SNO were broad while those for the ASO were narrow. The crystallization temperatures (Tc) shifted to lower temperatures (Table 2) as the rate of cooling increased.

|A |B |

|[pic] |[pic] |

Figure 2. Crystallization profile of ASO and SNO from -30 oC to 70 oC at different cooling rates: (A) ASO cooling at 5 oC/min to 20 oC/min (B) SNO cooling at 5 oC/min to 20 oC/min

According to Che Man and Tan [27], the cooling curves of vegetable oils and fats are subdivided into different exothermic regions which correspond to different fractions of a particular TAG type. During cooling, different fractions of the fat crystallize out according to how they are kinetically favoured. This means that the three crystallization peaks observed in this study may be due to sequential crystallization of different melting fat fractions. The position and magnitude of the crystallization peaks were found to be dependent on the cooling rate. At a slow cooling rate (5 oC/min), the crystallization process proceeds slowly allowing components of similar properties to have time to associate, co-crystallize and fractionate [29,30]. This leads to the formation of more discrete peaks and a reduction in overlapping. Also, at slow rates, triacylglycerols of similar fatty acid molecules have enough time for molecular rearrangement and organization leading to the formation of typically fewer crystals of higher purity[18,27]. As the cooling rates increase(10 oC/min to 20 oC/min), the high melting components crystallize more rapidly within the liquid phase of the melted fat and this phenomenon leads to a rapid increase in the viscosity of the melted fat, hence limiting mass transfer, and molecules are forced to crystallize rapidly into mixed crystals resulting in the merging and broadening of the peaks [31].Again, at a faster rate, high melting components may co-crystallize with low melting components and this causes a dilution in the crystallization temperatures and, hence, the shifting of the crystallization peaks to lower temperatures [18,27,32,33].

3.4 Isothermal Crystallization studies

In the DSC studies, two different crystallization temperatures, 16 oC and 20 oC, were used to study the isothermal crystallization behavior of the ASO and SNO. Figures 3 and 4 give the isothermal curves obtained for both ASO and SNO at 16 oC and 20 oC, respectively.

|A |B |

|[pic] |[pic] |

Figure 3. Isothermal crystallization peaks for (A) ASO and (B) SNO at 16 oC for 60 minutes

|A[pic] |B[pic] |

Figure 4. Isothermal crystallization peaks for (A) ASO and (B) SNO at 20 oC for 60 minutes

At 16 oC, both ASO and SNO showed single peak profiles. The crystallization of ASO was observed to occur faster with a sharper peak, while that of the SNO was slow and gave a broad peak. The fast crystallization at 16 oC of the ASO may be attributed to the presence of high melting fat fractions. The slow and broad nature of SNO peaks may be a result of the presence of a wide composition of fat fractions at the isothermal conditions. According to Vereecken et al (2010), a wide composition of different TAG compounds reduces the speed of crystallization since it increases the time required for molecular sorting at the solid-liquid interface [34]. This results in the broadening of the resultant peak.

At 20 oC both ASO and SNO gave a two-peak crystallization profile. The ASO gave a very sharp and narrow peak within the first 5 minutes and a broader peak at 20 – 30 minutes. This may be attributed to sequential crystallization of two different fat fractions or the occurrence of a polymorphic transition. The characteristic fast and sharp peaks observed for the ASO is a result of its limited fatty acid constituents while the broad peaks for the SNO may be attributed to the different fatty acid compositions.

3.5 Isothermal crystallization kinetics by the stop-and-return technique

A clear difference between the crystallization of ASO and SNO was observed under isothermal conditions. Therefore, the isothermal crystallization was further investigated by the DSC stop-and-return method. Data obtained were fitted using the Avrami model as shown in Figure 5. The isothermal data showed a good fit to the Avrami equation with the regression coefficient of ln[-ln(1-X)] vs. lnt higher than 0.97.

|A |B |

|[pic] |[pic] |

|C[pic] |D[pic] |

Figure 5. Melting profiles after certain isothermal times during the stop – and – return analysis. (A) ASO at crystallization temperature 16 oC (B) ASO at crystallization temperature 20 oC (C) SNO at crystallization temperature 16 oC (D) SNO at crystallization temperature 20 oC

Generally both ASO and SNO crystallized with a small Avrami exponent value (n = 0.14 – 1.42) at temperatures of 16 oC and 20 oC (Table 3). From the Avrami theory, the n value gives information on the nucleation and the mechanism of the crystal growth. A small n value signifies a faster nucleation and a more rapid crystal growth while a high n value indicates a complex mechanism for the crystal growth [18,35,36]. Therefore the smaller n values obtained for both fats indicate their fast nucleation and crystallization mechanism. This may be explained by the presence of high melting fatty acids such as stearic acid, which is present in high quantities, and are found in both ASO and SNO. Rashid et al (2012) [37] have discussed the influence of composition of fat blends and crystallization temperature on the crystallization behavior mechanism of fats.

|A[pic] |B[pic] |

Figure 6. Avrami equation plot for (A) ASO at different crystallization temperatures. 16 oC; y=0.41x – 0.646; r2 = 0.988, 20oC; y= 1.271x – 4.340; r2 = 0.973.(B) SNO at different crystallization temperatures. 16 oC; y= 1.415x – 4.079; r2 = 0.981, 20oC; y= 1.163x – 4.126; r2 = 0.983

To further understand the crystallization mechanism at temperatures of 16 oC and 20 oC, the peak maxima of the melting peaks as a function of the isothermal time were studied and the results are shown in Figure 6. It can be seen that, at 16 oC crystallization temperature, the ASO and SNO gave both low and high melting peaks at the beginning of the crystallization. The low melting peaks disappeared after some time (5 min for the ASO and SNO, and after 10 min for the second low melting peak of SNO and 40 min for the second low melting peak of the ASO). The disappearance of the low melting peaks may be due to a polymorphic transition from an unstable form to a more stable form as described by Rousseau and his co-workers [38].The melting profiles obtained from the stop-and-return procedures are shown in Figure 5 for a clear understanding of the source and growth of the various peak maxima.

At 20 oC, a multiple melting peak profile with a majority of the peaks occurring at high temperatures was observed for ASO. Additionally, a large number of the high melting peaks of the ASO stayed for a long time (20 min and 40 min) during the crystallization process before disappearing. The presence and persistence of a large amount of the high melting peaks for the ASO can be explained by the high amount of saturated fatty acid found in the ASO as discussed from the GC/MS analysis in the study.

|A[pic] |B[pic] |

Figure 7. Evolution of the peak maxima in the melting profiles as a function of the isothermal time (A) ASO and SNO at 16 oC crystallisation temperature (B) ASO and SNO at 20 oC crystallization temperature.

3.6 Crystal morphology

Polarized light microscopy was used to investigate the fat morphology directly after crystallization at room temperature, to further investigate the nature of the crystals formed.

Figure 8 shows the photomicrographs of the fat crystals for both ASO and SNO at 16 oC and 20 oC. As observed from the DSC analysis, there were some significant differences in the crystals formed during the solidification of ASO and SNO at 16 oC and 20 oC temperatures. At 16 oC, when temperature was low, crystal aggregation had already occurred and small crystal particles were observed for both ASO and SNO. The fat particles formed were evenly distributed within the crystal network. In comparing the morphologies of ASO and SNO at 16 oC, the crystals formed in ASO were spherical and the size of the particles formed bigger than that of the SNO. However, a larger number of crystals and a denser network was observed for the SNO implying a more stronger crystals interactions for the crystallization process of SNO.

At 20 oC, a fast crystallization process started in the ASO and this was observed in the appearance of a significant number of crystallized fat particles whilst SNO formed just a few crystals with smaller particle sizes. The micrographs obtained for ASO showed the formation of individual fat particles during the crystallization, which signifies a homogeneous nucleation process. The distinct fats formed in ASO could be explained by sequential crystallization of TAG compounds with different crystallization temperatures, while the continuous and denser crystallization of SNO may be due to the wide range of TAG compounds with similar crystallization temperatures. The presence of fat fractions with similar crystallization temperatures, as may be the case for SNO, leads to the formation of mixed fat crystals, hence the continuous crystallization.

|ASO (16 oC) | ASO (20oC) |

|[pic] |[pic] |

|SNO (16 oC) | SNO (20 oC) |

|[pic] |[pic] |

Figure 8. Morphology of ASO and SNO during crystallization at 16 oC and 20 oC

4. CONCLUSIONS

In this work, the physical properties as a result of change in temperature of the Allanblackia seed oil (ASO) and shea nut oil (SNO) were investigated and compared with their fatty acid composition. From the results obtained, it was revealed that the physical properties of both oils were very dependent on the nature of fatty acid present in the oil. Allanblackia seed oil showed high melting and crystallization temperatures as compared to the shea nut oil. This was attributed to the presence of a high amount of saturated fatty acids in the ASO. The crystallization peaks obtained for the Aallanblackia seed oil were narrow and sharp while those obtained for the shea nut oil were broad. However, the number of peaks obtained was the same for both oils and their crystallization temperatures were found to be dependent on the rate of cooling. There was a significant difference in the crystallization mechanism of the Allanblackia seed oil and the shea nut oil under isothermal conditions. The Allanblackia seed oil showed fast and sharp crystallization peaks, while shea nut oil gave a slow and broad crystallization peak. This behavior was attributed to the difference in their fatty acid composition. Data obtained from the peak maxima analysis revealed the possible occurrence of a polymorphic transition and sequential crystallization of low and high melting fat fractions during the crystallization process of Allanblackia seed oil and shea nut oil at both 16 oC and 20 oC temperatures.

Finally comparing the morphology of the crystalized oils at room temperature, the microscopy revealed shea nut oil crystallizes to form overlapping fat crystal particles while Allanblackia seed oils form discrete particles.

Conflict of interest

Authors have declared no conflict of interest

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Table 1: Fatty Acid Content (percentage by weight (wt %)) of the ASO and SNO Determined by GC/MS

| | ASO | SNO |

| | | |

|Fatty acid | | |

|C16:0 | 0.81±0.23 | 2.83±0.45 |

|C18:0 |56.93±0.61 |47.14±0.74 |

|C18:1 |42.16±0.16 |44.58±0.13 |

|C18:2 |N/D |4.20±0.12 |

|C20:0 |0.11±0.30 |1.09±0.21 |

|T.sat |57.78±0.14 |51.06±0.40 |

|T.unsat |42.16±0.15 |48.78±0.24 |

C16:0 – Palmitic acid, C18:0 – Stearic acid, C18:1 – Oleic acid, C18:2 – Linoleic acid and C20:0 – Arachedic acid. N/D – Not detected

Table 2: Temperature (Tp) and the Enthalpy Change ((Hc) Recorded for the ASO and SNO at Different Cooling Rates

|Peaks | a |b |c |

|Cooling rate (oC/min) |

Tp (oC) |

ΔHc (J/g) |

Tp (oC) |

ΔHc (J/g) |

Tp (oC) |

ΔHc (J/g) | |ASO |5

10

15

20 |17.82±0.02

16.98±0.02

16.38±0.01

15.80±0.16 |58.93±0.29

59.68±0.36

59.89±0.22

60.27±0.36 |7.13±0.02

6.49±0.01

6.03±0.12

5.67±0.05 |0.88±0.02

1.17±0.01

1.21±0.08

1.31±0.06 |-6.07±0.03

-6.57±0.01

-6.97±0.01

-7.52±0.00 |1.46±0.13

1.86±0.04

2.06±0.13

2.23±0.01 | |SNO

|5

10

15

20 |15.57±0.03

14.96±0.01

14.61±0.01

14.28±0.16 |49.69±0.48

66.34±0.15

66.59±0.03

67.21±0.16

|6.43±0.03

6.08±0.10

5.65±0.01

5.34±0.05 |0.30±0.89

0.47±0.06

0.55±0.08

0.76±0.09 |-6.93±0.14

-7.49±0.10

-7.84±0.13

-8.19±0.01 |2.26±0.03

2.45±0.02

2.60±0.03

2.79±0.17 | |

Table 3: Isothermal Crystallization Kinetics Parameters; Avrami Exponent (n), Crystallization Rate Constant (k) and Half-time of Crystallization (t1/2) Obtained from Fitting the Isothermal Crystallization Curve at 16 and 20 oC Temperature for ASO and SNO

Samples ID |Temp. (oC) |n |k |t 1/2 | |ASO |16 |0.41 |0.65 |1.17 | | |20 |1.27 |4.34 |0.24 | |SNO |16 |1.42 |4.08 |0.29 | | |20 |1.16 |4.13 |0.21 | |

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