Anthocyanins: the pigments that shift with pH

Where that colour actually comes from

Why does a blackberry stain your fingers crimson while a blueberry sitting in the same bowl looks almost black? The pigments doing the work belong to one family. What changes is the reading your eye gets.

Anthocyanins are water-soluble plant pigments, and between them they cover a long stretch of the red-to-blue range [1]. Blackberry reads crimson. Red cabbage reads violet. Blueberry reads blue-black. Certain cannabis varieties read purple [1]. Four plants, four impressions, one chemical family standing behind all of them.

Now the part that surprises most people. That whole spread of colour is not produced by dozens of unrelated compounds. It goes back to a handful of core structures, and those cores are the anthocyanidins [1]. The core carries the colour. Around it sits the rest of the molecule, and the whole group belongs to the flavonoid family, one of the large classes of plant compounds catalogued in the 2016 overview by Panche [1].

So the diversity you see in a fruit bowl is not chemical chaos. It is a small toolkit, used in different plants, under different conditions. Fewer parts than the colours suggest.

Water-soluble is worth slowing down on, because it explains something you may already have seen in a pan. These pigments dissolve in water rather than staying locked in the plant tissue [1]. Cook red cabbage and the water picks up colour. Crush berries and the juice carries it. That behaviour is a property of the molecule, not a quirk of one recipe [1].

The food industry reads that property as a practical advantage. Anthocyanins are used as natural food colours, and the reasoning is fairly plain: they dissolve in water, they come from plants, and they can be obtained in useful quantities [1]. Those three points together are what makes a colour workable at scale rather than only in a laboratory.

Two bulk sources get named in that context: grape skin and red cabbage [1]. Both are ordinary agricultural material rather than exotic inputs, which is part of why they end up in the supply chain. A colour that can be sourced in volume, from a plant, and mixed into a water-based product covers a lot of ground on a formulator's checklist.

Worth keeping in mind: a pigment that responds to its surroundings is a different proposition from a pigment that sits still. The next section is where that shows up, because acidity turns out to be the variable that does most of the talking.

• Blackberry: crimson [1]
• Red cabbage: violet [1]
• Blueberry: blue-black [1]
• Certain cannabis varieties: purple [1]
• Colour core: a handful of structures called anthocyanidins [1]
• Family membership: the flavonoids, a large group of plant compounds [1]
• Industrial role: natural food colours, chosen because they are water-soluble, plant-derived and available in bulk [1]
• Named bulk sources: grape skin and red cabbage [1]

Same pigment, different acidity

Here is the strange bit about anthocyanins. One molecule does not have one colour. It has a sequence of forms, and which form dominates depends on how acidic the solution around it is [1]. Read the sequence in the direction of rising pH and it becomes a ladder rather than a list.

Quick vocabulary check, because the rest of this only works if the terms are clear. pH is the scale for acidity, running from strongly acidic at the low numbers to alkaline at the high ones. A proton, in this context, is a hydrogen ion: the small positively charged piece a molecule can hand over or take back. A cation is a molecule carrying a positive charge. Deprotonation is what it sounds like, the loss of that proton. None of this is exotic chemistry. It is the same handful of moves, repeated.

At the bottom of the ladder, in strongly acidic conditions of roughly pH 1 to 3, the dominant form is the flavylium cation [1]. That is the positively charged version of the pigment, and it is the form associated with the red end of the range in the published description [1].

Move up into weakly acidic and neutral territory, roughly pH 4 to 6, and two things happen at once. The cation gives up a proton, which produces the quinoidal bases [1]. At the same time, a colourless form called the carbinol form accumulates [1]. That second point matters more than it first appears. Part of the pigment population is no longer contributing colour at all, so what you observe is a mixture: some coloured forms, some colourless, in the same solution [1].

Keep going into mildly alkaline conditions, around pH 7 to 8, and further deprotonation takes place, giving the quinoidal anions [1]. Same skeleton throughout. Different charge state, different appearance.

One important limit on all of this. The description above covers how the molecule behaves in a test tube [1]. It is chemistry measured in a controlled solution, not a rule for predicting the shade of a particular plant on a particular day. Panche's 2016 overview is describing the molecule's behaviour, and that is exactly how it should be read [1].

That caveat is easy to lose, so it is worth stating twice. A test-tube ladder tells you what forms exist and in which pH bands they take over [1]. It does not hand you the recipe for a specific flower, fruit or leaf. Real plant tissue is a crowded environment, and the cited work is not making a prediction about it.

Still, the ladder explains why the kitchen version of this experiment behaves the way it does. Put the water from cooked red cabbage in a glass, add something acidic, and you are pushing the solution down the pH scale toward the flavylium region [1]. Add something alkaline and you are pushing it up toward the quinoidal anion end [1]. The colour follows the acidity because the dominant form follows the acidity.

Notice what is not happening in that glass. Nothing is being added that carries its own colour. The pigment is not replaced. It is the same molecules, gaining and losing protons, and the visible result changes with them [1]. That is an unusual thing for a colourant to do, and it is the reason anthocyanins get used as a teaching example so often.

It also sets up the obvious question about living plants. If acidity decides which form dominates, then the acidity of the place where the plant stores its pigment ought to matter. That place has a name, and it is where the next section starts.

Inside the cell, in the vacuole

Plant cells are not solid blocks. A large share of the interior volume is taken up by the vacuole, a fluid-filled compartment bounded by its own membrane. This is where anthocyanins are held, and the conditions inside it are not neutral. Vacuoles are acidic, often sitting somewhere around pH 3 to 6 [1].

Compare that range with the ladder from the previous section and something clicks. A pH of 3 sits at the top of the strongly acidic band where the flavylium cation dominates [1]. A pH of 6 sits inside the weakly acidic to neutral band, where quinoidal bases form and the colourless carbinol form accumulates [1]. So the storage compartment spans a stretch of the ladder rather than parking at a single rung.

That acidity is not left to chance either. It is maintained by proton pumps in the vacuolar membrane [1]. Proton pumps are proteins that move hydrogen ions across the membrane, keeping the inside of the vacuole more acidic than what surrounds it [1]. Active work, not passive drift.

Think of it as a controlled environment with a mechanism attached. The plant does not merely happen to have an acidic compartment. It runs pumps to hold the acidity where it is [1]. For anyone who likes knowing why a system behaves consistently, that is the interesting detail: consistency has hardware behind it.

What this does not license is a shortcut. You cannot take a vacuolar pH figure, look it up on the test-tube ladder, and declare the colour of the plant. The pH sequence describes the molecule in a controlled solution [1], and the vacuole is a living compartment with its own contents. Two documented facts sitting side by side is not the same as a formula linking them.

This is where the honest answer is the useful one. From the sources cited here, we know the pigment sits in an acidic compartment, we know the approximate range, and we know what holds that range in place [1]. What we do not have from these sources is a quantitative bridge from vacuolar pH to the exact shade of a given blackberry, cabbage leaf or purple cannabis flower.

Which is fine. The plumbing is documented even where the arithmetic is not. And knowing the difference between the two is the whole point of reading the chemistry rather than the folklore.

One more practical consequence. Once the pigment leaves the vacuole and enters water in a pan or a glass, the pumps are no longer part of the picture. The molecule is now in whatever acidity you put it in, and the ladder takes over [1]. That is the cleanest way to hold the two ideas apart: inside the cell, a maintained compartment; outside it, an ordinary solution.

• Storage site: the vacuole, the large fluid-filled compartment inside a plant cell [1]
• Conditions inside: acidic, often around pH 3 to 6 [1]
• Mechanism: proton pumps in the vacuolar membrane maintain that acidity [1]
• Overlap: pH 3 to 6 covers the top of the flavylium band and the quinoidal base and carbinol band [1]
• Scope: the pH sequence describes the molecule's behaviour in a test tube [1]
• Open point: no numeric route in these sources from vacuolar pH to a specific shade

How the plant builds them

Colour has a supply chain inside the plant, and it starts further back than the pigment itself. Anthocyanins are made through the phenylpropanoid pathway, which is the shared route used by the rest of the flavonoid family as well [1]. One trunk, several branches.

That shared beginning is a structural fact, not a detail for specialists. It means anthocyanins are not built on a private production line. They come off a route that also supplies other flavonoids, so a plant investing in that trunk is supplying more than one class of compound [1]. Panche's 2016 overview places the whole family on that pathway [1].

Then there is the light part. The genes coding for the later enzymes in the pathway are strongly light-responsive [1]. Later enzymes means the steps near the end of the route, closer to the finished pigment. Light-responsive means their expression rises and falls with light exposure rather than staying flat [1].

So light is documented as an input, and documented at the level of gene expression [1]. What the cited work does not do is convert that into a number. There is no equation here that turns so many hours of light into so much pigment. The direction is described. The magnitude is not.

Worth saying plainly, because the gap gets filled with confident guesses elsewhere. Light-responsive genes near the end of the pathway [1] is a solid, specific statement. Anything that turns it into a prediction about a particular plant's final colour is going beyond what the source supports.

Hemp has its own entry in the flavonoid catalogue, and it is a genuinely unusual one. Cannflavin A and cannflavin B are named after the plant they were found in, and in the 2005 review by ElSohly they are described as hemp-exclusive flavonoids, with no other species shown to produce them [2]. A compound name that carries its own botanical origin is not something you meet every day.

Keep the two lines separate, though. Cannflavins are flavonoids, and so are anthocyanins, but they are not the same thing and cannflavins are not what makes a purple flower purple. The colour in certain cannabis varieties belongs to the anthocyanin side of the family [1], while the cannflavins are catalogued as hemp-specific flavonoids in their own right [2].

Put the pieces together and the picture is orderly rather than mysterious. A shared route builds the family [1]. Light-responsive genes sit at the late end of that route [1]. One species contributes two flavonoids that have not been shown elsewhere [2]. And the visible colour comes back to that small set of anthocyanidin cores [1].

• Route: the phenylpropanoid pathway, shared with the rest of the flavonoid family [1]
• Late steps: genes for the later enzymes are strongly light-responsive [1]
• What that means: light changes gene expression in the pathway [1], without a stated magnitude in the cited work
• Hemp-specific pair: cannflavin A and cannflavin B, named after the plant [2]
• Their status in the 2005 review by ElSohly: hemp-exclusive, with no other species shown to produce them [2]
• Boundary: cannflavins are flavonoids, not the anthocyanidin cores behind red, purple and blue colour [1]

Colour on the plant, numbers on the report

Colour is the first thing anyone notices about a plant and one of the last things a laboratory reports. That is not a contradiction. It reflects what an analysis is built to measure.

We have been working with cannabinoids since 2014, and the working method has not changed much in that time: describe what has been measured, name what has not, and let the analysis do the talking. Applied to pigments, that method gives a short and honest answer. A cannabinoid analysis reports cannabinoid content. Anthocyanins and anthocyanidins are a separate class of plant compound, sitting in the flavonoid family [1], and they are not lines on that kind of report.

So if you are holding a batch analysis, the pigment question is not the one it answers. It tells you which cannabinoids were found and in what amount. That is precisely its value, and pretending it covers more would make it less useful, not more.

The same discipline applies in reverse. A shade of purple, violet or crimson is an observation, not a measurement. It has no unit and no reference method behind it. The published chemistry gives us the structures, the pH ladder and the storage compartment [1], and none of that turns an appearance into a figure you can compare across plants.

What the chemistry does give is a clear-eyed way of reading colour. Colour comes from a handful of anthocyanidin cores [1]. Which form dominates depends on acidity, described for the molecule in a test tube [1]. The pigment sits in an acidic vacuole held there by proton pumps [1]. The pathway is shared across the flavonoid family, with light-responsive genes at the late end [1]. Four facts, each one checkable against the source.

That is also why anthocyanins keep showing up on ingredient lists as natural food colours [1]. Water-soluble, plant-derived, available in bulk from material like grape skin and red cabbage [1]. Three properties, one practical decision. No mystique required.

What the colour does not settle

Neither of the works cited on this page ranks a plant by its shade. Panche's 2016 overview describes the structures, the pH behaviour in solution, the vacuolar storage conditions and the biosynthetic route [1]. ElSohly's 2005 review catalogues cannflavin A and cannflavin B as flavonoids that have so far been found only in this plant [2]. That is the extent of what they cover.

So a few reasonable questions stay open. What exact shade a given plant will show, from these sources, is not derivable from a pH figure [1]. How much pigment a particular light exposure produces is not quantified in the description of light-responsive late-pathway genes [1]. And whether a purple appearance says anything about anything else in the plant is not a question these two works were built to answer.

We are comfortable leaving those lines blank. It is the same principle that applies to a bottle: read the report for what it measures, and take the rest as an open question until someone measures it. Since 2014, that has been the more reliable place to look.