Sorghum waxy and high protein digestibility traits and their relationship with malting and dough-based product making quality

Abstract

Sorghum is a critical cereal crop in Africa, owing to its heat- and drought- tolerance, its role in nutrition and potential in commercial food and beverage manufacture. However, sorghum has limitations for producing quality foods and beverages e.g. malt, breads and biscuits. This is due to its high starch gelatinization temperature and poor protein functionality, specifically its inability to form a viscoelastic dough like wheat. Novel sorghum lines with combined waxy (high amylopectin) and high protein digestibility (HD) traits have been developed through conventional breeding. These lines are hypothesized to have superior functionality in food and beverage applications due to their higher digestibility and improved dough functionality. The objective of this work was to determine the relationship between waxy and HD traits in sorghum and malting quality as well as dough-based product (injera-fermented flatbread and biscuit) making quality. Novel white tan-plant sorghum lines with differing endosperm traits: waxy, heterowaxy, waxy-HD, non-waxy-HD and non-waxy-normal digestibility traits were malted at a laboratory scale and their malting qualities were studied. Additionally, injera and biscuits were prepared using standard methods and their qualities were evaluated using a trained descriptive sensory panel (DSP) and instrumental texture analysis. The endosperm of the waxy lines had a pale waxed floor-like appearance (typical of waxy sorghum), high starch amylopectin and intermediate to corneous texture. Transmission electron microscopy revealed that the HD lines had protein bodies which were not densely packed in the protein matrix, of irregular shape and small size (0.48-0.56 mm diam.), and had a floury endosperm. Malt produced from waxy and heterowaxy sorghum lines generally had improved endosperm modification and starch granule degradation. Only non-waxy-HD and one waxy line malts exhibited clear evidence of endosperm protein degradation. Malt α- and β-amylase activities were not evidently affected by the traits. Malt from waxy lines had improved hot water extract (HWE) and free amino nitrogen (FAN). Principal component analysis (PCA) showed that the waxy lines were associated with high HWE, FAN, starch and protein loss. DSP showed that injera made from waxy sorghums were softer, more rollable, and flexible compared to normal sorghum injera and were much closer to teff injera reference. Instrumental texture analysis of fresh and stored injera revealed that waxy sorghums gave lower stress and higher strain compared to injera from non-waxy lines, indicating that the injera were softer and more flexible. There was no clear trend as to whether the HD-trait affected injera quality. The textural properties of the injera measured by texture analyser and DSP showed significant correlation. Also, the instrumental texture profile of the injera prepared using full-scale (traditional) and small-scale (microwave) methods were significantly correlated. DSP and instrumental texture analysis showed that sorghum biscuit quality was not affected by the traits. The waxy trait either alone or in combination with the HD-trait appears to improve sorghum malting quality, probably due to the better starch granule swelling property of amylopectin, which could facilitate hydrolysis by amylases and proteases. The improved injera quality is most likely due to the slower retrogradation and better water holding of amylopectin starch. This study shows that white tan-plant waxy sorghum can produce better quality malt and flatbreads than regular sorghums, and thus has considerable potential to partially replace barley malt and teff, respectively in these products in areas in Africa where sorghum is a major cereal crop. Hence, either white tan-plant waxy or normal sorghum can be used to partially replace wheat for biscuit making in regions that have shortage of wheat.