Chemical composition

Sesame seed has a high food value due to its high content of oil and protein. The composition is markedly influenced by genetic and environmental factors (Kinman and Stark, 1954; Lyon, 1972). The seeds contain 6-7% moisture, 17-32% protein, 48-55% oil, 14-16% sugar, 6-8% fibre and 5-7% ash. The proximate composition of sesame seeds is given in Table 17.1.

In general, Indian varieties tend to be lower in protein and higher in oil than Sudanese varieties, such as those generally appearing in the export market, and are commercially used in the USA. The hull content averages about 17% of the sesame seed, and contains large quantities of oxalic acid, calcium, other minerals and crude fibre. Thus, when using sesame for human food, it is advisable to remove the hull. When the seed is properly dehulled, the oxalic acid content is reduced from about 3% to less than 0.25% of the seed weight (Nagaraj, 1995). Screw-pressed, dehulled sesame contains about 56% protein, while the solvent extracted meal contains more than 60% protein. This is mostly used in feed, except in India where it is used as a food.

17.2.1 Lipids

Content

Sesame seeds contain more oil than many other oilseeds. Oil content varies with genetic and environmental factors. A wide range of the oil content from 37 to 63% has been reported in sesame seeds (Lyon, 1972; Swern, 1979; Bernardini, 1986). Oil content in seeds also varies considerably among different varieties and also growing seasons (Lyon, 1972; Yen et al., 1986). The oil content is also related to the colour and size of the seeds. White or light coloured seeds usually have more oil than the dark seeds and smaller seeds contain more oil than larger seeds (Seegeler, 1983). Rough-seeded cultivars generally have a lower oil content than smooth-seeded types (Yermanos et al., 1972).

Agronomic factors also influence the seed oil content. It increases with increasing length of photoperiod and early planting dates (Arzumanova, 1963; Abdel-Rahman et al., 1980). Likewise, the seeds from plants with a short growing period tend to have higher oil content than those from plants with a medium to long growing cycle (Yermanos, 1978). Heavy application of nitrogen fertilizer reduces oil content of sesame seeds (Singh et al., 1960).

Classification

The lipids of sesame seeds are mostly composed of neutral triglycerides with small quantities of phosphatides (0.03-0.13% with lecithin : caphalin ratio of 52 : 46). The phosphatides also contain about 7% of a fraction soluble in hot alcohol but are insoluble when cold. Sesame oil, however, has a relatively high percentage (1.2%) of unsaponifiable matter (Johnson and Raymond, 1964; Weiss, 1983). The glycerides are mixed in type, principally oleo-dilinoleo, linoleo-dioleo triglycerides and triglycerides with one radical of a saturated fatty acid combined with one radical each of oleic and linoleic acids (Lyon, 1972). The glycerides of sesame oil, therefore, are mostly triunsaturated (58 mol%) and diunsaturated (36 mol%) with small quantities (6 mol%) of monounsaturated glycerides. Trisaturated glycerides are almost absent in sesame oil. The unsaponifiable matters in sesame oil include sterols (principally comprising P-sitosterol, compesterol and sigma sterol), triterpenes (triterpene alcohols which include at least six compounds, of which three were identified, viz. cycloartanol, 24-methylene cycloartanol and amyrin), pigments, tocopherols and two compounds that are not found in any other oil, namely sesamin and sesamolin (Fukuda et al., 1981, 1988). Sesamin and sesamolin are responsible for the characteristic Baudouin or Villavecchia tests of sesame oil. Among the pigments spectroscopically identified, pheophytin A (^max = 665-670 nm) was found to markedly predominate over pheophytin b (^max = 655 nm) (Lyon, 1972). The pleasant aroma and taste principles contain C5-C9 straight-chain aldehydes and acetylpyrazine (Swern, 1979).

Fatty acid composition

Sesame oil contains about 80% unsaturated fatty acids. Oleic and linoleic acids are the major fatty acids and are present in approximately equal amounts (Lyon, 1972). The saturated fatty acids account for less than 20% of the total fatty acids. Palmitic and stearic acids are the major saturated fatty acids in sesame oil (Table 17.2). About 44 and 42% of linoelic and oleic acids and 13% saturated fatty acids are found in sesame oil (Smith, 1971). Arachidic and linolenic acids are present in very small quantities (Rao and Rao, 1981).

Table 17.2 Fatty acids composition of sesame oil (% of total fatty acids)

Fatty acid

Godin and Spensley

Yermanos

Seegeler

Maiti et al.

(1971)

(1978)

(1983)

(1988)

Palmitic

7-9

8.3-10.9

8.4-10.3

7.8-9.1

Stearic

4-5

3.4-6.0

4.5-5.8

3.6-4.7

Arachidic

8

-

0.3-0.7

0.4-1.1

Oleic

37-50

32.7-53.9

39.5-43.0

45.3-49.4

Linoleic

37-47

39.3-59.0

41.0-45.0

37.7-41.2

Endogenous antioxidants

Among the commonly used vegetable oils, sesame oil is known to be most resistant to oxidative rancidity (Budowski, 1950). It also exhibits noticeably greater resistance to autooxidation than would be expected from its content of tocopherols (vitamin E). This high stability to oxidation is often attributed to the presence of a large proportion of unsaponifiable matter. Moreover, the unsaponifiable matter itself includes substances such as sesamol and phytosterol that are normally not found in other oils. Sesamolin upon hydrolysis, yields sesamol. Sesame oil contans 0.5-1.0 % sesamin (Budowski et al., 1951) and 0.3-0.5% sesamolin (Budowski et al., 1950), with only traces of free sesamol (Beroza and Kinman, 1955; Budowski, 1964). Sesamol is released from sesamolin by hydrogenation, by acid or acid bleaching earth or other conditions of processing and storage (Budowski and Markley, 1951; Beroza and Kinman, 1955). Free sesamol is, however, removed by some blending earths or during the deodorization process, which results in decreased stability of sesame oil (Budowski and Markley, 1951; Mathur and Tilara, 1953; Budowski, 1964). Structures of natural antioxidants found in oil from sesame are depicted in Figs. 17.1 and 17.2.

Fig. 17.1 Structures of natural antioxidants found in sesame oil: (a) sesamin; (b) sesangolin; and

Fig. 17.1 Structures of natural antioxidants found in sesame oil: (a) sesamin; (b) sesangolin; and

Fig. 17.2 Structures of natural antioxidants found in sesame oil: (a) sesamolin; (b) sesamol; (c) sesamol dimer; and (d) sesamol dimer quinone.

Fig. 17.2 Structures of natural antioxidants found in sesame oil: (a) sesamolin; (b) sesamol; (c) sesamol dimer; and (d) sesamol dimer quinone.

Properties of oil

Sesame oil is deep to pale yellow in colour. It is fragrant or scented. It has a pleasant odour and taste. The aroma components were identified as C5 to C9 straight chain aldehyde or ketone derivatives (Lyon, 1972). Some of the important characteristics of sesame oil are given in Table 17.3. Sesame oil is dextrorotatory, which is unusual for an oil devoid of optically active fatty acid glycerides. The unsaponifiable fraction of the oil, however, does contain optically active minor constituents, which are responsible for the optical rotation of the oil.

Nutritional importance

Sesame oil is practically free of toxic components. The oil contains more unsaturated fatty acids than many other vegetable oils. The high proportion of unsaturated fatty acids renders sesame oil an important source of essential fatty acids in the diet (Langstraat and Jurgens, 1976). Linoleic acid is required for cell membrane structure, cholesterol transportation in the blood and for prolonged blood clotting properties (Vles and Gottenbos, 1989). Sesame oil is rich in vitamin E, but deficient in vitamin A. The crude oil contains a relatively low amount of free fatty acids. The minor constituents in sesame oil, sesamin and sesamolin, protect the oil from oxidative rancidity.

Table 17.3 Characteristics of sesame oil

Character

Andraos et al. (1950)

Lyon (1972)

Seegeler (1983) Weiss (1983)

Specific gravity (25%/25°C)

0.918

0.918-0.921

0.916-0.921

0.922-0.924

Refractive index (n50D)

1.463

1.472-1.474

1.463-1.474

1.458

(25°C)

(25°C)

(25°C)

(60°C)

Smoke point (°C)

165

-

166

-

Flash point (°C)

319

-

375

-

Solidifying point (°C)

-

-

-3 to -4

-3 to -4

Titre (°C)

22

20-25

20-25

22-24

Free fatty acids (as % oleic)

1.0

-

1.0-3.0

1.0-3.0

Unsaponifiable matter (%)

2.3

1.8

0.9-2.3

0.9-2.3

Iodine value

112

104-118

103-130

103-126

Saponification value

186

187-193

186-199

188-193

Reichert-Meissel value

0.51

-

0.1-0.2

0.1-1.0

Polenske value

0.4

-

0.10-0.50

-

Hydroxyl number

5.3

-

1.0-10.0

1.0-10.0

Thiocyanogen value

76

-

74-76

74-76

Hehner value

-

-

96.0

Content and characterization

Sesame seed contains 17-32% protein with an average of about 25% (Joshi, 1961; Lyon, 1972, Yen et al., 1986). The proteins in the seed are located mostly in the outer layers of the seed. Based on their solubility, sesame proteins have been classified as albumin (8.6%), globulins (67.3%), prolamin (1.3%) and glutelin (6.9%) fractions (Rivas et al., 1981).

As in most other seeds, globulin is the predominant protein fraction in sesame seeds (Guerra et al., 1984). It is composed of two components. a-Globulin is the major fraction and accounts for about 60-70% of the total seed globulin, while ß-globulin is a minor component contributing 25% to the globulin fraction (Nath and Giri, 1957; Rajendran and Prakash, 1988). a-Globulin is a high molecular weight protein (250 000-360 000 MW) and has a sedimentation coefficient of 11-13 S. It is an oligomeric protein composed of six dimeric units of molecular weight of about 50 000-60 000. The dimeric unit is of the A-B type linked by a disulphide bond (Robinson, 1987). The quaternary structure of a-globulin has been well established (Plietz et al., 1988). ß-Globulin is the minor component of sesame seed globulins. It has a molecular weight of 150 000 and is rich in acidic and hydrophobic amino acids (Plietz et al., 1988).

Nutritional quality

The essential amino acid composition of sesame seed proteins (Table 17.4) indicates that sesame proteins are rich in sulphur-containing amino acids, particularly methionine (Smith, 1971; Brito, 1981; Narasinga Rao, 1985) and also tryptophan (Johnson et al., 1979; Yen et al., 1986). Sesame proteins are, however, deficient in lysine (Evans and Bandemer, 1967; Cuca and Sunde, 1967; Narasinga Rao, 1985; Sawaya et al., 1985; Yen et al., 1986), which is unusual for oilseed proteins. Among other essential amino acids, sesame protein is borderline deficient for threonine, isoleucine and valine contents compared with Food and Agriculture Organization (FAO) reference values (Nath et al., 1957). During preparation of a protein isolate (>90% protein), there is some loss of methione, cystine and tryptophan.

Table 17.4 Essential amino acid composition of sesame meal proteins (g/16 g N)

Amino acid

Evans and

Smith

Rivas et al.

Gopalan

Narsinga

FAO/WHO

Bandemer

(1971)

(1981)

et al.

Rao (1985)

(1973)

(1967)a

(1982)

Arginine

12.0-13.0

11.9

12.5

12.0

-

-

Histidine

2.3-2.8

2.2

2.4

2.7

-

-

Isoleucine

3.3-3.6

4.3

3.9

4.0

4.2

4.7

Leucine

6.5-7.0

6.9

6.7

8.0

7.4

7.0

Lysine

2.5-3.0

2.8

2.6

2.7

2.6

5.5

Methionine

2.5-4.0

2.7

2.5

2.9

2.8

3.5b

Cystine

1.1-2.2

-

-

1.9

-

-

Phenylalanine

4.2-4.5

4.7

4.5

5.9

6.4

-

Tyrosine

-

-

3.7

3.7

-

6.0c

Threonine

3.4-3.8

3.6

3.4

3.7

3.1

4.0

Tryptophan

2.0-2.4

1.9

-

1.3

1.5

1.0

Valine

4.2-4.4

5.1

4.7

4.6

3.9

5.0

a Range for five varieties. b Methionine + cystine. c Phenylalanine + tyrosine.

a Range for five varieties. b Methionine + cystine. c Phenylalanine + tyrosine.

This may reflect the selective recovery or elimination of certain proteins by the isolation methods employed. The protein nutritive value of sesame is 15 to 42, relative to casein as 100 (Evans and Bandemer, 1967). Supplementation of sesame seed proteins with 0.2% lysine significantly increased their protein nutritive value, and nutritive value of sesame protein supplemented with 0.2% lysine + 0.1% methionine + 0.1% isoleucine was almost comparable to that of casein (Evans and Bandemer, 1967). The net protein utilization (NPU) of sesame meal has been reported to be 0.56 as compared with 0.74 for whole egg powder (Fisher, 1973).

Supplementation of sesame meal with 0.5% L-lysine increased the NPU to 0.63. When sesame protein was used at 20% level as the only source of protein in the chick diet, good growth was obtained by supplementing it with 0.5% lysine (Smith, 1971). Sastry et al. (1974) reported that supplementation of sesame flour with 1.25% lysine improved the nutritive value of proteins, making them comparable to that of skim milk powder. Supplementation of sesame diet at 18% protein level with threonine significantly improved the chick growth (Cuca and Sunde, 1967).

The protein efficiency ratios (PER) of sesame seed, meal and isolated protein are 1.86, 1.35 and 1.2, respectively (Narasinga Rao, 1985). Commercially prepared flour and press cake showed PER values of 0.9 and 1.03. Supplementation of sesame seed protein with lysine can increase its PER to 2.9.

The amino acid composition of sesame complements that of most other oilseed proteins. Tryptophan, which is limiting in many oilseed proteins, is adequate in sesame. The availability of amino acids from sesame seed protein is affected by the method of processing. Digestibility is enhanced by heat treatment under moist conditions, while screw pressing for oil recovery apparently has little adverse effect on available lysine. However, in vitro digestibility was reportedly the same for the isolated sesame protein before and after autoclaving, indicating a lack of trypsin inhibiters (Kinsella and Mohite, 1985).

The problems encountered during addition of limiting amino acids to achieve nutritional adequacy (Lis et al., 1972) are overcome by covalent attachment and the application of plastein reaction (Fujimaki et al., 1977). Lysine-enriched plasteins have been prepared from

Table 17.5 Sugar content of sesame seeds and defatted flour (% on dry weight)

(Aguilar and Torres, 1969)

Defatted flour (Wankhede and Tharanathan, 1976)

d-Glucose

1.55

3.63

d-Galactose

0.65

0.40

d-Fructose

0.24

3.43

d-Fructose

0.34

0.17

Raffinose

-

0.59

Stachyose

-

0.38

Planteose

0.06

0.23

Sesamose

-

0.14

Other sugars

-

0.16

Total sugars

-

11.26

sesame protein using N-e-cbz-lysine methyl ester and also enzymatic hydrolysates of casein or soybean proteins. Plasteins obtained with N-e-cbz-lysine methyl ester had a yield of 40% for sesame and the lysine content was 16-19% (Susheelamma, 1983).

The high level of sulphur-containing amino acids in sesame seed proteins is unique. It suggests that sesame protein should be more widely used as a supplement for methionine and tryptophan and should be an excellent protein source for baby and weaning foods. The use of sesame seed protein would eliminate the problems encountered when foods are supplemented with free methionine, which is unstable.

17.2.3 Carbohydrates

The carbohydrate content of sesame seeds is comparable to that of groundnut seeds and is higher than that of soybean seeds (Joshi, 1961). Sesame seeds contain 14-25% carbohydrates. The seeds contain about 5% sugars, most of which are of reducing type. Defatted sesame meal contains more sugars. The sugar contents of sesame seeds and defatted flour are given in Table 17.5. Sesame seeds are reported to contain 3-6% crude fibre (Ramachandra et al., 1970; Gopalan et al., 1982; Taha et al., 1987). The crude fibre is present mostly in husk or seed coat (Narasinga Rao, 1985). Wankhede and Tharanathan (1976) reported 0.582.34% and 0.71-2.59% hemicellulose A and B, respectively, in defatted flour. Hemicellulose A was found to contain galacturonic acid and glucose in the ratio of 1 : 12.9 while hemicellulose B contained galacturonic acid, glucose, arabinose and xylose in the ratio of 1:3.8:3.8:3.1.

17.2.4 Minerals

Sesame seed is a good source of certain minerals particularly calcium, phosphorus and iron (Table 17.6). The seeds contain a total of 4 to 7% minerals. Deosthale (1981) reported 1% calcium and 0.7% phosphorus in the seeds. They also contain sodium and potassium. Calcium is mostly present in the seed coat which is lost during dehulling. Further, the bioavailability of calcium from sesame is less than that from milk or bread, probably because of the high concentration of oxalate and phytate in the seed. Poneros-Schneier and Erdman (1989) reported the bioavailability of calcium from some food products, relative to CaCO3 as non-fat dry milk 100%; whole-wheat bread 95%; almond powder 60%; sesame seeds

Table 17.6 Mineral content of sesame whole seeds (mg/100 g)

Mineral

Joshi(1961)

Agren and Gibson (1968)

Gopalan et al. (1982)

Weiss (1983)

White seeds

Brown seeds

Calcium

1000

1017

1483

1450

1160

Phosphorus

700

732

578

570

616

Iron

20

56

-

10.5

10.5

Total

5700

5600

6200

5200

5300

65%; and spinach 47%. Sesame grown on selenium-rich soils also contains high selenium, although most of it is present in the hulls (Kinsella and Mohite, 1985).

17.2.5 Vitamins

Sesame seeds are an important source of certain vitamins, particularly niacin, folic acid and tocopherols (Gopalan et al., 1982; Weiss, 1983). The vitamin A content of seeds is, however, very low (Table 17.7). Vitamin E group includes several tocopherols, isomers and derivatives that differ in their biological activity (Table 17.8). The vitamin E activities of a-, P-, y- and 8-tocopherols and tocotrienol are in the ratio of 100, 40, 10, 1 and 30 (McLaughlan and Weihraugh, 1979). Sesame oil is rich in tocopherols. However, the proportion of 8-tocopherols is more than that of a-tocopherols. Therefore, the vitamin E activity of sesame oil is less than that of sunflower oil.

Table 17.7 Vitamin content of whole sesame seeds

Vitamin

Agren and Gibson (1968)

Gopalan et al. (1982) Weiss (1983)

Seegeler (1983)

White seeds

Brown seeds

Vitamin A (IU)

-

60a

30

Trace

Thiamin

(mg/100 g) Riboflavin

0.22

0.14

1.0

0.98

1.0

(mg/100 g) Niacin

0.02

0.05

0.34

0.24

0.05

(mg/100 g) Pantothenic acid

7.3

8.7

4.4

5.4

5.0

(mg/100 g) Folic acid

-

-

-

-

0.6

(Mg/100 g) Free

-

-

51

-

-

Total

-

-

134

-

-

Ascorbic acid

(mg/100 g)

-

-

-

-

0.5

Table 17.8 Vitamin E active compounds in sesame and sunflower oils (mg/100 g oil)

Compound

Sesame oil

Sunflower oil (Speek et al., 1985)

Muller-Mulot (1976)

Speek et al. (1985)

a-tocopherol

1.2

1.0

78.8

ß-tocopherol

0.6

<0.05

2.5

y-tocopherol

24.4

51.7

1.9

S-tocopherol

3.2

<0.05

0.7

Total tocopherol

29.4

52.8

83.9

Vitamin E activity

(a-tocopherol

equivalent)

-

14.9

79.0

17.2.6 Antinutritional factors

Sesame seed is nearly free of antinutritional factors and is suitable for human consumption as such or after processing. Sesame seeds, however, contain high amounts of oxalate (Deosthale, 1981; Narasinga Rao, 1985) and phytic acid (Prakash and Nandi, 1978; Johnson et al., 1979). Sesame seeds contain about 1-2% oxalic acid. Gopalan et al. (1982) reported 1.7% oxalic acid in the seeds. The high proportion of oxalate reduces the physiological availability of calcium from the seeds. The oxalic acid in sesame seeds is mostly present in the testa or the hull portion. The presence of testa imparts a slightly bitter taste to the whole seed or meal because of chelation of calcium by oxalic acid. Dehulling reduces the oxalic acid content of the seeds. Oxalic acid may also be removed from sesame meal by treating it with hydrogen peroxide at pH 9.5.

Sesame seeds contain a substantial amount of phosphorus. However, most of this phosphorus is tied up in phytic acid or as phytin, a calcium and magnesium salt of inositol hexaphosphate. The seeds have phytate levels among the highest found in nature (De Boland et al., 1975). Phytic acid is a strong chelating agent and binds dietary essential minerals such as calcium, iron and zinc to form phytate-mineral complexes (Reddy et al.,

1982). The formation of such complexes decreases the bioavailability of these minerals (Oberleas et al., 1966; Kon et al., 1973). The phytate in sesame meal is insoluble in water. O'Dell and De Boland (1976) extracted phytate from the meal by dilute HCl (0.3 m) and precipitated it with NaOH. The insoluble phytate had a composition of NaMg-phytate, suggesting that phytate in sesame meal exists as a magnesium phytate and not as phytin (CaMg-phytate).

Sesame oil contains two minor constituents, namely sesamin (0.5-1.0 %) and sesamolin (0.3-0.5%). Sesamolin upon hydrolysis yields sesamol (Godin and Spensley, 1971). Although the nutritional significance of sesamin and sesamolin is not clear, sesamol has been reported to be partially responsible for the resistance of sesame oil to oxidation (Weiss,

1983). Sesame plants seem to have an unusual capacity for lead accumulation in the seeds. Yannai and Haas (1973) reported that whole sesame seeds and kernels contained lead at the level of 0.13-0.22 mg/100 g. A high consumption of sesame (>200 g/day) is therefore considered to be harmful to humans.

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