The couple is accused of intentionally adding the industrial salt nitrite to the milk produced at Ma Wenxuan's dairy farm in Mafang Village, China's official Xinhua news agency reported.
Thirty-eight people in the town of Pingliang in Gansu province were sent to the hospital after drinking the nitrite-tainted milk.
A 36-day-old infant and two children under the age of 2-years-old died as a result.
Police said the couple being detained had a business dispute Ma Wenxuan.
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