Method of preparing animal food of increased palatability
BACKGROUND OF THE INVENTION 
     This Application is a continuation-in-part of copending U.S. application Ser. No. 461,831, now U.S. Pat. No. 3,857,968, filed Apr. 8, 1974, which is a continuation-in-part of U.S. application Ser. No. 346,364, filed Mar. 29, 1973, now abandoned; and it is related to subject matter disclosed and claimed in copending U.S. application Ser. No. 494,642, filed Aug. 5, 1974, which is a continuation of U.S. application Ser. No. 346,362, filed Mar. 29, 1973, now abandoned. 
    
    
     The present invention relates to animal foods, and, more particularly, to methods for producing animal foods having increased palatability. 
     There is a continuing effort being made to develop processes and formulations which increase the palatability of animal foods, while at the same time maintaining their nutritional value. While the development and production of nutritious animal foods has posed few problems to the art, there is a continuing problem of making these formulations palatable. Where the foods are unpalatable, animals often pass up the offered foods and do not take advantage of their nutritional value. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an animal food of improved palatability. 
     It is a further object of the present invention to provide a process for improving the palatability of animal food. 
     These and other objects are accomplished according to the present invention by incorporating into an animal food an effective amount of a palatability improving composition prepared by reacting an aqueous mixture of protein and emulsified fat with lipase and protease. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Animal foods normally contain from about 4 to 16 weight percent fat. While any portion or all of this fat content can be reacted according to this invention in preparing the palatability improving composition, it is necessary to the present invention only to provide an effective amount of the conditioned mixture of fat and protein (i.e., the palatability improving composition) for increasing the palatability of the animal food. The mixture of fat and protein can be reacted separately, or any portion of the total animal food composition comprising both fat and protein can be subjected to the action of the enzymes if desired. 
     The palatability improving composition can be incorporated into the animal food in any suitable manner. Thus, where the complete animal food or a portion thereof is conditioned, the palatability improving composition is incorporated by producing it in situ. Or, the palatability improving composition can be prepared separately from the remainder of the animal food and then mixed with or applied to the animal food. These procedures will be discussed in more detail below. 
     The fats employed in preparing the palatability improving composition according to the present invention are preferably animal fats such as those naturally present in meats, bleachable fancy tallow, chicken fat, butter oil and lard. While other fats can be employed, butter fat and bleachable fancy tallow are preferred. It will be apparent to those skilled in the art that certain fats and oils such as cocoa butter, which, naturally or after conditioning, are unpalatable to animals, are not preferred according to the present invention. 
     The protein employed in preparing the palatability improving composition can be any of those available in quantity at a reasonable cost. Where an emulsifier is added to the reaction mixture, it is preferable to employ a proteinaceous emulsifier which both supplies the protein and effects emulsification. Also, a meat slurry or other proteinaceous ingredient of the animal food may supply the necessary protein. 
     The process for conditioning the mixture of fat and protein according to the present invention broadly comprises emulsifying the fat and treating the mixture of protein and emulsified fat with lipase and protease. The reaction between the emulsified fat-protein mixture and the enzymes is presently believed to produce a complex array of reaction products. The fat reacts with the lipase to produce free fatty acids and mono- and diglycerides. The protein reacts with the protease to produce polypeptides and free amino acids. It is further possible that other reactions occur. The exact reactions and reaction products responsible for the unexpected improvement in the palatability of animal foods which is effected by the present invention are not presently identifiable. However, the reaction between the emulsified fat-protein mixture and the enzyme mixture produces a real and reproducible improvement, and Applicants do not wish to be bound to any specific theory as to which particular reaction or reaction product brings about the desired result of the present invention. 
     It is presently considered necessary to emulsify the fat before treatment with the enzymes. This is due to the fact that emulsification increases the fat-water interfacial area, thereby facilitating the heterogeneous reaction between the fat and the lipase. Emulsification can be obtained through the addition of an emulsifier; or, as in the case where the fat-protein mixture comprises a meat slurry, the meat slurry will impart a limited natural emulsifying effect. Where an emulsifier is added, it is preferably proteinaceous. Promine D soy isolate, available from Central Soya, is a particularly preferred proteinaceous emulsifier for use according to the present invention. Obviously, however, a non-proteinaceous emulsifier can be employed. Typical of the non-proteinaceous emulsifiers are sodium stearoly-2-lactylate and succinoylated monoglycerides. While the exact concentration of the emulsifer is not presently believed critical, it is typically employed in amounts ranging from about 0.5% to 20% by weight based on the weight of the fat. 
     The lipase and protease employed according to the present invention may be derived from any suitable source as long as they contain effective amounts of these enzymes when used at concentrations which will not adversely affect the palatability of the animal food. Preferably, if an enzyme mixture is employed, it should contain from about 20 to 250 lipase units per gram and from about 500 to 7000 protease units per gram. Also, however, separate sources for the lipase and protease can be employed, and the enzymes added either simultaneously or sequentially. A lipase unit is defined as that amount of the enzyme which will hydrolyze 0.885 grams of olive oil calculated as triolein to diolein and oleic acid in two hours at 37°C. A protease unit is defined as that amount of the enzyme which will digest 1 mg. of casein in 1 minute at 50°C at pH 7.5. Pancreatic lipase, which is an enzyme mixture containing about 220 lipase units per gram and about 6000 protease units per gram, is a particularly preferred enzyme mixture. The exact concentration at which the enzyme mixture, or the lipase and protease individually, are employed is not presently considered critical as long as the enzymes are present in amounts effective under the desired reaction conditions to cause an enzymatic reaction resulting in the production of the palatability improving composition. They are typically employed in amounts sufficient to supply from about 20 to 250 lipase units and about 500 to 7000 protease units per 100 grams fat. The enzymes can be admixed with the reaction mixture at any time before, during or after emulsification. Admixture after emulsification is, however, preferred. 
     To effect the reaction, the fat is admixed in liquid form with water, vigorously agitated to effect emulsification, combined with the protein and enzymes and maintained at suitable reaction conditions for a period of time sufficient to effect the reaction. In the case where the protein is to serve as an emulsifier, the protein is present in the water before emulsification. In this case, it is preferably added and thoroughly admixed prior to adding the fat but can be added subsequent to fat addition. Where the fat is normally solid or plastic, it is preferably melted to the liquid state. The relative amounts of fat and water necessary for the reaction are not presently considered critical, but are desirably present at a fat to water ratio of from about 1:100 to 10:1, and preferably from about 1:4 to 1:1. The reaction mixture may also contain a promoter such as calcium chloride and sodium chloride. 
     It has been found that temperatures within the range of from about 35° to 50°C are effective for both heating the reactants prior to admixture and for maintaining the reaction. This temperature range is, therefore, preferred; however, any reaction temperature effective to sustain the enzymatic reaction resulting in the production of the palatability improving composition can be employed. It is noted that lower temperatures (e.g., 20° to 35°C) can be employed with somewhat diminished results due to incomplete emulsification and reduced reaction rates. Also, somewhat higher temperatures (e.g., 50° to 65°C) can be employed but are generally more costly than the increased reaction rates will justify. The aqueous emulsified dispersion, which may also contain the promoter, is preferably brought to a boil for about 10 minutes and then reduced to 35°- 50°C for admixture with the fat. 
     After emulsification, the enzymes are admixed with the emulsion and constant efficient stirring is maintained for the reaction period to maintain a desirably high rate of reaction. The reaction will be continued for a period of time effective to sustain the enzymatic reaction resulting in the production of the palatability improving composition. Generally, it will be continued for a period of time ranging from about 5 minutes to 16 hours, typically from 15 minutes to two hours. Where the enzymes are added sequentially, the reaction in the presence of each enzyme will be continued for a period of time effective to sustain the enzymatic reaction to the degree effective to provide the palatability improving composition as the product of the combined reaction periods. 
     It has been determined that the pH during reaction has an effect on palatability, with alkaline reaction conditions producing the more preferred results; however, the pH can be maintained at any level which is effective to sustain the enzymatic reaction resulting in the production of the palatability improving composition. Typically, the pH is adjusted periodically to bring it to within the range of from about 4 to 9, preferably about 7 to 9, and most preferably from about 8.0 to 8.5. 
     After the desired period of reaction, the emulsion can be treated to inactivate the enzymes. Typically, it can be heated to an elevated temperature, on the order of about 70° to 95°C, for a period of time sufficient to inactivate the enzymes, on the order of about 5-15 minutes. Where the enzymes are added sequentially, it may be desirable to inactivate the first added enzyme, either the lipase or protease, before adding the second enzyme and continuing the processing. After completion of the reaction with the second of the two enzymes, the reaction mixture may again be heated to inactivate that enzyme. 
     The palatability improving composition is preferably maintained in the emulsified state for incorporation into the animal food. The emulsion can, if desired, be cooled or frozen and stored for extended periods of time. 
     The palatability improving composition can be incorporated into the animal food in any suitable manner. Application by spraying is particularly preferred for dry animal foods because it allows uniform surface application without breaking the emulsion. This makes it possible to obtain improved palatability with significantly smaller amounts of the palatability improving composition. Typical of a suitable device for spraying the emulsion onto the animal food is a spray gun of the kind commonly employed in spray painting. While the fat to water ratio of the emulsion is not believed to be critical during application of the emulsion to the animal food, it generally ranges from 1:5 to 1:1, and typically about 1:4. Where it is desired that the animal food have an outer coating of an unconditioned fat along with the palatability improving composition, the two materials can be applied sequentially or simultaneously. Preferably, the unconditioned fat is applied first, and the palatability improving composition is applied thereover. The animal food may be dried after incorporation of the palatability improving composition to reduce the moisture content to the desired level. 
     The palatability improving composition prepared in this manner is generally applied in any effective amount. It has been found in practice that amounts as low as about 0.1 percent by weight based on the total weight of the animal food has provided significant improvement in palatability for dogs. Generally, amounts of greater than about 5% by weight based on the total weight of the animal food are not employed unless the animal food so treated is later diluted with another material such as untreated animal food, meat scraps, water, or the like. 
     In the case where the fat-protein mixture comprises a proteinaceous ingredient of the animal food, such as a meat slurry, the reaction, according to the present invention, is preferably effected by heating the slurry to reaction temperature and admixing the enzymes therewith. Here, as in the case discussed above, the enzymes can be added simultaneously or sequentially with the reaction taking place in one or two steps. Constant, efficient stirring is maintained for the duration of the reaction. The natural emulsifying effect of the conventional meat slurry process may be sufficient to emulsify the fat in the slurry. However, it is preferred to add an emulsifier to obtain a more stable emulsion. The pH, temperature and reaction time for this embodiment are controlled in the same manner as in the embodiment wherein the protein comprises a proteinaceous emulsifier. 
     This enzyme-treated meat slurry, which is the palatability improving composition in this embodiment, is admixed with the other ingredients of an animal food, such as fat, carbohydrate, protein, vitamins and minerals in any effective amount to provide an animal food of improved palatability. The enzyme-treated meat slurry can generally comprise from about 1% to about 35%, and preferably from about 5% to 30%, of the animal food. As a guideline to an upper limit, it is noted that high concentrations seem to adversely affect the texture of the animal food, and should be avoided where the impairment of the texture outweighs the improvement in palatability. 
     While the palatability improving compositions prepared according to the present invention can improve the palatability of animal foods generally, they are especially suitable for use with nutritionally balanced foods, particularly dog foods, comprising protein, fat, carbohydrate, vitamins and minerals. Particularly significant and dramatic increases in palatability have been noted with dry animal foods of the type described in U.S. Pat. No. 3,365,297 to H.M. Burgess et al, and intermediate moisture animal foods such as those described in U.S. Pat. Nos. 3,482,985 and 3,615,652 to H.M. Burgess et al, and U.S. Pat. No. 3,623,884 to G.J. Haas. High moisture or canned-type animal foods can also be successfully treated according to the present invention. The disclosures relating to animal food formulations in the above mentioned patents are incorporated by reference. 
     The following examples are presented for the purpose of further illustrating and explaining the present invention and are not to be taken as limiting in any sense. Unless otherwise indicated, all parts and percentages are by weight. 
     EXAMPLE I 
     This example illustrates a preferred procedure for conditioning a fat-protein mixture according to the present invention. Promine D soy isolate (120 g), a proteinaceous emulsifier, was dispersed in 3.2 liters of an aqueous solution which was 0.0066 M with respect to CaCl 2  and 0.5 with regard to NaCl. This mixture was brought to a boil with a steam coil and held at this temperature for about 10 minutes. The mixture was then cooled to 50°C and the pH was adjusted to 8.0 - 8.5 and 800 grams of melted bleachable fancy tallow was added. The mixture was then homogenized by mixing in a Waring blender at high speed for 3 minutes. The pH was adjusted to the indicated range by the addition of sodium hydroxide when necessary. To the homogenized mixture was added 8 grams of pancreatic lipase 250, available from Miles Laboratories. The reaction mixture was incubated for about 2 hours while adjusting the pH to 8.0 - 8.5 every 30 minutes. At the conclusion of the conditioning reaction, the material was frozen. 
     EXAMPLE II 
     This example illustrates a preferred method according to the present invention for preparing an animal food of enhanced palatability. The frozen emulsion prepared in Example I was thawed in warm water for application to an animal food. In those instances where a quantity of the emulsion broke upon thawing, it was reemulsified by homogenizing in a Waring blender at high speed for about 3 minutes. This emulsion was applied to a commercially-available, granular, dry animal food having a coating amounting to 3% by weight of non-conditioned bleachable fancy tallow, a moisture content of 9% and having the following dry basis formulation: 40.4% corn; 23.1% soya meal; 20.8% wheat middlings; 11.6% of 50% meat meal, and 4.1% vitamin, mineral, dye and flavoring mix. The emulsion contained about 20% of the enzyme-conditioned fat-protein mixture and was sprayed onto the animal food by means of a spray gun into a cement mixer containing the dry food to give an enzyme-conditioned fat-protein mixture content of 1%. The animal food was then dried by heating for 5 to 10 minutes at 90°-110°C to reduce the final moisture content of the animal food to about 8-12%. 
     EXAMPLE III 
     This example illustrates the dramatic increase in palatability of animal foods which can be obtained according to the present invention. In each of the 21 runs listed in Tables I, II, and III, a fat was emulsified and conditioned with an enzyme mixture comprising protease and lipase. The fats were conditioned according to the procedure of Example I while varying the particular fats and reaction conditions as indicated in the tables. Non-proteinaceous emulsifiers were employed in runs 6, 7 and 21. The enzyme-conditoned fat was heated to 95°C where indicated to inactivate the enzymes. The enzyme-conditioned fats were applied by spraying in the indicated amounts onto an animal food according to the procedure of Example II to give a total fat coating on the animal food of 4%. The remainder of this fat was unemulsified, unconditioned bleachable fancy tallow (BFT). Animal foods prepared in this manner are identified by the letter L under the headings Treatment, Type in the tables below. 
     For comparison, three other types of animal foods were prepared: 
     1. type E, contained the same fat employed in the animal food identified by L in a given run. Here the fat was emulsified, but not conditioned with the enzyme mixture according to the present invention; 
     2. type U, contained the same fat employed in type L in a given run; however, the fat was neither emulsified nor conditioned with the enzyme mixture; and 
     3. as a base line control, in all of the runs except 4-8, 10 and 20 where the type U animal food was the control, an animal food containing the total 4% fat as a coating of unemulsified, unconditioned bleachable fancy tallow was employed. 
     According to this example, 40 dogs of mixed breed were fed the various foods as indicated in the tables and their relative preferences were recorded and objectively compared. The results of the test are expressed in terms of an average preference rating (APR). The APR values relate to comparisons with a control and are defined as follows: 
     
         APR Rating         Response                                               
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2.0 or greater   Strong preference                                        
0.80 to 1.99     Moderate preference                                      
0.0 to 0.79      No preference                                            
-0.00 to -0.79   No rejection                                             
-0.8 to -1.99    Moderate rejection                                       
-2.0 or less     Strong rejection                                         
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     The results of these tests are summarized in Tables I, II and III. It will be noted that Runs 2 and 3, which compare a type U fat to the baseline control indicate a preference between two different lots of bleachable fancy tallow. This is explained by the fact that no two samples are exactly alike, but vary according to their preparation and storage histories. 
     
                       TABLE I                                                     
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Fat         Treatment                                                     
                                   Emul- Post                             
Run  Type   wt%    Type pH  Enzymes                                       
                                   sifier                                 
                                         Heat APR                         
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     BFT    1.0    L    8.0  P.sup.a                                      
                                    S.sup.c                               
                                         no    1.18                       
     BFT    1.0    L    8.0  F.sup.b                                      
                                   S     no   0.65                        
     BFT    1.0    L    8.0 P      S     no   1.89                        
2    BFT    1.0    E    8.0 --     S     no   0.47                        
     BFT    1.0    U    8.0 --     --    no   -0.62                       
     BFT    1.0    L    7.94                                              
                            P      S     no   3.20                        
3    BFT    1.0    L    4.0 P      S     no   2.10                        
     BFT    1.0    U    8.0 --     --    no   1.43                        
     BFT    2.0    L    8.0 P      S     no   1.236                       
4    BFT    2.0    E    8.0 --     S     no   1.88                        
     BFT    2.0    U    8.0 --     --    no   0                           
     BFT    1.0    L    8.0 P      S     no   2.638                       
5    BFT    1.0    E    8.0 --     S     no   2.027                       
     BFT    1.0    U    8.0 --     --    no   0                           
     BFT    1.0    L    8.0 P       SSL.sup.d                             
                                         no   0.486                       
6    BFT    1.0    E    8.0 --     SSL   no   0.347                       
     BFT    1.0    U    8.0 --     --    no   0                           
     BFT    1.0    L    8.0 P       SMG.sup.e                             
                                         no   1.614                       
7    BFT    1.0    E    8.0 --     SMG   no   -0.208                      
     BFT    1.0    U    8.0 --     --    no   0                           
     BFT    1.0    L    8.0 P      S     yes  1.263                       
8    BFT    1.0    L    8.0 P      S     no   0.944                       
     BFT    1.0    U    8.0 --     --    no   0                           
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 .sup.a pancreatic lipase                                                 
 .sup.b fungal lipase                                                     
 .sup.c soy isolate proteinaceous emulsifier (Promine D)                  
 .sup.d sodium stearoyl-2-lactylate (Emplex)                              
 succinoylated monoglycerides (SMG)                                       
 
    
     
                       TABLE II                                                    
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Fat           Treatment                                                   
Run  Type      wt %   Type pH      Time (hrs)                             
                                           APR                            
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     Butter oil                                                           
               1.0    L    8.0-8.2 2       2.70                           
 9                                                                        
     Butter oil                                                           
               1.0    U    8.0-8.2 2       2.29                           
     Butter oil                                                           
               1.0    L    8.0-8.2 2       2.38                           
10   Butter oil                                                           
               1.0    E    8.0-8.2 2       1.35                           
     Butter oil                                                           
               1.0    U    8.0-8.2 2       0                              
     Butter oil                                                           
               1.0    L    8.0-8.2 2       0.27                           
11   Butter oil                                                           
               1.0    E    8.0-8.2 2       -0.08                          
     Butter oil                                                           
               1.0    U    8.0-8.2 2       -0.27                          
     Butter oil                                                           
               1.0    L    8.0-8.2 2       -1.1                           
12   Butter oil                                                           
               1.0    E    8.0-8.2 2       1.1                            
     Butter oil                                                           
               1.0    U    8.0-8.2 2       0.8                            
     Butter oil                                                           
               1.0    L    8.0-8.2 2       2.35                           
13                                                                        
     Butter oil                                                           
               1.0    E    8.0-8.2 2       2.30                           
     Butter oil                                                           
               0.5    L    8.0-8.2 2       2.20                           
14                                                                        
     Butter oil                                                           
               0.5    E    8.0-8.2 2       2.79                           
     Butter oil                                                           
               0.25   L    8.0-8.2 2       0.93                           
15                                                                        
     Butter oil                                                           
               0.25   E    8.0-8.2 2       0.27                           
     Butter oil                                                           
               0.10   L    8.0-8.2 2       1.21                           
16                                                                        
     Butter oil                                                           
               0.10   E    8.0-8.2 2       1.18                           
     Butter oil                                                           
               0.5    L    8.0-8.2 16      1.53                           
17                                                                        
     Butter oil                                                           
               0.5    E    8.0-8.2 16      1.75                           
     Butter oil                                                           
               1.0    L    8.0-8.2 2       2.15                           
18                                                                        
     Butter oil                                                           
               1.0    E    8.0-8.2 2       1.47                           
     Butter oil                                                           
               1.0    L    7.3     2       3.66                           
19   Butter oil                                                           
               1.0    L    4.0     2       2.72                           
     Butter oil                                                           
               1.0    U    8.0     2       3.10                           
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                       TABLE III                                                   
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Fat            Treatment                                                  
Run  Type       wt %   Type pH   Emul- Time  APR                          
                                 sifier                                   
                                       (hrs.)                             
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20   Chicken fat                                                          
                1.0    L    8.0  S     2     2.18                         
     Chicken fat                                                          
                1.0    L    8.0  S     4     1.77                         
     Chicken fat                                                          
                1.0    E    8.0  S     0     1.62                         
     Chicken fat                                                          
                1.0    U    8.0  --    --    0                            
21   Chicken fat                                                          
                1.0    L    8.0  SMG   2     1.208                        
     Chicken fat                                                          
                1.0    E    8.0  SMG   2     1.208                        
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     EXAMPLE IV 
     This example illustrates another preferred method according to the present invention for conditioning a fat-protein mixture according to the present invention. An admixture containing about 53.6% tripe, 15.8% trimmings, 3.9% defatted beef tissue, 1.7% lungs, 1.1% emulsifier, 12.9% propylene glycol, 6.4% corn syrup, and 4.6% water was pulverized in a homogenizer to obtain a meat slurry. For hundred pounds of this meat slurry is brought to 50°C with efficient stirring. The pH is adjusted to 8.0 - 8.5by the addition of 2N NaOH. Then, 0.36 pounds of Miles Laboratories pancreatic lipase 250,equivalent to 1% of the fat in the meat slurry, is added to the slurry. Constant efficient stirring is maintained for the 37 minute reaction period, during which the pH is monitored continuously. Each time the pH drops to 7.8, sufficient 2N NaOH is added to raise the pH to about 8.2. The pH is adjusted to 8.2 at the conclusion of the reaction, and the mixture is heated to 90°- 95°C to inactivate all enzymes. 
     EXAMPLE V 
     This example illustrates another preferred method according to the present invention for preparing an animal food of enhanced palatability. Three separate portions enzyme-treated meat slurry prepared in Example IV are blended with additional amounts of the untreated meat slurry to obtain combined slurries containing 11.2%, 33.6% and 67.2% of the enzyme-treated slurry. Then, 44.6 parts of each of these combined slurries is admixed with 1.9 parts of soya hulls, 5.9 parts of a vitamin, mineral, flavoring and dye mix, 15.7 parts sucrose, and 31.9 parts of soya flakes. These admixtures are then extruded through a cooled extruder to form three animal foods: the first containing 5% of the enzyme-treated meat slurry; the second, 15%; and the third, 30%. 
     EXAMPLE VI 
     This example illustrates the improvement in palatability which can be obtained according to the present invention. The three animal foods prepared according to the procedure of Example V are tested and compared to an animal food identical in all respects, except containing no enzyme-treated fat-protein mixture as a control. Forty dogs of mixed breed were fed the four animal foods and their preferences recorded in terms of the APR as explained in Example III. The results are shown below in Table IV. 
     
                       TABLE IV                                                    
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Run         % enzyme-treated                                              
                          APR                                             
            meat slurry                                                   
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1           0             0                                               
2           5             2.27                                            
3           15            2.35                                            
4           30            3.79                                            
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     Many modifications and variations of the present invention will become apparent to those skilled in the art upon reading the above disclosure. It is intended that all such modifications and variations be included within the scope of the present invention which is defined by the following claims.