Cell wall and tannin treatments for increased utilisation of cassava and sorghum in beverage and bioethanol production

Abstract

Diversification in the use of cassava and sorghum through commercial processing into value-added food and beverage products is an important means of developing the economies of African countries. However, potential applications of cassava and sorghum are limited by their unique structural properties and chemical composition attributes. With cassava tuber, its high fibrous material content constitutes a problem after starch hydrolysis in brewing. These cell wall materials (CWM) have high water holding capacity (85-90%), which results in poor wort filtration. Cellulolytic and hemicellulolytic enzyme treatment of cassava cake resulted in a reduction in total solids volume and in the amount of insoluble solids. The amount of free starch granules also increased with increase in enzyme concentration. Combination of different enzyme preparations showed a synergistic effect by significantly reducing the viscosity of the cassava cake (milled cassava tuber). These effects appear to be due to complementary enzyme activities that hydrolyse the parenchyma polymer cell walls, except for the lignified materials. As determined by GC, the residual CWM monosaccharide units consisted of arabinose, rhamnose, xylose, galactose and glucose, with glucose being the most sugar units followed by xylose. This suggests that cellulosic type material is the main components of cassava CWM. The application of tannin sorghum in beverage production is limited due to the condensed tannins interacting with other grain components, which negatively affects the processing functionality and nutritional properties of the beverages. Type II and III tannin sorghums were selected for investigation of tannin inactivation through steeping the flours in 0.1, 0.2 and 0.4% alkaline (NaOH) solution. Tannin inactivation was also investigated by admixing with soy protein isolate. Sodium hydroxide steeping of the flour resulted in a substantial reduction in assayable total phenols and tannins. The effect of NaOH on the tannins in the Type III sorghum also resulted in 60-80% reduction in α-amylase enzyme inhibition by the tannins. SPI treatment of Type III resulted in 76% reduction in α-amylase inhibition by tannins. Fermentability of the Type III sorghum by lactic acid bacteria improved with NaOH steeping. Applying this treatment in a mashing process resulted in a considerable reduction in starch liquefaction time with the Type III sorghum. The normal phase HPLC profile of the tannin molecules based on the degree of polymerisation showed an increase in the polymeric tannins with the NaOH treatment. LC/MS identified procyanidins ranging from dimer to hexamer in the NaOH treated Type II flour. These procyanidins were not identified in the Type III, which indicates tannin polymerisation in both Type II and III tannin sorghums to higher polymeric proanthocyanidins. The polymerisation mechanism is proposed to be by cross-linking of the tannin molecules resulting in the formation of higher proanthocyanidin polymers. These polymerised tannins are less reactive with amylases resulting in too large tannin polymers, which can be attributed to reduction in the availaible phenolic group reactive site. Enzymatic hydrolysis of cassava CWM will improve processing application of cassava in brewing. Sodium hydroxide pre-treatment of Type III sorghum flour will improve its processing functionality for increase utilisation in beverage production. The creation of new markets for these crops should help ensure sustainable food security through economic empowerment of the small-holder farmers in sub-Saharan Africa.