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Extraction and modification of hemicellulose from wheat bran to produce entrapment materials for the controlled release of chemicals and bioactive substances

Matavire, Thokozani Olga (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH SUMMARY: Hydrogels are a network of cross-linked hydrophilic polymers that form an insoluble three-dimension structure capable of imbibing large amounts of fluid and they can be used as entrapment matrices for substances. However, synthetic polymers used for the formation of entrapment matrices cause health and environmental concerns, which leads to a need for environmental benign alternatives. Hemicellulose biopolymer is a potential replacement for the petroleum based materials as it is abundant, renewable and biodegradable. Agro-residues such as wheat bran are an abundant and affordable source of hemicellulose as they are not primary food products and would reduce environmental concerns related to their disposal. However, hemicellulose is bound to other components in a complex structure and extraction methods need to be carefully considered to maintain functional properties required for formation and performance of entrapment materials as slow release devices. Furthermore, hemicelluloses are water soluble due to the presence of side groups along the backbone and low molecular weight and that limits its application as insoluble entrapment matrices. The solubility of hemicellulose can be modified physically, chemically and enzymatically to produce insoluble entrapment matrices or hydrogels for the delivery of chemical and bioactive substances. The aim of this study was to develop insoluble entrapment matrices from soluble hemicellulose extracted from wheat bran agro-residue as delivery devices for chemical and bioactive substances. The type of hemicellulose extracted from wheat bran was arabinoxylan. The extraction of soluble arabinoxylan from wheat bran was performed using alkaline method after pre-treatment of wheat bran to remove starch. A full factorial design was used to determine the effects sodium hydroxide (NaOH) concentration, solid loading, extraction time and temperature on xylan extraction. A face centered central composite design was subsequently used to optimize the significant (p<0.05) factors sodium hydroxide concentration, extraction time and temperature with yield and purity as the dependent variables. The highest arabinoxylan yield and purity in terms of arabinoxylan content was 63% and 53%, respectively. The extracted arabinoxylans contained no monomeric sugars indicating that they were in polymeric form and thus were suitable to form stable hydrogels. The extracted hemicellulose was used to form insoluble hydrogels using: (1) coacervation method based on neutralizing an alkaline xylan solution with acid and (2) side chain removing enzyme α-arabinofuranosidase that cleaved the arabinose side groups along the xylan backbone. The hydrogels were applied as delivery systems for gallic acid. The hydrogels were assessed for size, stability, encapsulation efficiency of gallic acid, antioxidant activity of encapsulated gallic acid and chemical structure. The wheat bran arabinoxylan hydrogels formed were in nanosize range (469 – 678nm), however chemically formed hydrogels were smaller compared to enzymatically formed hydrogels. Enzymatically formed hydrogels had smaller size distribution with a polydispersity index (PDI) of up to 0.3 whilst chemically formed hydrogels had a broad size distribution and PDI of 1. In addition, the zeta potential of enzymatically formed hydrogels was more negative compared to chemically formed hydrogels. The more negative zeta potential indicated agglomeration of hydrogels is less, hence enzymatically formed hydrogels were more stable than chemically formed hydrogels. The encapsulation efficiency was up to 72% and 59% for chemically and enzymatically formed hydrogels, respectively. It is worth noting that encapsulating gallic acid before and after formation of hydrogels resulted in differences in encapsulation efficiency. The enzymatically formed hydrogels better preserved the integrity of encapsulated gallic acid with an antioxidant activity of 91% as compared to 80% for chemically formed hydrogels. The release of gallic acid was sustained when the encapsulation of gallic acid occurred during the formation of hydrogels as compared to encapsulation after the formation of hydrogels. Despite enzymatic hydrogels having lower encapsulation efficiency, they showed sustained release of encapsulated gallic acid. In conclusion, chemically and enzymatically modified wheat bran arabinoxylan hydrogels were formed with the ability to entrap and release gallic acid.

‘n Hidrojel is ‘n netwerk van kruisgebonde hidrofiliese polimere wat ‘n onoplosbare drie-dimensionele struktuur vorm. Dit is in staat om groot hoeveelhede vloeistof op te neem en kan gebruik word om verbindings vas te vang. Daar bestaan egter kommer oor die gesondheids- en omgewingsimpak van die sintetiese polimere wat gebruik word in die produksie van sulke vasvang matrikse. Derhalwe bestaan daar ‘n behoefte aan omgewingsvriendelike alternatiewe. Hemisellulose biopolimeer is ‘n potensiële vervanging vir petroleum gebasseerde materiale omdat dit volop, herwinbaar en biodegradeerbaar is. Landboukundige residue soos koringsemels is ‘n volop en bekostigbare bron van hemisellulose omdat dit nie primêre voedselprodukte is nie en daar bestaan minder kommer met betrekking tot hul omgewingsvriendelike wegdoening. Hemisellulose is egter gebind aan ander komponente in ‘n komplekse struktuur. Hemisellulose ekstraksiemetodes moet derhalwe deeglik oorweeg word om die funksionele eienskappe te behou wat benodig word vir die vorming en werksverrigting van verstrikkingsmatrikse as stadige vrystellingsapparate. Hemiselluloses het lae molekulêre gewigte en is verder ook wateroplosbaar weens die teenwoordigheid van sykettings al langs die polimeer ruggraat af. Hierdie eienskappe beperk die toepassing daarvan as onoplosbare verstrikkingsmatrikse. Die oplosbaarheid van hemisellulose kan fisies, chemies en ensiematies gewysig word om onoplosbare verstrikkingsmatrikse of hidrojels te produseer vir die aflewering van chemiese en bio-aktiewe substanse. Die doel van hierdie studie was om onoplosbare verstrikkingsmatrikse te ontwikkel vanuit oplosbare hemisellulose, geekstraheer uit koringsemel landboukundige residue, om te dien as afleweringsisteme vir chemiese en bio-aktiewe substanse. Die tipe hemisellulose wat geekstraheer is uit koringsemels was arabinoxilaan. Die ekstraksie van oplosbare arabinoxilaan uit koringsemels is verrig deur gebruik te maak van ‘n alkaliese metode na afloop van vooraf behandeling van die koringsemels om stysel te verwyder. ‘n Volledige faktoriale ontwerp is gebruik om die effek te bepaal van natriumhidroksied (NaOH) konsentrasie, soliede lading, ekstraksie temperatuur en ekstraksietyd op xilaan ekstraksie. ‘n Gesig-gesentreerde sentraal saamgestelde ontwerp is gebruik om die statisties noemenswaardige (p < 0.05) faktore, naamlik natriumhidroksied konsentrasie asook ekstraksietyd en –temperatuur, te optimiseer met opbrengs en suiwerheid as die afhanklike veranderlikes. Die hoogste arabinoxilaan opbrengs en suiwerheid in terme van arabinoxilaan inhoud was 63% en 53%, onderskeidelik. Die geekstraheerde arabinoxilane het geen monomeriese suikers bevat nie, wat aandui dat hulle in polimeriese vorm was en daarom geskik was om stabiele hidrojels te vorm. Die geekstraheerde hemisellulose is aangewend om onoplosbare hidrojels te vorm deur gebruik te maak van: (1) ‘n faseskeidingsmetode gebasseer op die neutralisasie van ‘n alkaliese xilaanoplossing met suur en (2) die syketting verwyderende ensiem α-arabinofuranosidase wat die arabinose sykettings afgesplyt het al langs die xilaan ruggraat af. Die hidrojels is aangewend as afleweringsisteme vir gallussuur. Die hidrojels is geassesseer in terme van grootte, stabiliteit, inkapselingseffektiwiteit vir gallussuur, anti-oksidant aktiwiteit van gallussuur en chemiese struktuur. Die koringsemel arabinoxilaan hidrojels wat gevorm is, het groottes gehad wat gevarieer het in die nanogrootte gebied (469-678 nm). Chemies gevormde hidrojels was egter kleiner as ensiematies gevormde hidrojels. Ensiematies gevormde hidrojels het ‘n kleiner verspreiding in grootte gehad met ‘n polidispersie indeks (PDI) van tot 0.3, terwyl chemies gevormde hidrojels ‘n wye verspreiding in grootte gehad het en ‘n PDI van 1. Daarbenewens was die zeta potensiaal van die ensiematies gevormde hidrojels meer negatief in vergelyking met dié van chemies gevormde hidrojels. Die meer negatiewe zeta potensiaal het aangedui dat agglomerasie van hidrojels tot ‘n mindere mate plaasgevind het, met die gevolg dat die ensiematies gevormde hidrojels meer stabiel was as die chemies gevormde hidrojels. Die inkapselingseffektiwiteit was so hoog as tot 72% en 59% vir chemies en ensiematies gevormde hidrojels, onderskeidelik. Dit is noemenswaardig dat verskille in inkapselingseffektiwiteit waargeneem is na gelang daarvan of die inkapseling van gallussuur plaasgevind het voor of na die vorming van hidrojels. Die ensiematies gevormde hidrojels het die integriteit van die geïnkapselleerde gallussuur beter bewaar met ‘n anti-oksidant aktiwiteit van 91% in vergelyking met 80% vir die chemies gevormde hidrojels. Volgehoue vrystelling van gallussuur is waargeneem wanneer die inkapseling van gallussuur plaasgevind het gedurende die vorming van hidrojels in vergelyking met inkapseling na die vorming van hidrojels. Alhoewel ensiematies gevormde hidrojels laer inkapselingseffektiwiteit gehad het, het hierdie jels volgehoue vrystelling van geïnkapselleerde gallussuur getoon.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103529
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