Duncan Smith told BBC1's Andrew Marr Show yesterday that he would not back down after a flagship back-to-work scheme was deemed legally flawed.
He also spoke directly to graduate Cait Reilly, who challenged having to work for free at discount store Poundland or face losing jobless benefits.
"You have to learn early that if you commit to something you stay by and do it," he said.
Denying claims of 'slave labour', he insisted: "She was paid Jobseeker's Allowance by the taxpayer to do this.
"Most young people love this programme and I am sorry, but there is a group of people out there who think they are too good for this kind of stuff.
"Let me remind you that former Tesco chief executive Terry Leahy started his working life stacking shelves.
"The next time somebody goes in, those smart people who say there's something wrong with this, they go into their supermarket, ask themselves this simple question, when they can't find the food they want on the shelves, who is more important –them, the geologist, or the person who stacked the shelves?"
On Tuesday, in an interview with The Guardian, Reilly, who is currently working part-time in Morrisons, said she felt angry when ministers had previously branded her a "job snob".
"I didn't want to be on benefits. I hated it. I wanted to get a job as soon as I could. So for someone as high profile as that to say: 'Well, they're not trying, they're a job snob, they're lazy, they're a scrounger… ' It really made me angry."