FIELD OF THE INVENTION

The present invention relates to a process for separating proteins from biomass materials. More particularly the invention relates to a process, where biomass material comprising proteins is treated with a deep eutectic solvent (DES) for providing proteins and protein fractions with high yields and purity. The invention relates further to the use of the obtained proteins and protein fractions in foods and in animal feeds.

BACKGROUND OF THE INVENTION

There is constantly an outstanding protein shortage in several parts of the world, and further, a major part of the current protein production is not carried out in a sustainable way. The demand for protein products particularly in human nutrition and as animal feeds is enormous and continuously growing with the world population and rising living standards.

Many agricultural side-streams and waste materials, as well as side streams originating from the food industry contain high contents of protein. Said materials and side streams are currently utilized only to a limited extent. For value-added applications it is necessary to extract the proteins from the side stream matrix, but in many cases the currently used extraction methods with aqueous alkali solutions only provide low yields of extracted protein. The quality of the proteins may also be low after the alkali extraction, and typically unwanted compounds are co-extracted with the proteins necessitating further purification of the product.

In the alkali extraction method proteins are enriched from side streams of agrological origin with NaOH solutions at pH 10-12. Particularly, in the extraction of more recalcitrant starting materials, such as brewer's spent grain (BSG) or wheat bran (WB) the yields are not satisfactory. The high pH of the solution may lead to unwanted protein denaturation or even partial degradation. In general, one or several pH adjustment steps are needed in the work-up, leading to extensive use of acid and base chemicals as well as solvent water, which needs to be separated at a later point. Co-extraction of unwanted compounds is also a problem during the alkaline extraction of various feeds. A eutectic mixture forms a eutectic with a melting point much lower than any of the individual components. The phenomenon was first described in 2003 for a mixture of choline chloride and urea in a 1 : 2 mole ratio, respectively. Choline chloride has a melting point of 302°C and that of urea is 133°C. The eutectic mixture melts at 12°C.

WO 2011/155829 Al discloses a method for extracting materials from biological products using ionic liquids or deep eutectic solvents, particularly flavonoids, flavors and fragrances from plants, and antibiotics and colorants from microorganisms.

US 2010/0163018 Al relates to the fractionation of biomass materials with an ionic liquid to cellulose-enriched fractions and lignin-enriched fractions.

Despite the ongoing research and development of processing of agricultural side-streams and waste materials, as well as side streams originating from the food industry, there is still a need to provide an improved process for separating proteins from biomass materials.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved process for separating proteins and/or protein fractions from biomass materials.

Another object of the invention is to provide a process for separating proteins and/or protein fractions from biomass materials, where biomass material comprising proteins is treated with a deep eutectic solvent (DES) for providing proteins and/or protein fractions with high yields and purity.

Another object of the invention is the use of the obtained proteins and protein fractions in foods and in animal feeds. The present invention generally concerns a process for separating proteins and/or protein fractions from biomass materials, where biomass material comprising proteins is treated with a DES for providing proteins and/or protein fractions with high yields and purity.

Particularly, the invention relates to a process for separating proteins and/or protein fractions from biomass materials, where

in the first step biomass material is mixed with an aqueous DES comprising a hydrogen bond acceptor selected from alkali metal carboxylates and earth alkaline metal carboxylates, and urea as hydrogen bond donor to obtain biomass mixture, in the second step the biomass mixture is separated into a non-solubilized fraction and a liquid fraction; and

in the third step, proteins are separated from the liquid fraction using technique based on molecular size to yield a protein solution, or using technique based on isoelectric precipitation to yield a protein precipitate.

The invention also relates to the use of the obtained proteins and protein fractions in foods and in animal feeds.

Characteristic features of the invention are presented in the appended claims.

DEFINITIONS

The term "alkali metal or earth alkaline metal carboxylate" means here a compound having the structure I , where X denotes an alkali metal or earth alkaline metal selected from Na, K, Li, Be, Mg or Ca, preferably Na or K and Y denotes a mono- or polyvalent carboxylate, which may carry a straight alkane chain with a length between 1 and 20 carbons, an aromatic moiety, a branched alkane, a multivalent carboxylic acid, alcohol groups, alkenes and alkynes with chain lengths of 1 to 20 carbon units.

Structure I: [X] ^m Y _n °, in which m denotes the charge of the alkali or earth alkaline metal ion, + 1 or +2, and n denotes the number of carboxylate anions, 1 to 2, and o denotes the overall charge of the carboxylate anion, -1 or -2.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 illustrates an embodiment of the process.

Figure 2 illustrates the extraction efficiency of the process, where an embodiment using the DES NaAcO: Urea (molar ratio 1 : 2), containing 10 wt-% water, on brewer's spent grain at 3 extraction times is illustrated, and NaOH (pH 11) extraction is presented as reference.

DETAILED DESCRIPTION OF THE INVENTION

It was surprisingly found that a high degree of protein extraction can be achieved, particularly on an industrial scale, by using the process of the invention, where biomass material comprising proteins is treated with a deep eutectic solvent (DES).

Biomass material

Biomass material refers here to streams and materials of plant or animal or algal or microbial origin, which contain proteins. Preferably waste streams and side streams comprising plant material, animal material or algal material, particularly waste streams and side streams originating from agriculture and food industry are used . Examples of such biomass materials are side streams, such as press cakes from fruit industry, vine industry, beer industry and berry industry, and from edible oil pressing and refining industry, brewer's spent grain (BSG), rapeseed press cake (RSPC), wheat bran (WB), feathers, hair, hooves, claws, cartilage materials, and also beans and peas, which all contain high amounts of proteins.

The biomass material is suitably pretreated mechanically prior to introduction to the process of the invention.

Suitable mechanical pretreatment methods include grinding, milling, refining, crushing and cutting and any combinations thereof.

The biomass material may be dried prior to introduction to the process of the invention. The drying may be carried out using any drying methods known as such.

The biomass material may be sieved prior to using it as starting material in the process.

Suitably the average particle size of the biomass material entering the process is in the range of 0.05 - 10 mm, preferably 0.1 - 3 mm.

Deep eutectic solvent useful in the process

The DES useful in the present process is selected from DESs comprising a hydrogen bond acceptor selected from alkali metal carboxylates and earth alkaline metal carboxylates, and urea as hydrogen bond donor.

The term "alkali metal or earth alkaline metal carboxylate" means here a compound having the structure I, where X denotes an alkali metal or earth alkaline metal selected from Na, K, Li, Be, Mg or Ca and Y denotes a mono- or polyvalent carboxylate, which may carry a straight alkane chain with a length between 1 and 20 carbons, an aromatic moiety, a branched alkane, a multivalent carboxylic acid, alcohol groups, alkenes and alkynes with chain lengths of 1 to 20 carbon units.

Structure I: [X] ^m Y _n °, in which m denotes the charge of the alkali metal ion or earth alkaline metal ion, + 1 or +2, and n denotes the number of carboxylate anions, 1 to 2, and o denotes the overall charge of the carboxylate anion, -1 or -2. Preferably the alkali metal is selected from K or Na.

Suitably the carboxylate is a C1-C20 alkyl ester, preferably a C1-C5 alkyl ester, more preferably an acetate (AcO), formiate, oxalate or propionate, particularly preferably acetate.

Food grade DESs are preferred .

According to a preferable embodiment the DES is NaAcO: urea. The DES comprises a molar ratio of the alkali metal carboxylate or earth alkaline metal carboxylate to urea from 5 : 1 to 1 : 5, preferably from 1 : 1 to 1 : 3, respectively.

According to an embodiment the DES comprises 0.01-4 mol of water with respect to the molar amount of the alkali metal carboxylate or earth alkaline metal carboxylate, preferably 0.5-1.5 mol of water.

NaAcO: urea is preferably used as an aqueous solution containing 28-37 wt% of NaAcO, 54-62 wt% of urea and 8-15 wt% of water. The DES is suitably formed prior to introducing it to the process.

Process

The process of the invention for separating proteins and/or protein fractions from biomass materials comprises the steps, where

in the first step biomass material is mixed with an aqueous DES comprising a hydrogen bond acceptor selected from alkali metal carboxylates and earth alkaline metal carboxylates, and urea as hydrogen donor, to obtain biomass mixture,

in the second step the biomass mixture is separated into a non-solubilized fraction and a liquid fraction; and

in the third step proteins are separated from the liquid fraction using technique based on molecular size to yield a protein solution, or using technique based on isoelectric precipitation to yield a protein precipitate.

Preferably the biomass material is pretreated prior to introducing it to the process. The biomass material may be dried prior to introducing it to the process. Drying methods known in the art may be used. An embodiment of the invention is illustrated in Figure 1. Biomass material 10 and aqueous DES 20 are mixed in a vessel 100 to obtain a biomass mixture, water 30 is added to the biomass mixture and agitated, whereby an aqueous biomass mixture 40 is obtained. The aqueous biomass mixture 40 is separated into a non-solubilized fraction 50 and a liquid fraction 60 using a separator 200. The liquid fraction is separated in a separator 300 to a protein containing fraction 80 and non-protein residue 70. The protein containing fraction 80 is optionally dried in a drier (400) to yield dry protein product 90. In this embodiment water is added in the first step, however it is optional.

The separator 300 is a separator utilizing technique based on molecular size, or utilizing technique based on isoelectric precipitation.

First step

In the first step biomass material is mixed with an aqueous DES comprising a hydrogen bond acceptor selected from alkali metal carboxylates and earth alkaline metal carboxylates, and urea as hydrogen bond donor to obtain biomass mixture.

The temperature in the first step is from 0°C to 100°C, preferably from 20°C to 80°C.

The mixing time in the first step is from 30 min to 30 hours, preferably from 1 to 16 hours, particularly preferably from 1.5 to 4 hours.

The biomass mixture may contain water not more than 20 wt%, preferably 10-20 wt%, calculated from the total weight of the biomass mixture. Water may also be added to the biomass mixture or the biomass may contain the water. The amount of water in the biomass mixture may be decreased if necessary for example by distillation, evaporation etc.

The DES comprises 0.01-4 mol of water with respect to the molar amount of the alkali metal carboxylate or earth alkaline metal carboxylate.

In the first step the biomass mixture contains 1-40 wt%, preferably 3-25 wt% of the biomass material, calculated based on dry weight. In the first step the pressure is 100 mbar - 30 bar, preferably 1-20 bar. Inert gas, such as l\h, Ar, He or CO2 may be used for pressurization of the reaction vessel. Suitably any mixing devices may be used in the first step, such as high-shear mixers, homogenizers, processing grinders, helical mixers, high intensity mixers, fluidizers and twin-screw mixers may be used.

Second step

In the second step the biomass mixture is separated into a non-solubilized fraction and a liquid fraction.

The biomass mixture is suitably mixed and after the mixing is completed, the biomass mixture is subjected to separation. Suitably the temperature of the biomass mixture in the separation is 40-100°C, preferably 60-85°C. The separation of the non-solubilized fraction from the liquid fraction is carried out suitably using sieves, centrifuges, filters, pressure filters etc., and suitably with the aid of pressure.

The non-solubilized fraction is suitably washed with water to remove all free proteins.

The non-solubilized fraction may be subjected to optional fibrillation.

The non-solubilized fraction may also contain carbohydrates, lignin, phenolic compounds, starch, etc., and it may be subjected to further separation steps or it may be subjected to fermentation.

The DES may conveniently be removed from the washing liquid and recycled if desired.

According to an embodiment, in the second step water is added to the biomass mixture, which is then agitated and subjected to separation.

The amount of added water is 1-8, preferably 1-4 times the times the volume of the DES.

This amount of water also contains the amounts of water possibly present in the first step or originating from the first step.

The temperature of the added water is 0-100°C, preferably 20-80°C, particularly preferably 40-75°C. Third step

In the third step proteins are separated using technique based on molecular size from the liquid fraction to yield a protein solution.

Proteins are isolated and enriched from the liquid fraction, which comprises the DES, low molecular weight compounds and water in addition the proteins.

The technique based on molecular size is selected from ultrafiltration, dialysis, membrane filtration methods and combinations thereof, whereby a protein solution is obtained.

Alternatively, technique based on isoelectric precipitation to yield a protein precipitate may be used. The protein is precipitated from the protein solution, either in the presence of or after removal of DES components, by isoelectric precipitation, in which the pH is adjusted to the proteins' isoelectric point causing the protein to precipitate.

The obtained protein solution may be further purified if desired by methods known in the art, concentrated, washed, and if necessary enzyme treatments may be applied to depolymerize co-extracted polysaccharides to small saccharides, which can be easily removed by e.g. membrane technology.

The protein solution may be subjected to drying to obtain a dry protein extract.

The protein solution may be dried by freeze-drying, spray-drying, pressing, evaporating and by combinations thereof.

The obtained proteins may be used in human foods or nutritional supplements where they can serve as nutrients or performance proteins, in animal feeds, in chemical applications, and as specialty proteins.

It was surprisingly found that the selected DESs, particularly NaAcO: urea, were excellent and efficient solvents for solubilizing proteins, providing high yields of proteins

The process is fast, it can be reduced even to 2 h without significant yield losses.

Applying efficient washing protocols in the second step protein yields are increased, as washing the solid residue anew releases more protein from it. In differential scanning calorimetry (DSC) experiments with different highly pure commercial proteins in aqueous DES solutions (0-60 wt% NaAcO: urea, 1 : 2), it was noticed that the protein denaturation temperature increased with the DES concentration in aqueous DES solutions, indicating that the protein potentially can be extracted in a more intact and natural form by DES extraction. This stabilization of proteins results in improved protein quality.

With the process typically less sugars and other non-protein compounds are co-extracted with the proteins, compared to the NaOH extraction method, and the proteins yields are higher.

The non-solubilized fraction can easily be hydrolyzed to fermentable sugars, whereby it is a very suitable feedstock for saccharification for the further production of biofuels and bio- chemicals via the sugar platform pathway, thus eliminating the generation of any considerable waste fractions in this process.

The DESs also selectively dissolve biopolymers, and in part open-up the matrix of the biomass material to allow for a higher degree of protein extraction by exposing more protein for easy dissolution. In addition, NaAcO: Urea (1 : 2) with 10 wt% water was noticed to have a high selectivity for dissolving protein vs carbohydrates from BSG (BSG extract contained 1 part carbohydrates for every 10 parts of protein).

The selected DESs are cheap solvents and they may be of food grade and easily recyclable.

The process of the invention can be used for producing proteins and protein fractions, which can further be used for the manufacture of protein products both for human nutrition and for animal feeds, as well as for other applications, such as specialty protein products in a simple, economic and efficient way.

A wide range of low cost feedstock, particularly agro-based side streams available in vast amounts can now be used for the manufacture of valuable protein products.

The invention is now illustrated in the flowing examples. Examples

Example 1. Extraction of proteins from brewer's spent grain (BSG), rapeseed press cake (RSPC), and wheat bran (WB) using DES comprising sodium acetate and urea

Extraction experiments were carried out using a glass reactor equipped with mixer. Brewer's spent grain (BSG), rapeseed press cake (RSPC), and wheat bran (WB) were Wiley milled and treated with a DES comprising 90 % aqueous NaAcO : urea, (molar ratio 1 : 2). The DES solutions were prepared using dry salts, which were mixed with addition of water and heated up to the reaction temperature. The load of protein-containing starting material was 10 wt % in DES solution. The inside temperature in the vessel was 80°C, and the stirring speed of centrifugal stirrer paddles was 150 rpm and reaction time was adjusted to 16 hours. When the mixing was completed, the DES solution was filtered through 150 micro meter sieve from the solid residue, which was washed with hot water (8 times the amount of DES) to remove all free protein. The yield of extracted protein as compared to the total amount of protein in the starting material was 38.4 % from wheat bran, 44.4 % from RSPC and 37.5 % from BSG. Example 2. Extraction of proteins from brewer's spent grain (BSG) using DES comprising sodium acetate and urea

Extraction experiments were carried out using a glass reactor equipped with mixer. Brewer's spent grain (BSG) was Wiley milled and treated with a DES comprising 90 % aqueous NaAcO : urea, (molar ratio 1 : 2). The DES solution was prepared using dry salts, which were mixed with addition of 10 % water and heated up to the reaction temperature. The load of BSG was 5 wt % in DES solution. The inside temperature in the vessel was 80°C, and the stirring speed of centrifugal stirrer paddles was 150 rpm and the reaction time was adjusted to 1, 2 or 4 hours. When the mixing was completed, hot water (4 times the amount of DES) (80°C) was added to the DES solution, the solution was stirred with a magnetic stirrer and filtered through 150 micro meter sieve from the solid residue, which was further washed with an additional 200 ml of hot water (4 times the amount of DES) to remove all free protein. The protein extraction yields are presented in Figure 2. The reaction time could be reduced to 2 h without significant yield losses. Applying more efficient washing protocols after the initial extraction increases total protein extraction yields, as washing the solid residue anew in many cases released more protein from it. Extraction with aqueous NaOH solution (pH 11) at 30 °C and 5 wt % BSG loading is provided as reference.

Example 3. Separation and purification of proteins from aqueous DES solution 200 ml of extract obtained from the treatment of BSG in 90 % aqueous sodium acetate: urea (molar ratio 1 : 2), with general extraction parameters as described in examples 1 and 2 and 10 wt% extraction consistency and 20 h extraction time, was dialyzed using a dialysis membrane of regenerated cellulose with a molecular weight cutoff of 3 500 g/mol. The advance of the dialysis was followed by conductivity measurement, and the dialysis was deemed completed when the conductivity was below 2 μΞ^ιη. The dialyzed liquid was further freeze-dried to yield light brown solids of a fibrous texture. The final protein content of the solid dry product was analyzed to 45 wt% and the protein yield of the purification phase was 63.4 %, calculated on basis that the extract taken for dialysis had an analyzed protein content of 1.77 g/L.

Example 4. Extraction of proteins from brewer's spent grain (BSG) using DES comprising potassium acetate and urea

Extraction experiments were carried out using a glass reactor equipped with mixer. Brewer's spent grain (BSG) was Wiley milled and treated with a DES comprising 90 % aqueous KAcO : urea, (molar ratio 1 : 2). The DES solution was prepared using dry salts, which were mixed with addition of 10 % water and heated up to the extraction temperature. The load of BSG was 10 wt % in DES solution. The inside temperature in the vessel was 80oC, and the stirring speed of centrifugal stirrer paddles was 150 rpm and the reaction time was 4 hours. When the mixing was completed, hot water was mixed with the extraction mixture and the solution was stirred with a magnetic stirrer and filtered through 150 micro meter sieve from the solid residue, which was further washed with additional hot water to remove all free protein. The protein extraction yield was 65.6 %. Example 5. Extraction of proteins from brewer's spent grain (BSG) using DES comprising sodium formiate and urea

Extraction experiments were carried out using a glass reactor equipped with mixer. Brewer's spent grain (BSG) was Wiley milled and treated with a DES comprising 90 % aqueous Na-formiate: urea, (molar ratio 1 : 2). The DES solution was prepared using dry salts, which were mixed with addition of 10 % water and heated up to the extraction temperature. The load of BSG was 10 wt % in DES solution. The inside temperature in the vessel was 80oC, and the stirring speed of centrifugal stirrer paddles was 150 rpm and the reaction time was 4 hours. When the mixing was completed, hot water was mixed with the extraction mixture and the solution was stirred with a magnetic stirrer and filtered through 150 micro meter sieve from the solid residue, which was further washed with additional hot water to remove all free protein. The protein extraction yield was 63.8 %.