Document Type

Thesis - Open Access

Award Date

2025

Degree Name

Master of Science (MS)

Department / School

Biology and Microbiology

First Advisor

Bishnu Karki

Abstract

Thin stillage (TS) and whole stillage (WS) are major by-products of the corn ethanol industry. These by-products are generated in large quantities and contain nutrients that remain after yeast fermentation. Current industrial utilization of these by-products involves a series of downstream processing steps such as centrifugation and evaporation, which are energy-intensive and often result in nutrient losses. Traditionally, these by-products are dried to produce distillers dried grains with solubles. In this study, microbial fermentation was investigated as an alternative bioprocessing tool to expand the utilization of these by-products. The first objective was to explore the upcycling of TS, both alone and in combination of soybean hulls (SBH), a protein- and fiber-rich by-product of soy processing, through submerged fungal fermentation. TS and SBH served as carbohydrate and protein-rich growth medium for filamentous fungi. Four filamentous fungi, i.e., Aureobasidium pullulans (Ap), Aspergillus oryzae (Ao), Trichoderma reesei (Tr), and Aspergillus niger (An), were evaluated. Fermentation significantly increased the crude protein of pellets, and the addition of SBH to TS further improved nutrient recovery and total solids yield. In addition, SmF enhanced the total phenolic content and antioxidant activity, particularly in Tr- and An- fermented samples. Phytic acid reduction varied depending on the substrate and fungal strains, while total sugars decreased across all treatments, confirming substrate utilization. These findings suggest that both TS and TS+SBH can be effectively fermented with filamentous fungi to develop protein- enriched, nutritionally enhanced food and feed supplements. The second objective focused on amino acid enrichment of ethanol by-products through bacterial fermentation. TS and WS were used as fermentation media for Corynebacterium glutamicum (C. glutamicum) to produce lysine. Fermentation with C. glutamicum significantly increased lysine levels in both substrates, with notable utilization of glycerol, lactic acid, succinic acid, and acetic acid as carbon sources. Lysine production peaked at day 5 in TS-based fermentations, and the outcomes were significantly pH-dependent. Maintaining the fermentation broth at pH 7.0 did not provide additional benefits compared to uncontrolled pH conditions. Overall, this work establishes a biologically integrated, low-energy platform for converting underutilized ethanol and soybean processing by-products into nutrient-rich bioproducts. By reducing reliance on energy-intensive drying and improving nutrient recovery, this approach advances circular bioeconomy principles, enhances the economic sustainability of biorefineries, and creates new pathways for producing functional food and feed ingredients from industrial waste streams.

Library of Congress Subject Headings

Ethanol fuel industry -- By-products. 
Soybean. 
Lysine. 
Fermentation.      

Publisher

South Dakota State University

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Rights Statement

In Copyright