466 Modern Food Microbiology
pulses at 42
◦ C, a 5.91-log reduction occurred but when cinnamon powder (2%) or nisin (2.5%) was
added, a 6- to 8-log reduction occurred.
21
Using raw skim milk, APC was reduced by 7 logs with 80 kV/cm, 50 pulses at 52
◦ C, and both
nisin (38 IU/ml) and lysozyme (1,638 IU/ml) added.
57 Against vegetative cells of Bacillus cereus, a
treatment with 16.7 kV/cm, 50 pulses each at 2 µsec plus 0.06 ppm nisin, an increased reduction of
1.8 log units was observed over what PEF and nisin alone achieved.
43 A 5-log reduction of E. coli
0157:H7 was obtained in a simulated milk medium by 5 kV/cm + 1,200 IU nisin/ml at a w of 0.95.
59
It was found that NaCl and nisin reduced the effectiveness of PEF.
In a study that compared PEF with HHP and heat for controlling ascospores of Z. bailii in fruit juices,
two pulses of 32–36.5 kV/cm reduced vegetative cells 4.5–5 and ascospores 3.5–4 log cycles, and this
compared with a nearly 5-log reduction of vegetative cells by HHP but only a 0.5–1 log reduction of
ascospores with a 5-minute treatment at 300 MPa.
49 Overall, two pulses of 32–36.5 kV/cm reduced
vegetative cells or ascospores 3.5–5 log cycles for each of the five juices tested. The ascospores were
5–8 times more heat resistant than the vegetative cells.
Regarding E. coli, when 10
6 cfu/ml were added to pea soup and treated with two 16-pulse steps
at 35 kV/cm for a total of 2 sec, cells could not be detected by plate count.
48 In an earlier study,
bacteriophages of Lactococcus cremoris were found to be more sensitive to electric shock than four
species of bacteria, including spores of Bacillus subtilis.
14 More information on the PEF treatment of
foods can be found in reference 48.
ASEPTIC PACKAGING
In traditional canning methods, nonsterile food is placed in nonsterile containers, followed by
container closure and sterilization. In aseptic packaging, sterile food under aseptic conditions is placed
in sterile containers, and the packages are sealed under aseptic conditions as well. Although the
methodology of aseptic packaging was patented in the early 1960s, the technology was little used
until 1981, when the Food and Drug Administration approved the use of hydrogen peroxide for the
sterilization of flexible multilayered packaging materials used in aseptic processing systems.
In general, any food that can be pumped through a heat exchanger can be aseptically packaged.
The widest application has been to liquids such as fruit juices, and a wide variety of single-serve
products of this type has resulted. The technology for foods that contain particulates has been more
difficult to develop, with microbiological considerations only one of the many problems to overcome.
In determining the sterilization process for foods pumped through heat exchangers, the fastest-moving
components (those with the minimum holding time) are used, and where liquids and particulates are
mixed, the latter will be the slower moving. Heat-penetration rates are not similar for liquids and solids,
making it more difficult to establish minimum process requirements that will effectively destroy both
organisms and food enzymes.
Some of the advantages of aseptic packaging are as follows:
1. Products such as fruit juices are more flavorful and lack the metallic taste of those processed in
metal containers.
2. Flexible multilayered cartons can be used instead of glass or metal containers.
3. The time a product is subjected to high temperatures is minimized when ultrahigh temperatures
are used.
4. The technology allows the use of membrane filtration of certain liquids.
5. Various container headspace gases such as nitrogen may be used.
pulses at 42
◦ C, a 5.91-log reduction occurred but when cinnamon powder (2%) or nisin (2.5%) was
added, a 6- to 8-log reduction occurred.
21
Using raw skim milk, APC was reduced by 7 logs with 80 kV/cm, 50 pulses at 52
◦ C, and both
nisin (38 IU/ml) and lysozyme (1,638 IU/ml) added.
57 Against vegetative cells of Bacillus cereus, a
treatment with 16.7 kV/cm, 50 pulses each at 2 µsec plus 0.06 ppm nisin, an increased reduction of
1.8 log units was observed over what PEF and nisin alone achieved.
43 A 5-log reduction of E. coli
0157:H7 was obtained in a simulated milk medium by 5 kV/cm + 1,200 IU nisin/ml at a w of 0.95.
59
It was found that NaCl and nisin reduced the effectiveness of PEF.
In a study that compared PEF with HHP and heat for controlling ascospores of Z. bailii in fruit juices,
two pulses of 32–36.5 kV/cm reduced vegetative cells 4.5–5 and ascospores 3.5–4 log cycles, and this
compared with a nearly 5-log reduction of vegetative cells by HHP but only a 0.5–1 log reduction of
ascospores with a 5-minute treatment at 300 MPa.
49 Overall, two pulses of 32–36.5 kV/cm reduced
vegetative cells or ascospores 3.5–5 log cycles for each of the five juices tested. The ascospores were
5–8 times more heat resistant than the vegetative cells.
Regarding E. coli, when 10
6 cfu/ml were added to pea soup and treated with two 16-pulse steps
at 35 kV/cm for a total of 2 sec, cells could not be detected by plate count.
48 In an earlier study,
bacteriophages of Lactococcus cremoris were found to be more sensitive to electric shock than four
species of bacteria, including spores of Bacillus subtilis.
14 More information on the PEF treatment of
foods can be found in reference 48.
ASEPTIC PACKAGING
In traditional canning methods, nonsterile food is placed in nonsterile containers, followed by
container closure and sterilization. In aseptic packaging, sterile food under aseptic conditions is placed
in sterile containers, and the packages are sealed under aseptic conditions as well. Although the
methodology of aseptic packaging was patented in the early 1960s, the technology was little used
until 1981, when the Food and Drug Administration approved the use of hydrogen peroxide for the
sterilization of flexible multilayered packaging materials used in aseptic processing systems.
In general, any food that can be pumped through a heat exchanger can be aseptically packaged.
The widest application has been to liquids such as fruit juices, and a wide variety of single-serve
products of this type has resulted. The technology for foods that contain particulates has been more
difficult to develop, with microbiological considerations only one of the many problems to overcome.
In determining the sterilization process for foods pumped through heat exchangers, the fastest-moving
components (those with the minimum holding time) are used, and where liquids and particulates are
mixed, the latter will be the slower moving. Heat-penetration rates are not similar for liquids and solids,
making it more difficult to establish minimum process requirements that will effectively destroy both
organisms and food enzymes.
Some of the advantages of aseptic packaging are as follows:
1. Products such as fruit juices are more flavorful and lack the metallic taste of those processed in
metal containers.
2. Flexible multilayered cartons can be used instead of glass or metal containers.
3. The time a product is subjected to high temperatures is minimized when ultrahigh temperatures
are used.
4. The technology allows the use of membrane filtration of certain liquids.
5. Various container headspace gases such as nitrogen may be used.