The animal reproductive industry has been using liquid nitrogen (LN2) tanks to freeze and store frozen semen for several decades. More recently with the rapid development of embryo transfer technology in different species, it is being used for storing frozen embryos. Because LN2 tanks are a tool that farmers, artificial insemination technicians, embryo transfer practitioners, veterinarians, animal reproductive scientists, semen banks and researchers use everyday, this paper has been written to provide a basic knowledge and understanding of this amazing product and how to use it carefully and more efficiently.
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Cryogenic liquids or "Cryogens" are liquefied gases that have been cooled below ambient temperature, pressurized, and kept in their liquid state at very low temperatures. Cryogenic liquids have boiling points below -150°C (-238°F).
Examples of cryogens are the inert gases such as nitrogen, helium, neon, argon and krypton. Additional examples are the flammable gases such as hydrogen, methane, natural gas and oxygen.
Cryogens at room temperature and normal pressure are found as gases. In order to transform these gases into cryogenic liquid, they must be cooled below room temperature and then pressurized to liquefy them.
All the cryogens have three important characteristics:
· They are extremely cold. The vapors and gases released from these cryogens are also extremely cold.
· Very small amounts of these liquid cryogens can expand into very large volumes of gas that can displace air. For instance, 1 liter of liquid LN2 can form 695 - 700 liters of nitrogen gas when warmed to room temperature (20 - 21°C).
· According to WHMIS, cryogenic liquids are "compressed gases". Cryogens will create high gas pressures inside the storage containers, and therefore they require a relief vent/valve to allow the gases to escape from the container.
Nitrogen is a chemical element with the symbol N. Its atomic number is 7 in the periodic table. Nitrogen constitutes 78.03% by volume and 75.5% by weight of the Earth’s atmosphere making it the largest single component of the air. It is colorless, odorless, and tasteless. It is an important component of all living tissues and amino acids.
Nitrogen is classified as an "inert" gas due to its non-reactive nature with many materials. Nevertheless, nitrogen can form certain compounds under the influence of chemicals, catalysts, or high temperature. For example, when nitrogen is combined with hydrogen in the presence of a catalyst, ammonia is formed.
Liquid nitrogen (LN2), the cryogenic fluid used in the reproductive industry, is inert, colorless, odorless, non-corrosive, non-flammable, and extremely cold. It is produced industrially in large quantities by fractional distillation of liquid air.
When appropriately insulated from ambient heat, LN2 stays at -196°C or -320.5°F, which is the nitrogen boiling point. Because it maintains temperatures far below the water freezing point, LN2 is very useful in a wide range of applications as an open-cycle refrigerant and cryoprotectant including:
Rapid release of nitrogen gas into an enclosed space can displace oxygen in the air to levels below that required to support life, representing an asphyxiation hazard. The human carotid body, located near the bifurcation of the carotid artery, is an anatomical organ that measures changes in blood pressure and the composition of arterial blood flowing past it, including the partial pressures of oxygen and carbon dioxide. It is also sensitive to changes in pH and temperature. Unfortunately, when somebody aspirates air with higher concentration of nitrogen gas than normal, the carotid body works relatively slow and this low-oxygen concentration sensing system (hypoxia) will not react in an efficient way to prevent potential asphyxiation. When inhalation of nitrogen is excessive, symptoms such as dizziness, nausea, vomiting, loss of consciousness and death can happen. Death may happen from errors in judgment, confusion, or loss of consciousness that prevents self-rescue. At low oxygen and high nitrogen concentrations, unconsciousness and death may occur in seconds and without any warning. Because of this asphyxiation hazard, when working with LN2 in a confined or enclosed space it is recommended to monitor oxygen concentration with specialized ambient oxygen monitor-alarms that are available in the market (Fig. 1).
Figure 1. Example of an oxygen monitor to measure ambient oxygen concentration in confined rooms. This type of oxygen monitor activates when the oxygen concentration reaches below the set point, which is normally 19% or 19.5% O2. (Picture obtained from Alpha Omega Instruments website).
LN2 is capable of causing instant frostbite on contact with living tissue. Direct skin contact with LN2 causes severe frostbite (cryogenic burns) within moments to seconds depending on the form of liquid nitrogen (liquid vs. mist) and surface area of the nitrogen-soaked material. It is important to know that soaked clothing or cotton can cause more rapid skin damage than a spill of direct LN2 to the skin, which for a few seconds is protected by the Leidenfrost effect. The Leidenfrost effect is a phenomenon in which a liquid, in near contact with a mass significantly hotter than its boiling point, produces an insulating vapor layer which keeps that liquid from boiling rapidly. This effect happens when somebody dips a wet finger in molten lead or blowing out a mouthful of liquid nitrogen, both situations causing no injuries to the demonstrator.
The best practice to prevent frostbite when handling LN2 is to wear personal protective equipment such as a full-face shield over safety glasses, loose–fitting thermal insulated or leather gloves, long sleeve shirts, and trousers without cuffs.
Cryogens are shipped and stored in thermally insulated containers. The cryogenic containers are particularly designed to resist rapid temperature changes and extreme temperature difference. There are two types of tanks commonly used in the reproductive industry:
Dry shippers (Fig. 2) are Dewar flask containers designed for transportation of frozen biological specimens (semen or embryos) in LN2 vapor at temperatures around -190°C. They are made of lightweight aluminum and work by absorbing LN2 into a thick layer of hydro-phobic material that surrounds the inner cavity of the container where the semen is stored. The dry shippers must be properly charged by filling with LN2 to the point of saturation of the absorbing material and then removing the excess LN2 to provide maximum holding time. The long holding time of dry shippers is obtained because there is superior insulation created by a double-wall aluminum shell that is filled with foil and also vacuum sealed (Fig. 3). A unique lid plugs the dry shipper’s neck tube, helping to create the insulated interior environment of LN2 vapor.
Special cases or cartons are available from the manufacturers to protect the dry shippers from potential damage in transportation and to prevent the LN2 from spilling by ensuring the system remains upright at all times (Fig. 4).
Figure 2. Dry shipper container commonly used for transport of frozen semen or embryos.
Figure 3. Interior schematic view of a dry shipper. A - Aluminum wall. B - Locking tab. C – Neck tube. D – Vacuum chamber. E – Hydrophobic absorbent material shown in blue. F – Insulation shown in yellow. (Picture obtained from the MVE Bio-Medical Vapor Shippers catalogue, Chart Industries, Inc - See reference list below).
Figure 4. Dry shipper protective case.
It is the amount of time that the dry shipper will be capable of storing the specimen in LN2 vapor at the required temperature (-190°C) during transportation. The static holding time is measured in days and helps to calculate the shipping time of specimens. In the marketplace, there are dry shippers with static holding times that vary between 8 and 24 days. The static holding time depends on the LN2 evaporation rate of the shipper, which is calculated in litters evaporated per day (liters/day). At the same time, the evaporation rate depends on the dry shipper’s LN2 capacity measured in liters and the size of the neck opening. The liquid nitrogen capacity for dry shippers varies between the different models available; this usually ranges between 1.5 to 7 liters of LN2.
This is a measure of the maximum amount of straws that can be stored in a dry shipper for transportation. Usually dry shippers have only one canister, which will vary in diameter and height depending on the model. Therefore, the storage capacity will depend on the size of the canister, the type of straw being used (0.5 or 0.25 ml straws), and whether or not the straws are packaged on aluminum canes.
Biological specimens such as frozen semen and embryos are normally packaged in straws and stored in LN2 tanks (Fig. 5) with similar characteristics to a liquid Dewar flask described previously. Storage tanks are large, metal, vacuum-sealed liquid nitrogen refrigerators encased within an extremely efficient insulation system. They are made of aluminum and have two separate chambers: an inner and an outer chamber (Fig. 6). The space between the two chambers is filled with foil and special paper, and the air is removed to create a partial vacuum in this area. The vacuum increases the insulating effect and is the major effective property of the tank. A unique lid is used to plug the neck tube and create further insulation for the liquid nitrogen in the interior of the tank.
Figure 5. Left: Picture of a semen/embryo storage tank with a LN2 capacity of 22 liters and holding time of 40 days.
Right: Top view of the same storage tank after removing the lid. This tank has six canisters that are enumerated individually and can be seen in the interior of the tank.
Figure 6. Left: Interior schematic view of a 6 canister semen/embryo storage tank with the inner chamber with LN2shown in blue. A – Protective cover lid. B – Top. C – Aluminum wall. D – Neck tube. E – Locking tab. F – Lid or plug. G- Vacuum chamber. H – Bottom spider design for individual allocation of canisters. J – Insulation shown in yellow.
Right: Semen storage tank with a measuring stick for LN2. (Left picture obtained from the MVE SC Series - Aluminum Storage Tanks catalogue, Chart Industries, Inc).
As long as any LN2 is present inside the tank, LN2 storage tanks can keep semen and embryos at -196°C. The ideal scenario is to have the LN2 tank filled between half and full capacity. The recommended minimum volume of LN2 present in the tank is at least 5 cm (2 inches) in order to prevent any damage to the stored specimens. Ideally, the LN2 level should be monitored every week with a calibrated plastic measuring stick (Fig. 6).
The inner chamber is the area that stores the LN2 and contains the frozen specimens, and is actually held from the outer shell by the neck tube (Fig. 6). Storage tanks are very delicate. Any strong or excessive swinging motion could cause rupture of the neck tube creating vacuum loss and tank failure.
This is the amount of time that the tank will be capable of storing the specimen in LN2 at -196°C. The static holding time for storage tanks is measured in days and is used to calculate how often the tank needs to be refilled with LN2 and their estimated working time. The static holding time for these tanks varies considerably, and it may be as low as 17 days or as long as 340 days. The static holding time depends on the LN2 evaporation rate of the tank, which is calculated in liters evaporated per day (liters/day). The evaporation rate depends on the tank’s LN2 capacity measured in liters and the size of the neck opening. The liquid nitrogen capacity also varies between the different storage tank models available in the market place. In general, the LN2 capacity for the storage tanks used on farms, in veterinary clinics or by artificial insemination, technicians can range between 3.6 to 50 liters depending on the model and the manufacturer (Fig. 5). In the bovine artificial insemination industry, companies that collect, process, store and distribute bull semen require larger storage tanks with LN2 capacities in the hundreds or thousand of liters (Fig. 7). These tanks can also be used in frozen semen banks.
Figure 7. Top: Picture of liquid nitrogen tanks for storage of high volume of frozen semen/embryos. The left tank has a LN2 capacity of 370 liters, the middle one 756 liters, and the right one 1,672 liters.
Bottom: Interior schematic view of a high volume storage tank for semen or embryos. A – Offset neck design. B – Aluminum lid in yellow. C – Rotating interior tray. D – Stainless steel wall. E – Annular filling lines. F – Casters. G- Rack stand. H – Step-up platform. (Pictures obtained from the MVE Eterne Series – 190°C Vapor Storage Catalogue, Chart Industries, Inc).
This is a measure of the maximum straw holding capacity of the storage tank. Storage tanks normally have several canisters (6, 9 or 10 canisters) (Fig. 5) that vary in diameter and height depending on the tank model. The tank’s storage capacity will depend on the size of these canisters, the type of straw being stored (0.5 or 0.25 ml straws), and whether or not the straws are packaged on aluminum canes. As a rule of thumb, the manufacturers calculate the maximum storage capacity of tanks by using only 0.5 ml straws. When using aluminum canes, it is important to remember that five 0.5 ml straws can be stored in a 9.2-mm goblet, and that one aluminum cane can hold two 9.2 mm goblets (top & bottom level). This means that ten 0.5 ml straws can be stored per aluminum cane using 2 goblets (Fig. 8) and that several full aluminum canes are required to fill each canister (Fig. 9). The aluminum cane system is widely used for packaging frozen semen from different animals and it is commonly used in the reproductive industry worldwide. Each aluminum cane has a labeling tab on the top that allows the user to identify the 10 semen straws stored in that cane (Fig. 9).
Figure 8. Left: Aluminum cane and plastic goblets (9.2 mm diameter) with 0.5 ml straws before being assembled.
Right: After assembling the parts. Notice top and bottom location of the goblets in the cane.
Figure 9. Observation of the interior of a semen storage tank. One of the canisters has been retrieved to the center of the tank to show 7 aluminum canes (with 10 mm diameter goblets) located inside the canister and with extra room for storing more cane units.
On the other hand, some companies report maximum storage capacity of a tank as "Bulk storage" in 1 or 2 levels. In this type of storage, a much larger plastic goblet with a diameter very close to the internal diameter of the canister is used to store the straws (Fig. 10). One level bulk means that only one goblet full of straws is used in all the canisters (Fig. 10). Two level bulk means that two goblets full of straws are used in all the canisters, one on top of the other (Fig. 11). Because there is no need for using aluminum canes the storage capacity is increased automatically. With 2-level bulk storage, the tank capacity can be increased by 2 or 3 times, when compared with storage on aluminum canes. Bulk storage is routinely used by bovine artificial insemination companies to store large quantities of frozen semen for long periods of time. To locate and identify bulk stored semen, the visible portion of the canister handle is labeled and the information recorded.
Figure 10. Left (a): View of a 65 mm plastic goblet showing its high storage capacity, around 365 (0.5 ml) straws.
Right (b & c): Interior view of a semen storage tank that has 1-level bulk straw storage. Each canister is 79 mm in diameter and contains only one 65 mm diameter goblet full of 0.5 ml straws located at the bottom of each canister. The total capacity of this tank, using 65 mm goblets in 1 level bulk storage would be approximately 2,190 (0.5 ml) straws.
Figure 11. Interior view of a tank showing 2-level bulk straw storage. The canister is 79 mm in diameter and contains two 65 mm diameter goblets full of 0.5 ml straws, locating one goblet on top of the other inside each canister. The total capacity of this tank, using 65 mm goblets in 2-level bulk storage in 6 canisters would be approximately 4,380 (0.5 ml) straws.
As a rule of thumb, the user can nearly calculate the capacity of a storage tank for 0.25 ml straws by doubling the capacity mentioned for 0.5 ml straws. For instance if a storage tank has a maximum capacity of 720 (0.5 ml) straws, it can be inferred that the tank will have a capacity of approximately 1,400 (0.25 ml) straws.
The following are important procedures to ensure maximum holding time in a LN2 cryogenic storage tank:
As previously mentioned, frozen semen straws are often packaged in plastic goblets inside the semen storage tanks. Goblets are normally round in shape, although there are other shapes available, such as polygonal, hexagonal, etc. Round goblets are usually classified and differentiated by their diameter in millimeters (mm) (Fig. 12 and Table 1). The larger the diameter of a round goblet, the higher the straw storage capacity will be (Table 1).
Figure 12. Comparison of size and capacity of different plastic goblets used to store semen straws in storage tanks. The goblet containing yellow straws is 10 mm in diameter; the one having red straws is 13 mm in diameter; the one with transparent straws is 35 mm in diameter; and the one with blue straws is 65 mm in diameter.
The smaller goblets (7.1, 9.2, 10 and 13 mm) are normally put on canes for shipping, distribution and storage of semen (Fig. 8, Fig. 9 and Fig. 12). The larger goblets (35, 39, 42, 53 and 65 mm) do not require canes and are placed directly inside the canister (Fig. 10, Fig. 11and Fig. 12). As previously described, larger goblets are used for bulk storage of frozen semen.
The following examples will assist someone buying a semen/embryo storage tank to calculate the storage capacity quickly.
Using canes and small goblets, a tank manufacturer describes a tank with the following characteristics:
The user wishes to use 10 mm round goblets, aluminum canes that hold 2 round goblets per cane and is going to store semen packed in either 0.5 ml or 0.25 ml French straws.
A 10 mm goblet can store 7 straws (0.5 ml) or 14 straws (0.25 ml) inside (Table 1). This means that each aluminum cane can store a maximum of 14 (0.5 ml) straws or 28 (0.25 ml) straws. First determine how many aluminum canes can be stored per canister without being jammed. In a 56 mm diameter canister, it is possible to store 15 aluminum canes with 10 mm goblets. Therefore, the capacity per canister will be at least:
The tank has six canisters. This means that the storage capacity of the tank when full is at least:
To store this amount of straws, the user will require 90 aluminum canes and 180 goblets (10 mm diameter).Bulk storage.
The manufacturer describes the tank for bulk storage as:
The user wishes to use 53 mm round goblets that fit inside the 56-diameter canister and is going to store semen packed in either 0.5 ml or 0.25 ml French straws.
A 53 mm goblet can store at least 250 straws (0.5 ml) or 550 straws (0.25 ml) (Table 1).
If using 1-level bulk storage, only 1 goblet (53 mm diameter) would be placed per canister. The maximum capacity of the tank would be at least:
According to these calculations, when using one 53 mm goblet per canister, the 1 level bulk maximum storage capacity of the tank could not be reached. The explanation for this discrepancy may be that a 1 or 2 mm larger goblet (54 or 55 mm goblet) was used in the original capacity calculation allowing for storage of more straws per goblet/canister, or it may be that the capacity for the 53 mm goblet was a conservative estimate and it could have actually stored more straws than we calculated. In general, goblets are usually not packed to maximum capacity for storage purposes.
An alternative for more storage in a tank is to use 2–level bulk storage. Two goblets (53 mm diameter) are stored per canister instead of only one. Then the maximum capacity of the tank would be:
When using 2-level bulk storage, the tank’s capacity for 0.5 ml straws has been almost doubled from the originally listed 1-level capacity of 1,764 straws to a final capacity of 3,000 straws.
With the invention of computers, new devices such as computerized controlled rate freezers for cryopreservation of oocytes, semen and embryos (Fig. 13) have been created and developed in order to facilitate these procedures and to secure more control over the techniques. Liquid nitrogen is used to perform the freezing procedures with computerized freezers in order to achieve final temperatures colder than -80°C and ranging between -80°C and -196°C.
Figure 13. IceCube computerized controlled rate freezer used to cryopreserve semen, embryos and other specimens. On the right of the picture is a 50 liter LN2 cylinder (22 psig) connected to the IceCube freezer through an especially designed connecting hose.
The LN2 can be provided for computerized freezers either in relatively low volume liquid cylinders or through wall outlets that receive the cryogen from centralized storage tanks. Both types of LN2 reservoir can be attached to the freezer by a specially designed connecting hose (Fig. 13).
The LN2 cylinders (Fig. 14) are insulated, pressurized, vacuum-jacketed vessels that are commercially available with volume capacities that can vary from 35 liters up to 450 liters. When compared with ordinary storage tanks, these liquid cylinders are much larger containers. They are mainly used in the workplace where cryopreservation procedures for semen, embryos or other biological specimens are done periodically. LN2 cylinders come equipped with safety relief valves and rupture discs (Fig. 15) to protect them from pressure build-up. These cylinders can operate at pressures ranging between 22 and 350 psi. The pressure required for a computerized freezer such as the ones shown in Fig. 14 is normally 22 psi.
Figure 14. Top: Pictures of two commonly used LN2 cylinders available in the market. Both have a capacity of 50 liters of LN2.
Bottom: Schematic view of the interior parts of a cryogenic cylinder (Picture obtained from www-safety.deas.harvard.edu/services/nitrogen.html).
Figure 15. Top: Schematic top view of different design and configurations of liquid cryogenic cylinders. Middle: Top view of a basic LN2 cylinder with its respective parts.
Bottom: Measuring gauges that are normally found in a LN2cylinder. The left one shows the amount of cryogen left inside the cylinder and the right one the internal pressure. In this case the tank is empty with no pressure inside. (Top picture obtained from www-safety.deas.harvard.edu/services/nitrogen.html).
SERLNG Product Page
Centralized storage tanks are enormous mega containers that have a capacity to provide a cryogen to many users located in several different rooms or laboratories at the same time. They can contain between 2,000 and 60,000 liters. These types of tanks can be found in research institutions, hospitals, specialized private companies or universities (Fig. 16).
posted by Dave Arnold
Nils and I wrote an article on freezing for the September issue of Food Arts magazine. The piece included a short section on liquid nitrogen (LN or LN2), and I have greatly expanded it here for the blog. Important: we make no claims to inventing any of the techniques presented here. They are all pretty standard. Please, discuss who-came-up-with-what-first somewhere else. This is simply a primer.
Sections:
I. Introduction
II. Safety (very important stuff)
III. Getting and Storing LN
IV. Some Applications (cool stuff, most of the pictures)
I. Introduction:
Liquid nitrogen is about as cold as you can get in the kitchen, registering a whopping negative 196 degrees Celsius (-321° F) … and it’s non-diluting and non-contaminating to boot. Despite its preposterous coldness, liquid nitrogen has only 15% more cooling power than the same amount of ice at 0° Celsius. This counterintuitive fact leads many chefs to underestimate the amount of liquid nitrogen they need for a given task, like making ice cream.
Ice cream is typically the first thing people make with LN. Theoretically, it should be the best you can make because the mix freezes so quickly—and the quicker the freeze, the smaller the ice crystals, and the smoother the ice cream. But commercial ice cream machines make sufficiently small ice crystals for most palates, and it’s very easy to over-freeze portions of liquid nitrogen ice cream. So your LN ice cream results might not be as great as you expect.
But there are many other fantastic LN applications. You can turn fresh herbs into powder, separate citrus fruits and raspberries into jewel-like pieces, and freeze alcohol to make liquid centered treats. Be warned: liquid nitrogen is addictive and mesmerizing in the kitchen.
II. Safety:
Nitrogen makes up most of air we breathe. It is completely and utterly non-toxic—end of story. Yes, it is a chemical. Cows and carrots are just collections of chemicals.
The dangers of liquid nitrogen, of which you must be ever vigilant, are cold-burns, asphyxiation, and pressure-related explosions.
Dangers to Your Customers:
Don’t serve items to your customers too cold. You will burn their tongue, spoil their meal, and make them angry. I am speaking from first hand experience here (as the burnee, not the burner).
Dangers to You, Your Co-Workers, and Your Building:
Cold Burns:
If a small amount of liquid nitrogen touches your skin and rolls off, you will not be harmed. Lab technicians routinely dip their bare hands directly into liquid nitrogen (quickly! they don’t hold their hands in it). A layer of vapor instantly forms and temporarily protects your skin—this phenomenon is called the Leidenfrost Effect. Read a cool paper on the Leidenfrost Effect, written by a science teacher who dips his hand in molten lead here.
Burn danger occurs when liquid nitrogen remains in contact with your skin—whereupon liquid nitrogen burns the same way hot oil does. Painfully.
• Do Not wear clothing that can capture liquid nitrogen run-offs or spills, such as cuffed pants. I got my worst LN burn when my protective sleeve (which was not rated for cryogenic work—wrong choice) got too cold, cracked, and allowed LN to seep into the cloth cuff of my gloves.
• Do Not dispense LN into a container, such as a bain marie, that you are holding with your bare hands. You will get burned if the container gets super-chilled, which is likely. This hurts, too.
• Always wear safety goggles—LN can boil up and into your face at any moment, quickly blinding you. Forever.
• Never pour LN above your head, and do not kneel next to someone pouring LN.
• Do Not hold containers of LN with one hand, or in any other precarious way.
• Have a clear exit path in case something goes wrong—if you drop a whole container of LN, you and everyone else around you needs to get away from it quickly.
Explosion:
Never seal liquid nitrogen in a closed container. The pressure will rise, and unless the container can hold psi or better (nothing you have can) it will explode. In July a young cook in Germany lost both his hands and landed in a coma when he sealed LN in a closed container. Liquid nitrogen containers, called dewars, are either completely vented to the atmosphere or held between 10 and 22 psi with multiple safety valves. It’s worth repeating: don’t put LN in a container that is sealed or could become sealed by mistake.
• Let’s put that last part another way: Make sure there is no way your containers could become sealed. Example: a venting tube in a lid could get clogged, or crimped. A thermos lid you leave unscrewed can be sealed by your colleague. Note that even if an explosion isn’t strong enough to cause damage, LN spraying all over the place is hazardous.
• Never send LN through a pipe that could become sealed. You’ll make a pipe bomb.
• Never modify a dewar. In at Texas A&M, someone intentionally defeated the safety valves on a LN storage dewar. The resulting explosion destroyed several rooms, tore a hole in the ceiling, and ruptured plumbing that in turn flooded the entire building. Thankfully all this happened at 3am, and no one was hurt. The pdf report complete with pictures is: here.
Asphyxiation:
A small amount of liquid nitrogen turns into a large amount of nitrogen gas—LN expands by 700 times when it vaporizes. If you use a lot of LN in a closed space, it will displace the oxygen and suffocate you before you know what’s happening. This is the largest cause of death from LN. The danger is very, very real.
Unless you have been trained like an air force pilot—by coming up to the very brink of nitrogen-induced death under highly controlled circumstances—you can’t tell that you are suffocating on nitrogen. The panicky feeling you get from choking or staying under the water too long doesn’t come from lack of oxygen— it comes from the buildup of CO2 in your blood. Your lungs can get rid of CO2 just fine in a pure nitrogen environment, so your body doesn’t send those helpful distress signals. Also, breathing pure nitrogen is much worse than just holding your breath. Breathing nitrogen actually sucks the existing oxygen out of your bloodstream. Majorly bad news. So,
• Do Not use LN in an unventilated area. What constitutes an unventilated area? Read ASU’s guidelines here.
• Never go into an elevator with LN. If a dewar breaks in the elevator, you’re dead.
• Never carry LN in a vehicle’s passenger compartment. You get into an accident, the LN spills and vaporizes, you suffocate instantly.
• If you see someone passed out near an LN tank, do not try to help. Many people who die from nitrogen suffocation were trying to help co-workers. If the victim has passed out they are probably already dead. Call 911.
• Invest in an oxygen meter, like the one shown in the picture above.
• We can’t cover every eventuality here—be sensible.
Set up and enforce a safety program:
Make sure everyone who uses LN is properly trained. Print out and post basic LN safety rules. Some documents:
An overview of the hazards (plus gruesome details) from the US Chemical Safety Board.
The cryogenic fluids handling guide from Arizona State University.
III. Getting and Storing LN:
Unlike compressed gasses, liquid nitrogen isn’t stored under high pressure. In a cylinder of nitrogen gas, the pressure can easily top psi. It can sit around at room temperature practically forever without any loss. LN, on the other hand, is always freezing cold and is kept in an insulated container (called a dewar) under little, if any, pressure. Because LN is constantly boiling off, you’re always losing a little bit. The bigger and better insulated your dewar, the longer you can store your LN. Because LN is always boiling off and venting through those all-important relief valves, your dewar will occasionally makes little psssssssssssssssss sounds. Don’t be alarmed by the hissing noise—it’s your friend.
When you are filling a container or dewar remember this: you will use up a lot of LN just chilling down the vessel. Doesn’t matter if you are filling a dewar, a bain, or a Styrofoam cooler—when you first add LN, it will smoke furiously while the surface of the container gets chilled down to LN temperatures. After the container is chilled, it will fill up in an orderly and efficient way. Rule of thumb for LN efficiency: don’t let your containers run dry and warm up if you don’t have to. Keep them cold.
The easiest way to get LN is to snag some from a buddy who has a bunch already. Use a vented vacuum insulated coffee dispenser, never a thermos! A thermos can seal and explode. Many vacuum insulated coffee pots are inherently vented. Check to make sure yours is.
We use these pots quite a bit around the school. They are convenient, easy to pour from, and they only lose about 50 percent of their volume in 24 hours. I’ve heard people tell of getting these pots filled at their local gas supplier, but I have had no such luck.
Most chefs obtain liquid nitrogen from compressed gas/welding suppliers. You’ll have two choices: buy or rent a small 10-50 liter capacity dewar and have it filled on a regular basis (few places in New York still rent these dewars), or buy or rent a large 160-240 liter capacity storage dewar and have the company swap your empties for fulls when necessary. The large dewars are much more economical; up-front costs are higher ($ versus $) but they’re much cheaper to own. You’ll pay the same amount to swap out a 160 liter dewar as you will to fill a 50 liter dewar. If a small dewar develops a leak or breaks, it’s your problem—not an issue with large dewars because they are swapped out. You can usually rent the large dewars for a small monthly fee, plus the rather large refundable deposit. One annoyance with the large dewars: you will need to prove to the supplier that you own it. They won’t swap out your dewar if they think it’s a rental from a competitor or stolen off the street from the local utility company.
Small Dewars, 10-50 Liters:
10 liter dewars are small and convenient, but they don’t have nearly the holding time of larger dewars. Remember—the larger the dewar, the smaller the percentage of LN that evaporates per day. If you have to pay someone to come with a truck and fill your dewar, 10 liter dewars are uneconomical—go for 35-50 liters.
A 10-20 liter dewar really shines if you need to carry LN from place to place, or store the last of the LN from your big dewar while it’s out being refilled. Hint: you’ll want to order LN before you run completely dry, but you don’t want to throw away the last of your LN. Save the dregs with a 10- or 20-liter plug top dewar.
Liquid Withdrawal Device:
Dewars with liquid withdrawal devices will cost you more, but they’re worth it for their convenience. Here’s an image from dewar manufacturer Taylor Wharton:
The liquid withdrawal device just clamps onto an ordinary plug-top dewar. The clamp seals the top and allows the dewar to develop about 11 psi. Above that pressure, excess nitrogen is vented from a relief valve— which you will hear hissing. Sometimes, if the dewar is overfilled, the valve will get frozen open and you will lose all your LN. Try tapping the valve to shut it or gently heating it to unfreeze it. If the relief valve clogs and fails (as opposed to freezing open, which is not a safety issue, but a money issue), the secondary safety valve kicks in. Here is the operating manual for the liquid withdrawal device, including instructions on how to fill a dewar.
Big Dewars:
If you really want to be an LN ninja—and save money in the long run—buy a real dewar—160-240 liters. Here are the parts:
IV. Some Applications—the Fun Part:
Remember: these are basic techniques. No one is taking any credit. See the first paragraph of this primer.
Ice Cream:
LN ice cream is typically made in a Kitchen-Aid with a paddle attachment. Don’t use the whisk. All the mixer is really doing is stirring the LN into the mixture, not providing aeration. LN boils violently, so it provides both aeration and freezing. But the LN also floats—so without the mixer you will use a ton of LN and just freeze the surface. A liter of ice cream base at refrigerator temperature will take roughly a liter of LN to make ice cream. Turn the mixer on medium speed and add the LN slowly to prevent splashing. LN fog will be everywhere, and you will see nothing. Wait for the fog to disperse. Scrape down the sides of the bowl with a stiff metal spatula. If the mix is too soft, add more LN. If the mix is too hard or has hard lumps mixed with soft parts, turn the mixer on high for a bit to even out the texture.
Chilling Drinks and Other Liquids:
LN is a fabulous way to chill liquids without diluting them. Here is the simple rule: always add your liquid to LN rather than the other way around. LN floats. If you pour LN on top of a drink you will get a frozen layer on top with a warm layer on the bottom, and will use far more LN than necessary.
It doesn’t make sense to chill individual drinks with LN. It is very easy to overshoot when chilling small quantities, and you’ll freeze your liquid solid. LN is a great way to chill a lot of drinks—it’s as fast to chill 100 as 1. If you freeze your drink too solid, the heat from your hands, a torch, or hot water on the outside of the container can bring it back. Alcoholic drinks melt pretty quickly.
So How Frozen Should I Make My Drinks?
You need to be sure you aren’t going to freeze any tongues. Just because you can freeze vodka solid doesn’t mean you should. Straight liquor shouldn’t be served below -20°C (-18°C is better, a -20°C drink has a sting to it). The higher the water content, the more cold punch it will pack. A -10°C drink at 17% alcohol will taste a lot colder than a -10°C drink at 40% alcohol. Here is a visual guide:
By the way: you can use LN to freeze Pacojet containers in under a half hour, including tempering—but I shouldn’t print the technique because I’m sure it will void your warranty, and I don’t want all those broken Pacojet blades on my conscience.
Chilling Glasses:
LN is the best way to chill glasses, and it costs less than 10 cents a glass. I love this technique. The inside of the glass is chilled, but the stem and the base remain condensation-free and the outside of the glass doesn’t get so cold that your lips stick. To look badass, chill at least two glasses, one in each hand, simultaneously.
This technique works well with champagne flutes, highballs, rocks glasses, etc. Don’t use this trick with a martini glass; as you swirl you will send LN spraying into your face.
Blowing Smoke:
You cannot eat high-moisture foods right when they come out of LN. They will freeze to your tongue and cause you great pain. Low moisture foods—like dry meringues, marshmallows, some cookies, some cakes—don’t transfer a lot of energy from your tongue, so you can eat them almost right out of the LN. Added bonus: when you crunch these foods in your mouth, a plume of scented (and harmless) nitrogen fog shoots out of your nose and mouth.
Breaking Things Up: Raspberries, Citrus, and Powdered Fresh Herbs:
Things frozen in LN tend to get brittle. Use this fact to your advantage, as in the following three techniques:
Freezing the Un-freezable and Freezing in Cool Shapes:
Though this article is not about history and credit-bestowing, I have to make an exception here because Dani Garcia, of El Restaurante Calima in Spain, effectively owns the olive-oil process I’m about to explain. He has used LN to freeze olive oil and olive oil emulsions into more shapes and textures than you can imagine. LN-frozen oil doesn’t hurt your tongue because the oil doesn’t pack the heat wallop that water does.
The last technique I’ll show you is currently on the menu of our friend, Johnny Iuzzini of Restaurant Jean Georges. He makes chocolate squiggles and branches by piping chocolate into LN with a cornet.
This is a variation on the main idea that LN can make anything stay put by freezing it quickly. The possibilities are endless.
Let’s wrap this up. LN’s going to hook you quick. I remember seeing some Spanish chefs do a demo in . I noted to myself that they used liquid nitrogen like it was water, whereas I was rationing it like wine. Now that I have the big tank, I rip though it just like they do. And you will too.
For more information, please visit Liquid Dewar Cylinder.