the final extruded snack products of the invention are of expanded cellular construction and are characterized by a plurality of cell - defining walls . as indicated , the walls are made up of material including at least about 5 % by weight meat , and more preferably from about 7 . 5 - 15 % by weight meat . particularly preferred snack products have about 10 % by weight meat . the meat products can be selected from a variety of sources , but for reasons of costs and processability , the meat is selected from the group consisting of beef , pork and poultry . particularly preferred are the mechanically deboned poultry products . such extruded products are further characterized as having a moisture content of from about 1 - 10 % by weight , more preferably from about 2 - 6 % by weight , a specific gravity of from about 0 . 08 to 0 . 15 , and a bulk density of from about 5 - 9 pounds per cubic foot . moreover , they exhibit an instron compressibility rating of from about 1 . 0 - 5 . 0 kilograms force , as measured in a warner - bratzler shear cell , and have a water activity of less than 0 . 65 and preferably from about 0 . 2 - 0 . 4 . finally , the products of the invention have a protein content of from about 5 - 25 % by weight , more preferably from about 10 - 15 % by weight , and an as - extruded fat content ( i . e ., as the product exists after extrusion but before application of any topical flavorings or the like ) of from about 0 - 20 % by weight , more preferably up to about 4 % by weight , and most preferably below 3 % by weight by weight . the topical flavorings on the snack products of the invention are normally applied by spraying , and the quantity used typically ranges from about 0 . 1 to 0 . 5 % by weight of the total product . after such flavor coating , the products may be toasted using an impingement oven or similar device , in order to brown the surface of the snack pieces and enhance the crispness thereof . the snacks may assume virtually any desired shape which can be obtained via extrusion processing . for example , the products may be in the form of rods or cubes , and typically have a length of at least about 1 / 8 inch . alternately , the products may be in the form of rings or tubes . the extrusion process for fabricating the snack products of the invention involves first forming a mixture including at least about 15 % by weight meat ( such fraction including the normal moisture content of the meat being employed ), and more preferably from about 20 - 40 % by weight meat , and most preferably about 25 % by weight meat . the starting mixture also includes at least about 50 % by weight wheat flour , and more preferably from about 60 - 80 % by weight thereof . no additional moisture is required to be added to the starting mixture , if use is made of mdpm as a meat source . advantageously , the starting ingredients may be mixed in a preconditioner , without steam or water addition ; and the residence time in the preconditioner is preferably from about 60 - 180 seconds . in any event , the total moisture content of the mixture fed to the extruder should be from about 15 - 40 % by weight , and more preferably from about 20 - 30 % by weight . optional ingredients which may be added to the starting mixture include salt , surfactant , dextrose , fiber , antioxidants and sodium tripolyphosphate . these are normally present in very minor amounts of less than 5 % by weight . this mixture is then fed to the inlet of an extruder , most advantageously a twin screw extruder of the type commercialized by wenger manufacturing , inc . of sabetha , kans . as described previously , the starting ingredients for the mixture may be fed to a preconditioner apparatus to insure homogeneity of the final mixture , with the latter then being directed to the extruder inlet . during passage through the extruder , the mixture is subjected to elevated temperatures and pressures in order to cook and denature the meat protein of the mixture , and to form a final product . the maximum temperature of the mixture in the extruder barrel should be up to about 280 ° f ., more preferably from about 180 °- 250 ° f ., and most preferably from about 210 °- 240 ° f . the maximum pressure achieved in the extruder barrel should be up to about 2500 psig , and more preferably from about 500 - 1500 psig . the residence time of the mixture in the extruder barrel should be from about 10 - 60 seconds , and more preferably from about 15 - 45 seconds . as the material emerges from the endmost extruder die , it is normally cut via a rotating knife , with the knife speed determining product length . thereafter , the cut extrudate is directed to a dryer for drying the product down to essentially the final product moisture . a conventional multiple - stage dryer may be used to good effect for this purpose , with drying conditions being variable . after drying , the product may be flavored by the topical application of liquid or solid flavoring such as described previously . thereafter , if desired , the product may be subjected to a toasting process . the products of the invention exhibit a number of important properties making them eminently suited as desirable snacks . for example , they are relatively high in protein and low in fat , and are therefore nutritionally advantageous . moreover , because of the low moisture and water activity inherent in the products , there is virtually no possibility of microbial growth . as a consequence , the products of the invention , when properly packaged , should remain completely edible for many months . finally , when use is made of low cost ( at present prices about 17 cents per pound ) mdpm as a meat source , a high quality meat snack is provided which can be flavored to meet the dictates of consumer preference while still giving enhance nutritional benefits . these advantages are present in the invention largely because of the relatively high wheat flour content of the product , coupled with the relatively low fat content thereof . the following examples set forth a series of extrusion runs for the fabrication of snack products in accordance with the invention . it is to be understood that these examples are provided by way of illustration only , and nothing therein should be taken as a limitation upon the overall scope of the invention . in this series of tests , two starting mixtures were employed , namely a wheat flour based mixture having 97 % by weight wisdom wheat flour , 1 % by weight salt , 0 . 5 % by weight dimodan p . v . surfactant and 1 . 5 % by weight dextrose ; and a meat mixture including 93 . 98 % by weight mechanically deboned chicken product ( surimi ), 5 % by weight carboxymethyl cellulose ( fiber ), 0 . 5 % by weight sodium tripolyphosphate , 0 . 5 % by weight chicken flavoring , and 0 . 02 % by weight bht antioxidant . in extrusion mixture no . 1 , 75 % by weight of the wheat mixture was added to 25 % by weight of the meat mixture ; in extrusion mixture no . 2 , 73 . 16 % by weight of the wheat mixture was added to 26 . 34 % by weight of the meat mixture , and an additional 0 . 50 % by weight of chicken flavoring was added . in runs 1 and 2 , the respective extrusion mixtures were fed first to a conventional preconditioner ( mixing cylinder ) and then into the inlet of a wenger model tx - 52 twin screw extruder commercialized by wenger manufacturing , inc . of sabetha , kans . in run 3 , no preconditioner was used , and the material as fed directly to the tx - 52 . this type of extruder is described in u . s . pat . no . 4 , 875 , 847 , which is incorporated by reference herein . the tx - 52 extruder was equipped with a total of six heads , with heads 2 - 6 being jacketed for the introduction of heat exchange media in order to indirectly heat or cool the material passing through the extruder . the media included water ( w ), warm water ( w / temp ), or hot oil ( ho / temp ). the outlet end of the extruder was provided with a die spacer and a final apertured die . the internal screw of the extruder was equipped with two sets of spaced shearlock elements and a final , triple flighted conical screw adjacent the die . the specific die employed consisted of a wenger model no . 74010 - 271 backup die plate , together with a wenger model no . 55372 - 101 final die insert holder equipped with a pair of wenger model no . 74010 - 466 stainless steel die inserts . the backup die plate has two sets of circularly arranged apertures ( 8 per set , 5 / 32 &# 34 ; diameter ) respectively oriented in alignment with each of the extruder screws . the insert holder includes a pair of insert - receiving openings , while each insert includes a conical passage leading to a final die opening having a 5 / 32 &# 34 ; diameter . this die arrangement is typically used for breading products . the extrusion conditions recorded for this series of runs are set forth below : ______________________________________extruder : tx - 52 run # 1 run # 2 run # 3______________________________________extruder screw speed - rpm 406 403 403temp / control 2nd head w w wtemp / control 3rd head w w w / 63 ° f . temp / control 4th head ho ho ho / 87 ° f . temp / control 5th head ho ho ho / 128 ° f . temp / control 6th head w / w / w / 154 ° f . 149 ° f . 152 ° f . water to extruder 25 25 25 ( brooks ) extruder % load 25 % 25 % 30 % feeder rpm 11 11 13mixing cyl rpm 305 305pressure head # 5 psig 100 100 100extruder stability good good goodproduct stability good good goodformula no . 1 1 2______________________________________ in another series of runs , the extrusion mixture no . 1 described above was used , except that after drying , the extruded products were spray - coated with external flavorings , namely pork ( runs 1 and 2 ), chicken ( run 3 ) and beef ( run 4 ). the starting extrusion mixtures were fed to a preconditioner and then into the inlet of a model tx - 52 twin screw extruder , and in this instance having a total of nine heads , of which nos . 2 - 9 were jacketed for introduction of heat exchange media . the internal screw included a conical terminal screw section and was configured for a bread stick - type product . the final die was identical to that described in connection with example i . the data derived from this series of runs is set forth below . __________________________________________________________________________ run # 1 run # 2 run # 3 run # 4__________________________________________________________________________raw material rate pph 120 . 00 120 . 00 120 . 00 120 . 00feed screw speed rpm 11 11 11 11mixing cylinder rpm 300 300 300 298speedmoisture entering mcwb 20 . 70 23 . 44 23 . 21extruderextruder shaft speed rpm 400 397 396 398motor load % 40 30 25 31water flow to extruder ppm 0 . 090 0 . 500 0 . 400 0 . 350control / temperature ° f . w / 84 w / 86 w / 84 w / 862nd headcontrol / temperature ° f . w / 84 w / 86 w / 84 w / 863rd headcontrol / temperature ° f . ho / 106 ho / 118 ho / 127 ho / 1364th headcontrol / temperature ° f . ho / 111 ho / 126 ho / 133 ho / 1425th headcontrol / temperature ° f . ho / 108 ho / 86 ho / 176 ho / 866th headcontrol / temperature ° f . w / 70 w / 75 w / 70 w / 727th headcontrol / temperature ° f . w / 70 w / 75 w / 70 w / 728th headcontrol / temperature ° f . w / 158 w / 147 w / 140 w / 1449th headhead / pressure psig 8 / 550 8 / 300 8 / 250 8 / 300head / pressure psig 9 / 900 9 / 600 9 / 500 9 / 600knife drive speed rpm 6 7 7 7extrudate moisture mcwb 16 . 48 26 . 74 32 . 01 23 . 50final product pork pork chicken beefdescription stick stick stick stick__________________________________________________________________________ the extrudate from these runs was flavor - coated and then dried in a multiple pass dryer using an air temperature of 280 ° f ., with a retention time , first pass , of 7 . 5 minutes ; a retention time , second pass , of 7 . 5 minutes ; and a cooler retention time of 3 . 8 minutes . the moisture analyses of run 1 indicated that the product exiting the preconditioner had a moisture content of 20 . 7 % by weight mcwb ( moisture content , wet basis ); the extrudate prior to coating had a moisture content of 16 . 8 % by weight mcwb ; and the product after coating had a moisture content of 3 . 85 % by weight mcwb . in the case of run 2 , the extrudate had a moisture content of 26 . 74 % by weight mcwb and a moisture content after coating of 12 . 21 % by weight mcwb . in run 3 , the material from the preconditioner had a moisture content of 23 . 44 % by weight mcwb , the extrudate had a moisture content of 32 . 01 % by weight mcwb , and the product after coating had a moisture content of 11 . 33 % by weight mcwb . finally , in the case of run 4 , the moisture contents recorded after preconditioning , extrusion and coating were respectively 23 . 23 %, 23 . 50 % and 8 . 71 % by weight mcwb . the run 1 product exhibited a very large cell structure and was cut into 2 . 5 inch pieces as it emerged from the die . the run 2 product exhibited an excessively hard bite , indicating the need for changing the moisture content of the material in the barrel . runs 3 and 4 exhibited only minimal expansion . in a final series of tests , a formula identical with extrusion mixture no . 1 described above and using chicken flavoring was used , with various types of wheat being employed . the purpose of these tests was to determine which wheat variety gave the most optimum snack product . it was determined that soft white winter wheat gave the best product , although all wheats tested gave an acceptable snack . the most preferred run from this series of tests was conducted using a nine head wenger tx - 52 twin screw extruder and a wenger model mdl 1 dc preconditioner configured for relatively long retention time . the drawing figure is a schematic representation of the preferred dual screw configuration used in the tx - 52 . as shown , each screw 10 includes a series of axially interconnected screw sections with a number of spaced shearlock element sets 12 . a cut flight screw section 14 is also included , together with a high shear cone nose screw section 16 . the die assembly was a wenger back - up assembly , including a model 55361 - 001 back - up die plate and 55372 - 113 final die insert holder . a loop die ( 3 / 4 &# 34 ; diameter insert presenting an annular opening having an outer diameter of 5 . 75 mm , a pin diameter of 3 . 15 mm and an open area of 18 . 17 sq . mm ) formed a part of the die assembly giving an annular product . the extrusion and drying conditions recorded for this preferred run are set forth below : ______________________________________moisture 25 . 0raw material rate pph 128 . 00feed screw speed rpm 15 . 0mixing cylinder speed rpm 221steam flow to mixing cylinder ppm 0 . 0water flow to mixing cylinder ppm 0 . 0discharge temp . ° f . ambientdischarge moisture % 25 . 0shaft speed rpm 407motor load % 33zone temperature 2nd head ° f . w / 104zone temperature 3rd head ° f . w / 104zone temperature 4th head ° f . ho / 221zone temperature 5th head ° f . ho / 230zone temperature 6th head ° f . ho / 230zone temperature 7th head ° f . w / 122zone temperature 8th head ° f . w / 122zone temperature 9th head ° f . w / 189pressure - 9th head psig 700pressure - die psig 900knife drive speed rpm 6extrudate moisture % 17 . 9product rate lbs / hr 115 . 0dryer temperature ° c . 154retention time - top flight minutes 3 . 7retention time - bottom flight minutes 6retention time - cooling flight minutes 5 . 1______________________________________ the dried extrudate was then coated with various flavorings , including taco and cheese flavorings . this was accomplished by simply spraying a conventional coating mixture on the outer surface of the dried extrudate at levels of from about 0 . 1 to 0 . 5 % by weight . portions of these sprayed products were then subjected to toasting in an ordinary kitchen toaster oven ( temperature of about 250 ° f .) in order to crisp the surface of the products . the dried , flavored , and toasted extrudate products were then tested and determined to have a moisture content of 4 . 4 % by weight , and ash content of about 2 . 4 % by weight , a protein content of about 17 . 7 % by weight , and a fat content of about 0 . 5 % by weight . in addition , chicken flavored products made as outlined in this example were tested by a sensory panel for crispness , mouth feel and acceptability ; and were subjected to a tba ( thiobarburic acid ) rancidity test . the dried product had a crispness rating of 7 . 0 , a mouth feel rating of 7 . 5 , an acceptability rating of 7 . 5 , and tba results of 0 . 236 . the toasted product had a crispness rating of 8 . 0 , a mouth feel of 7 . 8 , an acceptability of 7 . 9 and tba results of 0 . 298 . the standard error of the means in all cases was 0 . 1 . in this example , 15 extruded samples of the type described in the foregoing operative example iii were analyzed for physical properties , namely , sectional , longitudinal and volumetric expansion , bulk density , specific gravity and instron compressibility . an additional 7 representative samples were tested for compressibility . section , longitudinal and volumetric expansion indices developed by alvarez - martinez , et al . ( j . food sci . 53 : 609 ) were used to characterize expansion of the products obtained in this study . sectional expansion index ( sei ) is a measure of radial expansion of the extrudate and is the ratio of the cross - sectional area of the extrudate to the cross - sectional area of the die . longitudinal expansion index ( lei ) is a measure of expansion in the axial direction and is described by the following equation : ## equ1 ## where a d is the open area of the die in square meters , l se is the specific length of the extrudate in m / kg , ρ d is the density of the dough behind the die ( assumed to be constant and equal to 1 , 200 kg / m 3 , and mc d and mc e are the moisture contents of the dough and extrudate , respectively . volumetric expansion index ( vei ) takes into account sei and lei to describe the expansion on a volumetric basis ( vei = sei × lei ). fifteen pieces from each treatment were measured for the expansion indices and the results are shown in the table below . bulk density was determined by filling a one liter volume container to level full capacity with product and weighing it in grams and converting it to lbs / ft 3 ( factor : 0 . 0624 ). bulk specific gravity is defined as the weight of a given volume of product relative to the weight of an equal volume of water at 4 ° c . thus , the gram / liter bulk density of the product was divided by 1 , 000 grams / liter bulk density of water to yield specific gravity . the results are shown in the table below . instron compressibility was determined with a warner - bratzler shear cell according to the method of faubion and hoseney ( cereal chemistry 59 : 529 , 1982 ). the warner - bratzler shear cell is attached to the instron universal testing machine and used to shear across the extruded tube perpendicular to its long axis ( or perpendicular to the axial direction of extrusion ). the cross - head speed was set at 5 cm / min . and the results expressed as the maximum force in kg required to shear through the piece . ______________________________________ bulk instron com - sample density specific pressibility # sei lei vei lb / cu . ft gravity kg force______________________________________ 1 11 . 30 0 . 49 5 . 54 7 . 67 0 . 12 3 . 85 2 4 . 22 0 . 45 1 . 92 7 . 33 0 . 12 3 . 60 3 7 . 96 0 . 54 4 . 27 7 . 21 0 . 12 1 . 25 4 9 . 22 0 . 56 5 . 19 7 . 32 0 . 12 4 . 00 5 10 . 31 0 . 56 5 . 77 7 . 30 0 . 12 2 . 80 6 10 . 50 0 . 47 4 . 98 7 . 02 0 . 11 4 . 05 7 10 . 05 0 . 43 4 . 35 6 . 99 0 . 11 3 . 95 8 8 . 64 0 . 53 4 . 60 6 . 72 0 . 11 4 . 15 9 13 . 25 0 . 56 7 . 38 6 . 77 0 . 11 2 . 2010 10 . 74 0 . 55 5 . 90 6 . 53 0 . 10 4 . 3511 8 . 48 0 . 53 4 . 54 6 . 76 0 . 11 1 . 8512 10 . 93 0 . 55 5 . 99 6 . 74 0 . 11 1 . 5013 8 . 45 0 . 48 4 . 07 6 . 66 0 . 11 2 . 0514 9 . 91 0 . 55 5 . 43 6 . 62 0 . 11 3 . 2515 10 . 75 0 . 46 4 . 98 6 . 55 0 . 10 1 . 9016 1 . 3017 2 . 0518 1 . 7519 1 . 7520 1 . 6021 0 . 9522 4 . 45______________________________________ accordingly , it was determined that products in accordance with the invention should have a sectional expansion index of from about 3 - 15 , and more preferably from about 7 - 13 ; a longitudinal expansion index of from about 0 . 3 - 0 . 7 , and more preferably from about 0 . 4 - 0 . 6 ; and a volumetric expansion index of from about 1 . 5 - 9 , and more preferably from about 4 - 7 . 5 .
a high protein , shelf - stable , meat - containing snack product is provided which is in the form of a cellular , expanded extrudate which can be dried and flavored to yield nutritional snacks having excellent organoleptic properties . the final snack contains at least about 5 % by weight meat , and at least about 50 % by weight wheat . the invention also comprehends a fabrication method wherein an initial mixture containing at least about 15 % by weight meat and at least about 50 % by weight wheat flour are mixed and passed through an extruder under time and temperature conditions to yield an expanded , cellular extrudate having the general appearance of a bread stick .