Method of preparing a dairy fraction enriched in growth factor and said product

Abstract

The present invention relates to a method of preparing a dairy fraction enriched in growth factor from a dairy starting material and said dairy fraction enriched in growth factor. In particular, the present invention relates to a method of preparing a dairy fraction enriched in growth factors by using an electrodialysis cell in combination with membrane filtration.

Description

Technical field of the invention The present invention relates to a method of preparing a dairy fraction enriched in growth factor from a dairy starting material and said dairy fraction enriched in growth factor. In particular, the present invention relates to a method of preparing a dairy fraction enriched in growth factors by using an electrodialysis cell in combination with membrane filtration. Background of the invention Milk is an abundant source of important molecules, such as proteins, lactose, fats, minerals, and vitamins that have nutritional values and biological properties. This is why milk and colostrum are the first food for mammals which nourishes immunologically immature neonates providing all the necessary energy and nutrients through a period of intensive growth and development. Apart from these molecules, milk and dairy solutions also contain various macromolecules such as growth factors having specialized functions as a bioactive ingredient in health-related applications, such as promoting cellular growth, differentiation and development of tissues in newborn, formation of bones and cartilages, treatment of skin disorders (for example psoriasis), gastrointestinal diseases, regulation of immune systems, wound healing etc. Growth factors (GF) are soluble proteins or polypeptides having the primary function of cellular proliferation and / or differentiation. The most abundant growth factors in milk and dairy solutions are insulin-like growth factor (IGF-I), transforming growth factor (TGF-B2), epidermal growth factors (EGF) and basic fibroblast growth factor (bFGF or FGF-2). However, fractionation of growth factors from dairy solutions is difficult and is challenged by several factors, such as the range of the molecular weights and the isoelectric points of the growth factors relative to the other compounds in milk and other dairy solutions. Fractionation of growth factors from dairy solutions are also challenged by most of the growth factors being bound or associated with other compounds in the dairy solution. In addition, the concentration of growth factors in dairy solutions are very low, less than 0.001 g / L in milk and less than 0.004 g / L in bovine colostrum. In milk, whey and colostrum, the protein concentration is high and therefore the concentration of growth factors in these dairy solutions based on the total protein content is even smaller. In milk, the content of growth factor of the total protein content is less than 6xl0'5 percent by weight, while the content of growth factor of the total protein content in milk is less than 4xl0'5 percent by weight. In colostrum, the content of growth factors of total protein content is less than 5%*10'5. Technologies known to enrich fractions from milk, colostrum or whey are known. For example, charge-based chromatographic processes and size-membrane separation processes. However, both these methods have disadvantages, for example, these methods give a low concentration of growth factors in the obtained product. Further, chromatography is a quite expensive process and membrane technology is associated with very low selectivity and fouling of the membranes due to accumulation of apposite charged molecules on the membrane surface. Hence, an improved method of enriching dairy solutions with growth factors would be advantageous, and in particular a more efficient method resulting in an increased separation of growth factors would be advantageous. Summary of the invention Thus, an object of the present invention relates to a method of preparing a dairy fraction enriched in growth factor from a dairy solution. In particular, the object of the method of the present invention is to concentrate growth factors from a dairy solution such that the obtained fraction of the dairy solution has a higher concentration of growth factors in relation to the total protein content than the concentration of growth factors in the dairy solution as such. Another object of the present invention is to provide a dairy fraction enriched in growth factor having a high concentration of growth factor of the total protein content. In particular, it is an object of the present invention to provide a method of preparing a dairy fraction enriched in growth factor from a dairy solution and said dairy fraction enriched in growth factor that solves the above mentioned problems of the prior art with. Thus, one aspect of the invention relates to a method of preparing a dairy fraction enriched in growth factor from a dairy starting material, the method comprises the following steps: a) providing a dairy starting material; b) defatting the dairy starting material and separating casein from the dairy starting material to obtain a dairy feed material; c) adjusting pH of the dairy feed material to a pH of 4.0 or below; d) passing the pH adjusted dairy feed material through an electrodialysis cell under electrical field, and wherein the electrodialysis cell comprises at least one filtration membrane and at least two ion exchange membranes, the filtration membrane is positioned between the at least two ion exchange membranes; and e) collecting a growth factor enriched fraction obtained after passage of charged compounds from the pH adjusted dairy feed material through the filtration membrane to a recovery compartment. Another aspect of the present invention relates to a dairy fraction enriched in naturally occurring growth factor, wherein the dairy fraction comprises one or more of alpha-lactalbumin and beta-lactoglobulin naturally occurring in a dairy material and the dairy fraction has a concentration of growth factors of at least 0.08*10'3% of the total protein content. Yet another aspect of the present invention is to provide a dairy fraction enriched in naturally occurring growth factor according to the present invention for use in promoting cellular growth, differentiation and development of tissues in infants, formation of bones and cartilages, treatment of skin disorders, treatment of gastrointestinal diseases, regulation of the immune system, or wound healing. Still another aspect of the present invention is to provide a nutritional composition comprising the dairy fraction enriched in naturally occurring growth factor according to the invention. Brief description of the figures Figure 1 shows a configuration of the electrodialysis with filtration membrane (EDFM) cell comprising one feed compartment, one recovery compartment comprising growth factor enriched fraction, an anion exchange membrane (AMX), a cation exchange membrane (CMX), an ultrafiltration membrane (UF), a positive charged protein (P-), negatively charged protein (P+) and neutral protein. Figure 2 shows total protein concentration in recovery compartment after 180 min of EDFM experiment for different pH conditions. Data with different letters are significantly different. Figure 3 shows MS chromatogram of serocolostrum 2.38X in feed (TO) and cationic fractions (recovered after 180 min of EDFM experiment). Figure 4 shows an enlarged section of figure 3. Figure 5 shows the concentration of IGF-I and TGF-32 in the recovery compartment after 180 min of EDFM experiment at different pH conditions. Data with different letters are significantly different. Lowercase letters indicate differences for IGF-I concentration; uppercase - for TGF- [32 concentration. Figure 6 shows enrichment of growth factors (IGF-I + TGF-32) from colostrum in the recovery compartment at T180. Figure 7 shows the concentration of TGF-32 and IGF-I in the a) acid whey as feed and b) recovery compartments during 180 min of EDFM of acid whey. Data with different letters are significantly different. Lowercase letters indicate differences for IGF-I concentration; uppercase - for TGF- [32 concentration. Figure 8 shows enrichment in growth factors (IGF-I + TGF-[32) in the feed (at TO and T180) and recovery compartment at T180. Data with different letters are significantly different. Figure 9 shows enrichment of o-lactalbumin in the feed (at TO and T180) and recovery compartment at T180. Data with different letters are significantly different. Figure 10 shows enrichment of growth factors (IGF-I + TGF-32) from whey in the recovery compartment at T180. Data with different letters are significantly different. The present invention will now be described in more detail in the following. Detailed description of the invention Definitions Prior to discussing the present invention in further details, the following terms and conventions will first be defined: Dairy fraction: In the context of the present invention the term "dairy fraction" refers to a fraction obtained from a dairy starting material (dairy solution) and the term "fraction" refers to a part of the dairy starting material. The dairy fraction obtained by the method of the present invention will have a concentration of growth factor and / or a concentration of growth factor of the total protein content that is higher than the concentration of growth factor and / or a concentration of growth factor of the total protein content in the dairy starting material that the fraction is obtained from. Dairy starting material: In the context of the present invention, the term "dairy starting material" refers to a dairy solution that the dairy fraction is obtained from. The dairy starting material is a dairy solution. In principle the dairy starting material may be any type of dairy solution that comprises growth factor and the present invention should not be limited the dairy starting material used to prepare the dairy fraction enriched in growth factors. However, preferably, the dairy starting material is in liquid form and in an embodiment of the present invention, the dairy starting material is selected from the group consisting of milk, colostrum, whey, and fractions hereof. In some embodiments of the present invention, the whey used as the dairy starting material is acid whey. By the term "fractions hereof" is meant any fraction that can be derived from milk, colostrum, and whey. For example, it could be concentrated milk, colostrum or whey, for example a milk protein concentrate or whey protein concentrate. In an embodiment of the present invention, the term "milk" may be selected from the group consisting of whole milk, reduced-fat milk, low-fat milk, fat-free milk, lactose reduced milk, lactose free milk, and buttermilk. Fat-free milk may also be referred to as skimmed milk or skim milk. The dairy starting material used for preparing the dairy fraction enriched in growth factor according to the present invention may be based on for example milk, colostrum or whey from any type of mammal, but preferably from cows, buffalos, goats, sheep, yaks, horses, or mixtures thereof. In a preferred embodiment of the present invention, the dairy starting material is from cows, i.e., bovine milk. The term bovine material and cow material refer to the same. Growth factor: The term growth factor refers in the context of the present invention to what is typically understood in the art as growth factor. The term "growth factor" is not to be limited to only one growth factor but is including one or more growth factors. Therefore, the growth factor according to the present invention are one or more soluble proteins or polypeptides having the primary function of cellular proliferation and / or differentiation. Hence, in an embodiment of the present invention, the growth factor is one or more selected from the group consisting of insulin-like growth factor (IGF-I), transforming growth factor (TGF-B2), epidermal growth factors (EGF) and basic fibroblast growth factor (bFGF). The most abundant growth factors in dairy solutions are IGF-I and TGF-B2 and in the examples is the content of these growth factors measured. However, the present invention should not be limited to only enriching IGF-I and TGF-B2. It is assumed that other growth factors are also present in the fractions enriched with growth factor of the present invention than IGF-I and TGF-B2. However, in an embodiment of the present invention the growth factors present in the dairy fraction enriched with growth factors are IGF-I and TGF-B2. The isoelectric point of IGF-1 is 7.8-8.5 while the isoelectric point for TGF-B2 is about 7.7. Method: In an aspect, the present invention relates to a method of preparing a dairy fraction enriched in growth factor from a dairy starting material, the method comprises the following steps: a) providing a dairy starting material; b) defatting the dairy starting material and separating casein from the dairy starting material to obtain a dairy feed material; c) adjusting pH of the dairy feed material to a pH of 4.0 or below; d) passing the pH adjusted dairy feed material through an electrodialysis cell under electrical field, and wherein the electrodialysis cell comprises at least one filtration membrane and at least two ion exchange membranes, the filtration membrane is positioned between the at least two ion exchange membranes; and e) collecting a growth factor enriched fraction obtained after passage of charged compounds from the pH adjusted dairy feed material through the filtration membrane to a recovery compartment. In the method of the present invention the dairy starting material is defatted because if fat is present in the dairy starting material it may clog the filtration membranes used. Further, fat has a bad influence in the energy efficiency during electrodialysis and increases the viscosity of the solution. In some embodiments of the present invention, defatting is not necessary. For example, if the dairy starting milk is skimmed milk having a very low fat content, it is not necessary to carry out a defatting step. If defatting is carried out, it may be by any suitable method known in the art. The present invention should not be limited to the method of carrying out defatting. However, in an embodiment of the present invention defatting of the dairy starting material is carried out by microfiltration and / or centrifugation. In addition, casein is separated from the dairy starting material. It is important to separated casein from the dairy starting material, since casein precipitates when pH is adjusted to a pH of 4.0 or below. Precipitation of casein is wished avoided since it will clog the spacer and / or filtration membrane used during the electrodialysis step. The spacer is part of the membrane filtration equipment, and a spacer is typically used in connection with membrane filtration to keep membrane sheets apart as well as enhance mixing by promote turbulence in the feed. In some embodiments of the present invention, separation of casein is not necessary. For example, if the dairy starting material is whey or a fraction of whey, it may not be necessary to separate casein as it has already earlier been separated in connection with obtaining the whey. The separation of casein may be by any suitable method known in the art and the present invention should not be limited to how to any specific method of separating casein. However, in an embodiment of the present invention separation of casein from the dairy starting material may be by using microfiltration and / or acidification and / or modification of ionic strength. In a preferred embodiment of the present invention, casein is separated by using acidification. Casein precipitate when pH is adjusted to be about 4.6 that is the isoelectric point of casein. Hence, when pH is adjusted to pH of 4.6 or below, casein precipitates and can therefore be separated. It may be an advantage to separate casein by using acidification instead of microfiltration and modification of ionic strength, since separation of casein and whey protein by using microfiltration may result in less efficient whey recovery and the composition of the whey may be temperature dependent. For example, microfiltration at cold temperatures may result in that some caseins (e.g. beta-casein) are separated but some casein is maintained. pH adjustment: In the method of the present invention, it is important to adjust pH of the dairy feed material to pH 4.0 or below. The inventors of the present invention have surprisingly found that if pH is decreased to 4.0 or below, the migration of growth factors, such as IGF-I and TGF-2, in an electrodialysis (EDFM cell) cell was increased. The inventors surprisingly found that when the pH was above 4.0, such as pH 5-6, there was no or low migration of the growth factors in an electrodialysis cell, but if pH was adjusted to be 4.0 or below, there was increased migration of growth factors. This was unexpected and surprising to the inventors, because the isoelectric point (pl) of IGF-1 and TGF-B2 is around 6.0. The inventors of the present invention found that the lower pH used as compared to the pl, would cause an overall positive charge of the growth factors to increase. This induces the migration of positivity charged growth factors through the filtration membrane and towards the electrode of opposite charge, such as towards the at least one ion exchange membrane. The ion exchange membrane that is placed in front of the electrode will stop the migration of the growth factors due to the low pore size of the ion-exchange membrane, In addition, the inventors of the present invention have found that adjusting pH of the dairy feed material to pH 4.0 or below decreases the migration of the total protein content to the recovery compartment. Therefore, it was found that the present invention where the pH of a dairy feed material was adjusted to pH 4.0 or below and subjected to electrodialysis with a filtration membrane, the concentration of growth factors of the total protein content was increased. The pH adjustment in the method of the present invention is preferably made by use of any known method of pH adjustment, for example by adding an acid such as a food grade acid. In the context of the present invention, the term "food grade acid" refers to an acid that is suitable for being added to a food product that is intended to be used for consumption. Any food grade acid may be used for the pH adjustment. However, in an embodiment of the present invention, the pH adjustment in step c) is by adding one or more acid selected from the group consisting of citric acid, acetic acid, lactic acid, malic acid, glucono-delta-lactone, hydrochloric acid, and phosphoric acid. The pH adjustment or acidification may also be by addition of carbon dioxide or using bipolar membrane electrodialysis. Preferably, the pH adjustment is obtained by adding one or more of citric acid, acetic acid, or hydrochloric acid. Typically, the acid is added in a solution. For example, the acid can be added as 20% acid solution meaning that the acid is present in a water solution in the ratio of 20:80 of acid:water. In an embodiment of the method of the present invention, the pH adjustment in step c) is to a pH of 3.85 or below and in a preferred embodiment to pH 3.7 or below. Most preferred, the pH was adjusted in step c) to a pH of 3.5 or below. In another embodiment of the method of the present invention, the pH adjustment in step c) is to a pH in the range of from 2.0 to 4.0, preferably to a pH in the range of from 3.85 to 2.2 and more preferably to a pH in the range of 2.4 to 3.7, most preferably to a pH in the range of 2.5 to 3.5. Concentrating dairy starting material: The dairy feed material may in an embodiment of the present invention be concentrated before subjected to electrodialysis. The concentration of the dairy feed material may be before or after pH adjustment of the dairy feed material. Hence, in an embodiment of the invention, the dairy feed material of step b) or the pH adjusted dairy feed of step c) is subjected to a concentration step before passed through the electrodialysis in step d). Due to the naturally, very low concentrations of growth factors in dairy solutions, such as milk and whey, the dairy starting material or dairy feed material is preferably concentrated before the electrodialysis in step d). The concentration is for example obtained by subjecting the dairy feed material or pH adjusted dairy feed material to one or more of the concentration processes selected from the group consisting of ultrafiltration, nanofiltration, reverse osmosis and evaporation. Preferably, the concentration is by using ultrafiltration and / or nanofiltration. The ultrafiltration and nanofiltration may in some embodiments of the present invention be in combination with diafiltration. In the context of the present invention, the term "diafiltration" is used when a liquid is added to the retentate obtained from the membrane filtration and the membrane filtration then is repeated. For example, diafiltration can be in combination with ultrafiltration or in combination with nanofiltration. If diafiltration is in combination with ultrafiltration, the diafiltration is by using an ultrafiltration membrane. The dairy feed material is first subjected to ultrafiltration to obtain a retentate and a permeate. Diafiltration is then performed on the ultrafiltration retentate by adding a liquid to the ultrafiltration retentate to obtain a mixture and subjecting said mixture to a second filtration step with an ultrafiltration membrane. If diafiltration is in combination with nanofiltration, the diafiltration is by using a nanofiltration membrane. Preferably, the concentration is by using a membrane having a molecular cut-off in the range of 0.1 kDa to 20 kDa, more preferably 1 kDa to 10 kDa. In an embodiment of the present invention, the dairy feed material was concentrated at least 1.2X by volume, such as 1.5X by volume. Preferably, the dairy feed material was concentrated from 1.2X to 3.OX by volume. The term "X" refers to the times the dairy feed was concentrated by volume, i.e., if the 100 ml of dairy feed material was concentrated 2.OX, it has been concentrated to 50 ml. The concentration of the dairy feed material is adjusted to the dairy starting material used to prepare the fraction enriched in growth factor. For example, the concentration from 1.2X to 3.OX by volume is preferably when colostrum is used as the dairy starting material. However, if whey or milk is used as the dairy starting material, the concentration may be at least 10X by volume, such as at least 15X by volume, preferably from 10X to 20X by volume. The higher concentration when using milk or whey as the dairy starting material is adjusted according to the lower concentration of growth factor in milk and whey. Electrodialysis with filtration membrane (EDFM): In the method of the present invention, the pH adjusted dairy feed material is passed through an electrodialysis cell under electrical field. The electrodialysis cell comprises at least one filtration membrane and at least two ion exchange membranes, where the filtration membrane is positioned between the at least two ion exchange membranes. The ion exchange membranes are selected from anion-exchange membranes and cation-exchange membranes. Electrodialysis (ED) is a membrane separation process in which ions species are induced to move by an electrical potential and are separated from water, macrosolutes and all uncharged solutes by means of ion-exchange membranes. Ion-exchange membranes are traditionally highly distended gels containing polymers with a fixed ionic charge, allowing passage of anions or cations and very little else. Since most of the ED membranes are made by chemical modification of polymers or by polymerisation of functional monomers and cross-linking agents, the pseudo-pores formed by the interstices within the polymer have a random size and do not allow the purification of molecules based on their size. At the opposite, dialysis, ultrafiltration and nano-filtration membranes allows the purification of molecules based on size. They, however, appear useless in ED since their electrical resistance in an ED system would be inappropriately high. The purification of molecules based on their size and ED are therefore most of the time performed separately. However, the inventors of the present invention have found that using electrodialysis with a membrane filtration makes it possible to separate growth factors from a dairy solution. The term "filtration membrane" means in the context of the present invention a membrane that may have a well-defined molecular weight cut-off. The filtration membrane in the electrodialysis cell is preferably an ultrafiltration membrane or a microfiltration membrane. Hence, in an embodiment of the present invention the filtration membrane is selected from the group consisting of ultrafiltration membranes and microfiltration membranes. In a preferred embodiment, the filtration membrane is an ultrafiltration membrane. The membrane used may be a charged or a neutral membrane. In another embodiment of the present invention, the filtration membrane has a molecular weight cut-off in the range of 150 kDa to 0.1pm. It was surprisingly and without being bound by any theory found by the inventors of the present invention that the use of a 100 kDa membrane did not result in efficiently migration of the growth factors. However, using a more open membrane (for example 300 kDa) resulted in efficient migration. In an embodiment of the present invention, the filtration membrane has a molecular weight cut-off in the range of 200 kDa to 0.1pm, such as 250 kDa to 0.1pm. Most preferably, the filtration membrane has a molecular weight cut-ff of 300 kDa. In an embodiment of the invention, the membrane is a polyethersulfone (PES) membrane. In an embodiment of the method of the invention, the electrodialysis cell comprises at least two filtration membranes. Each filtration membrane has a molecular weight cut-off that is either different from the other(s) or similar to the other. In a preferred embodiment of the present invention, the electrodialysis cell comprises at least one anion-exchange membrane and at least one cationexchange membrane. However, it is not necessary and the electrodialysis cell may for example comprise only anion-exchange membranes or cation-exchange membranes. In one embodiment of the invention, the electrodialysis cell comprises a filtration membrane stacked between an anion-exchange membrane and a cation-exchange membrane. See figure 1. The pH adjusted dairy feed is fed to the electrodialysis cell in a feed compartment placed between the anion-exchange membrane and the filtration membrane, and the growth factors and some proteins and other small molecules will migrate filtration membrane and can be collected in a cationic recovery compartment. Other proteins are not passing the filtration membrane and remain in the feed compartment where they can be collected from the opposite end of where the dairy feed material is introduced to the feed compartment. The electrodialysis cell also comprises a compartment for electrode rinsing solution. The electrode rinsing solution is preferably a mineral solution, such as a solution of NaCI or Na2SO4, but it could be another suitable rinsing solution. The concentration of the electrode rinsing solution is typically 20g / L. However, the electrodialysis cell may comprise several sets of filtration membrane placed in between a cation-exchange membrane and / or an anion-exchange membrane. The sets of membranes (filtration membrane, cationic membrane and / or anionic membrane) may be placed in the electrodialysis cell in parallel or be connected in series. For example, in another embodiment of the invention, the electro dialysis cell comprises a first filtration membrane (FM1) and a second filtration membrane (FM2) each stacked between a cation-exchange membrane (CEM) and a anion-exchange membrane (AEM). Hence, placed as CEM / FM1 / FM2 / AEM. This is referred to as to sets of membranes placed in parallel. In another embodiment of the present invention the sets of membranes are placed in series, i.e., a set of cation-exchange membrane, filtration membrane and an anion-exchange membrane that is placed one after each other connected in a series. The electrical field strength used is typically 1.0 to 10.0 V / cm and preferably about 2.5 to 3.5 V / cm. In an embodiment of the invention, the dairy feed material flows through the electrodialysis cell at a rate of between 0.1 to 20 L / min., but preferably between 0.1 to 3 L / min. The method of the invention is typically performed by continuous recirculation of the dairy feed material through the electrodialysis cell, but under different adaptions of the method. The method can also be performed on a continuing operation. Hence, after the feed material has been subjected to the electrodialysis cell with membrane filtration and a fraction enriched in growth factors is obtained, said fraction can be fed to the electrodialysis cell with membrane filtration to further concentrate the growth factors. It was surprisingly found by the inventors of the present invention that when colostrum was used as a feed, a decrease in pH before adding the dairy feed material to the electrodialysis cell resulted in increased migration of growth factors. In addition, it was found that decreasing pH resulted in a decrease in migration of alpha-lactalbumin, while the opposite was found for betalactoglobulin where a decrease of pH resulted in increased migration of betalactoglobulin. Without being bound by any theory, the inventors of the present invention believe that the reason for increased migration of beta-lactoglobulin when pH was decreased is due to beta-lactoglobulin is a monomer (about 18 kDa) at a pH below 3.5 while beta-lactoglobulin is an octamer (144 kDa) at pH values in the range of 3.5-5.5. In addition, it is believed that the charge of betalactoglobulin (which is positive under its pl) increases as the pH decreases, and thus its electrophoretic mobility increases allowing a better migration of betalactoglobulin to the recovery compartment at decreased pH. As earlier discussed, the fraction enriched in growth factor may be used as a "dairy feed material" and returned to another electrodialysis cell with another membrane configuration in order to make a new fraction that is further enriched in growth factor. In addition, the recovered feed solution that has not passed the membrane can be returned to the same electrodialysis cell for further enrichment of growth factor. Drying: In an embodiment of the present invention, the growth factor enriched fraction obtained in step e) is in a step f) dried to a powder. Drying to a powder may be by any method known in the art, for example by spray drying or freeze drying, preferably spray drying. In other embodiments of the invention the growth factor enriched fraction is concentrated before drying, such concentration may be by membrane filtration or evaporation. The concentration before drying may for example be by membrane filtration, such as by using ultrafiltration, nanofiltration or reverse osmosis. In embodiments of the invention, the membrane filtration may also be in combination with diafiltration. In the context of the present invention, the term "diafiltration" is used when a liquid is added to the retentate obtained from the membrane filtration and the membrane filtration then is repeated. For example, diafiltration can be in combination with ultrafiltration or in combination with nanofiltration. If diafiltration is in combination with ultrafiltration, the diafiltration is by using an ultrafiltration membrane. The dairy feed material is first subjected to ultrafiltration to obtain a retentate and a permeate. Diafiltration is then performed on the ultrafiltration retentate by adding a liquid to the ultrafiltration retentate to obtain a mixture and subjecting said mixture to a second filtration step with an ultrafiltration membrane. If diafiltration is in combination with nanofiltration, the diafiltration is by using a nanofiltration membrane. The dairy fraction enriched in naturally occurring growth factor: An aspect of the present invention relates to a dairy fraction enriched in naturally occurring growth factor, wherein the dairy fraction comprises one or more of alpha-lactalbumin (ALA) and beta-lactoglobulin (BLG) naturally occurring in a dairy material, and the dairy fraction has a concentration of growth factor of at least 0.08*10'3 percent of the total protein content. On the contrary, the content of growth factors of total protein content is less than 0.05%*10'3 in colostrum, less than 0.06%*10'3 in milk and less than 0.04%*10'3 in whey. By the term "naturally occurring" means in the context of the present invention that it is growth factors, alpha-lactalbumin and beta-lactoglobulin that are naturally occurring in the dairy starting material that has been concentrated. In addition, the term "dairy fraction" refers to a fraction (a part of) a dairy product. Hence, "a dairy fraction enriched in naturally occurring growth factor" does not cover growth factors as such dissolved in a solution, but the present invention relates to a fraction obtained from a dairy starting material that has a higher concentration of growth factor than present in the dairy starting material. Preferably, the dairy fraction enriched in naturally occurring growth factor comprises growth factors in an amount of at least 0.08*10'3 percent of the total protein content, such as in an amount of at least 0.10 *10'3 percent of the total protein content, preferably in an amount of at least 0.12 *10'3 percent of the total protein content more preferably in an amount of at least 0.15 *10'3 percent of the total protein content. Preferably, the dairy fraction enriched in naturally occurring growth factor comprises a total total protein content of at least 80 pg / ml, more preferably at least 85 pg / ml. In an embodiment of the invention, growth factor in the dairy fraction enriched in naturally occurring growth factor comprises a combination of insulin-like growth factor (IGF-I) and transforming growth factor (TGF-B2). If the dairy starting material is milk or whey, the growth factor content in said dairy starting materials are lower than in for example colostrum. However, the dairy fraction enriched in naturally occurring growth factor where the dairy fraction is obtained from milk or whey has a content of growth factors of at least 0.08 *10'3 percent of the total protein content, such as a content of at least 0.12 *10'3 percent of the total protein content, more preferably a content of at least 0.15 *10'3 percent of the total protein content and most preferably a content of at least 0.20 *10'3 percent of the total protein content. In an embodiment of the present invention, the dairy fraction enriched in naturally occurring growth factor has a content of growth factors in the range of 0.08*10'3 to 12*10'3 percent of the total protein content, such as in the range of 0.10 *10'3 10*10'3 percent of the total protein content percent of the total protein content, more preferably in the range of 0.15 *10'3 to 10*10'3 percent of the total protein content. In other embodiments of the invention where the dairy fraction is obtained from whey, the dairy fraction enriched in naturally occurring growth factor has a content of growth factose in the range of 0.08*10'3to 1.0*10'3 percent of the total protein content, such as in the range of 0.10 *10'3 0.6*10'3 percent of the total protein content percent of the total protein content, more preferably in the range of 0.15 *10'3 to 0.5*10'3 percent of the total protein content. If the dairy fraction enriched in naturally occurring growth factor where the dairy fraction is obtained from colostrum, the content of growth factor of the total protein content is higher. Hence, if the dairy starting material is colostrum the dairy fraction enriched in naturally occurring growth factor may comprise growth factor in an amount of at least 2.0*10'3 percent of the total protein content, such as in an amount of at least 2.5*10'3 percent of the total protein content, preferably at least 3.0*10'3 percent of the total protein content, and more preferably in an amount of at least 4.0*10'3 percent of the total protein content On the contrary, the concentration of growth factors in dairy solutions are very low, less than 0.001 g / L (i.e. less than 1000 ng / ml) in milk and whey and less than 0.004 g / L (less than 4000 ng / ml) in bovine colostrum. As discussed above, the dairy fraction enriched in naturally occurring growth factors include other nutrients, for example whey protein such as alphalactalbumin and / or beta-lactoglobulin. These nutrients are nutrients that also migrate the filtration membrane into the recovery compartment. Hence, in an aspect of the invention, the fraction furthermore comprises one or more of the whey proteins alpha-lactalbumin and beta-lactoglobulin naturally occurring in a dairy starting material. In an embodiment, the fraction furthermore comprises minerals naturally occurring in a dairy starting material used for making the diary fraction. However, the concentration of these other nutrients in the obtained dairy fraction enriched in growth factor is lower than in the dairy starting material the dairy fraction is obtained from. This is because some nutrients are retained in the feed compartment. Hence the solid content in the dairy fraction enriched in growth factor is lower than the solid content in the dairy starting material. This means that the ratio between growth factors and total protein in the dairy fraction enriched in naturally occurring growth factor is much higher than the ratio between growth factor and total protein in the dairy starting material. In an embodiment of the present invention, the ratio between growth factor and total protein in the dairy fraction enriched in naturally growth factor is at least 2 times higher than the dairy starting material. Preferably, the ratio between growth factor and total protein is at least 2.5 times higher. If the dairy starting material used is colostrum, the growth factors can be concentrated even more, such as at least 10 times, preferably at least 20 times, such as at least 40 times higher and most preferably at least 50 times higher in the dairy fraction enriched in naturally growth factor as compared to the dairy starting material. The inventors of the present invention surprisingly found that decreasing pH of the dairy feed material to below 4.0 resulted in a fraction having an increased concentration of growth factor of the total protein content. Further, said fraction will comprise whey protein such as alpha-lactalbumin and / or beta-lactoglobulin. In fact, the inventors of the present invention have found that the more the pH was decreased, the higher was the concentration of growth factor of the total protein content. In an embodiment, the concentration of growth factor of the total protein content in the dairy fraction enriched in naturally growth factor obtained from whey or milk is at least 0.10%*10'3, more preferably at least 0.12%*10'3, and most preferably at least 0.15%*10'3. In another embodiment of the present invention, the concentration of growth factor of the total protein content in the dairy fraction enriched in naturally growth factor obtained from milk or whey is in the range of 0.1%*10'3 to 0.5%*10'3, such as in the range of 0.12%*10'3 to 0.45%*10'3. In an embodiment of the invention, the dairy starting material used for preparing the dairy fraction is whey, and then the obtained dairy fraction enriched in naturally occurring growth factor comprises a ratio of ALA to BLG in the range of 1:0 to 1:1, such as 1:0.1 to 2:1, preferably, 1:0.2 to 3:1, more preferably 1:0.3 to 4:1. In an embodiment of the invention, the protein in the dairy fraction enriched in naturally occurring growth factor is essentially alpha-lactalbumin. For example, the "essentially" reers to at least 95% alpha-lactalbumin. This is relevant, if the dairy starting material used is whey or milk. In an embodiment of the invention, the protein in the dairy fraction enriched in naturally occurring growth factor is essentially beta-lactoglobulin. For example, the "essentially" refers to at least 95% beta-lactoglobulin. This is relevant, if the dairy starting material used is colostrum. In a furhter embodiment of the invention, the dairy fraction enriched in naturally occurring growth factor has a pH of 4.0 or below, preferably a pH of 3.85 or below, and more preferably embodiment a pH of 3.7 or below. Most preferred, the pH of the dairy fraction enriched in naturally occurring growth factor is 3.5 or below. In another embodiment invention, the dairy fraction enriched in naturally occurring growth factor has a pH in the range of from 2.0 to 4.0, preferably a pH in the range of from 3.85 to 2.2 and more preferably a pH in the range of 2.4 to 3.7, most preferably a pH in the range of 2.5 to 3.5. In a further embodiment of the present invention, the dairy fraction enriched in naturally occurring growth factor is a powder. In another embodiment of the present invention, the dairy fraction enriched in naturally occurring growth factor according to the invention comprises essentially no lactose. The fraction obtained from electrodialysis in combination with membrane filtration will only comprise low amounts lactose .This is because lactose is not charged and therefore will not all lactose migrate towards an electrode and pass the membrane. However, some lactose may diffuse through the membrane. By the term "essentially no" means in the context of the present invention, that the content of lactose in the dairy fraction is 1% by weight or less, such as 0.5% by weight or less and most preferably 0.1% by weight or less. The conductivity of the dairy fraction comprising naturally occurring growth factor is also different from the conductivity of the dairy starting material the dairy fraction is obtained from. For example, the conductivity of whey and colostrum is about 7.5-8.0 when pH is 3.05. On the contrary, the conductivity of a dairy fraction obtained from whey or colostrum at pH 3.05 is about 1.0 to 3.0. In an embodiment of the present invention, the conductivity of a dairy fraction is 3.5 or below when the pH is 4.0 or below, preferably 3.3 or below when pH is 4.0 or below. The inventors of the present invention surprisingly found that if the dairy fraction enriched in naturally occurring growth factor is derived from colostrum, the fraction comprised a low amount alpha-lactoglobulin but some beta-lactoglobulin passed the membrane to the recovery compartment and was present in the dairy fraction enriched in naturally occurring growth factor. Hence, most of the total protein content was present in the form of beta-lactoglobulin. In fact, At least 60% by weight of the total protein content is alpha-lactalbumin. On the contrary, if the dairy fraction enriched in naturally occurring growth factor is derived from whey or milk, the fraction did not comprise any beta-lactoglobulin but comprised alpha-lactalbumin. Hence, the beta-lactoglobulin did not pass the membrane to the recovery compartment and was present in the dairy fraction enriched in naturally occurring growth factor. Nutritional composition: The dairy fraction enriched in naturally occurring growth factor according to the preset invention may be used in a nutritional formulation. Hence, the present invention relates to a nutritional composition comprising the dairy fraction enriched in naturally occurring growth factor according to the invention. The nutritional composition may be any nutritional composition where growth factors may have a beneficial use. For example, the nutritional composition may be selected from the group consisting of a beverage, an infant formula, a follow-on formula, and growing up formula. It may be any type of composition for infant formulation, follow-up formulation and growing-up formulation, and may be both in liquid form or in powder form. For example, the nutritional composition comprising the airy fraction enriched in naturally occurring growth factor may be a final liquid nutritional formulation for infants and toddlers. However, it may also be in a dry composition, for example a powder, that later on can be reconstituted in water to make up the final liquid composition that is to be given to the infant or toddler. The nutritional composition may also be in the form of a concentrate. Use of the dairy fraction enriched in naturally occurring growth factor: The dairy fraction enriched in naturally occurring growth factor according to the invention may be used for preventing or treating different diseases, such as used in promoting cellular growth, differentiation and development of tissues in infants, formation of bones and cartilages, treatment of skin disorders, treatment of gastrointestinal diseases, regulation of the immune system, or wound healing. It should be noted that embodiments and features described in the context of one of the aspects of the present invention also apply to the other aspects of the invention. The invention will now be described in further details in the following non-limiting examples. Examples Example 1: Fractionation of growth factors from colostrum Materials: Food grade Neosepta ion-exchange membranes were used for ion-exchange membranes: Anion-exchange membranes (AMX) and cation-exchange membranes (CMX) from Aston (Tokyo, Japan) were used. The filtration membrane used for the electrodialysis was a polyethersulfone membrane (PES, MQ) 300 kDa from Synder (Vacacille, Ca, USA) Colostrum used was obtained from Centre de recherche en sciences animals de Deschambault (Quebec, Canada). Defatting, casein removal and concentration of colostrum to obtain a dairy feed material: Colostrum was defatted by using Skimmer 100 L / h (Delaval Canada, Ontario, Canada). Defatted colostrum was diluted 1:1 in 1 M HCI to precipitate casein. The acidified colostrum was subjected to centrifugation to remove casein and hence obtain serocolostrum. The term serocolostrum refers to colostrum that has been defatted and decaseinated. Afterwards, the serocolostrum was further concentrated by using membrane filtration. The serocolostrum was concentrated (2.38 by volume) using GEA membrane filtration pilot plant (model L, Dusseldorf, Germany) with a 2.13 m2 spiral wound 5 kDa PES ultrafiltration membrane (Synder Filtration Inc., Vacaville, Ca, USA) to obtain the dairy feed material. The concentration of the growth factors IGF-I and TGF-B2 was found by using ELISA and was found to be concentrated by 1.22X and 1.35X, respectively, as compared to their concentration in the serocolostrum. IGF-I in serocolostrum was found to be 252.07±10.34 ng / ml and TGF-B2 in serocolostrum was found to be 129.01±7.74 ng / ml. Electrodialysis with membrane filtration A MP type electrodialysis (ED) cell from ElectroCell Sysems AB company (Taby, Sweden) was used for all experiments of Electrodialysis with filtration membrane (EDFM). One PES 300 kDa membrane was stacked in between the AMX and CMX in the ED cell to obtain an EDFM configuration that comprised only one recovery (cationic) compartment, see figure 1. The configuration was chosen to allow migration of growth factors of interest (IGF-I and (TGF-B2)to the recovery compartment, and other proteins to remain in the feed compartment. The EDFM configuration consisted of three recirculation compartments; one for cationic proteins and growth factor recovery, one for the solution of dairy feed material (defatted, casein removed and concentrated colostrum), one for feed solution and one for electrode rinsing solution. Recovery and feed compartment comprised 500 mL KCI (2 g / L) and 500 mL of feed solution solution respectively, and the electrode rinsing solution compartment comprised 800 mL Na2SO4 (20 g / L) solution. The solutions were circulated using three centrifugal pumps (Model WMD-30LFY.115, Iwaki Walchem Corporation, Holliston, MA, USA) and the flow rates were controlled at 500 mL / min for the feed and recovery compartments and 1000 mL / min for the electrode rinsing compartment using flow meters (Aalborg Instruments and Control, Inc., Orangeburg, NY, USA). Electroseparation of serocolostrum was performed in a batch process with a constant electric field strength of 2.9 V / cm. for 180 min. Example 2: Analysing fractionation of growth factor dependent of pH An example was made to analyse the effect of pH on fractionating growth factor from colostrum by using EDFM as disclosed in example 1. Colostrum was defatted, casein removed and concentrated as disclosed in example 1. The pH of the feed starting material was adjusted to 4 different pH values: pH 4.25, 3.85, 3.45 and 3.05. The conductivity of the pH adjusted dairy feed material and recovery compartments were kept constant at their initial values. The 4 pH adjusted feed material was subjected to EDFM. The migration of growth factor and proteins collected in the recovery compartment was measured by using ELISA and LC-MS. Three independent EDFM experiments were carried out for each pH condition. After the EDFM, the cationic protein fractions (fraction in recovery compartment) and the feed solution recovered were freeze dried and stored at 4°C until further analysis. The total protein concentration in cationic recovery compartment and feed compartments were determined using microBCA (pBCA) protein assay from the samples withdrawn at the different time interval during EDFM (0, 30, 60, 120, 180 min.) The absorbance was read at 562 nm on a microplate reader. Concentration was determine with a standard curve in a range of 0-40 pg / mL of bovine serum albumin (BSA). The samples recovered at the end of EDFM (T=180 min) from the feed and the recovery compartments were analysed by RP-UPLC. samples were filtered through 0.22 pm PVDF filter into a glass vial. The sample was loaded (0.5 pL) onto an AdvanceBio RP-mAb Diphenyl column (2.1 x 100 mm, 3.5 micron, Agilent, Santa Clara, Ca, USA). The column was operated at a flow rate of 400 pL / min at 60°C. The gradient consisted of solvent A (LS-MS grade water with 0.1% formic acid) and solvent B (LC-MS grade CAN with 0.1% formic acid) starting at 30% for 3 min., ramping to 40% B until 7 min., then 50% up to 9 min. Ramping t 90% B until 9.50 min., holding until 10.50 min., then back to initial conditions until 12 min. A hybrid ion mobility quadrupole TOF mass spectrometer (6560 high-definition mass spectrometry (IM-Q-TOF), Agilent, Santa Claram US) was used to identify and quantify the relative abundances of protein, such as alpha Icactalbumin, betalactoglobulin, GMP etc. Sandwich ELISA was used to quantify amounts of growth factors (IGF-I and TGF-62) using precoated microplates with monoclonal antibodies specific for IGF-I and TGF-B2 respectively. The recovery yield of growth factors was calculated using the following equation: Total amount of growth factor in recovery compartment (ng} Recovery yield —--------------------------------------------------------x 100 Total amount of growth factor in feed solution (ng} To determine if there is an enrichment of growth factors in the recovery compartment, the following equation was used: Enrichment of growth factors (%) Total amount of growth factors in a compartment (ng) = -----------;--------------;-------------------------;---r------- X 100 Total protein in the same compartment (ng) Figure 2 shows the total protein in the recovery compartment after 180 min. of EDFM at the 4 different pH conditions tested. As shown in figure 2, the total protein migration from the feed to the recovery compartment decreased with a decrease in pH value. Consequently, the highest protein concentration was noticed in the recovery compartment at the highest pH condition tested, i.e., at pH 4.25. Comparing to pH 4.25, the protein migration rate was 26% lower with the value for pH 3.85, and 50% lower for pH conditions 3.45 and 3.05. For all the pH conditions tested, there was a linear increase in protein concentration as a function of time during the EDFM process (not shown). All major proteins present in the feed (serocolostrum) and the cationic recovery compartment were identified and quantified by RP-UPLC-MS-QTOF. The MS chromatograms are presented in figure 3 and in figure 4 is a selection of figure 3 showing the alpha-lactalbumin (ALA) and beta-lactoglobulin (BLG) peaks. Figures 3 and 4 show that out of the major proteins (GMP, BSA, ALA and BLG) identified in the feed, only ALA and BLG migrated to the recovery compartment, except for pH 3.05 where only BLG migrated to the recovery compartment. Figure 3 and 4 further show that migration of ALA decrease with a decrease in pH while the opposite was observed for BLG that increases with a decrease in pH value. This could be explained by BLG is a monomer at pH below 3.5 but exist as an octamer in the pH range of 3.5-5.5. In addition, the charge of BLG increases as the pH decreases and thus its electrophoretic mobility. Figures 3 and 4 also show that the migration of ALA from the feed to the recovery compartment decreased with a decrease in pH value. The migration of ALA during EDFM could be explained by its aggregation behaviour at acidic pH. With a decrease in pH, ALA undergoes significant structural changes leading to ALA aggregation. The total concentration of growth factors (IGF-I and TGF-B2) in the recovery compartment after 180 min. of EDFM at the 4 different pH values tested were quantified by ELISA and are presented in figure 5. Figure 5 shows that the migration of IGF-I and TGF-B2 increases as the pH decreases such that the higher concentration og growth factor in the recovery compartment was at pH 3.05 In table 1 below is the recovery yields of IGF-I and TGF-B2 in the recovery compartment after 180 min. of EDFM shown for different pH conditions. Tabei 1: pH condition IGF-I (%) TGF-P2 (%) 3.05 5.53±0.55a 1.50±0.25a 3.45 4.42±0.42b 2.00±0.70a 3.85 1.44±0.15c 0.27±0.08b 4.25 0.83±0.02c 0.20±0.05b The amount of growth factor in relation to total protein content in the recovery compartments after 180 min EDFM were also calculated for the 4 different pH conditions. This is shown in figure 6. Figure 6 shows that the content of growth factor in relation to the total protein content in the colostrum is very low. Further, figure 6 shows that the regardless the pH conditions during EDFM, the recovery fractions were significantly enriched with growth factors IGF-I and TGF-B2 based on the total protein content as compared to the feed serocolostrum. Further, the ratio of growth factors to total protein was highest at pH 3.05 and 3.45 (about 60 times higher than in feed solution). The growth factor to total protein ratio at pH 3.85 and 4.25 was found to be 14 times and 7 times higher, respectively, compared to the serocolostrum feed solution. Example 3: fractionation of growth factors from whey. Example 3 discloses analysis of fractionation of growth factors from acid whey by using EDFM. Skim milk (from Agropur Dairy Coperative, Lonqueuil, QC, Canada) was diluted 1:1 and acidified with IM HCI until the PH was 3.8, the acidified milk was centrifugated to remove casein and hence obtain acid whey. The acid whey was concentrated through two repetitions to an averagedl6.37X by volume using the same filtration system as disclosed in example 1. ELISA results showed that the TFG-B2 content was concentrated about 3.5X comparing to the initial concentration in acid whey. The concentrated acid whey was subjected to EDFM under same conditions as discussed in example 1 and where the concentrated acid whey used as feed material was pH adjusted to 3.05. Average concentrations of growth factors (TGF-B2 and IGF-I) in the feed and in the recovery compartment during EDFM (at T= 0, 30, 60, 120 and 180 min) was quantified by ELISA and presented in figure 7. Figure 7 shows that in the content of growth factor increased as the duration for EDFM increased. The recovery yield in the recovery compartment after 180 min if EDFM of acid whey was calculated. The recovery yield of TGF-B2 was 1.90±1.13% while the recovery yield for IGF-I was calculated to be 1.67±o.73%. Higher recovery yield of TGF-B2 than IGF-I for acid whey as feed material is opposite to the results for serocolostrum as the feed material. This is believed to be due to different concentrations of the two growth factors in colostrum and acid whey. In colostrum, IFG-I predominates overTGF-B2 almost a two-fold while in acid whey it is opposite. The content of growth factor and ALA, respectively, of the total protein content was also calculated. This is shown in figure 8 and 9 respectively. The content of total growth factors of the total protein content was 2.7X higher in the recovery compartment than in the acid whey feed solution and 6.IX higher in case of ALA to total protein content. The difference of total growth factor to total protein when having serocolostrum as feed versus using acid whey as feed (60X verses 2.7 times) could be explained by their compositions, i.e. the initial content of growth factor. Example 4: Analysis of fractionation of growth factors from whey at different pH values Example 4 discloses analysis of fractionation of growth factors from acid whey by using EDFM. The fractionation is performed as disclosed in example 3, but pH adjkusting to different pH values. The total amount of growth factors (TGF-B2 and IGF-I) obtained in the recovery compartment was quantified by ELISA and presented in figure 10. Figure 10 shows that the recovery of growth factor from whey is highest at a pH 3.4, but also that more growth factor is recovered when pH is 3.8 as compared to being 4.2. Hence, the pH should be adjusted to a pH of 4.0 or below. Figure 10 also shows that a pH as low as 2.6 results in a good fractionation of growth factor. Example 5: Analysis of conductivity of samples from feed and recovery compartments at TO and T180. An example was made to analyse the conductivity of samples from feed and recovery compartment at TO and T180 of EDFM of both serocolostrum and acid whey. The samples of feed and from recovery compartment when using serocolostrum and acid whey as feed was prepared according to example 1 and 3 above. The result is shown in table 2 below. Table 2: Sample Conductivity (mS / cm) Serocotostrum (2.38X) Whey. 1¾. 2 teg. 3 SD Reg. i [15.7X) Rg> 2 (17X) Bsb 3 (Mix) SD pH=3.05 Feed TO 7,37 7,67 8 7,68 0,32 7,73 7,79 7,07 7,53 0,40 FeedTlSO 6,16 6,21 6,15 6,17 0,03 5,87 5,89 4,46 5,41 0,82 Recovery TO 3,26 3,21 3,25 3,24 0,03 3,28 3,33 3,28 3,30 0,03 Recovery T180 3 2,83 2,9 2,91 0,09 1,18 1,13 0,98 1,10 0,10 pH=3.45 Feed TO 6,93 7,26 6,92 7,04 0,19 — — — — — Feed T180 6,02 6,06 6,02 6,03 0,02 — — — — — Recovery TO 3,27 3,21 3,15 3,21 0-,06 — — — — — Recovery T180 4,28 2,83 3,47 3,53 0,73 — — — — — pH=3.85 Feed TO 6,04 6,89 6,16 6,36 0,46 — — — — — FeedTlSO 4,76 5,99 5,82 5,52 0,67 — — — — — Recovery TO 3,17 3,15 3,09 3,14 0-,04 — — — — — Recovery T180 3,49 3,15 3,38 3,34 0,17 — — — — — 1 Feed TO 7,29 6,2 6,81 6,77 0,55 — — — — — FeedTlSO 5,78 5,76 6,18 5,91 0,24 — — — — — Recovery TO 3,22. 3,08 3,72 3,34 0,34 — — — — — Recovery T1S0 3,88 2,83 2,98 3,23 0,57 — — — — — Table 2 shows that the conductivity of the fraction from the recovery compartment is much lower than the feed solution for both serocolostrum and 5 acid whey as feed material. In addition, it was found that the conductivity of the solution in the recovery compartment (fraction according to the present invention) was decreasing as the pH of the feed material was decreasing .

Claims

1. A method of preparing a dairy fraction enriched in growth factor from a dairy starting material, the method comprises the following steps:a) providing a dairy starting material;b) defatting the dairy starting material and separating casein from the dairy starting material to obtain a dairy feed material;c) adjusting pH of the dairy feed material to a pH of 4.0 or below;d) passing the pH adjusted dairy feed material through an electrodialysis cell under electrical field, and wherein the electrodialysis cell comprises at least one filtration membrane and at least two ion exchange membranes, the filtration membrane is positioned between the at least two ion exchange membranes; and e) collecting a growth factor enriched fraction obtained after passage of charged compounds from the pH adjusted dairy feed material through the filtration membrane to a recovery compartment.

2. The method according to claim 1, wherein the dairy starting material is selected from the group consisting of milk, colostrum, whey, and fractions thereof.

3. The method according to claim 2, wherein the milk may be selected from the group consisting of whole milk, reduced-fat milk, low-fat milk, fat-free milk (skim milk), lactose reduced milk, lactose-free milk, and buttermilk.

4. The method according to any of claims 1 and 3, wherein the at least two ion exchange membranes are selected from the group of cationic membranes and anionic membranes.

5. The method according to any of the claims 1 to 4, wherein the filtration membrane in step d) is an ultrafiltration membrane or microfiltration membrane.

6. The method according to any of the claims 1 to 5, wherein the filtration membrane has a molecular weight cut-off in the range of 150kD to 0.1pm.

7. The method according to any of the claims 1 to 6, wherein the pH adjustment in step c) is to a pH of 3.7 or below.

8. The method according to any of the claims 1 to 7, wherein the dairy feed material of step b) or the pH adjusted dairy feed of step c) is subjected to a concentration step before passed through the electrodialysis cell in step d).

9. The method according to claim 8, wherein the concentration step is by using one or more of ultrafiltration, nanofiltration, reverse osmosis, and evaporation.

11. The method according to any of the claims 1 to 10, wherein defatting of the dairy starting material is obtained by microfiltration and / or centrifugation.

12. The method according to any of the claims 1 to 11, wherein the separation of casein from dairy starting material is obtained by microfiltration and / or acidification.

13. The method according to any of the claims 1 to 12, wherein the pH adjustment in step c) is by adding one or more acid selected from the group consisting of citric acid, acetic acid, lactic acid, malic acid, glucono-delta-lactone, hydrochloric acid and phosphoric acid, or by adding carbon dioxide or by using bipolar electrodialysis.

14. The method according to any of the claims 1 to 13, wherein the method comprises a further step f) where the growth factor enriched fraction obtained in step e) is dried to a powder.

15. A dairy fraction enriched in naturally occurring growth factor, wherein the dairy fraction comprises one or more of alpha-lactalbumin and betalactoglobulin naturally occurring in a dairy material, and the dairy fraction has a concentration of growth factors of at least 0.08*10'3% of the total protein content.

16. The dairy fraction enriched in naturally occurring growth factor according to claim 15, wherein the growth factor at least comprises a combination of insulinlike growth factor (IGF-I) and transforming growth factor (TGF-B2).

17. The dairy fraction enriched in naturally occurring growth factor according to any of the claims 15 or 16, wherein the fraction furthermore comprises minerals naturally occurring in a dairy material used for making the dairy fraction.

18. The dairy fraction enriched in naturally occurring growth factor according to any of the claims 15 to 17, wherein the dairy fraction enriched in naturally occurring growth factor is a powder.

19. The dairy fraction enriched in naturally occurring growth factor according to any of the claims 15 to 18 for use in promoting cellular growth, differentiation and development of tissues in infants, formation of bones and cartilages, treatment of skin disorders, treatment of gastrointestinal diseases, regulation of the immune system, or wound healing.

20. A nutritional composition comprising the dairy fraction enriched in naturally occurring growth factor according to any of the claims 15 to 19,21. A nutritional composition according to claim 20, wherein the nutritional composition is selected from the group consisting of a beverage, an infant formula, a follow-on formula, and growing up formula.

Images