Role of Acidulant and pH in Food Safety 

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Acidulants such as acetic, adipic, citric, fumaric, lactic, malic, phosphoric, tartaric acids, and glucono-delta-lactone are commonly used as food additives in processed foods and beverages. They are used for leavening functions in baked goods, control gel formation, and maintaining the viscosity of confections and gelatin desserts.

Besides imparting a sour taste to food, they also to aid in adjusting the pH, enhancing, or modifying the flavors and sweetness of sugar. The pH, used to indicate the hydrogen ion activity in a solution, is measured on a scale of 0 to 14, with seven being neutral.

How Acidulant and pH Affect Microbial Growth

The survival and reproduction of microbes depend on their ability to produce appropriate responses to their immediate environmental conditions. One of the most significant environmental parameters impacting growth and survival is pH.

Effective monitoring of pH in the food industry begins with testing raw materials and continues throughout production to the finished product. Microbes typically respond to acid stress by preventing a damaging drop in intracellular pH (pHi) below a threshold level necessary for viability.

  • Broadly, three distinct strategies are used to prevent such a critical drop in pH.
  1. Enzyme-catalyzed reactions employed by cells that consume protons. Decarboxylation reactions often serve this purpose since a proton is irreversibly incorporated into the reaction product following the removal of CO2. Examples are the decarboxylation of amino acids, such as glutamate, arginine, and lysine
  2. Reactions deployed by cells that produce basic compounds to help neutralize the low pH. The production of ammonia from urea or amine-containing amino acids such as arginine or glutamine is commonly used to counteract acidity
  3. Proton elimination from microbes. Protons can be effluxed from some bacteria using the F1Fo-ATPase while in some fungal or yeast species, a dedicated proton translocating efflux pump is used

In addition to these mechanisms for maintaining pHi, cells often deploy specific protective systems that help cope with acid stress. Modification of the lipid composition of the cytoplasmic membrane to reduce the permeability to protons is found in many microbes.

Acid is an important component of many foods, contributing to human safety as well as desirable flavor. The usual measure of the acid level in food is pH, defined as the negative logarithm of the concentration of hydrogen ions, expressed as gram equivalents per kilogram, essentially, molar concentration.

A value of seven indicates neutrality and is found for pure water. Lower values indicate acidity, and higher values than seven indicate basicity. The scale runs from 0 to 14. Few foods have values above eight.

An important dividing value for food safety is a pH of 4.6. Below pH 4.6, vegetative cells of Clostridium botulinum do not produce toxins, so foods with pH below 4.6 are considered high-acid foods and are safe from botulism. Unfortunately, other pathogens may survive, at least for a little while in high-acid foods.

Low-acid foods are at risk for botulism and so must be protected by severe preservation processes, such as thermal canning, designed to kill heat-resistant spores of C. botulinum. Spores are dormant forms of microbes that are highly resistant to extremes of temperature and other conditions that can kill vegetative or active cells.

These considerations explain why new or well-established preservation processes must be evaluated in the context of a target food’s pH. High-acid foods, with a pH below 4.6, are often pasteurized, meaning that vegetative pathogens are killed, but some spores and spoilage microbes may survive. Such foods are often refrigerated and have relatively short shelf lives because they can eventually spoil.

Low-acid foods require treatments adequate to kill C. botulinum spores and so are considered commercially sterile since a treatment that kills C. botulinum spores will kill nearly anything else. Commercially sterile foods are usually shelf-stable if they are properly packaged and protected from post-processing contamination, meaning they have extended shelf lives at ambient or room temperatures.

Canning is a familiar and mature preservation process for low-acid foods and is governed by Food and Drug Administration (FDA) regulations, known as Low Acid Canned Foods regulations. These require detailed filing with the FDA of the exact process and container description and then careful process control by properly trained operators and management.

Alternate processes for low-acid foods must demonstrate effectiveness equivalent to properly performed thermal canning. Examples include irradiation, pressure-assisted thermal sterilization, low-acid aseptic processing, and microwave sterilization. In general, the standard for low-acid foods is a 12 log reduction in C. botulinum spores.

By comparison, FDA requires a 5 log reduction in target organisms for almost all other types of preservation processes. The rationale has traditionally been that botulism is so hazardous and the risk is very high, whereas most other pathogens are somewhat less severe, and risks are somewhat mitigated by other measures, such as low pH.

Depending on the pH of the product, you may be able to use paper pH strips/litmus paper or a pH meter. Litmus paper relies on a color change in the paper to indicate product pH. Litmus paper can be used to measure pH if the product pH is less than 4.0. Litmus paper is an inexpensive way to test pH but can be inaccurate or difficult to read.

A pH meter measures the amount of hydrogen-ion (acid) in solution using a glass electrode immersed in the solution. A pH meter must be used when a product’s pH is greater than or equal to 4.0. If you are canning acidified foods, accurately monitoring and recording the product pH is key to knowing that you are selling a safe product.

When using a pH meter, there are several things to consider: accuracy, calibration, electrode, and temperature.

Accuracy is listed in a range of +0.xx pH units. A meter may read so many pH units above or below the actual pH of the product. Purchase a pH meter with an accuracy of +0.02 units or better. A pH meter with an accuracy of +0.01 is a good choice, while a pH meter with an accuracy of +0.10 is not accurate enough for all products.

All pH meters must be calibrated and checked against a known standard to assure accuracy. Standards are colored liquids of known pH. Purchase a meter that uses at least a 2-point calibration. For acidified foods, calibrate your meter with pH 4.0 and 7.0 buffers.

An electrode is the part of the instrument that is immersed in a solution. When considering which pH meter to purchase, consider the cost of replacement electrodes. Some electrodes have special non-clog tips and these may be useful when measuring the pH of food that are not easily blended.

To get an accurate reading, the pH meter must be calibrated at the same temperature as the samples being tested. More expensive meters will compensate for slight variations in sample temperature (too warm or too cold). If you calibrate the pH meter just before you monitor product pH and test the pH of room-temperature samples (after equilibrium pH has been reached), you do not necessarily need to purchase a meter with temperature compensation.

Before you decide on the equipment to purchase, do reach out to us at DKSH for a better understanding of acidulant and pH specifications as well as the right pH meters that will best fit your business needs.

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