Lactic acid (a.k.a. 2-hydroxypropanoic acid) Cheesemakers like it, athletes don't Roderick Edmonds Eton College, UK Molecule of the Month June 2026 Also available: HTML version.

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It must have something to do with milk.

Yes. The main sugar naturally present in milk is lactose, and lactic acid is formed when Lactobacilli bacteria ferment this. That's what happens when milk goes sour, but it's also the first stage in the production of cheese and yogurt.

Lactose	Lactic acid

Plenty of lactic acid here!
[Image: Roderick Edmonds]

I can't eat yogurt. I'm lactose intolerant! So were all the adult human population, until comparatively recently. Most mammals only consume lactose as infants, from their mother's milk. After weaning, they stop producing lactase, a key enzyme needed for the digestion of lactose. Around 10,000 years ago, humans began to keep cattle, originally as a source of meat and as draft animals. Mutations in the human genome then arose which caused the gene for lactase to be expressed throughout adulthood, allowing the consumption of milk as an extra source of nutrition. The mutations became established mainly in societies with a tradition of dairy farming. At least half the world's population are lactose intolerant today; the condition is particularly prevalent in those of East Asian descent, where historically dairy farming wasn't common. But remember this is a problem with lactose, not lactic acid.

[Image: Roderick Edmonds via Microsoft Copilot]

So why don't athletes like lactic acid?

It's fine for athletes who are lactose tolerant to eat yogurt. The problem is the accumulation of lactic acid in muscle tissue through anaerobic respiration.

But I'm aerobic, not anaerobic?

For the most part, yes, and that's all to the good. Aerobic respiration involves the production of adenosine triphosphate (ATP) via oxidation of glucose. ATP is the molecule which powers all the processes your body needs to function, and you can read more about it in the MOTM article from January 1998. The exact amount of ATP produced is still unclear, but it is around 31 molecules of ATP from the oxidation of one molecule of glucose.

C₆H₁₂O₆ + 6O₂ 6CO₂ + 6H₂O (+31 ATP)

But without oxygen, the only possibilities involve anaerobic respiration:

• Instead of reducing oxygen to water, reduce something else. Bacteria in stagnant ponds and sewage reduce sulfates to foul-smelling hydrogen sulfide (see Hydrogen sulphide - MOTM March 2009).
  
  
• Break down glucose to ethanol and carbon dioxide. This is the alcoholic fermentation performed by yeasts.
  

  C₆H₁₂O₆ 2C₂H₅OH + 2CO₂ (+2 ATP)

• Fermentation to lactic acid.
  

  C₆H₁₂O₆ 2C₃H₆O₃ (+2 ATP)

• The latter is what happens in the Lactobacilli, and also in your muscles when they are using energy faster than oxygen can be supplied. This often happens towards the end of a long race, where the runners can get cramps in their muscles due to lactic acid build-up.

Human production of lactic acid
[Image: Eton College library, reproduced with permission.]

So I can respire without oxygen? Not sustainably. The problem is that only two ATP molecules are produced, instead of about 30 from aerobic respiration. That's enough for some types of bacteria, but not for anything much more complex, and certainly not enough for a human. Pain and muscle fatigue are your body's ways of telling the brain you need to slow down, take deep breaths and generate a lot more ATP by oxidising that lactic acid. The pain of angina arises in a similar way when the heart muscles are respiring anaerobically because they aren't receiving enough oxygenated blood - but that's a more serious problem.

My sports coach talks about lactate.

Although "lactic acid" and "lactate" are often used interchangeably, this is incorrect. Lactate is the negative ion left behind when lactic acid loses a proton. It's a weak acid, so an equilibrium is set up, and any solution or body tissue which contains lactic acid will also contain lactate. Lactic acid is slightly less weak (dissociates more readily) than ethanoic or propanoic acids, because internal hydrogen bonding stabilises the lactate ion.

Can you make lactic acid in the lab?

Yes. Nowadays, this is usually done through biotechnology using Lactobacilli. They can ferment sugars other than lactose, so they don't need to be fed on milk. Cane sugar or corn syrup will do.

An older route is via the reaction of hydrogen cyanide with ethanal. This reaction is often studied in A-level chemistry as an example of nucleophilic addition. Hydrolysis of the resulting 2-hydroxypropanenitrile yields lactic acid.

And it's the same as the lactic acid in my muscles?

Not exactly. Lactic acid is a chiral molecule, with two non-superimposable mirror image forms.

Reacting ethanal with hydrogen cyanide will give an equimolar mixture (racemic mixture) of the two. The lactic acid produced by anaerobic respiration in mammals is exclusively the L, or (+) form, the one shown on the left of the diagram. As for bacteria, some produce one mirror image, some produce the other, and some can make both. This suggests that the enzymes needed must have evolved more than once during the history of life.

Tell me about poly(lactic acid).

PLA, for short. It's a polymer which can be made from sustainable resources, and which can be broken down by enzymes when we dispose of it. This makes it a promising replacement for polymers such as poly(ethene) which are not biodegradable, and whose raw material is crude oil or natural gas. PLA is a polyester. It forms when the carboxylic acid group of one lactic acid molecule reacts with the alcohol group of another, to form an ester link. Many lactic acid molecules link together in this way to form a polymer chain.

So it's quite easy to make?

As with most esters, heating the starting material (just lactic acid, here) with a strong acid catalyst will produce PLA but only as a component of an equilibrium mixture. This was tried during a sixth-form research project at Eton, using concentrated sulfuric acid as the catalyst and distilling off the water to shift the equilibrium.

The result is shown in the left-hand flask in the picture below. The trouble is, sulfuric acid readily dehydrates carbohydrates to leave a black mess of carbon, and that's what appears to have resulted here. If your organic chemistry experiments ever end up looking like this, well, sometimes it just happens that way.

Results of experiments to polymerise lactic acid, using sulfuric acid catalyst (left hand flask) and phosphoric acid catalyst (right hand flask).
[Image: Roderick Edmonds]

We repeated the experiment using phosphoric instead of sulfuric acid, and obtained a much better sample of PLA (shown in the flask on the right in the picture).

Is that how it's done commercially?

The industrial method first converts the lactic acid to a cyclic dimer. This is then polymerised using a catalyst of tin(II)octanoate. No water is released during the polymerisation, so there is no need to remove it to complete the reaction.

Dimer of lactic acid. [Image: Roderick Edmonds]

What can you make with PLA?

It's used in many types of packaging and containers. The picture shows a biodegradable cup made of PLA.

PLA cup [Image: Roderick Edmonds]	PLA cup processing [Image: Roderick Edmonds via Microsoft Copilot]

PLA is a good polymer to use in 3D printing, as it has a fairly low melting temperature as well as being biodegradable. The left-hand picture below shows a 3D printer set up with two spools of PLA, one of which has a red pigment added. The right hand picture shows a vase and some storage containers made with the printer.

3D printer loaded with PLA, and some printed articles. [Image: Roderick Edmonds]	Containers made from PLA by 3D printing. [Image: Roderick Edmonds]

Are there other polymers like PLA?

Yes, other hydroxycarboxylic acids can also be polymerised. Hydroxyethanoic acid and 3-hydroxybutanoic acid are two examples. A co-polymer of lactic and hydroxyethanoic acids is used to make surgical thread which will slowly degrade inside the body so that there is no need for a follow-up procedure to remove it.

Isn't 3-hydroxybutanoic acid the same as GHB?

Not quite, but they are isomers. GHB or gamma-hydroxybutanoic acid is another name for 4-hydroxybutanoic acid. It's a recreational drug closely related to the neurotransmitter gamma-aminobutyric acid or GABA (see GABA, MOTM for July 2015).

Bibliography

• Wikipedia: Lactic acid; Poly-lactic acid; Lactose; Lactose intolerance
• The Essential Chemical Industry - online: Degradable Plastics
• The Essential Chemical Industry - online: Biotechnology-in-the-chemical-industry
• C. Higgins, L-lactate and D-lactate - - clinical significance of the difference (October 2011)
• T.P. Coultate, Food; the chemistry of its components, RSC paperbacks, 2002
• S. Cotton, Chem. Rev. 32 (2023) 10-15.
• C. Durrani, Chem. Rev. 35(2026) 28-30.
• M. Fenkes, Bio. Sci. Rev. 29 (2016)

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