The blue wine trend is growing, but French wine researchers are casting doubts on whether or not it can actually be called wine. The founder of Vindigo, a blue wine out of Spain, says that the blue colour comes from natural pigment found in the grape, but wine experts don’t believe him.

Véronique Cheynier, director of research at the National Institute of Agricultural Research said that she doubted that Vindigo was true ‘wine,’ and was, in fact, a wine-based product.

“I don’t see how anthocyanin derived from red grape pulp can make this wine blue,” Cheynier said. “Even if anthocyanin-derived pigments that are blue in colour in an acidic medium have been successfully isolated in the lab, these pigments are only present in tiny quantities in grape skin pulp.” She said that the pigments are red when in an acidic solution and only turn blue at a pH of 7 or higher – most wines sit at 3 or 4.

New Zealand already has a blue wine available – the Italian Blumond. Blumond claims to use a similar process to Vindigo: “Firstly a base is created from a mixture of white grapes, which is then added to two organic pigments; indigo and anthocyanin – which comes from the very skin of the grapes used to make wine. This is how it gets is blue colour.” Blumond hasn’t caused the same stir at Vindigo, despite its claim to being the first blue wine in the world.