New Zealand dairy exports to China have come under scrutiny again after excessive levels of nitrates were found in a shipment of milk protein. The discovery is a further dent to the reputation of New Zealand in China, where many middle-class consumers pay high prices for imported products to avoid quality problems with domestic milk.

The agricultural regulator in New Zealand, the world’s largest dairy exporter, said yesterday it had revoked the export certificates for four consignments of milk protein bound for China. Just weeks ago Beijing ordered the recall of some New Zealand milk powder after Fonterra, the dairy group, said it had found bacteria that could cause botulism in some of its products.

That scare, which led to the resignation of the executive in charge of Fonterra’s milk processing business, saw products pulled from supermarket shelves in China, Australia, Vietnam and New Zealand.