Biomass and its potential for protein and amino acids : valorizing agricultural by-products

The use of biomass for industrial products is not new. Plants have long been used for clothes, shelter, paper, construction, adhesives, tools, and medicine. With the exploitation on fossil fuel usage in the early 20th century and development of petroleum based refinery, the use of biomass for industrial application declined. Since the late 1960s, the petroleum-based products have widely replaced the use of biomass-based products. However, depletion of fossil fuels, rising oil prices, and growing environmental awareness, push the attention and policy towards a transition from fossil into bio-based products. Bio-based products can also be obtained from protein. The amine group (-NH2) in protein shows attractive functionality for nitrogen-containing chemicals production. In petroleum based conversion of crude oil into chemicals, co-reagents such as ammonia have to be used, and various process steps are involved. With the amine in protein, various co-reagent introducing process steps can be by-passed. Biomass refinery for protein might not only be necessary for supplying feedstock for the chemical industry, before all, it is important to meet the world protein demand for food and feed. Chapter 1 illustrates the protein shortage in 2030 that we will encounter with the current uses of protein in the diet of both humans and animals. The worldwide protein production may provide this demand only if we consider the biomass refinery for protein and use the protein product in an effective and efficient way according the specific need of food, feed, and chemical industry. For this purpose, development in protein extraction technology from various types of biomass is essential. The thesis entitled “Biomass and its potential for protein and amino acids; valorizing agricultural by-products” describes possibilities for using agricultural by-products as protein and or amino acid resources. An overview on alkaline plant protein extraction was first presented, in Chapter 2, including the potential of addition of different types of enzymes. Protein extraction from common resources such as soybean meal and other oilseed meals were reviewed. Also new protein resources, like microalgae, were discussed on the applicability of alkali based methods for protein extraction. Most of the experimental studies opted for less than 100 min and 50-60°C as extraction time and temperature, respectively. A typical biomass to solvent ratio of 1:10 was selected in some studies. Alkaline pH was selected over acidic pH, because it is far away from the isoelectric point (IEP). Most proteins have the lowest solubility at their IEP, which commonly occurs at pH 4-5. Adding proteases during protein extraction increased protein yield. Two types of extraction methods were experimentally researched in this study; alkaline and combined alkaline and enzymatic. In Chapter 3, alkaline protein extraction method was used to extract protein from 16 types of biomass, mostly agricultural by-products. Aiming to maximise protein extraction yields, a three step extraction was performed at elevated temperatures; 25, 60, and 120 °C. Protein yield was correlated to biomass chemical composition through Partial Least Square (PLS) regression. The results showed that protein extractability depended crucially on the type of biomass used. Protein from cereals and legumes were highly extracted, compared to other biomass. High protein extractability coincides with the biological function of protein as a storage protein, as opposed to functional protein. Protein extraction was furthermore correlated to the composition of the biomass. Especially cellulose and oil hamper extractability of protein, whereas lignin has no significant influence, suggesting that alkaline treatment removed lignin sufficiently. In Chapter 4, the effect of proteolysis during protein extraction was studied. Based on their working pHs, both alkaline and acidic proteases tested. Oilseed meals from soybean, rapeseed, and microalgae were considered as protein resource. Proteases that worked at acidic pH assisted protein extraction; but, still, more proteins were extracted using proteases that work at alkaline pH. This finding is in line with the literatures study from Chapter 2 mentioning that more proteins can be extracted at alkaline pH. Protex 40XL, Protex P, and Protex 5L that work at alkaline pH assisted protein extraction, particularly for rapeseed and microalgae meals. To a lesser extent, these proteases also improved protein extraction yield of soybean meal and untreated microalgae. Having shown that proteolysis aids in protein extraction, proteases were also used to solubilise wheat gluten at alkaline pH. Solubilising wheat gluten is one of the bottle necks for wheat gluten application. In this thesis, wheat gluten was used to represent wheat dried distillers grains with solubles (DDGS). From our perspective, more biomass by-products, such as wheat DDGS derived from ethanol production, will be available, also due to the target to replace 10% fossil fuel with bio-based fuel in 2050. With high glutamic acid content, wheat gluten provides possibilities to serve as an amino acid resource. Glutamic acid, which currently is microbial produced, has potential as feedstock for bulk chemicals production. Large amounts of cheaper glutamic acid can be made available by enabling its production from biomass by-products, such as wheat DDGS. Several methods for producing glutamic acid from wheat gluten were developed and the results were presented in Chapter 5. We found that a combination of enzymatic and mild acid hydrolysis opens up new possibilities for the industrial production of glutamic acid from biomass. Finally, in Chapter 6, general knowledge obtained from this study is discussed and a perspective on biomass valorization for protein and/or amino acids is presented. It was concluded that biomass, and particularly agricultural residues, are potential resources for protein and/or amino acids. An outlook on protein and/or amino acids production from by-products was also provided in this chapter. For this, economic calculations were provided that focussed on the processing cost. Based on these calculations, overnight alkaline treatment at room temperature was most economical to extract protein from most types of biomass. Residual biomass following protein extraction can be used as animal feed or for energy usage to get to a more integrated biorefinery, thereby reducing protein production cost.