John R. N. Taylor

FACTORS AFFECTING SORGHUM PROTEIN DIGESTIBILITY

Abstract In the semi-arid tropics worldwide, sorghum ( Sorghum bicolor (L.) Moench) is cultivated by farmers on a subsistence level and consumed as food by humans. A nutritional limitation to its use is the poor digestibility of sorghum protein when wet cooked. The factors affecting wet cooked sorghum protein digestibility may be categorised into two main groups: exogenous factors (grain organisational structure, polyphenols, phytic acid, starch and non-starch polysaccharides) and endogenous factors (disulphide and non-disulphide crosslinking, kafirin hydrophobicity and changes in protein secondary structure). All these factors have been shown to influence sorghum protein digestibility. More than one factor may be at play at any time depending on the nature or the state in which the sorghum grain is; that is whether whole grain, endosperm, protein body preparation, high-tannin or condensed-tannin-free. It is proposed that protein crosslinking may be the greatest factor that influences sorghum protein digestibility. This may be between γ- and β-kafirin proteins at the protein body periphery, which may impede digestion of the centrally located major storage protein, α-kafirin, or between γ- or β-kafirin and α-kafirin.

Starch properties as affected by sorghum grain chemistry

To determine the relationship between sorghum grain polyphenol content, grain structure, and starch properties, starch was isolated from 10 sorghum varieties using an alkali steep and wet-milling procedure. SV2, a tannin-free variety with white pericarp, gave a white starch. Varieties having red or white pericarp and higher polyphenol levels gave pink starches. Hunter colour values (L, a, b) of starches were not correlated with grain polyphenol content. Grain appearance in terms of pericarp colour, or presence or absence of pigmented testa, did not relate to the intense pink colouration of sorghum starches. Starch amylose content was significantly negatively correlated (r  = −0.88, p < 0.001) to grain floury endosperm texture. Sorghum starches had higher peak viscosity (PV) in pasting than commercial maize starch. The time taken to reach peak viscosity from the initial viscosity rise was less for sorghum starches than maize starch. However, sorghum starches had a higher rate of shear thinning (Rst) than maize starch. There was a significant positive correlation between grain polyphenol content and starch PV (r  = 0.75, p < 0.05). Starch gel hardness was negatively correlated to pasting properties of Rst and paste breakdown (r  = −0.78 and −0.77 respectively) at p < 0.01. Peak gelatinisation temperature (Tp) occurred over a narrow range from 66 to 69 °C. Tp was negatively correlated to the floury endosperm portion of the grain (r  = −0.77) at p < 0.01. It is concluded that sorghum grain polyphenol content and grain characteristics influence its starch properties. © 2000 Society of Chemical Industry

Phenolic compounds and kernel characteristics of Zimbabwean sorghums

Sixteen sorghum varieties widely cultivated in Zimbabwe were examined for levels of phenolic compounds and kernel characteristics, to help identify those with desirable properties and develop suitable processing methods. Assays for polyphenols included the chlorox test, the vanillin- HCl, the ferric ammonium citrate and the butanol-HCl methods. Free phenolic acids were analysed using reverse-phase high performance liquid chromatography. Varieties DC-75, Mutode and Chirmaugute had the highest levels of condensed tannins. The polyphenols in Chibonda were mostly unextractable in methanol. No significant levels of polyphenols were found in 13 varieties. Phenolic acid content was related to pericarp colour. Endosperm texture and pericarp thickness were evaluated using video image analysis. Katandanzara and SV1 had relatively corneous endosperms ( 0.060 mm). Endosperm texture was not correlated with phenolic compounds. A positive correlation, however, was observed between pericarp thickness and polyphenol content (r> 0.64). Zimbabwean sorghums lack ideal agronomic and processing physico-chemical characteristics defined in terms of high polyphenols, plus hard endosperm and thin pericarp. Research is required to develop effective methods to process the available polyphenol-rich sorghums. # 1999 Society of Chemical Industry

INFLUENCE OF COOKING CONDITIONS ON THE PROTEIN MATRIX OF SORGHUM AND MAIZE ENDOSPERM FLOURS

ABSTRACT To understand the influence of the sorghum and maize endosperm protein matrix honeycomb structure on starch hydrolysis in flours, three-dimensional fluorescence microscopy was applied to floury and vitreous endosperm flours cooked under various conditions. Cooking caused the collapse and matting of the sorghum and maize vitreous endosperm matrices, with the effect being greater in sorghum. The effect of cooking was rather different in the floury endosperm in that the protein matrices expanded and broke up to some extent. These effects were a consequence of expansion of the starch granules through water uptake during gelatinization. Cooking in the presence of 2-mercaptoethanol caused an expansion of the vitreous endosperm matrix mesh due to breakage of disulfide bonds in the protein matrix. Mercaptoethanol also caused an increase in the proportion of β-sheet structure relative to α-helical structure of the endosperm proteins. Increased energy of cooking caused collapse of the sorghum matrix. Disul...

Developments in sorghum food technologies

Publisher Summary This chapter describes the major advances that have taken place in the development of sorghum food processing technologies, and examine the challenges that still have to be met. Tremendous strides have been made in developing sorghum food processing technologies. Two examples where the technologies have been successfully implemented are: the industrialization of sorghum brewing, taking it from a rural craft to a 20,000 liter batch scale, and brewing conventional beer with sorghum grain and enzymes. However, the implementation of other technologies, for example, composite breads, is notably lacking. One major factor limiting utilization of sorghum in Africa appears to be the unavailability of cost-efficient, reliable supplies of sorghum grain of acceptable quality for making high quality flour. The challenge, therefore, appears to revolve around a holistic approach to implementation, involving: economic studies, government programs, seed supply, grain production, selection of appropriate technologies, and training of operators, consumer awareness, and grain and product quality standards.

Physico-chemical Characterization of Grain Tef [Eragrostis tef (Zucc.) Trotter] Starch

Starch isolated from five grain tef (Eragrostis tef) varieties was characterized and compared with commercial maize starch. Grain tef starch is formed of compound granules, comprising many polygonal shape (2—6 μm in diameter) simple granules. The crude composition is similar to that of normal native cereal starches. The amylose content ranges from 24.9—31.7%. Gelatinisation temperature range was 68.0—74.0—80.0 °C, typical of tropical cereal starches, and resembling the temperature range of rice starch. The mean intrinsic peak viscosity (269 RVU), breakdown viscosity (79 RVU), cold paste viscosity (292 RVU) and setback viscosity (101 RVU) determined were considerably lower than that of maize starch. Tef starch has higher water absorption index (WAI) (mean 108%) and lower water solubility index (WSI) (mean 0.34%) than maize starch.

Properties of heat-treated sorghum and maize meal and their prolamin proteins.

The digestibility of sorghum protein is reduced when wet cooked. Size exclusion chromatography (SEC) together with other protein analytical techniques was applied to further elucidate the effects of cooking on the sorghum and maize and their prolamin proteins. Sorghum and maize meal and their respective tertiary butanol extracted kafirin and zein were wet heat treated by boiling or pressure cooking. As expected, the in vitro pepsin protein digestibility of sorghum meal and kafirin reduced with boiling and pressure cooking, whereas the decrease in maize meal and zein protein digestibility was much less. SDS-PAGE showed that the boiled and pressure-cooked kafirin was more polymerized than the corresponding zein preparations. SEC of kafirin also revealed a substantially increased high molecular weight peak with boiling and pressure cooking. In contrast, the high molecular weight peak was very small for control and wet heated treated zein. The highly polymerized kafirin occurs as a result of extensive disulfide bonding of kafirin monomers during cooking. Cooking sorghum meal and kafirin also resulted in a relative change in secondary structure from alpha-helical to beta-sheet, as determined by Fourier transform infrared spectroscopy. The pepsin indigestible kafirin residues were mainly in beta-sheet conformation. In contrast, the conformational changes were very small for cooked maize meal and zein. Disulfide bonds formed during heating cause polymeric kafirin formation and also promote realignment of kafirin into beta-sheet structures. These conformational changes apparently cause the lower proteolysis susceptibility of kafirin.

Kafirin microparticle encapsulation of catechin and sorghum condensed tannins.

To exploit the porous nature of previously developed kafirin microparticles, encapsulation of the bioactive polyphenols, catechin and sorghum condensed tannins, was investigated. The antioxidant release profiles of the encapsulated substances were studied under simulated gastric conditions. Kafirin microparticles encapsulating catechin or sorghum condensed tannins were similar in size to control kafirin microparticles (5-6 mum). TEM showed that kafirin microparticles encapsulating catechin had a rough porous surface. Microparticles encapsulating sorghum condensed tannins were irregular in shape, some apparently joined together, with a mixture of rough and smooth surfaces. Over a period of 4 h, catechin and sorghum condensed tannin encapsulated kafirin microparticles showed virtually no protein digestion but released approximately 70 and 50%, respectively, of total antioxidant activity. Thus, the use of kafirin microparticles to encapsulate catechin and sorghum condensed tannins has potential as an effective method of controlled release of dietary antioxidants.

Effects of processing sorghum and millets on their phenolic phytochemicals and the implications of this to the health‐enhancing properties of sorghum and millet food and beverage products

Sorghum and millet grains are generally rich in phytochemicals, particularly various types of phenolics. However, the types and amounts vary greatly between and within species. The food-processing operations applied to these grains, i.e. dehulling and decortication, malting, fermentation and thermal processing, dramatically affect the quantity of phenolics present, most generally reducing them. Thus the levels of phytochemicals in sorghum and millet foods and beverages are usually considerably lower than in the grains. Notwithstanding this, there is considerable evidence that sorghum and millet foods and beverages have important functional and health-promoting effects, specifically antidiabetic, cardiovascular disease and cancer prevention, due to the actions of these phytochemicals. Also their lactic acid bacteria-fermented products may have probiotic effects related to their unique microflora. However, direct proof of these health-enhancing effects is lacking as most studies have been carried out on the grains or grain extracts and not the food and beverage products themselves, and also most research work has been in vitro or ex vivo and not in vivo. To provide the required evidence, better designed studies are needed. The sorghum and millet products should be fully characterised, especially their phytochemical composition. Most importantly, well-controlled human clinical studies and intervention trials are required.

The protein compositions of the different anatomical parts of sorghum grain

A comprehensive picture of the distribution of proteins in two low-tannin sorghum cultivars was obtained by hand-dissection of the kernels into their anatomical parts: endosperm, germ and pericarp. The proteins in each part were characterised by Osborne-type fractionation, amino acid analysis and electrophoresis. Approximately 80% of grain nitrogen was in the endosperm, the majority being in the form of prolamin protein that was endosperm-specific. The second largest protein fraction in the endosperm was G3-glutelins. The germ contained approximately 16% of the grain nitrogen, most of which occurred as low molecular weight nitrogen and albumin and globulin proteins. The major location in the grain of albumins and globulins was the germ, and it was found that these proteins were rich in essential amino acids, especially lysine. Only approximately 3% of the grain nitrogen was found in the pericarp and the largest single fraction of this pericarp protein was unextractable under standard conditions.