in the present specification , the following words are given a definition that must be taken into account when reading and interpreting the description , examples and claims . the term “ stable ” used to describe the emulsion means that the emulsion is stable with respect to coalescence and / or inversion , both visually determined . the term “ solid matrix ” means a mixture of solid ingredients without added liquids the term “ easily disperses ” means disperses with a minimum of agitation or stirring . the properties and constitution of the emulsion are crucial in order to achieve the desired result with respect to dispersibility . accordingly , the emulsion has a water content of at the most about 30 % w / w . it is stable towards coalescence and inversion when extruded with a solid matrix of protein and carbohydrate in a weight ratio of from 1 : 0 . 5 to about 1 : 5 at a temperature at or above the melting point of the mixture of the emulsion and the solid matrix . the thus obtained extruded product is expanded into a vacuum chamber to obtain a solid product having at the most about 10 % water content . the emulsifier may be water soluble or fat soluble . suitable water soluble emulsifiers include caseinates such as sodium caseinate and potassium caseinate , acid caseinate , other milk fractions that are commonly used as emulsifiers and sugar esters . suitable fat soluble emulsifiers include lecithins and mono - and diglycerides such as citrem ®. preferably both water soluble and fat soluble emulsifiers are used . in this case , preferably both the water soluble emulsifier and the fat soluble emulsifier are added to the fat prior to emulsification . the weight ratio between fat and emulsifier may be in the range of from about 10 : 1 to about 18 : 1 depending on the amount of water employed . for example , a preferred ratio between fat and emulsifier is about 14 : 1 at 26 % water ( ideally 69 % fat , 5 % emulsifier and 26 % water ). if the ratio is too low , there is a risk that the emulsion inverts during homogenisation . if , however , the ratio is too high , it is difficult to stabilise small fat particles during the extrusion step . the fat component may constitute from 50 to 75 % w / w of the emulsion . any fat or mixture of fats generally suitable for nutritional compositions may be used . for example , if the composition is an infant formula , palm olein , coconut oil , canola oil , milk fat and mixtures thereof may be used . long chain polyunsaturated fatty acids may also be incorporated such as docosahexaenoic acid ( from fish oil ) and arachidonic acid ( from fungal oils such as that obtained from mortierella alpina ). oil soluble vitamins such as vitamin e may also be included in the emulsion if desired . the content of fat in the nutritional composition will vary according to the type of composition and is generally expressed in terms of the contribution made by fat to the overall energy of the composition . for example , in an infant formula , fat generally provides from 30 to 55 % of the total energy of the composition depending upon the type of product and the age group for which it is intended . the emulsion may be homogenized by any method known in the art to reach a mean particle size d [ 0 . 5 ] in the range from 0 . 2 to 10 μm . a multi - step homogenisation is preferred . in practice , the preferred mean particle size after homogenisation will depend upon the emulsifier being used and the composition of fats which form the emulsion . for example , for a mixture of vegetable fats emulsified with sodium caseinate , d [ 0 . 5 ] preferably lies in the range from 0 . 2 to 1 . 5 μm . the carbohydrate content of the nutritional composition will also be selected with reference to the type of composition being produced . again taking infant formula as an example , suitable carbohydrates include lactose , maltodextrin and starch and mixtures thereof . the content of carbohydrate in the nutritional composition will vary according to the type of composition and is generally expressed in terms of the contribution made by carbohydrate to the overall energy of the composition . for example , an infant formula generally contains from 7 to 15 g carbohydrate per 100 kcal of formula depending upon the type of product and the age group for which it is intended . prebiotic fibres may also be included if desired . the term prebiotic is used to designate a non - digestible food ingredient that beneficially affects the host by selectively stimulating the growth and / or activity of one or a limited number of bacteria in the colon , and thus improves host health . such ingredients are non - digestible in the sense that they are not broken down and absorbed in the stomach or small intestine and thus pass intact to the colon where they are selectively fermented by the beneficial bacteria . examples of prebiotics suitable for addition to infant formula include certain oligosaccharides , such as fructooligosaccharides , galactooligosaccharides , sialyl - lactose and oligosaccharides derived from animal milk . the protein component of the nutritional composition will likewise be selected with reference to the type of composition being produced . once more taking infant formula as an example , suitable proteins include milk proteins such as casein and whey , soy proteins , rice proteins and mixtures thereof . a mixture of 70 % whey to 30 % casein is particularly preferred . the proteins may be intact or partially hydrolysed . the content of protein in the nutritional composition will vary according to the type of composition and is generally expressed in terms of the contribution made by protein to the overall energy of the composition . for example , an infant formula generally contains from 1 . 8 to 4 . 5 g protein per 100 kcal of formula depending upon the type of product and the age group for which it is intended . the weight ratio between the emulsion and the solid matrix is from 1 : 0 . 5 to 1 : 5 . within this range , the selected weight ratio will depend on the type of nutritional composition desired . the water content of the mixture of emulsion and solid matrix entering the extruder may be 15 % or less . other ingredients typically present in nutritional compositions such as nucleotides , vitamins , minerals and trace elements may be added to the solid matrix prior to extrusion ( or to the fat prior to emulsification if fat soluble ). alternatively , some or all of these ingredients may be added after extrusion by dry mixing . any suitable extruder may be used such as , e . g . single screw , twin screw , multi screw , ring screw , planetary gear extruder , etc . or any other continuous or discontinuous mixing system with the possibility to shape the melt ( melted mixture ). furthermore , the term “ vacuum ” used in the present context is intended to denote a pressure between 0 . 01 and 250 mbar , preferably between 1 and 50 mbar . a particularly preferred range is between 10 and 25 mbar . extrusion is carried out at a temperature at or above the melting point of the mixture of the emulsion and the solid matrix . typically , this temperature will lie in the range between 80 and 90 ° c . and the extrusion will be carried out at a temperature in the range between 80 and 120 ° c . extruding at a temperature below the melting point tends to lead to instability of the emulsion during extrusion , which may in turn lead to a product with reduced solubility , flocculation or other unwanted effects such as complete phase separation when the product is reconstituted . conversely , extruding at an elevated temperature can adversely affect the nutritional quality of the product as measured for example by increased lysine blockage and browning of the product . the water content of the solid product leaving the vacuum chamber is 10 % or less . if desired , this may be subjected to a further drying step . for example , if it is desired to produce an infant formula containing probiotic bacteria , it will be necessary to further dry the product to a final water content of around 1 %. this may be effected by passing the extruded product from the vacuum chamber to a fluidised bed dryer for example . alternatively , or additionally , a source of heat such as a source of infra - red radiation may be provided in the vacuum chamber itself . as noted above , the product of the extrusion process is typically in the form of porous pellets or aggregates of relatively low density . unlike spray dried products , compositions produced according to the process of the present invention contain air trapped within the solid product . the incorporated air pockets have a non - circular shape as observed by fluorescence confocal microscopy . the structure can thus be compared to a frozen foam in which the air bubbles or pockets are non - spherical . if it is desired to produce a powder of higher density which takes up less room as well as looking more like the spray dried powders to which consumers are accustomed , the pellets / aggregates may be milled into a finely divided powder . nutritional compositions produced according to the process of the invention are found to have generally improved solubility / dispersibility compared to the same products produced by spray drying . at the same time , the nutritional quality of products made by the process of the invention ( expressed for example in terms of lysine blockage ) is found to be comparable to the same products produced by spray drying . further , implementing the process of the invention has a lower investment cost than the spray drying technology conventionally used to produce such products ( no requirement for spray drying towers or air heaters means less requirement for space ) and the process is simpler than spray drying making it overall more cost - efficient . the invention will now be further illustrated by reference to the following examples . the extruder used was a clextral type bc45h co - rotating intermeshing twin - screw extruder with a processing length of 800 mm ( l = 14 . 4 * d ), 4 barrels and a die plate with 32 holes of 1 . 5 mm diameter . the screw configuration for examples 1 and 2 was as shown in table 1 below preparation of the fat phase : the vegetable fat blend is melted and mixed with the citrem at 65 ° c . then the potassium caseinate is sieved through a sieve of 1 mm and added to the fat mixture with stirring . a yellow suspension is obtained . preparation of the water phase : the water is heated to 65 ° c ., the sodium citrate is added and the solution is stirred for 10 min . preparation and homogenisation of the emulsion : the fat phase is added to the water phase with vigorous stirring until all the fat is incorporated . an increase in viscosity is noted . the pre - emulsion is stirred for another 5 min then the temperature is raised to 95 ° c . and a multi - stage homogenisation is carried out at 85 ° c . under a pressure of 300 bar . the resulting highly viscous emulsion is pumped directly into the extruder . extrusion of infant formula formulation : 37 . 5 kg / h of an emulsion prepared as described above together with 70 kg / h of a solid matrix comprising 78 . 87 % maltodextrin de29 , 0 . 8 % k 2 hpo 4 , 12 . 22 % lacprodan di - 9224 whey protein isolate , 5 . 77 % potassium caseinate and 2 . 34 % calcium citrate , together with vitamin and trace element premixes are supplied to the extruder . barrel 1 of the extruder is maintained at 21 ° c ., barrel 2 at 60 ° c ., barrel 3 at 106 ° c ., barrel 4 at 109 ° c . and the die plate is maintained at 110 ° c . the mixture is extruded at a pressure of 0 . 4 bar and a temperature of 103 ° c . into a vacuum chamber at a pressure of 24 mbar yielding a solid product with a moisture content of 2 . 9 %. the mean size of the fat particles d [ 0 . 5 ] is 1 . 72 μm . the lysine blockage is 8 %. preparation of the fat phase : the milk fat is melted and mixed with the citrem and milk protein concentrate at 65 ° c . a yellow suspension is obtained . preparation of the water phase : the water is heated to 65 ° c ., the sodium citrate is added and the solution is stirred for 10 min . preparation and homogenisation of the emulsion : the fat phase is added to the water phase with vigorous stirring until all the fat is incorporated . an increase in viscosity is noted . the pre - emulsion is stirred for another 5 min then the temperature is raised to 95 ° c . and a multi - stage homogenisation is carried out at 85 ° c . under a pressure of 300 bar . the resulting highly viscous emulsion is pumped directly into the extruder . extrusion of growing up milk formulation : 37 . 5 kg / h of the emulsion together with 70 kg / h of a solid matrix with the composition 39 . 2 % maltodextrin de 29 , 0 . 8 % k 2 hpo 4 , 15 % milk protein concentrate 20 %, 22 . 5 % lactose and 2 . 5 % calcium citrate together with vitamin and trace element premixes are supplied to the extruder . the extrusion conditions are the same as for example 1 except that the die plate is heated to 105 ° c . the extruded product has a moisture content of 2 . 9 %. d [ 0 . 5 ] is 0 . 952 μm . lysine blockage is 11 . 8 %. preparation of the fat phase : the vegetable fat blend and potassium caseinate are stirred together . a yellow suspension is obtained . preparation of the water phase : the water is heated to 65 ° c . preparation and homogenisation of the emulsion : the fat phase is added to the water phase with vigorous stirring until all the fat is incorporated . an increase in viscosity is noted . the pre - emulsion is stirred for another 5 minutes then the temperature is raised to 95 ° c . and a multi - stage homogenisation is carried out at 85 ° c . under a pressure between 50 - 200 bar . the resulting highly viscous emulsion is pumped directly into the extruder . extrusion of infant formula formulation : 35 kg / h of an emulsion prepared as described above together with 72 kg / h of a solid matrix comprising 25 . 39 % maltodextrin de29 , 30 . 57 % skim milk powder , 16 . 02 % demineralised whey powder dwp28 , 26 . 68 % lactose , 2 . 34 % salts and minerals , together with vitamin and trace element premixes are supplied to the extruder . barrel 1 of the extruder is maintained at 30 ° c ., barrel 2 at 60 ° c ., barrel 3 at 110 ° c ., barrel 4 at 110 ° c . and the die plate is maintained at 110 ° c . the mixture is extruded at a pressure of 0 . 4 bar and a temperature of 105 ° c . into a vacuum chamber at a pressure of 24 mbar yielding a solid product with a moisture content of about 8 %. the mean size of the fat particles d [ 0 . 5 ] is around 2 μm . the lysine blockage is less than 11 %.
a process for preparing a nutritional composition including protein , fat and carbohydrate comprises emulsifying the fat in water with an emulsifier to form an emulsion having a solids content of at least 70 %, homogenising the emulsion and extruding the emulsion together with a solid matrix comprising the protein and the carbohydrate in a ratio between 1 part emulsion : 0 . 5 parts matrix to 1 part emulsion : 5 parts matrix into a vacuum chamber at a temperature at or above the melting point of the mixture of emulsion and matrix to obtain a solid product having a maximum water content of 10 %.