A police officer in Nantong, Jiangsu province, shows local residents how to distinguish gutter oil from the genuine article. The classes are part of a September campaign in the city to reduce the use of recycled gutter oil in public eateries. Xie Yu / China Daily

The Wenzhou restaurant's practice of recycling the oil used for the hotpot base and then mixing it with fresh oil will have shocked those customers who used to queue for hours to eat there.

However, the exposure of the restaurant's misdeeds last September triggered further investigation into the shady business, and it turns out that many other hotpot restaurants in the province have been employing the same trick.

The production of the so-called gutter oil is waning against the backdrop of enhanced supervision and enforcement. But the use of it has not been curbed.

Some hotpot restaurants have plenty of "incentive" to make their own "gutter oil", because the cost is low and in most cases customers fail to notice anything wrong with the carefully mixed oil. That the Wenzhou restaurant did not receive any customer complaints is a case in point.

But the recycled oil is by no means safer than the illegally manufactured substandard oil and can cause as much harm to customers.

Supervisory authorities at all levels need to do a better job, although the self-recycling of cooking oil can be difficult to trace and locate. Assigning undercover officials to collect evidence and rewarding citizens who report suspicious restaurants are worth a try.

The latest judicial interpretation stipulates that producers of the so-called gutter oil could face the death penalty, which should act as deterrent if there is stricter supervision to go with it.