Jam stain

From Joseph Bristow's TLS piece on Thomas Wright's new book about Oscar Wilde's reading:

It is evident that Wilde treated his copy of Sententiæ Artis (1886) by the aesthete-despising journalist Harry Quilter with some contempt. This piece of hack art-criticism took a bruising when Wilde was drafting his vitriolic review of it for the Pall Mall Gazette, and Wright notes that Wilde’s copy, which is in private hands, has a badly cracked spine and bumped corners. Similarly, Wilde’s copy of W. H. Mallock’s satire on modern intellectual fads, The New Republic (1877), which is held at Magdalen College, Oxford, has, appropriately enough, a jam stain visible on page 30. As Wright notes, Wilde “gorged himself on books and food simultaneously”.

[ED. What I really want to know - what kind of jam?!?]