METHOD OF ISOLATING A PROTEIN CONCENTRATE AND A FIBRE CONCENTRATE FROM FERMENTATION RESIDUE TECHNICAL FIELD OF THE INVENTION The present invention concerns a method of isolating a protein concentrate and a fibre concentrate from fermentation residue, in particular from fermentation residue that is obtained from an ethanol producing fermentation of a cereal selected from the group consisting of barley, corn, rice, wheat, rye, oat or combinations thereof. The present method comprises the steps of (a) preparing a suspension of the fermentation residue in water; (b) separating the suspension into a fibre concentrate containing at least 17 % crude fibre and less than 4% nitrogen by weight of dry matter and a proteinaceous juice containing more than 30% protein and less than 12% crude fibre by weight of dry matter; and (c) concentrating the proteinaceous juice to obtain a protein concentrate containing at least 15 wt. % protein.

Examples of fermentation residues that may suitably be employed as a starting material in the present method include brewer's or distiller's spent grain as well as resides from fermentative alcohol production.

Another aspect of the invention relates to a protein concentrate obtained from brewer's spent grain that is particularly suitable for feed and food applications.

BACKGROUND OF THE INVENTION In conventional processes of making commercial beer, barley malt is mixed with water to form a mash. The enzymes present in the malt are then allowed to break down the barley protein. Next, adjuncts such as cooked corn or rice, are added to the mash, to provide additional starch. Subsequently, enzymatic conversion of the starch to sugar is permitted to take place. After the starch conversion is more or less complete, the aqueous extract or wort is separated by filtration from the spent solids, which are commonly known as"brewer's spent grain. "The wort continues through the brewing process and eventually becomes beer.

Substantial quantities of brewer's spent grain are produced in the commercial production of beer. Typically, for each hectolitre of beer about 15-20 kg of wet spent grain is produced. The breweries usually dispose of the spent grain by selling it for cattle feed.

The brewer's spent grain contains all the solids that have been separated from the wort by filtration; it includes what is left of the barley malt and the adjuncts. The spent grain consists mainly of the pericarp and hull portions of the barley and of non-starchy parts of corn, provided corn grits were used as an adjunct. Although"spent"in terms of carbohydrate, brewer's spent grain is higher in protein, lipids, and fibre than was the original barley-adjunct mixture.

The crude fibre content of brewer's spent grain is approximately 150 g/kg dry matter, making it unsuitable as a feed for non-ruminant animals, e. g. pigs or chickens. Furthermore, spent grain as obtained from the brewery or distillery is a very bulky material due to the high water content (70-80% w/w), making handling and transport inefficient. The high water content also makes the spent grain material vulnerable to microbiological decay.

Attempts have been made in the past to develop processes for obtaining a dry pepsin- digestible protein-fraction from spent grain that can suitable be used as a component in animal feed for non-ruminants.

US 5,135, 765 (Kirin Beer Kabushiki Kaisha) is concerned with a process for producing a protein-rich product and a fibrous product from brewer's spent grain containing germ, husks and a proteinaceous material adhering to the husks, comprising: (a) providing a wet brewer's spent grain having a water content of at least 65 wt. %; (b) passing said wet brewer's spent grain through a roll mill to press said brewer's spent grain with simultaneous grinding of the germ and the proteinaceous material, thereby separating said germ and proteinaceous material from said husks; and (c) sieving the brewer's spent grain in water after passage through the roll mill to obtain a protein-rich product containing the germ and at least 50% proteinaceous material. Disadvantages of this method are the low separation efficiency and the intrinsic problems associated with the continuous operation of sieves.

US 4,938, 986 (Binding-Brauerei) describes a method of recovering roughage from brewery draff comprising: (a) adding water to draff in a weight ratio of about 1: 1, said draff containing a soft protein fraction and a hard roughage fraction; (b) mixing said draff and water to enhance separation of said soft protein fraction from said hard roughage fraction; (c) mechanically separating said soft protein fraction from said hard roughage fraction by using at least one of a screen press or a decanter; (d) removing water from said separated roughage fraction; and (e) drying said separated roughage fraction. Drawbacks of this recovery method are the large quantities of water used and the low separation efficiency (especially in case a decanter is used) WO 01/45523 (2B AG) describes a method of continuous separation of vegetable biomass into a fluid phase essentially comprised of aqueous juice and a solids containing phase of pulpy consistence, wherein the vegetable biomass is provided in a naturally moist state and admixed to a recycled portion of the aqueous juice for suspension therein, and wherein the flowing suspension of vegetable biomass in aqueous juice is processed for fibre production, characterized in that said processing involves the generation of a gradient of flow speed within the flowing suspension, which gradient is sufficient for acting upon cells of the vegetable biomass to open said cells and allow a content thereof to be liberated into the aqueous juice, whereas a residue is left that is mainly made up of fibres and ends up in the solids containing phase. The separation of spent grain in a raw protein and a raw fibre fraction is described in example 3. The method described in the PCT-application employs a high shear device that requires a high energy input and that needs regular maintenance.

Thus, each of the aforementioned prior art methods suffers from one or more drawbacks. The present invention aims to offer a method which does not suffer from these drawbacks.

It is known in the art to isolate high-protein fractions from spent grain and to use these in the production of food or animal feed. The nutritional value of these protein fractions is limited due to a relative shortage of the essential amino acid lysine.

Outside the brewing and distillers industry, large scale fermentation of cereals has been exploited as a commercially viable route to produce ethanol, e. g. fuel ethanol. Also in these fermentation methods a protein-rich fermentation residue is produced that contains high amounts of fibre that make it less suitable for use in animal feed and human food.

US 4, 810, 647 describes a process of producing ethanol and gluten from cereals by: (a) soaking crushed cereals in water; (b) mechanically separating the resulting mixture to obtain a sediment and a suspension containing at least 60% of the initially present nitrogen substances; (c) separating the suspension obtained in step (b) to isolate an insoluble fraction containing more than 50% of the initially present proteins and a supernatant fraction; (d) mixing the sediment from fraction (b) with the supernatant fraction of step (c) to produce a final suspension containing at least 90% of the initial starch; (e) submitting the starch contained in the final suspension to enzymatic hydrolysis; (f) fermenting the hydrolyse suspension with a strain of alcoholic yeast; (g) separating the fermented suspension to obtain a supernatant ethanol-containing fraction and a sediment which constitutes a proteinic foodstuff; and (h) distilling the supernatant fraction to recover the ethanol.

The process according to US 4,810, 647 is very complex in that it employs a large number of processing steps and it utilises large quantities of water. Furthermore, the separation of the fermented suspension in step (g) yields a proteinaceous sediment that contains a relatively low amount of protein (based on dry weight).

SUMMARY OF THE INVENTION The inventors have developed a simple, low cost and energy efficient method for isolating a protein concentrate and a fibre concentrate from fermentation residue. The method according to the invention enables a very effective separation of the proteinaceous and fibrous components contained in the fermentation residue and can be operated on a continuous basis for a prolonged period of time.

A critical element of the present method is the addition of water to the fermentation residue and the subsequent formation of a suspension prior to the actual separation. By bringing the fermentation residue into suspension, protein contained therein (e. g. attached to the husk), is allowed to disengage itself from the fibrous component within the same fermentation residue and to disperse into the aqueous phase. Thus, a suspension is obtained wherein a significant fraction of the protein is present as disengaged protein particles, i. e. as protein particles that are no longer physically attached to fibres, or as dissolved protein.

Surprisingly, it was found that neither conditions of high shear (nor of comminution) nor addition of chemicals are required to ensure that the bulk of the protein and fibres within the suspension become separated and dispersed into the aqueous phase. In the absence of conditions of high shear the size of the proteins and the fibres remains largely unchanged and subsequent separation of the suspension into a proteinaceous juice containing the dispersed protein and a fibre concentrate can surprisingly easily be achieved by a variety of mechanical separation techniques that are well-known in the art.

It is believed that due to the fact that the present method, unlike methods of the prior art, does not employ mechanical comminution of the fermentation residue material, very effective separation of the proteinaceous and fibrous component can be achieved.

Comminution effectively reduces the average size of particularly the fibrous components, making it generally more difficult to later on separate the fibre concentrate from the proteinaceous juice.

As a result of the highly efficient separation, the present method typically yields a fibre concentrate containing at least 17% crude fibre and less than 4% nitrogen by weight of dry matter and a proteinaceous juice containing more than 30% protein and less than 12% crude fibre by weight of dry matter.

Another crucial aspect of the present method resides in the use of a recirculated fraction of the proteinaceous juice to produce the suspension of fermentation residue which is subjected to the separation step. The use of such a recirculated fraction to produce the suspension offers the advantage that effective separation of the proteinaceous and fibrous components is achieved without the external supply of a large amount of water. Evidently, this has a favourable impact on the amount of energy, the amount of water and the size of the equipment required to subsequently dry the proteinaceous juice to a protein concentrate as well as on the total amount of wastewater that is generated in the process.

The fraction of the proteinaceous juice which is not recirculated is concentrated so as to obtain a protein concentrate containing at least 10 wt. % protein and an aqueous deproteinated stream. The protein concentrate so obtained may be dried to a highly nutritious and microbiologically stable product. The aqueous deproteinated stream, or a part thereof, may suitably be recirculated and combined with the fermentation residue at the beginning of the process The present method offers the important advantage that it enables effective separation of the proteinaceous and fibrous components without the need of excessive energy or water input, using a simple and highly robust configuration that can be operated without interruptions and with very little supervision for long periods of time. In the processes of the prior art high shear forces are often employed to ensure that proteinaceous material and germ are separated from the husk. In the method according to the present invention no high shear or comminution devices are employed The protein concentrate that can be obtained from the present process, has a very favourable nutritional profile which makes it particularly suitable for use in animal feed, especially in fodder for chickens, pigs or cows.

DETAILED DESCRIPTION OF THE INVENTION Accordingly, one aspect of the invention relates to a method of isolating a protein concentrate and a fibre concentrate from fermentation residue, said fermentation residue being obtained from an ethanol producing fermentation of a cereal selected from the group consisting of barley, corn, rice, wheat, rye, oat or a combinations thereof, said method comprising: a. combining the fermentation residue with a recirculated portion of a proteinaceous juice obtained from step d. to obtain a homogeneous suspension; b. providing a dispersion within the suspension of disengaged protein particles originating from the fermentation residue; c. separating the suspension into a fibre concentrate containing at least 17 wt. % crude fibre and less than 4% nitrogen by weight of dry matter; and a proteinaceous juice containing more than 30% protein and less than 12% crude fibre by weight of dry matter; d. recirculating a portion of the proteinaceous juice to step a. and concentrating the remaining portion so as to obtain a protein concentrate containing at least 15 wt. % protein and an aqueous deproteinated stream ; wherein the particle size of the fibres contained in the suspension remains essentially constant during steps a. and b.

The term"fibre"as used herein is synonymous to"crude fibre"and refers to the skeletal components of the plant cell that are largely resistant to digestion by enzymes in the digestive tract of non-ruminant mammals. Typical fibre components include pectin substances, gums and other carbohydrates as well as lignin and cellulose. Whenever reference is made in this document to the fibre content of or fibre concentration in a material, said content or concentration is determined by means of the analytical method described as ISO method EN ISO 6865: 2000 E, titled:"Animal feeding stuffs-Determination of crude fibre content-Method with intermediate filtration (ISO 6865 : 2000)" The terminology"particle size of the fibres"refers to the dimension of the individual fibres and fibre parts present in the suspension, more particularly to the volume of these particles.

As explained above, prior art methods generally apply techniques, such as grinding, cutting or hammering to disengage the protein from the fibres. As a result of the application of such comminution techniques, the individual fibres disintegrate into smaller fibres. In contrast, in the present method the particle size of the fibres remains essentially constant. In practice, the volume weighted average particle size of the fibres decreases by not more than 50%, preferably by not more than 40%, during steps a. and b. More preferably, during steps a. and b. , the volume weighted average particle size of the fibres decreases by not more than 30%, by not more than 20%, or even by not more thanl0%. Most preferably the decrease in volume weighed average particle size of the fibres observed during steps a. and b. does not exceed 5%. In order to establish the impact of the present process on the volume weighted average particle size of the fibres, the following method of analysis may suitably be employed (on fresh samples): laser diffraction method (mastersizer range 4-3470 um) In a particularly preferred embodiment, the present method employs a separation technique that also does not seriously affect the particle size of the fibres. In general, when the fibre concentrate and proteinaceous juice obtained from the separation step c. are recombined and the volume weighted average fibre particle size of the combined streams is compared with that of the suspension obtained in step a. , it can be concluded that the average particle size of the fibres has decreased by not more than 50%, preferably by not more than 40% during steps a. to c. More preferably, during steps a. to c. , said volume weighted average particle size decreases by not more than 30%, particularly by not more than 20%, more particularly by not more than 10%. In a most preferred embodiment the volume weighted average particle size decreases by not than 5% during steps a. to c.

The present method may suitably be employed to process fermentation residue of various origins. The composition of fermentation residue that can suitably be processed through the present method can be typified as follows (% on dry weight): Fibre 15-22 %, preferably 16-18 % Protein 15-40 %, preferably 20-35 % Fat less than 20 %, preferably 5-15 % Starch less than 10 %, preferably less than 5 % Other carbohydrates 40-70 % preferably 50-59 % These 5 components usually constitute at least 75 wt. %, preferably they constitute at least 85 wt. % of the dry matter contained in the fermentation residue. Typically, the fermentation residue contains up to 90 wt. % water, preferably it contains between 50 and 85 wt. % water.

As indicated above, the fermentation residue used in the present method should preferably contain a significant amount of water. Such a wet fermentation residue may be obtained as such, e. g. from a brewing or distilling process, or alternatively it may be reconstituted from (partially) dried fermentation residue. Particularly good results are obtained in the processing of fermentation residue if said fermentation residue is spent grain from a brewing or distilling process. In another preferred embodiment most of the solids present in the fermentation residue are derived from barley.

The separation step in the present method preferably employs a mechanical separation technique. Such a technique may advantageously achieve separation on the basis of differences in density and/or size. Examples of suitable equipment that may be used to separate on density are: centrifuges, hydrocyclones and decanters. Suitable apparatus for achieving separation on size includes: sieves, filters and membranes.

In the present method sieves enable highly efficient separation of the spent grain suspension into proteinaceous juice and fibre concentrate. Because sieves do not contain moving parts, their operation is highly reliable and requires little maintenance. Hence, in a particularly preferred embodiment of the invention, the suspension is passed over one or more sieves.

The term"sieve"as used herein refers to a sheet of solid material that is provided with a plurality of passages of such a size that water and small proteinaceous particles dispersed in said water can pass, whilst the fibre material of a larger particle size, contained in the suspension of fermentation residue, is denied passage through the sieve. The solid material of which the sieves have been made can suitably be chosen from the group consisting of metal, ceramics, cloth materials, plastics and combinations thereof. The sieve according to the present invention can, for instance, take the form of a screen, a perforated plate or a cloth. The passages of the sieve may be rectangular, round, triangular or take any other appropriate shape. In a particularly preferred embodiment the present method employs one or more vibrating screens to separate the suspension into the fibre concentrate and the proteinaceous juice. Vibrating screens were found to offer the advantage that separation efficiency can be maintained during prolonged periods of time.

Typically, the sieves employed in accordance with the present invention exhibit pore sizes of at least 0.05 mm, preferably of at least 0.07 mm and most preferably of at least 0.08 mm. Usually, the pore size of the sieves will not exceed 1 mm. Preferably the pore size does not exceed 0.7 mm, more preferably it does not exceed 0.5 mm.

The term"pore size"as used herein refers to the largest diameter of a passageway through the sieve. The basic principle of the present separation by means of sieves is to achieve a particle size based separation of small protein particles and bigger crude fibre particles.

The sieves employed in the present method yield one flow enhanced in fibrous components and a second flow enhanced in proteinaceous components. Typically, if only one screen is used, it is operated in such a fashion that the weight ratio between protein flow and the fibre flow is within the range of 1: 1 to 4: 1. More preferably, said ratio is within the range of 3: 2 and 3: 1. When using multiple screens, the aforementioned ratio is calculated on the basis of the total flow of proteinaceous juice and fibre concentrate generated with the help of these screens.

In an alternative embodiment of the invention the separation of the suspension into the proteinaceous juice and fibre concentrate is achieved under the influence of centrifugal forces, e. g. by employing one or more centrifuges and/or hydrocyclones. Separation under the influence of centrifugal forces may be achieved in a particularly advantageous manner by passing the suspension through one ore more hydrocyclones. In particular if one or more of these hydrocyclones are operated at a centrifugal force of at least 50 G, more preferably of at least 100 G, very effective separation can be achieved.

As a result of the particularly effective separation in the present method, a fibre concentrate exhibiting a nitrogen content of less than 3% by weight of dry matter and/or a fibre content of at least 18% by weight of dry matter may be obtained. Similarly, a proteinaceous juice can be obtained that contains less than 10%, preferably less than 8% fibre by weight of dry matter. The protein content of the proteinaceous juice obtained from the separation process can easily exceed 40%, or even 45%, calculated by weight of dry matter.

Preferably, the protein content of the proteinaceous juice is at least 60% higher, more preferably at least 80% higher and most preferably at least 100% higher than the protein content of the original fermentation residue, both protein contents being calculated on dry weight.

Like spent grain, surplus yeast is a co-product of breweries and distilleries. It was unexpectedly found that such surplus yeast may advantageously be used in the present method to produce a protein concentrate of particularly high nutritional value. Surplus yeast is an excellent source of lysine, which amino acid is underrepresented in spent grain.

Consequently, in a particularly preferred embodiment of the invention, surplus yeast obtained from a brewing or distilling process is combined with the proteinaceous juice or protein concentrate obtained from spent grain in a dry weight ratio of between 3 and 50%. Most preferably the surplus yeast is obtained from the same brewing process that yielded the spent grain.

The present method offers the important advantage that it does not require the application of a high amount of mechanical energy to form the suspension and/or to disperse the protein contained in the fermentation residue. Typically the mixing energy employed in steps a. and b. of the method does not exceed 2 kJ/kg, preferably it does not exceed 0. 5 kilkg, more preferably it does not exceed 0.3 kJ/kg. In a particularly preferred embodiment the mixing energy employed in steps a. to c. does not exceed 2 kJ/kg, more preferably it does not exceed 0.5 kJ/kg. In steps a. and b. application of some shear force is required to form and maintain the suspension. This may, for example, be achieved by simple stirring or by recirculation.

In the present method a significant part of the proteinaceous juice needs to be recirculated in order to enable the formation of a suspension. Generally between 40 and 98 % of the proteinaceous juice is recirculated, preferably between 50 and 90% of the juice is recirculated and most preferably between 60 and 80% is recirculated. Whenever reference is made to recirculation of a portion of the proteinaceous juice, this should be understood to also encompass recirculation of an aqueous deproteinated stream that is obtained in the course of concentrating the proteinaceous juice.

In a preferred embodiment of the invention, the recirculated proteinaceous juice is combined with the fermentation residue in a weight ratio of between 1: 1 and 200: 1 (dry matter content in mixing tank 0.1-10 wt. %) More preferably the proteinaceous juice and fermentation residue are combined in a weight ratio between 4: 1 and 40: 1 (dry matter content in mixing tank 0.5-4 wt. %), most preferably in a weight ratio between 9: 1 and 20: 1 (dry matter in mixing tank 1-2 wt. %).

It was found that the efficiency of the present method may be enhanced significantly by employing one or more sedimentation vessels in which the proteinaceous juice and/or fibre concentrate is allowed to settle out, thereby forming a bottom layer with a relatively high solids concentration and a top layer exhibiting a much lower solids concentration. The top layer may advantageously be recirculated to step a. , whereas the bottom layer is advantageously further dewatered. Typically, the residence time of the proteinaceous juice or fibre concentrate in the sedimentation vessel is in the range of 3-100 minutes. Preferably, said residence time is in the range of 5-60 minutes.

The recirculation flow may suitably be composed of partial or complete streams from potentially any of the different unit operations. Examples are the supernatants from the cyclones, settling tanks, the decanter and the fibre press. Naturally, the recirculation flow may also consist of a combination of the aforementioned streams.

The quantity of water that leaves the process in the form of the proteinaceous juice that is further concentrated preferably has a water content that approximately maintains the water balance and makes up both for the quantity of water that enters the process in the form of moisture contained in the fermentation residue and the quantity of water that leaves the process in the form of moisture contained in the fibre concentrate.

The fraction of the proteinaceous juice that is not recirculated may be concentrated by means of various techniques well-known in the art, including centrifugation, filtration, membrane filtration and decantation. Preferably said fraction of the proteinaceous juice is concentrated by means of centrifugation and/or decanting, most preferably by decanting. The aqueous deproteinated stream obtained after concentration usually contains less than 1 wt. % protein, preferably less than 0.5 wt. % protein.

The water content of the fibre concentrate obtained from the separation step is also advantageously reduced, preferably to less than 60 wt. %. Such reduction in water content may suitably be achieved by decanting, pressing and/or drying. Preferably a combination of these techniques is used to reduce the water content, e. g. by first removing water by pressure followed by drying.

Another aspect of the invention relates to the use of the dewatered fibre concentrate as a combustible in the present process or a process whose co-product is the present fermentation residue. In this particular embodiment of the invention optimum energy efficiency is achieved if the fibre concentrate has been dewatered to a water content of between 60 and 40 wt. % before it is burnt.

Alternative uses for the fibre concentrate obtained from the present method are animal litter and isolation material. In order for the fibre concentrate to be suitable for such applications, its water content should be reduced to less than 10 wt. %, preferably to less than 5 wt. %.

A further aspect of the invention concerns a protein concentrate obtained from brewer's spent grain and surplus yeast, said concentrate containing from 40-80% protein and less than 12% fibre calculated by weight of dry matter, the amino acid composition of said protein concentrate, calculated on protein, being characterised as follows: lysine 30-50 mg/g, preferably 35-45 mg/g methionine 12-30 mg/g, preferably 15-22 mg/g cysteine 12-30 mg/g, preferably 15-22 mg/g threonine 20-45 mg/g, preferably 25-40 mg/g tryptophan 8-18 mg/g, preferably 10-15 mg/g.

The protein concentrate of the invention may be obtained directly from the separation method described above, or it may obtained by blending a protein concentrate obtained from such a separation method with surplus yeast material. A small fraction of non-processed spent grain may also be added, provided the resulting fibre content does not exceed 10 wt. %.

Preferably the fibre content of the present concentrate is maintained below 8 wt. %, more preferably below 6 wt. % by weight of dry matter.

As mentioned hereinbefore a protein concentrate obtained from the combination of spent grain and surplus yeast exhibits a very favourable nutritional profile because the surplus yeast is an excellent source of lysine, which amino acid is underrepresented in the spent grain, especially if said spent grain mainly contains barley-derived material. Preferably spent grain and surplus yeast are used in such a ratio that the resulting concentrate meets the following equation (using amino acid concentrations in mg/kg protein): 0. 28 < [lysine]/ ([lysineJ+ [methionine] + [cysteine] + [threonine] + [tryptophan] ) < 0.5. In a particularly preferred embodiment, the latter ratio exceeds 0.3.

The fat that is naturally present in spent grain and surplus yeast is a nutritionally valuable component. The protein concentrate of the present invention typically contains between 6 and 20% fat, preferably between 8 and 15% fat by weight of dry matter, said fat preferably being derived from the spent grain and surplus yeast.

Yet another aspect of the invention concerns the use of the aforementioned protein concentrate in food or animal feed, especially feed for chickens, pigs or cows. Due to the exceptionally high nutritional value of the protein concentrate, it is preferably used in fodder for chicks, piglets or calves, most preferably in fodder for chicks or piglets.

The features and advantages of the present invention will be evident from the following, more detailed description of one embodiment of the invention and the accompanying figure.

Figure 1 is a diagram showing one embodiment of the method according to the invention. In Figure 1, the arrows show the various directions of flow of the fluids and pastes.

The vessel 1 contains a suspension 2. Said suspension 2 is obtained by combining a stream of fermentation residue 3 with a recirculated portion of a proteinaceous juice 4. The vessel 1 may suitably contain a stirring device to facilitate the formation of the homogeneous suspension 2. Alternatively, the stream of fermentation residue 3 may be combined with the stream of recirculated juice 4 by means of in-line mixing following which the combined streams are fed to the vessel 1. The feeding of vessel 1 may suitably occur in a continuous or semi-continuous fashion.

The suspension 2 is fed from vessel 1 to one or more sieves 5, in which the suspension is separated in a top flow that consists of proteinaceous juice 6 and a bottom flow that consists of fibre concentrate 7. A fraction of the proteinaceous juice 6 may be recirculated to the vessel 1.

At least a part of the proteinaceous juice 6 is fed from the sieves 5 to a sedimentation vessel 8 that is operated in a continuous fashion. In the sedimentation vessel 8, the proteinaceous juice 6 is allowed to separate into a low solids top fraction 9 and a high solids bottom fraction 10. The low solids top fraction 9 is advantageously recirculated to the vessel 1. The bottom fraction 10 is dewatered by feeding it into a centrifuge 11. The protein concentrate 13 that is obtained from the centrifuge is subsequently dried to a proteinaceous powder 22 in the drier 14. The wastewater 12 obtained from the centrifuge may be recirculated to vessel 1.

The fibre concentrate 7 obtained from the sieves 5 is fed into a sedimentation vessel 15, where it is allowed to separate into a low solids top fraction 16 and a high solids bottom fraction 17. The sedimentation vessel 15 is operated in a continuous fashion. The low solids top fraction 16 is wastewater, but may at least partly be recirculated to vessel 1. The high solids bottom fraction is fed to a press 18 for dewatering, generating a stream of wastewater 19. The dewatered fibre containing fraction 20 is fed to a drier 21, in which the water content is reduced to a sufficiently low level to obtain a combustible fibre concentrate 23.

The invention is further illustrated by means of the following examples.

EXAMPLES Comparative Example A Spent grains obtained from a lautertun from the Heineken@ Brewery in Zoeterwoude, the Netherlands were used as starting material.. The spent grains had a protein concentration of 24 wt. % and a crude fibre concentration of 21 wt. %, each calculated on dry matter.

A stream of spent grains was dosed continuously into a mixing vessel and mixed therein with a stream of 749 L/h water to obtain a dry matter concentration of 6.0 wt. % in the mixing vessel. The outlet stream of this vessel was pumped onto a vibrating sieve with a pore size of 0.100 mm. This continuous sieve was equipped with a continuous cleaning installation below the sieve.

The coarse or fibrous outlet of the sieve had a flow rate of 289 L/h and a dry matter content of 18.0 wt. %. Analysis of samples taken from the coarse outlet stream showed a protein concentration of 18 wt. % and a crude fibre concentration of 30 wt. %, each calculated on dry matter.

The proteinaceous stream at the bottom of the sieve was 743 L/h and had a dry matter concentration of 1.7 wt. %. Analysis of samples taken from this stream showed a protein concentration of 50 wt. % and a crude fibre concentration of 2.3 wt. %, each calculated on dry matter.

The fibre stream containing the coarse particles were fed into a screw press and dewatered to a dry matter concentration of 38 wt. %. The proteinaceous stream was fed into a decanter centrifuge and dewatered into a thickened product with a dry matter concentration of 29 wt. %.

Example 1 Comparative Example A was repeated using spent grains of comparable quality. The spent grains had a protein concentration of 26 wt. % and a crude fibre concentration of 20 wt. %, each by weight of dry matter. The flow of mixed spent grains suspension from the mixing vessel had a dry matter concentration of 4.8 wt. % and a flow rate of 494 L/h. This was initially achieved by addition of water to the mixing vessel.

Separation was performed by the same sieve as described in Comparative Example A.

The coarse or fibrous outlet of the sieve had a flow rate of 154 L/h and a dry matter content of 16 wt. %. Analysis of samples taken from the coarse outlet stream showed a protein concentration of 18 wt. % and a crude fibre concentration of 25 wt. %, each calculated on dry matter. The proteinaceous stream at the bottom of the sieve was 385 L/h and had a dry matter concentration of 1.9 wt. %. Analysis of samples taken from this stream showed a protein concentration of 51 wt. % and a crude fibre concentration of 3.1 wt. %, each calculated on dry matter.

The coarse fibrous fraction was fed into a screw press and the proteinaceous stream was fed into a decanter. The liquid stream from the screw press contained 1.2 wt. % dry matter and had a protein concentration of 35% by weight of dry matter and a crude fibre concentration of 7% by weight of dry matter. The latter stream was recirculated into the initial mixing vessel. After initial start-up with water, this amount of liquid recirculated was subtracted from the flow of fresh water added to the mixing vessel. The proteinaceous product from the decanter outlet had a dry matter concentration of 28 wt. % and a protein concentration of 61 % by weight of dry matter and a crude fibre concentration of 3. 1 % by weight of dry matter. The fibre product from the outlet of the screw press had a dry matter concentration of 40 wt. %, a protein concentration of 17 wt. % and a crude fibre concentration of 27 wt. %.

During this trial no accumulation of materials were observed and steady-state operation was achieved. The recirculation of the liquid stream from the screw press clearly did not adversely affect separation efficiency as the yields obtained were similar to those achieved in Comparative Example A.

Example 2 Spent grains obtained from a Mash filter from the Brand 0 Brewery in Wijlre (the Netherlands) were processed in the equipment depicted in figure 1.

At the beginning of the trial 185 kg tapwater was added to a stainless steel mixing tank with a net volume of about 200 1. Subsequently 21 kg of the spent grain (from cold storage) was added. The mixing tank was mildly agitated to obtain a homogeneous suspension. No external heating or cooling is applied during the trial. The dry matter content of the homogeneous suspension was 3.05 % by weight of dry matter.

The homogeneous suspension was pumped into a hydrocyclone (porcelain, type PZ- 100/15, manufactured by Dorr-Oliver) at a flow rate of 10.3 m3/h by means of a centrifugal pump. The top flow from the hydrocyclone was found to be 4.8 m3/h and the bottom flow 5.5 m3/h. Both the top and bottom flow were recirculated to the mixing tank. After running for 15 minutes samples were taken from feed, top and bottom flow for analysis and settling tests.

The sample taken from the protein-rich top flow had a dry matter content of 1.34 wt. % and was found to separate easily into two layers during the settling test. The bottom layer obtained after 100 minutes of settling time, representing 40% of the original sample, had a dry matter content 3.05 wt. % (a concentration factor of 2.28). The remaining liquid had a dry matter content of 0.3 wt. %.

The sample taken from the fibre-rich bottom flow has a dry matter content of 4.35 wt. % and was also found to separate easily into two layers during the settling test. The bottom layer obtained after 100 minutes of settling time, representing 43% of the original sample, had a dry matter content of 11.7 wt. % (a concentration factor of 2.69). The remaining liquid had a dry matter content of about 0.3 wt. %.

Example 3 Example 2 was repeated except that this time fresh spent grains from a lautertun were used (ex Heineken @ brewery Zoeterwoude, the Netherlands).

212 kg of tap water and 25 kg of the fresh spent grain were added to the mixing tank.

The resulting suspension displayed a dry matter content of 1.5 wt. % and was pumped through the hydrocyclone at a flow rate of 9.6 m3/h. The top flow from the hydrocyclone was found to be stable at 4.8 m3/h and the bottomflow 4.8 m3/h. After 15 minutes of running time, samples were taken from the feed, top and bottom flow for analysis and settling tests.

The sample taken from the protein-rich top flow had a dry matter content of 0.5 wt. % and was found to separate easily into two layers during the settling test. The bottom layer obtained after 60 minutes of settling time, representing 17% of the original sample, had a dry matter content 2.47 wt. % (a concentration factor of 4.94). The remaining liquid had a dry matter content of 0.15 wt. %.

The sample taken from the fibre-rich bottom flow has a dry matter content of 3.6 wt. % and was also found to separate easily into two layers during the settling test. The bottom layer obtained after 60 minutes of settling time, representing 40% of the original sample, had a dry matter content of 10 wt. % (a concentration factor of 2.78). The remaining liquid had a dry matter content of about 0.12 wt. %.