527
Chapter 41
Control by a Combination of
Methods (Hurdle Concept)
introduction
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 hightemperature 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.
Mechanisms of Antimicrobial Action
Many foods invariably contain different types of spoilage bacteria, and there is a good chance that
some can have pathogens (e.g., Staphylococcus aureus cells, Listeria monocytogenes, and pathogenic
clostridial spores). For safety and stability, the growth of pathogens and spoilage microorganisms
must be controlled during the storage life expected for these products. To achieve this goal, two
or more antimicrobial agents are employed together at a level at which the individual agents may
facilitate some growth of the concerned microorganism but not when used in combination. 1–3 As
Chapter 41
Control by a Combination of
Methods (Hurdle Concept)
introduction
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 hightemperature 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.
Mechanisms of Antimicrobial Action
Many foods invariably contain different types of spoilage bacteria, and there is a good chance that
some can have pathogens (e.g., Staphylococcus aureus cells, Listeria monocytogenes, and pathogenic
clostridial spores). For safety and stability, the growth of pathogens and spoilage microorganisms
must be controlled during the storage life expected for these products. To achieve this goal, two
or more antimicrobial agents are employed together at a level at which the individual agents may
facilitate some growth of the concerned microorganism but not when used in combination. 1–3 As