Chemical-free recovery of crude protein from livestock manure digestate solid by thermal hydrolysis

Ken Tasaki

doi:10.1186/s40643-021-00406-1

Abstract

Protein is becoming an increasingly important resource for a variety of commercial applications. Yet, a large volume of protein is being wasted. Notably, livestock manure solids have a significant content of protein which is not only underutilized, but prone to runoff and eventual breakdown to reactive nitrogen compounds, contributing to eutrophication. It would be desirable to remove protein before it causes environmental hazards and then convert it to value-added commercial applications. We have developed a novel thermal hydrolysis process (THP) to extract crude protein from livestock manure solid, or manure digestate solid (MDS) in particular, without the use of any chemical. We demonstrate the versatility of our new process to control the molecular weight (MW) distribution of the extracted protein hydrolysate (PH). The antioxidant activity of the crude protein hydrolysate (CPH) has been examined through Oxygen Radical Absorbance Capacity Assay. The results have shown that our CPH had its antioxidant capacity against the peroxyl radical similar to that of vitamin E and exhibited almost 7 times as strong inhibition against the hydroxyl radical as vitamin E. We also evaluated the nutritional value of our PH by analyzing its amino acid composition and the MW distribution through amino acid analysis, SDS-PAGE, and MALDI-TOF mass spectroscopy. The characterizations have revealed that the PH recovered from MDS had 2.5 times as much essential amino acids as soybean meal on dry matter basis, with the MW distribution ranging from over a 100 Da to 100 KDa. Finally, the protein powder was prepared from the extracted CPH solution and its composition was analyzed.

Highlights

Livestock manure solids contain a considerable amount of crude protein, from 12 to 48 wt%, depending on the animal and the growth period, according to the report by Pacific Northwest National Laboratories (PNNL) (Chen et al 2003)
The crude protein content in the original manure digestate solid (MDS) sample was higher than the value for cow manure solid previously reported by the PNNL group, 18.1 wt% (Chen et al 2003)
Once protein can be removed from manure solids, there will be less biologically nondegradable organic nitrogen compounds (BNON) left in the solids, easier for biological waste treatment

Summary

Introduction

Livestock manure solids contain a considerable amount of crude protein, from 12 to 48 wt%, depending on the animal and the growth period, according to the report by Pacific Northwest National Laboratories (PNNL) (Chen et al 2003). There is a substantially large volume of crude protein in livestock manure that could be potentially recovered (see Additional file 1: Table S1) (Livestock Wastes Subcommittee 1993; U.S Department of Agriculture 2020). Crude protein belongs to what is called biologically nondegradable organic nitrogen compounds (BNON) which are difficult to treat by conventional biological treatment processes (Kim et al 2000) It would be beneficial if crude protein is recovered from manure before it causes environmental problems such as eutrophication and the recovered crude protein is converted to value-added products from environmental and economical points of view

Methods

Results

Conclusion