Physical Methods to Control Microbial Growth - Online Tutor, Practice Problems & Exam Prep

Physical methods to control microbial growth include temperature manipulation, such as dry heat and moist heat, which denature proteins and enzymes. Dry heat, like incineration and hot air ovens, requires higher temperatures and longer times compared to moist heat methods like pasteurization and autoclaving. Low temperatures, desiccation, lyophilization, filtration, high pressure processing, and irradiation further inhibit microbial growth. Each method has unique applications and effectiveness, emphasizing the importance of selecting appropriate techniques for specific microbial control needs.

Physical Methods to Control Microbial Growth

In this video, we're going to begin our lesson on the physical methods to control microbial growth. It turns out that there are actually many different types of physical methods to control microbial growth. Below, we have an image that represents our map of the lesson or our outline of the lesson moving forward in our course on the physical methods to control microbial growth. Moving forward in our course, we're going to be following this map or outline by following the leftmost branches first.

We'll talk about temperature as a physical method, discussing dry heat, incineration in hot air ovens, and also moist heat such as boiling, pasteurization, and autoclaves. We'll also talk about using temperature but using low temperatures such as refrigeration and freezing. After we cover the temperature, we'll move on to the next type of physical method which is drying, and that includes techniques such as desiccation and lyophilization.

Then we'll move on to filtration such as liquid filters and air filters, and we'll also talk about high pressure processing, HPP. Last but not least, we'll talk about irradiation and the 2 main types, which are ionizing radiation and non-ionizing radiation. We'll move forward in our lesson covering these topics in this particular order. We’ll get to learn a lot more about each of these physical methods as we move forward in our course.

But for now, this concludes our brief introduction to some of the physical methods to control microbial growth, and I'll see you all in our next video to talk more about temperature physical controls specifically dry heat. See you all there.

Dry Heat

In this video, we're going to begin our lesson on dry heat as a physical method for controlling microbial growth. Dry heat is simply heat that has no moisture or liquid content, which is why it's called dry heat. Incineration is a type of dry heat that refers to the destruction by burning with a direct flame. This process turns the cell components into ashes.

Hot air ovens are another way to deliver dry heat. As their name implies, these ovens kill microbes with dry heat by destroying cell components and denaturing or inactivating proteins. Dry heat generally requires a higher temperature and more time to kill microbes than moist heat does.

We'll delve more into moist heat in our next lesson video. Moist heat can kill microbes faster and at lower temperatures than dry heat. However, an advantage of using dry heat over moist heat is that dry heat can be used on moisture-sensitive items that do not work well with moisture. For example, powders and oils, which cannot be used with moist heat because it can change their composition and are not compatible. However, dry heat can be used on them.

If we look at our image below, notice on the left-hand side we're showing you a laboratory tool being burnt with a direct flame. This is a form of incineration of a laboratory tool to help control the microbial growth on this tool. On the right, we're showing you a hot air oven. This hot air oven kills microbes on laboratory instruments. Laboratory instruments that are not sensitive to heat can be placed inside this hot air oven, and the microbial growth can be controlled with these hot air ovens.

This concludes our brief lesson on dry heat as a physical method for controlling microbial growth. We'll be able to get some practice and learn more about other physical methods of controlling microbial growth as we move forward in our course. So, I'll see you all in our next video.

Moist Heat

In this video, we're going to begin our lesson on moist heat as a physical method for controlling microbial growth. Most microbes require very specific temperatures for normal growth and reproduction to occur. Exceeding those temperatures by applying heat, either dry heat or moist heat, can destroy microbes. Moist heat is specifically a form of heat that has moisture, as its name implies, or you could define it also as heat that has liquid content in it.

Moist heat is capable of killing microorganisms by irreversibly denaturing their enzymes and proteins. Moist heat is typically going to be more effective than dry heat because it generally requires lower temperatures and less time to kill microbes than dry heat does. Dry heat usually takes longer to kill microbes, and it requires higher temperatures generally.

Examples of moist heat include boiling, pasteurization, which is really just a brief heat treatment, and it also includes pressurized steam. Later in our course, we're going to talk more about pasteurization, this brief heat treatment. What you should know about pressurized steam is that an instrument known as an autoclave, which somewhat resembles a pressure cooker, uses high temperatures and pressurized steam to sterilize heat and moisture-tolerant items.

An example of moist heat is boiling, where liquid is heated to a high temperature. Another example is pasteurization, a brief heat treatment. Here, we're showing you an image of milk being pasteurized. We'll talk more about pasteurization in our next lesson video. Pressurized steam is used in a device known as an autoclave to bring in heat and steam; the steam creates high pressures and heat, moist heat, in order to sterilize items that are heat and moisture-tolerant. The waste, air, and steam are able to exit through the back.

On the right, we are showing you a table that demonstrates that dry heat typically requires a lot more time to sterilize compared to moist heat. Using the same temperature of 121 degrees Celsius, moist heat is able to kill more effectively; it takes much less time, only 15 minutes here to kill or to sterilize the item, in comparison to 600 minutes or 10 hours when using dry heat. This shows that moist heat is more effective than dry heat. However, there are advantages and disadvantages to each of these methods. Even though moist heat can be more effective, moisture might not always be compatible with the item to be treated. Sometimes dry heat is a better method simply because of the compatibility of the items.

This here concludes our brief lesson on moist heat. We'll be able to apply these concepts and learn more about pasteurization and other physical methods to control microbial growth as we move forward in our course. I'll see you all in our next video.

Moist Heat:Pasteurization

In this video, we're going to continue to talk about moist heat as we talk more about pasteurization. Pasteurization is a process that was first discovered by Louis Pasteur and was also named after him. Pasteurization refers to the brief boiling of a specific product, such as milk or wine, to disinfect it and make it safe for consumption without significantly changing its properties, such as its taste or flavor.

Most pasteurization is completed by the method known as high temperature short time method, or HTST method for short, which exposes the product to temperatures of about 165 degrees Celsius for only about 15 seconds. The temperature used in the HTST method is typically not high enough to sterilize the product, and some microbes are likely to remain even after HTST pasteurization. However, another method implements even higher temperatures and shorter times than the HTST method, and this is the ultra high temperature, or UHT pasteurization method, which uses such high temperatures that it's capable of actually sterilizing the product and killing all of the microbes. For example, the UHT method would use temperatures up to 280 degrees Celsius for just 3 seconds.

We can also visualize the HTST and the UHT pasteurization methods, which were discovered by Louis Pasteur. Notably, milk extracted from cows contains many different types of microorganisms, and may be unsafe to drink directly. Pasteurization is a process that helps control the microbial growth in the milk.

There are two methods of pasteurization: the high temperature short time method, or HTST method, is implemented with temperatures around 165 degrees Celsius for approximately 10 to 20 seconds. This process yields milk safe to consume without significantly changing the flavor and taste. However, as the HTST method does not use temperatures high enough for sterilization, it is important to implement preservation techniques such as refrigeration to help delay the growth of the microbes. Milk that is refrigerated typically has gone through the HTST pasteurization process.

On the other hand, the ultra high temperature pasteurization method, or UHT method, uses much higher temperatures, around 280 degrees Celsius, for only 3 seconds. The high temperatures are enough to sterilize and kill all of the microbes, making the milk shelf-safe. Therefore, this milk can be stored at room temperature and sometimes is found on grocery store shelves not requiring refrigeration.

This concludes our brief lesson on pasteurization, and we will get some practice applying these concepts and learn more about other physical methods of controlling microbial growth as we move forward in our course. See you all in our next video.

Pasteurization is what type of microbial control method?

• A. A physical, dry heat control method.
• B. A chemical, heat control method.
• C. A physical, moist heat control method.

Low Temperatures

So now that we've talked about both dry heat and moist heat as a means of controlling microbial growth, in this video we're going to talk about using low temperatures as a physical method to control microbial growth. Depending on the type of microbe, low temperatures can have varying effects, because recall that some microbes are capable of growing really well at low temperatures. However, generally, low temperatures are going to slow the growth of microorganisms. Recall from our previous lesson videos that psychrophiles and psychrotrophs are groups of microbes that are exceptions because they are actually capable of growing pretty well at freezing temperatures below 0 degrees Celsius. Other than these exceptions, most other organisms are going to generally grow slower with lower temperatures. Low temperatures can be, in many cases, a sufficient means of controlling microbial growth.

Refrigeration generally is going to delay, or once again slow the growth of many pathogens and spoilage microbes, which is why we refrigerate many of our foods in order to help delay the growth and slow the growth until we're able to eat those items. We can store those items in a refrigerator for longer periods of time by keeping them at these low temperatures. Freezing usually is going to preserve foods and other products by inhibiting microbial growth. But freezing does not always kill the microorganisms. Sometimes you're able to freeze foods and other products, and it can really extend the life of those items by a lot. But once you thaw those items, because the microbial growth is only being inhibited by the cold temperatures, sometimes thawing them will allow those microbes to continue to grow after those items have been thawed.

Recall from our previous lesson videos that the term "preservation" is referring to the process of delaying the spoilage of perishable products. Perishable products, once again, are items that are likely to go bad quickly, such as specific types of vegetables and fruits. If we take a look at our image down below, we can get a better understanding of how low temperatures can be used as a physical method to control microbial growth, specifically refrigeration and freezing. On the left-hand side, we are showing you refrigeration. Refrigeration can be used to store many of our foods that we eat on a daily basis, but also refrigeration can also be used in a laboratory setting. Here, this is showing you an image of a cold lab that is going to be maintained right around 2 degrees Celsius, and experiments are going to be performed at 2 degrees Celsius inside this cold room, in this lab.

On the right, we are showing you images of freezing. Of course, freezing is going to imply using temperatures below 0 degrees Celsius. Lab specimens can be stored in a freezer for really long periods of time because it inhibits the microbial growth completely in many cases, again, except for the exceptions of the psychrophiles and psychrotrophs. Generally, low temperatures such as refrigeration and freezing are going to help slow the growth of microorganisms. It is a means of controlling microbial growth. This here concludes our lesson on low temperatures, and we'll be able to get some practice applying these concepts and learn more about other physical methods of controlling microbial growth as we move forward in our course. I'll see you all in our next video.

Desiccation

In this video, we're going to begin our lesson on the process of desiccation, and how it is used as a physical method to control microbial growth. Although some microbes can survive for years without water, most microbes require water at a pretty constant rate in order to grow effectively. The process of desiccation is the process of drying out or removing moisture or water from something in order to prevent it from growing effectively. Generally, the process of desiccation results in the inhibition of microbial growth. Recall from our previous lesson videos when we talked about osmosis and tonicity that the addition of solutes such as salt, for example, can create what's known as a hypertonic environment. Hypertonic environments are capable of drawing out water from cells via osmosis in order to dehydrate those cells. The dehydration of those cells generally results in the inhibition of growth.

If we take a look at our image down below, we can get a better understanding of this process of desiccation, which again is like drying out. We have these images of the cactus and the sun to remind you of dried-out areas. Notice over here on the left, we're just showing you an image of desiccated soil, again to help you associate desiccation with drying out. Over here, we're showing you some meat that's actually being salted in order to create a hypertonic environment on the outside of the meat. Notice all of this salt that's on the outside of the meat, and this is called salt curing. It can help to preserve the meat, in order to delay microbial growth and inhibit microbial growth.

Essentially, what happens by adding the salt is creates a hypertonic environment, which, recall from our previous lesson videos, is an environment that has more solutes on the outside of the cells. Water, in terms of osmosis, always flows from hypotonic solutions towards hypertonic solutions. So, if the outside is hypertonic, water flows towards the outside of the cell via osmosis. When water leaves these cells, it desiccates or dries out the cells. Basically, what we get is what's called a, dried out or a crenated red blood cell here. Ultimately, this is also going to happen with microbes as well, and it will dry out and dehydrate those microbes and inhibit their growth. And really, that's the main idea behind the salting out here, is that it helps to inhibit and prevent microbial growth to preserve the meat.

This here concludes our brief introduction to desiccation as a physical means of controlling microbial growth, and we'll be able to get some practice applying these concepts and learn more about other physical methods of controlling microbial growth as we move forward in our course. So I'll see you in our next video.

How does desiccation control microbial growth?

• A. Desiccation removes all moisture from living cells, inhibiting or killing microbes.
• B. Desiccation drops the temperature so low, that only psychrophiles can survive.
• C. Desiccation utilizes ultra-heated steam to kill pathogenic microbes.
• D. Desiccation incinerates all living cells, killing all microbes.

Lyophilization

In this video, we're going to introduce lyophilization as a physical method of controlling microbial growth. And so lyophilization is really just the process of freeze-drying. Basically, the process involves freezing the product and then drying out the product in a vacuum. Now, through the process of lyophilization, it is going to remove water from the product via sublimation. And sublimation is referring to the transition of water directly from a solid state to a gas state without actually becoming a liquid. Now lyophilization is widely used for preserving foods, such as, for example, coffee, milk, meats, vegetables, and a lot of other foods as well without the need for refrigeration. Now overall, through the process of lyophilization, the quality of the product is going to end up being a lot better than usual typical drying methods. And so, if we take a look at our image down below at lyophilization, notice on the left-hand side over here we're showing you a fancy piece of equipment that is a lab freeze dryer. And here we're showing you some scientists that are using a lab freeze dryer to preserve artifacts with freeze drying or lyophilization. And over here on the right, we're showing you some freeze-dried strawberries and some freeze-dried ice cream as well, over here. This is the ice cream and this is the strawberries. And so, basically, lyophilization is a common means of preserving many different types of foods, and so it is a means of controlling microbial growth through freezing and drying, almost a combination of using low temperatures and desiccation. And so, this here concludes our brief lesson on lyophilization, freeze drying, and we'll be able to get some practice applying these concepts and learn more about other physical methods of controlling microbial growth as we move forward. So I'll see you all on our next video.

Filtration

In this video, we're going to talk about filtration as a physical method to controlling microbial growth. And so filtration is the process of using filters with pores that are incredibly small, small enough to actually physically remove microbes from either liquid or air. Now once again, the pores that are in the filter are incredibly small, and they're way smaller than the microbes themselves. And so the microbes, in relation to the pores, are way too large to pass through those pores of the filter. And therefore, the microbes are going to get trapped in the filter. And that is what separates the microbes from the liquid and air that they are in because the liquid and air can pass through the filter very easily, but the microbes are too large and cannot pass. They get trapped. And so filtration can be used to remove microbes from heat-sensitive liquids when heat is not an option. Now a very specific type of filter known as a high efficiency particulate air filter or a HEPA filter, for short, is a specific type of filter that will remove airborne particles and microbes from the air.

And so if we take a look at our image down below, notice that the left-hand side over here is focused on liquid filtration. And the right-hand side over here is focused on air filtration. And so focusing in on the left side first, notice that we have this apparatus right here, there is a vacuum pump with a flask here, and then there's this container at the top. And notice that the container at the top contains the contaminated liquid but it also contains the filter itself, which is horizontally here in orange color. And so the contaminated liquid is going to have microbes in it along with the liquid. And so, zooming into this region, you can see this over here, notice that the microbes are in green. So we can label these as the microbes. And the microbes are simply too large to pass through the pores of the filter. And so you can see the microbe filter again is here in orange, and there are some pores in it, but the microbes are too large to get through. However, the liquid itself is capable of getting through. And so the liquid is going to pass through this filter, but the microbes are going to get trapped in the filter itself. And so this separates the microbes from the liquid, and the liquid that gets through is going to be sterile liquid.

And so over here what we're showing you are some images of, some cells that are being trapped in a filter. And so here at the top, we're showing you Micrococcus luteus bacteria on a polycarbonate filter. And so notice that these bacterial cells that you see right here are simply too large to pass through these tiny, tiny little pores that are neighboring them. And so the microbes get trapped in the filter, but once again the liquid is capable of passing through. Down below, we're showing you some sperm cells on a polycarbonate filter and again, the sperm cells that you see here are simply too large to pass through these tiny little pores that you see all around them. Now over here on the right-hand side, again, we're showing you air filtration, and specifically you want to think about the HEPA filters, the high efficiency particulate air filters. And so HEPA filters are commonly used on airplanes to ensure that the air that you're breathing on airplanes is free of microbes that may be found in the atmosphere. Filter, or a HEPA filter for short.

And so this here concludes our brief lesson on filtration as a physical method to control microbial growth by physically removing microbes from liquids or air. And so we'll be able to get some practice applying these concepts moving forward and also learn about other physical methods of controlling microbial growth moving forward in our lesson. So I'll see you all in our next video.

High Pressure Processing

In this video, we're going to talk about high pressure processing as a physical method for controlling microbial growth. High pressure processing is commonly abbreviated as HPP. High pressure processing, or HPP, as its name implies, uses really high pressures, somewhere around 120,000 pounds per square inch, or PSI, which is a unit of pressure. It uses these high pressures to destroy microbes and control microbial growth. High pressure processing is able to kill microbes by altering the molecular structure of proteins. It can also be used to disinfect specific products while still preserving features like flavors, colors, and nutrient values as well. That's a benefit of using HPP.

It is important to note that some microbes, specifically endospores that are capable of tolerating high pressures, can survive high pressure processing. High pressure processing does not always kill; it does not always sterilize a particular product. If we take a look at our image down below, we can get a better understanding of a device that creates immense amounts of pressure. You can put your item to be treated inside the instrument, and it will create extreme amounts of pressure. Again, that pressure can ultimately alter the molecular structure of proteins, which kills microbes. However, since some microbes can tolerate high pressures, it does not always kill all microbes and hence not always sterilize.

This concludes our brief lesson on high pressure processing as a physical method for controlling microbial growth. We'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.

Irradiation

In this video, we're going to briefly introduce irradiation as a method of controlling microbial growth. Irradiation is the process by which an object is exposed to radiation. It can be used to destroy microbes and control microbial growth. Radiation is defined as the emission or transmission of energy as either electromagnetic waves or as moving subatomic particles. In this video, we will briefly introduce two types of radiation:

• Ionizing radiation: This type of radiation has penetrative power and is capable of penetrating deeper into objects. It has sufficient energy to remove electrons and, therefore, can ionize atoms or molecules. Since electrons are being removed, there is an imbalance of protons and electrons. Ionizing radiation, with its penetrative power and high energy, can harm cells directly by destroying molecules within the cells such as DNA and membranes. It is also capable of creating reactive oxygen species (ROS), which are chemicals that are highly reactive and can lead to damage within the cells themselves. Thus, ionizing radiation can cause direct cellular damage or create reactive oxygen species that cause internal cellular damage.
• Non-ionizing radiation: As the name implies, non-ionizing radiation lacks the energy to ionize atoms or molecules. It has less penetrative power and therefore must be used directly on the microbes. It cannot penetrate through packaging or other barriers. Examples of non-ionizing radiation include UV ultraviolet light, which can damage DNA and proteins within cells to help control microbial growth. Additionally, microwaves can generate lethal amounts of heat to kill microbes.

We can enhance our understanding of irradiation by referring to an image of the entire electromagnetic spectrum. The spectrum shows that waves start off really long and become progressively shorter. The shortest wavelengths, having the highest amount of energy and most penetrative power, represent ionizing radiation. Conversely, waves that are longer have less energy and less penetrative power, representing non-ionizing radiation. For instance, radio waves have very long wavelengths and are therefore considered non-ionizing radiation.

In the video, we also show an image of a laboratory using ultraviolet light to sterilize the environment and eliminate microbes within a culture hood. Moreover, we present an image contrasting two oranges: one non-irradiated orange showing microbial growth on its surface, and another that has been irradiated showing no microbial growth. This visual aid demonstrates how irradiation can control microbial growth and enhance safety in the use of products and consumption of specific types of food.

This concludes our brief lesson on irradiation as a method to control microbial growth, and we will have opportunities to apply these concepts as we progress in our course. I'll see you all in our next video.

This method of physical microbial control combines the removal of all moisture with extremely low temperatures.

• A. Desiccation.
• B. Lyophilization.
• C. Irradiation.
• D. Pasteurization.

How does irradiation control microbial growth?

• A. Some types of radiation can create lethal amounts of heat.
• B. Radiation penetrates cells causing damage to DNA and cell membranes.
• C. Radiation creates reactive oxygen species which damage cellular processes.
• D. All of the above are ways that irradiation controls microbial growth.

A heat-sensitive liquid in a laboratory has been contaminated with E. coli bacteria. Which form(s) of microbial growth control could you use to remove this E. coli population from the liquid?

• A. Desiccating the liquid.
• B. Freezing the liquid.
• C. Filtering the liquid.
• D. Boiling the liquid.

How does high pressure processing (HPP) control microbial populations?

• A. Extreme pressure damages the proteins within microbes until they can no longer function.
• B. Extreme pressure destroys DNA and membranes within microbes.
• C. Extreme pressure removes water from microbial cells via sublimation.
• D. Extreme pressure combines with extreme heat to pasteurize the product and kill microbes.

Here’s what students ask on this topic:

What are the differences between dry heat and moist heat methods in controlling microbial growth?

Dry heat and moist heat are both effective methods for controlling microbial growth, but they operate differently. Dry heat methods, such as incineration and hot air ovens, require higher temperatures and longer exposure times to kill microbes. They work by oxidizing cell components and denaturing proteins. Moist heat methods, like boiling, pasteurization, and autoclaving, are generally more effective because they require lower temperatures and shorter times. Moist heat kills microbes by irreversibly denaturing their enzymes and proteins. An advantage of dry heat is its suitability for moisture-sensitive items like powders and oils, whereas moist heat is more effective for sterilizing heat and moisture-tolerant items.

How does pasteurization differ from sterilization in microbial control?

Pasteurization and sterilization are both methods used to control microbial growth, but they differ significantly in their processes and outcomes. Pasteurization involves brief heat treatment, typically using the High Temperature Short Time (HTST) method at around 165°C for 15 seconds, or the Ultra High Temperature (UHT) method at 280°C for 3 seconds. Pasteurization reduces the number of viable pathogens to safe levels but does not kill all microbes, so refrigeration is often required to preserve the product. Sterilization, on the other hand, aims to kill all forms of microbial life, including spores, and is achieved through methods like autoclaving, which uses pressurized steam at 121°C for 15 minutes. Sterilized products are free from all living microbes and do not require refrigeration for preservation.

What is the role of low temperatures in controlling microbial growth?

Low temperatures play a crucial role in controlling microbial growth by slowing down the metabolic activities of microbes. Refrigeration, typically at around 4°C, delays the growth of many pathogens and spoilage microbes, making it a common method for preserving perishable foods. Freezing, at temperatures below 0°C, inhibits microbial growth more effectively by halting metabolic processes. However, freezing does not necessarily kill all microbes; some, like psychrophiles and psychrotrophs, can survive and even thrive at low temperatures. Once thawed, these microbes can resume growth, so proper handling and storage are essential to maintain food safety.

How does filtration work as a physical method to control microbial growth?

Filtration controls microbial growth by physically removing microbes from liquids or air using filters with very small pores. These pores are smaller than the microbes, preventing them from passing through while allowing the liquid or air to flow freely. Liquid filtration is often used for heat-sensitive solutions, where the liquid is passed through a filter to remove contaminants. High-Efficiency Particulate Air (HEPA) filters are used for air filtration, trapping airborne particles and microbes. Filtration is particularly useful in laboratory settings and for sterilizing medical and pharmaceutical products without the use of heat.

What is lyophilization and how does it control microbial growth?

Lyophilization, also known as freeze-drying, is a process that combines freezing and drying to preserve perishable materials and control microbial growth. The process involves freezing the product and then reducing the surrounding pressure to allow the frozen water in the product to sublimate directly from the solid phase to the gas phase. This removal of water inhibits microbial growth, as most microbes require water to survive and reproduce. Lyophilization is widely used for preserving foods, pharmaceuticals, and biological samples, maintaining the quality and stability of the product without the need for refrigeration.