Does anyone know if there is a scripture reference for this, if not what is the thinking behind it?
We see at Luke 16: 19-21:
"There was a rich man who was dressed in purple and fine linen and who feasted sumptuously every day. And at his gate lay a poor man named Lazarus, covered with sores, who longed to satisfy his hunger with what fell from the rich man’s table; even the dogs would come and lick his sores."
There was presumably a custom among the rich, of cleaning one's fingers between one course of meal and another with some inferior kind of bread, which would then be discarded , as we do with tissue papers today . (I am sorry I am unable to quote any reference in support of the prevalence of such a custom at the time of Jesus.)