Patent Publication Number: US-2012045546-A1

Title: Method for producing an acidified milk product

Description:
TECHNICAL FIELD 
     The present invention relates to a method for producing an acidified milk product with improved texture and pleasant mouth feel. 
     BACKGROUND OF THE INVENTION 
     The market for acidified milk products, which includes fermented milks, such as spoonable, set-type and liquid (drinkable) yoghurt, is increasing worldwide and there is an interest in improving the quality and economics of this product. 
     Acidified milk products are generally produced by acidification of a milk base, such as a pasteurized fresh milk or a reconstituted milk from milk protein powder and water. Acidification may take place through addition of a chemical, such as glucono delta-lactone (GDL) or lactobionic acid (LBA), or it may be caused by fermentation of the milk with lactic acid bacteria. 
     Optionally, the acidified milk product is mixed with a sugar syrup solution, and/or subjected to a homogenization treatment. 
     Transglutaminase (TGase or TG) can be used to increase the viscosity and gel stiffness of fermented milk products, such as e.g. yoghurt. Whereas TGase significantly increases the gel firmness of yoghurt, the mouth thickness of the yoghurt tends to become watery and thin. 
     It is an objective of the present invention to provide a method for manufacturing of a fermented milk product with improved mouth feel compared to a standard acidified milk product. 
     SUMMARY OF THE INVENTION 
     The present inventors have observed that when yoghurt is produced using an exopolysaccharide (EPS) producing lactic acid bacterial culture in combination with TGase, it is possible to produce a yoghurt with high gel stiffness as well as a pleasant high mouth thickness and a pleasant mouth feel. 
     Surprisingly, it has been found that using a combination of an EPS producing culture (which normally provides a ropy texture to the yoghurt) together with TGase, it is possible to produce a yoghurt with high gel firmness, high mouth thickness and a short texture, rather than the ropy texture normally induced when just using the starter culture. 
     Thus, the present inventors have surprisingly found that a fermented milk product produced with transglutaminase and a polysaccharide producing microorganism has e.g. improved mouth feel and reduced ropyness compared to a fermented milk product produced the same way, but without transglutaminase treatment. 
     Consequently, the present invention relates to a method producing an acidified milk product, said method comprising: 
     a) providing a milk substrate;
 
b) treating the milk substrate with an enzyme having transglutaminase activity; and
 
c) fermenting the milk substrate with a microorganism which produces a polysaccharide, such as an exopolysaccharide (EPS).
 
     The method of the invention will decrease the ropyness of an acidified milk product, and improve the texture and/or the mouth thickness and/or mouth feel of an acidified milk product, thus resulting in a product having a pleasant mouth feel. 
     Another aspect of the invention related to an acidified milk product obtainable by any method of the invention. The acidified milk product produced by any method of the present invention may be drinkable, i.e. to be consumed as a beverage, and/or spoonable, i.e. to be consumed using a spoon. 
     In a further aspect, the invention relates to the use of transglutaminase for improving the mouth feel and/or decreasing the ropyness of an acidified milk product, such as set-type or spoonable yoghurt, especially of an acidified milk product which has been produced using a microorganism which causes a high/ropy texture in the acidified milk product, or of an acidified milk product which has been produced using a microorganism which produces a polysaccharide, such as an exopolysaccharide (EPS). 
     DETAILED DISCLOSURE OF THE INVENTION 
     In its broadest aspect, the present invention relates to a method for producing an acidified milk product, said method comprising: 
     a) providing a milk substrate;
 
b) treating the milk substrate with an enzyme having transglutaminase activity; and
 
c) fermenting the milk substrate with a microorganism which produces a polysaccharide, such as an exopolysaccharide (EPS).
 
     In another aspect, the invention relates to a method for decreasing the ropyness of an acidified milk product, said method comprising: 
     a) providing a milk substrate;
 
b) treating the milk substrate with an enzyme having transglutaminase activity; and
 
c) fermenting the milk substrate with a microorganism which produces a polysaccharide, such as an exopolysaccharide (EPS).
 
     In yet another aspect, the invention relates to a method for improving the texture and/or the mouth thickness and/or mouth feel of an acidified milk product, said method comprising: 
     a) providing a milk substrate;
 
b) treating the milk substrate with an enzyme having transglutaminase activity; and
 
c) fermenting the milk substrate with a microorganism which produces a polysaccharide, such as an exopolysaccharide (EPS).
 
     In a further aspect, the invention relates to a method for obtaining an acidified milk product with a pleasant mouth feel, said method comprising: 
     a) providing a milk substrate;
 
b) treating the milk substrate with an enzyme having transglutaminase activity; and
 
c) fermenting the milk substrate with a microorganism which produces a polysaccharide, such as an exopolysaccharide (EPS).
 
     Interesting embodiments of any method of the invention are:
         a method, wherein the microorganism produces a polysaccharide (such as EPS) which causes a high/ropy texture in the acidified milk product.   a method, wherein the acidified milk product is produced substantially without, or completely without any addition of a thickener and/or stabilizer, such as pectin, gelatine, starch, modified starch, carrageenan, alginate, and guar gum.   a method, wherein step b) is performed before or during step c).   a method, wherein the microorganism is a lactic acid bacterium.   a method, wherein the microorganism belongs to a species selected from the group consisting of:  Streptococcus thermophilus, Lactobacillus delbrueckii  subsp.  Bulgaricus, Lactococcus lactis, Lactococcus lactis  subsp.  cremoris, Leuconostoc mesenteroides  subsp.  cremoris, Pseudo leuconostoc mesenteroides  subsp.  cremoris, Pediococcus pentosaceus, Lactococcus lactis  subsp.  lactis biovar. diacetylactis, Lactobacillus casei  subsp.  Casei, Lactobacillus paracasei  subsp.  Paracasei, Bifidobacterium bifidum , and  Bifidobacterium longum.      a method, wherein the milk substrate is subjected to pasteurization before acidification and the enzyme treatment is performed before pasteurization.   a method, wherein the milk substrate is subjected to heat treatment prior to treatment with the enzyme having transglutaminase activity.   a method, wherein glutathione is added to the milk substrate prior to treatment with the enzyme having transglutaminase activity.   a method, wherein a protein hydrolyzate is added to the milk substrate prior to treatment with the enzyme having transglutaminase activity.   a method, wherein the protein hydrolyzate is selected from the group consisting of: a milk protein hydrolyzate (such as casein hydrolyzate, NZ Case or Maxicurd); a peptone (such as potato peptone); a yeast extract; and a hydrolyzed yeast extract.   a method, wherein the acidified milk product is selected from the group consisting of: a set-type fermented milk product, a drinkable fermented milk product, and a spoonable fermented milk product.   a method, wherein the acidified milk product has a milk solid non-fat content of less than 8%.   a method, wherein the acidified milk product has a fat content of less than 2%.   a method, wherein the acidified milk product has a fat content of less than 0.5%.   a method, wherein the enzyme having transglutaminase activity is recombinantly produced.   a method, wherein the enzyme having transglutaminase activity is obtained from a bacterium belonging to the genus  Streptomyces.          

     In a further aspect, the present invention relates to an acidified milk product obtainable by any method of the invention. The product of the invention may be packaged, e.g. in a sealed container having a volume in the range of 25 to 1500 ml. In preferred embodiments, the acidified milk product of the invention is free, or substantially free of stabilizers and thickeners like HM pectin, LM pectin, starch, modified starch, gelatin, CMC, Soya Bean Fiber/Soya Bean Polymer, Alginate. By substantially free should be understood that the product comprise less than 20% (w/w) (e.g. less than 10%, less than 5% or even less than 2% or 1%) stabilizers or thickeners. 
     In a last aspect, the present invention relates to the use of transglutaminase for improving the mouth feel and/or decreasing the ropyness of an acidified milk product, such as set-type or spoonable yoghurt. 
     Interesting embodiments of this invention are:
         the use of transglutaminase for improving the mouth feel and/or decreasing the ropyness of an acidified milk product which has been produced using a microorganism which causes a high/ropy texture in the acidified milk product, such as set-type or spoonable yoghurt.   the use, wherein the acidified milk product has been produced using a microorganism which produces a polysaccharide, such as an exopolysaccharide (EPS).   the use of transglutaminase for improving the mouth feel and/or decreasing the ropyness of an acidified milk product which has been produced using a microorganism which produces a polysaccharide (e.g. an exopolysaccharide (EPS)), such as set-type or spoonable yoghurt.       

     The terms “ropy” or “ropyness” refers to the texture of the acidified milk product. E.g., the texture of a yoghurt is assessed by sensory evaluation. A spoonful of yoghurt is pulled from the sample and the ropy property is evaluated. The longer the thread becomes before it breaks the ropier the product. Ropyness can also be assessed instrumentally: Using a rheometer with a bob-cup system a flow curve measuring shear stress as a function of shear rates from 10 to 350 s-1 (up and down sweeps). Hysteresis loop area between the upwards and downwards flow curve (shear rates from 0 to 241 s-1) was calculated in % of the area under the upper curve. The hysteresis loop area is correlated to the sensory perceived ropyness. 
     The term “short texture” refers to the opposite of ropy texture. So when the ropyness of a yoghurt decreases, the texture becomes “shorter”. 
     The term “acidified milk products” refers to any milk-based product which has been acidified, and includes fermented milk products, and acidified milk drinks. 
     The term “fermented milk product” includes yoghurt. The term “yoghurt” typically covers a milk product produced by fermentation by a starter culture comprising the combination of a  Lactobacillus  species (e.g.  L. bulgaricus ) and  Streptococcus thermophilus  or any other appropriate combination of microorganisms. 
     “Acidified milk drinks” according to the present invention include any drinkable product based on acidified milk substrates, thus including fermented milk drinks and liquid yoghurt drinks. Acidified milk drinks according to the invention are drinkable in the sense that they are in liquid form and consumed as beverages, i.e. they are suitable for drinking instead of being eaten with a spoon (“spoonable”). “In liquid form” means that the products are in the fluid state of matter thus exhibiting a characteristic readiness to flow. Thus, the shape of a liquid is usually determined by the container it fills, in contrary to e.g. a gel-like substance, which is soft, but not free flowing, such as e.g. yoghurt or pudding. Acidified milk drinks according to the invention may have a viscosity allowing the consumer to drink the products using a straw if desired. 
     In an interesting aspect, acidified milk drinks according to the invention have a viscosity measured as discharge time from a 10 ml pipette which is substantially the same as the discharge time of an acidified milk drink produced without transglutaminase. In this context, a discharge time which is substantially the same means that it is less than 20% increased, preferably less than 15% increased and more preferably less than 10% increased. 
     An acidified milk product according to the present invention may have a pH of less than 4.6, preferably less than 4.4, more preferably less than 4.2 and even more preferably about pH 4.0 or less. In one aspect, the acidified milk product has a pH of less than 3.8, such as less than 3.6. 
     In the methods of the present invention, acidification is performed as a fermentation with a microorganism and/or by addition of an acid, such as an organic acid (e.g. lactic acid, lactobionic acid or GDL). In a preferred embodiment, the milk substrate is acidified by fermentation with a microorganism. Optionally, such acidification by fermentation is combined with chemical acidification of the milk substrate, e.g. by addition of an acid as described above. 
     An acidified milk product according to the invention may have a fat content of 0 to 2%, preferably below 1.5%, below 1% or below 0.5%, more preferably of about 0.1% or less. The acidified milk product may have a milk solid non-fat content of less than 20%, preferably less than 8.5%, less than 8%, less than 7.5%, less than 7%, less than 6.5% or less than 6%, and more preferably of about 5%. 
     An acidified milk product according to the invention may have a protein content of between 0.5 and 4%. In one preferred aspect, the acidified milk product has a protein content of below 1%. In another preferred aspect, the acidified milk product has a protein content of between 2% and 3%. 
     An acidified milk product according to the invention may have a shelf life of more than 7 days, preferably more than 14 days, more preferably more than 28 days, such as more than 3 months. By the term “shelf-life” as used herein should be understood the time-period from the finalisation of a product and until this product, when stored properly and under the conditions recommended by the manufacturer, becomes unacceptable to the consumer. 
     Mouthfeel (or mouth feel) is a product&#39;s physical and chemical interaction in the mouth, an aspect of food rheology. It is evaluated from initial perception on the palate, through swallowing to aftertaste. The term describes all tactile observations related with the texture and sensation of texture in the mouth (or sensations occurring in the oral cavity, related to the oral tissues and their perceived condition, e.g. coating), including the characteristic “creaminess” which usually refers to the mouthfeel of cream. (Barnes et al., 1991, Journal of Dairy Science 74:2089-2099, Lawless and Heyman (1999) Sensory evaluation of food: principles and practices. Aspen Publishers, Inc., Gaithersburg, Md.). 
     “Mouth thickness” may be described as the degree of thickness when swallowing the yogurt at normal-high eating rate; high mouth thickness corresponds to a thick product that takes a long time to swallow (Meilgaard, M., Civille, V. G., Carr, B. T., eds. 1999. Sensory Evaluation Techniques (3rd Editition). New York: CRC Press.). 
     The term “spoonable” should be understood as to be consumed using a spoon. The term “spoonable fermented milk product” includes “stirred yoghurt”. The term “stirred yoghurt” specifically refers to a yoghurt product which sustains a mechanical treatment after fermentation, resulting in a softening and liquefaction of the coagulum formed under the fermentation stage. The mechanical treatment is typically but not exclusively obtained by stirring, pumping, filtrating or homogenizing the yoghurt gel, or by mixing it with other ingredients. Stirred yoghurts typically but not exclusively have a milk solid non-fat content of 9 to 15%. The term “set-type fermented milk product” includes a product based on milk which has been inoculated with a starter culture, e.g. a yoghurt starter culture, and packaged next to the inoculating step and then fermented in the package. The term “drinkable fermented milk product”, “acidified milk drink”, “fermented milk drink” and the like includes beverages such as “drinking yoghurt” and similar. The term “drinking yoghurt” typically covers a milk product produced by fermentation by the combination of a  Lactobacillus  species (e.g.  L. bulgaricus ) and  Streptococcus thermophilus . “Drinking yoghurt” is typically consumed by drinking the yoghurt, e.g. directly from the packaging or from a glass/cup or the like. Drinking yoghurt typically have a milk solid non-fat content of 8% or more. Furthermore, the live culture count for drinking yoghurt drinks is typically at least 10E6 cell forming units (CFU) pr ml. 
     “Milk substrate”, in the context of the present invention, may be any raw and/or processed milk material that can be subjected to acidification according to the method of the invention. Thus, useful milk substrates include, but are not limited to, solutions/suspensions of any milk or milk like products comprising protein, such as whole or low fat milk, skim milk, buttermilk, reconstituted milk powder, condensed milk, dried milk, whey, whey permeate, lactose, mother liquid from crystallization of lactose, whey protein concentrate, or cream. Obviously, the milk substrate may be milk. The term “milk” is to be understood as the lacteal secretion obtained by milking any mammal, such as cows, sheep, goats, buffaloes or camels. In a preferred embodiment, the milk is cow&#39;s milk. 
     In one aspect of the present invention, the milk substrate is more concentrated than raw milk, i.e. the protein content is higher than in raw milk. In this aspect, the protein content is more than 5%, preferably more than 6%, such as more than 7%, more preferably more than 8%, such as more than 9% or more than 10%. Preferably, the lactose content is also higher than in raw milk, such as more than 7%, more than 8%, more than 9%, more than 10%, more than 11% or more than 12%. In a preferred embodiment of this aspect, the milk substrate is a concentrated aqueous solution of skim milk powder having a protein content of more than 5% and a lactose content of more than 7%. 
     In the context of the present invention, percentages defining the content of the milk substrate or the content of the acidified milk product are mass percentages, i.e. the mass of a substance (e.g. protein or lactose) as a percentage of the mass of the entire solution (milk substrate or acidified milk product). Thus, in a milk substrate having a protein content of more than 5%, the mass of the proteins constitutes more than 5% of the mass of the milk substrate. 
     Preferably, at least part of the protein in the milk substrate is proteins naturally occurring in milk, such as casein or whey protein. However, part of the protein may be proteins which are not naturally occurring in milk. 
     The term “hydrolyzate” refers to any substance produced by hydrolysis. The term is not intended to be limited to substance produced by any specific method of hydrolysis. The term is intended to include “hydrolyzates” produced by enzymatic as well as non-enzymatic reactions. For example, any of the known hydrolytic enzymes (e.g., proteases, serine proteases, metalloproteases, hydrolases, etc.) are capable of producing hydrolyzates within the meaning of how the term is used in the present context. Similarly, non-enzymatic methods of hydrolysis (e.g., acid/base hydrolysis, etc.) also produce hydrolyzates within the meaning of how the term is used in the present application. The term “protein hydrolyzate” refers to a hydrolyzate produced by hydrolysis of a protein of any type or class. Any known protein may be hydrolyzed to produce a protein hydrolyzate within the meaning of the term. A “protein hydrolyzate” may be produced by enzymatic as well as non-enzymatic methods and may include protein fragments (e.g., polypeptides) that range in size from two to 100 or more amino acids. Further, as used herein, a “protein hydrolyzate” is not limited to a single product compound, but may include a heterogenous distribution or mixture of hydrolysis products (e.g., protein fragments). It may also include a homogenous compound or purified fraction of hydrolysis products. The term “protein” refers to any composition comprised of amino acids and recognized as a protein by those of skill in the art. The terms “protein,” “peptide” and “polypeptide” are used interchangeably herein. The term “milk protein hydrolyzate” refers to a hydrolyzate produced by hydrolysis of a milk protein of any type or class, e.g. a casein or a whey protein. 
     Proteases useable in the present invention is in particular proteases from the IUBMB Enzyme Nomenclature class EC 3.4.-.-, especially from subclasses EC 3.4.21.-, EC 3.4.22.-, EC 3.4.23.- and EC 3.4.24.-. The classes comprises serine proteases,  Bacillus  proteases, Cysteine proteases, Aspartic proteases, metalloproteases, proteases classified in EC 3.4.21.62, EC 3.4.22.2, EC 3.4.23.4, EC 3.4.24.28, Neutrase®, Alcalase®, subtilisin A (Type VIII), papain, chymosin, Colorase N, Optimase or Protease N “Amano”. The protease may be used in purified form, e.g. isolated from a microorganism, such as a protease originating from a lactic acid bacterium, or it may be produced in situ by a microorganism, such as the lactic acid bacterium used for the fermentation. 
     Prior to fermentation, the milk substrate may be homogenized and pasteurized according to methods known in the art. 
     “Homogenizing” as used herein means intensive mixing to obtain a soluble suspension or emulsion. If homogenization is performed prior to fermentation, it may be performed so as to break up the milk fat into smaller sizes so that it no longer separates from the milk. This may be accomplished by forcing the milk at high pressure through small orifices. 
     “Pasteurizing” as used herein means treatment of the milk substrate to reduce or eliminate the presence of live organisms, such as microorganisms. Preferably, pasteurization is attained by maintaining a specified temperature for a specified period of time. The specified temperature is usually attained by heating. The temperature and duration may be selected in order to kill or inactivate certain bacteria, such as harmful bacteria. A rapid cooling step may follow. 
     “Fermentation” in the methods of the present invention means the conversion of carbohydrates into alcohols or acids through the action of a microorganism. Preferably, fermentation in the methods of the invention comprises conversion of lactose to lactic acid. 
     In the context of the present invention, “microorganism” may include any bacterium or fungus being able to ferment the milk substrate. The microorganisms used for most fermented milk products are selected from the group of bacteria generally referred to as lactic acid bacteria. As used herein, the term “lactic acid bacterium” designates a gram-positive, microaerophilic or anaerobic bacterium, which ferments sugars with the production of acids including lactic acid as the predominantly produced acid, acetic acid and propionic acid. The industrially most useful lactic acid bacteria are found within the order “Lactobacillales” which includes  Lactococcus  spp.,  Streptococcus  spp.,  Lactobacillus  spp.,  Leuconostoc  spp.,  Pseudoleuconostoc  spp.,  Pediococcus  spp.,  Brevibacterium  spp.,  Enterococcus  spp. and  Propionibacterium  spp. Additionally, lactic acid producing bacteria belonging to the group of the strict anaerobic bacteria, bifidobacteria, i.e.  Bifidobacterium  spp., are generally included in the group of lactic acid bacteria. These are frequently used as food cultures alone or in combination with other lactic acid bacteria, 
     Lactic acid bacteria are normally supplied to the dairy industry either as frozen or freeze-dried cultures for bulk starter propagation or as so-called “Direct Vat Set” (DVS) cultures, intended for direct inoculation into a fermentation vessel or vat for the production of a dairy product, such as an acidified milk product. Such cultures are in general referred to as “starter cultures” or “starters”. 
     Commonly used starter culture strains of lactic acid bacteria are generally divided into mesophilic organisms having optimum growth temperatures at about 30° C. and thermophilic organisms having optimum growth temperatures in the range of about 40 to about 45° C. Typical organisms belonging to the mesophilic group include  Lactococcus lactis, Lactococcus lactis  subsp.  cremoris, Leuconostoc mesenteroides  subsp.  cremoris, Pseudoleuconostoc mesenteroides  subsp.  cremoris, Pediococcus pentosaceus, Lactococcus lactis  subsp.  lactis biovar. diacetylactis, Lactobacillus casei  subsp.  casei  and  Lactobacillus paracasei  subsp.  paracasei . Thermophilic lactic acid bacterial species include as examples  Streptococcus thermophilus, Enterococcus faecium, Lactobacillus delbrueckii  subsp.  lactis, Lactobacillus helveticus, Lactobacillus delbrueckii  subsp.  bulgaricus  and  Lactobacillus acidophilus.    
     Also the strict anaerobic bacteria belonging to the genus  Bifidobacterium  including  Bifidobacterium bifidum  and  Bifidobacterium longum  are commonly used as dairy starter cultures and are generally included in the group of lactic acid bacteria. Additionally, species of Propionibacteria are used as dairy starter cultures, in particular in the manufacture of cheese. Additionally, organisms belonging to the  Brevibacterium  genus are commonly used as food starter cultures. 
     Another group of microbial starter cultures are fungal cultures, including yeast cultures and cultures of filamentous fungi, which are particularly used in the manufacture of certain types of cheese and beverage. Examples of fungi include  Penicillium roqueforti, Penicillium candidum, Geotrichum candidum, Torula kefir, Saccharomyces kefir  and  Saccharomyces cerevisiae.    
     In a preferred embodiment of the present invention, the microorganism used for fermentation of the milk substrate is  Lactobacillus casei  or a mixture of  Streptococcus thermophilus  and  Lactobacillus  species, e.g.  L. delbrueckii  subsp.  bulgaricus.    
     Optionally, the fermented milk substrate may be subjected to heat treatment to inactivate the microorganism. 
     Fermentation processes to be used in production of acidified milk products are well known and the person of skill in the art will know how to select suitable process conditions, such as temperature, oxygen, amount and characteristics of microorganism(s) and process time. Obviously, fermentation conditions are selected so as to support the achievement of the present invention, i.e. to obtain a fermented milk product suitable in the production of an acidified milk drink. 
     Likewise, the skilled person will know if and when additives such as, e.g., carbohydrates, flavors, minerals, enzymes (e.g. rennet, lactase and/or phospholipase) are to be used in production of acidified milk products according to the invention. 
     Optionally, the fermented milk substrate may be diluted to obtain the acidified milk drink. In one embodiment, the fermented milk substrate is diluted at least 1.5 times, preferably at least 2 times, at least 2.5 times or at least 3 times. It may be diluted with water or an aqueous solution of any kind. “Diluted at least 1.5 times” in the context of the present invention means that the fermented milk substrate is diluted so that its volume is increased by at least 50%. 
     In one embodiment, a syrup is added to the fermented milk substrate. “Syrup” in the context of the present invention is any additional additive ingredient giving flavor and/or sweetness to the final product, i.e. the acidified milk product. It may be a solution comprising, e.g., sugar, sucrose, glucose, liquid sugar of fructose, aspartame, sugar alcohol, fruit concentrate, orange juice, strawberry juice and/or lemon juice. The mixture of the fermented milk substrate and the syrup may be homogenized using any method known in the art. The homogenization may be performed so as to obtain a liquid homogenous solution which is smooth and stable. Homogenization of the mixture of the acidified milk substrate and the syrup may be performed by any method known in the art, such as by forcing the milk at high pressure through small orifices. 
     In another embodiment of the invention, water is added to the fermented milk substrate, and the mixture of fermented milk substrate and water is homogenized. 
     The methods of the present invention comprise treatment of the milk substrate with an enzyme having transglutaminase activity. The enzyme treatment may be performed prior to fermentation, such as before inoculation with the microorganism. The enzyme treatment may be performed at the same time as the fermentation. In one embodiment, the enzyme is added before, at the same time or after inoculation of the milk substrate with a microorganism, and the enzyme reaction on the milk substrate takes place at essentially the same time as it is being fermented. Alternatively, the enzyme treatment may be performed after fermentation. If the acidified milk substrate is mixed and optionally homogenized with the syrup, the enzyme treatment may be performed before or after this. The enzyme may be added at the same time or after the syrup, but before homogenization, or it may be added after the acidified milk substrate and the syrup have been mixed and homogenized. 
     In a preferred embodiment, enzyme treatment is performed before or during fermentation. In a more preferred embodiment, the milk substrate is subjected to pasteurization prior to fermentation, and the enzyme treatment is performed before pasteurization. The pasteurization may thus inactivate the enzyme. 
     In another preferred embodiment, the milk substrate is subjected to heat treatment, such as pasteurization, prior to treatment with transglutaminase. The heat treatment may be performed so that more than 50%, preferably more than 60%, more than 70% or more than 80%, of the whey protein in the milk substrate is denatured. In the context of the present invention, whey protein is denatured when it sediments at pH 4.5. In a more preferred embodiment, the milk substrate is subjected to heat treatment followed by homogenisation prior to treatment with transglutaminase. Optionally, the fermented milk substrate may be subjected to heat treatment, such as pasteurization, to inactivate the microorganism. Another heat treatment may be performed after the enzyme treatment so as to inactivate the enzyme. 
     In another preferred embodiment, yeast extract or a reducing agent such as glutathione is added to the milk substrate prior to treatment with transglutaminase. 
     Another heat treatment, such as a pasteurization, may be performed after the enzyme treatment so as to inactivate the enzyme. 
     The enzyme having transglutaminase activity is added in a suitable amount to achieve the desired degree of protein modification under the chosen reaction conditions. The enzyme may be added at a concentration of between 0.0001 and 1 g/L milk substrate, preferably between 0.001 and 0.1 g/L milk substrate. Dosing in units, the enzyme may be added at a concentration of between 0.5 TGHU (TransGlutaminase Hydroxamate Units) and 20 TGHU TGase/g protein in the milk substrate, preferably between 2 and 10 TGHU TGase/g protein in the milk substrate. 
     In another preferred embodiment, yeast extract or a reducing agent such as glutathione is added to the milk substrate prior to treatment with transglutaminase. 
     The enzymatic treatment in the methods of the invention may be conducted by adding the enzyme to the milk substrate and allowing the enzyme reaction to take place at an appropriate holding-time at an appropriate temperature. The enzyme treatment may be carried out at conditions chosen to suit the selected protein modifying enzyme according to principles well known in the art. The treatment may also be conducted by contacting the milk substrate with an enzyme that has been immobilized. 
     The enzyme treatment may be conducted at any suitable pH, such as, e.g., in the range of pH 2-10, such as, at a pH of 4-9 or 5-7. It may be preferred to let the enzyme act at the natural pH of the milk substrate, or, if acidification is obtained because of fermentation, the enzyme may act at the natural pH of the milk substrate during the fermentation process, i.e. the pH will gradually decrease from the natural pH of the unfermented milk substrate to the pH of the fermented milk substrate. 
     The enzyme treatment may be conducted at any appropriate temperature, e.g. in the range 1-80° C., such as 2-70° C. In one embodiment of the present invention, the enzyme treatment may preferably be conducted at a temperature in the range 40-50° C. In another embodiment, the enzyme treatment may preferably be conducted at a temperature of below 10° C. 
     Optionally, after the enzyme has been allowed to act on the milk substrate, the enzyme protein may be removed, reduced, and/or inactivated by any method known in the art, such as by heat treatment and/or reduction of pH. 
     Optionally, other ingredients may be added to the acidified milk product, such as color; stabilizers, e.g. pectin, starch, modified starch, CMC, etc.; or polyunsaturated fatty acids, e.g. omega-3 fatty acids. Such ingredients may be added at any point during the production process, i.e. before or after fermentation, before or after enzyme treatment, and before or after the optional addition of syrup. 
     In the context of the present invention, an enzyme having transglutaminase activity may be an enzyme which catalyzes the acyl transfer between the gamma-carboxylamide group of peptide-bound glutamine (acyl donor) and primary amines (acyl acceptor), e.g. peptide-bound lysine. Free acid amides and amino acids also react. Proteins and peptides may thus be cross linked in this way. Transglutaminase may also, e.g. if amines are absent, catalyze the deamination of glutamine residues in proteins with H 2 O as the acyl acceptor. 
     A transglutaminase may also be referred to as, e.g., protein glutamine-gamma-glutamyl transferase, Factor XIIIa, fibrinoligase, fibrin stabilizing factor, glutaminylpeptide gamma-glutamyltransferase, polyamine transglutaminase, tissue transglutaminase, or R-glutaminyl-peptide:amine gamma-glutamyl transferase. The group of transglutaminases comprises but is not limited to the enzymes assigned to subclass EC 2.3.2.13. In the context of the present invention, transglutaminase may also be referred to as TGase or TG. 
     A transglutaminase to be used according to the invention is preferably purified. The term “purified” as used herein covers enzyme protein preparations where the preparation has been enriched for the enzyme protein in question. Such enrichment could for instance be: the removal of the cells of the organism from which an enzyme protein was produced, the removal of non-protein material by a protein specific precipitation or the use of a chromatographic procedure where the enzyme protein in question is selectively adsorbed and eluted from a chromatographic matrix. The transglutaminase may have been purified to an extent so that only minor amounts of other proteins are present. The expression “other proteins” relate in particular to other enzymes. A transglutaminase to be used in the method of the invention may be “substantially pure”, i.e. substantially free from other components from the organism in which it was produced, which may either be a naturally occurring microorganism or a genetically modified host microorganism for recombinant production of the transglutaminase. However, for the uses according to the invention, the transglutaminase need not be that pure. It may, e.g., include other enzymes. 
     In a preferred aspect, the transglutaminase to be used in the method of the invention has been purified to contain at least 20%, preferably at least 30%, at least 40% or at least 50%, (w/w) of transglutaminase out of total protein. The amount of transglutaminase may be calculated from an activity measurement of the preparation divided by the specific activity of the transglutaminase (activity/mg EP), or it may be quantified by SDS-PAGE or any other method known in the art. The amount of total protein may, e.g., be measured by amino acid analysis. 
     In one embodiment of the methods of the invention, the enzyme having transglutaminase activity is recombinantly produced. 
     In other embodiments of the present invention, the enzyme having transglutaminase activity may be of animal, of plant or of microbial origin. Preferred enzymes are obtained from microbial sources, in particular from a filamentous fungus or yeast, or from a bacterium. For purposes of the present invention, the term “obtained from” as used herein in connection with a given source shall mean that the enzyme originates from the source. The enzyme may be produced from the source or from a strain in which the nucleotide sequence encoding the enzyme has been inserted, i.e. a recombinant strain. In a preferred embodiment, the polypeptide obtained from a given source is secreted extracellularly. 
     The enzyme may, e.g., be obtained from a strain of  Agaricus , e.g.  A. bisporus; Ascovaginospora; Aspergillus , e.g.  A. niger, A. awamori, A. foetidus, A. japonicus, A. oryzae; Chaetomium; Chaetotomastia; Dictyostelium , e.g.  D. discoideum; Mucor , e.g.  M. javanicus, M. mucedo, M. subtilissimus; Neurospora , e.g.  N. crassa; Rhizomucor , e.g.  R. pusillus; Rhizopus , e.g.  R. arrhizus, R. japonicus, R. stolonifer; Sclerotinia , e.g.  S. libertiana; Trichophyton , e.g.  T. rubrum; Whetzelinia , e.g.  W. sclerotiorum; Bacillus , e.g.  B. megaterium, B. subtilis, B. pumilus, B. stearothermophilus, B. thuringiensis; Chryseobacterium; Citrobacter , e.g.  C. freundii; Enterobacter , e.g.  E. aerogenes, E. cloacae Edwardsiella, E. tarda; Erwinia , e.g.  E. herbicola; Escherichia , e.g.  E. coli; Klebsiella , e.g.  K. pneumoniae; Miriococcum; Myrothesium; Mucor; Neurospora , e.g.  N. crassa; Phytophthora , e.g.  P. cactorum; Proteus , e.g.  P. vulgaris; Providencia , e.g.  P. stuartii; Pycnoporus , e.g.  Pycnoporus cinnabarinus, Pycnoporus sanguineus; Salmonella , e.g.  S. typhimurium; Serratia , e.g.  S. liquefasciens, S. marcescens; Shigella , e.g.  S. flexneri; Streptomyces , e.g.  S. antibioticus, S. castaneoglobisporus, S. lydicus, S. mobaraensis, S. violeceoruber; Streptoverticilium , e.g.  S. mobaraensis; Trametes; Trichoderma , e.g.  T. reesei, T. viride; Yersinia , e.g.  Y. enterocolitica.    
     In a preferred embodiment, the enzyme is a transglutaminase obtained from a bacterium, e.g. an Actinobacterium from the class Actinobacteria, such as from the subclass Actinobacteridae, such as from the order Actinomycetales, such as from the suborder Streptomycineae, such as from the family Streptomycetaceae, such as from a strain of  Streptomyces , such as  S. lydicus  or  S. mobaraensis . In another embodiment, the enzyme is a transglutaminase obtained from a fungus, e.g. from the class Oomycetes, such as from the order Peronosporales, such as from the family Pythiaceae, such as from the genera  Pythium  or  Phytophthora , such as from a strain of  Phytophthora cactorum.    
     According to the present invention, transglutaminase activity may be determined by any method known in the art, such as by incubating the enzyme with gamma-carboxamid group of protein- or peptide-bound glutamine and an amine group, e.g. protein- or peptide-bound lysine, in a buffer at various pH and temperatures, e.g. 50 mM MES at pH 6.5 at 37° C. for 30 minutes. The detection of enzyme activity can be followed by the release of ammonia (e.g. kit obtained from Roche NH3-11877984) or using hydroxylamine as amine group donor (the amount of Glutamic acid gamma-hydroxamate formed in the reaction is detected as a red complex with ferric ions under acid conditions measured at 510 nm) or by determination of the epsilon-(gamma-glutamyl)lysin by amino acid analysis. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising”, “having”, “including” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     REFERENCES 
     
         
         WO09016257A (Novozymes); US2009061046A (NovoZymes); WO9421129A (Novozymes); WO2007/060288A, WO05016027A; WO0110232A; EP0671885; U.S. Pat. No. 4,289,789 
         Food Control, Volume 16, Issue 3, March 2005, Pages 205-209 
         Journal of Dairy Science, Vol. 85, No. 7, 1705-1708 
         Journal of Dairy Research (2008) 75 450-456 
         International Dairy Journal, Volume 16, Issue 2, February 2006, Pages 111-118 
       
    
     All references cited in this patent document are hereby incorporated herein in their entirety by reference. 
    
    
     
       DRAWING 
         FIG. 1  depicts the gel stiffness as a function of TGase dosage, see the example. 
     
    
    
     EXAMPLE 
     Yoghurt was made from 3×200 mL whole milk: 
     Sample 1: Whole milk added 2% of Skim milk powder (SMP)
 
Sample 2: Whole milk added 1% of SMP and 0.2 Units TGase per gram of substrate protein (U/g).
 
Sample 3: Whole milk added 1% of SMP and 0.5 U/g TGase.
 
     In all cases fresh whole milk from Arla Foods (Arta Ekspress, “Sødmælk”) was used. The milk was added skim milk powder and allowed to hydrate overnight at 5 C. Then the milk was heat treated at 90 C for 20 min and cooled to the fermentation temperature of 43 C. After cooling to 43 C, the necessary amount of TGase (Ajinomoto Activa YG) was added to the milk. Hereafter the culture (YL-F800, Chr. Hansen A/S) was added and the milk was incubated at 43 C until reaching a pH of 4.7. Then the sample was cooled at 5 C and stored until measurement. One the next day the rheological properties of the yoghurt were measured using a StressTech rheometer. Measurements were done at 13 C. In one test the gel stiffness of the yoghurt was measured by a so-called frequency sweep, measuring the complex modulus (G*) of the gel as function of oscillation frequency. The value of G* at 1 Hz was used a the “gel stiffness” of the yoghurt. The viscosity of the yoghurt was measured by a so-called constant rate measurement, measuring the shear stress of the yoghurt as function of shear rate. The shear stress at a rate of 300 s −1  has previously been found to correlate well with sensory perceived “Mouth thickness”, and thus this value was used to evaluate “mouth thickness” of the yoghurts. The area that is formed between the shear stress curve, when increasing shear rate and decreasing it again it often referred to as the “hysteresis loop”. It has previously been found that the ratio of the “hysteresis loop” area divided by the shear stress at 300 s −1  is a measure of the ropyness of the yoghurt, and thus this value (the ratio) was used to describe ropyness of the yoghurts. 
       FIG. 1  clearly shows that when TGase is added to the yoghurt, even when replacing 1% SMP, the gel stiffness of the yoghurt is significantly improved (as a function of TGase dosage). 
     The table below (Table 1) shows the shear stress at 300 s-1 (“Mouth thickness”) and the ropyness (as measured as described). It is clear that the “mouth thickness” of the yoghurt increases, whereas the ropyness decreases. This shows that TGase together with an EPS producing culture can produce a yoghurt with high gel stiffness, high “mouth thickness” and a shorter texture, than anticipated from using the EPS producing culture without TGase. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Ratio of 
               
               
                   
                 Shear stress 
                 Hysteresis 
                 Loop area: 
               
               
                   
                 at 300 s −1   
                 Loop area 
                 Upper shear 
               
               
                 Sample 
                 (Pa) 
                 (—) 
                 stress (—) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 YF-L800 
                 100.50 
                 11835.73 
                 0.44 
               
               
                 (2% added SMP) 
               
               
                 YF-L800 
                 105.97 
                 12416.99 
                 0.43 
               
               
                 (1% added SMP + 0.2 U/g TG) 
               
               
                 YF-L800 
                 124.43 
                 11611.65 
                 0.36 
               
               
                 (1% added SMP + 0.5 U/g TG) 
               
               
                   
               
            
           
         
       
     
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.