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Extraction of amaranth starch from an aqueous medium using microfiltration

Amaranth starch granules are very small, which makes them suitable for a range of specialty applications. This starch is difficult to extract by wet milling due to the strong association between the starch and protein, the high protein content of the seed, and the small granule size. At the time of this research, no commercial amaranth starch extraction methods existed. The recently developed Al-Hakkak process has been successfully used to extract amaranth starch on a laboratory-scale. The work reported here forms part of the Al-Hakkak process scale-up investigations being performed by AgResearch Ltd, for the Biopolymer Network Ltd. During the Al-Hakkak process, an aqueous stream (known as starch-milk) containing insoluble starch granules, soluble carbohydrates, soluble proteins, and lipids, is produced. On the laboratory-scale the starch is recovered from the starch-milk using a high-speed centrifuge. However, at pilot and industrial scales density-based processes, such as centrifuges, settling tables, or hydrocyclones, may not be practical due to the small size, and low mass, of the amaranth starch granules. The research reported here investigated microfiltration as an alternative to density-based processes for separating the amaranth starch-milk into (i) a starch-rich concentrate and (ii) an aqueous stream containing the soluble proteins and carbohydrates. A Millipore ProFlux M12 Tangential Filtration System, fitted with a 1000 kDa regenerated cellulose membrane, was used as the experimental apparatus. It was shown that microfiltration has the potential to recover amaranth starch from the starch-milk produced during the pilot-scale Al-Hakkak process. The selected membrane retained all the starch granules, but also retained more protein than desired (protein retention was 67 % and the starch-rich concentrate had a dry-basis protein content of 12 %). Diafiltration was used to decrease the protein content of the starch-rich concentrate and after six washes the protein content had stabilised at 4 %, which was significantly higher than the 0.1 % previously reported for the laboratory-scale Al-Hakkak process. Analysis of the feed liquor, and diafiltered concentrate, revealed the presence of some non-starch insoluble material. This material, which may have been protein-based, was present in the starch-milk produced using the pilot-scale method but not the laboratory-scale method, and its presence determined the final protein content of the diafiltered concentrate. During processing to reach steady-state conditions membrane flux declined from 60 to 15 L m-2 h-1 over the first 45 minutes. This decrease was predominantly caused by the soluble components of the feed stream, and to a lesser extent by the starch granules. During concentration, flux had a three stage relationship with volumetric concentration (VCF). During the first stage flux decreased almost linearly with increasing VCF, in the second stage flux increased with increasing VCF, and in the third stage flux was independent of VCF. The second stage (flux increase) is unusual, and could form the subject of a separate study. The optimal transmembrane pressure was approximately 100 – 150 kPa, above which flux increased non-linearly with pressure. However, the flux-pressure relationship was weak, suggesting that higher operating pressures may be sustainable. The membrane proved very difficult to clean. A multi-step cleaning cycle was developed which adequately cleaned the membrane between runs. Key cleaning steps were: a cold water rinse to remove loosely bound material, a protease wash to remove protein, a sodium hydroxide wash to “pre-treat” any remaining starch granules, an amylase wash to degrade the starch granules, and a final sodium hydroxide wash to remove residues from the previous step. Additional research is needed to determine why the starch-milk from the pilot-scale process contains insoluble non-starch material, and to improve the process to prevent its inclusion, or remove it. Microfiltration should then be re-evaluated as a potential starch recovery process. An alternative membrane material, and larger pore size, should be trialled with the aim of decreasing protein retention and improving cleanability.
Type of thesis
Middlewood, P. G. (2011). Extraction of amaranth starch from an aqueous medium using microfiltration (Thesis, Master of Science (Technology) (MSc(Tech))). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/5373
University of Waikato
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