I can accept that, but when I did type in 'nel burro' the next time, I was wrong again being told that it was 'nello burro'. Thus the confusion and I think it should be one way or the other. I think too that this is a Duolingo error which has gone unnoticed or ignored. Thank you for your explanation, I appreciate it.
I read an explanation that "nello" is used for words beginning with a z and ones beginning with an s + a consonant. Ex: Nello Zucchero (in the sugar), Nello zaino (in the backpack), and Nello scarico (in the drain), Nello specchio (in the mirror).
For "nel", nel comes before other masculine words beginning with a consonant. Ex: Nel giornale (in the newspaper), Nel mondo (in the world), Nel documento (in the document), Nel burro (in the butter). This really helped me! I hope it helps you and anyone else! :)
It's called compound butter --burro composto http://www.simplyscratch.com/2011/10/roasted-garlic-and-rosemary-butter.html
I played it several times. It sounded to me like mela burro. Being a southern US transplant I finally decided on the improbable "apple butter" This lady has trouble with the ending of some words and also with "gi" which sound more like an Engish "d" rather than a "j". Is this dialect variation?