Repsol YPF

Until recently, the disadvantage had been the high cost of the material, however, nowadays you can buy vacuum packers at prices which are not that different to the cost of a small everyday electrical appliance. This system allows us to store foods which once cooked could be kept for three or four days in the fridge, for two weeks without changing their flavour or characteristics. 

Nowadays, many households continue to make homemade preserves. This system enables them to store both preserves and jams and vegetables for a considerable length of time. Animal products require more complicated hygiene processes and in this sense it is preferable to forget making meat and fish preserves and buy them in shops. 

Sterilising the jars
Before using the jars to make preserves, they must be sterilised to eliminate possible bacteria. They are left to dry and once the food is ready, they are filled and placed in a saucepan to boil them and vacuum seal them. 

It is recommended that you put a cloth at the bottom of the pan to protect the jars, so that when the water boils the jars do not suffer when they move and do not break when they hit the base of the pan. The water level should be maintained at five centimetres below the lid of the jars. They are left to boil for 20 or 30 minutes and once this time has elapsed, the jars can be cooled down in the same pan. 

Another homemade resource involves heating the jars in a bowl over a pan of boiling water for one or two hours from when the water starts to boil. Once the process is complete, they are left to cool down in the water and are then ready to store. The food should be stored in cool places without direct sunlight. If on opening a lid you notice that it is swollen, it is sensible to throw away the contents because it will undoubtedly not be in any state to be eaten. 

Currently, there is a wide range of automatic machines with sealing systems on the market which enable us to easily vacuum pack freshly bought food at home and then, either freeze or store it. Vacuum packed fresh meat, retains its fresh appearance for longer and if in addition to being in a vacuum, it is frozen, it can last twice as long as it would if it were in the freezer without having been vacuum packed. The explanation is that on creating the vacuum, all the water vapour is also removed and perfect frost-free freezing is achieved.

The importance of the type of film
 The size of these devices is similar to that of a toaster and the process involves putting the food in a specially designed bag and placing it in the vacuum machine, so that the open end is on top of the sealer. You just have to lower the pressurised lid and push a button to start the short sealing process. Other models have one or several gas tubes placed gently in the bag, to blow a predetermined amount of gas into the chamber, subsequently the electrical resistance of the sealer seals the open end and several seconds later, when the bag is completely sealed, it allows air to enter the vacuum chamber, causing the bag, which has until now been inflated, to contract and become more or less wrinkled. 

An important element when it comes to vacuum packing food is the type of film that is used to preserve the food, as whether or not the atmosphere inside the bag is effectively maintained will depend on its physical attributes. Normally multi-layered film is used, so that one of them has a high meeting point and mechanical resistance, another has low gas permeability  and a third one is characterised by a low melting point making it easy to seal the bag. 

Linked to this system is vacuum cooking, a technique which has a series of advantages: despite the pressure exerted by the plastic when it retracts due to the action of the reduced pressure, the food does not lose its original shape or succulence, as the impermeable containers prevent loss of moisture. Likewise, there is a lower chance of the food going rancid or losing colour. Particularly in the case of vegetables, as the pigments are not oxidised, they maintain their bright colour. It must also be taken into account that during cooking, low temperatures are used for a long time, which means that much more tender textures are obtained, particularly when it comes to preparing meat.