in a preferred embodiment of the process , the crystallization additive is one of the compounds shown in formulae 1 to 3 , or a mixture thereof : r 3 —( o — c 2 h 4 ) n — o — so 3 m ( formula 3 ) where n is an integer from 1 to 12 , m is sodium or potassium and r 1 , r 2 and r 3 are a linear , branched or cyclic , saturated or unsaturated c 8 to c 20 alkyl group or an aryl group . in a preferred embodiment of the aforementioned compounds , n = 2 and r 1 , r 2 and r 3 are linear , saturated c 8 to c 18 alkyl groups . in a further embodiment of the process , the crystallization additive is a sorbitan fatty acid ester or a mixture of different sorbitan fatty acid esters , preferably polyethoxylated sorbitan fatty acid esters . in a particularly preferred embodiment , the crystallization additive is a polyethoxylated sorbitan stearate , and in particular a polyethoxylated sorbitan tristearate according to formula 4 : the concentration of the crystallization additive in the solution from which the recrystallization takes place is preferably at least 50 ppm based on the total mass of the solution , particularly preferably at least 100 ppm , most preferably at least 400 ppm . in order to achieve an optimum dosing and distribution of the crystallization additive , it is preferably used in the form of an aqueous solution or emulsion , in which case the concentration of the crystallization additive in the solution or emulsion is preferably 2 to 15 % by weight . in a preferred embodiment of the process according to the invention , the solution from which the recrystallization takes place additionally comprises an antifoam . the antifoam has the function of suppressing the foam which is formed when handling the methionine solution and suspension and which is intensified and / or caused by some of the aforementioned crystallization additives . moreover , a synergistic effect arises for the attained bulk densities of methionine when simultaneously using antifoam and crystallization additives , as a result of which bulk densities of more than 600 g / l are achieved , the negative consequences of accumulation processes are simultaneously avoided and the process according to the invention can thus also be carried out in continuous mode . the antifoam preferably comprises silicone oil , preference being given to using a silicone oil with a kinematic viscosity of 0 . 65 to 10 000 mm 2 / s ( measured at 25 ° c . in accordance with din 53018 ), particularly preferably from 90 to 1500 mm 2 / s . the antifoam can further contain constituents which are effective as emulsifiers , for example mixtures of polyethoxylated fatty acids and polyethoxylated fatty alcohols . the antifoam can likewise comprise silica . in a preferred embodiment , the antifoam is an aqueous solution which comprises 5 to 10 % by weight of silicone oil , 0 . 05 to 1 % by weight of silica , 0 . 5 to 5 % by weight of a mixture of polyethoxylated fatty acids , and 2 to 7 % by weight of a mixture of polyethoxylated fatty alcohols . preferably , the antifoam is used in a mixture with the crystallization additive , the crystallization additive being admixed in a concentration of preferably 2 to 15 % by weight . in order to achieve a continuous , stable dosing of the antifoam , it is preferably further diluted with water prior to being used . the use of silicone oil antifoams leads to silicon being able to be detected in the methionine prepared by the process according to the invention using a suitable analysis method ( e . g . x - ray photoelectron spectroscopy , abbreviated to xps ). therefore , a further object of the present invention is d , l - methionine obtained by the process according to the present invention , wherein a silicone oil antifoam is used in said process . surprisingly , it has been found that the presence of potassium ions in the solution from which the recrystallization takes place is important for the crystallization success . preferably , the potassium ion concentration in the solution from which the recrystallization takes place is 1 to 30 g / kg , particularly preferably 2 to 14 g / kg , most preferably 5 to 10 g / kg . the potassium preferably passes into the recrystallization solution with the crude methionine . the potassium concentration can be adjusted for example by introducing washing water during the crude methionine filtration and / or by introducing freshwater to the pure filtrate used for dissolving the crude methionine and / or by introducing potassium into the pure filtrate used for dissolving the crude methionine . according to the invention , the crude methionine is dissolved in an aqueous solution before the recrystallization . this is effected preferably by heating the solution to a temperature of at least 95 ° c ., particularly preferably by heating to boiling temperature . to dissolve the crude methionine , it is possible to use , for example , freshwater , the filtrate of the pure methionine , or the condensate of the vacuum crystallization described below or mixtures thereof . according to the invention , crystallization additive and the antifoam are added to the aqueous matrix used for dissolving the crude methionine . in one possible embodiment of the process , the crystallization additive and the antifoam are also added to the solution from which the crude methionine is precipitated out . preferably , the recrystallization takes place by introducing an 85 to 110 ° c .- hot crude methionine solution into a 35 to 60 ° c .- warm methionine suspension , the temperature of the mixture that is formed as a result being kept constant between 35 and 60 ° c . in this connection , the volume ratio of the introduced crude methionine solution to the initial charge of methionine suspension is preferably in the range from 1 : 1 to 1 : 10 , particularly preferably from 1 : 3 to 1 : 6 . in a further preferred embodiment of the process , the recrystallization is carried out in two stages . for this , in the first recrystallization stage , an 85 to 110 ° c .- hot crude methionine solution is introduced into a 60 to 80 ° c .- warm methionine suspension and the temperature of the mixture that is formed as a result is kept constant between 60 and 80 ° c . it is particularly preferred here to remove some of the methionine suspension from the first recrystallization stage and to return it again to the recrystallization via a circulation circuit , the temperature of the suspension in the circulation circuit being increased by 6 to 12 ° c . the 60 to 80 ° c .- warm methionine suspension obtained in the first recrystallization stage is introduced , in a second recrystallization stage , into a 35 to 60 ° c .- warm methionine suspension , the temperature of the mixture that is formed as a result being kept constant between 35 and 60 ° c . the volume ratio of the introduced methionine suspension to the initial charge of methionine suspension is preferably in the range from 1 : 1 to 1 : 10 , particularly preferably from 1 : 3 to 1 : 6 . besides a first or a first and second recrystallization stage , the process according to the invention can also involve further recrystallization stages . in the event of a multistage procedure , all stages can be charged in parallel with crude methionine at the same temperature difference between crude methionine solution and initial charge of methionine suspension . the multistage recrystallization can also be carried out such that the recrystallization stages are successively charged with the methionine solution from the proceeding stage , the temperature difference between crude methionine and methionine solution being selected such that the methionine solution from one recrystallization stage can be used as crude methionine for the next recrystallization stage . this has the advantage of reduced formation of undesired platelet - like crystals as a result of excessively large temperature differences . the multistage recrystallization of course also involves mixed forms of parallel and consecutive charging of the recrystallization units . the preferred temperature control for the process according to the invention arises from the temperature - dependent solubility behaviour of methionine shown in fig6 . in economic terms , it is expedient to cool the methionine solutions to an end temperature of from 30 to 50 ° c . since , in so doing , both the amount of methionine remaining in solution can be minimized , and also the use of expensive cooling media for the purposes of further cooling the methionine - containing solutions is avoided . in a preferred embodiment of the process , the recrystallization is carried out by vacuum crystallization . here , the pressure in the first recrystallization stage is preferably 100 to 1000 mbar , particularly preferably 150 to 400 mbar . if a two - stage recrystallization is carried out , the pressure in the second recrystallization stage is preferably 35 to 200 mbar , particularly preferably 35 to 100 mbar . preferably , the water evaporated in the vacuum crystallization is condensed and is reused for dissolving further crude methionine . in one preferred embodiment of the process , some of the methionine suspension is removed from the first and / or one of the other recrystallization stages and is returned again via a circulation circuit . in the first crystallization stage , the hot methionine solution is preferably added to the circulated colder suspension in a volume ratio of 1 : 3 to 1 : 6 . upon this rapid cooling , a high supersaturation is produced , as a result of which , on the one hand , relatively large crystals grow isometrically , or else new , small , platelet - like crystals are formed . the small platelet - like crystals are also dissolved again in the recirculation line by increasing the temperature by 6 to 12 ° c ., the isometric relatively large crystals being retained . separating off the pure methionine from the mother liquor of the recrystallization preferably takes place by filtration , for example pressure or vacuum filtration , or by means of centrifuges , for example trailing - blade , pusher - type or screen centrifuges . the process according to the invention can either be carried out continuously or else discontinuously or semicontinuously . the attached fig1 to 4 show electron micrographs of crystalline methionine . fig1 shows crude methionine as is obtained from the crude methionine precipitation without the addition of crystallization additives . fig2 shows crude methionine from the crude methionine precipitation with the addition of an antifoam according to ep 1 256 571 a1 . fig3 shows methionine as is obtained without the addition of crystallization additives , without the presence of potassium by simple cooling . fig4 shows pure methionine as is obtained with the process according to the invention . fig5 shows , by way of example and in diagrammatic form , an arrangement for carrying out the process according to the invention in a preferred two - stage recrystallization . in container a , crude methionine is dissolved with an aqueous matrix , which can comprise the filtrate of the pure methionine , at a temperature of from 90 to 100 ° c . the temperature is adjusted via a circulation pump and an external heat exchanger . the crystallization additive according to the invention including antifoam is added continuously to the aqueous matrix . the methionine solution is heated to 100 to 110 ° c . via one or more heat exchangers b and then fed to the circulation circuit of the first vacuum crystallizer d . the circulated suspension has a temperature of 60 to 70 ° c . the ratio of amount fed in to circulation amount is in the range from 1 : 3 to 1 : 6 . the average residence time of the mixture in the circulation circuit is 5 to 15 sec . the mixture is heated to 65 to 75 ° c . via a heat exchanger c , as a result of which fine and in particular platelet - like methionine crystals rapidly dissolve because of their relatively large specific surface area . the mixture then passes to the first vacuum crystallizer d , in the top region of which , at a pressure of 180 to 200 mbar , water evaporation and cooling of the mixture occurs . this results in crystallization of dissolved methionine . the methionine crystals settle out in the vacuum crystallizer at differing rates . small , platelet - like crystals settle out more slowly than coarse , isometric crystals . the suspension for recirculation is removed in the upper region of the vacuum crystallizer , where predominantly smaller , platelet - like crystals are found on account of the slower settling rate . the coarse , isometric crystals are removed in the lower region of the vacuum crystallizer d and fed to the circulation circuit of the second vacuum crystallizer e . the suspension circulated here has a temperature of 30 to 50 ° c . the ratio of amount fed in to circulation amount is in the range from 1 : 3 to 1 : 6 . the pressure in the vacuum crystallizer e is 60 to 80 mbar . in vacuum crystallizer e , further methionine is crystallized , as a result of which the average particle size of the methionine crystals in particular is increased . if required , the methionine suspension can be passed to an interim container f in order to permit a postprecipitation of methionine . finally , the methionine is isolated in a suitable solid / liquid separation step g , where the filtrate obtained can , if required , be returned to container a . the examples below aim to explain the invention in more detail . recrystallization in the presence of a crystallization additive according to the invention compared with a known crystallization additive 60 g of methionine , 305 g of water and 35 g of crude methionine filtrate were introduced into a flask and circulated via a heat exchanger by pumping at a temperature of 40 ° c . as a result of the potassium carbonate present in the crude methionine filtrate , the potassium ion concentration was ca . 7 g / kg . a solution , heated to 90 ° c ., of 150 g of methionine in 990 . 5 ml of water and 109 . 5 g of crude methionine filtrate was added to this suspension at a rate of 18 ml / min , during which the temperature of the initial charge of suspension was kept at 40 ° c . after adding 650 ml of the hot solution , 500 ml of suspension were removed and then a further 500 ml of the hot solution were metered in at a rate of 18 ml / min . the resulting suspension was discharged , the amount of foam was determined , and the methionine was filtered off and washed with 300 ml of acetone . after drying the methionine , the bulk density was determined . the recrystallization experiments were carried out in the presence of the following additives , the stated concentration being established by adding the additive to both starting solutions / suspensions . the concentration data give the total active ingredient content of the additive without water based on the total mass of the solution or suspension . additive 1 was an aqueous mixture of antifoam and crystallization additive according to ep 1 451 139 a1 , consisting of 2 % by weight of hydroxyethylcellulose and 2 % by weight of a polyethoxylated fatty acid ( c 18 h 37 —( co )— o —( ch 2 — ch 2 o ) 7 — h ). additive 2 was an aqueous mixture of a crystallization additive and an antifoam composition according to the present invention , consisting of 6 . 1 % by weight of silicone oil with a kinematic viscosity of 1000 mm 2 / s ( ak 1000 , wacker - chemie gmbh ), 0 . 25 % by weight of hydrophobicized silica ( sipernat d10 , evonik degussa gmbh ), 2 . 6 % by weight of a polyethoxylated fatty acid mixture ( intrasol ® fs 18 / 90 / 7 , ashland deutschland gmbh ), 3 . 7 % by weight of a polyethoxylated fatty alcohol mixture ( 2 . 35 % by weight of marlipal ®, sasol germany gmbh , 1 . 35 % by weight of brij c2 , croda chemicals europe ) and 5 . 1 % by weight of a fatty alcohol sulphate ( sulfopon ® 1218 g , oleochemicals ) according to the formula : the table below shows the ascertained amounts of foam and methionine bulk densities as a function of type and concentration of the mixtures used as crystallization additives , the total active ingredient content ( without water ) being given . it is observed that the crystallization additive according to the invention at a low concentration improves the bulk density as effectively as the additive according to ep 1 451 139 a1 and that the additive according to the invention , in contrast to the additive according to ep 1 451 139 a1 , retains its effectiveness even at a high concentration . recrystallization in the presence of pure antifoam , pure crystallization additives , and mixtures of antifoam and crystallization additive recrystallization experiments according to the procedure from example 1 were carried out with the addition of pure crystallization additives according to the invention , of mixtures of the crystallization additives with the antifoam and of the pure antifoam . the table below shows the amounts of foam and methionine bulk densities ascertained here . the pure antifoam ( comparative example 1 ) was used in the form of an aqueous mixture consisting of 6 . 1 % by weight of silicone oil with a kinematic viscosity of 1000 mm 2 / s ( ak 1000 , wacker - chemie gmbh ), 0 . 25 % by weight of hydrophobicized silica ( sipernat d10 , evonik degussa gmbh ), 2 . 6 % by weight of a polyethoxylated fatty acid mixture ( intrasol ® fs 18 / 90 / 7 , ashland deutschland gmbh ), 3 . 7 % by weight of a polyethoxylated fatty alcohol mixture ( 2 . 35 % by weight of marlipal ®, sasol germany gmbh , 1 . 35 % by weight of brij c2 , croda chemicals europe ). 2 ) c n h 2n + 1 — o — so 3 na , where n = 12 to 18 ( sulfopon ® 1218g , oleochemicals ) 3 ) c n h 2n + 1 — o — c 2 h 4 — so 3 na , where n = 8 to 18 ( hostapon ® sci 85 , clariant ) 4 ) c n h 2n + 1 —( oc 2 h 4 ) 2 — o — so 3 na , where n = 12 ( disponil ® fes 27 , cognis ) 5 ) c n h 2n + 1 —( oc 2 h 4 ) 12 — o — so 3 na , where n = 12 ( disponil ® fes 993 , cognis ) comparative example 6 ) c n h 2n + 1 —( oc 2 h 4 ) 30 — o — so 3 na , where n = 12 ( disponil ® fes 77 , cognis ) for the mixtures of the antifoam with the crystallization additives , in each case 5 . 1 % by weight of the corresponding crystallization additive was added to the aforementioned mixture and the water fraction was reduced by 5 . 1 % by weight of : the results show that the pure antifoam does not result in an improvement in bulk density ( entry 1 ). the crystallization additives 2 to 5 according to the invention improve the bulk density to values & gt ; 500 g / l , but in the majority of cases bring about increased foaming . the combinations 7 to 9 according to the invention of antifoam and crystallization additives lead to bulk densities & gt ; 600 g / l , the combination 10 according to the invention leads to bulk densities & gt ; 500 g / l , without increased foaming arising . recrystallization in the presence of antifoam and crystallization additives or antifoam and mixtures of crystallization additives further recrystallization experiments according to the procedure from example 1 were carried out with mixtures of a antifoam and a crystallization additive or mixtures of an antifoam and several crystallization additives . for this purpose , the following mixtures were used : 8 ) ( 1 )+( 3 ) in concentrations of 200 , 400 , 1200 , 2000 and 4000 ppm 9 ) ( 1 )+( 4 ) in concentrations of 200 , 400 , 1000 , 1200 , 2000 and 4000 ppm 10 ) ( 1 )+( 5 ) in concentrations of 200 , 400 , 1000 , 1200 , 2000 and 4000 ppm 11 ) ( 1 )+(( 3 )+( 2 ) at the ratio of 1 : 1 ) in concentrations of 200 , 400 , 1200 , 2000 and 4000 ppm 12 ) ( 1 )+(( 4 )+( 2 ) at the ratio of 1 : 2 ) in concentrations of 200 , 400 , 1200 , 2000 and 4000 ppm the results summarized in the tables above show that — in contrast to the process described in ep 1 451 139 a1 — an increase in the concentration of the tested additives does not lead to a decrease in bulk density or at least not to a significant decrease in bulk density . recrystallization experiments were carried out with the anionic surfactants ( 13 ) sodium dodecylbenzenesulfonate and ( 14 ) dioctyl sodium sulfosuccinate known from jp 46 019610 b . here , the pure surfactants were used in a concentration of 400 ppm each . the experimental data show that these surfactants lead to results which are worse than the results for the surfactants tested in example 2 . recrystallization experiments according to the procedure from example 1 were carried out with the addition of nonionic surfactants . the following sorbitan based surfactants were used in the recrystallization experiments , where the surfactants were each used in a concentration of 400 ppm . with the non - ionic surfactant polyethoxylated sorbitan monostearate ( tween ™ 65 from croda ) in a concentration of 400 ppm a methionine bulk density of 616 g / l was achieved . 1000 g of a 95 ° c .- hot solution of 100 g of methionine in 900 g of water were added dropwise , with stirring , to a 40 ° c .- warm suspension of 20 g of methionine in 180 g of water over 2 h , during which the temperature of the initial charge of suspension was kept at 40 ° c . the experiments were carried out in the presence of 400 ppm of total active ingredient content based on the total mass of the solution / suspension of a mixture according to the invention of a crystallization additive and of an antifoam and of an amount of potassium hydrogen carbonate corresponding to the potassium ion concentration given in the table . the mixture according to the invention of a crystallization additive and of an antifoam consisted of an aqueous solution of 6 . 1 % by weight of silicone oil with a kinematic viscosity of 1000 mm 2 / s ( ak 1000 , wacker - chemie gmbh ), 0 . 25 % by weight of hydrophobicized silica ( sipernat d10 , evonik degussa gmbh ), 2 . 6 % by weight of a polyethoxylated fatty acid mixture ( intrasol ® fs 18 / 90 / 7 , ashland deutschland gmbh ), 3 . 7 % by weight of a polyethoxylated fatty alcohol mixture ( 2 . 35 % by weight of marlipal ®, sasol germany gmbh , 1 . 35 % by weight of brij c2 , croda chemicals europe ) and 5 . 1 % by weight of a fatty alcohol sulphate ( sulfopon ® 1218 g , oleochemicals ) according to the formula : where n = 12 to 18 . the concentration of the pure crystallization additive was 117 ppm . the bulk density of the precipitated methionine was determined after filtration and drying . the addition of potassium ions accordingly leads to an improvement in the bulk density even at a low concentration of the fatty alcohol sulphate used as crystallization additive .