Further strikes are expected at Grampian Country Food Group following clashes over pay and pension cuts.
Little progress had been made in talks with the company since workers went on a 24-hour strike last Thursday, said bosses at the Transport and General Workers Union (T&G).
“The T&G will not allow our members or our representatives to be treated with contempt and are not going to go away, no matter how long it takes," said a spokesman.
The strike, which involved 1,400 pork and poultry workers, was the result of a dispute over pay, pension cuts and the "increasing casualisation of the workforce", claimed the union.
The six sites involved were: Cambuslang near Glasgow; Coupar Angus in Perthshire; Fairview near Edinburgh; Llangefni on Anglesey; Wiveliscombe in Somerset; and the Grampian-owned Welsh Country Foods on Anglesey.