56
Modern Food Microbiology
Relative Humidity of Environment
The RH of the storage environment is important both from the standpoint of a w within foods and
the growth of microorganisms at the surfaces. When the a w of a food is set at 0.60, it is important that
this food be stored under conditions of RH that do not allow the food to pick up moisture from the air
and thereby increase its own surface and subsurface a w to a point where microbial growth can occur.
When foods with low a w values are placed in environments of high RH, the foods pick up moisture
until equilibrium has been established. Likewise, foods with a high a w lose moisture when placed in
an environment of low RH. There is a relationship between RH and temperature that should be borne
in mind in selecting proper storage environments for foods. In general, the higher the temperature, the
lower the RH, and vice versa.
Foods that undergo surface spoilage from molds, yeasts, and certain bacteria should be stored under
conditions of low RH. Improperly wrapped meats such as whole chickens and beef cuts tend to suffer
much surface spoilage in the refrigerator before deep spoilage occurs, due to the generally high RH of
the refrigerator and the fact that the meat-spoilage biota is essentially aerobic in nature. Although it
is possible to lessen the chances of surface spoilage in certain foods by storing under low conditions
of RH, it should be remembered that the food itself will lose moisture to the atmosphere under such
conditions and thereby become undesirable. In selecting the proper environmental conditions of RH,
consideration must be given to both the possibility of surface growth and the desirable quality to be
maintained in the foods in question. By altering the gaseous atmosphere, it is possible to retard surface
spoilage without lowering the RH.
Presence and Concentration of Gases in the Environment
Carbon dioxide (CO 2 ) is the single most important atmospheric gas that is used to control microorganisms in foods.
15,35 It along with O 2 are the two most important gases in modified atmosphere
packaged (MAP) foods, and this is discussed in Chapter 14.
Ozone (O 3 ) is the other atmospheric gas that has antimicrobial properties, and it has been tried over a
number of decades as an agent to extend the shelf life of certain foods. It has been shown to be effective
against a variety of microorganisms,
9 but because it is a strong oxidizing agent, it should not be used
on high-lipid-content foods since it would cause an increase in rancidity. Ozone was tested against
Escherichia coli 0157:H7 in culture media, and at 3 to 18 ppm the bacterium was destroyed in 20 to 50
minutes.
10 The gas was administered from an ozone generator and on tryptic soy agar, the D value for
18 ppm was 1.18 minutes, but in phosphate buffer, the D value was 3.18 minutes. To achieve a 99%
inactivation of about 10,000 cysts of Giardia lamblia per milliliter, the average concentration time was
found to be 0.17 and 0.53 mg-min/L at 25
◦ C and 5
◦ C, respectively.
53 The protozoan was about three
times more sensitive to O 3 at 25
◦ C than at 5
◦ C. It is allowed in foods in Australia, France, and Japan;
and in 1997 it was accorded GRAS (generally regarded as safe) status in the United States for food use.
Overall, O 3 levels of 0.15 to 5.00 ppm in air have been shown to inhibit the growth of some spoilage
bacteria as well as yeasts. The use of ozone as a food sanitizing agent is presented in Chapter 13.
Presence and Activities of Other Microorganisms
Some foodborne organisms produce substances that are either inhibitory or lethal to others; these
include antibiotics, bacteriocins, hydrogen peroxide, and organic acids. The bacteriocins and some
Modern Food Microbiology
Relative Humidity of Environment
The RH of the storage environment is important both from the standpoint of a w within foods and
the growth of microorganisms at the surfaces. When the a w of a food is set at 0.60, it is important that
this food be stored under conditions of RH that do not allow the food to pick up moisture from the air
and thereby increase its own surface and subsurface a w to a point where microbial growth can occur.
When foods with low a w values are placed in environments of high RH, the foods pick up moisture
until equilibrium has been established. Likewise, foods with a high a w lose moisture when placed in
an environment of low RH. There is a relationship between RH and temperature that should be borne
in mind in selecting proper storage environments for foods. In general, the higher the temperature, the
lower the RH, and vice versa.
Foods that undergo surface spoilage from molds, yeasts, and certain bacteria should be stored under
conditions of low RH. Improperly wrapped meats such as whole chickens and beef cuts tend to suffer
much surface spoilage in the refrigerator before deep spoilage occurs, due to the generally high RH of
the refrigerator and the fact that the meat-spoilage biota is essentially aerobic in nature. Although it
is possible to lessen the chances of surface spoilage in certain foods by storing under low conditions
of RH, it should be remembered that the food itself will lose moisture to the atmosphere under such
conditions and thereby become undesirable. In selecting the proper environmental conditions of RH,
consideration must be given to both the possibility of surface growth and the desirable quality to be
maintained in the foods in question. By altering the gaseous atmosphere, it is possible to retard surface
spoilage without lowering the RH.
Presence and Concentration of Gases in the Environment
Carbon dioxide (CO 2 ) is the single most important atmospheric gas that is used to control microorganisms in foods.
15,35 It along with O 2 are the two most important gases in modified atmosphere
packaged (MAP) foods, and this is discussed in Chapter 14.
Ozone (O 3 ) is the other atmospheric gas that has antimicrobial properties, and it has been tried over a
number of decades as an agent to extend the shelf life of certain foods. It has been shown to be effective
against a variety of microorganisms,
9 but because it is a strong oxidizing agent, it should not be used
on high-lipid-content foods since it would cause an increase in rancidity. Ozone was tested against
Escherichia coli 0157:H7 in culture media, and at 3 to 18 ppm the bacterium was destroyed in 20 to 50
minutes.
10 The gas was administered from an ozone generator and on tryptic soy agar, the D value for
18 ppm was 1.18 minutes, but in phosphate buffer, the D value was 3.18 minutes. To achieve a 99%
inactivation of about 10,000 cysts of Giardia lamblia per milliliter, the average concentration time was
found to be 0.17 and 0.53 mg-min/L at 25
◦ C and 5
◦ C, respectively.
53 The protozoan was about three
times more sensitive to O 3 at 25
◦ C than at 5
◦ C. It is allowed in foods in Australia, France, and Japan;
and in 1997 it was accorded GRAS (generally regarded as safe) status in the United States for food use.
Overall, O 3 levels of 0.15 to 5.00 ppm in air have been shown to inhibit the growth of some spoilage
bacteria as well as yeasts. The use of ozone as a food sanitizing agent is presented in Chapter 13.
Presence and Activities of Other Microorganisms
Some foodborne organisms produce substances that are either inhibitory or lethal to others; these
include antibiotics, bacteriocins, hydrogen peroxide, and organic acids. The bacteriocins and some