Sorghum grain : development of methodologies for end-use quality evaluation

Abstract

Sorghum [Sorghum bicolor (L) Moench] is the second major cereal crop in Southern Africa after maize. Despite its importance as a cereal crop it is underutilised in Southern Africa because of insufficient characterisation and development of end-use quality evaluation methods for available cultivars, and the unattractive colour of some products from sorghum. This research aimed at developing simple methodologies for determination of important parameters of sorghum end-use quality, namely kernel hardness and kernel colour. The current research also set out to determine the cause for darkening of porridges made from white tan-plant sorghums and how this may relate to polyphenol oxidase activity. Sixteen Zambian sorghum cultivars grown during the 2008 and 2009 seasons of predominantly medium size, and ranging in colour from white to brown and red, with or without a pigmented testa were used. Endosperm texture of the sorghums determined by visual endosperm examination ranged from soft to hard. Abrasive hardness index values ranged from 6.28 to 19.64 and percentage water absorption ranged from 8.43 to 26.56%. Percent water absorption was significantly and positively correlated (r= 0.85, p <0.001) with endosperm texture and negatively correlated with abrasive hardness index (r=-0.89, p<0.01). The percent water absorption method could separate soft grains from hard grains just as well as endosperm texture and abrasive hardness index. The simplicity of the percent water absorption method makes it potentially usable by farmers and traders in remote areas where it can contribute to meaningful end-use quality assessment. The relatively dark colour of food products from white tan-plant (food-grade) sorghums can compromise their acceptability. The relationship between white tan-plant sorghum polyphenol oxidase activity (PPO) and porridge colour was investigated. Sorghums (including 28 white tan-plant samples grown in Zambia over two seasons), wheat and white maize were studied. Sorghum grain was intermediate in PPO between wheat and maize. When white tan plant sorghum and maize flours were cooked into porridges, they became darker with lower L* values. More importantly, the transition from white tan-plant sorghum flour to porridge caused a much larger reduction in mean L* value (27.9) than that with white maize (16.9). There were significantly negative correlations between all white tan-plant sorghum PPO activity and porridge L* values (r = -0.657, p < 0.01) and between Zambian white tan-plant sorghum PPO activity and porridge L* values (r = -0.710, p < 0.001). It is apparent that PPO activity in white tan-plant sorghums is an important determinant of the relatively dark colour of food products made from them, as is the case in wheat. Grain colour is an important quality indicator in sorghum-based foods and rural sorghum farmers in sub-Saharan Africa need simple, more accessible methods for its determination. The development of a simple quantitative method for sorghum grain colour assessment by analysis of the grains and their sodium hydroxide (NaOH) extracts was studied. Sixteen Zambian sorghums (white, red and brown types which were either tannin or non-tannin) from the 2008 and 2009 seasons and a Sudanese white tannin sorghum type were assessed for surface colour using Tristimulus colorimetry before and after treatment with NaOH. The NaOH extracts were also analysed using UV-visible spectrophotometry and reverse phase HPLC. Tristimulus colorimetry of the grain surface was able to distinguish white from coloured sorghums (brown and red) but was unable to separate tannin from non-tannin sorghum. UV-visible spectrophotometry and reverse phase HPLC of NaOH extracts from a representative set of the grains consisting of five sorghum types (red tannin, brown tannin, white tannin, red non-tannin and white non-tannin) separated the tannin from non-tannin sorghums regardless of grain surface colour. However, UV-visible absorption of NaOH extracts from the sorghum grains could not be related to grain surface colour as determined using Tristimulus colorimetry. Eleven phenolic compounds consisting of two flavan-3-ols, five anthocyanins and four 3-deoxyanthocyanins were identified in the NaOH extracts using UPLC/PDA/MS. NaOH extract from Framida *SDS[3845]23-2-1 (red tannin) contained all of the eleven compounds identified. The flavan-3-ols (catechin and +-catechin-3-O-gallate) were present in NaOH extracts of all the five sorghums. While some anthocyanins could be identified in NaOH extracts from all five sorghums, only Framida *SDS[3845]23-2-1 (red tannin), Sima (white non-tannin) and MMSH625 (red non-tannin) contained any 3-deoxyanthocyanins. Total peak area due to anthocyanins and 3-deoxyanthocyanins was higher for NaOH extracts from tannin sorghums compared to non-tannin. This was in agreement with the UV-visible spectrophotometry of the NaOH extracts which also separated the tannin from non-tannin sorghums. This shows that with NaOH treatment, it is possible to separate tannin from non-tannin sorghums. Colorimetry of NaOH extracts from sorghum may therefore be considered as a potentially simple and cheap alternative method for distinguishing tannin from non-tannin sorghums. The findings of this research provide a platform for the development of a system which promotes an integrated and inclusive approach in using the methodologies developed for sorghum end-use quality evaluation. The methodologies will form an integral part of the system which can be applied along the sorghum value chain in sub-Saharan Africa and involve various stakeholders such as the gene bank, sorghum breeders, cereal scientists and researchers, sorghum food processors, traders, rural farmers and consumers. The efficient application of this system could lead to increased sorghum production and utilisation and contribute significantly to food and nutrition security.