Sorghum as an alternative source of starch: characterisation and properties for food applications

Cereal grains are considered as a primary source of energy worldwide. They are widely grown and consumed as a staple mainly in its whole form. Over the years, these grains have shown evidence of strong nutritional benefits and proven to be potential sources for novel ingredients in the food industry. There has been scarce literature on sorghum starch in comparison to other starches. The objective of this study has been to investigate cereal starches, particularly gluten free sorghum starch, isolation, characterisation and relative analysis with other major cereal grains.<br><br>Utilising wheat, rice, sorghum and maize, extraction procedures for the isolation of cereal starches have been evaluated. The heating temperature required for steeping of extracting solution to break the lipid-protein matrix was affected by grain size. Another issue that came up with starch extraction was the formation of the insoluble proteinaceous layer, following the repeated centrifugation, which was difficult to separate and showed the tendency to settle with the starch. The method established is relatively simple, involving flour steeping steeping in a mildly alkaline solution at 25 degrees temperature, followed by washing, multiple sieving, repeated centrifugation until no further protein layer was formed, sedimentation and drying. The starches collected for the four cereal starches were oven dried and the recoveries ranged between 30 and 36%.<br><br>In the food industry, the granular morphology, gelatinisation characteristics and crystallinity of the cereal grains are considered to be of primary importance. Previous research has demonstrated inconsistent results as to the changes of these properties when granular starch is exposed to various pH conditions. Hence, the aim of this analysis has been to investigate and overcome this problem, in a view to providing clarification, particularly in regard to sorghum starch. These properties were analysed by exposing laboratory extracted sorghum starch to buffer solutions of pH 3.0, 5.0, 9.0 and 11.0. Distilled water was used as a control. Starch were then incubated at these pH values at various times between 3 and 48 hours followed by drying using a freeze dryer. The resultant samples were examined by differential scanning calorimetry, X-ray diffraction and scanning electron microscopy.<br><br>The results indicated that gelatinisation temperatures increased by 2 to 3°C following three hours of treatment at pH 3.0 and lesser but still significant increases at 11.0. After 24 and 48 hours the increase in gelatinisation temperature for the pH 11.0 treatment became greater than that of acidic treatments and the gelatinisation enthalpy became predominantly lower. Both granular and crystal morphology showed no significant change after 3 hours of treatment at any pH. The measurement of relative crystallinity was problematic in the samples used. In addition, it was observed that while in alkali condition sorghum starch was noticeably yellow in appearance which could be dispersed by neutralising the slurry. In conclusion, the preliminary results found here do indicate that starch characteristics are influenced by treatment of the granules under the more extreme pH conditions studied here. On this basis, a further series of studies are now recommended.<br><br>Spray drying pilot plant scale was utilised to produce the microcapsules. The microstructural and morphological characteristics of microencapsulated cereal starches were analysed using environmental scanning electron microscope. Particle sizing and crystallinity were investigated using the laser beam scattering and X-ray diffraction. In order to optimise the conditions of inlet and flowrate of spray drying, preliminary designs were established. The characteristics of microcapsules and loss of starch during processing by spray drying was investigated and it was found that the microcapsules had good structure and integrity when loading rates were within the initial designed model. The optimum processing conditions during spray drying were developed by varying inlet air temperature and feed flowrate.  Moisture content and process yield were influenced positively by increasing inlet temperatures. It was inferred that the inlet and outlet temperature played a vital role in the loss of starch as, the feed flow rate decreased and resulted in the accumulation of burnt residue whilst using high inlet temperature conditions.<br>In summary, a promising application of microencapsulation technology for encapsulated sorghum starch was found. Spray drying has also been found to provide a convenient approach where high and consistent yields of microcapsules were produced. Accordingly, further studies are warranted, and it is recommended to encompass a wider range of gluten-free food products.<br><br>The overall conclusions are that the starch extraction method adapted in this investigation was a practical approach, producing relatively pure, white starches. The characteristics of the four cereal starches showed many similarities, but there were some variations in the properties, indicating that there may be different applications for their incorporation into food formulations. Therefore, sorghum starch offers potential as novel food ingredient warranting further sensory evaluation and larger scale probability studies.