In this riveting exploration, we dive into how pH affects the growth and demise of microbes, using sodium acetate, propionic acid, and sorbic acid as our guides to elucidate the mechanisms of antimicrobial action of organic acids. Additionally, we discuss the crucial role of stomach acid in annihilating infectious food poisoning bacteria, highlighting its germicidal functions. Furthermore, we'll explore how variations in acid resistance among food poisoning bacteria relate to the minimum number of bacteria required to cause disease. Lastly, it's fascinating to note that food poisoning bacteria are generally vulnerable to acids, a characteristic intrinsic to their very nature as pathogens.
The Impact of pH on Microbial Growth Control
When discussing the relationship between microbial growth and pH, it's worth noting that the response varies by microorganism. However, for a broad understanding, keep the number pH 5.0 in mind.
Generally, microbial growth cannot be effectively inhibited if the pH is above 6.0. Based on my experience with food companies in growth control experiments, lowering the pH to around 6.0 typically does not significantly inhibit microbial growth. Microbial growth starts to be gradually suppressed when the pH drops below 6.0. Yet, at pH levels above 5.0, microbes usually continue to grow, albeit slowly. Once the pH dips below 5.0, however, most microbes cease to proliferate.
Thus, if you're aiming to control microbial growth in food through pH adjustment, targeting a pH below 5.0 is crucial. Classic examples of such foods include beer, soy sauce, juices, and carbonated drinks.
In this way, lowering the pH below 5.0 can almost completely inhibit the growth of many microorganisms (spoilage and pathogenic bacteria), with the exception of certain bacteria such as lactic acid bacteria. However, controlling the pH below 5.0 also brings out acidity, which can diminish the product's value—a significant dilemma for food product developers.
Of course, strictly speaking, the minimum growth pH varies for each microorganism. The table below, taken from the FDA's website, lists the minimum growth pH for representative foodborne pathogens. Not shown in the table, lactic acid bacteria, for example, often thrive even at a lower pH, typically below pH 4.0.
It's generally good to understand that most foodborne pathogens have a minimum growth pH below 5.0, but above pH 4.0. For instance, while the figure for Salmonella is listed as 3.7, this number should not be misconstrued as the pH range where Salmonella can actively proliferate, which would be between pH 4.0 and 5.0; rather, it indicates whether Salmonella could divide at a pH of 3.7. For practical knowledge in food quality control, it's beneficial to recognize that controlling the pH below 5.0 is generally effective.
Antimicrobial Effects of Organic Acids
When controlling microbial growth in food through pH, organic acids are often employed. Common organic acids used to inhibit microbial growth in foods include acetic acid, lactic acid, sorbic acid, propionic acid, and benzoic acid. Vegetables left out can spoil due to spoilage bacteria like Pseudomonas that cling to them. However, in pickled vegetables, these spoilage bacteria cannot proliferate due to low water activity (the concept of water activity will be explained in a future article).
On the other hand, salt-tolerant lactic acid bacteria also reside on the surface of vegetables. Therefore, in pickled vegetables, only lactic acid bacteria can proliferate, leading to lactic acid fermentation. This fermentation by lactic acid bacteria reduces the pH of the pickles, resulting in sour pickles. When the pH drops below 4.6, as explained in the article on botulinum, even if botulinum spores are present, they cannot grow.
Similarly, most spoilage bacteria cannot grow in such low pH conditions. Thus, in pickles, lactic acid, one of the organic acids, suppresses the growth of foodborne pathogens and spoilage bacteria. This mechanism is what has been adapted to other foods in the form of organic acids used as food additives.
Antimicrobial Mechanism of Organic Acids as Preservatives
Here, we will explore the mechanism by which organic acids inhibit microbial growth.
A common chemical structure in organic acids is the carboxyl group, which acts as a hydrophilic part. Attached to this carboxyl group are various hydrophobic functional groups. The simplest of these is the methyl group linked in acetic acid. When the carbon chain extends to two carbons, lacking an ethyl group, it forms propionic acid. Similarly, in lactic acid, a hydrophobic functional group is attached to the carboxyl group. Benzoic acid, for example, includes a benzene ring. When considering the inhibition mechanisms of microbial growth by organic acids, it is not necessary to delve into the specifics of each hydrophobic functional group's structure. The crucial point is that all these contain hydrophobic functional groups.
So, how do organic acids inhibit microbial growth? Generally, the addition of organic acids in food must sufficiently lower the food's pH to be effective. The antimicrobial and bactericidal mechanisms are thought to operate as follows:
Organic acids like acetic acid are in a chemical equilibrium (R-COOH → RCOO- + H+), where in an acidic environment they exist in a non-dissociated state (R-COOH). Typically, in their non-dissociated form, the influence of the R group makes them more hydrophobic.
Firstly, understanding the basic properties of cell membranes is necessary. Microbial cell membranes, made up of a phospholipid bilayer, easily allow hydrophobic compounds to pass through. However, charged ions like hydrogen ions cannot pass through the hydrophobic phospholipid bilayer. Thus, the organic acids, becoming hydrophobic, easily infiltrate the microbial cells.
Microbial cell membranes do not permit the passage of hydrophilic substances or ions, but hydrophobic substances can pass through more easily.
Therefore, non-dissociated acetic acid can easily enter cells. Once inside, where the pH is near neutral, the organic acid dissociates into RCOO- and H+.
As a result, an excess accumulation of hydrogen ions [H+] occurs inside the cell, which the cell attempts to expel. However, as illustrated in the diagrams below, the phospholipid bilayer forming the cell membrane does not allow charged ions like hydrogen ions to pass through. Therefore, to expel hydrogen ions, the cell must use proteins in the cell membrane, a process that requires energy. This energy expenditure involves ATP, the chemical energy used by microbes.
Through this process, microbes deplete their ATP and slow down their proliferation rate. This is considered the mechanism by which organic acids inhibit microbial growth. As mentioned, despite the various structures of hydrophobic functional groups in different organic acids, the inhibition mechanism is based on these common principles.
The Role of the Stomach as a Germicide
Next, let's explore an important role of the stomach, specifically the significance of stomach acid in killing infection-causing foodborne bacteria. The stomach produces gastric acid, primarily composed of hydrochloric acid. This hydrochloric acid reduces the pH in our stomachs to below 3.0. In such a highly acidic environment, microbes are killed. A critical function of our stomach, then, is to sterilize pathogenic microbes that enter along with our food.
We've learned in high school biology that the stomach's role is largely in digestion. Certainly, the stomach does have the ability to physically soften food. However, digestion can occur even without this function. People who have had a total gastrectomy, for example, due to stomach cancer, can still digest food because the enzymes that digest food are secreted in the small intestine. However, if the stomach is entirely removed, it's no longer possible to kill microbes with stomach acid. Therefore, individuals without a stomach are more susceptible to infection-based food poisoning. Additionally, even when people eat the same food contaminated with infectious foodborne bacteria, some get infected while others do not. Susceptibility to food poisoning can indeed depend on one's immune strength, but the differences in germicidal power within the stomach also significantly influence this.
The Variation in Acid Resistance Among Infectious Foodborne Pathogens Relates to the Minimum Infectious Dose
When comparing enterohemorrhagic Escherichia coli (EHEC) and Salmonella, the latter requires a higher number of bacteria to cause infection. This difference in the minimum infectious dose is also linked to the varying strengths of these two microbes against acid. In the case of Salmonella, many cells can enter the stomach, but due to their relative weakness to acid, the majority are killed off. Therefore, for some surviving Salmonella to reach the intestines, a higher number of cells need to enter the stomach with the ingested food, compared to EHEC.
On the other hand, enterohemorrhagic Escherichia coli is more resistant to acid. Therefore, even a small number of cells entering the stomach can result in some surviving and reaching the small intestine. Thus, the difference in acid resistance among infectious foodborne pathogens indeed relates to the minimum number of bacteria required to initiate an infection.
