Last reviewed: 8 May 2026
The synthetic indigo blue in UK frozen desserts — and how it differs from E131 and E133
If you've ever scanned a bright-blue UK ice lolly, a tub of bubblegum-flavoured ice cream, blue cake icing or a blue-raspberry sherbet, you've probably seen one of three blue dyes on the back of the pack: E131 Patent Blue V, E132 Indigo Carmine, or E133 Brilliant Blue FCF. They look almost interchangeable on a label, but they're three structurally different molecules with different stability, different typical uses, and different regulatory backstories. This page is the disambiguation guide for E132 specifically — what it is, where it tends to turn up on UK labels, and how it differs from its two sister blues.
E132 is the disodium salt of indigo-5,5'-disulfonic acid. In plain English: it's chemically derived from indigo — the same parent compound that gives blue jeans their blue — by adding two sulfonate groups, which makes the molecule water-soluble and food-grade-friendly. Food-grade E132 is synthesised in a factory rather than extracted from indigo plants, so the colour you see on a UK ingredients label is industrially produced, not botanically harvested.
Structurally, indigo carmine is an indigoid dye. That puts it in its own class — distinct from the triarylmethane family that includes E131 (Patent Blue V) and E133 (Brilliant Blue FCF), and distinct from the azo dyes that make up most of the Southampton Six (E102 Tartrazine, E104 Quinoline Yellow, E110 Sunset Yellow, E122 Carmoisine, E124 Ponceau 4R, E129 Allura Red). That structural class matters: regulatory and hypersensitivity findings about azo dyes don't automatically transfer to E132.
Names you'll see for the same molecule on different packaging:
Visually, E132 in solid form is a dark blue-purple powder. In water it dissolves to a deep blue. Mixed with a yellow dye (most often E102 Tartrazine) it produces the green colour used in some sweets and lollies — that pairing is one of the cases where you'll see E132 listed alongside a Southampton Six dye, even though E132 itself isn't on that list.
If you scan three different blue UK products you'll often find three different E-numbers. They aren't interchangeable, and which one a manufacturer picks depends mostly on stability and shade.
None of the three is in the Southampton Six. All three have their own specific concerns and we cover each on its own page rather than lumping them together.
E132's poorer light and heat stability shapes where you find it on UK shelves. Manufacturers tend to reach for it in cold-chain and ambient products rather than in anything baked or pasteurised at high temperature:
Where you generally won't see E132: mainstream colas, mass-market hard sweets and most baked goods. Those tend to use E133 Brilliant Blue FCF instead, because E133 holds its colour better through heat, light exposure and acidic carbonated environments. If a product needs a blue shade that survives a long ambient shelf life on a sunny shop window, E133 is the safer engineering choice; E132 belongs in the freezer cabinet or the icing piping bag.
Indigo carmine has two well-known weaknesses that food technologists work around. It has poor light stability — direct sunlight or strong UV will progressively bleach the dye, with the colour shifting from a deep blue toward a dull greenish-yellow as the indigoid chromophore breaks down. And it has poor heat stability above roughly 60°C — sustained heating during baking or pasteurisation degrades the molecule, again toward a dull off-colour.
The practical signal for shoppers: if you find a blue product where the colour looks washed-out, patchy or has gone yellow-green at the edges, you're looking at degraded E132. The dye is still in the product, but in a partly broken-down form. The more vivid, more durable blue you see in mainstream confectionery and drinks is almost always E133 rather than E132 for exactly this reason.
Indigo carmine has a longer medical history than most food dyes. It's been used since the late 19th century in renal-function testing: the dye is injected intravenously, and the time taken for blue urine to appear gives a rough indication of kidney clearance. That use has largely been replaced by modern imaging and laboratory tests, but indigo carmine still appears in some specialty applications — for example, helping surgeons distinguish fistula tracts or visualise certain anatomical structures intra-operatively.
The intravenous medical-injection use has produced a documented signal of transient hypotensive reactions — short-lived drops in blood pressure shortly after injection, generally reversible without intervention. Those reports concern the injected medical dose and route, not oral food intake; the doses, exposure route and pharmacokinetics are different. We mention it because it's part of the public record on the molecule, not because it's a directly transferable food risk.
That said, the medical literature on E132 is one reason its safety profile has been studied for longer than many newer food dyes — there's a back-catalogue of clinical observations stretching back over a century, alongside the modern toxicology dossiers EFSA and FDA rely on.
The published literature on E132 reactions is small but real. Case reports describe urticaria (hives), occasional asthma exacerbations, and gastrointestinal upset in a small number of individuals exposed to the dye. The reports are scattered across food, pharmaceutical and clinical-injection contexts, and they don't suggest a population-wide pattern.
One specific cross-reactivity worth knowing about: a subset of people with aspirin / salicylate sensitivity appear to react to several synthetic colours, including some blue dyes. The mechanism isn't fully understood and the affected population is small, but it's why dye-elimination diets in salicylate-sensitive patients sometimes include E132 alongside the more frequently cited dyes.
Indigo carmine is not on the UK / EU list of 14 mandatory allergens that must be highlighted on labels. NHS allergy guidance and FSA labelling rules don't require it to be flagged separately from the rest of the ingredient list.
The 2007 Southampton study and the resulting FSA labelling rule identified six dyes linked to a small but measurable increase in hyperactivity in some children: E102 (Tartrazine), E104 (Quinoline Yellow), E110 (Sunset Yellow FCF), E122 (Carmoisine), E124 (Ponceau 4R) and E129 (Allura Red AC). UK products containing any of those six must carry the on-pack warning: "may have an adverse effect on activity and attention in children".
E132 is not on that list. The Southampton study didn't test it, and it isn't subject to the FSA on-pack warning. Two reasons not to over-read that:
If you've been avoiding the Southampton Six because of the hyperactivity warning, E132 doesn't fall into that group. It's a separate question with separate evidence.
UK / EU: permitted as a food colour. EFSA published a re-evaluation of E132 in 2014 and confirmed an Acceptable Daily Intake of 5 mg/kg body weight per day. That's the working ADI used by UK / EU regulators today. On UK labels the dye must be declared as "E132", "Indigotine" or "Indigo Carmine".
US: permitted as FD&C Blue No. 2, subject to the FDA's batch-by-batch certified-colour-additive system, which requires each manufactured lot to meet specifications before it can be sold for food use.
Worldwide approval status varies by jurisdiction and food category, but indigo carmine is one of the more broadly accepted synthetic blues globally.
E132 is synthetic, with no animal-derived starting material in standard manufacture, so it's typically suitable for vegan, vegetarian, halal and kosher diets. As with any colourant, the certifying authority for a finished product will look at the whole formulation and processing context, not just the dye in isolation. If you eat to a strict standard, the certification mark on the pack is the authoritative answer; the E-number on its own is a starting point.
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