## 3.2 Biochemical conversion techniques

Definition

Biochemical (e.g. anaerobic digestion, aerobic and anaerobic fermentation and enzymatic conversion) processes occur at mild conditions (lower temperature and pressure) using microorganisms or enzymes.

Enzymes are the working force in micro-organisms. Depending on the specific technique micro-organisms or isolated enzymes are employed. The use of enzymes has great advantages to classical chemical conversion processes. The reactions with enzymes occur in water and do not need high temperatures or large pressures. This results in a low energy use and low carbon dioxide footprint. A difficult step in enzyme mediated conversions is the isolation of the end product from the watery solution, often by centrifugation or membrane filtration. The use of enzymes to convert biomass is crucial for the biobased economy. A few examples are given as illustration.

Example 1: Anaerobic digestion

As the name anaerobic refers, the anaerobic digestion is carried out by microorganisms that can only live in an oxygen free environment. The decomposition of biowaste occurs in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis as shown in the figure below.

Anaerobic treatment processes require the presence of a diverse closely dependent group of bacteria to bring about the complete conversion of complex mixtures of substrates to biogas (made up mostly of methane and carbondioxde). Every step in the process involves different bacteria groups. The last group is for example called the methane forming bacteria. Biogas can be combusted to produce renewable electricity and heat. Another option is to upgrade the gas to natural gas quality, thus facilitating the inlet of the (green) gas in the natural gas pipeline network.

Dutch farmers exploiting an anaerobic digester are supported by subsidies (MEP, SDE). Nevertheless it is hard to make a profit due to low electricity prices and increasing costs for co-products, such as maize (corn) or glycerol.

Example 2: Yeasts converting C5 sugars

Ethanol can be used as biofuel or basic chemical compound to produce other chemicals. The common way to produce ethanol is by converting feedstocks such as maize (corn) and sugarcane. However, these feedstocks can also be used for human consumption. A lot of effort and research is therefore dedicated to produce ethanol from nonfood feedstocks like straw or wood chips. One technical challenge producing ethanol from cellulose economically is a robust organism to utilize the different sugars present in cellulosic biomass. Unlike starch where glucose (C6 sugar) is the only sugar present, cellulosic biomass has other sugars such as xylose and arabinose, usually called C5 sugars. As shown in the illustration below woody biomass consists of cellulose, hemi-cellulose and lignin. Cellulose is the long chain of C6 sugars. Hemi-cellulose contains C6 sugars but also a minor amount of C5 sugars.

The three main components of wood: cellulose, hemi-cellulose and lignin. There is one C5 sugar present in hemi-cellulose. The C5 sugar has a ring with 5 angles.

After enzymatic hydrolysis yeasts ferment C6 sugars to ethanol whereas C5 sugars are not fermented to ethanol by any natural microorganism in sufficiently high concentrations. A Dutch company, Royal DSM, has developed the combined fermentation of C6 and C5 sugars from wheat straw on an industrial scale. The combined fermentation results in a 40% increase in ethanol yield per ton of straw, which can result in significant cost cuts in the production of bio-ethanol from cellulosic feedstock. So, the company succeeded in producing industrial yeast strains that are capable of co-fermenting glucose and certain C5 sugars. In the video you can see an example how micro-organisms and enzymes can convert straw into ethanol.

https://youtu.be/niZls2dpHjM