Lactic Acid is an organic acid found in nature and in many foods and milk sugars. It is produced by the metabolism of the human body and is formed when there is a situation of oxygen deficiency and sugar consumption. For example during muscular exertion and anaerobic respiration, the process by which cells produce energy without oxygen around.
The name describes the structure of the molecule:
- Lactic comes from the Latin word 'lactis', meaning milk. This is because lactic acid was first isolated from sour milk.
- Acid indicates that the compound can donate a proton (H+) in a solution.
It is generated by pyruvic acid under anaerobic conditions in skeletal muscles, the brain, red blood cells and the kidney. It is eliminated in a relatively short time, from a few minutes to a few hours. Lactate (produced from lactic acid) disappears very quickly at a rate of 320mmol/l/h, mostly through liver metabolism and in reconversion lactate reverts to pyruvate (1).
Industrially, it is obtained by fermentation from pyruvate using lactate dehydrogenase. It is produced by fermentation with Lactobacillus plantarum, a micro-organism widely used in the food industry, which can produce quantities of lactic acid in a lower pH environment. Another bacterium, Pediococcus pentosaceus, helps with fermentation.
The synthesis process takes place in different steps:
- Fermentation. Lactic acid is most commonly produced through the fermentation of carbohydrates (such as glucose, sucrose or lactose) by lactic acid bacteria. Carbohydrates are broken down into pyruvate, which is then converted into lactic acid.
- Chemical synthesis. Lactic acid can also be chemically synthesised from acetaldehyde and hydrogen cyanide, followed by hydrolysis.
Lactic acid comes in the form of a colourless to yellow odourless syrupy liquid or as a fine white powder.
What it is used for industrially and where it is used
Food
In foodstuffs it is labelled in the European list of food additives with the number E270 and has an acidifying and antibacterial function. It is used as an acidity corrector, antioxidant in meat, wine, beverages, confectionery, vegetables (onion, olive, cucumber), marinated and canned foods, grain processing, fruit preservation.
As a pH regulator it maintains the colour of food, improves the quality of the product in which it is incorporated and enhances taste.
Medical
Lactic acid is a pharmaceutical intermediate used in the production process of pharmaceuticals and medicines such as erythromycin and infusion therapy. The antifungal property of lactic acid, and in particular its inhibition of mycotoxigenic fungi, depends on the ability of the strain to produce specific organic acids, and these acids may differ from strain to strain (2).
This study summarises the natural antimicrobial compounds produced by lactic acid bacteria that are responsible for maintaining safe and stable food products and highlights a wide range of potential applications (3).
In dermatology, it is used in the same way as other alpha-hydroxy acids (citric acid, glycolic acid, tartaric acid, malic acid) to combat the appearance of keratoses and acne (4).
Cosmetics
- Buffering agent. It is an iingredient that can bring an alkaline or acid solution to a certain pH level and prevent it from changing, in practice a pH stabiliser that can effectively resist instability and pH change.
- Humectant. Hygroscopic compound used to minimise water loss in the skin and to prevent it from drying out by facilitating faster and greater absorption of water into the stratum corneum of the epidermis. The epidermis is the most superficial of the three layers that make up human skin (epidermis, dermis and hypodermis) and is the layer that maintains hydration in all three layers. In turn, the epidermis is composed of five layers: horny, the most superficial, granular, spinous, shiny, and basal. Humectants have the ability to retain the water they attract from the air in the stratum corneum and have the function of moisturising the skin. They are best used before emollients, which are oil-based.
- Skin conditioning agent. It is the mainstay of topical skin treatment as it has the function of restoring, increasing or improving skin tolerance to external factors, including melanocyte tolerance. The most important function of the conditioning agent is to prevent skin dehydration, but the subject is rather complex and involves emollients and humectants that can be added in the formulation.
Lactic acid is able to maintain the pH value between 4 and 5 and as a wetting agent, is an organic acid which is part of the alpha hydroxy acids recently used as surface peeling agents.
A polymer of lactic acid of synthetic origin, poly-l-lactic acid has been shown to improve facial wrinkles and is used as a filler for those who do not want to undergo cosmetic surgery (5).
Tanning industry
Lactic acid is used in the degreasing of hides and in their decalcination, i.e. the process of removing the lime attached to the leather fibres. Lactic acid improves the quality of degreasing as it is able to lower the pH.
Textile industry
Lactic acid reduces the chromate content of wool and prevents oxidation of the fibre. In silk and rayon finishing, it is used as a pH regulator to lighten and increase the colour of the yarn.
Other uses :
- Beekeeping. As an antidote to Varroa, a parasitic mite that attacks bee larvae.
- Agriculture. As a herbicide
- Animal husbandry. To inhibit bacteria and effectively increase the weight of livestock.
- Beer. To inhibit bacteria, promote yeast development, enhance flavour and improve beer yield.
For more information:
Typical optimal characteristics of the commercial product Lactic acid
| Appearance | Colourless to yellow liquid |
| Assay | 88.3% |
Boiling Point
| 227.6±0.0 °C at 760 mmHg |
Melting Point
| 18ºC |
| Density | 1.3±0.1 g/cm3 |
| Stereochemical | ≥98% |
Residue on ignition
| < 0.1% |
Heavy Metal
| <10mg/kg |
| Lead | < 0.5mg/kg |
| Cyanide | < 5mg/kg |
| Chloride | < 0.1% |
| Arsenic | < 3mg/kg |
| Sulfate | < 0.25% |
| Iron | < 10mg/kg |
| Safety |  |
- Molecular Formula: C3H6O3
- Linear Formula CH3CH(OH)COOH
- Molecular Weight: 90.078 g/mol
- Exact Mass 90.031693
- UNII: 3B8D35Y7S4
- CAS: 50-21-5 152-36-3 598-82-3 31587-11-8 92129-90-3 1104201-86-6 1148011-07-7 1384255-85-9 947236-64-8
- EC Number: 200-018-0 295-890-2 209-954-4 608-645-5
- DSSTox Substance ID DTXSID7023192
- IUPAC 2-hydroxypropanoic acid
- InChI=1S/C3H6O3/c1-2(4)3(5)6/h2,4H,1H3,(H,5,6)
- InChl Key JVTAAEKCZFNVCJ-UHFFFAOYSA-N
- SMILES CC(C(=O)O)O
- MDL number MFCD00004520
- PubChem Substance ID 24901202
- ChEBI 78320
- ICSC 0501
- FEMA 2611
- RTECS OD2800000
- UN 3265
- NCI C80130
- NSC 367919
- FEMA Number: 2611
- Beilstein Registry Number 1209341
- Council of Europe no. 4c
- Flavis number 8.004
Synonyms:
- DL-Lactic acid
- 2-hydroxypropanoic acid
- 2-hydroxypropionic acid
- Milk acid
- Polylactic acid
- lactate
- 1-Hydroxyethanecarboxylic acid
- Aethylidenmilchsaeure
- alpha-Hydroxypropionic acid
- (+-)-2-Hydroxypropanoic acid
- Tonsillosan
- Acidum lacticum
- Racemic lactic acid
- Propanoic acid, 2-hydroxy
- (+/-)-Lactic acid
- Ethylidenelactic acid
- Lactovagan
References_______________________________________________________________
(1) Pundir CS, Narwal V, Batra B. Determination of lactic acid with special emphasis on biosensing methods: A review. Biosens Bioelectron. 2016 Dec 15;86:777-790. doi: 10.1016/j.bios.2016.07.076.
(2) Guimarães A, Venancio A, Abrunhosa L. Antifungal effect of organic acids from lactic acid bacteria on Penicillium nordicum. Food Addit Contam Part A Chem Anal Control Expo Risk Assess. 2018 Sep;35(9):1803-1818. doi: 10.1080/19440049.2018.1500718.
(3) Ibrahim SA, Ayivi RD, Zimmerman T, Siddiqui SA, Altemimi AB, Fidan H, Esatbeyoglu T, Bakhshayesh RV. Lactic Acid Bacteria as Antimicrobial Agents: Food Safety and Microbial Food Spoilage Prevention. Foods. 2021 Dec 17;10(12):3131. doi: 10.3390/foods10123131.
(4) Tang SC, Yang JH. Dual Effects of Alpha-Hydroxy Acids on the Skin. Molecules. 2018 Apr 10;23(4):863. doi: 10.3390/molecules23040863.
(5) Breithaupt A, Fitzgerald R. Collagen Stimulators: Poly-L-Lactic Acid and Calcium Hydroxyl Apatite. Facial Plast Surg Clin North Am. 2015 Nov;23(4):459-69. doi: 10.1016/j.fsc.2015.07.007.
Lactic Acid 
Nicotinamide adenine dinucleotide 
