The unexpected ingredients in your shop-bought pesto

Traditionally, pesto is made from four key ingredients: basil, pine nuts, parmesan, olive oil and maybe lemon juice and garlic.But a Which? investigation has found that shop-bought jars of pesto can contain some surprising ingredients you wouldn't expect to find, including bamboo fibres and sugar.

As part of a comparative investigation into the difference between supermarkets' standard and premium products, we reviewed the contents of jars of pesto from supermarkets Asda, Co-op, M&S, Morrisons, Sainsbury's, Tesco and Waitrose.

The standard pestos that we investigated were all made in Italy and contained between 42% and 49% basil, but they also contained cheaper ingredients in place of the traditional elements, such as cashew nuts instead of pine nuts, or a mix of both. Olive oil was also substituted with sunflower oil, and parmesan with less expensive Grana Padano and Pecorino Romano cheeses.

Pesto with potato and bamboo

All standard pestos, apart from Waitrose (£1.35), used thickeners, such as potato flakes, nut flour, vegetable or even bamboo fibres. Co-op (£1.19), Sainsbury's (£1) and Tesco (£1) standard pestos also contained sugar, and Co-op, Morrisons (£1) and Tesco listed water as an ingredient.

The premium pestos we reviewed were all labelled 'alla Genovese' and by definition have to contain the traditional ingredients of basil, olive oil, pine nuts and parmesan. The five we examined all contained between 38% and 50% basil, plus the requisite extra-virgin olive oil and Parmigiano Reggiano (parmesan) as well as pine nuts.

However, even the premium pestos contained some surprising added extras. The Morrisons (£1.95), Sainsbury's (£1.50) and Tesco (£2) premium alla Genovese pestos all contained vegetable or bamboo fibres as thickeners, as well as added sugar, and Morrisons Pesto alla Genovese also listed water as an ingredient.

Price no guarantee of purity

Our investigation showed that price doesn't necessarily mean that your pesto will be free of less-expensive and possibly unwanted ingredients. Check the ingredients list - some premium pestos contained thickeners and sugar, and the most expensive standard jar pesto, M&S (£2.10), contained carrot fibres.

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).

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.'

Which? members can find out more about the differences between more budget and premium food favourites in the May issue of Which? magazine, out next week.

Can't see our full ingredients table below? See it here.