For billions of years, nature has perfected the means to deal with the recycling of materials. Like a good housewife who saves as much as she can, she knows how to avoid too much and too little.

The evolution of ruminants is one of the best examples of this thrifty behavior of nature: animals such as deer, goats, cows and antelopes have developed a unique digestive system, made up of four different stomachs, to convert even the toughest grasses and leaves in milk, wool and protein foods.

Remember, when you were a kid, how one cringe at the thought of regurgitating one’s own food to chew it on before it was swallowed again? Well, cows spend about eight hours of their day doing just that, helping their microbial community process raw plant foods.

A cow is therefore a natural example of a consolidated bioprocess where cellulose (in plant material) is hydrolyzed and transformed into various products in a single container (the cow). So what can we learn from nature, and more specifically from cows, in the face of the 83 million tonnes of agricultural, municipal and fruit waste produced each year in South Africa?

According to microbiologists at Stellenbosch University, the concept of a biorefinery based on the four-stomach digestive system of ruminants may well be the key to setting up simple and robust small-scale biorefinery operations in South Africa. South and Africa.

Professor Emile van Zyl, distinguished professor of microbiology at SU, says first world countries have made significant strides in recent decades in developing advanced, capital-intensive technologies to produce bioethanol from hardware. vegetal. Yet, as long as relatively cheap fossil fuels are available, scaling up these technologies will remain commercially unsustainable.

“South Africa and Africa cannot afford the enormous capital costs of large-scale cellulosic ethanol plants and the associated technological challenges,” he explains.

This is why, in a recent review published in the journal Catalysis, they propose the introduction of the rumen microbiome in the anaerobic digestion process. Currently, mixed anaerobic microbial cultures are used to break down organic matter in order to generate mainly biogas, and much research is aimed at finding the most efficient microbes and identifying the parameters of their optimal functioning. Yet, the researchers argue, mammalian ruminants have naturally evolved to perform anaerobic digestion of plant material.

Additionally, instead of producing biogas, they suggest removing this latter part of the digestion process and instead using microbial hosts to produce important industrial organic acids, such as acetic, propionic, butyric, valeric and caproic acids. from agricultural waste. Yeast biotechnology can also be used for the conversion of malic acid in grape and apple pomace into higher value lactic, citric, fumaric and succinic acids. For example, the current value of organic acids can range from about US $ 600 per metric ton for acetic acid to over US $ 2,000 per metric ton for 4-6 carbon carboxylic acids. With the increasing demand for bioplastics, organic polymers such as polylactate are reaching values ​​of over US $ 3,500 per metric tonne.

Professor Marinda Viljoen-Bloom, one of the co-authors, says that while the application of rumen microbes for digestion of plant material is not a new concept, it remains a challenge to replicate the complicated interactions found in ruminants in a bioreactor. In the Biofuels Research Group of the Department of Microbiology, they are specifically studying ways to add value to South African waste streams: For his PhD, Dr Sesethu Njokweni explored the anaerobic production of volatile fatty acids from agricultural waste, while doctoral student Annica Steyn is building a recombinant yeast strain capable of efficiently converting malic acid into higher-value organic acids.

Did you know?

The most important sources of organic waste in South Africa are sugarcane bagasse (5.35 million metric tons), invasive plants (11.30 million metric tons) and fruit waste (1.3 billion metric tons). Microbiologists at the University of Stellenbosch are studying the use of mammalian rumen in the anaerobic digestion process to break down or separate organic wastes into their original building blocks, from where they can then be converted into various products of great value – just as a cow does with processing hard plant matter into the building blocks on which milk production is based.


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