Native speakers have explained this to me that since they like to run words on into each other, "de" becomes "ze" and "te" become "che" but that's for ease of speech. Therefore if you are speaking super slowly you would say the de and te....For example if you say "Eu quero suco de uva" full speed you just rock the "ze" rolling the words, but if you are taking your time and say "eu quero...suco.......de......uva" it's de
We do the same thing in English. We pronounce every letter when speaking slowly, but mix things together when speaking fast. If you say, "I can't play basketball," at regular speed in English, we don't pronounce the t. It's hard for English Language Learners to hear sometimes.