An investigation into supermarket own-brand pesto has found that bamboo fibres, potato flakes and nut flour are being used as cheaper alternatives to traditional ingredients.

Pesto, which is traditionally made from basil, pine nuts, parmesan, olive oil – and sometimes lemon juice and garlic – has become a staple food in the UK.

However, a Which? investigation into pesto sold by Asda, Co-op, M&S, Morrisons, Sainsbury’s, Tesco and Waitrose found that supermarket own-labels used cashew nuts instead of pine nuts, sunflower oil instead of olive oil and cheaper grana padano cheese instead of parmesan to cut costs.

The analysis found the most authentic and traditional pestos on sale were the Asda Extra Special Genovese Basil Pesto, costing £1.39, and the Waitrose 1 Pesto alla Genovese, priced at £2.70.

All standard supermarket pestos, with the exception of Waitrose, used thickeners such as potato flakes, bamboo fibres and nut flour; Sainsbury’s, Tesco and Co-op recipes also contained sugar, while Tesco lists water as an ingredient of its standard pesto.

Premium pesto recipes, advertised as “pesto alla Genovese”, must contain the traditional quartet of basil, olive oil, pine nuts and parmesan, but some supermarkets added extra ingredients: sugar, vegetable fibres and bamboo were all added to Morrisons, Sainsbury’s and Tesco Pesto alla Genovese recipes.

Nikki Stopford, director of research at Which?, said: “Pesto has become a staple food in the UK but our research shows that many shoppers may be getting more than they bargain for when it comes to the pesto they are buying – and not necessarily in a good way.

“Our advice is not to assume that all pesto contains the same traditional ingredients. Check the ingredients list if authenticity is important to you, or if you are trying to avoid certain contents, such as added sugar.”

Recipe: Rachel Roddy’s basic pesto

• 50g basil
• 30g pine nuts or almonds
• 100ml olive oil
• 1–2 garlic cloves
• 50g parmesan or pecorino, grated

Pulse the basil, pine nuts, olive oil, and garlic together in a blender until you have a paste, then stir in the cheese.