the process of this invention for making 5 - keto - l - ascorbic acid comprises reacting methyl 2 , 5 - diketogluconate - 5 , 5 - dimethyl ketal ( described in belgian pat . no . 861 , 693 , granted june 9 , 1978 ) with an inorganic base ; e . g ., an alkali metal or alkaline earth metal basic salt , representative of which are the bicarbonates , carbonates , hydroxides and alkoxides wherein the alkoxide group has from one to four carbon atoms . the reaction is conducted in a solvent selected from the group consisting of water , alcohols having from one to four carbon atoms and mixtures of said solvents . the favored bases are sodium or potassium methoxide and sodium or potassium hydroxide . preferred bases are sodium or potassium bicarbonate . the preferred solvent is methanol because of the fact that the 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal sodium or potassium salt precipitates from the reaction mixture and is readily recovered . the temperature of the reaction can range from about 0 ° c . to the reflux temperature of the solvent . there appears to be a relationship between the strength of the base used and the optimum temperature for a given reaction . the stronger bases , such as potassium or sodium methoxide and hydroxide , permit the use of temperatures at the lower range with production of satisfactory yields of the desired ketal . when using a weak base such as sodium or potassium bicarbonate a temperature of at least about 20 ° c . should be used and desirably a temperature of at least about 40 ° c . the preferred temperature when using a weak base is the reflux temperature . the reaction period is , of course , related to the temperature and / or base used . in general , the stronger the base and the higher the temperature , the less time is required for the reaction . in most instances , a reaction period of from about 2 to about 24 hours is used . the methyl 2 , 5 - diketogluconate - 5 , 5 - dimethyl ketal and inorganic base are generally reacted in the ratio of from about 1 . 0 to about 1 . 5 equivalent moles of base per equivalent of dimethyl ketal reactant . while the proportion of reactants used is not critical , the use of less than one equivalent of base per equivalent of dimethyl ketal reactant is not desirable since it tends to reduce the yield of desired product . the use of more than 1 . 5 equivalent moles of base does not appear to improve the yield of the desired product . the preferred procedure comprises reacting equimolar amounts of methyl 2 , 5 - diketogluconate - 5 , 5 - dimethyl ketal and sodium bicarbonate in methanol for a period of about 5 hours . the 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal salt ; e . g ., sodium ( or potassium ) salt , thus produced is then neutralized in liquid medium , e . g . water , to produce 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal . the nature of the neutralizing agent is not critical . both inorganic and organic acids having pk & lt ; 2 , such as those enumerated below can be used . preferred as neutralizing agents are the acid forms of sulfonic acid cation exchange resins . such neutralizing agents are favored because of the ease with which they can be removed from the neutralizing reaction mixture , thus affording a simple procedure for recovering the desired product . suitable cation exchange resins are the sulfonic acid - type which are copolymers of sulfonated polystyrene and divinyl aromatic compounds such as divinylbenzene and divinyl toluene . such resins are described in u . s . pat . nos . 2 , 579 , 974 ; 2 , 191 , 853 ; 2 , 366 , 007 and 2 , 518 , 420 . the 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal thus obtained is then treated with a strong acid in the presence of water to produce 5 - keto - l - ascorbic acid . the acid reactant can be a strong ( pk & lt ; 2 ) inorganic or organic acid . representative of the inorganic acids useful in this step are the mineral acids . representative of organic acids useful for regenerating the keto function are trifluoroacetic acid , trichloroacetic acid , methanesulfonic acid and p - toluenesulfonic acid . neutralization of the 5 - keto - l - ascorbic acids with alkali metal or alkaline earth metal bases such as are enumerated above affords the corresponding alkali metal or alkaline earth metal salts . in water , at ph 2 - 3 , 5 - keto - l - ascorbic acid exhibits no free carbonyl group in the infrared or by c 13 nuclear magnetic resonance spectroscopy , suggesting it exists as a fully hydrated 5 - keto moiety . the ultraviolet absorption spectrum in water at ph 2 . 5 exhibits a single maximum at 247 mμ . thus , in water , 5 - keto - l - ascorbic acid exists in the hydrated form to the extent of at least 95 %, the remaining 5 % or less being the enol form . catalytic or chemical reduction of 5 - keto - l - ascorbic acid affords l - ascorbic acid . catalytic reduction can be accomplished using raney nickel or noble metals at hydrogen pressures ranging from subatmospheric to super - atmospheric pressures . chemical reduction can be carried out using amine boranes ( ammonia , primary or secondary amine boranes ), borohydrides of sodium , calcium or zinc , or cyanoborohydrides of sodium or zinc . the use of alkali metal borohydrides , e . g ., sodium or potassium borohydride , in strong acid media , e . g . trifluoroacetic acid , permits direct conversion of 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal to l - ascorbic acid . reduction of 5 - keto - l - ascorbic acid in water at ph 2 . 5 with sodium cyanoborohydride afforded a mixture of ascorbic and erythorbic acids in the ratio of about 3 : 1 . reduction of 5 - keto - l - ascorbic acid by means of zinc cyanoborohydride in water at ph 2 afforded a mixture of ascorbic and erythorbic acids of approximately 2 : 1 . the mixtures were totally racemic . the 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal and 5 - keto - l - ascorbic acid are effective antioxidants as determined in the schaal oven test at 63 ° c . in soybean oil (&# 34 ; handbook of food additives &# 34 ;, t . e . furia , editor , chemical rubber company press , cleveland , ohio , 2nd ed ., p . 201 , 1975 ). comparison of the antioxidant effectiveness of 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal with that of related antioxidants is provided below : ______________________________________ days to peroxide valueantioxidant of 70 meq / kg of oil______________________________________0 . 02 % erythorbic acid 130 . 02 % erythorbyl laurate 200 . 02 % 5 - keto - l - ascorbicacid - 5 , 5 - dimethyl ketal 23none 11______________________________________ when intimately mixed ; i . e ., dissolved in , added as a solution , dispersed as solid particles , with oxidizable substances , especially with deterioration - prone consumables such as food products , edible oils , essential oils , fats , nuts , flavorings , vitamins and pharmaceuticals , 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal and 5 - keto - l - ascorbic acid , and the alkali metal and alkaline earth metal salts thereof , when used at a level of from 10 to 1 , 000 ppm , based on the weight of the consumable substance , effectively retard their oxidation . the favored amount is from 10 to 200 ppm . 5 - keto - l - ascorbic acid , preferably as the sodium salt , and 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal are also valuable inhibitors of nitrosamine formation such as occurs in meat products containing nitrite , e . g ., frankfurters and bacon . the addition of from about 250 to about 1000 ppm of sodium 5 - keto - l - ascorbate or 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal to the meat prior to or during curing effectively reduces nitrosamine formation . the procedure described by fiddler et al ., j . ag . food chem . 26 , 653 ( 1978 ) enables one to determine the nitrosamine inhibiting effect of said compounds . a further method which can be used is that described by walters et al ., z . lebensm . unter - forsch . 162 , 377 ( 1976 ) which makes use of model systems to determine nitrosamine inhibition of a given compound . the present invention is illustrated by the following examples . it should , however , be understood that the invention is not limited to the specific details of these examples , variations of which are possible as indicated by the above disclosure . to a solution of methyl 2 , 5 - diketogluconate - 5 , 5 - dimethyl ketal ( 2 . 52 g ., 10 . 0 mmoles ) in anhydrous methanol ( 10 ml .) at 0 ° c . under a nitrogen atmosphere was added sodium methoxide ( 9 . 0 ml . of 1 . 0 n ) in methanol ( 10 ml .). the mixture was stirred at room temperature overnight and the resulting gel - like crystals filtered , washed with anhydrous methanol and then dried in vacuo ( room temperature ) for three days . the resulting white , friable powder hydrated upon standing in the air to give 1 . 1 g . ( 42 %) of the monohydrate of the title product . ir ( kbr ): 3413 ( s , oh ), 1733 ( s ), 1597 ( s ) cm - 1 . 1 h nmr ( 60 mhz ) δ d . sbsb . 2 o tms ( ppm ): ## str1 ## 3 . 68 ( s , 2 , -- ch 2 -- oh ), 3 . 35 [ s , 6 , ( ch 3 o 2 )]. 13 c nmr ( d 2 o ) ( ppm ): 176 . 5 ( s ), 176 . 1 ( s ), 113 . 2 ( s ), 100 . 5 ( s ), 77 . 5 ( d ), 59 . 6 ( t ), 49 . 8 and 49 . 3 ( q ). uv ( h 2 o ): λ max 269 mμ ( ε = 9 . 01 × 10 3 ). analysis : calc &# 39 ; d . for c 8 h 11 ona . h 2 o : c , 36 . 93 ; h , 5 . 03 . found : 37 . 23 ; h , 4 . 37 . to a solution of methyl 2 , 5 - diketogluconate - 5 , 5 - dimethyl ketal ( 11 . 46 g ., 45 . 5 mmols ) in anhydrous methanol ( 500 ml .) was added sodium bicarbonate ( 7 . 65 g ., 91 mmols ). the mixture was heated at reflux under nitrogen for five hours and then stirred at ambient temperature overnight . the jelled reaction mixture of the sodium salt of the title compound was broken up and 75 ml . of amberlite 15 resin ( a sulfonic acid cation exchange resin available from rohm and haas co . ), acid form , previously washed with methanol , was added . the mixture was stirred until a clear solution formed ( about one hour ) and the resin then removed by filtration . the filtrate was evaporated in vacuo to an oil and then refrigerated overnight . the crystalline mass which formed was ground and vacuum dried . yield , 9 . 70 g . ( 96 . 9 %) of product which was homogeneous by thin layer chromatography ( silica gel ; n - butanol : acetic acid : water , 2 : 1 : 1 ). m . p . 123 . 5 °- 124 . 0 ° c . ; [ α ] d 25 - 23 . 3 ° ( c 0 . 54 , h 2 o ). u . v . ( ch 3 oh ): λ max 250 mμ ( ε = 5 . 34 × 10 3 ). 1 h nmr ( 60 mhz ) δ dmso - d . sbsb . 6 tms ( ppm ): ## str2 ## 3 . 47 ( s , 2 , -- ch 2 -- oh ); 3 . 25 [ s , 6 , ( ch 3 o ) 2 ]. 13 c nmr ( d 2 o ) ( ppm ): 172 . 7 ( s ), 156 . 0 ( s ), 118 . 8 ( s ), 101 . 6 ( s ), 74 . 8 ( d ), 59 . 4 ( t ), 50 . 4 and 49 . 9 ( q ). anal . calc &# 39 ; d . for c 8 h 12 o 7 : c , 43 . 64 ; h , 5 . 49 . found : c , 43 . 79 ; h , 5 . 44 . to 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal ( 1 . 0 g ., 4 . 54 mmols ) was added trifluoroacetic acid ( 19 ml .) and water ( 1 ml .) at ambient temperature . the mixture was stirred for 30 minutes and the trifluoroacetic acid and water removed under high vacuum to give a light yellow oil . thin layer chromatography in the system silica gel , butanol : acetic acid : water ( 2 : 1 : 1 ) showed the oil to be homogeneous . the oil was taken up in water ( 10 ml .) the solution treated with activated charcoal ( 200 mg .) and filtered . refrigeration of the filtrate for 3 days afforded a crystalline material which was removed by filtration and dried in air . yield = 568 mg ., 65 % as monohydrate . u . v . ( h 2 o , ph 2 . 65 ): λ max 242 , a = 0 . 82 , 300 ( a = 0 . 10 ). 1 h nmr ( 60 mhz ) δ dmso - d . sbsb . 6 tms ( ppm ) 4 . 58 ( s , 1 ), 3 . 63 ( a b quartet , 2 , j = 11 . 8 hz ). 1 h nmr ( 60 mhz ) δ d . sbsb . 2 o tms ( ppm ): 3 . 45 ( s , 2 ), 4 . 27 ( s , 1 ). 13 c nmr ( dmso - d 6 ) ( ppm ): 169 . 7 ( s ), 152 . 0 ( s ), 119 . 0 ( s ), 92 . 3 ( s ), 76 . 7 ( d ), 76 . 3 ( d ), 65 . 8 ( t ), 62 . 1 ( t ). 5 - keto - l - ascorbic acid - dimethyl ketal ( 475 mg ., 2 . 16 mmols ) was dissolved in water ( 1 ml . ), cooled to 0 ° c . and trifluoroacetic acid ( 19 ml ., previously cooled to 0 ° c .) added slowly ( ph ˜ 2 . 5 ). the reaction mixture was stirred at 0 ° c . for one hour , and at 25 ° c . for 30 minutes after which the solvent was removed under high vacuum at below room temperature . the residue was taken up in water ( 30 ml .) and treated with 542 mg . ( 32 mmols ) sodium cyanoborohydride . the mixture was stirred at 25 ° c . for 2 hours and excess dowex 50 , a sulfonic acid cation exchange resin ( 20 ml ., acid form , available from the dow chemical co .) was added to decompose the excess cyanoborohydride . the resin was removed by filtration , and the methanol removed by distillation . the residue was re - dissolved in methanol ( 20 ml .) and the methanol removed by distillation . this step was repeated two more times to affect removal of boric acid residues . the final solid isolated comprised 388 mg . ( 100 %) of an iodine reducing material , shown by thin - layer chromatographic analysis ( silica gel , n - butanol : acetic acid : water , 2 : 1 : 1 ) to be a mixture of ascorbic and erythorbic acids . a portion of the residue was per - trimethylsilylated with trimethylsilylchloride in pyridine according to the procedure of bacon et al ., j . lab . and clin . med ., 73 , 1030 ( 1969 ). the procedure comprised dissolving a 5 - 20 mg . sample of the residue in a mixture of n - trimethylsilylimidazole , n , o - bis -( trimethylsilyl ) acetamide and trimethylchlorosilane ( 3 : 2 : 2 v / v ) in pyridine ( available from pierce chemical co ., as tri - sil tbt ). the reaction mixture is allowed to stand for 15 minutes and is then injected directly onto the glpc column . gas liquid phase chromatographic ( glpc ) analysis of the pertrimethylsilylated derivative showed a mixture of erythorbic acid and ascorbic acid in a ratio of 26 : 74 . the glpc analysis was accomplished using a 5 &# 39 ;, 1 / 8 &# 34 ; o . d . column packed with 3 % ov - 210 on chromasorb whp , 100 / 120 mesh , isothermally at a temperature of 135 ° c . and a carrier gas flow rate of 30 ml ./ minute . ov - 210 is a silicon oil polymer containing 50 % 3 , 3 , 3 - trifluoromethyl groups and is marketed by analabs inc ., a subsidiary of new england nuclear , 80 republic drive , north haven , ct 06473 . chromasorb whp is a registered trademark of the johns - manville co ., 22 e . 40th street , new york , ny 10016 for a diatomite support which is acid washed and dimethylchlorosilane treated . 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal ( 195 mg ., 0 . 887 mmols ) was dissolved in trifluoroacetic acid - water ( 5 . 0 ml . of 95 : 5 ) and stirred at room temperature ( ph ˜ 2 ) under nitrogen for 15 minutes . removal of solvent under high vacuum at below room temperature afforded a foam residue which was re - dissolved in water ( 5 . 0 ml .). after the sequential addition of anhydrous zinc chloride ( 60 . 4 mg ., 0 . 443 mmols ) and sodium cyanoborohydride ( 371 . 6 mg ., 5 . 913 mmols ) the solution was stirred at room temperature under nitrogen for 16 hours . analysis of the reaction mixture by thin layer chromatography ( silica gel ; n - butanol : acetic acid : water ; 2 : 1 : 1 ) showed the presence of l - ascorbic acid . the reaction mixture was stirred with 15 ml . of dowex 50 ion exchange resin ( sulfonic acid form ) for 3 hours after which the resin was removed by filtration and the filtrate stripped in vacuo to a white solid residue . the residue was placed in a 50 ml . roundbottom flask charged with methanol ( 25 ml .) and fitted with a soxhlet extractor containing 10 ml . of ag 1 - x8 resin ( a quaternary ammonium ion exchange resin , hydroxide form , available from rohm and haas company ). the solution was heated at reflux for 5 hours to absorb boric acid residues on the ion exchange resin , cooled , and the methanol removed by rotatory evaporation . the residue was derivatized with trimethylsilyl chloride in pyridine according to the procedure of example 4 . glpc analysis of the derivatized reaction mixture ( 5 &# 39 ;, 3 % ov - 210 column at 135 ° c . and flow rate of 30 ml ./ min .) showed the presence of ascorbic acid and erythorbic acids in a ratio of 2 : 1 . the procedure of example 1 is followed but with the modifications in reaction conditions noted below to afford , depending upon the base used , the alkali metal or alkaline earth metal salts of 5 - keto - l - ascorbic acid dimethyl ketal . ______________________________________ equiv - alent moles dmg */ timesolvent base base t ° c . ( hours ) ______________________________________ch . sub . 3 oh naoch . sub . 3 2 : 1 22 18ch . sub . 3 oh naoch . sub . 3 1 : 2 25 20ch . sub . 3 oh koch . sub . 3 1 : 1 22 20c . sub . 2 h . sub . 5 oh naoc . sub . 2 h . sub . 5 1 : 1 20 24c . sub . 4 h . sub . 9 oh koc . sub . 4 h . sub . 9 1 . 5 : 1 25 20c . sub . 4 h . sub . 9 oh naoch . sub . 3 1 : 1 30 18isopropanol ko -- i - c . sub . 3 h . sub . 7 1 : 1 . 5 22 22ch . sub . 3 oh khco . sub . 3 1 : 2 reflux 5ch . sub . 3 oh na . sub . 2 co . sub . 3 1 : 2 reflux 6ch . sub . 3 oh naoh 1 : 1 50 5ch . sub . 3 oh nah 2 : 1 20 18h . sub . 2 o k . sub . 2 co . sub . 3 1 : 2 reflux 4h . sub . 2 o khco . sub . 3 2 : 1 80 5ch . sub . 3 oh -- h . sub . 2 o ( 1 : 1 ) koh 1 : 2 25 24ch . sub . 3 oh -- h . sub . 2 o ( 95 : 5 ) nahco . sub . 3 1 : 2 reflux 5ch . sub . 3 oh koch . sub . 3 1 : 1 20 18h . sub . 2 o ca ( oh ). sub . 2 1 : 1 . 5 75 10h . sub . 2 o caco . sub . 3 1 : 1 reflux 18h . sub . 2 o ba ( oh ). sub . 2 1 : 1 80 8h . sub . 2 o baco . sub . 3 1 : 1 . 5 reflux 18ch . sub . 3 oh ca ( och . sub . 3 ). sub . 2 1 : 1 reflux 6ch . sub . 3 oh sr ( oh ). sub . 2 1 : 2 reflux 5h . sub . 2 o sr ( hco . sub . 3 ). sub . 2 1 : 1 reflux 6h . sub . 2 o lioh 1 : 1 60 6c . sub . 2 h . sub . 5 oh lioh 1 : 1 . 2 reflux 5ch . sub . 3 oh mg ( oh ). sub . 2 1 : 1 reflux 6______________________________________ * dmg = methyl 2 , 5diketogluconate - 5 , 5 - dimethyl ketal . the procedure of example 2 is repeated but using the following acids for neutralization of the sodium salt : dowex 50 ( sulfonic acid cation exchange resin , acid form , available from the dow chemical co .). amberlite irc - 50 ( carboxylic acid cation exchange resin , acid form , rohm and haas co .). the procedure of example 3 is repeated but substituting the acids listed below for trifluoroacetic acid to give the title compound . the procedure of example 3 is repeated but the aqueous solution of 5 - keto - l - ascorbic acid obtained is neutralized with dilute ( 1 n ) sodium hydroxide to give the sodum salt which is recovered by lyophilization . in like manner the potassium , lithium , calcium , barium , magnesium and strontium salts are prepared . to a solution of methyl 2 , 5 - diketogluconate - 5 , 5 - dimethyl ketal ( 10 . 0 g ., 39 . 7 mmols ) in anhydrous methanol ( 100 ml .) was added sodium bicarbonate ( 3 . 36 g ., 40 . 0 mmol ). the mixture was stirred vigorously at reflux for 4 hours , cooled to 40 ° c . and concentrated sulfuric acid ( 1 . 08 ml ., 20 mmol ) added dropwise to the stirred solution . on cooling to ambient temperature , ether ( 50 ml .) was added and the precipitated sodium sulfate removed by filtration . workup as in example 2 afforded 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal in 89 % yield by iodine titration . to a rapidly stirring solution of 5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal ( 3 . 72 g ., 16 . 9 mmols ) in 50 ml . of trifluoroacetic acid at ice bath temperature was added sodium borohydride pellets ( 0 . 750 g ., 19 . 8 mmol ). the mixture was stirred at 0 ° c . for 3 hours under nitrogen , the trifluoroacetic acid removed by rototory evaporation and the residue taken up in 50 ml . of methanol . the methanol was treated with 1 . 65 ml . of concentrated hcl and the precipitated sodium chloride removed by filtration . the filtrate was concentrated by rototory evaporation with subsequent addition of methanol three times to affect removal of trimethylborate , finally affording a tan solid shown by tlc ( silica gel , ethyl acetate - ethanol - water , 5 - 3 - 2 ) to be a mixture of l - ascorbic acid and d - erythorbic acid . a portion of the solid ( 10 mg .) was per - trimethyl silylated with tri - sil tbt as in example 4 . glpc analysis of the per - trimethylsilylated derivative showed a mixture of d - erythorbic and l - ascorbic acids in a ratio of 23 : 77 . the crude solid was chromatographed on silica gel ( 180 g .). elution with ethyl acetate - ethanol - water ( 5 - 3 - 1 ) afforded 2 . 64 g . ( 76 %) of l - ascorbic acid ( by tlc and glpc analysis according to the procedures described above ). it was shown to be 95 % pure by iodine titration . a sample was crystallized from methanol - ethyl acetate , 1 -- 1 . 1 h nmr ( d 2 o ) δ4 . 75 ( d , 1 ); 3 . 76 ( multuplet , 1 ); 3 . 49 ( multuplet , 2 ).