As useful as we may find our household freezers or fridges, used on an industrial scale the mechanical systems just don’t compare to advanced cryogenic equipment on the market at present.
The efficiency of the food freezing or chilling process is crucial to food production levels, yield and quality; therefore food processors are continually on the look-out for new innovations to help them improve their processes.
Fast freezing and effective chilling using liquid nitrogen or liquid carbon dioxide has been widely used in the food industry since the 1960s.
Gas companies manufacture cryogenic equipment specifically suited to food preservation, using liquefied gases to freeze or chill food efficiently, whilst retaining the goodness and quality of the product. The freezing and chilling process
The initial freezing process of food has a huge impact on the quality of the product when defrosted. The longer the freezing process takes, the more chance there is of the water contained in the cells of the food turning into large ice crystals, which can damage the cell membranes and tissues, resulting in a loss in vitamins, nutrients and flavours.
Additionally, a long freezing process and damaged cells can lead to a loss in the form and structure of the product, and can also increase the drip loss, meaning the product loses moisture.
The speed of the freeze is also important when considering the efficiency of a food processing plant; the faster the freezing process, the more batches frozen.
If the freeze takes longer than 5cm/h, as is the case with a conventional freezer, the damaging large ice crystals form. The answer, unfortunately, is not as simple as decreasing the temperature in order to get a quicker freeze, as this will also cause the aforementioned negative effects.
Similarly, as far as the chilling rather than freezing of food is concerned, the temperature needs to be brought down quickly to retain the goodness and appearance of the product, and food processors need a quick turnaround of product, rather than having to wait for each batch to be cooled before being able to process the next.Cryogenic processes
By using cryogenic nitrogen or carbon dioxide as fast freezing or chilling agents, only small ice crystals form and all of the negative effects can be greatly reduced.
For example, according to Messer Group statistics, drip loss can be as much as 5% with conventional systems, but is less than 1% with cryogenic freezing.
Larger quantities can be frozen or chilled faster, less space occupied, and individual pieces, like raspberries for example, can be individually frozen without moulding together, as is a common problem with a slow freeze process.
Mechanical freezers are usually designed for a specific cooling capacity, whereas cryogenic systems have a broad capacity range, meaning food can either be chilled or frozen, depending on how the equipment is set.
The low temperature of the cryogenic gases means that on contact with the (relatively warm in comparison) food product, they vaporise. The cold gas removes the thermal energy from the food, freezing it in a matter of minutes.
Cryogenic liquid nitrogen boils at -196°C under atmospheric pressure. When applied to the surface of the foodstuff, the nitrogen boils and therefore evaporates, absorbing heat from the product.
Liquid carbon dioxide, or carbonic acid, is liquefied at a pressure of 5.18 bar. When the liquid carbon dioxide expands to atmospheric pressure, dry ice snow, and gas are produced.
When the dry ice comes into contact with the surface of the food it sublimes, removing the heat from the food. Liquid CO2 also inhibits the growth of bacteria, making it suited to the freezing of food.Gas choice and equipment
The range of cryogenic equipment available for cooling and freezing food is vast.
Revenue from the food and beverage sector forms a substantial part of industrial gas companies’ income, and the sector has become even more crucial to earnings in light of the financial crisis, as it is one of few businesses which has remained relatively stable.
Therefore, the gas companies are continually striving to meet the needs of food processors, finding new technologies and incorporating them into cryogenic systems, in order to maximise efficiency, hygiene and quality.
Different gases and different equipment suits different foods. There is a classic range of freezers, but there are hundreds of variations on these which can cater to specific needs. The principle is the same whether freezing or chilling with cryogens, therefore some of the freezers can be adjusted to chill the food, rather than freeze it.
Tunnel freezers are efficient at processing large quantities of food. The freezer houses a conveyor belt, which moves food along past nozzles, which blast it with liquid nitrogen or liquid carbon dioxide.
Spiral freezers are useful for areas where there is limited space. Aptly named, the spiral freezer has a conveyor belt running over several levels, like a spiral staircase. Cryogen is injected in, and each individual piece of food is uniformly chilled or frozen; it is particularly useful for delicate products.
Immersion freezing is the fastest method on the market, it is usually used with another freezing system to boost effectiveness. Food is taken on a conveyor belt through a bath of liquid nitrogen, and immediately hardens. The system is particularly useful for food which is still hot – it can be frozen without any discolouring or loss of structure.
The flighted freezer is specifically designed to individually quick freeze (IQF) pieces of food, like meatballs or sausages.
The individual food products are transported through the freezer, and flighted conveyor belts tumble the food pieces along, keeping them separate from each other and ensuring each and every one of them is targeted by the cryogen.
Liquid carbon dioxide is a popular choice for systems designed specifically for chilling rather than freezing.
Vulnerable food products, like whole chickens for example, are chilled using equipment manufactured specifically for the purpose, in order to avoid an uneven chill, which can result in frozen or warm spots.
Even flour, dough and fat can be gently frozen using a cryogenic chilling system.
Another strand of the food business is refrigerated transportation. Once food is frozen, it must be kept frozen so as not to compromise its quality; therefore from factory to supermarket, food is transported in cryogenic vehicles, which utilise the unique cooling properties of CO2 to ensure the cold chain is not broken.
As with the in-situ cryogenic fridges and freezers, new technologies are continually being sought to ensure the quality of food being transported is of the very best it can possibly be.
Food processors are in constant search of the next revolutionary product to improve yield, whist keeping the quality of the product in all areas of the process.
In response to increasing pressure to combat climate change, those in the food refrigeration business are introducing new green technologies to their systems.
The Linde Group’s SnowCool® is an intelligent refrigeration system that makes it possible for a variety of perishable goods to be transported at different temperatures, ranging from ambient temperature, right down to -21°C or colder if needed, in the same truck.
This means that if five different types of products requiring five different chilling temperatures are to be transported, there is no longer a need for five different vehicles; they can all be transported in one vehicle, meaning diesel emissions are dramatically cut.
Goods are loaded into food grade fibreglass boxes. Once parameters for each box have been determined, a defined amount of CO2 snow (dry ice) is generated by injecting pressurised liquid CO2 into the box using a filling gun.
The decrease in pressure causes the liquid CO2 to expand and convert to solid flakes and vapour.
Some companies have also started using recycled CO2 from other industrial processes, in their food industry applications.
The Linde Group has said that it uses the by-products from ammonia, ethanol, titanium dioxide and gasoline production and compresses, purifies and liquefies it for use in food and beverage processing, instead of releasing it into the environment.
Even food refrigeration companies who don’t have direct access to CO2 being produced in large industrial processes are opting to approach companies to ask if they can capture the CO2, which would otherwise be released into the atmosphere.