In the food industry, we all know and love the 'good bacteria' like the ones that make our yogurt and fermented foods. However, there's another side to the story. Some lactic acid bacteria can cause our food to spoil. Today, I'll be delving into these not-so-friendly lactic acid bacteria that pose problems in our food.
What is Lactic Acid Bacteria?
For those unfamiliar, I won't go into too much depth today, but there's another article that gives an easy-to-understand breakdown of what this bacteria is and its characteristics.
How Do We Distinguish Lactic Acid Bacteria from Other Gram-Positive Bacteria Using the Catalase Test?
Why focus on Lactic Acid Bacteria?
As we've increased our consumption of chilled packaged foods, there's been a rise in complaints and incidents related to spoilage caused by these bacteria. Traditional microbiology textbooks often highlight the role of Pseudomonas, a type of Gram-negative bacteria, as the main culprit in spoiling vegetables and meat. This bacteria thrives in oxygen-rich environments and quickly multiplies.
But our modern food distribution system has changed. We now consume a wide variety of processed and packaged foods. Think of foods that are heated, contain preservatives, vacuum-sealed, and distributed at lower temperatures. In these products, it's not Pseudomonas but lactic acid bacteria that's causing the problem. While many of us associate lactic acid bacteria with the good stuff – like yogurt and cheese – they're not always our friends.
Take cavities, for instance. Did you know that cavities are caused by a type of lactic acid bacteria called Streptococcus mutans? These bacteria can combine sucrose to form high molecular polysaccharides, creating their living space (dental plaque) on our teeth. Once settled, they produce lactic acid, which being acidic, erodes our teeth leading to cavities.
Real-world Examples
Lactic acid bacteria aren't just beneficial microorganisms for humans. In fact, in the context of actual food distribution, they often play a role as spoilage microorganisms, causing problems.
The following chart illustrates the changes in the total bacterial count and lactic acid bacteria when ham is vacuum-sealed. The fluctuations in the total bacterial count and the lactic acid bacteria count are almost identical. This means that in vacuum-sealed ham, the majority of the bacteria responsible for spoilage are lactic acid bacteria.
The graph you see above is derived from data extracted from the following research paper, which I've summarized in this graph.
Samelis et al.
Selective effect of the product type and the packaging conditions on the species of lactic acid bacteria dominating the spoilage microbial association of cooked meats at 4℃.
Food Microbiology, 2000, 17, 329-340
Now, let's take a look at the chart below, which shows the changes in the total bacterial count and lactic acid bacteria when bacon went through a similar chilled distribution process. Once again, you'll notice that the total bacterial count and lactic acid bacteria count closely mirror each other. In other words, when bacon goes through chilled distribution, it's predominantly lactic acid bacteria that make up the spoilage flora.
The graph you see above is derived from data extracted from the following research paper, which I've summarized in this graph.
Samelis et al.
Selective effect of the product type and the packaging conditions on the species of lactic acid bacteria dominating the spoilage microbial association of cooked meats at 4℃.
Food Microbiology, 2000, 17, 329-340
It's important to note that lactic acid bacteria don't only cause spoilage in meat products.
Seafood products can also undergo spoilage due to lactic acid bacteria. The chart below depicts changes in the total bacterial count and lactic acid bacteria count when smoked salmon is stored in chilled conditions. Once again, you can observe that the total bacterial count and lactic acid bacteria count follow a similar trend. In other words, when it comes to the spoilage flora of smoked salmon, the majority of it is comprised of lactic acid bacteria.
The graph you see above is derived from data extracted from the following research paper, which I've summarized in this graph.
Leroi et al.
Effect of Salt and Smoke on the Microbiological Quality of Cold-Smoked Salmon during Storage at 58C as Estimated by the Factorial Design Method.
Journal of Food Protection, Vol. 63, No. 4, 2000, Pages 502–508
The table below provides information about the changes in the total bacterial count and lactic acid bacteria count when pickled herring undergoes spoilage. Once again, you can observe that the total bacterial count and lactic acid bacteria count are closely matched. In other words, when pickled herring spoils, it's primarily due to the presence of lactic acid bacteria.
The table you see above is derived from data extracted from the following research paper, which I've summarized in this graph.
Lyhs et al.
Lactobacillus alimentarius: a specific spoilage organism in marinated herring.
International Journal of Food Microbiology 64 (2001) 355–360.
But why Seafood?
It's easy to understand why lactic acid bacteria might spoil mammalian products, but why seafood? There are two main reasons to consider:
- The bacteria can contaminate seafood from river waters to coastal seawaters
- Secondary contamination might occur during processing in factories after the seafood is caught.
Even a slight contamination with lactic acid bacteria can make it the dominant spoilage organism. This might explain why we frequently encounter this bacteria as a spoilage agent in seafood.
Lactic acid bacteria play a dual role in our food industry. While they help in making some of our favorite fermented foods, they can also be a cause of spoilage, especially in our modern food distribution system. So, next time you're enjoying that yogurt or dealing with spoiled food, remember these tiny organisms are working behind the scenes!
Why Lactic Acid Bacteria Are a Common Cause of Food Spoilage
Understanding the Resilience of Lactic Acid Bacteria
1. Lactic Acid Bacteria are Gram-positive
Firstly, lactic acid bacteria belong to the group known as Gram-positive bacteria. While Gram-negative bacteria can't handle dry environments like in factories very well, Gram-positive bacteria, like lactic acid bacteria, naturally thrive in dry land environments. This means if there's contamination by these bacteria in a factory, they can survive for long durations. Even in dry factory conditions, they linger and can cause secondary contamination. Moreover, compared to Gram-negative bacteria, they can survive and multiply even in foods with low water activity, like those preserved with salt or sugar.
2. Their Acid Production Makes Them Hard to Control
Imagine if these bacteria contaminate a sausage or ham packaging line in a factory. Lactic acid bacteria acidify their environment by producing lactic acid. If the surrounding environment becomes too acidic, it can threaten their own survival. To combat this, they have a defence mechanism where they produce amines by decarboxylating amino acids, like creating histamine from histidine. This process, commonly seen in foods like cheese and fermented sausages, helps them manage the internal acidic conditions.
Preservatives like sorbic acid and sodium acetate, salts of organic acids, are often used as they can enter microbial cells easily by turning non-dissociative (or hydrophobic) in low pH. Once inside the cell, they release hydrogen ions. However, because lactic acid bacteria can effectively get rid of excess internal hydrogen ions, these organic acid preservatives struggle to inhibit their growth.
3. They Can't Be Easily Controlled with Vacuum or Gas-Packing
If you vacuum-pack products like sausages and ham, aerobic bacteria can't grow. Only certain bacteria, including lactic acid bacteria, can. In many countries, many products like sausages are packed with carbon dioxide. However, lactic acid bacteria are known to resist high concentrations of CO2.
4. Many of Them Grow Even at Low Temperatures
Considering the distribution process, foods like vacuum-packed or gas-packed sausages and ham are usually stored at low temperatures. Only psychrotrophic bacteria, those that grow at low temperatures, can multiply in such conditions. Many lactic acid bacteria are capable of growing even below 10°C, while others, like Staphylococcus aureus, can't.
The graph you see above is derived from data extracted from the following research paper, which I've summarized in this graph.
Hamasaki, Y. et al.,
Behavior of psychrotrophic lactic acid bacteria isolated from spoiling cooked meat products. Appl.Environ.Microbiiol., 69, 3668-3671(2003).
4. Lactic Acid Bacteria: A Friend and Foe
Lactic acid bacteria exhibit impressive resilience across various conditions. That's why humans have used them for fermenting foods for ages. They're tough, which is why they're popular in fermentation. Their resilience also explains their role in probiotics. If they were vulnerable to environmental factors, fermented foods might not have been part of human history.
However, when they're on the opposing side, especially from a food spoilage perspective, they become quite a challenge to handle.
Closing Thoughts:
Many professionals in food companies often ask about preservatives that can control the growth of spoilage lactic acid bacteria. But as discussed, handling these bacteria is quite a challenging task.
