The food safety ministry said Tuesday worker error is suspected to have caused the recent mixing of cleaning solution in some Maeil Dairies Co.'s milk products, requesting a one-month business suspension on the company's plant responsible for the incident.

Last Thursday, some employees at an unidentified company reported stomach pain and other symptoms after consuming Maeil Dairies' 200 milliliter sterilized milk product, prompting an investigation by the Ministry of Food and Drug Safety at the dairy company's plant in Gwangju, about 270 kilometers south of Seoul.

The investigation found that a cleaning solution mixed with the product after a sterilizer valve opened for about one second at the plant on Sept. 19, possibly affecting up to around 50 milk products per second, according to the ministry.

It said the valve is suspected to have been opened due to a worker error while cleaning the sterilizer.

The agency ordered the company to come up with preventative measures and requested relevant authorities to take administrative steps under the Livestock Products Sanitary Control Act -- a monthlong business suspension and disposal of affected products.

On Monday, Maeil Dairies issued an apology over the incident, noting that about 50 products were affected and they were all sent to one of its clients. As of Monday, Maeil Dairies had recalled more than 15,000 products.