Italian pesto sold by UK supermarkets contains substitute and often cheaper ingredients, an investigation by consumer group Which? has found.
The sauce is traditionally based on four key ingredients of basil, pine nuts, parmesan and olive oil.
A test of 12 own-brand standard and premium prestos at seven supermarkets found carrot and bamboo fibres among extra ingredients.
Which? urged consumers to check the labelling.
The watchdog's investigation reviewed the ingredients of 190g pesto jars from Asda, Co-op, Marks and Spencer, Morrisons, Sainsbury's, Tesco and Waitrose.
It found all of the standard pestos were made with between 42% and 49% basil, but also contained cheaper alternatives to the traditional ingredients. They included:
- Cashew nuts instead of pine nuts, or a mix of both, with olive oil also substituted with sunflower oil.
- Parmesan cheese was replaced with cheaper Grana Padano and Pecorino Romano cheeses by Sainsbury's, Morrison's and Tesco (all £1), the Co-op (£1.19) and Marks and Spencer (£2.10).
- All the standard pestos, aside from Waitrose's £1.35 version, used thickeners such as potato flakes, nut flour, vegetable oil and bamboo fibres.
- Standard pesto from Sainsbury's, Tesco and the Co-op also contained sugar.
Which? said its investigation also showed that a higher price did not guarantee authentic ingredients.
Marks and Spencer's standard jar, the most expensive at £2.10, contained carrot fibres.
In response, a spokesman for the supermarket said the carrot fibre "ensures that the oil and basil does not separate, naturally improving its shelf life for customers".
'Surprising' additional ingredients
All of the supermarkets' premium pestos - labelled 'alla Genovese' in reference to the sauce's origins in Genoa - used traditional recipes, but also contained an unusual range of extra ingredients.
Morrisons (£1.95), Sainsbury's (1.50) and Tesco (£2) all added vegetable or bamboo fibres as thickeners, as well as sugar.
The products with the most authentic and traditional ingredients were Asda Extra Special Genovese Basil Pesto (£1.39) and Waitrose 1 Pesto alla Genovese (£2.70).
Asda said it was "pleased" at the result and that its ingredients were "clearly labelled" on the pack.
A Co-op spokesperson said the company aims for "top quality products" and that its pesto is "very low in sugar containing just 1% of your daily intake".
Which? director of research Nikki Stopford said customers should "not assume" the ingredients of pesto.
"Check the ingredients list if authenticity is important, or if you are trying to avoid certain contents, such as added sugar," she said.
What's in a traditional pesto sauce?
- Genoese basil
- Ligurian extra virgin olive oil
- Garlic (preferably from Vessalico)
- Italian pine nuts
- Parmesan cheese
- Pecorino cheese
- Coarse salt