Why is F0 value so important in sterilization processes?

Why is F0 value so important in sterilization processes?

All the details to understand what F0 value is and what role it plays in the sterilization of preserved foods

One of the most difficult concepts to understand when producing preserved food or ready meals is the F0 value. That is why we have decided to further explore this topic.

If at any time you get lost with any of the terms mentioned throughout this post, we recommend you to download our dictionary of terms related to the world of packaged food .

Sterilization of packaged food

Packaged food sterilization is a preservation technique that consists of exposing hermetically packaged foods to high temperatures for a certain period of time in order to completely destroy their microorganisms, pathogenic or non-pathogenic, including spores.

The objective is reducing to almost zero the probability of infections and disease transmission after consuming those foods, avoiding endangering your consumers’ health and your business. And the fact is, failing to properly eliminate microorganisms and spores from a gourmet preserved food or ready meal can lead to food poisoning and, in the worst case, botulism, a disease caused by Clostridium botulinum that causes difficulty in breathing, muscle paralysis and can even be fatal.

Therefore, you are probably wondering how long and at what temperature should I expose my preserved food to ensure the total elimination of microorganisms and spores? Well, the answer is variable and at this point the F0 value takes action.

The role of F0 value in food sterilization

Local producer with autoclave, F0 value

Just as we use horsepower to quantify the power of a car engine, we use F0 value in the food industry to quantify the sterility level of foods.

The F0 value is defined as the thermal lethality time required to eliminate all microorganisms present in foods, by exposing them to a temperature of 121.1ºC and it is expressed in minutes. In fact, F0 can also be expressed as F121.1, and both forms are correct. When we talk about F0 with other temperatures, for example 110ºC, then we will be referring to F110.

This value is calculated from the time/temperature ratio through a very technical formula that we will not explain in this post. It is important to clarify that, at a microbiological level, the same F0 can be achieved in different ways, experimenting different combinations of time and temperature. For example, we can obtain an F0=3 by exposing a product to 121.1ºC for 3 minutes, or 115ºC for 12 minutes or 124ºC for 2 minutes. And this occurs thanks to F0‘s formula.

Target F0 value and actual F0 value

There are two types of F0 values that we will always talk about in our preserved food production.

On one hand we have the Target F0 value, which is the F0 value that food experts have determined as essential to eliminate the most heat-resistant pathogenic microorganisms, and it will vary according to the type of recipe we make and the requirements we establish for ourselves. If we want a longer expiration date then we will set a higher target F0 value, on the other hand, if we want a shorter expiration date we will set a lower target F0 value.

Secondly, we have the actual F0 value, the F0 value we really achieved in our autoclave thermal processing, and it is calculated by applying the F0 formula to the temperature data obtained from each second of our cycle. As you can imagine, calculating this value is not a simple task, but it has vital importance.

The actual F0 value may be higher or lower than the target F0 value. This information will help us to know how close or how far we are from reaching the target F0 value when sterilizing our preserved foods and ready-to-eat meals. Keeping an accurate control of the actual F0 value for each batch will help us to know we are working in a correct way.

F0 vs P0

At this point, we believe it is necessary to make a clarification. As we have already mentioned in other articles, F0 and P0 are reference thermal values. But don’t get confused. The F0 value is the one corresponding to sterilization processes, because it takes temperatures above 100ºC as its starting point. However, it is possible to sterilize certain acidic foods without exceeding 100ºC.

And P0 value, however, is used in pasteurization processes because it generally involves temperatures below 100ºC.

How is the reference value of F0 calculated?

As we have said, the target F0 value will depend on the type of recipe and the expiration date we are aiming for, but in general the standard value for food processing is 121.1ºC for 3 minutes. But, what is the origin of 121.1ºC temperature? And, why 3 minutes and not 5?

In order to explain this, it is necessary to understand how F0 value is calculated and this is done from heat transfer theory and thermal resistance of microorganisms (thermal destruction kinetics). For many decades, scientists and experts in the field have been experimenting on this subject, exposing all types of food and packaging to different combinations of temperature and time in order to determine how the resistance of different microorganisms evolves and how they are destroyed.

Formula for F0 value calculation
Δt = time interval between two subsequent measurements of T
T = temperature of sterilized product at time t
z = temperature coefficient

F0 value, therefore, is useful to evaluate the efficiency of a thermal process intended to eliminate microorganisms present in foods. It allows us to know to what extent we have eliminated spores, bacteria and fungi from our preserved foods and/or ready-to-eat meals. In other words, it guarantees we are eliminating microorganisms from our food.

Remember that there are many types and species of microorganisms, but in terms of food, the elimination of Clostridium botulinum is almost always taken as an important reference, because it is one of the most dangerous pathogenic microorganisms for human health and one of the most heat-resistant. It is not easy to eliminate it.

Thus, in order to find an optimum F0 value to eliminate Clostridium botulinum, different tests were carried out varying time and temperature. And reference values were established to evaluate microorganism resistance to different variations in temperature and time:

  • D value, as decimal reduction time. This value indicates how sensitive a microorganism is at the same temperature. This value refers to the time required to eliminate 90% of microorganisms present in a particular food. 1D represents a 90% reduction, 3D is 99.9% and 6D is 99.9999%.
  • Z-value, as the thermal resistance constant. This value indicates how the sensitivity of a microorganism may fluctuate when temperature is modified. A microorganism is not eliminated as quickly at 120ºC for 1 minute as at 110ºC for 1 minute.

And consequently it was discovered, as you can imagine, that D and Z values are closely related to F0 value. In fact, these values are included in the formula for calculating F0. Different combinations of temperature and time can achieve similar results.

Therefore, time will depend on temperature and vice versa. In order to achieve the same sterilization results, the same actual F0 value, at higher temperature will need less time and at lower temperature more time will be required.

On the basis of these studies, it was determined that, to ensure public health in packaged products, an optimal reduction of Clostridium botulinum should be 12 logarithmic units (12D). A decrease of 12D means there is a 1 in 1,000,000,000,000,000 chance that a spore will survive in a unit of treated product. In addition, it was determined that to eliminate Clostridium botulinum in 12 logarithmic units (12D), foods must be exposed to a temperature of 121.1ºC (250ºF) during 3 minutes.

Since Clostridium botulinum is a very resistant microorganism and the most dangerous, it was determined that the F0 value that ensures an acceptable level of elimination of most microorganisms corresponds to 121ºC for 3 minutes.

F0 value related to Clostridium botulinum elimination

Thus, we could say that we already have the answer to the question posed at the beginning of this article. But we don’t. Exposing foods to high temperatures during a long period of time can lead to nutritional and organoleptic changes (color, smell, texture and flavor) in preserved foods or ready-to-eat meals.

Moreover, heat does not penetrate all foods in the same way. It will depend on its ingredients, shape, packaging type and even the exact location of products inside an autoclave. A fruit jam is quite different from a fish pâté. Besides the changes in texture or pH between foods, the microorganisms present in fruits are different from those found in foods of animal origin.

F0 or reference value in commercial sterilization

Vegetable exhibit to illustrate reference F0 value for preserved food sterilization

We already know that F0 value represents the reference point in commercial sterilization in order to reduce the number of microorganisms to an acceptable level, taking into account a temperature of 121.1°C (250°F) and a Z value equal to 10.

Therefore, depending on the specific characteristics of your preserved foods or ready-to-eat meals, you will have to regulate and add variations in F0 value, experiment with time and variations in temperature increase, which is known as the additional Σ%F0 records, until almost all microorganisms are eliminated, but, above all, until you obtain the desired product in terms of appearance, smell, taste and texture.

Because, as you may already know, if you expose foods to high temperatures or cook your products for a long time, they may have a burnt taste, and their color and texture may change. Therefore, once you are absolutely sure about the characteristics you want in your preserved foods or ready meals, you will have to make several tests to correct the F0 value in the autoclave until you get the product you are looking for.

But don’t worry, you don’t have to start from scratch. Our food advisory service will provide you a starting point so that you know in advance how to proceed with your product. In addition, you can also consult this document where we list recommended F0 values for a wide variety of foods.

The reference temperature for commercial sterilization

Consequently, it is established that the reference temperature for commercial sterilization is 121.1ºC (250°F) and regarding acid food sterilization is 93.3ºC.

Sterilization below 100ºC?

Exactly, because when it comes to foods with an acid pH, i.e. pH<4.5, it is more difficult for microorganisms to develop in it, and they are eliminated more easily. Therefore, by applying temperatures below 100ºC, we eliminate almost all microorganisms without needing subsequent refrigeration like pasteurization.

It is important to keep in mind that microorganisms are never totally eliminated, although they are almost eradicated. For this reason, in commercial sterilization terms, the probability of a preserved product being contaminated by microorganisms is acceptably remote.

And what about pasteurization?

In pasteurization, P0 values are used, which are calculated following a similar formula, establishing a reference temperature of 70ºC.

F0 value in commercial sterilization

As we have said, the value of F0=1 at a temperature of 121.1ºC for 1 minute was assigned by international convention. From this point, different values of time and temperature are established to achieve the same results, taking into account that for commercial sterilization it is always necessary to choose a treatment with F0 at least greater than 3, since heat is not distributed instantaneously in the same way throughout the interior of an autoclave and there are different levels of contamination and microorganisms that are more resistant than Clostridium botulinum.

Thus, as you can see in the table below, the same F0 is achieved for 3 minutes at 121ºC compared to 30 minutes at 111ºC or 300 minutes at 101ºC.

For this reason, it is very important to have an autoclave that operates with regulation through target F0 values and with a record of actual F0 value, so that we can produce our packaged foods knowing what we are doing and ensuring food safety of our products at all times.

TERRA Food-Tech® autoclaves allow programming a cycle with a target F0 value with different reference temperatures, and when they start working, they add the sterilization value assigned to each moment according to the current temperature, until the target F0 value is achieved. They also report the actual F0 value achieved.

F0 value for F10-121 values corresponding to 1 minute of heating
Table with F-values achieved with 1 minute of processing at several temperatures

TERRA Food-Tech® Autoclaves: target F0 control and actual F0 recording

TERRA Food-Tech® autoclaves with regulation through target F0 values and recording of actual F0 value

So, going back to the beginning of this topic, we are sure that you understand much better now why working with F0 value to sterilize (or P0 value to pasteurize) your preserved foods or ready-to-eat meals is both essential and necessary. And you will also understand that:

  1. The target F0 value your recipe needs will depend on the type of food, packaging and expiration date you want to achieve.
  2. The same target F0 can be met in many different ways, and depending on the type of recipe, it will be more convenient to use shorter times at higher temperatures or longer times at lower temperatures.
  3. In order to achieve a longer expiration date it is necessary to set larger target F0 values.
  4. Health authorities require that the actual F0 value obtained in each batch of preserved foods be controlled and recorded to ensure consumer health.
  5. If you have an autoclave that can be programmed by target F0 and maximum processing temperature, you will be able to modify your production process while respecting the organoleptic qualities of foods and achieving the same best-before date.

Therefore, in order to determine the time and temperature your preserved foods or ready-to-eat meals should be exposed to, you will have to perform several tests until you find the most appropriate target F0 value.

And in order to perform these tests, we recommend you to have an autoclave with adjustable F0 value, which allows you to enter time and temperature variables until you get the product you want. Like TERRA Food-Tech® autoclaves, which allow you to record these values for 50 different recipes or programs and display the actual F0 value achieved in each batch, as well as determining maximum processing temperature.

But if this task seems difficult for you, remember that the purchase of any of our autoclaves includes a first food advisory service provided by one of our experts, who will help you in everything you need, so that you can prepare your preserved foods and ready meals according to your preferences, but above all, with total food safety.

If you would like to know more about F0 value, about our autoclaves or concerning our food consulting services, please do not hesitate to contact us. We will be glad to help you.

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