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About 450 members of the Food and Allied Workers Union and employees of dairy producer, Parmalat, are on a protected national strike demanding higher wages.
FAWU wants a 9% increase, while the company is offering 7 percent.
The union says in a statement that operations at plants in East London and Port Elizabeth are at a standstill.
Workers in the Western Cape and Gauteng also joined the strike, which started on Tuesday.