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College of Agriculture & Life Sciences


Dressings, sauces, marinades, and similar food products depend on their acidity to prevent spoilage. They may consist of naturally acid foods, such as fruit juice or tomatoes, or they may be formulated by combining acid foods with other foods to achieve the desired acidity. Some foods, such as vinegar and certain pickled vegetables, may develop acidity from microbial fermentation.

Because foods without adequate acidity may allow the growth of microorganisms that cause foodborne illness, the federal government regulates the manufacture of these products. Title 21 of the Code of Federal Regulations, Parts 114 & 108 (21CFR114 & 21CFR108), regulate acidified foods. Processors in North Carolina are subject to regulatory inspection by both the North Carolina Department of Agriculture (NCDA) and the U.S. Food and Drug Administration (FDA).


Table of Contents


Categories of Foods Preserved by Acids

Naturally acid foods and fermented foods along with jams, jellies, preserves, and certain dressings and sauces are exempt from the provisions of 21CFR114. Generally, if a food is formulated predominantly from acid foods, it qualifies for exemption. If, however, the food contains a mixture of acid and low-acid foods, the regulation applies.

Acidity and pH

The acidity of a food is indicated by its pH value. The pH scale (Figure 1) ranges from O to 14, with pH 7 being neutral. Any pH below 7 falls in the acidic range while those above pH 7 are in the basic range. The lower the pH reading, the more acid the food.


Figure 1. The pH scale.


The regulation requires that foods preserved by acidity have a pH of 4.6 or less. At these levels, production of deadly toxins by the organism that causes botulism are inhibited. Foods that have a pH greater than 4.6 are called low-acid foods. Most fruits and fruit products are acid foods, and most vegetables and meats are low-acid foods. Remember, low-acid foods have high pH readings. Table 1 lists the pH values of some common foods.


Table 1. pH Values of Common Foods


Formulating Acidified Foods

Acid foods depend on one or more food acids, such as citric, malic, or acetic acid, to achieve stability. Most acidified foods, including dressings and sauces, use vinegar (acetic acid) to attain the desired acidity. Vinegar is a familiar and effective source of food acid.

When low-acid foods are used in formulations, it is important that they be properly acidified before they have a chance to spoil. Their pH must reach its equilibrium value before spoilage begins. The rate of acid uptake by low-acid foods can be influenced by factors such as piece size or the presence of a waxy peel. These factors can often be overcome simply by cutting the low-acid food into smaller pieces. When oil is used in the formulation, the low-acid components should reach an equilibrium pH of 4.6 or less before the oil is added.

Most foods possess a chemical property called buffering capacity that allows them to resist changes in pH. At certain pH levels, greater amounts of acid must be added to the food to continue to reduce the pH. Buffering can be a useful property because it prevents changes of pH with minor variations in the amount of acid added.

Measuring pH

The pH of a food is usually determined using a pH meter. Electrodes from the meter are inserted into solution to measure the pH electronically. A variety of pH meters are available from scientific equipment suppliers. Prices range from less than $100 to over $1,000, depending on the type of meter and its features. A portable meter is shown in Figure 2.


Figure 2. Portable pH meter.


The pH can also be estimated colorimetrically using pH test papers, which change their color according to the acidity of the solution in which they are placed. These papers are available from scientific supply houses for a few dollars. Regulations allow calorimetric monitoring for foods with pH levels below 4.0. To provide a safety factor, acid and acidified foods are normally formulated to a pH well below 4.6. Most acidified foods have a pH of less than 4.0.

Before the pH of a food is measured, the food should be in liquid form or prepared as a puree in a blender. Distilled water may be added to aid in mixing the components thoroughly. Be sure to test a sample representative of the whole. If an oil layer is present, it may need to be decanted so that the nonoil phase can be measured. To measure the equilibrium pH of a low-acid food, first separate the low-acid food portion from the acidifying portion. Then prepare and measure the low-acid portion.

When measuring the pH of a prepared sample, carefully follow the instructions provided with the pH meter and also be sure to follow these guidelines:

Caution: Do not rub the electrodes with the tissue as rubbing produces a static charge that can produce erroneous readings.

Controlling Spoilage

Properly acidifying food to a pH of 4.6 or less will inhibit the growth and formation of toxins from the bacteria that cause botulism. Acidification cannot take the place of proper sanitation and care in manufacturing. The manufacturer must therefore adhere to the highest standards of cleanliness and product protection. These standards are covered under another regulation: 21CFR110, often referred to as "Good Manufacturing Practices."

Even if manufactured with proper acidification and sanitation, a food product may still be spoiled by bacteria, yeasts, and molds. To prevent this spoilage, processors usually heat acid acidified foods to 180°F and package them hot. This process kills yeast and mold spores on the products and in the container and cap. Before the product is placed in cardboard cases it should be cooled to prevent "stack burn." Some products should not be heated. In those cases, more acid should be added, chemical preservatives should be used, or both. Sodium benzoate and potassium sorbate are the preservatives commonly used. They are often used together to take advantage of their combined effects.

Meeting Other Requirements

The potential for improper formulation in the manufacturing of acidified foods has been designated a public health hazard. To reduce the likelihood of foodborne illness, the acidified foods regulation requires plant registration and the filing of the scheduled process for each product with the FDA. The information filed must come from a recognized process authority who is qualified by training and experience. In addition, persons supervising the manufacturing of acidified foods must attend an approved school and must earn a certification as a supervisor for acidified foods. The regulation also addresses other important aspects of manufacturing, such as monitoring and record keeping.

Labeling the Product

Regulations dealing with food labeling are extensive and complicated. Many are in a state of change. Before purchasing labels, consult the regulatory authorities.

Where to Get Help

A number of public agencies can help in understanding and meeting requirements for the preservation of acidified foods.

For assistance with training, certification, and technical matters, contact:

North Carolina Cooperative Extension Service
Department of Food Science

Campus Box 7624
North Carolina State University
Raleigh, NC 27695-7624

For copies of regulations and label review, contact:

North Carolina Department of Agriculture
Food and Drug Protection Division

4000 Reedy Creek Road
Raleigh, NC 27607-6468

For information on registration and filing processes, contact:

U.S. Food and Drug Administration (FDA)
LACF Registration Coordinator (HFF-233)
Center for Food Safety & Applied Nutrition

200 C Street, S.W.
Washington, DC 20204

For advice on starting a business and developing a business plan, contact:

North Carolina Small Business and Technology
Development Center

820 Clay Street
Raleigh, NC 27605


Further Reading

The following publications provide useful information on preservation of acidified foods.

Acidified and Fermented Foods
by Ralph Costilow
Pickle Packers International, Inc.
St. Charles, IL 60174

Canned Foods
Food Processors Institute
1401 New York Ave. N.W., Suite 400
Washington D.C. 20005

A Complete Course in Canning
by A. Lopez
The Canning Trade, Inc.
Baltimore, MD 21218-4576


Keys to Formulating Safe Acidified Foods

  • Formulate the product carefully. Be sure to add enough acid to reach a pH of 4.6 or less.



Prepared by

John E. Rushing and Patricia A. Curtis
Extension Food Science Specialists


Published by



5/96—IM—GBB—260179 (Reprint) AG-479

Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Last modified on April 3, 2000 by Terri Appelboom,
NCSU/CALS Communication Services


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