This section will address the various methods used to preserve food products. Included will be a description of various heating processes as well as preserving methods using cold temperatures. The detail on each category will depend on its popularity.

Foods Preserved By Heat

Retorted-pressure process

It is possible to destroy bacterial cells and spores by exposure to heat for various lengths of time. The severity of the process depends on the pH of the food. Table 1 lists various foods and their respective pH value. Foods that have a pH exceeding 4.6 (low acid) must be processed under pressure in order to reach “commercial sterility”. Which is the condition achieved by application of heat which renders such food free of viable forms of microorganisms of public health significance, as well as any microorganisms of non-health significance capable of reproducing in food under normal storage conditions. The canning of low-acid foods requires a large capital investment therefore further detail is not necessary for this publication.

Acid/Acidified Foods

Acid foods have a natural pH of 4.6 or less such as tomatoes. Because of the natural acidity these products can be cooked at atmospheric temperatures (212°F.) or less, usually around 190°F., and filled hot for preservation. Many fruits and their juices are processed in this manner.

Sometimes acids or acids foods are added to low acid foods to yield products that have a final equilibrium pH of 4.6 or less: These foods are called acidified and have a specific regulation in the Code of Federal Regulations (CFR), see page 127. Many products that new processors are trying to produce fall in this category. Relishes, most hot sauces and many pickled products are termed acidified foods. Strict regulations have been set up for these products; see part 114 CFR under regulations.

Foods Preserved by Cold


This method of preservation is growing in popularity. It involves blending and/or processing of foods and then chilling them to temperatures just above the freezing point of water, approximately 32-36°F. The growth of most microorganisms is retarded at these temperatures and many may be killed. There are however, certain microorganisms that are able to grow slowly and the spores do survive. Any abuse in holding temperatures upwards of 38°F. may cause spoilage problems and decreased shelf-life. Certain organisms capable of causing food-borne illness may also become a problem.


This is probably one of the safest forms of food preservation. Frozen food is normally held at O°F. The freezing and storage process kills most bacteria, but the spores are able to survive. Once frozen foods are thawed they should be utilized promptly.

The equipment for freezing foods can be quite costly and the energy to operate it is also expensive. Small scale operations are in existence, but usually most freezing operations require a high capital investment.

Foods Preserved by Reducing Moisture


Bacteria, yeasts and molds are like people, in that they must have moisture to survive. Dehydrated and freeze-dried foods have a low moisture level and therefore are shelf stable at room temperature. Moisture is removed in dehydrated foods by hot air, whereas freeze-dried products are produced under a vacuum with minimal heat involved.

Reducing Water Activity

Water activity (aw) is probably a new term for most people. It is essentially a measure of free or available water in a food system. It is the equilibrium relative humidity of food divided by 100. Pure water has an aw of 1.0. Most foods will be at .97 to .99, but dissolved substances such as sugar and salt will lower this reading by binding the moisture. Other substances which can reduce water activity include: starches; phosphates; polyhydric alcohols such as sorbitol; mamitol; propylene glycol and glycerol. Dried foods have an aw of 0.60 or less. Bacteria need more water than most yeasts and molds. The osmophilic yeasts can grow where there is very little moisture available. Table 2 illustrates the minimum water activity of certain types of microorganisms.

High sugar and acid are the two main preserving factors for jams and jellies. Certain cheese based dips are also preserved by lowering the aw and controlling pH.

Jams, Jellies & Related Products

Jellies, jams and preserves are popular items with many specialty food processors. Producing a quality product that jells properly is not always easy. Sometimes one ends up with ice cream topping instead of jelly.

The four essential ingredients for proper jelly manufacture are fruit, pectin, acid and sugar. These ingredients must be present in proper quantities to yield an acceptable jellied product.

Some fruits, such as apples, have enough natural pectin to make a high quality product. Others require added pectin especially if they are used for making jelly; which has to firm. Most processors prefer adding pectin to all formulations so that they can use fully ripe fruits; reduce cooking time; and increase yields.

Sugar concentration and pH are two very critical factors in jelly manufacture. Normally a pH of 3.1 with a sugar content of 67.5 % is considered optimum.

There are several excellent references on jelly manufacture including, “Handbook for the Fruit Processing Industry” from Hercules, Inc. and A Complete Course in Canning, listed under references.

Other Methods of Preservations


This process involves sterilizing food at high temperature for a short time, cooling it in a sterile chamber and then packaging the product in a sterile container under aseptic conditions. This system has historically only been used on high acid products. However, there are certain low-acid products being produced today. The equipment is expensive and technological expertise is a must for aseptic processing.


The use of ionizing radiation has been cleared for use on raw fruits, raw vegetables, spices and raw pork. This is a cold process with little heat involved. It will extend shelf-life of the above named products. Consumer acceptance and the high cost of an irradiation facility will delay the use of irradiation in our food system.


Preservatives of various types have been used for many years. Smoking and salt were two of the earliest substances used. There are many others in use today.

Preservatives can be divided into two categories according to function; (1) those which act to control or prevent growth of microbes, and (2) the ones used to control deteriorative chemical reactions such as rancidity. Salts of various acids such a propionic, sorbic, and benzoic are some of the most important microbe inhibitors used in foods. Calcium propionate is added to, bread and other baked products to retard mold growth. Potassium sorbate also is effective against mold as well as yeasts in cheeses, syrups, soft drinks, jams, mayonnaise and pickles. Sodium benzoate is an especially effective preservative against yeasts and bacteria in high acid foods such as sauces and fruit drinks. It is also popular for use in carbonated beverages. Table 3 gives the use levels of the various preservatives.

Parabens are yet another category of preservatives that are active against yeasts and molds. The methyl and propyl parabens antimicrobial effectiveness is best at a pH of 7 or above.

Sulfur dioxide has been used as a sanitizing agent in wine-making since ancient times. Today sulfur dioxide and its sulfite salts are useful in controlling microbial growth and maintaining color in a wide range of food products. Sulfur dioxide and the sodium and potassium forms of sulfite, bisulfite and meta bisulfite are all considered GRAS (Generally Recognized As Safe) for use in foods except fresh fruit and vegetables (other than potatoes) although their status is under active review by FDA.

Sulfiting agents are effective antimicrobials preservers of quality, including color and flavor, however, their safety has come under close scrutiny. This is due to adverse reactions experienced by certain asthmatics, including several deaths. It is estimated that about 8% of the asthmatic population is sensitive to these substances. If you use a sulfiting agent, include it in the ingredient statement. Properties and use limits of some commonly used preservatives are listed in Table 7.

Table 7. Properties and Use Limits of Commonly Used Preservatives.
Agent Optimum pH Range Targeted Organism Use Limits
Sodium or Potassium Benzoate 2.5 – 4.0 yeasts & bacteria do not exceed 0.1% by weight
Potassium Sorbate 6.0 or less molds & yeasts do not exceed 0.1% by weight
Propionates 5.0 or less molds do not exceed 0.3% by weight
Parabens 7.0 or above yeasts do not exceed 0.1% by weight
Sulfites and Sulfur Dioxide 4.5 or less yeasts, mold & bacteria depends on product, not a popular choice
Nitrites/Nitrates 5.0 – 5.5 Clostridium botulinium spores Mostly used in Cured meat products.
Nitrites do not exceed 200 ppm.
Nitrates do not exceed 500 ppm.

Nitrites, another important preservative used in foods, act to prevent the growth of Clostridium botulinum in cured meats. Nitrites are also responsible for fixing the color in these products as well as enhancing flavor. Nitrites combine with amines in the stomach to form nitrosamines, some of which are known carcinogens. Reduced levels of sodium nitrate are being utilized along with other compounds like hypophosphate, which is believed to block the formation of nitrosamines.

The second category previously mentioned deals with those preservatives which act by inhibiting detrimental chemical changes. Rancidity is one of the most common types of food spoilage which results from this type of reaction in fatty foods. Two of the more common antioxidants include BHA (butylated hydroxyanisole) and BHT (butylated hydroxytoluene). Propyl gallate is often used in combination with BHA or BHT. In addition, citric acid, phosphoric acid and ascorbic acid are often added to enhance the effectiveness of BHA and BHT.

Chelating or sequestering agents are added to foods to bind trace metals which can act as catalysts in food spoilage. These agents play an important role in preventing clouding of what should be a clear soft drink. The prevention of fruit discolorations is aided by the addition of small amounts of these preservatives. Calcium disodium ethylene diamine tetracetate, otherwise known as EDT A, is gaining popularity for its ability to prevent or reduce discoloration, clouding and rancidity. It is used in a variety of products such as cream style corn, black-eyed peas, salad dressings and beer.

Preservatives are normally used in small quantities, therefore it is necessary to be sure the product is dissolved completely and evenly distributed throughout the batch. This is best accomplished by mixing the preservative in a quart or gallon container and adding that to the total mix.

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