A manipulating apparatus (10) receives a plurality of previously frozen pieces of pH modified foodstuffs or workpieces (23) and physically manipulates or stresses the workpieces while they remain in a frozen state. The physical manipulation causes relative movement between various points within the volume of the workpieces (23) and kills microbes within the foodstuff. One preferred pH modifying arrangement includes a supply of NH₃ gas (11) and a pump (9) for placing the comminuted foodstuffs to be processed and the NH₃ gas together under an operating pressure for a period of time sufficient to increase the pH of the foodstuffs.