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Water activity and pH can be directly controlled in foods. Inhibitors to growth can be created by adding chemicals -- additives or substances such as salt. In addition, one can adjust the atmosphere using special packaging techniques. Very often food processors use a combination of controls, rather than relying on only one. This is because a one-control system carried to the extreme, can be harsh, making food unacceptable to consumers. The use of multiple controls is called the hurdle concept. Microbiological controls using pH, water activity, inhibitors, and atmosphere will be described in this section.
Every microorganism has a minimum, optimum, and maximum pH for growth. Yeasts and molds can grow at low pH, but 4.6 is generally considered the level that will prevent the growth and toxin production for pathogens. Some pathogens, in particular E. coli 0157:H7, can survive acidic conditions for extended periods of time, even if their growth is inhibited. pH is considered primarily a means of growth inhibition and not a method for destruction of existing pathogens. However, at low pH values many microorganiams will be destroyed if held at that pH for significant time.
A pH 4.6 is used as a divider between high-acid and low-acid foods. Some foods that are naturally low-acid are processed in a way that makes them a high-acid food. This is called acidification.
Naturally high-acid foods include peaches (pH 4.0), orange juice (pH 3.5), and apples (pH 3.5). In general, most fruits are high-acid. However, some tropical fruits including pineapple, might fall in the pH range above 4.6 depending upon growing conditions.
Examples of low-acid foods (pH above 4.6) are: fresh fish (pH 6.3), canned green beans (pH 5.0), bread (pH 5.5), and fresh ham (pH 6.2). Low-acid foods include protein foods, most vegetables, starch-based foods, and a variety of others.
Examples of foods that are processed to make them high-acid include pickled fish and pickled peppers that use vinegar (acetic acid) to lower the pH (a process called acidification); and olives and sauerkraut that use a fermentation process to produce lactic acid that reduces pH.
Acidification is the direct addition of acid to a low-acid food. The target is usually a pH of 4.6 or lower. These foods are called acidified foods and are covered under 21 CFR Part 114. There are instances where the target pH is higher than 4.6 and the food requires multiple barriers, such as refrigeration. These foods are not covered by the acidified food regulation.
Fermentation is a process where certain non-pathogenic microorganisms are used to promote chemical changes in the food. The action of these microorganisms result in the production of acid or alcohol. Bacteria typically produce acetic or lactic acid. Yeast typically produce alcohol.
The production of acid or alcohol through fermentation serves two purposes. One is to impart certain qualities to the food to produce a desired taste or even texture. An example of this is yogurt. Its unique flavor and texture is due to the fermentation process. The other purpose is to preserve the food, such as with pickled products. These foods are not regulated by the acidified food regulations. However, the food must be fermented to a pH of 4.6 or below to be considered safe at non-refrigerated storage temperatures.
Acidification is the direct addition of acid to a low-acid food. There are a variety of organic acids that are used -- acetic, lactic, and citric. Which one is selected, is dependent on the desired attributes of the finished product. Other examples of acidified foods are: pickled pig's feet, pickled onions, pickled asparagus, and fresh pack cucumber pickles that are acidified rather than fermented to get the desired pH.
Rather than using acids to acidify foods, naturally acid foods, such as tomatoes, can be added to acidify low-acid ingredients. An example is the use of tomatoes in spaghetti sauce containing whole vegetables like celery, onions, or peppers. Canned tomatoes generally have a pH of about 4.2, while the other vegetables are low-acid. The finished food would be below pH 4.6.
A food is considered to be acidified, and covered by the FDA regulations, if the pH of the finished food differs from the pH of the acid raw material. For example, the raw material tomatoes has a pH 4.2. If the finished product pH is 4.5 then the food would be acidified, because acid from the tomatoes was used to acidify the vegetables. On the other hand, if the finished product pH was still 4.2, then no significant amount of acid from the tomatoes was used to acidify the vegetables. In this case the product might not be covered under the acidified food regulations and might not be considered an acid food by formulation. Some of these foods are mustard, catsup, salad dressing, and other condiments -- all of which are shelf stable.
Processors of acidified foods are required to register and file a scheduled process with FDA. The process needs to be scientifically established to ensure that the final pH is always below 4.6. Processors need to test each lot of finished product for equilibrium pH. That means a natural pH balance has been reached by all ingredients -- which can take as long as 10 days in foods with very large particulates. Foods that require several days to reach equilibrium pH might need to be refrigerated during that time to prevent the growth of C. botulinum or other pathogens. In order to speed up the testing process, the product can be blended to a uniform paste. When a food that contains oil is blended, the oil should be removed before blending. Another way to do this is to measure the pH before the oil is added to the product because the oil does not affect the final pH.
If a processor is using acidification, they must measure pH. A pH meter is what most food processors use. Food processors can also use indicator solutions, indicator paper, or titration as long as the finished pH is below 4.0. If a pH meter is used, it must be calibrated properly.
The pH meter can be a two electrode or a single combination electrode, where both functions are combined into one electrode system. One is the reference electrode and one is the measuring electrode. When not in use, the electrodes are stored submerged in distilled water or other solutions as recommended by the manufacturer. The instrument must be checked each day of use with two different buffer solutions -- one on either side of the expected equilibrium pH. After calibration, the electrodes should be rinsed off with distilled water and then they can be used for testing. The operation and calibration of the pH meter must follow the meter manufacturer's instructions.
Direct Acidification and Bath Acidification
There are several different methods of adding the acid to the product. One is direct acidification, where predetermined amounts of acid are added to individual finished product containers during production (adding vinegar to home canned tomatoes is an example of this). With this method, one must control the acid to food ratio. This is probably the most common method used for acidified vegetables. Another method of acidification is batch acidification. Acid and food are combined in large batches and allowed to equilibrate. The acidified food is then packaged.
The necessary frequency of monitoring the finished product for pH would be less for batch acidification than for direct acidification. This is because there is variability from jar to jar with direct acidification that one does not have with batch acidification.
Acidified foods and acid foods by formulation must be heat treated sufficiently to inactive spoilage microorganisms and vegetative pathogens. There are two reasons for this - (1) to prevent spoilage triggering economic loss and (2) the action of the spoilage microorganisms can raise the pH, compromising the safety of the product.
In wine and beer production, yeast is used to ferment the product to alcohol. The alcohol preserves the product. In the production of sauerkraut, fermented sausages, cheese, sweet pickles, olives, and buttermilk, lactic acid is produced by bacteria during fermentation. Molds are used to ferment some foods, such as soy sauce, tamari sauce, and other oriental foods, mainly for taste and to achieve other desirable characteristics.
In practice, fermentation is an art. One needs to encourage growth of the right microorganisms and discourage the growth of the wrong microorganisms that cause spoilage. This is usually accomplished by adding salt or a starter culture to the food, or in some cases slightly acidifying it. A starter culture can be either yeast or bacteria.
In many fermented foods, there is no process to eliminate acid-producing bacteria. That is common for many but not all fermented foods. It is the reason why most fermented foods must be refrigerated -- so culture bacteria do not spoil the product.
WATER ACTIVITY CONTROL
Like pH, every microorganism has a minimum, optimum, and maximum water activity for growth. Yeasts and molds can grow at a low water activity, however 0.85 is considered the safe cutoff level for pathogen growth. A water activity of 0.85 is based on the minimum water activity needed for S. aureus toxin production.
Table 1. Water activity and foods
Requirements for Control
Requires refrigeration or another barrier to control the growth of pathogens
0.60 and 0.85
Intermediate Moisture Foods
Does not require refrigeration to control pathogens. Limited shelf-life because of spoilage, primarily by yeast and mold
Low moisture foods
Extended shelf-life, even without refrigeration
Foods above a water activity of 0.85 require refrigeration or another barrier to control the growth of pathogens. Foods with water activities between 0.60 and 0.85 are classified as intermediate moisture foods. These do not require refrigeration to control pathogens, but have a limited shelf-life because of spoilage, primarily by yeast and mold. For the most part, foods with a water activity below 0.60 have an extended shelf-life, even without refrigeration. These foods are called low moisture foods.
Examples of moist foods (those with water activities above 0.85) include: most fresh meats, fruits, and vegetables. The big surprise here is bread because most think that it is a dry, shelf-stable product. Actually, it has a relatively high water activity. It is only safe because of the multiple barriers of pH, water activity, and mold growth, which is favored over pathogen growth. In other words, it turns green before it becomes unsafe to eat. Examples of moist foods include:
Examples of intermediate moisture foods (water activity between 0.60 and 0.85) are:
Intermediate Moisture Food
Heaveily salted fish, such as cod
Some unique foods like soy sauce appear to be a high moisture product, but because salt, sugars or other ingredients bind the moisture, their water activities are quite low. Soy sauce has a water activity of about 0.80. Because jams and jellies have a water activity that will support the growth of yeast and mold, they are mildly heat-treated immediately before packaging, to prevent spoilage. Examples of low moisture foods (water activity below 0.60) are:
Low Moisture Food
The water activity of some of the intermediate and low moisture foods is naturally low, such as for molasses and flour. Other intermediate and low water activity foods, like dried fruit, salted fish, strawberry jam, crackers, soy sauce and noodles, begin with a high water activity food and through processing, the water activity is reduced.
Control of Water Activity
Some foods require careful control of water activity, while others do not. For example, jam would not be jam -- in other words it would not gel -- and could not be marketed unless the sugar needed to reduce the water activity was present. These types of foods do not require control of water activity in order to be safe. On the other hand, one could market a more moist dried fruit or less salty fish, which might be unsafe from a water activity standpoint. For that reason, control of water activity is important in these foods. Two primary ways to reduce water activity in foods including drying or adding salt or sugar to bind the water molecules.
Drying is one of the oldest methods of food preservation. While open air-drying is still practiced in many parts of the world, there are four primary methods of drying in the U.S.:
Hot air drying -- used for solid foods like vegetables, fruit, and fish
Spray drying -- used for liquids and semi-liquids like milk
Vacuum drying -- used for liquids like juice
Freeze-drying -- used for a variety of foods.
The other method of reducing water activity in foods is adding salt or sugar. Some examples of these types of foods include soy sauce, jams, and salted fish. Special equipment is not needed to do this. For liquid or semi-liquid foods, like the soy sauce or jam, the process involves formulation control. For solid foods like fish or cured ham, salt can be applied dry, in a brine solution or brine injected.
There are several approaches to controlling water activity. First, one can accomplish this by having a scientifically established process for drying, salting, or formulation that ensures a water activity of 0.85 or below, and then closely following that process. Second, one can take finished product samples and test for water activity.
Sometimes the chosen food preservation method does not provide protection against the growth of all microorganisms. In these cases, additional protection might be provided by the addition of chemicals. Chemical preservatives include benzoates, sorbates, sulfites, nitrites, and antibiotics. Examples include:
Hummus that uses sodium benzoate to inhibit yeast and mold.
Bread that uses calcium propionate to inhibit mold.
Smoked fish that uses sodium nitrite and some of the ingredients in the wood smoke to inhibit C. botulinum.
Chemical preservatives work through one or more of four actions on microorganisms. They can denature protein, inhibit enzymes, and alter or destroy the cell walls or cell membranes.
Some of the more commonly used chemical preservatives are:
Benzoates, including benzoic acid, sodium and potassium benzoate, and parabens, inhibit yeast or mold.
Sorbates, including sorbic acid, and sodium and potassium sorbate, inhibit mold.
Propionic acid used in breads, cakes and cheese inhibit mold.
Sulfites, including sulfur dioxide used in a variety of foods including lemon juice, seafood, vegetables, molasses, wines, dried fruit and fruit juices, used primarily as an antioxidant but also has antimicrobial properties.
Nitrites are used in, cured meats and smoked fish, usually in combination with salt or sugar and inhibit the growth of C. botulinum.
Salt prevent the growth of pathogens, especially C. botulinum.
Nisin and natamycin two antibiotics approved for use directly in food are used as antimicrobials in cheese.
These and other chemical preservatives, including the approved uses and use levels, are included in the FDA's Food Additive Status List.
Control for the Use of Chemical Preservatives
The control for the use of chemical preservatives is addressed through product formulation. The food processor must carefully control the quantity of food additive for each batch of product.
However, in some cases it might not be that simple. For example, smoked fish rely on a certain water phase salt level -- the percent salt in the water portion of the fish. The water phase salt level that one needs to control C. botulinum is not usually achieved by just formulation. It requires a combination of factors, such as brining time, brine strength, drying time, and temperature. As in water activity, processing conditions need to be scientifically established and followed. An alternate or duel approach for control is to test the finished product for water phase salt.
Packaging is different from the other methods of control. Although packaging is sometimes used to control microbiological growth, it is limited to the control of spoilage microorganisms. Packaging is generally not considered a method for controlling the growth of pathogens. In fact, packaging can contribute to product safety concerns.
From a food safety standpoint, packaging serves two functions: (1) it prevents contamination of the food or (2) it extends the effectiveness of food preservation methods. For example, packaging maintains the atmosphere in a controlled or modified atmosphere package or a vacuum package or it prevents rehydration of a dried food.
Types of Packaging
Vacuum Packaging. A number of foods, such as smoked fish are vacuum packaged. Vacuum packaging is where air is mechanically extracted from the package immediately prior to sealing. The product is placed in a low oxygen permeable bag. The bag is placed in the vacuum machine where air is mechanically evacuated from the package and a heat seal is formed. The film is held tight against the product. There is no air or atmosphere left in the package. Vacuum packaging machines were traditionally only found in food manufacturing. Today, however, they are used in hospital foodservice operations and retail food stores.
Modified Atmosphere Packaging. Food, such as fresh pasta, can be packaged in a modified atmosphere package. Modified atmosphere packaging involves a one-time gas flushing and sealing process. Primarily, three different gases are used for flushing, either singularly or in combination -- nitrogen, carbon dioxide, and oxygen. Each one of these gases has a different function.
Nitrogen takes the place of oxygen and therefore reduces the growth of aerobic spoilage microorganisms.
Carbon dioxide is lethal to many microorganisms and is used to extend shelf-life by destroying spoilage microorganisms.
Oxygen is essential for aerobic spoilage microorganisms. However, its presence even at reduced levels gives added safety by inhibiting C. botulinum. Striking the right balance between safety and extended shelf-life is complex -- usually at levels around 2%-4% oxygen. However, oxygen present in the package can be depleted by the growth of spoilage microorganisms which reduces the level to below the 2% safety level. For this reason, these foods should have a "use-by" date.
Controlled Atmosphere Packaging. Controlled atmosphere packaging is an active system that retains the atmosphere in the package throughout its shelf-life by using an oxygen scavenger in the packaging. Beef jerky is an example of this type of packaging. A packet containing iron filings that absorbs oxygen from the package is part of the packaging. Oxygen absorption is useful for extended shelf-life foods because most packaging is somewhat permeable to oxygen.
There are a variety of packaging films that provide different rates of oxygen permeability. One place that these are used is in the extended shelf-life storage of produce. This type of packaging is used for vegetables such as lettuce. When plant materials respire, they take in oxygen and give off carbon dioxide. If the amount of available oxygen is limited by the film one can reduce the respiration rate and extend the shelf-life.
One does not even need special packaging to reduce the available oxygen and create an anaerobic environment. Packing food in oil, like garlic in oil, can create that same environment. However, one must consider other safet concerns, such as the outgrowth of C. botulinum if this method is used.
Reduced Oxygen Packaging. All of these packaging forms are grouped into a category called reduced oxygen packaging. Reduced oxygen packaging is used to prevent the growth of spoilage microorganisms in order to extend the shelf-life of the product. There are other product quality benefits as well, such as reductions in rancidity, shrinkage, and color loss.
There are concerns with these packages. The extended shelf-life provides more time for toxin production or pathogen growth. Lower oxygen levels favor the facultative and anaerobic pathogens over the aerobic spoilage microorganisms. For this reason, one might get toxin production before the food spoils -- something that is less likely to happen in traditional packaging.
The major concern is C. botulinum, although there is some reason for concern with other pathogens. With that in mind, these packaging techniques should not be used unless barriers for C. botulinum are present. These barriers include:
water activity below 0.93 with adequate refrigeration to control other pathogens;
pH below 4.6; salt above 10%;
high levels of competing microorganisms;
thermal processing in the final container; and
freezing with frozen storage and distribution.
Each of these barriers by itself or in combination can be effective in the control of C. botulinum growth.
Vacuum packaged raw meat and poultry, as well as fermented cheeses, are examples of foods where competing microorganisms are used to inhibit C. botulinum toxin development. With fermented foods, such as cheese, the live starter culture grows during temperature abuse producing acid that prevents the growth of C. botulinum. Vacuum packaged raw fish, on the other hand, is not safe under refrigeration because it might contain type E C. botulinum spores that are not controlled by refrigerated temperatures.
Typically foods are thermally processed in metal cans but can also be thermally processed in a glass jar or flexible, retortable pouch. These are shelf-stable food. Flexible retort packagea look similar to some flexible pouch packaged smoked fish that requires refrigeration. This can make it confusing for consumers and retailers to determine how to store. This is why one must always read the product label to determine how the food must be stored for safety.
One barrier not previously mentioned as a singular barrier is refrigeration. This is because it is a weak barrier. Temperatures in retail and home refrigerators often do not control temperatures adequately to prevent C. botulinum growth. Sous vide foods, which are vacuum packed, partially cooked, refrigerated foods, do rely on refrigeration as a single barrier. Because of this, refrigeration must be tightly controlled throughout processing, storage, and distribution in order for sous vide foods to be safe. Foods prepared in an institutional setting for use in that setting might be the only acceptable use of this processing technique.
The hurdle concept is frequently used to control C. botulinum growth in reduced atmosphere packages. Examples include:
Vacuum packaged, hot smoked fish that uses salt, nitrites, heat damage, and smoke to control type E C. botulinum and refrigeration to control type A C. botulinum. Guidance materials, such as FDA's Fish and Fishery Food Hazards and Controls Guide, recommend time, temperature, and salinity levels for smoked fish.
Deli meats that rely on both nitrites and refrigeration to prevent C. botulinum growth.
Canned pasteurized blue crabmeat that has been vacuum packaged. The process for the pasteurized crabmeat uses heat to destroy type E C. botulinum and refrigeration to control type A C. botulinum.
These foods are safe because of their established barriers.