History Of Lactic Acid 4b2r6x
This document was ed by and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this report form. Report r6l17
Overview 4q3b3c
& View History Of Lactic Acid as PDF for free.
More details 26j3b
- Words: 4,733
- Pages: 11
http://www.lactic.com/index.php/lacticacid
HISTORY OF LACTIC ACID Lactic acid, also known as 2-hydroxypropionic acid, is present in almost all forms of organized life and was probably also found in primitive life forms which appeared on Earth at the beginning (Brin, 1965). However, notwithstanding this relative abundance, lactic acid has been, for a long time, difficult to identify. As its name implies, it is in milk, or rather in curdled milk, that, in 1780, Carl Wilhelm Scheele, a Swedish chemist, found an acid that he purified by crystallizing its calcium salt. Scheele named it "Mjölksyra" – or "milk acid" – believing that it was a normal component of milk but never realizing that it was in fact the fermentation product of rancid milk (Dobbin, 1931). It is only in 1813 that Henri Braconnot of Nancy University (), who worked on the acidic components of fermented food, found a product that he named "nanceic acid", the name referring to the French town (Nancy). He was convinced that this acid was different from the one discovered by Scheele and concluded that, during fast fermentation whereby alcohol is first produced, only vinegar accumulates and nanceic acid is produced only in the slow fermentation process (Braconnot, 1813). A few years later, in 1817, Vogel, a German chemist very clearly showed that nanceic and lactic acid were absolutely identical (Holten, 1971). It was then generally accepted that lactic acid was produced by fermentation but nobody could explain how until Cogniard-Latour observed small "globules" able to multiply in a medium containing sugar while producing gas and alcohol. Then, in 1857, Pasteur, who was then advisor in a distillery experimenting with beet juice fermentation, described what he called "lactic yeast",a greyish substance, made of small globules aligned in short segmented ligaments, believed to be the originator of lactic acid ("lactic fermentation") which plays an essential part in the growth of yeast cells and in the production of alcohol ("alcoholic fermentation"). Lactic acid was not yet a widely used consumer product, but merely a pharmaceutical speciality until, 1881, when Charles Avery established the Lactate Company near Boston, U.S.A. The objective of this company was to replace the tartric acid used in bread making. However, the Avery factory faced problems and could not start production. It was finally in that the first lactic acid sales were recorded , around 1900, by Boeringher. From then on, many companies invested in lactic acid production but most of them had to stop because of their product's low quality which was unacceptable for the market. ♠ back to top
Lactic acid occurrence The lactic acid molecule is most frequently found in vegetals, in micro-organisms and in the animal kingdom where it is present as an intermediary for the carbo-hydrates and amino acids metabolism.
L(+) or D(-) Although, quantitatively speaking, the lactic acid L(+) form is certainly the one most frequently occuring in nature, the D(-) form is also present as well as, in some cases, a mix of both optical isomers in variable proportions. Lactic acid is found in virtually all tissues, physiological fluids and excretions. It is also found in human blood (1.4 µmol/ml), in sperm (4.1 µmol/ml), in sweat (4.0 – 40.0 µmol/ml), in the cerospinal fluid (1.6 µmol/ml) and in urine (0.3 µmol/ml) (Geigy, 1968).
In the human body
Although in the human body it is the L(+) form that predominates, it is by no way the only one. Indeed, in urine, the lactic acid D(-) can be detected if it has been ingested by the subject. The lactic acid D(-) metabolism occuring in the liver, it is generally recommended that this form is not added to food for children under the age of 3 because of their hepatic immaturity although physiological consequences in relation with its ingestion have not been proved (Vrese, 1990). In organisms, lactic acid L(+) is formed by a reduction reaction of pyruvic acid. This hydrogen transfer is done by a NADH-dependent enzyme, deshydrogenase lactate. The hydrophobic nature of lactate and pyruvate molecules allows free diffusion through the cell membranes which establishes a direct relation between these two substances concentration in the blood stream and the redox equilibrium of the cells' NADH/NAD+ system (Hohorst, 1965). In aerobic conditions, the energy feed to muscle cells is brought about by oxidative degradation of glucose which generates energy compounds such as triphosphate adenosin (ATP) and phosphate creatin. In anaerobic conditions, or in case of increased energy demands (intense physical exertion, for instance), the required energy is supplied by glycolysis wich induces the build up of lactic acid in the muscles. This excess acid diffuses then in the blood stream and will be absorbed in the liver and kidneys to again synthesize glucose ("gluconeogenesis") and glycogen ("glyconeogenesis"). Evidently, this metabolic path by itself is using energy produced by oxidative degradation of lactic acid. Already in 1930, Meyerhof established that the ratio between used lactic acid and oxidized lactic acid was 5 to 1 (Meyerhof, 1930). Glucose again synthesized in the liver or in kidneys is in turn fed in the blood stream which brings it to muscular cells where it is transformed into energy or lactic acid thus closing the so-called "Cori cycle" (Alpert, 1965; Brecht, 1967). Reichard et al. (1963) determined that at least 40% of the lactic pool exists as glucose and about 6% of the total glucose volume is recycled via the Cori cycle. Globally, one estimates that the average turnover of lactic acid in a adult male is about 120 to 150 g per day (FDA, 1978).
In micro-organism Lactic acid is also found in large quantitities in the micro-organism world. Several bacterial species produce large quantities of lactic acid, among which the best known are Lactobacillus, Sporolactobacillus, Enterococcus, Lactococcus, Bacillus, Streptococcus, Pediococcus, Leuconostoc or bifidobacteria which widely differ from the others by the metabolic path they go through. Bifidobacteria are, for all practical purposes, exclusively anaerobic when other bacteria are generally microaerophilic. The two lactic acid isomers may be produced by of a same bacterial specie as in the case of Lactobacillus for instance. Indeed, Lb. maltaromicus, Lb. agilis, Lb. sharpae, Lb. amylophilus, Lb. murinus produce only the L(+) acid whereas Lb. delbrueckii, Lb. vitulinus, Lb. coryniformis only produce the D(-) isomer and Lb. plantarum or Lb. helveticus are examples of simultaneous producers of both molecular forms. Ph conditions as the age of cultures may influence the L(+)/D(-) ratio (Garvie, 1967). The stereospecificity of the acid is influenced by the presence of one and/or the other specific dehydrogenase lactate which, as happens in mammal metabolism, transforms pyruvate into lactate. No hydrogenase lactate was found that would simultaneously produce both lactic isomers. Therefore, the existence in the same pure culture of the L(+) and D(-) forms shows the presence of genes of the two dehydrogenase lactates. However, an enzyme, called racemase, which transforms an isomer into the other form was observed a long time ago in certain micro-organisms such as Clostridium butylicum or Lactobacillus sake (Hiyama, 1968). Fermented substrates used to produce lactic acid are generally hexoses. In connection with the bifidobacteria fermentation, which follows a very special process, the lactic fermentation may occur following two different ways, i.e. a homolactic fermentation or a heterolactic one, both of them related
to the energy cycle of the cell and producing lactic acid respectively as the only product or lactic acid plus various co-products such as ethanol, carbon dioxide and acetate. Homolactic fermentation follows the metabolic path of glycolysis which proceeds in accordance with the EMDEN-MEYERHOF-PARNAS scheme. The general equilibrium of this path is given by following equation:
This equation shows an anaerobic and exothermic reaction whose yield is often higher than 90%. The heterolactic fermentation does not work along this path, because of the unavailablity of the specific enzymes of glucose catabolism according to Emden-Meyerhof-Parnas (aldolase and triose phosphate isomerase), but goes through oxidative degradation of pentoses phosphates allowing glucose metabolism via the hexose monophosphate shunt. In the same way, micro-organisms favouring this path ferment the D-xylose, the L-arabinose and the D-ribose to form lactic acid and acetic acid (Bernstein, 1953). The general equilibrium of glucose heterofermenting catabolism may be expressed as follows:
♠ back to top
Molecular properties Lactic acid is a weak organic acid and one of the smallest molecules with an asymetrical carbon in location α of the carboxylic function. This feature allows its existence with two different stereoisometric forms called L(+) and D(-). The plus and minus signs show the polarized light rotation, respectively to the right and to the left, an effect called dextro- and levo-rotatory. The L and D symbols show the spatial configuration of the molecule with reference to the two glyceraldehyde isomers.
When containing no water, both these optically active acids form white solid crystals which melt at significantly higher temperature (52.7 – 52.8°C) than their racemic mix containing 50% of each isomer (16.4°C) (Walter, 1988). In addition to its optical activity, the lactic acid molecule has simultaneous carboxylic and hydroxyl groups allowing it to react either as an acid or as an alcohol. This dual functionality initiates the spontaneous formation on intermolecular esters (oligomers) with discharge of water leading to a thermodynamic equilibrium subject to the solution concentration and age. The speed of this condensation reaction is related to the temperature, the possible presence of catalyst species (Lewis acid) and to the equilibrium gap of the solution under consideration. As for every esterification reaction, it is reversible by adding water (hydrolysis of formed oligomers). In addition, the formation of lactic acid oligomers during heating and concentration of the lactic acid solution make it impossible to precisely measure the boiling point of the pure product at atmospheric pressure. Lactic acid is a weak acid with a pKa of 3.86 at 25°C. However, the tendency of every monoacid to dissociate is a function of the pH of the medium as compared to the pKa of the contemplated acid. Indeed, the more the pH increases, the more the acid molecule will be dissociated. On the other hand, with the same pH, different acids will be more dissociated if their pKa is low as can be seen on fig. 1. We will see, later in this chapter, that such considerations are of prime importance because the bacteriostatic or bactericide potential is larger when the acid is protonized, i.e. not dissociated.
Variation of the % of protonized molecules as a function of pH for some monoacids. ♠ back to top
Thermodynamic characteristics of lactic acid Dissociation constant (Ka) :
0.000137 (at 25°C) Heat of dissociation (ΔH) :
- 63 cal/mol (at 25°C) Free energy of dissociation (ΔF) :
~5000 cal/mol Heat of solution (ΔH) :
+ 1868 cal/mol (for crystalline L(+) lactic acid at 25°C) Heat of dilution (ΔH) :
-1000 cal/mol (for dilution with a large volume of water) Heat of fusion (ΔH) :
+ 2710 cal/mol (for racemic lactic acid) + 4030 cal/mol (for L(+) lactic acid) Entropy of solution (ΔS) :
+ 6.2 cal/mol/°C Entropy of dilution (ΔS) :
- 3.6 cal/mol/°C Entropy of fusion (ΔS) :
+ 9.4 cal/mol/°C (for racemic lactic acid) + 12.2 cal/mol/°C (for L(+) lactic acid) Heat of combustion (ΔHc0) :
- 321220 cal/mol (for crystalline L(+) lactic acid at 25°C) - 325600 cal/mol (for liquid racemic lactic acid at 25°C) Heat of formation (ΔHf0) :
-
165890 163000 164020 164080
cal/mol cal/mol cal/mol cal/mol
(for (for (for (for
crystalline L(+) lactic acid at 25°C) liquid lactic acid) lactic acid in dilute solution) dissociated and diluted lactic acid)
Heat capacity () :
0.338 cal/g/°C (for crystalline lactic acid at 25°C) 0.559 cal/g/°C (for liquid lactic acid at 25°C) Absolute entropy (S0) :
34.0 cal/mol/°C (for crystalline L(+) lactic acid at 25°C) 45.9 cal/mol/°C (for liquid racemic lactic acid at 25°C) Entropy of formation (ΔSf0) :
- 137.2 cal/mol/°C (for crystalline lactic acid at 25°C) - 125.3 cal/mol/°C (for liquid lactic acid at 25°C) Free energy of formation (ΔFf0) :
- 124980 cal/mol (for crystalline L(+) lactic acid at 25°C) - 126500 cal/mol (for liquid racemic lactic acid at 25°C) (Holten, 1971). ♠ back to top
Antimicrobial activity The purpose of an antimicrobial agent is to reduce or even to completely eliminate all microbial activity. This can be achieved either by inhibitting the contamining micro-organisms (bacteriostatic and fungistatic action) or by killing these micro-organisms (bactericidal and fungicidal action). For the food industry, an antimicrobial processing is normally considered at two stages. First, food decontamination, in particular cattle carcasses at slaughterhouse, and, second, shelf-life increase of fresh or semi-processed foodstufff. Consumers, aware of this shelf-life notion, will only accept food based on poorly objective considerations like their visual appearance and color or their organoleptic properties (odor, flavor). Indeed, foodstufff with extended shelf-life obtained by vacuum packaging may not be consumed because of its aspect, odor or flavor as the lack of oxygen creates anaerobic conditions changing the food bacterial flora and, in certain cases, may even provide growth potential for pathogenic contaminants. A well-designed packaging is thus often only part of the solution to ensure that the integrity of the foodstufff remains unaffected over a given period of time. Such considerations, leading to the stengthening of hygienic conditions in food processing, are justified by the extensive consumption of processed food by developed countries consumers which commands the increasingly strict application of GMP rules (Good Manufacturing Practices) and HAC (Hazard Analysis of Critical Control Points).
Although other inhibition mechanisms may be considered (Shelef, 1994), the two main ones are : (a) lactic acid capacity to reduce the medium’s pH (Freese, 1973; Hunter, 1973; Salmond, 1984) and (b) the effect of lactates – mainly sodium lactate – on water activity in foodstufffs with intermediate humidity (Debevere, 1989 ; Loncin, 1975).
Acting on intracellular pH The anti-microbial activity is larger in the presence of organic acids than inorganic ones. It is also well known that inhibition increases with the medium pH decrease. This is explained by the hydrophobic feature of most organic acids, often called "volatile fatty acids", which allows free diffusion of the protonied form through the cell's membrane. As, generally, the intracellular pH is higher than the extracellular one, the acid undergoes dissociation as soon as it enters the cell's cytoplasm and then decreases the intracellular pH by the proton release phenomenon. This dissociation effect is described, for a monoacid such as lactic acid, by the relation shown hereunder which shows the ratio between the dissociated form ([A-]) and the non-dissociated one (or protonized form, [HA]) as an exponential function of the neighbouring pH.
In order to counter the decrease of its cytoplasmic pH, resulting from the ionisation of the entered acid, the cell allocates the main part of its energy content to eliminate these newly formed protons which results in slower growth kinetics (Cassio, 1987; Ten Brink, 1980; Ten Brink, 1985). Preceeding reflexions take only into the inhibition effect of the proton on microbial growth. However, various studies showed that bacteria growth is affected by the anion as well as by the proton (Eklund, 1983) and that lactate anion acts itself differently from other anions produced by fermentative processes such as propionate, sorbate or acetate. Additional weight is being given to this in the case of non-fermented foodstuffs which pH is ordinarily close to neutrality; it gives a ratio between dissociated and not-dissociated forms which is clearly in favor of the non-dissociated form. In addition, for lactic acid salts such as sodium and potassium lactates, an inhibition phenomenon can not be assigned to the acid's dissociation and to the subsequent production of protons as these are entirely dissociated in the aqueous phase.
Acting on water activity When certain studies, such as the Loncin (1975) one, state that sodium lactate would have a more marked deflationary effect on water activity (aw) than is the case for other organic acids and of sodium
chloride with equal concentrations, Chirife and Ferro Fontan (1980) reported a much lesser effect. Indeed, according to the latter, sodium lactate decreases water activity less than sodium chloride, but this effect would however be higher than the one obtained from tartarate or sodium citrate at equal concentrations. Houstma et al. (1983) reports that sodium lactate and chloride have exactly the same effect for concentrations ranging from 0.089 to 0.89 mol/L. If the sodium lactate deflationary effect on water activity seems thus more or less equivalent to the one of sodium chloride, it is however necessary to consider that, according to Houstma et al. (1993) the inhibition effect of sodium lactate is more intense than for the corresponding chloride which tends to confirm the specific inhibition effect of the lactate ion on cell growth. Indeed, from this vast study of gram-positive, gram-negative bacteria and yeasts, it appears that the sodium lactate minimal inhibition concentration is far lower than the one of chloride with the exception of Campylobacter jejuni, Trichosporon beigelii and Torulaspa delbrueckii strains.
Other effects Various effects other than those already mentioned have been reported. Hydroxycarboxylic acids are known for their chelating properties and lactic acid, in particular, is commonly used by the food industry for this purpose as are polyphosphates, citric and ethylenediaminetetracetic (EDTA) acids (Kabara, 1991). Iron chelation, was, for instance, shown as a possible contribution to the antilisterial effect of lactate (Shelef, 1994) and to the stabilization of food lipides by the antioxidizing effect obtained (Nnanna, 1994).
Comparing lactate and other organic acids Lactic acid is generally thought of as less efficient than sorbic, acetic, propionic and benzoic acids, for example (Baird-Parker, 1980; Minor, 1970; Chung, 1970). Based only on its pKa, one may indeed expect that lactic acid displays a higher pH deflationary effect but a more limited growth inhibition effect as, for a given pH, lactic acid will be more ionized than the other acids even when it has been demonstrated that the inhibitor form is the protonized form (not ionized) of the molecule (see supra). Such considerations must however be accepted with due carefulness as testified by Brackett (1987) who shows following classification as a function of the type of effect observed on the growth of Yersinia enterocolitica : (a) antimicrobial activity based on pH : propionic > lactic > acetic > citric; (b) antimicrobial activity based on the concentration of the non-dissociated form (for monoprotic acids only) : lactic > propionic > acetic; (c) antimicrobial activity based on equal molar concentrations : citric > lactic > propionic > acetic. The author concluded that, based on statistics, there are no significant differences between these acids. In addition, numerous publications report synergetic effects between organic acids mentioned above and, more particularely, for acetic and lactic acids which therefore should be used concominantly (Rubin, 1978; Adams, 1988; Dickson, 1992). Various mixes of lactic acid, citric acid and potassium sorbate showed synergy either by slowing the growth of Salmonella, Yersinia enterocolitica, Pseudomonas fluorescens, two strains of lactic bacteria (Restaino, 1981) and four strains of osmophilic yeasts (Restaino, 1982) or by preventing entirely their growth. Processed product
Effect
Reference
Veal
Discoloration at concentrations higher than 1.25%
Woolthuis,
Veal
Discoloration at concentrations higher than 1.25%
Snijders, 1
Veal (head)
No effect on color at concentration of 1%
Cudjoe, 19
Color, look
Veal (tongue)
Improvement of look at concentration of 2%
Visser, 198
Beef
No effect on color at concentration of 2%
Hamby, 19
Beef
No effect on color at concentration of 2%
Prasai, 199
Beef
Discoloration at concentrations higher than 1%
Snijders, 1
Pork
Discoloration at concentrations higher than 1.5%
Pork
Color is darker and less red with a mix of 1% lactic acid and 1% acetic acid
Mendonca,
Chicken
Color is lighter at concentration ranging from 1 to 5%
Van Der M 1989
Chicken
Look deteriorates at concentration of 3%
Kolsarici, 1
Crayfish
No effect on color at concentration of 1%
Dorsa, 199
Catfish
Look deteriorates at concentration higher than 2%
Marshall, 1
Veal
Little effect on flavor at concentrations less than 2%
Woolthuis,
Veal
No effect on flavor at concentrations less than 2%, Altering of flavor at concentration of 4%
Snijders, 1
Veal (tongue)
No effect on flavor at concentration of 2%
Visser, 198
Beef
No effect on flavor at concentration of 1%
Acuff, 1987
Beef
No effect on flavor at concentration of 1%
Dixon, 198
Beef
Improvement of flavor at concentration of 1%
Hamby, 19
Chicken
No effect on flavor at concentration ranging from 1 to 5%
Van Der M 1989
Pork
No effect on flavor at concentration less than 2%
Epling, 199
Crayfish
No effect on flavor at concentration of 1%
Dorsa, 199
Beef
Improvement of aroma for concentration of 1%
Prasai, 199
Chicken
Aroma deteriorates for concentration of 3%
Kolsarici, 1
Catfish
Odor deterirates for concentrations higher than 2%
Marshall, 1
Flavor
Odor, aroma
Table 1: impact of lactic acid on organoleptic profile of foodstuffs. Processed product
Effect
Reference
Beef
Sodium lactate emphasizes red color (specially with 2% concentration)
Papadopou 1991
Beef (minced)
Sodium lactate does not alter color at concentration of 2 to 3%
Egbert, 19
Pork (saussages)
Sodium lactate improves color for concentration of 1.8%
Lamkey, 1
Pork (saussages)
Sodium lactate keeps red color unaltered at concentration of 2 to 3%
Brewer, 19
Pork (saussages)
Sodium lactate does not alter color
Brewer, 19
Color, look
Pork (saussages)
Potassium lactate does not alter color at concentration of 1.2%
Bradford, 1
Pork (minced)
Sodium lactate keeps red color at concentrations of 2 to 3%
Brewer, 19
Beef
Sodium lactate improves flavor
Papadopou 1991
Beef (minced)
Potassium lactate does not alter flavor at concentrations of 2 to 3%
Egbert, 19
Pork (saussages)
Sodium lactate gives a less acidic and more salted taste at concentrations of 2 to 3%
Brewer, 19
Pork (saussages)
Sodium lactate prevents the weakening of flavor for concentrations of 1 to 2%. At 3% it increases it substantially. The more concentration increases, the more salted taste. Bitter and acidic tastes are not altered
Brewer, 19
Pork (saussages)
Potassium lactate does not alter flavor for concentration of 1.2%
Bradford, 1
Pork (minced)
Sodium lactate emphasizes flavor up to 3% concentration. Salted taste of sodium lactate is not as much as the one of sodium chloride
O'Connor
Pork (saussages)
Sodium lactate slows the appearance of unwanted odors
Lamkey, 1
Pork (saussages)
Sodium lactate prevents unwanted odors for concentrations of 2 to 3%
Brewer, 19
Flavor
Odor, aroma
Table 2: Impact of sodium and potassium lactates on the organoleptic profile of foodstuffs. ♠ back to top
Industrial Production
As explained earlier, numerous micro-organisms produce lactic acid as the main metabolite (homofermentation) or mixed with other by-products (heterofermentation). The utilization of these strains is also the sole industrial way to produce large quantities of this acid because the chemical synthesis process inevitably produces a racemic mix of the two optical isomers in equal volumes. Currently, one uses equally well the fungi (Rhyzopus Oryzae) and bacteria to make lactic acid. Not going into detailed analysis of the numerous research work performed to modelize lactic fermentation, it is however useful to describe its basic operation because it has to do with certain important technological choices for the lactic acid industry. Indeed, lactic acid is secreted during the cellular growth as well as when it has stopped (stationary phase) as shown by the Luedeking and Piret (1959) kinetic model whose equation is given below :
This equation clearly shows that the lactic acid production velocity, or instant productivity (δP/δt) is proportional to the instant growth speed (δX /δt) but also to the cell population (X) growing or not. Factors α and β are specific constant coefficients for the strain under consideration for temperature and pH established conditions. This model has been modified in several instances (Friedman, 1970 ; Hanson, 1972 ; Rogers, 1978 ; Jorgensen, 1987). In addition, a very complete model was suggested by Aborhey and Williamson (1977) which takes into , among others, the lactic acid inhibition. If, at present, the industry still operates batch fermentation systems, rather well described by kinetic models mentioned hereabove, numerous research workers have investigated continuous fermentation techniques to attempt to shorten non-productive operations which are inevitably related to batch systems (filling, lag phase, emptying,…). Continuous systems make use of different processes such as the immobilization or inclusion of entire micro-organisms in or at the surface of a substructure (Stenroos, 1982 ; Boyaval, 1985 ; Roy, 1986), cell recycling using centrifugation or making use of membrane techniques (ultra-filtration, micro-filtration)(Prigent, 1984 ; Boyaval, 1988 ; de Raucourt, 1989). Knowing that the α constant in the Luedeking-Piret model is generally much larger than the β constant, it is of interest to operate a system with growth kept constant. In addition the β . X factor pleads in favour of a system maximizing cell density. Both these observations are acknowledged when one adopts lactic acid production in chemostat coupled with a cell recycling system which is thus preferred to immobilized systems in which inhibition problems are more important. On the other hand, lactic acid can be purified either by precipitation of metal lactates followed by a neutralyzing reaction with sulfuric acid (Maesato, 1985) or by esterification with alcohol, distillation and hydrolysis of the formed ester (Boroda, 1966), or by electro-dialysis (Jacquement, 1968). A more recent purification process is to extract lactic acid by liquid-liquid extraction making use of at least one organic solvent not miscible with water in the presence or not of at least one Lewis base such as a ternary amine for instance. With this process, lactic acid must be recovered in a second step with a liquid-liquid backextraction. This step allows to re-transfer the lactic acid to water (Baniel, 1972 ; Baniel, 1980). Finally, lactic acid in acid- and/or ammonium lactate or metal lactate-form may be purified by processing it on cationic and/or anionic ion-exchange columns (Napierala, 1972 ; Maesato, 1985 ; Shkurino, 1986 ; Obara, 1987a ; Obara, 1987b ; Zelenava, 1982). It should be mentioned that these purification steps generally start with solutions of diluted lactic acid in water. As explained before, this is because the bearing molecule structure is made of an hydroxile
function and a carboxylic acid group. Indeed this double functionality is at the origin of condensation reactions producing` lactic acid oligomers. These oligomerization reactions tend towards equilibrium but are all the more probable that the temperature and concentration of the starting aqueous solution are high. This explains why lactic acid was, for a long time, thought of as a weakly volatile substance and one which can not be distilled under atmospheric pressure. In fact, lactic acid condenses to form oligomers with a high boiling point for this pressure. Certain research work on lactic acid distillation by vapour-carrying at 160-200°C show that it is possible to distillate it with yields of about 75 to 85%. However such drastic conditions are detrimental to the product quality, degradation and racemization can not be avoided. Certain variants of this theme have been suggested (Sepitka, 1962 ; Shishkini, 1977). Current industrial-scale processes are however based on esterification followed by distillation and hydrolysis as well as on liquid-liquid extraction using a ternary amine and, in a lesser way, calcium lactate crystallization with sulfuric acid acidification plus gypsum (calcium sulfate) filtration. ♠ back to top
Toxicology Most mammals, humans included, tolerate daily oral intakes of lactic acid higher than 1500 mg/kg of body weight. Values for LD50 vary from 1810 mg/kg for the Guinea pig up to more than 4800 mg/kg for the mouse (WHO, 1967). Clinical signs of a toxic dose of lactic acid are, among others, the subject’s excitation state and tachycardy. Various studies show that no lactic acid accumulation is recorded in the rat fed with 1 to 2 g/kg of body weight during 14 to 16 days. In the same way, dogs perfectly tolerate oral doses of 600 to 1600 mg of lactic acid per kg of body weight when ingested 42 times over a 10-weeks time-span (WHO, 1967). This absence of toxicological contra-indication explains why lactic acid and lactates have been long accepted as G.R.A.S. (Generally Recognized As Safe) food additives in the U.S. and may thus be used freely (U.S. Government, 1987). In Europe, the 95/2/CE Directive, published in the March 18th, 1995 issue of the Official Gazette, deals with the use of additives by the food industry. In the case of lactic acid, sodium, potassium and calcium lactates, the quantum satis principle is prescribed i.e. the necessary and adequate quantities may be added to reach the desired effect but not more. ♠ back to top
HISTORY OF LACTIC ACID Lactic acid, also known as 2-hydroxypropionic acid, is present in almost all forms of organized life and was probably also found in primitive life forms which appeared on Earth at the beginning (Brin, 1965). However, notwithstanding this relative abundance, lactic acid has been, for a long time, difficult to identify. As its name implies, it is in milk, or rather in curdled milk, that, in 1780, Carl Wilhelm Scheele, a Swedish chemist, found an acid that he purified by crystallizing its calcium salt. Scheele named it "Mjölksyra" – or "milk acid" – believing that it was a normal component of milk but never realizing that it was in fact the fermentation product of rancid milk (Dobbin, 1931). It is only in 1813 that Henri Braconnot of Nancy University (), who worked on the acidic components of fermented food, found a product that he named "nanceic acid", the name referring to the French town (Nancy). He was convinced that this acid was different from the one discovered by Scheele and concluded that, during fast fermentation whereby alcohol is first produced, only vinegar accumulates and nanceic acid is produced only in the slow fermentation process (Braconnot, 1813). A few years later, in 1817, Vogel, a German chemist very clearly showed that nanceic and lactic acid were absolutely identical (Holten, 1971). It was then generally accepted that lactic acid was produced by fermentation but nobody could explain how until Cogniard-Latour observed small "globules" able to multiply in a medium containing sugar while producing gas and alcohol. Then, in 1857, Pasteur, who was then advisor in a distillery experimenting with beet juice fermentation, described what he called "lactic yeast",a greyish substance, made of small globules aligned in short segmented ligaments, believed to be the originator of lactic acid ("lactic fermentation") which plays an essential part in the growth of yeast cells and in the production of alcohol ("alcoholic fermentation"). Lactic acid was not yet a widely used consumer product, but merely a pharmaceutical speciality until, 1881, when Charles Avery established the Lactate Company near Boston, U.S.A. The objective of this company was to replace the tartric acid used in bread making. However, the Avery factory faced problems and could not start production. It was finally in that the first lactic acid sales were recorded , around 1900, by Boeringher. From then on, many companies invested in lactic acid production but most of them had to stop because of their product's low quality which was unacceptable for the market. ♠ back to top
Lactic acid occurrence The lactic acid molecule is most frequently found in vegetals, in micro-organisms and in the animal kingdom where it is present as an intermediary for the carbo-hydrates and amino acids metabolism.
L(+) or D(-) Although, quantitatively speaking, the lactic acid L(+) form is certainly the one most frequently occuring in nature, the D(-) form is also present as well as, in some cases, a mix of both optical isomers in variable proportions. Lactic acid is found in virtually all tissues, physiological fluids and excretions. It is also found in human blood (1.4 µmol/ml), in sperm (4.1 µmol/ml), in sweat (4.0 – 40.0 µmol/ml), in the cerospinal fluid (1.6 µmol/ml) and in urine (0.3 µmol/ml) (Geigy, 1968).
In the human body
Although in the human body it is the L(+) form that predominates, it is by no way the only one. Indeed, in urine, the lactic acid D(-) can be detected if it has been ingested by the subject. The lactic acid D(-) metabolism occuring in the liver, it is generally recommended that this form is not added to food for children under the age of 3 because of their hepatic immaturity although physiological consequences in relation with its ingestion have not been proved (Vrese, 1990). In organisms, lactic acid L(+) is formed by a reduction reaction of pyruvic acid. This hydrogen transfer is done by a NADH-dependent enzyme, deshydrogenase lactate. The hydrophobic nature of lactate and pyruvate molecules allows free diffusion through the cell membranes which establishes a direct relation between these two substances concentration in the blood stream and the redox equilibrium of the cells' NADH/NAD+ system (Hohorst, 1965). In aerobic conditions, the energy feed to muscle cells is brought about by oxidative degradation of glucose which generates energy compounds such as triphosphate adenosin (ATP) and phosphate creatin. In anaerobic conditions, or in case of increased energy demands (intense physical exertion, for instance), the required energy is supplied by glycolysis wich induces the build up of lactic acid in the muscles. This excess acid diffuses then in the blood stream and will be absorbed in the liver and kidneys to again synthesize glucose ("gluconeogenesis") and glycogen ("glyconeogenesis"). Evidently, this metabolic path by itself is using energy produced by oxidative degradation of lactic acid. Already in 1930, Meyerhof established that the ratio between used lactic acid and oxidized lactic acid was 5 to 1 (Meyerhof, 1930). Glucose again synthesized in the liver or in kidneys is in turn fed in the blood stream which brings it to muscular cells where it is transformed into energy or lactic acid thus closing the so-called "Cori cycle" (Alpert, 1965; Brecht, 1967). Reichard et al. (1963) determined that at least 40% of the lactic pool exists as glucose and about 6% of the total glucose volume is recycled via the Cori cycle. Globally, one estimates that the average turnover of lactic acid in a adult male is about 120 to 150 g per day (FDA, 1978).
In micro-organism Lactic acid is also found in large quantitities in the micro-organism world. Several bacterial species produce large quantities of lactic acid, among which the best known are Lactobacillus, Sporolactobacillus, Enterococcus, Lactococcus, Bacillus, Streptococcus, Pediococcus, Leuconostoc or bifidobacteria which widely differ from the others by the metabolic path they go through. Bifidobacteria are, for all practical purposes, exclusively anaerobic when other bacteria are generally microaerophilic. The two lactic acid isomers may be produced by of a same bacterial specie as in the case of Lactobacillus for instance. Indeed, Lb. maltaromicus, Lb. agilis, Lb. sharpae, Lb. amylophilus, Lb. murinus produce only the L(+) acid whereas Lb. delbrueckii, Lb. vitulinus, Lb. coryniformis only produce the D(-) isomer and Lb. plantarum or Lb. helveticus are examples of simultaneous producers of both molecular forms. Ph conditions as the age of cultures may influence the L(+)/D(-) ratio (Garvie, 1967). The stereospecificity of the acid is influenced by the presence of one and/or the other specific dehydrogenase lactate which, as happens in mammal metabolism, transforms pyruvate into lactate. No hydrogenase lactate was found that would simultaneously produce both lactic isomers. Therefore, the existence in the same pure culture of the L(+) and D(-) forms shows the presence of genes of the two dehydrogenase lactates. However, an enzyme, called racemase, which transforms an isomer into the other form was observed a long time ago in certain micro-organisms such as Clostridium butylicum or Lactobacillus sake (Hiyama, 1968). Fermented substrates used to produce lactic acid are generally hexoses. In connection with the bifidobacteria fermentation, which follows a very special process, the lactic fermentation may occur following two different ways, i.e. a homolactic fermentation or a heterolactic one, both of them related
to the energy cycle of the cell and producing lactic acid respectively as the only product or lactic acid plus various co-products such as ethanol, carbon dioxide and acetate. Homolactic fermentation follows the metabolic path of glycolysis which proceeds in accordance with the EMDEN-MEYERHOF-PARNAS scheme. The general equilibrium of this path is given by following equation:
This equation shows an anaerobic and exothermic reaction whose yield is often higher than 90%. The heterolactic fermentation does not work along this path, because of the unavailablity of the specific enzymes of glucose catabolism according to Emden-Meyerhof-Parnas (aldolase and triose phosphate isomerase), but goes through oxidative degradation of pentoses phosphates allowing glucose metabolism via the hexose monophosphate shunt. In the same way, micro-organisms favouring this path ferment the D-xylose, the L-arabinose and the D-ribose to form lactic acid and acetic acid (Bernstein, 1953). The general equilibrium of glucose heterofermenting catabolism may be expressed as follows:
♠ back to top
Molecular properties Lactic acid is a weak organic acid and one of the smallest molecules with an asymetrical carbon in location α of the carboxylic function. This feature allows its existence with two different stereoisometric forms called L(+) and D(-). The plus and minus signs show the polarized light rotation, respectively to the right and to the left, an effect called dextro- and levo-rotatory. The L and D symbols show the spatial configuration of the molecule with reference to the two glyceraldehyde isomers.
When containing no water, both these optically active acids form white solid crystals which melt at significantly higher temperature (52.7 – 52.8°C) than their racemic mix containing 50% of each isomer (16.4°C) (Walter, 1988). In addition to its optical activity, the lactic acid molecule has simultaneous carboxylic and hydroxyl groups allowing it to react either as an acid or as an alcohol. This dual functionality initiates the spontaneous formation on intermolecular esters (oligomers) with discharge of water leading to a thermodynamic equilibrium subject to the solution concentration and age. The speed of this condensation reaction is related to the temperature, the possible presence of catalyst species (Lewis acid) and to the equilibrium gap of the solution under consideration. As for every esterification reaction, it is reversible by adding water (hydrolysis of formed oligomers). In addition, the formation of lactic acid oligomers during heating and concentration of the lactic acid solution make it impossible to precisely measure the boiling point of the pure product at atmospheric pressure. Lactic acid is a weak acid with a pKa of 3.86 at 25°C. However, the tendency of every monoacid to dissociate is a function of the pH of the medium as compared to the pKa of the contemplated acid. Indeed, the more the pH increases, the more the acid molecule will be dissociated. On the other hand, with the same pH, different acids will be more dissociated if their pKa is low as can be seen on fig. 1. We will see, later in this chapter, that such considerations are of prime importance because the bacteriostatic or bactericide potential is larger when the acid is protonized, i.e. not dissociated.
Variation of the % of protonized molecules as a function of pH for some monoacids. ♠ back to top
Thermodynamic characteristics of lactic acid Dissociation constant (Ka) :
0.000137 (at 25°C) Heat of dissociation (ΔH) :
- 63 cal/mol (at 25°C) Free energy of dissociation (ΔF) :
~5000 cal/mol Heat of solution (ΔH) :
+ 1868 cal/mol (for crystalline L(+) lactic acid at 25°C) Heat of dilution (ΔH) :
-1000 cal/mol (for dilution with a large volume of water) Heat of fusion (ΔH) :
+ 2710 cal/mol (for racemic lactic acid) + 4030 cal/mol (for L(+) lactic acid) Entropy of solution (ΔS) :
+ 6.2 cal/mol/°C Entropy of dilution (ΔS) :
- 3.6 cal/mol/°C Entropy of fusion (ΔS) :
+ 9.4 cal/mol/°C (for racemic lactic acid) + 12.2 cal/mol/°C (for L(+) lactic acid) Heat of combustion (ΔHc0) :
- 321220 cal/mol (for crystalline L(+) lactic acid at 25°C) - 325600 cal/mol (for liquid racemic lactic acid at 25°C) Heat of formation (ΔHf0) :
-
165890 163000 164020 164080
cal/mol cal/mol cal/mol cal/mol
(for (for (for (for
crystalline L(+) lactic acid at 25°C) liquid lactic acid) lactic acid in dilute solution) dissociated and diluted lactic acid)
Heat capacity () :
0.338 cal/g/°C (for crystalline lactic acid at 25°C) 0.559 cal/g/°C (for liquid lactic acid at 25°C) Absolute entropy (S0) :
34.0 cal/mol/°C (for crystalline L(+) lactic acid at 25°C) 45.9 cal/mol/°C (for liquid racemic lactic acid at 25°C) Entropy of formation (ΔSf0) :
- 137.2 cal/mol/°C (for crystalline lactic acid at 25°C) - 125.3 cal/mol/°C (for liquid lactic acid at 25°C) Free energy of formation (ΔFf0) :
- 124980 cal/mol (for crystalline L(+) lactic acid at 25°C) - 126500 cal/mol (for liquid racemic lactic acid at 25°C) (Holten, 1971). ♠ back to top
Antimicrobial activity The purpose of an antimicrobial agent is to reduce or even to completely eliminate all microbial activity. This can be achieved either by inhibitting the contamining micro-organisms (bacteriostatic and fungistatic action) or by killing these micro-organisms (bactericidal and fungicidal action). For the food industry, an antimicrobial processing is normally considered at two stages. First, food decontamination, in particular cattle carcasses at slaughterhouse, and, second, shelf-life increase of fresh or semi-processed foodstufff. Consumers, aware of this shelf-life notion, will only accept food based on poorly objective considerations like their visual appearance and color or their organoleptic properties (odor, flavor). Indeed, foodstufff with extended shelf-life obtained by vacuum packaging may not be consumed because of its aspect, odor or flavor as the lack of oxygen creates anaerobic conditions changing the food bacterial flora and, in certain cases, may even provide growth potential for pathogenic contaminants. A well-designed packaging is thus often only part of the solution to ensure that the integrity of the foodstufff remains unaffected over a given period of time. Such considerations, leading to the stengthening of hygienic conditions in food processing, are justified by the extensive consumption of processed food by developed countries consumers which commands the increasingly strict application of GMP rules (Good Manufacturing Practices) and HAC (Hazard Analysis of Critical Control Points).
Although other inhibition mechanisms may be considered (Shelef, 1994), the two main ones are : (a) lactic acid capacity to reduce the medium’s pH (Freese, 1973; Hunter, 1973; Salmond, 1984) and (b) the effect of lactates – mainly sodium lactate – on water activity in foodstufffs with intermediate humidity (Debevere, 1989 ; Loncin, 1975).
Acting on intracellular pH The anti-microbial activity is larger in the presence of organic acids than inorganic ones. It is also well known that inhibition increases with the medium pH decrease. This is explained by the hydrophobic feature of most organic acids, often called "volatile fatty acids", which allows free diffusion of the protonied form through the cell's membrane. As, generally, the intracellular pH is higher than the extracellular one, the acid undergoes dissociation as soon as it enters the cell's cytoplasm and then decreases the intracellular pH by the proton release phenomenon. This dissociation effect is described, for a monoacid such as lactic acid, by the relation shown hereunder which shows the ratio between the dissociated form ([A-]) and the non-dissociated one (or protonized form, [HA]) as an exponential function of the neighbouring pH.
In order to counter the decrease of its cytoplasmic pH, resulting from the ionisation of the entered acid, the cell allocates the main part of its energy content to eliminate these newly formed protons which results in slower growth kinetics (Cassio, 1987; Ten Brink, 1980; Ten Brink, 1985). Preceeding reflexions take only into the inhibition effect of the proton on microbial growth. However, various studies showed that bacteria growth is affected by the anion as well as by the proton (Eklund, 1983) and that lactate anion acts itself differently from other anions produced by fermentative processes such as propionate, sorbate or acetate. Additional weight is being given to this in the case of non-fermented foodstuffs which pH is ordinarily close to neutrality; it gives a ratio between dissociated and not-dissociated forms which is clearly in favor of the non-dissociated form. In addition, for lactic acid salts such as sodium and potassium lactates, an inhibition phenomenon can not be assigned to the acid's dissociation and to the subsequent production of protons as these are entirely dissociated in the aqueous phase.
Acting on water activity When certain studies, such as the Loncin (1975) one, state that sodium lactate would have a more marked deflationary effect on water activity (aw) than is the case for other organic acids and of sodium
chloride with equal concentrations, Chirife and Ferro Fontan (1980) reported a much lesser effect. Indeed, according to the latter, sodium lactate decreases water activity less than sodium chloride, but this effect would however be higher than the one obtained from tartarate or sodium citrate at equal concentrations. Houstma et al. (1983) reports that sodium lactate and chloride have exactly the same effect for concentrations ranging from 0.089 to 0.89 mol/L. If the sodium lactate deflationary effect on water activity seems thus more or less equivalent to the one of sodium chloride, it is however necessary to consider that, according to Houstma et al. (1993) the inhibition effect of sodium lactate is more intense than for the corresponding chloride which tends to confirm the specific inhibition effect of the lactate ion on cell growth. Indeed, from this vast study of gram-positive, gram-negative bacteria and yeasts, it appears that the sodium lactate minimal inhibition concentration is far lower than the one of chloride with the exception of Campylobacter jejuni, Trichosporon beigelii and Torulaspa delbrueckii strains.
Other effects Various effects other than those already mentioned have been reported. Hydroxycarboxylic acids are known for their chelating properties and lactic acid, in particular, is commonly used by the food industry for this purpose as are polyphosphates, citric and ethylenediaminetetracetic (EDTA) acids (Kabara, 1991). Iron chelation, was, for instance, shown as a possible contribution to the antilisterial effect of lactate (Shelef, 1994) and to the stabilization of food lipides by the antioxidizing effect obtained (Nnanna, 1994).
Comparing lactate and other organic acids Lactic acid is generally thought of as less efficient than sorbic, acetic, propionic and benzoic acids, for example (Baird-Parker, 1980; Minor, 1970; Chung, 1970). Based only on its pKa, one may indeed expect that lactic acid displays a higher pH deflationary effect but a more limited growth inhibition effect as, for a given pH, lactic acid will be more ionized than the other acids even when it has been demonstrated that the inhibitor form is the protonized form (not ionized) of the molecule (see supra). Such considerations must however be accepted with due carefulness as testified by Brackett (1987) who shows following classification as a function of the type of effect observed on the growth of Yersinia enterocolitica : (a) antimicrobial activity based on pH : propionic > lactic > acetic > citric; (b) antimicrobial activity based on the concentration of the non-dissociated form (for monoprotic acids only) : lactic > propionic > acetic; (c) antimicrobial activity based on equal molar concentrations : citric > lactic > propionic > acetic. The author concluded that, based on statistics, there are no significant differences between these acids. In addition, numerous publications report synergetic effects between organic acids mentioned above and, more particularely, for acetic and lactic acids which therefore should be used concominantly (Rubin, 1978; Adams, 1988; Dickson, 1992). Various mixes of lactic acid, citric acid and potassium sorbate showed synergy either by slowing the growth of Salmonella, Yersinia enterocolitica, Pseudomonas fluorescens, two strains of lactic bacteria (Restaino, 1981) and four strains of osmophilic yeasts (Restaino, 1982) or by preventing entirely their growth. Processed product
Effect
Reference
Veal
Discoloration at concentrations higher than 1.25%
Woolthuis,
Veal
Discoloration at concentrations higher than 1.25%
Snijders, 1
Veal (head)
No effect on color at concentration of 1%
Cudjoe, 19
Color, look
Veal (tongue)
Improvement of look at concentration of 2%
Visser, 198
Beef
No effect on color at concentration of 2%
Hamby, 19
Beef
No effect on color at concentration of 2%
Prasai, 199
Beef
Discoloration at concentrations higher than 1%
Snijders, 1
Pork
Discoloration at concentrations higher than 1.5%
Pork
Color is darker and less red with a mix of 1% lactic acid and 1% acetic acid
Mendonca,
Chicken
Color is lighter at concentration ranging from 1 to 5%
Van Der M 1989
Chicken
Look deteriorates at concentration of 3%
Kolsarici, 1
Crayfish
No effect on color at concentration of 1%
Dorsa, 199
Catfish
Look deteriorates at concentration higher than 2%
Marshall, 1
Veal
Little effect on flavor at concentrations less than 2%
Woolthuis,
Veal
No effect on flavor at concentrations less than 2%, Altering of flavor at concentration of 4%
Snijders, 1
Veal (tongue)
No effect on flavor at concentration of 2%
Visser, 198
Beef
No effect on flavor at concentration of 1%
Acuff, 1987
Beef
No effect on flavor at concentration of 1%
Dixon, 198
Beef
Improvement of flavor at concentration of 1%
Hamby, 19
Chicken
No effect on flavor at concentration ranging from 1 to 5%
Van Der M 1989
Pork
No effect on flavor at concentration less than 2%
Epling, 199
Crayfish
No effect on flavor at concentration of 1%
Dorsa, 199
Beef
Improvement of aroma for concentration of 1%
Prasai, 199
Chicken
Aroma deteriorates for concentration of 3%
Kolsarici, 1
Catfish
Odor deterirates for concentrations higher than 2%
Marshall, 1
Flavor
Odor, aroma
Table 1: impact of lactic acid on organoleptic profile of foodstuffs. Processed product
Effect
Reference
Beef
Sodium lactate emphasizes red color (specially with 2% concentration)
Papadopou 1991
Beef (minced)
Sodium lactate does not alter color at concentration of 2 to 3%
Egbert, 19
Pork (saussages)
Sodium lactate improves color for concentration of 1.8%
Lamkey, 1
Pork (saussages)
Sodium lactate keeps red color unaltered at concentration of 2 to 3%
Brewer, 19
Pork (saussages)
Sodium lactate does not alter color
Brewer, 19
Color, look
Pork (saussages)
Potassium lactate does not alter color at concentration of 1.2%
Bradford, 1
Pork (minced)
Sodium lactate keeps red color at concentrations of 2 to 3%
Brewer, 19
Beef
Sodium lactate improves flavor
Papadopou 1991
Beef (minced)
Potassium lactate does not alter flavor at concentrations of 2 to 3%
Egbert, 19
Pork (saussages)
Sodium lactate gives a less acidic and more salted taste at concentrations of 2 to 3%
Brewer, 19
Pork (saussages)
Sodium lactate prevents the weakening of flavor for concentrations of 1 to 2%. At 3% it increases it substantially. The more concentration increases, the more salted taste. Bitter and acidic tastes are not altered
Brewer, 19
Pork (saussages)
Potassium lactate does not alter flavor for concentration of 1.2%
Bradford, 1
Pork (minced)
Sodium lactate emphasizes flavor up to 3% concentration. Salted taste of sodium lactate is not as much as the one of sodium chloride
O'Connor
Pork (saussages)
Sodium lactate slows the appearance of unwanted odors
Lamkey, 1
Pork (saussages)
Sodium lactate prevents unwanted odors for concentrations of 2 to 3%
Brewer, 19
Flavor
Odor, aroma
Table 2: Impact of sodium and potassium lactates on the organoleptic profile of foodstuffs. ♠ back to top
Industrial Production
As explained earlier, numerous micro-organisms produce lactic acid as the main metabolite (homofermentation) or mixed with other by-products (heterofermentation). The utilization of these strains is also the sole industrial way to produce large quantities of this acid because the chemical synthesis process inevitably produces a racemic mix of the two optical isomers in equal volumes. Currently, one uses equally well the fungi (Rhyzopus Oryzae) and bacteria to make lactic acid. Not going into detailed analysis of the numerous research work performed to modelize lactic fermentation, it is however useful to describe its basic operation because it has to do with certain important technological choices for the lactic acid industry. Indeed, lactic acid is secreted during the cellular growth as well as when it has stopped (stationary phase) as shown by the Luedeking and Piret (1959) kinetic model whose equation is given below :
This equation clearly shows that the lactic acid production velocity, or instant productivity (δP/δt) is proportional to the instant growth speed (δX /δt) but also to the cell population (X) growing or not. Factors α and β are specific constant coefficients for the strain under consideration for temperature and pH established conditions. This model has been modified in several instances (Friedman, 1970 ; Hanson, 1972 ; Rogers, 1978 ; Jorgensen, 1987). In addition, a very complete model was suggested by Aborhey and Williamson (1977) which takes into , among others, the lactic acid inhibition. If, at present, the industry still operates batch fermentation systems, rather well described by kinetic models mentioned hereabove, numerous research workers have investigated continuous fermentation techniques to attempt to shorten non-productive operations which are inevitably related to batch systems (filling, lag phase, emptying,…). Continuous systems make use of different processes such as the immobilization or inclusion of entire micro-organisms in or at the surface of a substructure (Stenroos, 1982 ; Boyaval, 1985 ; Roy, 1986), cell recycling using centrifugation or making use of membrane techniques (ultra-filtration, micro-filtration)(Prigent, 1984 ; Boyaval, 1988 ; de Raucourt, 1989). Knowing that the α constant in the Luedeking-Piret model is generally much larger than the β constant, it is of interest to operate a system with growth kept constant. In addition the β . X factor pleads in favour of a system maximizing cell density. Both these observations are acknowledged when one adopts lactic acid production in chemostat coupled with a cell recycling system which is thus preferred to immobilized systems in which inhibition problems are more important. On the other hand, lactic acid can be purified either by precipitation of metal lactates followed by a neutralyzing reaction with sulfuric acid (Maesato, 1985) or by esterification with alcohol, distillation and hydrolysis of the formed ester (Boroda, 1966), or by electro-dialysis (Jacquement, 1968). A more recent purification process is to extract lactic acid by liquid-liquid extraction making use of at least one organic solvent not miscible with water in the presence or not of at least one Lewis base such as a ternary amine for instance. With this process, lactic acid must be recovered in a second step with a liquid-liquid backextraction. This step allows to re-transfer the lactic acid to water (Baniel, 1972 ; Baniel, 1980). Finally, lactic acid in acid- and/or ammonium lactate or metal lactate-form may be purified by processing it on cationic and/or anionic ion-exchange columns (Napierala, 1972 ; Maesato, 1985 ; Shkurino, 1986 ; Obara, 1987a ; Obara, 1987b ; Zelenava, 1982). It should be mentioned that these purification steps generally start with solutions of diluted lactic acid in water. As explained before, this is because the bearing molecule structure is made of an hydroxile
function and a carboxylic acid group. Indeed this double functionality is at the origin of condensation reactions producing` lactic acid oligomers. These oligomerization reactions tend towards equilibrium but are all the more probable that the temperature and concentration of the starting aqueous solution are high. This explains why lactic acid was, for a long time, thought of as a weakly volatile substance and one which can not be distilled under atmospheric pressure. In fact, lactic acid condenses to form oligomers with a high boiling point for this pressure. Certain research work on lactic acid distillation by vapour-carrying at 160-200°C show that it is possible to distillate it with yields of about 75 to 85%. However such drastic conditions are detrimental to the product quality, degradation and racemization can not be avoided. Certain variants of this theme have been suggested (Sepitka, 1962 ; Shishkini, 1977). Current industrial-scale processes are however based on esterification followed by distillation and hydrolysis as well as on liquid-liquid extraction using a ternary amine and, in a lesser way, calcium lactate crystallization with sulfuric acid acidification plus gypsum (calcium sulfate) filtration. ♠ back to top
Toxicology Most mammals, humans included, tolerate daily oral intakes of lactic acid higher than 1500 mg/kg of body weight. Values for LD50 vary from 1810 mg/kg for the Guinea pig up to more than 4800 mg/kg for the mouse (WHO, 1967). Clinical signs of a toxic dose of lactic acid are, among others, the subject’s excitation state and tachycardy. Various studies show that no lactic acid accumulation is recorded in the rat fed with 1 to 2 g/kg of body weight during 14 to 16 days. In the same way, dogs perfectly tolerate oral doses of 600 to 1600 mg of lactic acid per kg of body weight when ingested 42 times over a 10-weeks time-span (WHO, 1967). This absence of toxicological contra-indication explains why lactic acid and lactates have been long accepted as G.R.A.S. (Generally Recognized As Safe) food additives in the U.S. and may thus be used freely (U.S. Government, 1987). In Europe, the 95/2/CE Directive, published in the March 18th, 1995 issue of the Official Gazette, deals with the use of additives by the food industry. In the case of lactic acid, sodium, potassium and calcium lactates, the quantum satis principle is prescribed i.e. the necessary and adequate quantities may be added to reach the desired effect but not more. ♠ back to top
Related Documents 171j1w
History Of Lactic Acid 4b2r6x
November 2019 36
Assay Of Lactic Acid 622723
November 2019 58
Estimation Lactic Acid 5q154x
December 2019 34
Lactic Acid Industrial Production 163s6o
December 2019 35
Lactic Acid Bacteria Project 252z68
December 2019 38