Sous Vide And Blast Chilling Information

Why Sous Vide?

Sous vide, meaning ‘under pressure’ or ‘under vacuum’ in French, refers to a style of cooking where the food product is vacuum sealed  (cryovac**) in an air-tight bag and cooked in a water bath at a precise temperature. The purpose of sous vide cooking is to cook a food item until it reaches the desired temperature throughout, without overcooking the outside of the product. This is often done over a longer period of time than conventional cooking methods such as steaming, roasting and pan frying.

When using traditional cooking methods, the food product must be heated at a higher temperature than the desired core temperature. For example, you may wish to roast a leg of lamb until it reaches 60°C throughout, but to do this you would need to cook the product in an oven which is heated to 180-200°C. This means the outside of the product, which is more exposed to the heat, will reach higher temperatures during cooking, resulting in a product which is less cooked internally than it is externally. Sous vide cooking resolves this issue as the food product is placed in a water bath which is set at a temperature equal to the desired core temperature of the product. The product cannot get any hotter than the temperature of the water it is being cooked in.

This can be especially useful when cooking a variety of foods. Vegetables and fruit can be cooked at a temperature lower than boiling point, allowing the risk of overcooking to be removed and their shape and firmness to be retained when eaten,  as the cell structure is not as readily denatured at lower temperatures. At the same time, when meat is cooked using sous vide, the collagen between the muscle fibre is broken down without the fibres becoming tough from overcooking. This allows for a more tender piece of meat which has not lost its moisture.

Sous vide is not just useful for cooking. It is an invaluable tool when reheating foods, especially for catering purposed. Foods such as curries and stews can be cooked in advance, chilled, then placed in vacuum bags (cryovac bags) for storage and reheating. A bag of food reheated to the desired serving temperature using sous vide will not lose any moisture in the reheating process and will not cook any further. This also means that the food can be held in the water bath until ready for service, removing issues with timing when large quantities of meals need to be served.

Despite the many advantages of sous vide cooking, there are some minor limitations. Steak and other meat will not develop the brown ‘crust’ which is often desired and will need to be sealed quickly in a hot pan after cooking in the water bath in order to achieve this.

When setting up your kitchen for sous vide cooking, you will require some new equipment. You can purchase a free standing bench top unit such as one from the Roner series which consists of a thermostat and bowl in the same device. These units, which come in two sizes, provide the water circulation and temperature stability needed for sous vide cooking and are made from stainless steel. Alternatively, there are a variety of portable thermal circulators available which provide the same temperature accuracy, but are able to be used with any heat resistant pot or container (size subject to the machines limitations).

**CRYOVAC – Cryovac is a brand name.  It represents one of the first Companies to make food vacuum machines and food vacuum bags.


When looking at ways to freeze foods in the commercial food industry, there are a variety of options to choose from. Blast freezing has become more popular as it reduces the amount of time needed to freeze a product, but if time is not an issue for you, is there really much need for it? The answer is ‘yes’ and here’s why:

There are various factors which influence the effectiveness of freezing on food preservation. Air velocity, degree of contact between the food and cooling medium, packaging and product thickness can all play a part in the speed at which an item of food is frozen.

In the case of blast freezing, the air velocity plays a large part in increasing the speed when compared with conventional chest freezing. Slower freezing methods, such as chest freezing, create large ice crystals in food tissues which rupture cell membranes, causing loss of moisture once thawed. This is especially the case with fish, where the flesh is more fragile.

A chest freezer has no air velocity as it is considered to be a still air freezer. The chest freezer is a unit which keeps a chamber at a constant temperature well below the freezing point of water (typically -12 to -18°C) using vapour compression. When food is placed in the unit it slowly freezes as the temperature of the food lowers to that of the air inside the freezer unit.

Foods which would typically take around 12-24 hours to freeze in a traditional still air freezer can be frozen in around 4 hours in a blast freezer. In blast freezing, foods are placed in a unit then subjected to a series of cold air flows at speeds of around 10-15 metres per second, which bring the temperature of the foods to below -24°C, sometimes as low as -30°C or -45°C, quite rapidly.

Although both methods of freezing will preserve the food item by suspending microbial growth, rapid freezing methods, such as blast freezing, allow foods to reach temperatures which limit microbial growth sooner, while also having a better effect on maintaining the structural integrity of the food items. Faster freezing methods allow foods to be thawed to a state which are more like their initial fresh state.

Recently, at University, we conducted an experiment with fish fillets to assess the differences between blast and chest freezing. We froze some fish fillets in a blast freezer and some in chest freezer; then stored both sets of fillets in a chest for two weeks. The fillets were arranged on trays in different configurations – one to a tray, two to a tray (stacked on top of each other) and four to a tray (also stacked). Here’s a summary of what we found:

  • As expected, the freezing time for the fish fillets was much slower in the chest freezer, especially for the fillets at the bottom of the stack of four.
  • The freezing time for fillets in the blast freezer was faster and the freezing time was not significantly different for fillets frozen at the bottom of a stack.
  • As the ice crystals formed during freezing were larger in the case of the chest freezer, there was greater deterioration of cells, causing greater amounts of fluid to be lost when thawed (% loss calculated).
  • Further fluid was lost when baked, as a result of the deterioration in cell structure.
  • The sensory properties of the fish, its flakiness, juiciness and chewiness, became less appealing as the freezing time increased. The fillet which had the longest freezing time, bottom of a stack of four in the chest freezer, was the most unappealing. It had lost the majority of its flakiness and moisture and had become very tough and chewy, taking a long time to be macerated enough to swallow.

It was concluded that blast freezing can be seen to be the better method of freezing when compared to still air chest freezing. The sensory properties were far less appealing in the case of the fish frozen in the chest freezer. The blast freezer would also allow for more fillets to be frozen simultaneously, as the difference in the sensory properties was less noticeable with increased fillets per tray, compared to that of the chest freezer fillets. It was also much quicker – increasing time efficiency.

These results do not just relate to fish. When considering freezing in the food industry, it is important to remember the final product and its effect on the consumer’s satisfaction. Foods frozen in a blast freezer will be closer to their fresh counterparts when defrosted, allowing for a better sensory experience when consumed.

Rely Services offer a range of machines from Irinox that will quickly chill or freeze foods. Irinox are leaders in blast chilling and freezing technology and their machines will allow you to be more efficient, productive and profitable in the kitchen, not to mention ensuring supreme levels of food quality and safety.