This document discusses methods for controlling microorganisms through lowering the pH, specifically by adding organic acids. It defines pH and explains how most microbes grow best around pH 7 but some bacteria can grow at pH lower than 4. Organic acids like sorbic acid, benzoic acid, acetic acid, and propionic acid are commonly used to lower the pH of foods and inhibit microbial growth. Their effective pH ranges are listed. The document also provides information on measuring pH and discusses how lowering pH through acidification or fermentation can preserve foods.

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CONTROL OF MICROORGANISMS by
lowering ph (by adding organic
acids)

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Contents
 Introduction
 Type of CONTROL OF MICROORGANISMS
 Methods of controlling Microbes
 Conditions Influencing Antimicrobial
Activity
 CONTROL OF MICROORGANISMS by lowering ph
(addition of organic acids)
 pH
 PRINCIPLES FOR USING Disinfectants
 Influence of pH on the preservation of
foods
 Lower pH
 Acidification
 Measuring pH
 Organic Acids
 SORBIC ACID
 BENZOIC AC1D
 Acetic Acid
 PROPIONIC ACID

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CONTROL OF MICROORGANISMS
Sterilization
Sterilization is a treatment that kills or removes all living cells, including viruses and
spores, from a substance or object.
Disinfection
Disinfection is a treatment that reduces the total number of microbes on an object or
surface, but does not necessarily remove or kill all of the microbes.
Sanitation
Sanitation is the reduction of the microbial population to levels considered safe by
public health standards.
Antiseptic
Antiseptic is a mild disinfectant agent suitable for use on skin surfaces.
-cidal
A suffix meaning that “the agent kills.” For example, a bacteriocidal agent kills
bacteria.
-static
A suffix that means, “The agent inhibits growth.” For example, a fungistatic agent
inhibits the growth of fungi, but doesn’t necessarily kill it.
Type of CONTROL OF MICROORGANISMS
Controlling microorganisms can either be positive or negative:
POSITIVE control - we want to make microbes grow: Industrial Fermentations;
beer, wine and bread making
NEGATIVE control - we want to destroy them by (1) physical or chemical means
or (2) antibiotics
Usually we mean negative control.

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Methods of controlling Microbes
Two methods are used in this contex:
1. Physical methods
2. Chemical methods
Conditions Influencing Antimicrobial Activity
Several critical factors play key roles in determining the effectiveness of an
antimicrobial agent, including:
 Population size
 Types of organisms
 Concentration of the antimicrobial agent
 Duration of exposure
 Temperature
 pH
 Organic matter
 Biofilm formation
CONTROL OF MICROORGANISMS by lowering ph
(addition of organic acids)
Factor pH is generally related to the chemical way of controlling microbes because pH
is generally reduced by adding the organic acids, which are a type of disinfectant.
pH
The pH scale was introduced by the Danish chemist Søren Sørensen in 1909, when he
controlled the quality of beer brewing. This scale is now widely used in chemistry,
geology, ecology, medicine, industry and agriculture.
In aqueous solution, the pH is defined as:
pH = - log [H+
]
Acids can be used to control microbes, sometimes they work by destruction of
microbial cells, or they work because they create extremes of pH in which microbes

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cannot grow - as with the use of vinegar in canning of vegetables to prevent bacterial
growth.
PRINCIPLES FOR USING Disinfectants
 Most agents are weak acids.
 Most work when in the un-ionized form.
 Most are not effective above their pKa's.
 Most affect flavor.
 Most are affected by processing, order of addition.
Influence of pH on the preservation of foods
Weathering reactions, whether chemical, enzymatic or microbiological, are largely
influenced by the pH of the medium.
Effect of pH on the reactions of impaired food
Weathering
reactions
Minimum
pH
Optimum pH Maximum pH
Growth of mold 1.5 to 3.5 4.5 to 6.8 8 to 11
Growth of bacteria 4.5 6.5 to 7.5 9.0
Growth of yeasts 1.5 to 3.5 4.0 to 6.5 8.0 to 8.5
Enzymatic browning 5 6 to 6.5 7
The above table shows that most microorganisms grow best at a pH close to 7 (4 to
7.5). Some bacteria, including lactic and acetic bacteria, are able to grow at pH lower
than 4.
Lower pH
The reduction of pH of food products can be achieved by two methods.
1. The first is a direct method is to acidification of the product by adding one or
more acids, organic acids are most commonly used acetic acid (vinegar) and
citric acid.
2. The second is an indirect method based on the use of microorganisms in
fermentation.

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Acidification
The acidification is the addition of an organic acid (as acetic acid) or an acidic
ingredient (such as lemon) to a food that is initially slightly acidic. The acid or acidic
ingredient must be added to the proportions determined for the pH of the finished
product is below 4.5.
The 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 is inhibited. 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. 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.
pH Values of Common Foods
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.
Organic Acids
Various organic acids and their salts are common antimicrobials in foods.
 Food preservatives
 Safe in foods
 Mold control in foods and drinks
Agent pH Range
Benzoic acid 2.5-4.0
Sorbic acid 3.0-6.5

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Proplonic acid 2.5-5.0
Acetic acid 3.0-5.0
EFFECTIVE PH RANGE OF COMMON ORGANIC ACIDS
SORBIC ACID
 pKa = 4.8
 Usage level limited to 0.1%.
 Broad spectrum against yeast and molds. Lactic acid bacteria at low pH.
 Major usage in salad dressings, fruit juices, cottage cheese.
 Potassium salt is the most widely used form.
o Potassium sorbate
o Sodium sorbate
o Sorbic acid
BENZOIC AC1D
 pKa = 4.2
 Usage level limited to less than 0.1% by regulation.
 Most active against yeast and molds.
 Major uses in beverages, juices, non-standard "Jellies". margarine.
 Acid form relatively insoluble and not used much.
o Benzoic acid
o Sodium benzoate
Acetic Acid
 pKa = 4.76
 Usage level limited by taste.
 More effective against yeast and bacteria.
 Major usage in "Pickled" foods. Also In "natural" breads.
 Sodium diacetate used in some breads.
o Sodium diacetate.
PROPIONIC ACID
 pKa = 4.87
 Usage limited to 0.1%.
 Effective against molds, but no effect an yeast.
 Major usage in broad and baked goods.
 Calcium salt most used form.
o Propionic acid.
o Calcium propionate.
o Sodium propionate.

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References
1. Montville T.J. and Matthews K. R., 2005. Food Microbiology: an
Introduction. ASM Press. New York.
2. Nakai S. A. and Siebert K. J., 2003. Validation of bacterial growth inhibition
models based on molecular properties of organic acids. International Journal
of Food Microbiology. 86: 249-255.
3. Russell J. B. and Diez-Gonzalez F., 1998. The effects of fermentation acids on
bacterial growth. Advances in Microbial Physiology. 39: 205-234.