Expression of oil from sesame seedsO.O. AJIBOLA, O.K. OWOLARAFE, O.O. FASINA and K.A. ADEEKO

Department of Agricultural Engineering, Obafemi Awolowo University, lle-lfe, Nigeria. Received 15 December 1989;accepted 28 July 1992.

Ajibola, O.O., Owolarafe, O.K., Fasina, O.O. and Adeeko, K.A.1993. Expression of oil from sesame seeds. Can. Agric. Eng.35:083-088. Investigation was carried out on the effects of heatingtemperature (40 to 85°C), heating time (5.0 to 27.5 min), moisturecontent (6.1 to 12.0%) and applied pressure (15 and 20 MPa) on theyield, color, refractive index, and specific gravity of oil expressedfrom sesame seeds using a laboratory press. The yield of oil wasaffected but the color, specific gravity and refractive index were notaffected by processingconditions within the range considered. Theoil yieldwas increased withdecreasein moisture contentof sampleafter heating. Highest oil yield (based on total mass expressed) of33.5% corresponding to an expression efficiency (basedon seedoilcontent) of 65.7% was obtained when sesame seeds were conditioned to moisture content of 6.1%, heated at 85°C for 20 min, andexpressed at a pressure of 20 MPa. Theaverage values of refractiveindex andspecific gravity were1.47and0.918respectively while thecolorof the oil was yellow (i.e. a Munsell color notationof 2.5Y).

Ont ete etudies les effets de la temperature de chauffage (de 40 a85°C),de la duree de chauffage (de 5,0 a 27,5 min), du pourcentagede matiere humide (de 6,1 a 12%) et la pression appliquee (15 et 20MPa) sur le rendement, la couleur, l'indice de refraction et la densitede Thuile extraite des graines de sesame par le moyen 'un pressoirde laboratoire. Le rendement etait influence mais la couleur, ladensite et l'indice de refraction n'ont subi aucune influence due auxconditions de traitement avec les differentes variations prises encompte. Lerendement etait plus grand quand ily avait une reductionde la matiere humide dans les echantillons apres chauffage. La plusgrande quantite d'huile (compte tenu de la totalite obtenue) etant33,5% et correspondant a une efficacite d'extraction de 65,7% (parrapport aupourcentage d'huile dans les graines) aeteobtenue quandles graines de sesame etaient conditionnees a 6,1% de matiere humide, chauffees a 85°Cpendant 20 min et subiesa une pression de20 MPa au moment d'extraction. Les valeurs moyennes pour l'indicede refraction et la densite etaient respectivement 1,46 et 0,918. Lacouleur de l'huileetaitjaune(i.e.unenotation decouleurMunsell de2,5Y).

INTRODUCTION

Sesame seed (Sesamum indicum) is one of the oilseeds thathas not been commercially exploited in the world despite itshigh oil content of about47% (UNIDO 1977). Sesame seedis consumable, has laxative properties and can be processedinto biscuit. The oil obtained from sesame seed is goldenyellow in color and requires little or no refining. Theoil canbe used in salad dressings, and the manufacture of cosmeticssuch as soap, hair conditioners, and bath oil. The cake obtained after expression can be used in the preparation oflivestock feed.

The mechanical expression of oil from oilseeds is the mostcommon method used for the removal of oil from oil-bearingmaterials. This method is economical for small and mediumscale oil processors in the developing countries (Singh et al.1984). Although the use of a hydraulic press during expres

sion results in lower oil removal efficiency, it is preferred tothe screw press by small scale producers because of lowerinitial and running costs. The amount of oil expressed usingthe hydraulic press is affected by the pre-pressing and pressing conditionssuch as particle size, moisturecontent, heatingtemperature, heating time, applied pressure, and pressingtime (Khan and Hanna 1983). Steinbock (1948) explained,that with the exception of very small sized seeds like sesame,reduction of oilseeds to flakes by rolling is essential with thesize and hardness of the seeds determining the number ofstages for the flaking operation. Heating is essential becauseit completes the breaking down of oil cell walls, lowers theviscosity of the oil to be expelled, coagulates the protein inthe meal, and adjusts the moisture contentof the meal to theoptimum level for pressing (Ward 1976). Oil yield has beenshown by several researchers to be affected by the temperature and duration of heating (Ajibola et al. 1990; Fasina andAjibola 1989; Sivakumaran et al. 1985; Singh et al. 1984).High heating temperatures and long heating times may havenegative effects on the quality of expressed oil and cakeresidue(Anjou 1972;Ohlson 1976).It is thereforeimportantto identify theoptimal heattreatment required forprocessingsesame seed.

The moisture content of oilseeds before pressing affectsthe yield and quality of oilexpressed. Cloudy oil is obtainedfrom seeds with high moisture content. Therefore, moisturecontent adjustment of the seeds is necessary before pressing.Vaughan (1970) explained that the effectof moisture contentonexpression efficiency canbe related to mucilage development in the outer epidermal cells of the seeds. The mucilagecoating can impede theflow of oil from thecotyledon tissue.High oil yields ranging between 37.0 and 39.6% were obtained from samples of conophor nut with post-heatingmoisture content between 8 and 10% (dry basis) (Fasina andAjibola 1989), while maximum oilyields of about 40% wereobtained when the moisture contents of melon seed sampleswere reduced by about 5% (dry basis) (Ajibolaet al. 1990).Singh et al. (1984) observed that the moisture content ofsunflower seed was the most significant factor affecting theexpression of oil from the seed.

Pominski et al. (1970) reported that the optimum pressurefor maximum oil yield may be dependent on the pre-pressingoperation and the type of seed being expressed. They foundthat the amountof oil expressedfrom peanuts tended to leveloff at expression pressure of 15 MPa and above, whileAdeeko and Ajibola (1990) found a pressure of 20 MPa asoptimum pressure for the expression of oil from groundnut.The oil yield from conophor nut was found to increasewithincrease in applied pressurefrom 10MPato 25 MPa(Fasina

CANADIAN AGRICULTURAL ENGINEERING Vol. 35, No. 1, January/February/March 1993 83

and Ajibola 1989). No study is known to have identified theeffects of processing factors on yield of oil expressed fromsesame seeds.

Specific gravity, refractive index, and color are some of theproperties of oil that are used for identification of oil qualityand its possible uses. The specific gravity is of diagnosticvalue in the consideration of the quality or purity of the oiland it is also used in assessing the weight of oil in bulkshipments or oil stored in large tanks (Whiteley 1951). Therefractive index allows the rapid sorting of oil suspected ofadulteration because of the ease and rapidity with which thevalue can be determined (Whiteley 1951).

The objective of the study was to determine the effects ofheating temperature, heating time, moisture content, and applied pressure on the yield, specific gravity, refractive index,and color of oil expressed from sesame seeds.

MATERIALS AND METHODS

Laboratory press

Expression of oil from heated samples was carriedout usingthe laboratory press shown in Fig. 1. The press consisted of alever arm (B) with a weight container (A) attached to its end.Knownweights were placed in the weight containerto generate the required pressures at the pressing ram (C). Thepressing cylinder(E)wasmadefroma mildsteelpipehavingan 80 mm interior diameter. The pipe was welded to a metalbar (J) that served as the base of the pressing cylinder. Holes(3 mm in diameter) were drilled in the section of the metalbar within the pipe's circumference with the total area of theholes forming about 25% of the area of this section. The holesallowed for passage of oil from the pressing cylinder to thecollecting cylinder (H) through the funnel (G). A metal disc(D) was placed in the cylinder to distribute the pressurefrom

1700

the pressing ram (C) evenly on the sample. A hydraulic jackwas used to lower and lift the lever arm at the beginning andend of pressing respectively.

Test procedure

The sesame seeds used for this study were collected from amarket in Benue State. The seeds were cleaned manually toremove foreign materials. Size reduction of the seeds was notcarried out, as recommended by Steinbock (1948), due to thesmall size of the seeds. The seeds were conditioned to the

various moisture contents used in the study. Calculatedamounts of water were sprayed on the samples to increase themoisture contents to the desired levels. The moisture wasthen allowed to equilibrate by placing the conditioned seedsin plastic bags and storing them in a refrigerator for a minimum duration of 48 h. The actual moisture contents of thesamples were determined after conditioning.

Heating at the different temperature and time combinations used in the study was achieved by spreading an 80 gsample in a thin layer in a closed container and placing it ina forcedair circulationovenpreset at the desiredtemperature.A thermometerwas used to verify the oven temperature.Themoisture lost from the samples was noted by weighing thesamples after heating. The samples were then wrapped incheesecloth and introduced into the pressingcylinderfor oilexpression. A collecting cylinder was used to collect the oilexpressed during pressing.

The factors and their corresponding ranges used in thestudy were selected based on a review of literature and preliminary laboratory investigations. Heating temperatures of40, 55, 70, and 85°C, heating times of 5.0, 12.5, 20.0, and27.5 min, moisture contents of 6.11, 9.1, and 12.0% wetbasis, and expression pressures of 15 and 20 MPa were used.The duration of pressing was 10 min. The amount of oil

Floor level

v////////////////,Fig. 1. Schematic diagram of laboratory press (not to scale, dimensions in mm).

84 AJIBOLA, OWOLARAFE, FASINA, and ADEEKO

expressed was determined by subtracting the weight of thepressed cake from that of the unpressed sample. The oil yieldwas taken as the percent ratio of the expressed oil to theweight of material expressed, while the expression efficiencywas the percent ratio of the expressed oil to the total oil in theunpressed sample. All experiments were replicated twice.

The moisture content of samples was determined by ovendrying 100 g of samples at 130°C for6 h as recommended byYoung et al. (1982) for oilseeds with high oil content. Thepycnometer method was used in the determination of thespecific gravity while a Zeiss 74111 Butyrorefractometer wasused in the determination of the refractive index (AOAC1984). A Munsell color chart was used for color grading ofthe expressed oil sample (Anon 1975).

RESULTS AND DISCUSSION

The average oil yields obtained with the different pre-pressingconditions considered are givenin TablesI and II. Theoilyield increased with increase in pressurefrom 15 MPa to 20MPa at each combination of heating temperature, heatingtime, and pre-heating moisture content. Generally, the response of oil yieldto the pre-pressing conditions wassimilarat the two applied pressures. The increase in oil yield due topressure increase was attributed to the greater force appliedto the oil-containing sacs.

The post-heating moisture content of sesame seed at the

Table I. Average* oil yield (%) at expressionpressure of 15 MPa for the differentprepressing conditions

Temperature

(°C)

40

55

70

85

Heating time

(min)

5.0

12.5

20.0

27.5

5.0

12.5

20.0

27.5

5.0

12.5

20.0

27.5

5.0

12.5

20.0

27.5

♦Standarderror = 0.19

Pre-heating moisture content

(% wet basis)

6.1

22.7

25.3

26.3

28.8

25.1

29.2

29.7

30.6

25.1

30.1

31.1

31.7

26.1

27.3

31.4

28.9

9.1

7.7

12.3

12.9

13.7

10.0

13.5

16.0

17.5

10.3

16.4

20.5

24.4

8.0

17.7

24.5

26.0

12.0

1.4

1.8

1.9

2.3

1.9

2.1

2.5

2.8

1.6

3.3

6.5

12.0

1.9

3.9

6.6

12.8

Table II. Average* oil yield (%) at expressionpressure of 20 MPa for the differentpre-pressing conditions

Temperature

(°C)Heating time

(min)

Pre-heating moisture content

(% wet basis)

40

55

70

85

5.0

12.5

20.0

27.5

5.0

12.5

20.0

27.5

5.0

12.5

20.0

27.5

5.0

12.5

20.0

27.5

6.1

25.7

29.1

30.2

30.3

27.9

29.5

29.6

30.9

29.5

31.4

31.3

32.5

28.2

31.9

33.5

32.0

9.1

9.6

11.4

12.1

14.1

10.4

13.4

16.8

21.2

14.7

21.4

28.1

30.8

15.7

22.5

31.2

31.8

12.0

1.7

1.8

2.3

3.9

1.6

2.3

5.8

7.7

1.9

7.4

8.4

12.9

3.5

4.8

11.5

17.0

Standard error = 0.47

different pre-pressing conditions to which the sample wassubjected is presented in Table III. The effects of these pre-pressing conditions on the post-heating moisture contentwere evaluated using FACTOR, a factorial analysis of variance procedure, in theMSTAT-C package (MSTAT-C 1990).The analysis (Table IV) shows that pre-heating moisturelevel, heating temperature, heating time and interactions ofthese factors significantly affected the post-heating moisturecontent.

The separate and interactive effects of such processingfactors as pressure, heating temperature, heating time andmoisture content on the oil yield were evaluated using FACTOR in MSTAT-C (1990). The analysis shows that all thefactors considered had significant effects on the oil yieldfrom sesame seed at the levels considered in the study (TableV). Most of theinteractions of thefactors alsohadsignificanteffects on the oil yield. The most significant factor, at thelevels considered, was pre-heating moisture content of thesample.

At all the heating temperatures used in the study, the oilyield increased with an increase in heating time from 5.0 to27.5 min and with decrease in pre-heating moisture contentfrom 12.0 to 6.1% wet basis. The increase in oil yield withincrease in heating time at any heating temperature may beattributed to the breakdown of more oil cell walls, the coagulation of proteins, or the adjustment of the seed moisturecontent to the optimum level. However,at a heating tempera-

CANADIAN AGRICULTURAL ENGINEERING Vol. 35, No. 1, January/February/March 1993 85

Table III. Effects of pre-heating moisture content,temperature and heating time on average*post-heating moisture content (% wet basis)

Temperature Heating time Pre-heating moisture content(°C) (min) (% wet basis)

Table V. Statistical analysis of the effects of processingconditions on oil yield

Source Degrees of Sum of Mean Ffreedom squares square value

40

55

70

85

Pressure (A)

Heating temp. (B)

1 299.47 299.5 416.3

6.1 9.1 12.0 3 1285.63 428.5**

595.8

Heating time (C) 3 1607.72 535.9 745.1**5.0 5.9 8.9 12.5 PreheatingMC (D) 2 18607.88 9303.9 12935.0**

12.5 5.8 8.3 10.9 A*B 3 62.38 20.8 28.9**20.0 5.6 8.1 10.3 A*C 3 2.35 0.8 1.1*27.5 5.6 7.8 10.0 A*D 2 19.19 9.6 13.3**

B*C 9 261.72 29.1 40.4**5.0 5.8 8.7 11.4 B*D 6 339.31 56.6 78.6**

12.5 5.5 8.0 11.0 C*D 6 299.44 49.9 69.4**20.0 5.0 7.3 9.6 A*B*C 9 14.98 1.7 2.3*27.5 4.8 6.8 9.0 A*B*D 6 88.22 14.7 20.4**

A*C*D 6 31.44 5.2 7.3**5.0 5.6 8.5 10.7 B*C*D 18 213.47 11.9 16.5**

12.5 4.9 7.4 9.0 A*B*C*D 18 45.46 2.5 3.5**20.0 4.5 6.5 7.8 Error 96 69.05 0.7

27.5 3.9 6.0 6.7

Total 191 23247.69

5.0 5.4

• 4.7

8.1

6.7

10.7

8.712.5 Significant at 95% level20.0 3.7 6.7 8.8 **Significant at 99.9% level27.5 3.2 5.6 4.7

Standard error = 0.14

Table IV. Statistical analysis of the effects of processingconditions on post-heating moisture content

decrease in oil yield with increase in pre-heating moisturelevel may be due to the explanation given by Vaughan (1970)that the effect of moisture level on expression efficiency canbe related to the development of mucilage (a sticky substance) on the outer walls of the particles and that the additionof more water results in swelling of the mucilage. The mucilage produced a cushioning effect which might haveprevented the rupturing of the oil cell walls and hence hindered oil expression.

At pre-heating moisture levels of 9.1 and 12.0%, there wasincrease in oil yield with increase in heating temperaturewhereas at pre-heating moisture level of 6.1%, the responseof oil yield to increasein heatingtemperature was not significant. About 85% of the samples expressed at 15 MPa and75% of those expressedat 20 MPa and having pre-pressingmoisture content of 6.1% yielded between 26 and 32% oil.The lower mucilage content of the samples, due to the relatively lower moisture content of 6.1%, may have caused thebehaviour obtained at this moisture level.

It was observed that, similar to the findings on oil expression from conophor nut (Fasina and Ajibola 1989), the oilyield from sesame seed at any pressure was dependent on themoisture content of the sample after heating. Figures 2 and 3show the effect of moisture content after heating on oil yieldat expression pressures of 15 MPa and 20 MPa respectively.Each figure contains data obtained at all the heating temperature - heating time combinations used in the study. The oilyield increased with decreases in post-heating moisture content up to about 4% (wet basis).

When unhealed samples at moisture levels of 6.1,9.1, and

Source Degrees of Sum of Mean F

freedon squares square value

Heating temp. (A) 3 69.68 23.23 521.3**Heating time (B) 3 76.59 25.53 573.0**Preheating MC (C) 2 279.79 139.89 3139.7**A*B 9 16.68 1.85 41.6**A*C 6 8.54 1.42 31.9**B*C 6 12.21 2.03 45.7**A*B*C 18 3.47 0.19 4.3**Error 48 2.14 0.05

Total 95 469.09

Significant at 99.9% level

ture of85°C and a pre-heating moisture level of6.1%, forthetwo pressure levels considered, the maximum oil yield wasobtained from samples heated for 20.0 min. Samplesheatedfor 27.5 min gave a lower oil yield at the two pressure levels(Tables I and II). The increasing hardness of samples, thatwas observed during the experimental work, with increase inheat treatment may have contributed to the lowering of oilyield with increase in heating time beyond 20 min. The

86 AJIBOLA, OWOLARAFE, FASINA, and ADEEKO

40

32

e 24

Q)

= 16

/V

°9>

<b

%<*>

o oo o

"e o °ooOo o

—I—

12

Moisture content after heating (%w.b.)

16

Fig.2. Effect of post-heating moisture content on oil yieldfrom sesame seed at expression pressure of

15 MPa.

32

^ 24

Q)

5 16

Fig. 3.

o>r

Oq o

Oo(9c

0 4 8 12

Moisture content after heating (%w.b.)

Effect of post-heating moisture content on oil yieldfrom sesame seed at expression pressure of20 MPa.

16

12 were expressed at 20 MPa, the oil yields obtained werelower than heated samples at the same pre-pressing (post-heating) moisture content (Table VI). This againre-emphasizes the importance of heating of oilseeds beforeexpression in orderto achieve highoil yields. The highestoilyield of 33.5%, corresponding to an expression efficiency of65.7%, was obtained when a sample at pre-heating moisturecontent of 6.1% was heated at 85°C for 20 min to a post-heating moisture contentof 3.7%and expressed at a pressure of20 MPa.

Processing conditions at the levels considered in the studydid not have significant effect on the physical properties(specific gravity, refractive index and color) as shown inTable VII. The average value of 0.918 obtained for the specific gravity agrees well with the published value of 0.9187(Swern 1964). The average value of the refractive index was1.47 while the color of the oil obtained was yellow (2.5Y onthe Munsell color notation).

Table VI. Comparison of oil yields from heated andunheated samples expressed at 20 MPa

Post-heating moisture Oil yield from Oil yield fromcontent heated samples unheated samples

(% wet basis) (%) (%)

6.1

9.1

12.0

25.0-30.5

7.5-8.5

20.1

6.2

0

Table VII. Some physical properties of sesame oil

Physical properties Average

Refractive index

Specific gravityColor

1.470

0.918

2.5Y

Range

1.469-1.470

0.917-0.919

2.5Y

Standard

deviation

0.0152

0.0266

CONCLUSIONS

The findings of this study show that it is possible to expressoil from sesame seeds using simple techniques that could beadapted for the small and medium scale oil processors. Thehighestoil yieldof about 33.5%(corresponding to anexpression efficiency of 65.7%) was obtained when sesame seedswere conditioned to a moisture content of 6.1%, heated at85°C for 20 min and expressed at a pressure of 20 MPa.

Within the rangeof this study, the oil yield increased withanincrease inheating temperature (40 to85°C), heating time(5 to 27.5 min) and pressure (15 to 20 MPa), and with adecrease in moisture level (12 to 6%). The quality of oilexpressed (color, specific gravity and refractive index) wasnot affected by the processing conditions.

ACKNOWLEDGEMENTS

This study was sponsored by the Federal Ministry of ScienceandTechnology of Nigeria through GrantNo. 1426 KH and agrant from the National Farming Systems Network, Nigeria.

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AJIBOLA, OWOLARAFE, FASINA, and ADEEKO