This is a two hour presentation highlighting the hazards and controls for reduced oxygen packaging in retail grocery and foodservice operations. It was presented at NEHA in 2012. It is sort of a mini version of the ROP HACCP workshop that takes 1.5 days.
Addressing food safety in the reduced oxygen packaging process requires the HACCP system. Basically HACCP stands for Hazards analysis and critical control points. It means for any hazard that exists, one or more controls must be provided.
Let’s take a moment to define two important terms in HACCP. The first is “control point”. A control point is any action or activity that can be used to prevent, eliminate or reduce a significant hazard.
A critical control point is any essential or critical action or activity that can be used to prevent, eliminate or reduce a significant hazard.
So, what controls can be used to prevent, eliminate or reduce reduced oxygen packaging hazards? Because this section covers a lot of material it will be split into three parts. First we’ll look at the main two controls used in determining potentially hazardous foods. These are pH (acidity) and water activity.
Bacteria can only grow under a defined range of food acidity. Extensive research has helped determine some of the minimum pH levels that permit growth. Note that the lowest pH for growth of any of these pathogens is Salmonella and Yersinia at pH 4.2. For the sporeformers, listed in red, the minimum pH for growth is 4.6 for Clostridium botulinum. Note that the remaining ROP Hazard, Listeria monocytogenes has a pH minima for growth of 4.39. Keep in mind these pH values are obtained with all other growth factors such as temperature, nutritional media, salt level, and oxygen level, are at their optimum.
Acidity of a food is measured as pH. Technically it is the log of the hydrogen ion concentrations of a food. Acidity or pH is measured using an inexpensive meter. The cost is usually under $100. The pH scale goes from 0-14. Lemon juice would be near 2, meats at near 6, and pure water at 7. Few foods are at a pH from 8-14.
Bacteria can only grow sufficient water is available for growth. However, it is not as simple as how much water is on a food.
Water is chemically reactive. Some chemical reactions can bind water making it impossible for microorganisms to transport it into the cell. If water cannot get into the cell it cannot be used. Because of this fact water is referred to as “free” or “bound”. Only free water is available for microbial growth.
The measurement of free water is called water activity and it’s measured using specialized instrumentation. The instruments cost several thousand dollars. Calibration is important and the manufacturer’s instructions must be followed. Samples are placed in the meter and after 5 or 10 minutes a reading is provided.
Here is a table of the water activity growth minima for food pathogens. Note that the lowest Aw for growth of any of these pathogens is Staphylococcus aureus toxin formation at 0.88. The Aw minima for ROP Hazards include Listeria monocytogenes at Aw 0.92 and Clostridium botulinum type E at Aw 0.97. Once again, keep in mind these pH values are obtained with all other growth factors such as temperature, nutritional media, salt level, and oxygen level, are at their optimum.
As mentioned earlier, salt can bind water, reducing the Aw of a food. Since the measurement is linear with Aw, we can look at the water phase salt or WPS as a number to determine growth or no growth of pathogens. WPS is valuable in formulating foods.
If we look at the table we would need to formulate a food with 5% salt to inhibit the growth of psychrotrophic Clostridium botulinum and 10% salt to inhibit Listeria monocytogenes. Five percent is quite salty tasting and few foods can tolerate this level and remain palatable.
In summary, it requires a pH of 4.6 or below to prevent the growth of psychrotrophic Clostridium botulinumand 4.39 or below to prevent the growth of Listeria monocytogenes. For water activity a food must be 0.97 or below for psychrotrophic Clostridium botulinum and 0.92 or below to inhibit the growth of Listeria monocytogenes. For salt, it requires 5% to inhibit psychrotrophic Clostridium botulinum and 10% to inhibit the growth of Listeria monocytogenes.
Let’s look at hurdles and barriers. First a hurdle is any control that can reduce or slow microbial growth or toxin formation. A barrier is a complete prevention of growth or cell destruction. Sometimes multiple hurdles can become a barrier.
The potentially hazardous foods tables found in the Food Code in Chapter 1 is one such multiple hurdle assessment tool. This is Table A. Table A is used for foods that have received a heat treatment to destroy vegetative bacteria. In reduced oxygen packaging that would be cook-chill or sous vide foods. Ready-to-eat vacuum packaged foods were most likely cooked, but they have been potentially contaminated with Listeria monocytogenes. Therefore the growth limits are based on the sporeformers. Note that it uses the interaction of both pH and water activity. Look at the green highlighted squares. These are the growth limits of Clostridium botulinum. Basically either a pH of 4.6 or less OR a water activity 0.92 or less. However, when the pH is between 4.7-5.6 AND the water activity between 0.92-0.95.
This is Table B. Table B includes the growth limits for vegetative bacteria and spore formers. Table B applies to vacuum packaged foods, since both Clostridium botulinum AND Listeria monocytogenes are potential hazards. The green zones are either pH or water activity barriers to growth. Essentially foods with a pH 4.2 or below OR water activity of 0.88 or below will not be a potentially hazardous food. The yellow zone is the multiple growth barrier zone.
Non-potentially hazardous foods are EXEMPT from both ROP code and ROP HACCP.
This part will look at temperature, both refrigeration temperature and temperature of cooking.
Of the eleven pathogens in this list four are capable of slow growth under normal food code refrigeration temperatures of 41 degrees Fahrenheit or below. Bacillus cereus grows very poorly at low temperatures in reduced oxygen atmospheres and Yersinia enterocolitica is an infrequent pathogen in the US and controlling for Listeria monocytogenes will control Yersinia. For these reasons we are left with psychrotrophic Clostridium botulinum and Listeria monocytogenes as the major hazards for refrigerated foods during extended storage.
Basically, the temperatures required to prevent the growth of pCB and LM are 38 and 34 degrees Fahrenheit respectively.
Here is a graph of the log growth of Listeria monocytogenes at either 4,6,or 8 degrees Celsius. Note that there is relatively no growth at 4 degrees Celsius in one week. However, approximately 1 log of growth occurred at 6 degrees Celsius and approximately 4 logs of growth occurred at 8 degrees Celsius.In 2003 the FDA began to focus on the risk of Listeria monocytogenes in refrigerated extended storage foods, especially those that are ready-to-eat. A risk assessment concluded that refrigerated ready-to-eat foods could be stored at 41 degrees Fahrenheit for up to 7 days safely. This work led to new regulations called Datemarking.
Here is a figureof the time it takes to toxin formation from psychrotrophic Clostridium botulinum based on storage temperature. The data comes from four different mathematical models. The most conservative data, the dashed red line, comes from Dr.’s Skinner and Larkin of the FDA. The bottom table corresponds to their work. Looking there we can see that it will take at least 9 days for toxin to develop in a food stored refrigerated at 41 degrees Fahrenheit.
Based on the Skinner Larkin model we can see that reduced oxygen foods are safe from potential toxin formation at 41°F for nine days. And the Date-marking section of the Food Code permits ready-to-eat meat refrigeration at 41°F for up to 7 days. Thus these two hurdles, time and refrigeration temperature become a barrier.
There are several other time-refrigeration temperature multiple hurdle controls that are permitted in the Food Code. These will be reviewed later.
When looking at the combined hurdles of pH and refrigerated storage at 41 degrees Fahrenheit or below, both psychrotrophic Clostridium botulinumand Listeria monocytogenes cannot grow at all. These foods would be potentially hazardous, but they would have two hurdles to both hazards.
In summary cooking foods to approved food code temperatures will destroy Listeria monocytogenes. Cooking foods to 194°F for 10 minutes will destroy 6 logs of spores of psychrotrophic Clostridium botulinum. These are both BARRIERS to growth of these hazards.
While proper cooking will destroy Listeria monocytogenes, undercooking is a potential hazard. Here is a slide of the recommended fish sous vide cooking temperatures for some fish. Note that the highest cooking temperature recommended was only 140°F; the lowest was just 122°F. Low cooking temperatures combined with long sous vide cooking times can be potentially hazardous by allowing microbial growth rather than destruction.
High temperatures are used for minimizing the potential for processing contamination with Listeria monocytogenesduring cook-chill bagging. Temperatures at or above 135°F are the minimum permitted. However, temperatures that are closer to 165°F are more lethal and are recommended.
High temperatures are also used to prevent the growth of any potential hazard after foods are removed from packaging and placed into hot holding. The food code requires foods be hot held at or above 135°F.
In summary there are numerous temperature based hurdles and barriers to the growth of both Listeria monocytogenes and psychrotrophic C. botulinum.
There are two common food preservatives that will prevent or minimize the growth of Listeria monocytogenes and Clostridium botulinum. They are lactate and nitrite respectively. Lactate is a salt form of lactic acid. Nitrite is “NO2”. Both preservatives are generally recognized as safe or GRAS chemicals.
Lactates and diacetates inhibit the growth of Listeria monocytogenes by lowering the cell’s intracellular pH.The USDA permits up to 4.8% lactate and 0.25% diacetate as anti-listerial preservatives for meats and poultry, except in infant formulas. Lactates are not approved for use in many foods outside of deli meats and hot dogs.
This is a graph of the growth of Listeria monocytogenes in a deli meat (red line) compared to one with Listeria monocytogenes that has sodium lactate at 2.5% and sodium diacetate at 0.15% (blue line). Note that essentially no growth occurred in the preservative sample over at least 18 weeks at 39 degrees F. You should also note that the preservative was not lethal, it just inhibited growth.
Nitrites have been used as an anti-botulinum preservative for many years. It was discovered accidentally when curing meats with sea salt prevented botulism. Later it was found that nitrite salts were the active preservative. The USDA permits up 200 ppm nitrite in cured meats. Curing salts with nitrite is also be called Prague powder 1.
Here is a graph of four meat samples containing approximately 10,000 spores per gram. The sample marked “X” contains 200 ppm nitrite. As you can see no growth occurred after 7 days of incubation, while the samples without nitrite grew considerably. Experiments like this have been repeated for hundreds of days and still there is no growth in the presence of nitrite.
The main method of utilizing these barriers to Listeria monocytogenes and Clostridium botulinum growth is to purchase cured deli meats formulated with lactates. An operator seeking to cure their own meats or add lactates to their own products would need a variance.
Both Clostridium botulinum and Listeria monocytogenes grow very slowly at refrigerated temperatures. Many spoilage organisms grow considerable faster at refrigerated temperatures. Spoilage bacteria end up consuming nutrients in the food and producing byproducts faster than the pathogens. Thus, they quickly outgrow them and due to the competition, the pathogens grow even slower or not at all.
Here is a growth curve of Listeria monocytogenes starting at approximately 1,000 cells per gram. The top line is Listeria monocytogenes with less than 100 cells per gram of background spoilage bacteria. The bottom line is Listeria monocytogenes growth in the presence of 1,000,000 cells per gram background spoilage bacteria. Note that after 10 days at 5° Celsius there was almost no Listeria growth when competing spoilage bacteria are present. Without competing flora the culture grew 5 logs within 10 days.