Ascorbic acid is made by two fundamentally different routes depending on whether nature or a factory is doing the work. In living organisms, plants build it from simple sugars through a multi-enzyme pathway, while most animals convert glucose into it through a different chain of reactions in the liver or kidneys. Humans lost the ability to run that final reaction millions of years ago, which is why we need dietary vitamin C at all. On the industrial side, virtually all of the world’s ascorbic acid starts as glucose from corn starch and passes through a hybrid process of microbial fermentation and chemical conversion, a method that has been refined for nearly a century.
How Plants Build Vitamin C
The main production line in plants is called the Smirnoff-Wheeler pathway, named after the researchers who mapped it out in the late 1990s. It starts with glucose and runs through a series of sugar intermediates, converting them step by step until the final product, L-ascorbic acid, emerges. The pathway involves around ten enzymatic steps, and it operates in nearly every green plant studied so far. Understanding its component enzymes has implications for plant biology, nutritional science, and biotechnology, particularly in efforts to breed or engineer crops with higher vitamin C content.1PubMed. Structural insights into the Smirnoff-Wheeler pathway for vitamin C production in the Amazon fruit camu-camu
Not all plants rely exclusively on this single route, though. Over the past 25 years, researchers have found evidence for at least three alternative pathways that feed into ascorbic acid production. These involve different sugar starting materials and intermediates. Feeding experiments, gene expression analyses, and transgenic studies across multiple plant species show that these alternative routes contribute to vitamin C levels and can even help plants tolerate environmental stresses like salt, heat, and drought.2Journal of Experimental Botany. Alternative pathways leading to ascorbate biosynthesis in plants: lessons from the last 25 years Some of these alternative intermediates overlap with the pathway animals use, which suggests that parts of the animal route may also operate in plants.3Trends in Plant Science. How Is Ascorbic Acid Made? From Nature to Industry
The practical takeaway is that vitamin C content in fruits and vegetables is not just a matter of one genetic switch being turned up or down. Multiple biosynthetic routes feed into the final pool, and environmental conditions like sunlight, temperature, and soil quality shift how much each pathway contributes. That is why the same variety of pepper or strawberry can have quite different vitamin C levels depending on where and how it was grown.
How Animals Make It, and Why Humans Cannot
Most vertebrates synthesize their own ascorbic acid through a pathway that converts glucose to vitamin C in the liver (for mammals) or kidneys (for reptiles and some birds). The animal pathway is chemically distinct from the main plant route. Its final two steps are handled by the enzyme regucalcin for the penultimate reaction and L-gulonolactone oxidase, often shortened to GULO, for the last step.4PubMed Central. The evolution of vitamin C biosynthesis and transport in animals A goat, for instance, can churn out grams of ascorbic acid per day entirely on its own. Dogs, cats, cows, and most other mammals do the same.
Humans are among the exceptions. So are other higher primates, guinea pigs, and certain bat species. In every case studied, the inability to make vitamin C traces to mutations in the GULO gene, which has accumulated insertions, deletions, and premature stop codons that render it nonfunctional.5PubMed. Conserved or lost: molecular evolution of the key gene GULO in vertebrate vitamin C biosynthesis The gene is still there in our DNA as a pseudogene, a kind of molecular fossil. It simply no longer produces a working enzyme.6PubMed Central. The genetics of vitamin C loss in vertebrates
The prevailing explanation for why this loss persisted is dietary compensation. Ancestral primates likely ate fruit-rich diets that supplied plenty of vitamin C, so there was little survival pressure to maintain a working GULO gene. Over millions of years, the gene accumulated damage that was never repaired. The result is that humans, unlike most of the animal kingdom, are completely dependent on food or supplements for their vitamin C.
Fungi and Microalgae Take a Different Approach
Plants and animals are not the only organisms that make ascorbic acid or its close relatives. Fungi have their own version of the story, though it comes with a twist. Most ascomycete and basidiomycete fungi do not produce L-ascorbic acid at all. Instead, they make a closely related molecule called D-erythroascorbic acid, using a pathway that appears to have evolved in the common ancestor of fungi.7PubMed. Characterisation and biosynthesis of D-erythroascorbic acid in Phycomyces blakesleeanus D-erythroascorbate is structurally similar enough to L-ascorbate that it likely fills similar antioxidant roles inside fungal cells, but it is a distinct compound with different biological activity in mammals.
Some microalgae, meanwhile, do produce genuine L-ascorbic acid. This matters for aquatic food webs and also for biotechnology researchers exploring whether microalgal fermentation could become a commercially viable source of vitamin C.
The Industrial Process
Given that humans cannot make their own vitamin C and that extracting it from fruit would be absurdly expensive at scale, the world’s supply comes almost entirely from industrial synthesis. Global production runs to roughly 150,000 to 200,000 metric tons per year, with the vast majority manufactured in China. The process that made large-scale production possible was developed in the 1930s by Tadeus Reichstein and is still the foundation of modern manufacturing, though it has been significantly updated.
The classic Reichstein process starts with D-glucose, typically derived from corn starch. The glucose is first hydrogenated to D-sorbitol using a metal catalyst. Then comes the step that makes the whole process economically viable: a bacterium, traditionally Gluconobacter oxydans, ferments the sorbitol into L-sorbose. This microbial step replaced what would otherwise be a difficult and expensive chemical oxidation. From L-sorbose, a series of chemical reactions, including protection of certain hydroxyl groups, oxidation, and acid-catalyzed rearrangement, converts the molecule into 2-keto-L-gulonic acid, which is then easily converted to L-ascorbic acid through lactonization.
The modern variant used in most Chinese factories extends the biological portion further. Rather than relying on chemistry to get from sorbose to the key intermediate, a second fermentation step uses a consortium of bacteria. In this two-step fermentation, Gluconobacter oxydans handles the first conversion of sorbitol to sorbose, and then a co-culture of Ketogulonicigenium vulgare and Bacillus megaterium converts sorbose into 2-keto-L-gulonic acid.8PubMed Central. Microbial Interactions in a Vitamin C Industrial Fermentation System: Novel Insights and Perspectives The B. megaterium does not actually produce the intermediate itself but supports the growth and productivity of K. vulgare by supplying nutrients and growth factors. The final chemical step, converting 2-keto-L-gulonic acid to ascorbic acid, remains straightforward chemistry.
Toward One-Step Fermentation
The holy grail of vitamin C manufacturing would be a single microorganism that takes in glucose and spits out ascorbic acid directly, skipping both the intermediate chemical steps and the need for bacterial consortia. Researchers have been working toward this for years using metabolic engineering, and recent progress is promising if not yet commercially competitive.
One approach reconstructed the entire ten-gene vitamin C biosynthesis pathway from the plant Arabidopsis thaliana inside the bacterium Escherichia coli. The engineered strain produced vitamin C directly from glucose, reaching at least 1.53 milligrams per liter in shake flask cultures.9PubMed Central. Metabolic engineering of Escherichia coli for direct production of vitamin C from D-glucose That yield is far below what a conventional industrial fermenter achieves, but it represents a proof of concept: you can transplant the plant pathway into a microbe and get it to work.
A parallel effort did something similar in baker’s yeast (Saccharomyces cerevisiae), reconstructing a vitamin C pathway and applying protein engineering and metabolic engineering strategies to improve output.10PubMed Central. One-Step Biosynthesis of Vitamin C in Saccharomyces cerevisiae Yeast has some practical advantages over E. coli for industrial fermentation, including tolerance to acidic conditions and established large-scale infrastructure from the ethanol industry. Neither system is ready to replace the existing two-step process, but they point toward a future where vitamin C production could be simpler, potentially cheaper, and more adaptable to different feedstocks.
Why Ascorbic Acid Breaks Down So Easily
One of the persistent practical challenges with ascorbic acid, whether you are a food manufacturer, a supplement company, or just someone storing orange juice, is that the molecule degrades readily. It is sensitive to oxygen, heat, light, and the pH of its surroundings. In acidic water exposed to air, ascorbic acid first oxidizes to dehydroascorbic acid, which then breaks down further into smaller compounds.11Journal of Agricultural and Food Chemistry. Degradation of Ascorbic Acid in Aqueous Solution Ultraviolet light accelerates this process, increasing the formation of dehydroascorbic acid and its breakdown products even in acidic model juice systems.12PubMed. Ultraviolet-induced oxidation of ascorbic acid in a model juice system: identification of degradation products
This instability explains a lot of everyday phenomena. Freshly squeezed juice loses vitamin C over hours if left at room temperature. Cooking vegetables in water and heat destroys a significant fraction of their ascorbic acid content. Supplements stored in humid, warm environments degrade faster than those kept cool and dry. The same instability is also why ascorbic acid works as an antioxidant: it is so eager to donate electrons to reactive molecules that it sacrifices itself in the process, neutralizing free radicals before they can damage other molecules.13PubMed. Ascorbic acid: The chemistry underlying its antioxidant properties
To get around this instability, manufacturers often use stabilized derivatives of ascorbic acid rather than the pure compound. Ascorbyl palmitate, for instance, is a fat-soluble form that resists degradation better in oily environments and is widely used in cosmetics and food products. Sodium ascorbyl phosphate is another derivative with improved stability, commonly found in serums marketed for skin brightening.14PubMed Central. Stability of ascorbyl palmitate in topical microemulsions These derivatives are converted to active ascorbic acid in the body or at the skin surface, functioning as pro-vitamins that solve the shelf-life problem while still delivering the active molecule where it is needed.
Synthetic vs. Natural Vitamin C
A perennial question for consumers is whether the ascorbic acid made in a factory is the same as the vitamin C in an orange. Chemically, the answer is unambiguous: industrially produced L-ascorbic acid is the identical molecule, with the same structure, same chirality, and same biological activity. But identity at the molecular level and equivalence in the body are separate questions, and researchers have tested both.
Steady-state bioavailability studies in humans have consistently shown no difference between synthetic and food-derived vitamin C. People absorb, circulate, and excrete it in the same way regardless of the source. Some short-term pharmacokinetic studies have detected small, transient differences in absorption rates, but these appear to have minimal real-world impact on vitamin C status.15PubMed Central. Synthetic or Food-Derived Vitamin C—Are They Equally Bioavailable? Animal studies have been more mixed, with some showing differences depending on the species and the body compartments measured, but the human data, which is what matters for the supplement-buying public, is quite consistent.
That said, fruits and vegetables deliver more than just ascorbic acid. They come with fiber, bioflavonoids, minerals, and other compounds that may interact with vitamin C in the body or confer their own separate health benefits. So while the ascorbic acid molecule itself is the same whether it comes from a tablet or a bell pepper, the nutritional context is not.
Stereoisomers and the Shape That Matters
Ascorbic acid exists in multiple spatial configurations, and only one of them is the vitamin C your body uses. The biologically active form is L-ascorbic acid, which is the form plants make, the form animals make, and the form industrial processes are designed to produce. Its mirror image, D-ascorbic acid, has little antioxidant value in human biology and is not recognized as vitamin C.
A more practically relevant cousin is erythorbic acid, also known as D-isoascorbic acid. This molecule is an epimer of L-ascorbic acid, meaning it differs in the arrangement of atoms around just one carbon. Erythorbic acid is widely used in the food industry as an antioxidant preservative, particularly in cured meats to prevent the formation of harmful nitrosamines. Your body absorbs erythorbic acid rapidly, much as it does ascorbic acid, but clears it from the bloodstream faster. Prolonged ingestion of erythorbic acid neither helps nor harms your vitamin C status.16American Journal of Clinical Nutrition / Oxford Academic. Effects of erythorbic acid on vitamin C metabolism in young women In other words, the erythorbic acid in your deli meat is not doing double duty as a vitamin supplement, but it is not depleting your vitamin C either.
Discovery and Why It Took So Long
Sailors had been dying of scurvy for centuries before anyone understood what was missing from their diets, and even after citrus fruits were recognized as a cure, it took remarkably long for the responsible molecule to be identified. In 1928, Albert Szent-Györgyi isolated a substance from adrenal glands that he called hexuronic acid. He did not initially realize it was the anti-scurvy factor. That connection was established a few years later, and in 1933, Norman Haworth worked out the chemical structure.17PubMed. The discovery of vitamin C Both men received Nobel Prizes for their contributions, Szent-Györgyi in physiology or medicine and Haworth in chemistry.
The Reichstein process for industrial synthesis followed almost immediately, in 1933-1934, making vitamin C one of the first vitamins to be mass-produced. The speed of that translation from discovery to factory was unusual for the era and reflected both the severity of vitamin C deficiency diseases and the relative simplicity of the molecule compared to, say, vitamin B12, which would not be synthesized industrially for decades.
What Ascorbic Acid Actually Does in Your Body
The reason we cannot survive without dietary vitamin C is that it serves as a cofactor for a range of enzymes involved in essential biochemical reactions. It is required for the hydroxylation of collagen, which is why scurvy’s hallmark symptoms include bleeding gums, poor wound healing, and eventually the breakdown of connective tissue. Beyond collagen, ascorbic acid participates in the biosynthesis of carnitine, the conversion of dopamine to norepinephrine, peptide amidation, and tyrosine metabolism.18Journal of Faculty of Pharmacy of Ankara University. AN OVERVIEW OF ASCORBIC ACID BIOCHEMISTRY It also enhances the absorption of non-heme iron from the gut by reducing iron from its ferric to its ferrous form, which is why pairing iron-rich plant foods with a source of vitamin C is a standard dietary recommendation.19PubMed Central. Vitamin C-Sources, Physiological Role, Kinetics, Deficiency, Use, Toxicity, and Determination
Its antioxidant role, while the most publicly recognized, is in some ways the least specific. Ascorbic acid donates electrons to neutralize reactive oxygen species, but the same willingness to give up electrons is what makes the molecule so unstable in a bottle of juice. The chemistry that makes it biologically useful is the same chemistry that makes it a headache for food processors and supplement manufacturers. That tension between reactivity and stability runs through every aspect of how ascorbic acid is made, stored, formulated, and used.