Valorisation of Sago Hampas Hydrolysate for Very High Gravity (VHG)Bioethanol Fermentation in a Stirred Tank Bioreactor

Nur Adila, Muradi and Dayang Salwani, Awang Adeni and Nurashikin, Suhaili (2022) Valorisation of Sago Hampas Hydrolysate for Very High Gravity (VHG)Bioethanol Fermentation in a Stirred Tank Bioreactor. PhD thesis, UNIVERSITI MALAYSIA SARAWAK.

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Abstract

Very high gravity (VHG) ethanolic fermentation is a high potential technology used in bioethanol production. However, the technology is often linked with the excessive amount of glucose that is entirely supplied at the beginning of the culture, causing the fermentation process to be slow. The high concentration of glucose in the fermentation medium also intensifies the osmotic pressure, which has a detrimental effect on yeast cells. The production of sago hampas was estimated to be 10 tons per day in Sarawak. Due to no proper treatment of this waste product, the disposal of sago hampas is polluting the environment. Sago hampas hydrolysate (SHH) was proven to be an excellent bioethanol substrate in normal gravity fermentation, but the feasibility of sago hampas in VHG fermentation is not yet been discovered. Hence, this study was carried out to elevate bioethanol production from SHH under VHG conditions by employing multiple modes of fermentations which are batch, fed-batch and repeated batch modes. The basic fermentation media consisted of 250 g/L SHH and commercial glucose (SHH+CG)supplemented with 5 g/L yeast extract. Fermentations were performed in a 2-L stirred tank bioreactor by using Saccharomyces cerevisiae. Our results showed that the maximum yeast cell concentration achieved in fed-batch mode was significantly improved by 1.5-fold compared to batch mode. The ethanol yield attained in the fed-batch culture represents an increment of 22% over that achieved in the batch culture. Additionally, the bioethanol productivity achieved in the fed-batch culture was enhanced by 1.8 times compared to the productivity attained in the batch culture. Nonetheless, the residual glucose in the fed-batch fermentation was 30 g/L which contributed to wastage in the cost of raw materials.Consequently, additional supplementation of the fermentation medium is necessary to increase yeast tolerance towards inhibitors, glucose consumption, and bioethanol production. The effect of supplementing the basic media with various nutrients in bioethanol fermentation under VHG conditions was investigated. The nutrients added were magnesium sulphate (0.12 /L), urea (3 g/L), glutamic acid (5 g/L), and peptone (5 g/L). Our results showed that culture supplemented with peptone has significantly improved yeast growth by 0.9-fold, glucose consumption efficiency by 10%, and bioethanol production by 13% compared to the control culture with the production of 126.20 ± 3.0 g/L. An ample amount of active yeast cells were produced from VHG fed-batch bioethanol fermentation using SHH+CG supplemented with yeast extract and peptone. The cells were recycled to start a new cycle of bioethanol fermentation in a repeated batch system, which reduced the time and the operational cost of the bioethanol fermentation. The yeast was allowed to naturally sediment at the bottom of the fermenter for 6 h. After sedimentation, the concentration of viable cells recorded was 1.21 x106 ± 0.135 cells/mL. The fermentation broth was harvested from the bioreactor, fresh media was added to the bioreactor, and repeated batch fermentation was carried out. The results of the repeated batch had attained similar bioethanol fermentability to the fed-batch experiment. The ethanol yield (YE/s) was 0.475 ± 0.02, and the bioethanol concentration (PE) recorded was 135.86 ± 2.19 g/L. Meanwhile, the results showed glucose consumption efficiency improved by 26% and 67% enhancement in PE compared to the single batch fermentation. The results of this study demonstrated that SHH is a feasible substrate for bioethanol fermentation under VHG conditions. The employment of fed-batch and repeated batch systems have significantly improved yeast growth and bioethanol productivity in VHG fermentation than in batch mode. Keywords: Bioethanol, sago hampas hydrolysate, very high gravity (VHG) fermentation, fed-batch system, repeated batch

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