Liquid Sterilization Guide for Lab Autoclaves

Second post in series, “Understanding Laboratory Autoclaves”

What is one of the most common applications of laboratory autoclaves? Processing liquids. Think of how many beakers of liquids must be sterilized on a daily basis in universities, research centers, and biotech labs all over the world to ensure the smooth operation of research and experimentation. And it’s also one of the most unique since medical autoclaves rarely sterilize liquids. In this blog post we will dive into the lake of lab liquids for autoclaves, also known as steam sterilizers. If you missed our last blog post in which we introduced lab autoclaves and how to process solids/hollows and isothermals, check out Sterilization of Solids/Hollows & Isothermal Processing.


Autoclaving liquids presents some unique challenges. Liquids take longer to sterilize than other media since liquids have a high heat capacity. As opposed to solids, liquids take a lot longer to heat up and cool down, and the total cycle time is increased dramatically as a result.

With liquids, there is no requirement for air removal from the chamber in order to ensure successful sterilization whereas with solids, we do need to remove all air due to possible hollow spaces inside a load. In the case of solids, steam is the sterilization medium and must penetrate inside the load and make contact with all surfaces, which is why all air must be removed. If air were to remain inside the load (think, inside a tall beaker or flask), that would compromise steam’s ability to reach all surfaces of the load.

Liquids, however, which have no hollow spaces, use steam as a medium to heat up the liquid, but the actual sterilization takes place by the liquid sterilizing itself due to its increased temperature. Therefore, there is no requirement to remove chamber air.

In addition, due to the delicate nature of rapidly heating and cooling liquids, special precautions must be taken to avoid their boiling over inside the autoclave. The boil-over effect, as it’s commonly known, is problematic because it causes the loss of liquids and because it creates a mess inside the autoclave. Obviously, no researcher enjoys cleaning up splattered hot liquid or exploded glassware at the end of a liquid cycle, which are both unpleasant and potentially dangerous.

Liquid Cooling

Reducing the cooling times for liquid sterilization is of paramount importance not only to save time, but more importantly, to protect the integrity of the load and ensure the liquids are not “overcooked” (except for water, which cannot be overcooked). There are two cooling applications available for liquid cooling:

  • Fast Liquid Cooling will cool down the load 75% faster than leaving the load to cool under ambient (regular room temperature) conditions. After the sterilization phase is complete, chamber pressure is increased by forcing compressed air through a microbiological filter into the chamber. This increase in pressure prevents boil-over, spills and cracks in containers that would normally occur under low-pressure conditions. Since the pressure is increased, the temperature can then be safely reduced. Cold water is circulated through the chamber jacket1 to cool the chamber and load [link anchor to large autoclave article section about jackets]. This combination of introducing cold water and pressurized air helps reduce the temperature of the liquid load quickly and safely.
  • Super Fast Liquid Cooling In addition to the introduction of high-pressure air inside the chamber and cold water circulating in the walls of the chamber, a fan can be used to further accelerate cooling by speeding up the circulation of air, which transfers the heat from the load to the cold chamber walls more efficiently. Super fast liquid cooling can reduce liquid cycle times by as much as 90%.
The solid red line represents temperature for fast cooling, and the dotted red line represents temperature for standard cooling. The solid blue line represents pressure for fast cooling, and the dotted blue line represents pressure for standard cooling

Two Temperature Sensors

The temperature in the chamber can reach the sterilization temperature of 121℃ way before the liquid inside the container will reach the same temperature. And the reverse is true as well: the chamber cools down much faster than the liquid load. In order to account for this disparity, we use two flexible temperature sensors, which are placed inside the liquid load, to give an accurate reading of the temperature of the liquid, thus allowing us to know when the liquid’s temperature actually reaches the proper level for sterilization. The two flexible temperature sensors are also used to control the conditions inside the chamber during the cooling phase. As a result, we are able to avoid dangerous messes caused by boil-over, and ensure safe conditions for opening the door. The reason we use two temperature sensors (in different vessels) is for safety. Should anything happen to one of them, for example container breakage, then there will be a difference between the two readings, thus prompting the autoclave operator to halt the cycle and keep the door locked until the temperature and pressure inside the chamber return to safe conditions for opening the door.

Fo Feature for Heat-Sensitive Liquid Media

One last point about liquid sterilization. Laboratory autoclaves are designed to be able to perform the Fo feature (pronounced F-zero). This is a feature that reduces exposure time of the load to high temperatures, in this case delicate heat-sensitive liquid media. In minimizing the overall time that liquid media are exposed to high temperatures, the integrity of the load is maintained, as well as reducing overall cycle time and energy costs for the lab. It allows the autoclave operator to consider the heat energy contributed during the heat-up time as contributing to sterilization. This is calculated by empirically tested tables of Fo values.

For example, the Fo feature is useful for a very delicate liquid, which can be easily caramelized and therefore disqualified for use. The autoclave operator can calculate a lower cycle time for sterilizing the liquid based on the Fo empirical tables. If a normal sterilization holding time is set to 15 minutes, it may be reduced by seconds/minutes based on the Fo calculation. This will help prevent overcooking the liquid while still ensuring proper sterilization.

For those interested in the science behind this concept, here is the actual equation that is used to calculate Fo times:

Fo = ΔtΣ10(T-121)/Z   Fo values are calculated using this equation. Δt is the time interval between Fo measurements.  T is the load temperature at time t and Z = 10oC. The Fo is a cumulative term, as represented by the Σ in the equation, which is determined from measurements of the load temperature (T) for the period of the sterilization process.

Fluid Sterilization

We discussed the unique challenges of autoclaving liquids, foremost among them preventing the boil-over effect. In order to achieve this, we investigated fast liquid cooling and superfast cooling, which lower overall cycle time for processing liquids. We then looked at how the two temperature sensors prevent dangerous spills and ensure proper liquid sterilization. Finally, we explored the Fo feature for heat-sensitive liquid media, which maintains the integrity of the load during processing and saves precious processing time and energy costs.

Stay tuned for our next blog post in which we will explore basic features of laboratory autoclaves.


1 In some benchtop lab autoclaves a jacket is not used for cooling, but rather two cooling pipes inside the chamber are used. Water is pumped through the cooling pipes to cool down the chamber.



Autoclave Training in Bangkok, Thailand

Tuttnauer training provide users, maintenance staff, technicians and salespeople with the skills and knowledge to safely operate and maintain Tuttnauer’s equipment. The training program takes into account the mixed abilities of students.

This month we are in Bangkok, Thailand providing in-depth training. We have a great group of 36 technicians and salespeople from Thailand, Singapore, Taiwan, Brunei, Malaysia, Indonesia and Korea.

The training is divided into two sessions:

February 14 - 17, 2017 is dedicated to Tabletop and Lab autoclaves. We cover the following topics:

  • Introduction to sterilization
  • Sterilization cycles
  • TTA/Lab piping diagram
  • Bacsoft controller and electrical diagrams

February 20 - 24, 2017 is dedicated to large Horizontal Autoclaves where we cover the same topics as in the first session, but with a focus on large autoclaves.

On successful completion of training, a certificate is provided to each attendee.

Get to Know Our Customers

Training is a great opportunity to get to know our customers. The Town in Town hotel is where it all happens. The group stays in the hotel during training period and the training also takes place in the hotel conference room. We have already made new social connections and ties and new friendships also form between our group members that come from different countries.


Town in Town hotel is where it all happens

We invite course participants to stay in touch also after the course ends. You can find us on our social media channels: Facebook and Linkedin. We invite you to join the discussion or just get the latest industry and company updates.


Introducing GS Line of Single Door Autoclaves


Following the success of the Double Door GS autoclave we are introducing the Single Door GS Autoclave. Available in 160 liter volume and 250 liter volume (1 StU and 2 StU, respectively), the GS autoclaves are designed to be more affordable for medical centers that require single door installations or double door pass-through installations, but are operating on limited budgets.

We have maintained Tuttnauer's high quality for hospital grade autoclaves, providing reliable, heavy-duty high performance machines that are also designed to fit into narrow spaces. The rounded chamber and coiled jacket reduce the manufacturing costs, thus allowing for a more affordable price tag without compromising on quality.

The convenient Single Door Pass-Through installation is just the newest addition to our family of reliable and high-performance autoclaves that our customers have come to expect from Tuttnauer.


Tuttnauer at Arab Health 2017

We were happy to participate in Arab Health 2017, Dubai. The four-day event was a great experience and a perfect opportunity to introduce Tuttnauer’s autoclaves, and other sterilization and disinfection solutions for medical facilities. This year we presented a range of steam autoclaves and the PlazMax, a low temperature sterilizer for heat and moisture sensitive equipment.

Many visitors joined Tuttnauer’s stand to discuss and experience our products. We were glad to welcome visitors from the Middle East, India, Africa, Asia and even a good number of Europeans and Americans. We were happy to meet our customers and partners as well as to make new business connections.

A warm thank you to all of our partners, customers and friends whom we met at Arab Health!

Looking forward to seeing you again at Arab Health 2018!

Nabil Aqel, Henk Ras and the Tuttnauer Team


Solids & Hollows Sterilization Guide for Lab Autoclaves

First post in series, “Understanding Laboratory Autoclaves”

Laboratories and research facilities simply cannot function without their autoclaves. How else can they properly sterilize all of the instruments, materials, and media to run their experiments and daily tasks? In short, laboratory autoclaves, also known as steam sterilizers, are designed for the specific needs of research institutes, and universities, as well as microbiological, pharmaceutical, food, and chemical labs. These autoclaves must be able to provide high quality, dependable performance that will allow for the smooth daily operation of the life sciences’ cutting-edge research and testing, which will ultimately allow for scientific creativity to flourish.

In this blog post we will take a look at the special needs of lab autoclaves, which are different than hospital sterile processing needs. The hospital SPD mainly autoclaves packages of surgical tools around the clock, constantly trying to keep up with the demand of sterilized surgical trays for the operating room (OR). In contrast, most laboratories autoclave a wider variety of materials and media, including liquids, pipette and glass hollows, solid instruments, biohazard and waste, and agar preparation, and at a much lower overall capacity.

In this post we will take a look at some of the most common applications of laboratory sterilizers:

  • Glassware, Hollow, and Tip Applications
  • Isothermal Processing

(In our next post, we will discuss autoclaving liquids, and in a future post, we will be covering common BSL laboratory applications, such as biohazard, waste, and dangerous pathogens.)

Glassware, Hollow, and Tip Applications

The tricky part about sterilizing glassware, hollows, and pipette tips is removing the air before sterilization. Since these loads are essentially containers of various sizes, they inherently contain air inside of the containers, and this air must be removed in order to ensure complete sterilization of the load. The longer, or taller, the sides of the container, the harder it is to remove all of the air inside. Think of a short, fat beaker versus a tall, skinny beaker. Which one do you think would be harder to sterilize? The tall, skinny one.

Enter the laboratory autoclave equipped with a vacuum pump. Instead of relying on the basic technology of gravity displacement to remove air (which is what a Class N tabletop autoclave uses), we need a more advanced system to remove air; indeed this is what the vacuum pump does with fractionated pre-vacuum air removal pulses. These air removal pulses successfully remove more than 99% of the air inside the chamber, thus eliminating air pockets in the load, especially in those tricky spots like the bottom of a very tall flask.

In addition to removing air efficiently, the vacuum pump also helps dry the load quicker and more effectively than other drying methods. This is crucial for the complete drying of porous loads and hollow instruments, such as pipette tips. 

How does this work exactly? During the drying phase, the vacuum pump keeps the pressure low inside the autoclave chamber. This low pressure state reduces the boiling temperature, thus forcing the quick evaporation of all hot moisture inside the chamber. The vacuum extracts the vapor (moisture) from the chamber, and presto, the load is completely dry!

Isothermal Processing

Isothermal Processing is another application that laboratory autoclaves are designed to handle. This cycle is most often used in the preparation of agar and other biological media. The temperature settings for this cycle range from 60°C to 95°C and allow for the gentle heating and cooling down of liquid loads, like agar. Agar is a delicate medium that can be easily overcooked/caramelized if proper precautions are not taken.

That’s a Wrap

Laboratory autoclaves for life sciences are designed to meet the high standards of today’s busy research institutes, universities, and biotech companies. As we’ve explained here, these autoclaves are constructed to sterilize the different types of media processed on a daily basis in your lab.

First we studied the challenges of air removal in glassware, hollow, and tip applications. We discussed how using a vacuum pump, via pre-vacuum air removal pulses, achieves greater than 99% air removal and eliminates air pockets inside even the tallest hollow containers. In addition, the vacuum pump dries the load more efficiently by keeping the pressure low and thus quickly extracting the vapor moisture.

And finally we spoke about isothermal processing, in which the temperature range of 60°C to 95°C allows for the preparation of agar and other isothermal media that are sensitive to heat.

Join us in our next blog post where we will explore liquid sterilization in laboratory autoclaves, another common lab application.

Comments and questions are welcome, as always, in the comment section below.