In his first comments on the system since the recent Geylang Serai food poisoning case, he said it had worked well and rejected suggestions to switch to issuing 'Pass or Fail' grades.

He was also not in favour of taking away the licences of stallholders who do not score the top A or B grades.

He revealed that since its introduction in 1997, the grading system has resulted in better ratings for many hawkers.

At the start, many more food sellers scored C and D than A and B. Today, the majority of more than 5,600 stallholders in 106 food centres are rated A and B. Only seven are graded D, and 14 per cent, C.

There are two reasons for this, the minister said. One is that the current system prods those who get the lower C and D grades to shape up, especially if the stalls around them are rated higher.

He felt a simple Pass or Fail system would not tell the stallholder where he stood compared to others around him.

The other reason is that the grading system comes with a regime of inspections, hygiene awareness campaigns and enforcement.

There have been suggestions to penalise stallholders rated C or D, but Dr Yaacob thought this too harsh. 'Don't forget, these are people who depend on hawking as their livelihood,' he said.

He preferred to encourage those stallholders to improve cleanliness and was confident it would work.

Read the full story in Thursday's edition in The Straits Times.