Eiwitkarakterisering van Tef [Eragrostis tef (Zucc.)Trotter] en zijn geschiktheid als glutenvrij graan
- HabtuShumoy Abraha
Wist u dat overgevoeligheidsreacties een sterk opkomend probleem is in de westerse gemeenschap? Gluten, afkomstig uit granen, is één van de veertien officieel erkende allergenen door de Europese wetgeving. Om deze redenen wordt er actief onderzoek gedaan naar minder gekende graansoorten zoals Eragrostis tef ter vervanging van de frequent gebruikte glutenbevattende graansoorten zoals tarwe. Tijdens dit onderzoek werd een eiwitkarakterisering uitgevoerd op zeven verschillende tef variëteiten. Dit werk bevat tal van nieuwe gegevens die samen een beeld scheppen over de glutenvrije eigenschappen van tef.
1. Wat zijn gluten en wat kunnen ze veroorzaken?
Gluten zijn in de westerse wereld goed gekend omwille van hun negatieve effecten bij gluten-gevoelige personen. We kennen allemaal wel iemand die hier dagdagelijks rekening mee moet houden. Maar wat zijn gluten nu eigenlijk? Gluten zijn eiwitten die voornamelijk aanwezig zijn in veelvuldig gebruikte graansoorten in de voedingsindustrie zoals tarwe, gerst en rogge (Spaenij-Dekking et al., 2005). Graaneiwitten werden door Osborn reeds in 1924 onderverdeel in vier groepen. De twee niet-gluten eiwitten zijn albumine en globuline, terwijl de gluteneiwitten de prolamines en de glutelines omvatten (Zilic et al., 2011 & Singh et al., 2001). Gluteneiwitten zijn echter van het belang voor de fysische eigenschappen van het eindproduct in de bakkerij-industrie. Tijdens de fermentatiestap in de broodbereiding produceert gist koolstofdioxide, dit is een gas dat door het glutennetwerk wordt vastgehouden wat resulteert in een gerezen deeg (The National Academies, 1996). Niet tegenstaande van deze positief functionele eigenschappen van gluten wordt toch actief onderzoek gedaan naar minder gekende graansoorten. Deze granen kunnen omwille van gezondheidsaspecten dienen ter vervanging van de gluten bevattende graansoorten. Gluten kunnen immers drie verschillende ongewenste reacties veroorzaken in het lichaam zoals allergie, intolerantie en coeliakie, een glutengevoelige auto-immuunziekte van de dunne darm (Spaenij-Dekking et al., 2005).
2. Eragrostis tef
Eén van de graansoorten waar een groeiende interesse voor bestaat, is Eragrostis tef (Fig. 1), een graan afkomstig uit Ethiopië. Tef is één van de kleinste graansoorten ter wereld. De massa van één graan is slechts 0,6-0,8% van de massa van tarwe. Omwille van deze redenen, wordt tef gemaald tot volkorenbloem waarbij de buitenste lagen mee vermalen worden (Gebremariam et al., 2014). In Ethiopië wordt tef voornamelijk gebruikt voor de bereiding van injera, een zacht, rond en gefermenteerd brood dat deel uitmaakt van het traditioneel dieet in Ethiopië. De interesse in tef is gegroeid in de afgelopen jaren doordat het hoge gehaltes bevat aan essentiële aminozuren, mineralen en vitaminen, maar voornamelijk omdat tef beschouwd wordt als een glutenvrij graan, hoewel dit nooit officieel met cijfers is bewezen (Zhang et al., 2016).
3. Eiwitkarakterisering van tef
Granen zijn opgebouwd uit vijf hoofdcomponenten, namelijk koolhydraten, eiwitten, vet, vezels en as. In dit werk werd dieper ingegaan op de eiwitten aanwezig in tef. Voor de eiwitkarakterisering van zeven verschillende tef variëteiten werd allereerst het ruw eiwit gehalte in de bloem bepaald. Er werd een gehalte gevonden dat overeenkomstig was met het eiwitgehalte in tarwe wat dus zeer positief was.
Vervolgens werden de vier eiwitfracties gescheiden op basis van de Osborn techniek. Hiervoor werden resultaten bekomen die afnamen in de volgorde van globuline > albumine > prolamine > gluteline (Fig. 2). Uit deze resultaten kon al reeds afgeleid worden dat de niet-gluteneiwitten in grotere hoeveelheden aanwezig zijn in tef dan de gluteneiwitten. Uit de resultaten van tarwe, werd een duidelijk veel hoger gehalte aan gluteneiwitten teruggevonden.
Daarnaast werd ook ‘in vitro’-eiwitverteerbaarheid (IVPD) getest op zowel de bloem als op de injera. Dit was informatie die tot nu toch nog ongekend was voor tef. Door middel van een literatuurstudie konden onze resultaten vergeleken worden met deze van andere graansoorten. Hieruit bleek dat de IVPD van tef hoger was dan van gerst, mais en haver terwijl de IVPD van tarwe en rogge hoger waren dan van tef. Wel bleek dat na bereiding van de bloem tot injera, het gehalte aan IVPD lichtjes positief was toegenomen (Fig. 3).
Zoals eerder vermeld werd nooit eerder het gehalte aan gluten in tef bepaald. Een casestudie in Nederland waarbij bij coeliakiepatiënten na het consumeren van tef, de reacties geëvalueerd werden, is het enige wat tot nu toe gekend is over gluten in tef. In dit werk werd door middel van een sandwich-enzym linked immunosorbent assay (ELISA) het gehalte aan gluten in de tef bloem bepaald. Volgens de Europese commissie mogen voedingsmiddelen als glutenvrij gelabeld worden als het glutengehalte lager is dan 20 mg/kg. Zoals te zien is in figuur 4 liggen de waarden voor alle zeven de tef variëteiten onder deze drempel waardoor tef als glutenvrij kan beschouwd worden.
Tenslotte werd ook via sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) de aanwezigheid van potentieel andere allergische eiwitten aanwezig in tef bepaald. In de literatuur zijn nog geen allergenen officieel gedetecteerd in tef terwijl er voor tarwe 51 officieel gekend zijn. Uit ons onderzoek bleek wel dat ongeveer 42 allergische eiwitten mogelijks kunnen voorkomen in tef. Belangrijk is hierbij te vermelden dat heel wat van deze allergische eiwitten na voedselbereiding waren verdwenen. Hierdoor werden nog slechts 17 allergische eiwitten teruggevonden die mogelijks kunnen voorkomen in injera.
4. Conclusie
Gluten is een vaak gekend probleem in de westerse maatschappij waardoor men actief op zoek is naar nieuwe granen die als glutenvrij kunnen beschouwd worden. Uit ons onderzoek bleek tef een veelbelovende vervangende graansoort. Onderzoek is nog steeds lopende, aangezien het gebruik van glutenvrij graan ook invloed heeft op de fysische eigenschappen van bakkerijproducten zoals brood. Ook de aanwezigheid van andere allergische componenten moet nog verder onderzocht worden, maar uit eerste resultaten blijken ook deze al veelbelovend te zijn.
Bronnen
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