<P> Examples of hurdles in a food system are high temperature during processing, low temperature during storage, increasing the acidity, lowering the water activity or redox potential, and the presence of preservatives or biopreservatives . According to the type of pathogens and how risky they are, the intensity of the hurdles can be adjusted individually to meet consumer preferences in an economical way, without sacrificing the safety of the product . </P> <Table> <Tr> <Th_colspan="3"> Principal hurdles used for food preservation (after Leistner, 1995) </Th> </Tr> <Tr> <Th> Parameter </Th> <Th> Symbol </Th> <Th> Application </Th> </Tr> <Tr> <Td> High temperature </Td> <Td> </Td> <Td> Heating </Td> </Tr> <Tr> <Td> Low temperature </Td> <Td> </Td> <Td> Chilling, freezing </Td> </Tr> <Tr> <Td> Reduced water activity </Td> <Td> </Td> <Td> Drying, curing, conserving </Td> </Tr> <Tr> <Td> Increased acidity </Td> <Td> pH </Td> <Td> Acid addition or formation </Td> </Tr> <Tr> <Td> Reduced redox potential </Td> <Td> </Td> <Td> Removal of oxygen or addition of ascorbate </Td> </Tr> <Tr> <Td> Biopreservatives </Td> <Td> </Td> <Td> Competitive flora such as microbial fermentation </Td> </Tr> <Tr> <Td> Other preservatives </Td> <Td> </Td> <Td> Sorbates, sulfites, nitrites </Td> </Tr> </Table> <Tr> <Th_colspan="3"> Principal hurdles used for food preservation (after Leistner, 1995) </Th> </Tr> <Tr> <Th> Parameter </Th> <Th> Symbol </Th> <Th> Application </Th> </Tr>

Which of these is described as one of the oldest methods used for food preservation