What is the source of the plum flavor in (this) plum porter? I had a plum porter lately, it has a really intense plum flavor and aroma and even a bit of plums in the taste, so I thought there were some plum products in it. However, there is nothing in ingredients list which could give the plum note to the drink.
How does the plum falvor originate here?
I contacted the brewery and they replied that

To get the plum flavour we use a natural plum extract.

Curious that it's not on the ingredients list.
PS. It's St.Peter's Plum Porter.