Control by a Combination of Methods (Hurdle Concept)

Many of the factors used to kill microorganisms or control microbial growth in foods are also able to facilitate microbial growth and survival. These factors, which include temperature, A_w, pH (acidity), Eh (redox potential), and some preservatives, have a range in which microbial growth varies from optimal to minimal. Beyond the range, depending on the factor, microorganisms either do not grow or die. When only one method is used to preserve food, the condition beyond the growth range of microorganisms is generally used. Examples of such methods are high- temperature treatment to produce a sterile food (preferably in a container to prevent post-heat contamination) or drying a food below an A_w of 0.6. Although, microbiologically, these products can have a very long shelf life and be safe, their acceptance and nutritional qualities are generally lower, and they might not be preferred by many consumers. This is particularly true for consumers interested in foods that are fresh, natural, healthy, and convenient and not harshly processed or harshly preserved. There is a general interest in producing foods that are low in salt, sugar, and fat and that can be stored for a desirable period of time by refrigeration or chilling or even at room temperature and be eaten readily or by microwave heating. There is also an interest in producing such fresh-like foods for the U.S. Army and astronauts.