Speed of the essence when it comes to cryogenic temperature control

In different parts of the food industry, various techniques to give foodstuffs the required temperature during or after mixing have been developed. These include adding chilled water, brine, or water ice to the product, using over-chilled or frozen ingredients, or most commonly using mechanically refrigerated mixing equipment.

Spraying or injecting a cryogenic fluid onto the product in the mixer while it is being mixed is also an efficient and safe ways of chilling. Industrial gas company Air Liquide specialises in the latter – utilising liquid nitrogen or liquid carbon dioxide to chill produce quickly and effectively. There are several benefits to using cryogenic freezing, according to Stephen Crawford, who is an Air Liquide senior engineer and expert in food cryogenics.

“It’s not hard to implement,” said Crawford. “Either liquid nitrogen or liquid carbon dioxide can be sprayed onto merchandise being chilled either from the top or the bottom. Both methods are used in industry and can be installed on existing equipment.

“Usually they are injected into some sort of mixer – for example, protein mixing, such as beef or chicken mixing, which is being made into patties or nuggets. It’s cheaper to install top mixing than it is bottom mixing, but the bottom injection method is more efficient.
“We use this type of injection to maintaian temperature. As food is mixed, you get friction between the product and the blades so heat is generated. You need to maintain temperature below 4˚C so you don’t get bacterial growth.”

Another benefit is that the gases have direct contact with the food. If an ammonia chiller is being used in a mixer, users might be able to cool down the walls of the mixer but there is no direct heat transfer that is possible when liquid nitrogen is injected into the mixer itself.
“From a heat transfer point of view it’s much more efficient,” said Crawford.

As mentioned, speed can be a key. Crawford cites the example of one of Air Liquide’s clients that specialises in producing goat cheese, a produce that is temperature sensitive.

“When the milk comes out the goat, it’s at room temperature or above. The longer it takes to get down to 4˚C, the shorter the shelf life will be,” he said. “The company puts it into a mixer and injects liquid nitrogen at -196°C through a cryo-injector. They get liquid nitrogen coming up, which rapidly cools the milk. As a comparison, mechanical chillers can only reach a temperature of -35°C with ammonia refrigerant.

“What would have taken them hours if you had put buckets of it in a mechanical chiller happens in just a few minutes in the mixer. You have blades inside stirring it. You don’t end up freezing one portion and having another portion still warm. They are constantly stirring it while injecting liquid nitrogen. It brings down the temperature of the whole product much quicker.”

Some of the mainstays of cryogenic chilling are chicken nuggets and meat patties, which are popular with those who process fast-food items. However, cryogenic chilling can also be used for dough mixing.

Most large flour mills have the flour stored in big silos outside. Industrial bakers blow it around pneumatically to get it into the mixer. It is the most efficient way for them to move the flour. The flour then comes into its mixture where it is combined with water and other ingredients. It’s mixed mechanically. But the temperature of the product in pastry has an impact on the texture and the final outcome, according to Crawford.

“If you’re trying to make a product that has a certain amount of ingredients and you mix them all together at 15˚C instead of 20˚C, the texture of the final product will be quite different even though the ingredients are the same,” said Crawford. “The bakeries find – especially if they have days like in the middle of summer where it is 35˚C – when you introduce that into the dough, the dough is far too hot and melts the butter. If you can inject a small amount of cryogen, then, depending on the temperature, it can make all the difference. If it is 20˚C you know to inject nitrogen or carbon dioxide for 10 or so seconds, or if it’s 35˚C you might have to inject for 40 seconds. During the process, it brings the dough to a consistent mix. This means there will be the right chemical reactions with the yeast. Over the years, Air Liquide has acquired a deep knowledge of process parameters through hundreds of references in cryogenic chilling worldwide and knows how to implement the right recipe to reach desired outcomes.”

Crawford also talks of the safety aspects the cryogenic chilling can offer. Most large commercial chillers have ammonia in them, which means they have a refrigeration cycle, so they’ll have a pump that is compressing the ammonia. Like a fridge at home, the wires on the back get hot. There’s a heat pump that takes the heat energy from inside the box and puts it into the grill at the back. An industrial refrigerator works the same way but ends up with large cooling towers to get rid of all the excessive heat.

“This means factories have these ammonia lines running through their plants which you wouldn’t want to spring a leak,” said Crawford. “However, with cryogenics, if you have a minor leak of nitrogen, it must be repaired, but it is not an immediate hazard. Eighty per cent of the air we breathe is nitrogen. Ammonia is a corrosive and toxic chemical and customers require ammonia sensors around the place. Often they spend a lot of money maintaining it, and then you have the cooling towers and all the other issues with the cooling water side of it.”

Some synergies are also possible with other processes down the chain, such as food freezing and modified atmosphere packaging that also use nitrogen, carbon dioxide and mixtures thereof, which further reduce the cost of operations compared to mechanical chilling.

“Another advantage is that it is very reliable because there are very few moving parts when working with cryogenic chilling,” said Crawford. “The injectors can be retrofitted on customer’s existing mixers.

“With only a valve to open and close, there is no compressor that needs to be maintained like on a mechanical system. The servicing requirements just aren’t there. The cooling equipment we have is better. There is nothing hard about what we are doing. It’s an easy method for people to learn to do. We can even maintain the equipment for our customers.”