Combined Processes (the ‘hurdle effect ’) Many food products owe their safety and shelf-stability not to a single preservation technique but to the combined effect of a number of mechanisms, acting simultaneously. While each mechanism alone would be insuffi cient to provide the desired protection, the combination does the job. As an example, one can assume that the shelf life and safety of dry sausage is provided by the combined effect of low water activity, relatively low pH, smoking, salt, low temperature and perhaps spices, each one of these ‘ hurdles ’ being not ‘ high ’ enough to achieve the expected preservation when acting alone. Additional examples of preservation by the combined effect of processes will be discussed in preceeding articles. While the ability to preserve food by combining a number of preserving factors has been known for many years in different cultures, the concept of the hurdle effect is relatively new.