Rice straw, a worthless by-product?

14 April 2022 | Lucas Unger

Are rice straw just worthless by-products of the rice harvest?

Rice belongs to the biggest base foods in the world. In particular in Asia, it can be found in everyday meals. This is why its cultivation is of high importance in countries such as Vietnam and is grown in large quantities. However, after harvesting the rice grains from the paddies, it leaves behind a huge amount of by-products, the actual rice plant. Traditionally it is removed by burning it on the fields. However, this process is very harmful for the environment, human health and impacts the worldwide climate negatively. But is the leftover rice plant really just a worthless by-product?

In consideration of the increasing climate struggles we are facing, scientist from all over the world have been working on solutions to reduce the emissions of greenhouse gases to net zero. The burning of rice straw contributes heavily to climate change and destroys a feedstock which holds a great potential of producing high-value products: rice straw biomass. Biomass is the term that describes plant-based materials that can be used as sources for renewable energy. The conversion of rice straws into biofuels not only reduces the emission of greenhouse gases by not burning them on the fields but also by making us less dependent on fossil fuels.

Using microorganisms as factories for renewable energy

Traditionally, various chemicals were used in the process to convert rice straw into high value products. These processes, however, are not sustainable and usually create a lot of toxic and environmental harming by-products themselves while requiring huge amounts of energy. Therefore, industry stepwise replaces the chemical treatments with microorganisms which use the initial, seemingly worthless raw materials as a source of energy. As any kind of living organism, just alike humans or animals, microorganism require a food source to survive. Most microorganism prefer to feed on energy-rich sources such as the sugar glucose. However, if they are exposed to other energy sources, they are able to adapt their metabolism to those. This way agricultural waste products can serve as energy sources for microorganism. While metabolising the waste into the relevant nutrients they require, the microorganisms also create by-products which we can further utilize in other processes. By controlling the feedstock and the metabolism conditions, microorganisms can be directed to generate high-value products for us.

The complex rice straw mesh contains valuable sugar sources

To put it simply, the rice straw plant is a heterogenous mix of sugars forming a complex structured mesh which we call lignocellulose. Just like we do not like to eat raw food, we cannot offer the rice straw in a whole to the microorganism. The meal needs to be prepared to make it attractive to be eaten. A pre-treatment is necessary to entangle this mesh mess and extract the single components before serving it as a meal. So let's have a look at what the rice straw has to offer.
Rice straw mainly consists of cellulose, hemicellulose and lignin forming a robust cell wall structure and further extractives including proteins, pectins, chlorophyl, fats, oils and waxes (Figure 1). Because of its heterogeneous and complex structure and the tight connection between the different components, breaking it apart requires often multi-stage extraction methods with physical, chemical and biological pre-treatments.

Fig 1

Figure 1. Structure and composition of rice straw. (A) Rice plant [1] with insert showing rice straw components [2]. (B) Composition of rice straw with weight % determined from multiple reports [3].

The most abundant component of rice straw is cellulose which is built of many glucose units. Instead of forming a linear chain, the building blocks are connected to a complex three-dimensional net-like structure which provides the mechanical strength to the plant cell wall. The second component is hemicellulose consisting of a heterogeneous group of sugars (xylose, arabinose, mannose, galactose) making up for 25% of the rice straw. Lignin is the third important component of rice straw and presents the second most abundant substance on earth. Next to the just above described components, rice straw possesses further macromolecules which cannot be utilized by the microorganisms and need to be separated to achieve a pure feedstock. This complicates the extraction process.

In our article “Separating the lignocellulosic biomass components", we describe the idea behind the pre-treatment of the lignocellulosic biomass of rice straw in more detail. Furthermore you can find out more about the high-value products which can be created out of rice straw in the following articles: Bioethanol: a carbon dioxide neutral energy source; Malic acid is part of your daily life.

1. Itoh, J., et al., Rice plant development: from zygote to spikelet. Plant Cell Physiol, 2005. 46(1): p. 23-47.
2. Klass, D.L., Biomass for renewable energy, fuels, and chemicals. 1998, San Diego: Academic Press.
3. Satlewal, A., et al., Rice straw as a feedstock for biofuels: Availability, recalcitrance, and chemical properties. Biofuels, Bioproducts and Biorefining, 2018. 12(1): p. 83-107.