Much of the grain appeared to be mouldy and mixed with bits of dirt and other foreign matter. The clip circulated on Weibo, China’s Twitter-like platform.

There is currently no evidence suggesting that the country is facing an insufficient food supply.

But the video showing the stockpiled corn triggered public questions about whether the official grain reserves were safe and sufficient enough to meet domestic demand.

The ban on carrying mobile phones into granaries further fuelled online speculation that the state-owned grain reserve agency could be hiding a deterioration in the quality of stockpiles and was moving to prevent exposure.

In response, Sinograin issued a statement on Weibo on Sunday night, confirming the clip showed the Zhaozhou warehouse but adding that the parent company was not trying to conceal any problems.

It also admitted that the local ban was a crude step and said it had severely criticised the Heilongjiang subsidiary.

The subsidiary also said it would allow the public to visit local granaries on certain days, but did not give a timetable or registration guidelines.

“We never prohibit outsiders from bringing their mobile phones into the warehouse,” Chen Yuan, a press officer with Sinograin in Beijing, told the South China Morning Post. “However, from the perspective of personnel safety, we do not recommend using mobile phones in the grain-storage area.

“Frequent use of mobile phones may cause a lack of concentration, which could result in a safety hazard.”

The official statement said that the company had tightened the internal management on mobile phones, “in response to some customers’ practice of overgeneralisation and influencing public opinion during grain sales”, making an indirect reference to the controversial clip.

In an earlier statement on July 14, Sinograin said that none of the problems alleged in the video had been found after an investigation.

“After preliminary verification, the quantity and quality problems of the batch of corn reflected in the video are basically inconsistent with the facts,” Sinograin said in the statement.

It confirmed that it found some foreign materials in certain parts of the pile, which were left in the corn after the warehouse failed to clean them up. “[But] the foreign matter did not affect the quality and quantity of the whole corn warehouse,” the state stockpile manager said.

The food supply for the country’s 1.4 billion people has long been a top priority for Beijing, and the central government has promoted the huge national reserves of grains as the key guarantee of China’s food security.

Sinograin, however, has always kept the overall size of grain reserves a secret. China has designated corn, wheat and rice as strategic grains that it relies on to ensure adequate supply.

The latest available data was from a food security white paper issued by the State Council Information Office last year, which showed that the storage capacity of China’s warehouses had reached 910 million tonnes in 2018.

China also said the country’s summer grain output reached a historic high of 142.81 million tonnes this year, up 0.9 per cent from last year, citing this as further evidence of food security.

But a nearly 30 per cent jump in futures prices since January still suggests a domestic supply gap in corn – a crucial ingredient in animal feed, alcohol and ethanol fuel.

In response to the higher prices, China stepped up corn purchases from the United States, with a deal for 1.937 million tonnes confirmed by the US Department of Agriculture (USDA) on Thursday.

Ma Wenfeng, a senior analyst with Beijing Orient Agribusiness Consulting, said that the soaring corn price pointed to a possible decline in the total output of summer grains, in contrast with the official statistics and comments.

He determined that the output of summer grains might have declined by up to 4.6 per cent from a year ago to 135.17 million tonnes, 7.64 million tonnes lower than the figure cited by Beijing, and also the lowest since 2013.