This invention relates to a composition which has an alkali(ne earth) metal hydroxycarboxylate and an aliphatic carboxylic acid which has a lower pKa than that of the hydroxycarboxylic acid. The hydroxycarboxylate salt, when used e.g. in an aqueous medium which allows dissociation of the salt and the aliphatic carboxylic acid into anions and cations, dissociates into the metal ions and hydroxycarboxylate ions, and the acid into the corresponding carboxylate ions and hydrogen ions. The relative pKa values of the two acids ensure that the aliphatic acid forms the alkali(ne earth metal salt thereby liberating the free hydroxycarboxylic acid in situ. This method masks the unpleasant odors of acids for ensiling etc without causing distress to operatives during storage and use.

The invention relates to a method of storing and using hydroxycarboxylic 
acids, especially lactic acid, releasably, bound on a support. 
It is well known that one of the most common hydroxycarboxylic acids is 
lactic acid. This acid has a melting point of 26.degree. C. in its purest 
form but is usually a hygroscopic and syrupy liquid under ambient 
conditions due to the inevitable presence of water and minor impurities 
therein when produced on a commercial scale. Similar problems occur with 
gluconic acids (pKa 3.7). The hygroscopic and syrupy nature of such 
compounds presents significant difficulties during storage and 
transportation. In fact, hydroxyacids such as lactic acid also present 
corrosion problems. Yet this acid has a number of commercial uses in 
various industries such as e.g. baking, confectionery, sauce-making, 
meat/meat products, beer, wine and cider, soft drinks and fruit juices, 
salads, mayonnaise and dressings, pickles, marinades, pharmaceuticals e.g. 
infusion solutions, dialysis solutions and cephalosporin antibiotics, 
cosmetics, surface treatment of metals, metal plating, leather, synthetic 
resins, printing inks and to a lesser extent as an animal feed supplement. 
Thus, the industry would like to store and transport this or similar acids 
in a readily hanbdleable form e.g. a powder provided that such powder can 
be readily converted to the acid at the point of use without any adverse 
effect on the formulations where such acids are used. 
It has now been found that these problems can be mitigated by storing such 
hydroxy carboxylic acids in the form of their alkali(ne earth) metal salt 
either as such or admixed with another carboxylic acid of defined physical 
properties such that the free hydroxycarboxylic acid is released at the 
point of use when such admixture is brought into contact with an 
appropriate solvent system. 
Accordingly, the present invention is a composition comprising an alkali(ne 
earth) metal carboxylate of a hydroxycarboxylic acid which is a liquid or 
a semi-solid at ambient temperature, and an aliphatic carboxylic acid 
which has a lower pKa than that of the hydroxycarboxylic acid. 
The hydroxycarboxylic acid may be a mono-, di- or poly-hydroxycarboxylic 
acid. The present technique is particularly suitable for hydroxycarboxylic 
acids such as lactic acid (pKa 3.08) and gluconic acid (pKa 3.7). 
A feature of the invention is that the alkali(ne earth) carboxylate of the 
hydroxy carboxylic acid, when used in a medium capable of allowing 
dissociation of the salt or free carboxylic acids into anions and cations 
e.g. in aqueous systems, dissociates into the alkali(ne earth) metal ions 
and hydroxycarboxylate ions, and the aliphatic carboxylic acid in turn 
dissociates into the corresponding carboxylate ion and hydrogen ions. 
However, due to the relative differences in the pKa values (i.e. 
dissociation constants), the aliphatic carboxylate ion combines 
preferentially with the alkali(ne earth) metal ions to form the alkali(ne 
earth) metal salt of the aliphatic carboxylic acid and releases the free 
hydroxycarboxylic acid in situ. 
The hydroxycarboxylic acids which can benefit by this technique suitably 
include acids which contain a hydroxy group(s) either alone or in 
combination with other functional groups such as e.g. amino groups. 
Specific examples of such carboxylic acids are lactic acid (pKa 3.08) and 
gluconic acid (pKa 3.7)which are the most suited to this technique due to 
their syrupy and hygroscopic nature. 
The process works particularly efficiently if the alkali(ne earth) metal 
salt of the aliphatic carboxylic acid so formed readily soluble in the 
solvent system used to generate the hydroxycarboxylic acid. However, for 
practical reasons, it may be preferable, though not essential, to form a 
substantially insoluble alkali(ne earth) metal salt in order to enable 
easy separation of the hydroxycarboxylic acid solution from the insoluble 
salt by decantation or filtration immediately prior to use. Such an 
insoluble salt can be formed by the appropriate selection of reactents 
and/or solvents. It should be noted, however, that in the process of 
separating the hydroxycarboxylic acid solution from the precipitate, some 
of the yield of available hydroxycarboxylic acid may be lost due to 
occlusion on the precipitate. 
Particularly suitable alkali(ne earth) metal salts for use in the 
compositions of the present invention are those of sodium, potassium, 
calcium and magnesium. 
The aliphatic carboxylic acids which have a pKa value lower than that of 
the functionalised carboxylic acid under comparable conditions would be 
suitable for admixing with the alkali(ne earth) metal hydroxycarboxylate. 
Such aliphatic carboxylic acids may be mono-, di- or poly-carboxylic acids 
and may be saturated or unsaturated. Particularly suitable for this 
purpose are the di- and poly-carboxylic acids, especially the unsaturated 
carboxylic acids due to their ability to form alkaline earth metal salts 
which have very low solubility in aqueous systems, e.g. water. Specific 
examples of the preferred aliphatic carboxylic acids include trans-fumaric 
acid (pKa 3.03), maleic acid (pKa 1.83), malonic acid (pKa 2.83) and 
methyl-malonic acid (pKa 3.07). 
For instance, if fumaric acid, which has substantially low volatility, is 
intimately mixed with a calcium lactate salt and stored as such, the 
problems of instability and volatility are immediately alleviated. 
However, when the fumaric acid admixed with the calcium lactate is brought 
into contact with a suitable solvent, e.g. water, at the point of use and 
at room temperature, a rapid exchange takes place and the lactic acid is 
liberated immediately into the aqueous solution in situ leaving behind a 
substantially insoluble precipitate of calcium fumarate which can, if so 
desired, be readily removed by filtration or decantation. 
The aqueous solution containing lactic acid and some calcium fumarate can 
then be used as desired. 
The above reaction can be represented as follows: 
##STR1## 
The alkali(ne earth) metal hydroxycarboxylate and the aliphatic carboxylic 
acid in the composition may be combined together in various ways. For 
instance, if the aliphatic carboxylic is a solid, this can be intimately 
mixed with the calcium salt of the hydroxycarboxylic acid and form a solid 
mixture. However, if the aliphatic carboxylic acid is a liquid, this 
liquid can be used to impregnate the solid calcium carboxylate of the 
hydroxycarboxylic acid. The admixed or impregnated calcium salt of the 
hydroxycarboxylic acid can then be stored and used as desired. 
The amount of aliphatic carboxylic acid present in the composition along 
with the alkali(ne earth) metal salt of the hydroxycarboxylic acid is 
limited only by the physical ability of the two to be admixed or for the 
former to be impregnated on the latter. The hydroxycarboxylate salt may, 
for instance, be admixed or impregnated with 1 to 90% w/w, preferably 
40-60% w/w of the aliphatic carboxylic acid based on the total weight of 
the alkali(ne earth) metal salt of the hydroxycarboxylic acid. An 
equimolar mixture of the aliphatic carboxylic acid and the 
hydroxycarboxylate salt is most preferred. 
Each of the alkali(ne earth) metal hydroxycarboxylate and the aliphatic 
carboxylic acid admixed therewith may be, if not a liquid, in the form of 
a powder or granules, or can be shaped into any other convenient shape or 
form, e.g. pellets. The physical shape of the two will be determined by 
the desired speed of release of the hydroxycarboxylic acid once they are 
in contact with the appropriate solvent system. It would be clear that for 
a slow release system, the admixture of the two will be highly compacted. 
Whichever form of the components is used in the composition, it will be 
clear that in order for the hydrocarboxylic acid to be released from the 
salt, the admixture has to be brought into contact with an aqueous or 
non-aqueous system, e.g. water, which is capable of allowing the salt and 
the carboxylic acid to dissociate in said system. Upon intimately mixing 
the alkali(ne earth) metal hydroxycarboxylate and the aliphatic carboxylic 
acid with the solvent system, the exchange of ions takes place. 
Thus according to a further embodiment, the present invention is a method 
of releasing a hydroxycarboxylic acid in situ in a solvent system capable 
of allowing a salt of the hydroxycarboxylic acid and an aliphatic 
carboxylic acid of lower pKa value than the hydroxycarboxylic acid to 
ionize in said system, said method comprising bringing into contact with 
the solvent system an alkali(ne earth) metal salt of the hydroxycarboxylic 
acid and the aliphatic carboxylic acid whereby a solution comprising the 
free hydroxy carboxylic acid in the solvent is generated.

The following Examples will, for the sake of convenience, be directed to an 
admixture of an aqueous solution of calcium lactate with fumaric acid with 
a view to generating lactic acid in situ, but should in no way be 
construed as limiting the generic inventive concept disclosed herein. 
EXAMPLE 
A mixture of calcium lactate (6.7 g, 1 mol water) and fumaric acid (3.3 g) 
was dissolved in 100 ml of water to form a solution. The solution was then 
stirred for one hour and filtered through a Whatman glass microfibre 
filter. The free acid content of the filtrate was measured by titrating 
against a 0.1N aqueous sodium hydroxide solution. The free acid content of 
the filtrate was assumed to be that contributed by lactic acid alone since 
fumaric acid is not soluble in water. The experiment was repeated twice. 
The efficiency of conversion of calcium lactate to free lactic acid in 
situ in these experiments was calculated on this basis to be about 94% as 
follows: 
The measured amounts of free acid in the mixtures from the two experiments 
were 4.33 and 4.42%. After stirring, the mixtures were almost clear, so 
the total weight of the mixture was 
EQU 100+6.7+3.3=110 g 
(no loss of weight was observed during filtration). The actual amounts of 
lactic acid that could be detected by titration in 110 g were 4.76 g and 
4.86 g respectively corresponding to the yield of 4.33 and 4.42% 
respectively. The maximum amount of lactic acid that could be present, 
based on calcium lactate (6.7 g, 1 mol water of crystallization) intake, 
is 5.11 g. Therefore, the efficiency of the conversion was: 
EQU (4.76+4.86).times.100/(2.times.5.11)%=(4.81.times.100)/5.11%=94.1% 
The above experiments show that lactic acid can be generated efficiently in 
situ from a salt thereof admixed with fumaric acid in an aqueous solution.