the milk whey protein solution to be used in the invention may be a whey protein solution in which the protein content in the solids is from 80 % to 100 %, preferably from 90 to 100 %, and the solid content in the aqueous solution is from 3 % to 30 %, preferably from 7 % to 20 %. the raw material , the production method and the like of the milk whey protein are not particularly limited , and a substance obtained by condensing and separating whey produced when cheese or casein is produced from cow milk , a substance obtained by adding water to a substance obtained through drying and powdering of the above condensed and separated substance and dissolving the resulting powder in water or the like can be used . when the protein content in the solids is 80 % or less , the rate of the transglutaminase reaction decreases . probably , the reason is considered that the amount of a substance that inhibits the transglutaminase activity is relatively increased when the protein content in the solids is lower . in general , it is considered that as the concentrations of the substrate and the enzyme are higher , the rate of the enzymatic reaction increases , however , in the case where a milk whey protein is used as a substrate , when the condition does not fall within the above - mentioned range , the rate of the enzymatic reaction decreases . further , in the case where the solid content is low , the volume of a tank to be used during the enzymatic reaction becomes large , therefore , an economical advantage is deteriorated . the transglutaminase to be used in the invention is not particularly limited in terms of its origin as long as it has a transglutaminase activity , and for example , a transglutaminase derived from a microorganism belonging to the genus streptomyces or the like ( see , japanese patent application publication no . jp - a - 64 - 27471 ), a transglutaminase derived from a mammal such as a guinea pig ( see , japanese patent no . jp - b - 1 - 50382 ), a transglutaminase derived from a fish such as codfish ( see , nobuo seki et al ., abstracts for the autumn meeting of the japanese society of fisheries science , 1988 , p . 167 ), a transglutaminase obtained by a genetic recombination technique utilizing biotechnology ( see , japanese patent application publication no . jp - a - 1 - 30889 and japanese patent application publication no . jp - a - 6 - 225775 ) or the like can be used . in addition , a transglutaminase which is commercially available from ajinomoto co ., inc . under the trade name of “ activa ” may be purchased and used . as described above , as the transglutaminase , any can be used regardless of its origin and type as long as it is a transglutaminase . further , either a transglutaminase of a type which requires calcium for the expression of its activity ( a calcium - dependent transglutaminase ) or a transglutaminase of a type which does not require calcium for the expression of its activity ( a calcium - independent transglutaminase ) can be used . however , it is preferred that a transglutaminase derived from a microorganism is used for the reasons that it can be used without calcium , it can be obtained in a large amount and the like . the activity unit of the transglutaminase used in the invention is measured and defined as follows . that is , the transglutaminase is allowed to act in a reaction system containing benzyloxycarbonyl - l - glutamylglycine and hydroxylamine as substrates at a temperature of 37 ° c . in tris buffer at ph 6 . 0 , and resulting hydroxamic acid is formed into a ferric complex in the presence of trichloroacetic acid , and then an absorbance at 525 nm is measured . the amount of hydroxamic acid is calculated from a calibration curve , and an enzyme which produces 1 μmol of hydroxamic acid per minute is defined as 1 unit ( 1 u ) ( see , japanese patent application publication no . jp - a - 64 - 27471 ). in the invention , the modification of a whey protein by a transglutaminase can be performed by adding a transglutaminase to a whey protein solution and thereafter allowing the enzymatic reaction to proceed by , for example , letting the resulting mixture stand or stirring it . the addition amount of a transglutaminase is preferably from 0 . 5 to 5 units per gram of the protein although it depends also on the reaction time and temperature . when the addition amount is less than 0 . 5 units per gram of the protein , it is difficult to obtain an effect or a required reaction time is prolonged . on the other hand , when the addition amount exceeds 5 units , an effect relative to the addition amount is significantly decreased , therefore , an economical advantage is lost . the temperature during the enzymatic reaction is preferably from 50 ° c . to 65 ° c . when the temperature is lower than 50 ° c ., the enzymatic reaction proceeds slowly , and also the growth of microorganisms cannot be suppressed , therefore , it is not preferred from a hygiene point of view . when the temperature exceeds 65 ° c ., inactivation of a transglutaminase becomes prominent , therefore , an effect relative to the addition amount is significantly decreased . the required reaction time depends also on the addition amount of a transglutaminase and the temperature during the reaction , however , in the case of 50 ° c . or higher , a sufficient effect can be obtained if the transglutaminase is allowed to react for about 10 minutes to about 5 hours . a solution of the thus obtained modified whey protein is added to a raw material milk of a yogurt in an amount of 0 . 05 % to 3 %, preferably 0 . 1 % to 1 % of the raw material milk in terms of the weight of the whey protein . also from this , it is found that the used amount of a transglutaminase relative to a raw material milk is very small and the present invention is economically advantageous . after a solution of the thus obtained modified whey protein is added to a raw material milk of a yogurt , the resulting mixture is sterilized by heating , and then a lactic acid bacterium is inoculated into the mixture , and fermented milk curds are obtained under a general condition . as the lactic acid bacterium , a lactic acid bacterium commonly used for a yogurt may be used . further , as for the inoculation of a lactic acid bacterium , a lactic acid bacterium may be added or a commercially available yogurt containing a lactic acid bacterium may be added as such . subsequently , the curds are crushed by a procedure such as stirring or high - pressure emulsification to make the resulting product smooth , whereby a stirred type yogurt is obtained . in this connection , the modified whey protein solution may be heated before it is mixed in a raw material milk of a yogurt so as to effect sterilization and inactivate a transglutaminase . further , the modified whey protein solution is dried thereby obtaining modified whey protein powder and the powder may be directly mixed in a raw material milk or an aqueous solution of the powder may be mixed in a raw material milk of a yogurt . other features of the invention will become apparent in the course of the following descriptions of exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof . to 10 parts of a whey protein isolate ( alacen 895 manufactured by fontera co , protein content : 94 %), 90 parts of water was added and mixed , whereby a whey protein solution was obtained . further , a transglutaminase preparation for a yogurt manufactured by ajinomoto co ., inc ., “ activa ” tg - yg ( transglutaminase activity : 100 units / g , glutathione contained : 3 . 6 %) was added thereto in accordance with table 1 , and the resulting mixture was left to stand at a temperature of 55 ° c . for 60 minutes , whereby a modified whey protein solution was obtained . on the other hand , 53 parts of commercially available pasteurized cow milk , 4 . 8 parts of powdered skim milk and 39 . 5 parts of water were mixed , whereby a yogurt mix was obtained . to 94 parts of this yogurt mix , 3 parts of the above - mentioned modified whey protein solution ( corresponding to the addition of a whey protein in an amount of 0 . 5 % of the raw material milk ) was added . then , the temperature of the mixture was immediately raised and maintained at 95 ° c . for 5 minutes . thereafter , the mixture was cooled to 43 ° c ., and 3 parts of a commercially available stirred type yogurt ( containing a lactic acid bacterium ) was added thereto and the temperature of the mixture was maintained at 43 ° c . thereby allowing the fermentation to proceed . when the ph of the mixture reached 4 . 6 , the mixture was cooled to 20 ° c ., and passed through a high - pressure homogenizer ( lab 1000 , apv co .) at a preset pressure of 0 bar , whereby a stirred type yogurt was obtained . this stirred type yogurt was packed in a container and stored under refrigeration for 7 days , and then , evaluation was performed . the evaluation results are shown in table 2 and table 3 . as a control , a yogurt was prepared in the same procedure as above except that 3 parts of water was added in place of 3 parts of the whey protein solution . as shown in the above , compared with the yogurt obtained by using a whey protein which was not modified by a transglutaminase , the yogurt obtained by using a whey protein which was modified by a transglutaminase showed no water separation , and gave a preferred rich feel without deteriorating a smooth texture . according to the present invention , a stirred type yogurt , which shows little water separation , gives a rich feel and has a smooth structure can be economically produced , therefore , the invention is extremely useful in the field of food industry . where a numerical limit or range is stated herein , the endpoints are included . also , all values and subranges within a numerical limit or range are specifically included as if explicitly written out . obviously , numerous modifications and variations of the present invention are possible in light of the above teachings . it is therefore to be understood that , within the scope of the appended claims , the invention may be practiced otherwise than as specifically described herein . all patents and other references mentioned above are incorporated in full herein by this reference , the same as if set forth at length .
mixing milk with a reaction product , which is prepared by dissolving at a high concentration highly pure milk whey protein having been separated from cow &# 39 ; s milk , etc . and concentrated and then treating with transglutaminase , then inoculating with a lactic acid bacterium and fermenting economically affords a yogurt of agitated type , which is enriched in whey protein , shows little water release and has a rich feeling and a highly smooth texture .