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It can be found in many types of food but is mainly a problem with unpasteurised milk, soft cheeses and chilled ready-to-eat snacks like pre-packed sandwiches.

https://news.sky.com/story/amp/spain-issues-international-alert-after-countrys-biggest-ever-listeria-outbreak-11790743

Please explain the use of coordinating conjunction "but" in "but is mainly..."

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JigneshbharatiIt can be found in many types of food but is mainly a problem with unpasteurised milk, soft cheeses and chilled ready-to-eat snacks like pre-packed sandwiches.

Listeria (bacteria) can be found in many types of food (where it is not a problem), but it (=listeria) is (mainly) a problem with ....

It's a normal use of 'but' to show contrast. Some of the relevant parts are implied. The writer thinks the reader will know what those are, so he does not bother to supply them.

Listeria is not a problem in some types of food,
but listeria is a problem in other types of food.

CJ

Comments  

This is elliptical. The complete sentence would be something like:


"It can be found in many types of food but this organism is mainly a problem with unpasteurized milk, soft cheeses and chilled ready-to-eat snacks like pre-packaged sandwiches." (The words, "this organism", are omitted for brevity and stylistic considerations and are understood from the context.)

 CalifJim's reply was promoted to an answer.