Sous-vide: Pinpointing systematic cooking

(by Prof. Dr. Thomas Vilgis, Max Planck Institute in Mainz)

The Process

The combination of high heat and long cooking times is often the cause for the number 1 cooking mistake where protein rich foods are concerned:
Fish or meat can become too dry or leathery.
Even poaching at temperatures less than 100°C is not always a good idea. The direct contact of the water with the item being cooked can change both the taste, as well as the aroma if the composition of the cooking broth does not correspond to the item being cooked.
In addition, the consistency of the product's surface will also change.

Therefore, exact temperature and heat transfer control without having direct contact with water makes Sous-vide the ideal cooking solution!

Using the Sous-vide technique at correspondingly low cooking temperatures (usually 50-85°C) especially appealing culinary conditions can be achieved.
The process is very simple:
Season products lightly (if at all), vacuum-seal the products and cook in a water bath at the appropriate temperature.
After a certain time, the following applies:

Water temperature = core temperature

Meat is cooked at a desired core temperature, e.g. at 55°C, in a water bath at exactly 55°C.
Depending on the thickness, the core will reach this temperature eventually.
A major advantage of Sous-vide cooking is therefore undisputed:
The cooking process and especially the cooking condition can be predicted with great accuracy.
Sous-vide cooking is therefore a physically accurate, very simple and efficient method to achieve the desired result.
Naturally, the water temperature can be set slightly higher than the core temperature, e.g. water temperature 56°C, core temperature 55°C.
For this purpose, the core temperature of the product being cooked is continuously measured with a temperature probe (needle).
When the core temperature is reached, the product being cooked must be removed from the water bath to avoid a further increase of the core temperature.

Sous-vide physics of proteins

Meat and fish largely consist of different proteins, which fulfil certain biological functions in living animals:

> Muscles for movement
> Stabilising connective tissue
> Myoglobin for oxygen transport

Therefore, every cooking process must be adapted to these very different types of "bio-materials". Proteins are "sensitive" molecules, which react to heat with dramatic changes in molecular state and this must be executed in a very gentle way.
The molecular arrangement of proteins in meat is a finely balanced matter. Small energy changes are sufficient to throw these forms out of "balance".
Under intense heat, proteins become denatured and meat juices exit the tissue completely (juice separation).
The meat becomes tough, the fish dry. When cooking at low temperatures, most proteins are not completely destroyed; therefore, sufficient "loose water pockets" are retained in which the cellular water can "collect".
The water retention is ensured and the meat stays juicy.

How important is the temperature stability?

Temperature fluctuations must be avoided because the cooking temperature is always close to the temperature at which some proteins are destroyed. However, fish and meat consist of different types of protein, which react differently to temperatures.
This can be conveniently observed on a piece of salmon or salmon trout. If the temperature is kept constant at 49°C, the colour remains completely unchanged. Water barely escapes and the consistency is tender and the sensation in the mouth is "gel-like" perfect.
If the temperature only briefly rises above 50°C, small white dots appear on the surface of the salmon; albumin coagulates on the surface of the fish.
Although hardly any water exits and the consistency is not affected, such results are not acceptable in top restaurants.

Temperature stability is imperative for perfect cooking and even small variations of 1°C± can have a negative effect!

Cooking table: Thickness of the product being cooked

Cooking tables facilitate work in the kitchen because they provide guidelines for cooking times for certain dimensions of the products to be cooked.
However, they can only serve as rough guidelines since every cook knows:

Not all meats are the same.

When cooking at the temperature limit of the proteins, however, even small differences in protein composition are of crucial importance.
Many of the cooking tables in circulation do not consider these differences. The perceived accuracy of these values, for example the size guidelines for meat in a water bath at 60°C for a core temperature of 54°C is approximately given in the table. Thereby, an initial temperature of 5°C for the meat is assumed.


Thickness / mm
Time / min

Table: Values for the meat cooking times that are cooked in a Sous-vide water bath at 60 °C.
The desired core temperature of 54 °C in the corresponding dimensions stipulates the cooking time through the physics of heat conduction.

The start temperature of the product to be cooked
Although the cooking time is determined mainly by the size of the product, the starting temperature also plays a big role.
Therefore, it is quickly evident that a product with a temperature of 5°C needs a longer cooking time than one that is already at room temperature or has been pre-warmed. An adequate amount of water in the water bath is essential for all the Sous-vide cooking processes. Especially when, for example, due to hygiene reasons the initial temperature of the product is low. The product always cools the water. If the cooling effect is too high in a too small amount of water, this can have a negative effect on the cooking because the cooking time must be increased and the surface of the product being cooked is warmed for a longer period.

Collagen rich meat

The cooking times of collagen-rich meat such as a beef shoulder cut, calf (leg) and similar must be longer.
Three protein strands are twisted into a spiral, like ropes, in the connective tissue collagen.
At sufficiently high temperatures, these can untwist themselves, but this takes time and a higher temperature.
In the simplest form of collagen, the unwinding process begins at 55°C; other types of collagen must be heated to 68°C:
> Collagen turns into gelatine.
Although the muscle proportion of the meat at 68°C can no longer bind water completely, the gelatine binds the released water instead. Gelatine is a water binder and tenderizer. This correlation of the proportions allows perfect Sous-vide cooking temperatures at modest temperatures between 55°C and 75°C.

Fluid in the cooking bag

Often, juices leak out during cooking into the cooking bag. This shows that a small amount of fluid loss is difficult to avoid. If, however, too much water collects, this is an indication that the cooking temperature was a few degrees too high. Several trial runs, each at a slightly lower temperature provide conclusive results.
Unfortunately, the optimal temperatures are not theoretically easy to predict since the precise protein composition from animal breed to animal breed varies. In addition, breeding, feed and the method of animal husbandry all have an effect on the optimal temperature.
On the other hand, the fluid that has leaked out possesses a perfect composition of flavours, minerals and salts.
Therefore, no osmosis occurs - the meat cooks "in its own juice". For this reason, it is not advisable, especially for longer cooking times of 20-60 minutes, to heavily salt in advance.
The salt concentration on the surface would be much too high. The meat being cooked at the de-naturalisation limit would be exposed to further dehydration through osmosis.

Is a stable vacuum necessary during the cooking process?

The protective plastic film is wrapped tightly around the product to be cooked, air bubbles between the film and the product to be cooked are avoided, and the warm water contact is perfect. The heat transfer is optimised.
Due to the increased external pressure, the original structure of the meat remains better intact. This is especially important for types of fish, which due to their extremely small connective tissue composition, tend to fall apart even at low temperatures. Through a well-maintained vacuum, meat juices cannot easily penetrate into the bag. A stable vacuum also provides for the exclusion of ambient oxygen, therefore an oxidation process is unlikely. During Sous-vide cooking of vegetables and fruits, the vacuum is even more important, as we will show below, because the absence of ambient oxygen is essential, for example, for the preservation of colour and structure.

Germs and bacteria

How can cooking at low temperatures be reconciled with hygiene regulations? Cooking temperatures below 60° C are not sufficient to sterilise the food. Germs are often spread from the outside. The meat from healthy animals, which was butchered under conventional conditions, is free of germs on the inside. Thus, there are a number of simple measures to immediately nip the bacteria problem in the bud. Absolute cleanliness is necessary. Hands can transmit up to 20,000 germs. Normally, this is not serious because the immune system is designed to deal with a certain number of germs without any problems. However, the bacterial population grows so strongly under certain conditions and at certain temperatures that the number of bacteria can rapidly increase above a tolerable level.
The salmonella problem shows: At 55° C a germ dies only after one hour, at 60° C after only half an hour. Therefore, temperature and typical cooking times of the Sous-vide method involve certain dangers, though they may be minimised by the choice of products and hygiene.

Figure: Bacteria can hardly be avoided in the cooking relevant temperature ranges. The temperature range 40-60°C is especially interesting for Sous-vide cooking. Here, mesophilic- and thermophilic-bacteria need to be avoided. Among these are staphylococcus and salmonella, which are mostly transmitted by hand contact.

Conserve and store -storage in the cooking bag

The product to be cooked, depending on the quality and temperature, can be stored well in a vacuum. The storage period, of course, depends on the food itself. The Sous-vide process has several advantages for this purpose. Firstly, by means of the vacuum, all oxidising processes, which involve oxygen, are slowed down greatly. Nevertheless, rules must be adhered to and the pieces may not be stored in the vacuum for too long. In addition, techniques after removal of the air, such as flushing a small amount of inert gas through back flushing into the bag (e.g. carbon dioxide or nitrogen) help to prolong the shelf life. Cooked meat that is not used immediately can be stored at normal refrigeration temperatures. Possibly, a new vacuum packaging makes sense if too much fluid has leaked into the bag.
The "flavouring" of meat that has been fried for a very short period is possible with its own roasted aromas.
By means of browning (short grilling), the surface is, quasi, sterilised and germ free.
Immediate vacuum packing and rapid cooling ensures that the meat can be stored for several days and then cooked the Sous-vide way.

Sous-vide for fruit and vegetables

The Sous-vide has been proven beneficial, for example, for the colour retention of vegetables. The colour of vegetables also suffers from the oxidation processes. If the oxygen is largely removed by the vacuum, this is very advantageous for the colour retention of, e.g. artichokes or apples as these quickly turn brown when exposed to oxygen.
If vegetables or fruits are pre-cooked at low temperatures and without direct contact with water, the cell structure remains stable. Thus, melons, bananas, figs, etc. can be cooked easily without becoming too soft or mushy.
Pectin does not dissolve. It remains, such as hemicelluloses and glycoprotein, in the cellulose structure.
Thereby, new ways of controlled and prepared cooking of vegetables is opened up by the Sous-vide cooking method.
Therefore, potatoes, for example, can be cooked, depending on the type, at lower temperatures between 60°C and 70°C, i.e. pectin content and starch to protein ratio. This preserves the cell structure and the French fries resulting for top restaurants have a better bite to them. Colours and aromas of vegetables remain more natural when they are pre-cooked in a vacuum at modest temperatures.

In summary:

1) The water content is the key to the shelf life of food. The rule of thumb: high water content produces a short shelf life - that is self-evident. Bacteria and germs can multiply better in a moist environment.

2) Large pieces have a longer shelf life than small pieces. The bacterial growth occurs on the surface. There is an unfavourable surface to volume ratio for smaller pieces.

3) The lower the storage temperature, the less spoiling. Psychrophiles in foods can be avoided through meticulous cleaning.

4) The shelf life is strongly dependent on the type of meat.
> Beef (high, four to five weeks)
> Pork (small, one to two weeks)
> Poultry and fish (very low, one week)
The reasons are also water content and protein composition. The vacuum is often gladly used as a maturity medium for pieces of beef. Here it is assumed that during storage, further maturing processes take place without further loss of fluid.

5) Pre-cooked meat (such as through browning or grilling) has a lower shelf life than fresh products. It is important to note that pre-cooked pieces should only be vacuum packed after they have cooled. Due to the rapid pressure drop, steam formation increases at higher temperatures because the boiling point of the remaining water drops at a low pressure as well.

6) Carbohydrate rich foods should be sealed by gas (with carbon dioxide and nitrogen) so that through high contact pressure, sticking during cooking and storage is avoided.

7) Good packaging material does not allow fluid loss - this applies to all foods packaged according to the Sous-vide method.
This is also sometimes noted as an economic factor.

8) The packaging material should consist of a tear and puncture resistant plastic. More recently, this is also offered. This consists of a polymer material whose molecules are strongly oriented and cross-linked. If it is heated, the molecules become more mobile, thus more "elastic" and pull together. Thus, the already very tight film shrinks even further together and puts pressure on the food. Less fluid leaks out and cooking is therefore, further perfected.