Both animals and plants are living organisms, which means that upon slaughter or harvest, these foods begin a series of biological changes that affect their quality and safety in our food supply.
For example, after harvest, much of the sugar in corn turns into starch, and the corn loses some of its natural sweetness. As for fish—after capture, a phospholipid called trimethylamine oxide splits off from fat stores and begins changing into trimethylamine, the substance responsible for "fishy" odor.
Fats, carbohydrates, proteins, enzymes, and other naturally occurring compounds in living matter continue some of their activity as products enter our food supply. In addition, microorganisms that are present may spring into action, causing spoilage. All of these factors affect the quality and shelf life of food.
Food preservation is the step(s) we take to slow the spoilage process and keep food fresh.
Preservation is not new, although some of our techniques have evolved. Drying foods in the sun dates back to the ancient world. Fermentation, as used in making vinegar, wine, and beer, is also centuries old.
According to the American Chemical Society (ACS), "The Incas stored their potatoes and other foods at high altitudes where they lived in the Andes Mountains. The food froze at these cold temperatures, and because of the reduced atmospheric pressure, the water dissipated more quickly than it would have at sea level." This technique evolved into freeze-drying, says the ACS.
Meanwhile, the process of canning foods stems back to the early 1800s, when "the Frenchman Nicolas Appert 'made the seasons stand still' by inventing a technique to preserve foods in glass jars," explains the ACS.
As you'll see in this CE program, these longstanding methods build on some basic food science. Controlling water availability, acidity, and temperature are all approaches to preserving food. Basic food preservation techniques target keeping organic food compounds in their original or "fresh" form, as well as controlling the growth of microbes.
Source: American Chemical Society. Early methods of food preservation.
Upon completion of this CE program, you will be able to:
Food spoilage is deterioration of quality that comes from biological, chemical, or physical changes. As food spoils, it does not necessarily become unsafe; it simply loses characteristics that make it appealing.
Food can become unsafe as well, through contamination from biological, chemical, or physical hazards. Sometimes, bacteria or other biological agents that are already present on foodstuffs (as contamination) begin to multiply, and/or produce toxins, that make the food unsafe. Food spoilage is generally apparent through the senses, whereas food contamination with pathogens typically is not.
In some situations, spoilage or deterioration of a food product make it more vulnerable to pathogenic growth. Also, some of the same factors that contribute to spoilage can simultaneously contribute to growth of pathogens. Thus, the two ideas, while not identical, are related. This is why some of the control techniques for preserving freshness and keeping food safe to eat coincide.
Here are some examples of spoilage:
Stale Bread: Bread that has become stale has lost moisture. This is a physical change defined by food scientists as a form of spoilage. Technically speaking, the bread becomes dry because starch molecules in the bread are slowly forming crystals—and capturing moisture from gluten in the bread to do so.
Chilling Injury: A type of deterioration that occurs in fresh produce is called chilling injury, which is browning and pitting that results from extended cold storage (typically below 37°F).
Chocolate Bloom: This is the dull, white streaking we see on a chocolate bar, which actually reflects changes in the crystalline structure of molecules. Bloom is safe, but unattractive. Often, bloom occurs when chocolate is subjected to low or high temperatures (below 27°F or above 80°F).
Freezer Burn: A well-known form of spoilage is freezer burn, characterized by dried-out, light-colored spots on meat. What happens is that ice crystals form in the meat and migrate to the surface, drawing out moisture as they go. Freezer burn, like chocolate bloom, is safe but unappealing. It tends to cause a leathery texture and spotty appearance on meat. Generally, it occurs at frozen meat temperatures above 0°F, so steady, very cold temperatures can help prevent it. Air-tight wrapping helps prevent it as well.
Sour Milk: Milk undergoes gradual change as bacteria ferment its lactose to produce alcohols and acids. Flavor changes, and so does the smell of the product. The characteristic "sour milk" odor comes from lactic acid and related acids produced by bacteria.
Soft Vegetables: Have you noticed that fresh vegetables such as carrots, broccoli, celery, and many others become soft with age? Certain molds and bacteria produce enzymes that cause this change. The crispness of vegetables comes from its cellulose or fiber. The enzymes from microorganisms break down that fiber into smaller molecules that lack woody characteristics, reducing the crisp texture.
Slimy Fruit: Common on aging fruits and vegetables, this is caused by small sugar-like carbohydrates produced by bacteria. Their slime alters both texture and flavor of these foods by changing their natural sugars to related carbohydrates.
Sulfur or Ammonia Odors in Meat: Bacteria in meat bring about a number of changes. For example, types of Clostridium and other bacteria break down proteins into amino acids, and then into foul-smelling byproducts. The amino acid cysteine breaks down into hydrogen sulfide, along with other components. Enzymes in meat also contribute to conversion of amino acids into other unpleasant-smelling compounds with equally unpleasant names like cadaverine and putrescine.
Rancid Fats: What happens when sweet butter or other fats develop a sour flavor? Certain bacteria and fungi convert the fat molecules into glycerol and various acids. The acids impart characteristic sour flavors.
Moldy Cheese: Everyone has seen patchy mold growth on foods, from cheese to produce to bread to leftovers. Molds, unlike some forms of spoilage, generally pose a food safety risk. This is because some molds produce toxins (mycotoxins) that can cause illness.
Food preservation techniques have their basis in controlling factors that favor spoilage processes. Many of the same factors also favor the growth of pathogens, and will thus be familiar to you if you have studied foodservice sanitation.
The objectives of food preservation are to:
Common preservation techniques focus on the chemical composition of the food. Others employ temperature control. Some use basic physics.
According to the Purdue University Department of Food Science, "The two most important chemical composition factors that affect how a food is preserved are water content and acidity."
We will take a look at these factors next.
Water is not only about moisture; it's about available water, i.e., water that is not bound to other constituents of the food and is available to enter into chemical reactions, such as spoilage processes, enzymatic reactions, or growth of pathogens.
The highest possible value of available water is 1.0, which represents 100% water, or pure water.
Bacteria, yeasts, and molds need a minimal level of available water in order to flourish. Here are some preferred available water levels for microorganisms:
How much available water is in foods? Here are some examples:
In general, bacterial pathogens need an available water level of at least 0.85 to be able to multiply. As you can see, many fresh foods indeed offer high availability of water.
This is why drying or dehydrating food works as a simple preservation technique. When available water is too low, bacteria cannot flourish.
Other simple techniques that reduce available water are adding salt or sugar to a food. These tie up some of the water, reducing availability for spoilage activity. There are other food additives that accomplish this as well. Fructose, glycerol, sorbitol, mannitol, and propylene glycol are examples of additives that attract and hold available water within a food. These additives are classified by food scientists as humectants. By reducing water availability, they help to preserve foods and extend shelf life.
You probably remember from chemistry classes that we can express acidity as pH, on a scale of 0 to 14. This is actually a logarithmic measure of hydrogen ions. A pH of 7 is neutral. A pH below 7 is acid, whereas a pH above 7 is alkaline.
If you have taken a ServSafe® or similar sanitation class, you've seen acidity addressed as one of the factors that contribute to growth of pathogens. The acronym for these conditions is: FAT TOM, which stands for potentially hazardous Food, Acidity, Temperature, Time, Oxygen (for those pathogens that require oxygen), and Moisture (or available water).
For foodborne pathogens, the "A" actually means "slightly acid or near neutral". Here are some preferred pH ranges for microorganisms:
As you can see, getting the pH of a food below about 4.2 inhibits growth of microorganisms. This means it can also work as a preservation technique.
What pH do common foods begin at? Vegetables typically run at 4.0 – 7.0, in the ideal range for bacterial growth. Meats run at pH 5.0 – 7.0. Milk is 6.3 – 8.5. Eggs are at 7.1 – 7.9. In fact, many common foods feature a pH range that's favorable to both spoilage bacteria and pathogens.
Adding acid, such as vinegar, can work as a preservation technique because it reduces pH. This is why we see it in old-fashioned preservation techniques, such as pickling. In food processing, we also see pH control agents or acidulants in the form of malic acid, tartaric acid, citric acid, and others.
Because many spoilage microorganisms as well as pathogens prefer danger-zone temperatures to flourish, keeping food cold can extend shelf life while supporting food safety.
Many foods don't lend themselves to other preservation techniques, as we want to enjoy them in their fresh state. Examples are vegetables, fruit, and in some cases, seafood, poultry, and meat.
For these foods, which are both perishable and potentially hazardous, maintaining cold temperatures is a ready option. Refrigeration (at or below 41°F) does not halt spoilage and microbial growth, but it does slow down these processes. For purposes of preservation and safety, it's important to control temperature throughout the farm-to-fork cycle.
The longer a food is exposed to undesirable temperatures, the more spoilage and/or pathogenic growth can occur. The effects of time and temperature abuse are cumulative. Time and temperature exposure over the life of a food in the farm-to-fork chain affect both freshness and safety.
From an economic standpoint, refrigeration is expensive, as costs for warehousing, transporting, and storing foods at the site of a consumer's home or foodservice operation are greater than for shelf-stable foods. The tradeoff is desired freshness.
Freezing food (typically at -10°F or below) does halt microbial growth, but it does not destroy bacteria. Interestingly, freezing reduces available water by solidifying it into ice. Some bacterial species can become active again upon thawing.
Food technologists employ a number of techniques to achieve rapid cooling or freezing. The Individual Quick Frozen (IQF) process, for instance, uses carbon dioxide to freeze foods (from 160°F to -10°F) in about 10 minutes. Shortened time in the danger zone makes the food safer and preserves quality.
Purdue University scientists point out that freezing, too, has an economic cost. However, quality is maintained for extended periods of time. They add, "Consumers perceive food items such as frozen meals and desserts as more convenient than making them from scratch at home. In addition, frozen fruits and vegetables are perceived as fresher than canned."
Evolving from ancient sun-drying, many of today's food processing techniques still focus on the basic concept of removing moisture from foods to extend shelf life.
Some foods, such as fruit pieces for cereals and potato flakes, are dried in a revolving, heated drum. This is called drum drying.
Another technique called spray drying is used for dairy products, dried eggs, and instant coffee. A fluid product is atomized and sprayed through a heated chamber so that evaporation takes place quickly, forming a powder on dried food that retains a great deal of quality.
In freeze-drying, technologists evaporate off moisture from a frozen food, in a vacuum. Coffee, ice cream, fruits, and spices are among the foods that can be freeze-dried.
The craft of charcuterie has taken hold in the upscale foodservice marketplace over recent years. Some people view it as a back-to-basics concept because it is essentially a traditional culinary art of preserving food by hand.
Charcuterie means drying and curing meats. It is often used for beef, pork, and poultry. Dating back to practices that began as many as 6,000 years ago, it uses the basic food preservation concepts of removing moisture from meats through the use of salt to prevent microbial growth. Andy Frame, quoted in Food Safety News, calls charcuterie “the opposite of fast food”. It has taken off as a do-it-yourself culture, he points out, with chefs and consumers alike making their own prosciutto, lardo, capicola, soppressata, and other specialty uncooked meats.
Is it safe? Not necessarily, reports Food Safety News, because the parameters of available water and pH require careful control. One consultant cited in Food Safety News suggests it is important to buy a water activity meter for about $2,000 and spend money on lab testing to verify control systems. The USDA says meat pH must drop to 5.3 within an acceptable time and temperature combination to manage hazards.
In 2015, the Portland Press Herald reported that hundreds of pounds of cured meat had been embargoed by local health officials due to food safety concerns. They placed the burden on proving safety on restaurant operators, according to the story.
Food Safety News points out that foodservice sanitation requirements for charcuterie vary by state, and may call for a variance.
A key hazard in charcuterie is failing to reduce available water adequately, according to Food Safety News. Have there been foodborne illnesses associated with dry, cured meats? Yes.
Food Safety News summarizes these findings:
Thus, charcuterie, as any method of food preservation, requires controls and verification systems (a HACCP plan) to ensure success in applying the basic science of food preservation to ensure food safety.
Ordinary air contains 78% nitrogen, 21% oxygen, and 0.03% carbon dioxide. Modifying proportions of these gases can have an impact on food spoilage and shelf life. Modified Atmosphere Packaging (MAP) controls levels of gases to inhibit the growth of bacteria by sealing a food product in a package with specific mixtures of gases.
In a typical MAP solution, oxygen may be extensively replaced with carbon dioxide and/or nitrogen gases. For fruits and vegetables, replacing some of the oxygen with carbon dioxide can delay ripening to support lengthened timeframes from farm to fork. Below oxygen levels of 8%, fruits and vegetables do not produce ethylene, a compound that promotes ripening. A typical oxygen ratio for produce is 1-5%.
On the other side of the coin, researchers have also been testing very high levels of oxygen for produce, such as 70-100% oxygen mixtures. This method, called oxygen shock, shows some promise in inhibiting the growth of both aerobic and anaerobic bacteria. However, researchers have also identified instances where the approach actually stimulates growth of pathogens. Further research is underway to determine precise factors for success.
Gas mixtures are specific to the food involved. For example, packaging of snack products may rely heavily on nitrogen gas, which does not specifically deter the growth of pathogens, but does prevent rancidity.
A risk with extremely low-oxygen mixtures is proliferation of anaerobic bacteria within the package, such as Clostridium botulinum. It forms toxins, which are then present in the packaged food and can cause foodborne illness.
Various packaging films used for MAP allow varying amounts of breathability. Selecting the right packaging film is a piece of the puzzle in creating a controlled environment for safe food. Some amount of oxygen is often allowed in the management of the Clostridium risk.
Also, the gas environment may change over time. Fruits and vegetables respire or breathe, modifying their own atmosphere. Available water and pH, of course, are also factors that will influence growth or control of pathogens. All of these factors are taken into account when a MAP packaging solution is designed for any particular food.
Technologies include permeable films, films with antimicrobial properties, edible coatings, and intelligent packaging that uses sensors to indicate the quality of a packaged produce.
MAP packaging is used for fresh produce, meats, seafood, cheese, and some convenience and snack foods. As with any other food, a consumer or foodservice operator needs to follow standard operating procedures for MAP-packaged foods, keeping foods refrigerated as directed and observing package dates. It’s important to note that once a MAP package has been opened, the controlled environment is gone; food may spoil much faster from this point on.
An emerging food preservation technique gaining in popularity today is called High Pressure Processing (HPP). Another term for this technique is Pascalization, named for the 17th century French scientist, Blaise Pascal, who identified the effects of pressure on fluids.
Very high pressure can inactivate bacteria, molds, yeasts, thus reducing spoilage and growth of pathogens. Applications to guacamole increase shelf life from 3 to 20 days.
Another technique, called High Hydrostatic Pressure in Combination with Temperature (HPT), adds a heat step early in the process. This destroys most of the enzymes in the food, thus further extending shelf life. HPT protects food quality as well, and shows the most promise for the longest shelf life.
Phys.org notes that HPP does “not greatly affect the nutritional value, taste, texture, and appearance” of food. Like pasteurization, it is considered a natural processing technique because it does not use chemicals.
HPT also requires low energy and water usage, making it an environmentally friendly solution.
As briefly mentioned earlier, a number of chemical food additives can help with food preservation. Some of these are on the GRAS (Generally Recognized as Safe) list; others are approved by the FDA for use as food additives.
Examples include some additives that inhibit growth of bacteria and molds, such as sodium or calcium proprionate, potassium sorbate, sodium acetate, sodium benzoate, and sulfite. Sodium nitrite is used for cured meats, and specifically inhibits growth of Clostridium bacteria.
Another category of chemical preservatives is antioxidants. These work by limiting the amount of oxygen available to microbes. BHT, BHA, vanillin, and the popular vitamins alpha-Tocopherol (vitamin E) and ascorbyl palmitate (a form of vitamin C) fall into this category.
Some food additives used for food preservation have come under fire in recent years. For example, CNN reported in 2014 that butylated hydroxyanisole (BHA) is “reasonably anticipated to be a human carcinogen” according to the National Institutes of Health. Likewise, CNN reported there is speculation that propyl gallate may be an endocrine disruptor, a compound that interferes with normal hormonal functioning.
As highly publicized in October 2015, the World Health Organization (WHO) concluded that processed meats, such as ham, bacon, or hot dogs, raise the risk of colon cancer. There was no definitive conclusion as to why. TIME magazine, however, summarizes three ideas. One is that nitrates or nitrites used as preservatives may turn into nitrosamines, which can damage cellular DNA.
Aside from common chemistry, another common approach to food preservation is the application of heat, also called thermal processing. The idea with heat processing is to destroy both spoilage microorganisms and pathogens.
Absolute destruction of all microbes would be termed sterilization, and is not practical due to the destruction of food quality that would accompany extended processing at very high heat. However, food scientists use the term commercially sterile to mean that food is, practically speaking, preserved and safe.
They also quantify destruction of pathogens on a logarithmic scale. A logarithm is just an exponential count of surviving bacteria. Jumping from each log number to the next, 90% of existing bacteria have been destroyed. In food processing, log counts correspond to scientific standards with generous padding for safety, and may be expressed as "a 3-log reduction" or "a 4-log reduction" in bacterial counts. Established safety standards mean that, for all practical purposes, there are no bacteria remaining.
One familiar heat-processing procedure is blanching for fruits and vegetables. Quick application of heat, such as dipping in boiling water, inactivates enzymes prior to freezing. This is important because some enzymes would otherwise continue to be active, causing food spoilage. Another familiar procedure is canning, both at home and commercially, which uses heat to destroy microbes.
Every heat processing technique follows established time and temperature standards required to achieve desired results. As in food service, the combination of time and temperature are important controls for destruction of microbes. The two factors work in tandem, and government agencies set standards based on numerous scientific studies.
Invented by the French scientists Louis Pasteur and Claude Bernard in 1864, pasteurization involves applying a specified amount of heat for a specified length of time to preserve food. The process eliminates public health hazards (pathogens), while also extending shelf life.
Pasteurization is best known for its application to milk, but it is also common today for juices, maple syrup, soy sauce, and a variety of liquid foods, as well as for raw shell eggs.
Food safety officials vehemently advise use of pasteurized milk and dairy products due to the common presence of pathogens such as Listeria, Salmonella, and E. coli. Listeriosis from unpasteurized milk or cheese made from unpasteurized is a serious public health issue, especially for pregnant women, who may experience stillbirth as a result of foodborne illness. Milk pasteurization also helps prevent diseases such as tuberculosis, diphtheria, and typhoid fever, according to the FDA.
Milk pasteurization may use one of a variety of time and temperature combinations, such as:
All of these combinations are effective at making milk safe. Juice, likewise, has time and temperature standards for pasteurization, such as 180°F for 15 seconds. UHT pasteurization is combined with aseptic packaging to create shelf-stable products that do not require refrigeration as long as packaging remains intact (creamers, juice boxes, soups, milk products, and sauces, for example).
Another application of pasteurization is for fresh shell eggs. This uses a patented process with a series of warm-water baths to eliminate the risk of bacteria (such as Salmonella) and viruses (such as Avian flu) without cooking the eggs. Again, this is based on a combination of time and temperature.
The USDA says, "In-shell pasteurized eggs may be used safely without cooking," and the FDA exempts pasteurized shell eggs from the definition of a potentially hazardous, temperature control for safety (TCS) food. These are safe to eat undercooked or even raw, or to use in recipes calling for raw eggs (e.g., Caesar salad dressing, eggnog, smoothies, raw cookie dough, etc.).
As with other preservation techniques, pasteurization increases shelf life or storage time. For eggs, this is particularly important, as an estimated 4 out of 5 Salmonella Enteritidis illnesses trace back to raw or undercooked eggs.
Two of the newer preservation techniques are pulsed light and pulsed electric field.
Pulsed Light Technology (PLT) was first developed by the FDA in 1996. Its aim is to destroy microbes with minimal effect on food quality. The idea is to destroy the cell membranes of microbes through very short bursts of high-intensity white light, at roughly 20,000 times the brightness of the sun. PLT is used with some produce, as well as with packaging materials, to render them aseptic.
Pulsed Electric Fields (PEF), used for fruit juices, milk, and liquid eggs, apply high voltage electricity for short times to similarly inactivate microbes.
Advantages of these techniques are maintenance of nutritional value and characteristics that make food aesthetically fresh.
According to the FDA, “A great deal of research remains to be done before pulsed light technology will be suitable for commercial use.”
Irradiation, generally applied at the end of food processing, exposes food to gamma rays for a short time.
"The energy from irradiation moves through the food, much as microwaves travel through food in a microwave oven. Food irradiation is sometimes nicknamed cold pasteurization, because it applies energy to destroy pathogens...but it does not specifically heat the food." (Grossbauer, 2001)
Irradiation involves low doses for short times, and does not make food radioactive. The process has negligible effects on nutrient values, and serves as an adjunct to food preservation techniques through the flow of food. It does not destroy all pathogens, but rather serves as a deterrent. For example, it can destroy trichinella parasites in pork, or reduce bacterial counts of E. coli on ground beef.
In fresh root vegetables like potatoes, it inhibits sprouting. For bananas, it delays ripening, allowing more time for transport and sale.
In 2008, the FDA approved irradiation for fresh greens, such as spinach and iceberg lettuce. Citing foodborne illness outbreaks in greens (e.g., E. coli in spinach), the FDA suggested this end-stage process could help ensure safety of the food supply.
By 2015, the FDA had approved irradiation for many foods, including: beef, pork, poultry, lobster, shrimp, crab, oysters, clams, mussels, and scallops, fresh fruits and vegetables, seeds for sprouting, shell eggs, spices, and seasonings. A Radura symbol is required on packaging for irradiated foods. Individual ingredients, such as spices, used in multi-ingredient foods do not have to be labeled.
Some consumers have hesitancy about irradiation. Here are positions from leading organizations.
An emerging technology in food safety is called nanotechnology, defined by the National Nanotechnology Initiative as "a science that involves the design and application of structures, devices and systems on an extremely small scale, called the nanoscale – that is, billionths of a meter, or about 1-millionth the size of a pinhead."
Nanoparticles have a variety of applications in the food supply, as well as in packaging and cooking utensils. Many nanoparticles are used to destroy bacteria.
According to UnderstandingNano.com, "Nanosensors are being developed that can detect bacteria and other contaminates, such as Salmonella, at a packaging plant. This will allow for frequent testing at a much lower cost than sending samples to a lab for analysis. This point-of-packaging testing, if conducted properly, has the potential to dramatically reduce the chance of contaminated food reaching grocery store shelves."
Safety of the emerging technology is still under review, reports David Biello in Scientific American. The FDA does not require proof of safety for nanoparticles in food, he reports, but instead requires manufacturers to demonstrate that products containing them are safe.
Nanotechnology can be used to protect against spoilage, increase availability of nutrients, or even function as an antioxidant.
In a review article, He and Hwang note that safety concerns include food allergy and heavy metal release. They say, "The fate and potential toxicity of nonmaterioals are not fully understood at this time." They recommend continued research on health, safety, and environmental impact. The FDA position is that it "does not make a categorical judgment that nanotechnology is inherently safe or harmful."
The quest for food preservation is nearly as old as food itself. Food preservation methods serve to extend shelf life, while also controlling proliferation of bacteria, viruses, and other microbes that could cause foodborne illness.
Simple principles guide many of our preservation methods. Controlling available water, pH, and temperature are among them. Pasteurization is one of the temperature-based methods that has been in use for nearly a century and a half.
Some of today's techniques use common food ingredients (such as salt, sugar, or vinegar), preservatives, and even vitamins, for their antioxidant properties, to modify microbial growth factors. Other techniques use the physics of electricity, light, or irradiation to preserve food.
Some food preservation techniques can be used at home, such as simply freezing food, while other techniques require precision controls in a processing environment, such as pasteurization.
Regardless of the methods, the objectives remain the same: retaining food quality for as long as possible, and ensuring safety of the food supply.
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