Control by a Combination of Methods (Hurdle Concept) ◾ 529
fruits), low A w (sugar in fruits and added), and anaerobic packaging are used to reduce microbial
numbers as well as the growth of survivors. However, the greatest challenge will be the long-term
preservation of meat, dairy, and fish products at refrigerated temperatures or even at ambient temperatures, when refrigeration is not always available, such as during a military operation, a work
relief during a natural disaster, or in the rural areas of many developing countries. To achieve this
goal, it will be necessary to study the effectiveness of combining different factors to control target
microorganisms in a food. In conducting such studies, one should also recognize that the factors
could have additive, synergistic, or even adverse (neutralizing) effects when used in combination.
Current Status
These aspects have been studied in a rather limited way. Results of some of these studies are briefly
described here. 1–5
Low-Heat Processing
Low heat (≤100°C) does not kill many pathogenic and spoilage bacterial spores. They can be heat
activated, which leads to germination and outgrowth. However, if the pH of the food is reduced
to 4.5 or NO 2 and NaCl are added, the heat-shocked spores will not germinate.
Low Storage Temperature
Clostridium botulinum grows at 35°C (95°F) at an A w of 0.95. However, if the storage temperature
is reduced to 20°C (68°F), it does not grow unless the A w is increased to 0.97. Similarly, Lis. monocytogenes grows at 25°C (77°F) in a broth containing 6.5% NaCl in three days but fails to grow
under similar conditions at 14°C (57.2°F).
Low pH
Clo. botulinum grows at pH 7.0, 37°C (98.6°F), and an A w of 0.94; however, when the pH is
dropped to 5.3, no growth is observed, even at an A w of 0.99. A Salmonella strain grows at pH
5.8 at an A w of 0.97, but when the pH is reduced to 5.0, an A w of 0.99 is required for growth. Clo.
botulinum produces a toxin during incubation at 16°C (60.8°F) in 28 days at pH 5.5, but at pH
5.2 under the same conditions, no toxin is produced.
In using acid to reduce pH, it is important to recognize that organic acids, such as acetic, propionic, and lactic acids, are more effective than HCl and phosphoric acid. Also, acetic or propionic
acids are more effective than lactic acids. Some acids, such as citric and phosphoric acids, act by
both lowering pH and chelating divalent cations that are important for microbial functions.
Low A w
Some of the examples used show how a reduced A w interacts with other parameters in adversely
affecting microbial growth. Sta. aureus can grow at an A w of 0.86 and high pH. However, it fails
to grow even at an A w of 0.93 (in the presence of NaCl) at pH 4.6, a pH that favors growth of this
bacterium at higher A w . Similarly, Sta. aureus grows at 12°C, pH 7.0, and an A w of 0.93. However,
if the A w is reduced to 0.90, it will not grow under the same conditions.
fruits), low A w (sugar in fruits and added), and anaerobic packaging are used to reduce microbial
numbers as well as the growth of survivors. However, the greatest challenge will be the long-term
preservation of meat, dairy, and fish products at refrigerated temperatures or even at ambient temperatures, when refrigeration is not always available, such as during a military operation, a work
relief during a natural disaster, or in the rural areas of many developing countries. To achieve this
goal, it will be necessary to study the effectiveness of combining different factors to control target
microorganisms in a food. In conducting such studies, one should also recognize that the factors
could have additive, synergistic, or even adverse (neutralizing) effects when used in combination.
Current Status
These aspects have been studied in a rather limited way. Results of some of these studies are briefly
described here. 1–5
Low-Heat Processing
Low heat (≤100°C) does not kill many pathogenic and spoilage bacterial spores. They can be heat
activated, which leads to germination and outgrowth. However, if the pH of the food is reduced
to 4.5 or NO 2 and NaCl are added, the heat-shocked spores will not germinate.
Low Storage Temperature
Clostridium botulinum grows at 35°C (95°F) at an A w of 0.95. However, if the storage temperature
is reduced to 20°C (68°F), it does not grow unless the A w is increased to 0.97. Similarly, Lis. monocytogenes grows at 25°C (77°F) in a broth containing 6.5% NaCl in three days but fails to grow
under similar conditions at 14°C (57.2°F).
Low pH
Clo. botulinum grows at pH 7.0, 37°C (98.6°F), and an A w of 0.94; however, when the pH is
dropped to 5.3, no growth is observed, even at an A w of 0.99. A Salmonella strain grows at pH
5.8 at an A w of 0.97, but when the pH is reduced to 5.0, an A w of 0.99 is required for growth. Clo.
botulinum produces a toxin during incubation at 16°C (60.8°F) in 28 days at pH 5.5, but at pH
5.2 under the same conditions, no toxin is produced.
In using acid to reduce pH, it is important to recognize that organic acids, such as acetic, propionic, and lactic acids, are more effective than HCl and phosphoric acid. Also, acetic or propionic
acids are more effective than lactic acids. Some acids, such as citric and phosphoric acids, act by
both lowering pH and chelating divalent cations that are important for microbial functions.
Low A w
Some of the examples used show how a reduced A w interacts with other parameters in adversely
affecting microbial growth. Sta. aureus can grow at an A w of 0.86 and high pH. However, it fails
to grow even at an A w of 0.93 (in the presence of NaCl) at pH 4.6, a pH that favors growth of this
bacterium at higher A w . Similarly, Sta. aureus grows at 12°C, pH 7.0, and an A w of 0.93. However,
if the A w is reduced to 0.90, it will not grow under the same conditions.