Large amounts of rapeseed from the production of biodiesel can be used in feed for Nile tilapia, one of the most important fish species in global aquaculture.
"The world needs food and the world needs energy. Therefore it is so exciting to upgrade by-products from biodiesel, in this case rapeseed meal, to a better protein source for fish", said Trond Storebakken. He is professor at the Aquaculture Protein Centre, a Norwegian Centre of Excellence that is affiliated with the Norwegian University of Life Science (UMB).
The Chinese PhD student Youling Gao recently proved that high amounts of rapeseed meal can be used efficiently in feed for tilapia. "We observed fast growth, efficient feed utilization, and found no health problems", Storebakken said.
The access to protein rich rapeseed meal will increase as the demand for rapeseed oil for biofuel increases. The disadvantage with rapeseed meal from biofuel production is that unwanted substances called glucosinolates are concentrated in the meal.
These substances are natural pesticides for the plant and prevents the rapeseed against being eaten by insects. They’re not poisonous in themselves, but are transformed to toxins by an enzyme in the rapeseed when the plant is damaged or attacked by insects. The same type of enzymes can be activated when rapeseeds are processed into feed.
Once rid of these glucosinolates, rapeseed meal will be an even better protein source for farmed fish. Recently APC tried to destroy the glucosinolates by subjecting rapeseed meal to solid-state fermentation. In the trials preformed by APC it was difficult to obtain stable conditions during fermentation, but the tilapia grew better on fermented rapeseed meal, than on untreated rapeseed meal.
The rumen is the solution
Scientists from APC therefore move on to find out how to efficiently utilize rapeseed meal from the production of biodiesel in feed for tilapia, with the help of fermentation. Ruminants have an effective way of getting rid of glucosinolates, by letting microbes in the rumen ferment the feed.
"Inspired by what’s going on in the rumen, we’re now trying out wet fermentation with two different cultures of microbes. While the first culture of microbes will remove the fiber network that protects the glucosinolates, the next will destroy them once released", said Olav Fjeld Kraugerud, postdoctor at APC.
The conditions at the lab are naturally not identical to the rumen, but the main principles are the same.
Rapeseed is the most important protein producing plant on world basis next to soy. But the utilization of this plant to other animals than ruminants is limited because of glucosinolates and phytic acid. These are the substances APC wants to remove.
"With the large interest we’re seeing on using the oil from rapeseeds to produce biodiesel, the protein is left over and we have to utilise it for food", said Storebakken.
Nile tilapia is a very important fish for farming in the Southeast Asia and South America, but its origin is Africa. Norwegian research institutions have been crucial to facilitate the production of tilapia, as the food research institute Nofima was a central developer of a selective breeding program for tilapia.
"Today, tilapia from this breeding program are extremely fast growing, and they efficiently utilise feed based on inexpensive plant ingredients. That’s why we want to utilise the inexpensive rapeseed meal from biodiesel production, that is not suitable for direct human consumption, in feed for exactly this species", Storebakken said.
The tilapias that APC used in the experiments are farmed at UMB in Norway. APC uses tilapia in nutritional research, where they work on different aspects of feed and feeding of tilapia.