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Wet decortication of sesame seeds by new methods


Yehia G. Moharram, Hussein O. A. Osman, and Yaldes I. A. Abou-EI-Khier

The process of dehulling sesame seeds not only is important because it eliminates oxalates in the hull, but it is also a preliminary step for preparing an attractive white, protein-rich flour, low in fibre content and free of bitterness. It also increases the yield of oil and improves the nutritional value of sesame products [1]. In Egypt, sesame seeds are used mainly for preparing tehina (sesame butter) and halawa tehinia (a sesame sweet), for some types of confectionery such as semsimia, and in the baking industry after dehulling and roasting.

El-Taibany [2] has described the dehulling and roasting of sesame seeds. After a sifting process, the clean seeds are soaked in fresh water for six to ten hours, depending on the sesame variety and the temperature of the water. The hulls of the soaked seeds are separated from the kernels in a brine solution. The washed kernels are usually roasted in steam-heated open roasters at 120 C for two hours. This time-consuming method needs a great deal of water and leads to a 19% seed loss.

The main aim of this work was to investigate new, more efficient methods of preparing sesame seeds for further processing, which could be used on an industrial scale.

Materials and methods

The two varieties of sesame seed (Sesamum indicum) used in this study, Sudany and Balady, were obtained from the ICA plant of the Bisco Misr Company in October 1984.

Physical and chemical methods

The water absorption of the two varieties of sesame seed was measured by the difference in weight of the seeds before and after soaking in water and Iye. The colour of the seeds before and after decortication by water or by lye and roasting for different periods at various temperatures was examined visually.

The moisture, salt (NaCI) content, and pH of the seeds before and after decortication were determined according to the methods of the AOAC [3].

Technological methods

Decortication

Figure 1 (see FIG. 1. Flow sheet for the decortication and roasting of sesame seeds) outlines the processes used for the wet decortication and roasting of sesame seeds. Trials were conducted both of decortication using water alone and of decortication with Iye. For the water decortication, the procedure and the dehulling unit of the Bisco Misr Company were used, as described by El-Taibany [2].

In the lye decortication procedure, the clean sesame seeds were immersed in the lye solution at various temperatures without stirring. The lye-treated wet seeds were then subjected to the following trials for hull separation: (1) manual decortication after the seeds were washed with fresh water, (2) rubbing of the lye-treated seeds in a saturated saline solution (NaCI) to separate the hulls from the kernels according to their specific gravities, after which the kernels were collected and washed with fresh water to remove both lye and salt, and (3) washing with fresh water to remove the remaining lye from the whole seeds, then roasting, followed by mechanical screening to separate the hulls from the kernels.

The factors studied included the use of NaOH, Na2CO3, and a mixture of both, as well as the temperature and the concentration of the lye used in soaking the seeds. The temperatures investigated were room temperature (about 22-25 C) and five-degree intervals from 30 to 65 C. Water was the soaking medium for the control samples.

Drying and roasting

Both the seeds that were treated with lye and washed with fresh water and the kernels obtained from water and lye decortication were subjected to roasting in a steam-heated open roaster equipped with continuously moving paddles to agitate the kernels. The roasting time was nearly two hours at 90-100 C. A control experiment was carried out in the laboratory using an electric oven instead of an open roaster to follow the changes in moisture content and colour of kernels during roasting. The cooled roasted seeds were sieved with a mechanical rotary sieve equipped with a suction air fan to remove the dust.

Results and discussion

Hull loosening

The effect of different concentrations of NaOH, Na2CO3, and a mixture of the two at various temperatures in loosening sesame hulls compared with water is shown in table 1 and table 2 . The lye solutions prepared from NaOH, Na2CO3, and the mixture were found to be more efficient and more rapid in loosening the hulls than the tap water used industrially. Both varieties of sesame seed needed the same soaking time for the same lye concentration. However, the percentage of lye inhibition and hull removal was lower for the Balady variety than for the Sudany variety. Generally, the percentage of lye concentration and the soaking time required for decortication as well as the percentage of lye imbibition and the colour changes of the kernels were much lower when NaOH was used than when Na2CO3 or the mixture was used.

The lye mixture of NaOH and Na2CO3 has a less corrosive effect on sesame than NaOH solutions. These results agree well with those reported by Moharram et al. [4], who were working on a laboratory scale.

If this technique were applied industrially, one would need to be aware of (1) the corrosive effect of the high lye concentration on equipment and pipes, (2) the hazards of handling the hot (60 C) lye solution, (3) the energy costs required to heat the lye to 60 C and to keep it at that temperature during the operation, and (4) the cost of the large quantities of water required to wash off the lye and the problems of its disposal.

The short time required for hull removal by this method appears to be an advantage in the laboratory, but industrially it may not be practical because of the hazards and disadvantages mentioned. We can conclude that the best medium for soaking sesame seeds industrially is a mixture of 3% Na2CO3 and 0.04% NaOH at 35 C with a seed-to-lye ratio of 1:3 (w/v). The suitable soaking time is 40 minutes. This is one eighth the time currently required industrially to loosen the sesame hulls. In other words, the time required to treat one ton of seeds in water would be sufficient to treat at least six to seven tons if the suggested lye mixture were less than that for water, thus allowing re-use of the lye solution after adjusting its volume and concentration. This is also an economic advantage.

TABLE 1. Effects of soaking in NaOH and Na2CO3 solutions for the removal of hulls from two varieties of sesame seed

  Temperature (C)

Sudany

Balady

Soaking time required (min.) Lye
imbibition
(%)
Loss in hull
removal (%)
Soaking time required
(min.)
Lye
imbibition
(%)
Loss in hull
removal
(%)
NaOH solution
0.125% 20 40 57 9.7 23 39 5.3
40 4 39.7 6.1 6 39 4.2
60 2 39 5.3 2 37.6 4.9
0.25% 20 23 43 9.5 22 38 4.7
40 4 49 7.7 4 37 4.5
60 2 39 5.8 2 38 5.1
3% 20 4 39.5 5.5 4 31 4.2
60 3 32 6.8 3 26.5 5
6% 20 2 34.6 5 2 27 4
60 6 28 5.3 6 25.5 4.3
Na2CO3 solution
3% 20 65 71 8.1 52 62 3.7
40 6 63.3 7.7 6 51 2.8

The ratio of seed to solution was 1:3 (w/v).

TABLE 2. Effects of soaking in a mixture of 0.04% NaOH and 3% Na2CO3 compared with soaking in water for the removal of hulls from sesame seed (Sudany variety)

Soaking medium and
temperature (C)
Soaking time required (min. ) Lye
imbibition ( % )
Moisture
( % )
Loss in hull
removal
( % )
Lye concentration (N)
Before Soaking After removal of seeds
Tap water
20 330 95 45 8.92
Lye mixture
22 148 75 38.63 6.31 0.18 0.11
30 41 67 37.8 4.72 0.18 0.14
35 39 67.2 37.9 4.62 0.18 0.14
40 34 67 37.85 4.92 0.18 0.14
45 32 65 37.9 4.46 0.18 0.14
50 26 65 38 6.13 0.18 0.14
55 21 60 37.3 5.91 0.18 0.14
60 17 60 37.3 5.71 0.18 0.14
65 10 37 4 96 4.96 0.18 0.14

The ratio of seed to solution was 1:3 (w/v).

Hull removal

Soaking the sesame seeds in the lye-mixture solution loosened the hulls, but the hulls still needed to be separated from the kernels. Trials were undertaken to determine a method of separation suitable for industrial use. The Sudany variety was used in these experiments.

Soaking in saturated bane

Soaking in saturated brine was also used to remove the hulls of the seeds treated with the lye mixture after washing with running water to remove the Iye. Figure 2 (see FIG. 2. Removal of hulls by soaking in brine) summarizes the procedures. It was found that (1) agitation for four minutes was important for hull removal; (2) agitation during soaking in the lye mixture to loosen the hulls caused disintegration of the seeds (fig. 2, B); (3) the use of the concentrated brine for both washing and rubbing the lye-treated seeds removed the hulls and produced roasted kernels with a salt content of 10.53% (C-1); and (4) the optimum conditions for removing the hulls after loosening them in the lye mixture were washing in water for three minutes to remove the lye and lower the pH from 9.8 to 7.1, soaking in 24 Baum brine solution 40 seconds rather than one minute to separate the hulls, and washing in water four rather than seven minutes to remove salt. Under these conditions, the salt content of the kernels after roasting was 2.5% (C-3) .

The data show that concentrated brine can be used successfully to remove the hulls of lye-treated seeds with the same equipment used for this purpose in the ICA plant, but with better results and marked savings in energy, the amount of water used' and the processing time. To apply this new suggested method on a small scale, a multipurpose wet dehulling unit was designed (see photograph).

The disadvantages of using brine to separate the hulls include (1) the quantity of water needed for preparing the brine and for washing, (2) the disposal of waste water and its effects on the drainage system, (3) the quantity and quality of salt required and problems that may arise from its preparation and storage, (4) the presence of a high salt content in the roasted kernels, which could adversely affect the quality of the tehina and halawa tehinia produced, and (5) the loss of hulls and small sesame seeds in the waste water.

Agitation and sieving

Because of the disadvantages of using brine, sieving of roasted seeds was also tried, and factors affecting this procedure were studied.

After the lye-treated seeds were washed in running water, some of the seeds were agitated with iron paddles for four minutes before roasting. The other seeds were roasted directly. In both cases, the roasting was carried out as in the ICA plant at 120 C for two hours in a steam-heated open roaster equipped with continuously moving paddles. The cooled, roasted seeds were sieved using a mechanical rotary sieve.

The following observations were made: The hulls of the unagitated seeds adhered strongly to the kernels and were not removed. The kernel yield was 94% in agitated seeds and 20.3% in unagitated seeds; the kernel yield of the agitated seeds approximated the amount of kernels present in the batch. The kernels obtained either with or without agitation were dark in colour and dull in appearance. According to these data, agitation is an important step in hull removal. The change in kernel colour and appearance may be the result of the time and the temperature of the roasting process.

Roasting conditions

Laboratory roasting

Sesame seeds were roasted at 95 C in an electric oven in the laboratory without agitation. Samples were drawn during roasting to estimate the moisture content, and the roasting was stopped when the moisture content was considered suitable for grinding to prepare tehina. The effects of this experiment are illustrated in table 3 and table 4.

TABLE 3. Changes in the moisture content of sesame seeds during laboratory roasting

Roasting time (min.)

Moisture content (%)

Water-treated

Lye-treated

0

42

34

20

32.8

28.5

40

28.8

24

60

21

17

80

16

12.4

90

12

9a

100

8.4a

6

115

3.6

1.5

120

2.3

-

125

1.4

-

a. At this level of moisture, the hulls were completely separated by sieving.

TABLE 4. Effect of sieving on hull removal after roasting in the laboratory (at 95 C) and steam roasting in the plant (120 C)

  Roasting time
(min. )
Moisture at hull removal ( % ) Roasting time to 1.8-2.4% moisture (min. ) Yield ( % ) Kernel colour Kernel appearance Other
observations
Laboratory  
lye-treated 90 8.4 115 95 better bright hull completely removed
water-treated 100 10 120 94 better bright hull completely removed
Plant  
Iye-treated 110 8 120 91 slightly dark slight corrosion
water-treated 110 10 120 95 slightly dark hull completely removed

The following points can be drawn from these results: (1) The moisture content of lye-treated seeds was lower and decreased more rapidly during roasting than that of the water-treated seeds. (2) The hulls separated completely upon sieving when the seed moisture content reached 8%-10%. At this level of moisture the kernel yield was 95% in the lye-treated seeds and 94% in the water-treated seeds. The time required to achieve this was 90 minutes for the lye-treated seeds and 100 minutes for the water-treated seeds. (3) Because at this level of moisture the kernels cannot be milled for tehina production, roasting was extended for another 15 minutes for the lye-treated kernels and 20 minutes for the water-treated kernels. (4) Both the appearance and the colour of the kernels were more desirable than previously obtained with other methods.

Industrial-scale roasting

The previous work was repeated on an industrial scale at 120 C using an open roaster. The data in table 4 indicate that the hulls can be separated by sieving after 110 minutes of roasting. After the kernels were re-roasted for 20 minutes, the yield was 91.9%, which was less than that obtained in the laboratory. This difference might be because agitation of the seeds during roasting may have led to a slight corrosion of the kernels.

To examine this assumption, lye-treated agitated sesame seeds were roasted in the laboratory at 95 C without agitation for 110 minutes and then sifted with the ICA plant sieve. The yield of kernels was 95%, with 2.4% moisture content. The colour of the kernels was improved and nearly the same as that of kernels separated in brine.

Summary

The hulls of lye-treated sesame seeds can be separated either with concentrated brine or by sieving agitated roasted whole seeds. Although removal of the hulls with brine produced kernels with a better appearance and colour than those obtained by sieving, sieving not only avoided the problems of the use of brine but also lowered the costs of sesame dehulling and decreased the temperature and time of roasting compared to methods currently used industrially. The hull fraction recovered can also be increased. The losses in dehulling were 5.8% instead of 8.3% on a dry-weight basis, and the kernels were very low in salt when sieving rather than concentrated brine was used.

A flow sheet for applying this suggested procedure can be obtained from the author.

Recommendations

The two new methods for sesame decortication suggested in this study depend on soaking in a lye mixture of 0.04% NaOH and 3% Na2CO3 for loosening the sesame hulls (table 1).

The differences between the two methods lie in the technique of hull removal. In the first method, the steps are (1) soaking a lye solution to loosen the hulls, (2) washing with water to remove the lye, (3) agitating for four minutes, (4) dipping in brine to separate the hulls, and (5) washing to remove the salt.

In the second method, the washed, lye-treated seeds are roasted at 95 C and then sieved to remove the hulls. The kernels obtained are free from salt. The costs and time for this method are less than those of the method used at present in the industry. The yield of roasted kernels is five times that produced by the present method.

The following techniques can therefore be recommended, with the hope that they will be adopted by the industry:

- the use of the suggested lye-mixture solution instead of water for loosening the sesame-seed hulls,
- the use of the modified brine technique to remove the hulls,
- sieving of the roasted, hull-loosened seeds.

References

  1. Villages AM, Gonzalez A, Calderon R. Microbiological and enzymatic evaluation of sesame protein. Cereal Chem 1968;45;379-85.
  2. El-Taibany AM. Technological studies on the separation of oil from tehina and halawa tehinia. M.Sc. thesis. Faculty of Agriculture, Alexandria University, Alexandria, Egypt, 1970.
  3. Association of Official Analytical Chemists. Official methods of analysis. Washington, DC: AOAC, 1975: 464, 1015.
  4. Moharram YG, Moustafa AM, Osman HOA. Studies on wet dehulling of Ehyptian sesame seeds by lye solutions. Lebensmittel Wissenschaft und Technologie 1981;14: 137-40.

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