To make it even more confusing, sap is actually one of the few words that can be used both with de and het. Here's a link to the Dutch dictionary that shows that. http://www.vandale.nl/opzoeken?pattern=sap=nn#.VVHR3PmqpBd
There are no steady rules, just some guidelines (which you can find here http://www.dutchgrammar.com/en/?n=NounsAndArticles.04 )
Does the 'd' at the end of 'brood' make a 't' sound like it does in German?