Flaxseed Hull: Chemical Composition and Antioxidant Activity during Development

January 1, 2014 Human Health and Nutrition Data 0 Comments

Flaxseed Hull: Chemical Composition and Antioxidant Activity during Development

Year: 2014
Authors: Al Hjaili, A.D. Sakouhi, F. Sebei, K. Trabelsi, H. Kallel, H. Boukhchina, S.
Publication Name: J. Oleo Sci.
Publication Details: Volume 63(7):681-9.


Changes in the chemical composition and antioxidant activity of flaxseed hull during maturation were investigated.  P129 hull variety was studied at four maturation stages.  Significant variation in proximate composition of the hull was observed during development.  The main methyl esters were linolenic acid, oleic acid and linoleic acid.  The highest polyunsaturated fatty acids were found to be 67.14 per cent at the first stage of maturity.  Flaxseed hull oil was of good quality, containing an abundance of omega 3 essential fatty acids.  The iodine value increased, while the saponification value of oil decreased during seed development.  The decrease in ascorbic acid content was steady.  The maximum level of total phenolic acid content was reached at 7 DAF.  The antioxidant activity of oilseed was assessed by means of 2.2 diphenyl 1 picrylhydrazyl radical scavening assay.  Radical scavenging activity for green hull was 52.74 per cent and mature hull was 69.32 per cent. (Authors abstract)
The separation of flaxseed into hull and cotyledon fractions has been the subject of several studies.  When the moisture content of the seed was adjusted to 20 g per kg prior to grinding, the yield of cotyledon was 718 g per kg with an increase of 20 and 28 g per kg for protein and oil, respectively.  The hull, on a full fat basis, comprised 41.4 per cent  of the seed, however, on an oil and moisture free basis; it constituted essentially 60 per cent of the flaxseed meal.  It had a nitrogen value approximately one third that of the cotyledon fraction which contained 96.7 per cent of the total oil. The hull, including the seed coat and endosperm, constitutes 36 per cent of the total weight of hand dissected flaxseed or 22 per cent of the seed when obtained mechanically.  Flaxseed hull is difficult to digest and therefore hinders access to the lipids.  Oil content of flaxseed hulls varies from 26 per cent to 30 per cent depending on processing conditions representing approximately 18 per cent of the total seed oil.  This oil from hulls obtained by dry abrasive hulling contained significantly higher levels of palmitic acid and lowest level of stearic and oleic acids compared to those from the whole seed.  Flaxseed hull is enriched in SDG compared to the cotyledons.  The objectives of the present study were to investigate the phytochemical profile and the antioxidant activity during hull development to increase its value and contribution to the development of new omega 3 products.
It is shown that linolenic acid was the major fatty acid (C18:3) during flaxseed hull development.  The high amount of C18:3 in flaxseed hull oil suggest an important activity of ∆ 15 desaturase.  The fatty acids C18:1 and C18:2 were also well represented with mean values, respectively, of around 21 and 16 per cent of total fatty acids.  The changes in physicochemical characteristics of flaxseed hull oils during development are presented.  A decrease was observed in saponification value as maturity progressed.  The saponification values of flaxseed hull oil vary from 198 mg KOH per g oil to 178 mg KOH per g oil.  In all samples studied, free fatty acid contents were less than 3 during all stages of flaxseed development.  High FFA content causes problems in the transesterification process.  During flaxseed hull development, Iodine value was found in the range  160 to 170.  During the early stages of flaxseed development, total phenolic acids content in flaxseed hull oil was higher (128.3 mg gallic acid per 100 g oil) and then decreased during maturation to achieve the harvest values (62.40 mg gallic acid per 100 g oil). 
The DPPH test aims at measuring the capacity of the flaxseed hull oil to scavenge the stable free radical 2, 2 diphenyl 1 picrylhydrazyl (DPPH) by donation of hydrogen atom or an electron.  If the extracts have the capacity to scavenge the DPPH free radical, the initial blue to purple solution will change to a yellow colour due to the formation of disphenylpicrylhydrazine.  The results of scavenging activity of this study were 52.74 per cent to 78.55 per cent during flaxseed hull development.  This result showed more ripened of hull had a higher of DPPH.  It is possible that the antioxidative activity of flaxseed hull oil are caused, at least in part, by the presence of polyphenols and other yet to be
discovered antioxidant compounds.  According to studies, lignan occurs mainly in the hull of flaxseed. 
This research describes a wide range of physicochemical quality characteristics of flaxseed hull oil.  Oil and protein contents were highly accumulated on the third stage.  Whereas ascorbic acid, total phenolic acid and unsaponifiable contents reached their maximum on the first stage of flaxseed hull development.  According to the results, more ripened stages of flaxseed hull oil resulted lower of total phenolic acid and flavanoids content but higher of DPPH value. (Editors comments)

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