Antinutrients and Phytochemicals in Food - American Chemical Society

Chapter 10

Cyanogenic Glycosides of Flaxseeds

Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on April 1, 2015 | http://pubs.acs.org Publication Date: April 1, 1997 | doi: 10.1021/bk-1997-0662.ch010

Fereidoon Shahidi and P. K. J. P. D. Wanasundara Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X9, Canada

Cyanogenic glycosides are secondary metabolites that are found in various plant tissues and produce HCN upon hydrolysis. They are widely distributed in the plant kingdom and are synthesized during metabolism of aromatic amino acids such as phenylalanine and tyrosine and branched amino acids such as leucine, isoleucine and valine. Flaxseed contains linamarin, linustatin and neolinustatin. Cyanogenic glycosides can be quantified in the intact form by chromatographic methods or indirectly by determining the content of HCN released due to their decomposition. The ability of cyanogenic glycosides to release HCN is due to their enzymic hydrolysis which may cause cyanide poisoning. Therefore, removal of cyanogenic glycosides is necessary to improve the nutritional value and safety of cyanogen containing foods including flaxseeds.

Cyanogenesis is the ability of the living tissues to produce hydrocyanic acid (HCN) and widely observed in the plant kingdom. H C N does not occur in the free form in higher plants but is released from cyanogenic precursors due to enzymic hydrolysis. The cyanogenic compounds of plants are usually carbohydrate derivatives, specifically β-glycosides of ct-hydroxynitriles (cyanohydrins). Cyanolipids are alternate sources of H C N in plants but are limited in nature. Cyanogenic glycosides of plants are nitrogen-containing secondary metabolites and are found in leaves, roots, seeds or other plant tissues. Several of the cyanophoric plants play important economic, dietary and nutritional roles and are extensively incorporated into human food and animal feed. Since the cyanogenic compounds are largely localized within the vacuoles of the cyanophoric plant cells, and their hydrolytic enzymes are cytoplasmic, damaging the cells leads to destruption of cellular integrity and results in enzymic hydrolysis of cyanogenic compounds and concomitant evolution of HCN. Therefore, cyanogenic glycosides are considered as part of the plant defence mechanism against

© 1997 American Chemical Society

In Antinutrients and Phytochemicals in Food; Shahidi, F.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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pest/insect damage (7). There are also evidences that cyanogenic glycosides in seeds serves as a form of stored nitrogen which can be converted to amino acids when there is a great demand for nitrogen such as germination (2). Table I summarizes the sources of cyanogenic glycosides and their estimated content of H C N released upon acid hydrolysis. Flaxseed is traditionally used for oil extraction for industrial purposes, but has recently been investigated for its potential use in value-added products. Presence of biologically-active phytochemicals such as cc-linolenic acid, lignans and soluble fibre has generated new and increased interest about the nutritional and therapeutical value of flaxseed. Due to the presence of a wide spectrum of biologically active phytoche micals in flaxseed, this oilseed has also been identified as an item for the "Designer Foods" project in the United States (5). The Canadian grown flaxseed contains, on the average 41% oil (on a moisture-free basis), 26% protein (%N χ 6.25), 4% ash, 5% acid detergent fibre and 24% nitrogen-free extract (4). However, presence of cyanogenic glycosides in flaxseed is a concern and limits its use of these seeds in large quantities in foods and livestock feed formulations. Therefore, it is desirable to remove cyanogenic glycosides from foods via processing or by employing biotechnological means. Chemistry of Cyanogenic Glycosides Both the vegetative parts and seeds of flax contain cyanogenic glycosides. Linamarin (2-[(6-0-P-D-glucopyranosyl)-oxy]-2-methylpropanenitrile) and lotaustralin ([(2R)-[(6-0-P-D-glucopyranosyl)-oxy]-2-methylbutanenitrile]) are the monosaccharide cyanogenic glycosides which may be present in flaxseed (Figure 1 ; 1 and 5). Smith et al. (6) have reported that two disaccharide cyanogenic glycosides (Figure 1) may also be isolated from flaxseed meal, namely linustatin (2-[(6-0-β-ϋglucopyranosy 1- β-D-glucopyranosyl)-oxy] -2-methylpropanenitrile) and neolinustatin ([(2R)-[(6-0-P-D-glucopyranosyl-P-D-glucopyranosyl)-oxy]-2-methylbutanenitrile]). Recent studies by Oomah et al (7) and Wanasundara et al (8) have shown the presence of linamarin, linustatin and neolinustatin in the seeds. The content of these three glycosides depends on the cultivar, location and year of production, with cultivar having the most important effect. The predominant cyanogenic glycoside of the Canadian cultivars is linustatin (213 to 352 mg/100 g seed) which accounts for 54 to 76% of the total content of cyanogenic glycosides (Table II). The content of neolinustatin ranges from 91 to 203 mg/100 g seed and linamarin is present in less than 32 mg/100 g seed (7) and are not present in some cultivars. Table III presents cyanogenic glycoside content of commercial flaxseed products available in Canada. Linamarin and lotaustralin are cyanohydrins of acetone and 2-butanone, respectively. The unstable cyanohydrin moiety is stabilized by a glycosidic linkage to D-glucose in linamarin and lotaustralin and to D-gentiobiose in linustatin and neolinustatin (Figure 1). Therefore, a close structural relationship exists among linamarin, linustatin, lotaustralin and neolinustatin. Linustatin and neolinustatin have been isolated as levorotatory crystalline solids. The IR spectra of these compounds show strong hydroxy 1 absorption (3400 cm ) and a weak absorption due to C ^ N (2240 c m ) with no carbonyl absorption. The C N M R spectra indicate that these -1

-1

, 3



Figure 1.

L - Isoleucine

H

2

( R )

" Lotaustralin

Ο - β - D - glucose

( R )

Neo

. linustarin

Cyanogenic glycosides found in food plants and their relationship to amino acids.

NH

Η


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ANTINUTRIENTS AND PHYTOCHEMICALS IN FOOD

Table I.

Cyanogenic food plants and their yield of H C N


Plant

1

H C N yield (mg/100 g fresh weight)

Almond (amygdalin) bitter seed young leaves

290 20

Apricot, seed (amygdalin)

60

Bamboo (taxiphyllin) stem, unripe tops of unripe sprouts

300 800

Cassava (linamarin and lotaustralin) less toxic clones, bark of tuber inner part of tuber leaves very toxic clones, bark of tuber inner part of tuber leaves

69 7 77 84 33 104

Flax, seedling tops (linamarin, linustatin and neolinustatin) Lima bean, mature seed (linamarin) Puerto Rico, small black Burma, white American, white Peach (prunasin) seed leaves Sorghum (dhurrin) mature seed etiolated shoot tips young green leaves Wild cherry, leaves (amygdalin) 1

91

400 210 10 160 125

0 240 60 90-360

Adapted from Ref. (75). Name of cyanogenic glycoside in each case is given in parenthesis.



1

Adapted from Réf. (P)

Andro Flanders A C Linora Linott McGregor Noralta NorLin Norman Somme Vimy

Cultivar 16.7 ± 3.8 13.8 ± 3.7 19.8 ± 5.4 22.3 ± 8.2 25.5 ± 4.0 20.3 ± 3.4 ND ND 27.5 ± 12.1 31.9 ± 8.3 342 282 269 213 352 271 295 231 322 262

± ± ± ± ± ± ± ± ± ±

38 55 28 29 56 34 46 63 46 31

Linustatin 203 ± 24 147 ± 22 122 ± 20 161 ± 2 5 91 ± 19 163 ± 18 201 ± 37 135 ± 37 149 ± 25 115 ± 2 1

Neolinustatin

Cyanogenic glycosides, mg/100 g seed

Cyanogenic glycoside content of flaxseed cultivars

Linamarin

Table II.

1


550 432 402 396 464 455 496 365 489 409

± ± ± ± ± ± ± ± ± ±

53 47 51 54 76 50 81 97 78 54

Total

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ANTINUTRIENTS AND PHYTOCHEMICALS IN FOOD

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Antinutrients and Phytochemicals in Food - American Chemical Society

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