Sainsbury's Basics chopped tomatoes now contain more nutrients than the retailer's premium and standard chopped tomatoes, its packaging supplier has claimed.

The supermarket this week announced a packaging switch that saw its Basics tomatoes moved from a can into a SIG Combibloc.

The SIG carton is filled aseptically, meaning it is cooked in a very hot oven for a short time to sterilise the product and provide the long shelf life.

The Tetra Recart pack, which has been used for Sainsbury's premium and standard tomatoes since 2007, is retorted, meaning it is cooked at a lower temperature but for longer.

The shorter cooking time of the SIG cartons meant the cheaper Basics tomatoes would retain more of their nutrients than the premium lines, said Cindy Haast, SIG Combibloc marketing manager, North Europe.

However, Tetra Pak retail manager Ian Williamson disputed this, claiming that the acidity of tomatoes - unlike that of beans - meant cooking time made very little difference to the nutrient value.