Rape cake: more than just a by-product

A small but growing quantity of European rape seed goes into oil-mills that use cold pressing as a simple and cheap method of plant oil production. Their main product, rapeseed oil, sells as a food, a feeding stuff or as fuel. Rape cake or rape expeller is the solid by-product of an oilpress and makes up the bulk of its output: one tonne of rape seed produces about 660 kg of rape cake. At current market prices, this represents more than a quarter of the mill’s overall turnover.

Production in Europe
Overall pressing capacity is increasing rapidly due to additional small on-farm mills as well as new enterprises for the production of biodiesel with a monthly output of several thousand tonnes. The largest capacities for oil seed processing in the EU are in Germany. This is true for the crushing of oilseeds to produce refined plant oils as well as for oil-mills using cold pressing. In March 2007, researchers at the Technology and Support Centre in Straubing (TFZ) counted 577 German pressing facilities with an estimated annual production of approximately 700.000 tonnes of rape cake – three years ago, production figures had been at about 252.000 tonnes.

In other European countries, the pressing of oil seeds is still a fairly marginal business. Luc Meinrad from the French agricultural organisation TRAME estimates that hundreds of farmers have started pressing their own rape seed, but mostly on a very small scale of a few kilos per hour. In the U.K., a number of on-farm enterprises are producing for the local market. David Proudley, non-food crops adviser at the British National Farmers Union (NFU), expects an increase with the establishment of decentralised biodiesel production. “At present, there is a great interest in the market,” he says. In the Netherlands, rape cake is imported from Germany. A Dutch trader estimates annual domestic production at about 40,000 tonnes, but sees it on the rise.

Full of fat and protein
The most striking difference between rape cake (produced by pressing rape seeds) and rape seed meal (produced by solvent extraction) is the fat content. A single pressing leaves about 12 percent fat in the cake; products from small facilities contain up to 20 percent fat. A second pressing, sometimes with the help of steam, reduces the fat content to about 8 percent. In comparison, hexane extraction produces rape seed meal with only one to three percent fat. Also, rape cake is a largely untreated natural product. Nutrient content as well as anti-nutritive factors of the rape seed – such as glucosinolate content – considerably influence its quality.

The largest market for rape cake is its use as animal feed. It contains high levels of Omega 3 fatty acids, and this is also true of milk and meat products from livestock fed with rape cake. Omega 3 fatty acids are valuable in human nutrition, making rape cake a beneficial ingredient in feeding stuffs used in the production of premium foods. Friedrich Schoene from the Thuringian State Institution for Agriculture (TLL) recommends rape cake as a source of energy and protein in the feeding of high-performing livestock. For dairy cows, about 2.5 kg per day are feasible, depending on the fat content of the ration. Its use is however limited in pig production by its glucosinolate content. Up to ten percent rape cake produced from varieties with low glucosinolate content can be added to pig rations. Broilers can be fed about five percent rape cake.

Many on-farm mills deal directly with local livestock farmers, but feed manufacturers are becoming increasingly active. Farmers who prepare their own feed rations prefer cake with low fat contents. With producers of compound feeds, this is not always the case. Miguel Diaz from the feed manufacturer Deuka sees a fat content of twelve percent as the upper limit, while a Dutch feed trader says: “Decent rape cake contains at least ten percent fat.” In his view, rape cake with high oil content is a cheap source of energy.

Quality counts
For unrestricted access to the feed market, oil-mills need to assure the quality and traceability of their produce. The quality assurance scheme QS offers a relatively simple approach: the control requirements are adapted to small producers, with the German Federation of Decentralized Oil-mills (BDOel) coordinating the system.

Rape cake used to be the problem child of many a pressing facility as it sold at rock-bottom prices. In the wake of high prices for soya bean meal and rape meal, rape cake presently sells for up to 160 euros per tonne in Germany. However, with an increasing number of oil-mills, experts are already warning of overproduction and price deterioration in the future. “Rape cake will remain a product mainly for the regional feed market,” reckons Wiencke von Schenk, who monitors the sector for ZMP, the German information service on agricultural markets. Researchers and associations of oil producers are already starting to explore alternative market opportunities.

Rape cake: burn it…
There is currently a certain amount of research being carried out into the use of rape cake as a source of bioenergy. Due to its high fat content, the fuel value is high, at about 5.31 kWh per kilo compared to 4.00 kWh per kilo for firewood. Nevertheless, rape cake is not an easy fuel: large ash quantities and a low melting point of the ash require appropriate technology, while emissions of dust and nitrogen oxides exceed limits in some EU countries. Some options currently under discussion are the purification of waste gases and the co-firing of rape cake with other fuels. Heating stations and power plants, where exhaust gas purification is state of the art, show some interest in rape cake as an additional source of fuel, as Karl-Josef Gross, head of the feed department at the German Oil-mill Association has found out from a recent survey. Co-firing in a large coal power plant, for instance, may use about 20,000 tons of rape cake per year. The booming market for pellets as household fuels may offer a chance for rape cake as a component in so-called biomass pellets. But it’s still early days: “Before we can work on getting biomass pellets legally accepted for home use, we need to address the technological challenges,” says Janet Witt, who conducts research on the topic at the Institute for Energy and Environment in Leipzig. Moreover, a considerable gap exists between the prices that have been calculated for rape cake as fuel and the present value of the product as feed for livestock.

…or use it for biogas
Technically, rape cake is quite well-suited to anaerobic digestion. About 460 m³ of methane per tonne of dry mass can be expected, a yield approximately 40 percent higher than that of maize silage as substrate. However, biogas plants generally have to count rape cake as a residue or by-product rather than as a renewable raw material. In countries with schemes promoting agricultural biogas production, e.g. Germany and Austria, biogas plants using only energy crops pay for substrates whereas plants that are run with residues generally levy fees for accepting the material. If synergy effects are exploited well, a combination of biodiesel and biogas production may be economical. Suntechnics Bioenergy is planning a project for decentralised biodiesel production which feeds its by-products rape cake and glycerol into an on-site biogas fermenter. In return, the biodiesel facility draws its production heat from the biogas plant’s energy unit.

Rape cake for human foodstuffs
In the future, technologies for processing rape cake into protein food for humans may well yield premium prices for rape expeller. Researchers at Fulda University of Applied Sciences have successfully fermented rape cake to produce a substance comparable to tofu. At the Fraunhofer Institute for Process Engineering and Packaging, scientists intend to develop a system of fractionating rape cake and extract pure proteins for human and animal nutrition. Recently, news of bread baked with rape cake have been circulating.

The bottom line
For the time being, the feeding trough remains the most profitable destination for rape cake. Friedrich Schöne (TLL) has calculated the potential returns on the German market: while rape cake for animal feed sells for up to 180 euros per tonne, the same product used as fuel might fetch anything between zero and 80 euros. For biogas plants, the value is about 40 to 60 euros. Using rape cake as an organic fertiliser turned out to be particularly uneconomical: if the cost for transport and application are included, every tonne of rape cake would result in a loss of ten to thirty euros.

For Germany, current market prices for rape cake are surveyed by the ZMP and published monthly in UFOP-Marktinformationen Ölsaaten und Biokraftstoffe.

(Cf. news of 2007-03-02 and 2007-05-30.)

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nova-Eigenrecherche; Foto: C.A.R.M.E.N. e.V..

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