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Wageningen Universiteit (2005)

Endogenous phenolics and starch modifying enzymes as determinants of sorghum for food use in Burkina Faso

Dicko, Mamoudou Hama

Titre : Endogenous phenolics and starch modifying enzymes as determinants of sorghum for food use in Burkina Faso

Auteur : Dicko, Mamoudou Hama.

Université de soutenance : Wageningen Universiteit

Grade : Doctor Thesis 2005

Résumé
The objective of this thesis is to screen for biochemical determinants in sorghum varieties cultivated in Burkina Faso to identify the best sorghum varieties to be used as source of bioactive components or for specific local foods, e.g. "tô", thin porridges for infants, granulated foods "couscous", and local beers "dolo". The results revealed that sorghum varieties have different contents of starch, amylose, amylopectin and that germination decreased more the content of amylose than amylopectin. While on averagea‑amylase activity increased in all varieties by 2-20fold,b‑amylase activity did not uniformly increase after germination and even decreased in some varieties. The majority (82%) of sorghum varieties cultivated inBurkina Fasoare low proanthocyanidins (condensed tannins) containing sorghums. Some varieties had relatively high content(> 0.4%, w/w),of phenolic compounds such as 3-deoxyanthocyanidins and flavan-4-ols, which are of particular interest for food, colorant and pharmaceutical industries. Independent of grain germination, all sorghum varieties are among the most prominent natural sources of antioxidants due essentially to their phenolic contents. Sorghum varieties are highly polymorph in their expression of phenolic biosynthesizing enzymes, e.g. phenylalanine ammonia lyase, and phenolic modifying enzymes, e.g. peroxidases and polyphenol oxidases. Several peroxidase isoenzymes are expressed both before and after germination, and display a high activity in vitro . These enzymes are the main potential oxidases involved in the oxidation of endogenous phenolic compounds in sorghum grain. The major sorghum grain peroxidase isoenzyme representing more than 80% of total peroxidase activity was characterized at the molecular level. The enzyme, localized in chromosome 1 of sorghum, is a monomeric glycoprotein containing a non-covalently bound type-b heme. The catalytic properties and primary structure of the enzyme are similar to cereal peroxidases, in particular to barley peroxidase 1. Agronomic characteristics of grains (presence of absence of pigmented testa layer and color of the glumes) and plants (red or tan) could be linked to sorghum grain contents in phenolics and starch modifying enzymes. On average, varieties resistant to stresses have greater content and diversity in phenolic compounds and higher oxidative enzyme activities than stress susceptible varieties. The screened biochemical parameters could be linked to the preferences of varieties for specific local foods. For instance, among varieties used for "tô", "dolo", couscous and thin porridge preparation, the "dolo" varieties had the highest average content and diversity in phenolics as well as the highest antioxidant activities. Varieties good for infant porridges preparation have low amylose content and higha-amylase activity after germination.

Mots clés : SORGHUM / STARCH / TROPICAL FOODS / GERMINATION / PHENOLIC COMPOUNDS / ANTIOXIDANTS / ENZYMES / BURKINA FASO

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