What is vacuum packing?

Vacuum packing is a food preservation technique that takes place by eliminating, from inside a container, air (up to 99.9%) by suction. Consequently, the food present inside the container is found in a negative environmental pressure condition.

The vacuum technique is fairly recent. It is applied to products of vegetable, animal origin, for doughs and for cooked foods; the same can also be associated with other conservation treatments, such as cooking, brine (dry or wet), refrigeration and atmospheric modification with the addition of inert gases; some excellent results can also be obtained by freezing vacuum-packed foods.

Many do not know that vacuum packing is also used for storing wine, directly in already opened bottles; to do this, an airtight rubber stopper is inserted in the neck and a manual pump to extract the air. Recently, bottle-type wine pinning machines have also been patented, which automatically create the vacuum in the container. This allows the drinks to fully preserve their original bouquet.

Obviously, the vacuum treatment of beverages aims at eliminating as much air as possible, but it is far from the percentages indicated for food vacuum. This is mainly due to the rigidity of the container (glass) which does not allow to extract completely the air contained in it.

Vacuum-frozen foods

Why vacuum even a frozen food?

With regard to this last technique, which is certainly little used compared to the others, many readers will wonder what their usefulness may be. The answer is quite simple; it is not a question of shelf-life (product life or shelf life), but of maintaining the organoleptic and taste characteristics.

The "vacuum-packed" foods, especially the cooked ones, and then frozen (better if lowered in temperature), totally preserve the original flavor and aroma. With this technique the classic drawbacks of negative temperature conservation are avoided, which are:

  • Deposition ("cooking" with the cold) of the food surface and alteration of color and consistency
  • Acquisition of a smell and taste better identifiable as "taste / aroma of fridge or freezer".

It can be inferred that vacuum-packed frozen foods can prolong their integrity longer than normal and make use of greater organoleptic and gustatory characteristics even close to the conservative deadline (for a product processed at home, no more than 1-3 months, depending on the temperature).

Means and tools

To induce the vacuum in food, two fundamental components are needed: the machine and the containers.

The vacuum packing machine, also called vacuum, is an instrument that extracts the air from the bag containing the food and seals it by heat sealing in two strips, respectively placed at the top of the container. There are two versions: bell and bar; the bell type is certainly the most effective.

Vacuum bags are plastic (or polyethylene) or aluminum bags, suitable for foodstuffs. They can be smooth and single-portion, embossed and in rolls, to cook, or even anti-UV (usually used for meat).

The procedure for vacuum packing is quite simple. Place the food in the bag, place it in the machine and then, once started, wait for the air to be extracted and thermo-welded. WARNING!

The most common mistake in the practice of vacuum packing is to dirty the vertices of the bag; doing so, the heat-sealing will not be completely effective and will suck (more or less quickly) the outside air through the defective closure.

Effectiveness

The effectiveness of the vacuum preservation method is mainly linked to the elimination of oxygen. This gas, which is present in the air for only 21%, is a powerful oxidizing agent and causes food browning. Furthermore, a part of the microorganisms responsible for food degeneration is of the aerobic type, meaning that it lives and multiplies thanks to the presence of oxygen (a bit like human beings!). In practice, thanks to vacuum packing, food preservation is prolonged by acting on two very distinct fronts: bacterial inactivation and reduction of non-enzymatic oxidation of food.

Contraindications

Certainly, reading what has been written so far, it would seem that vacuum packing is the definitive solution to all food storage problems. Obviously it is not so!

First of all (fortunately, today less than in the 90s) a good professional bell vacuum has a decidedly inaccessible cost. First choice instruments can be found ranging from € 1500 up to almost € 13000; it's not little! Then, as if that were not enough, even vacuum bags (plastic or aluminum) are certainly not among the "cheapest" items; we specify that these are "disposable" containers, at least in theory! The multiple use of these envelopes causes a considerable increase in the risk of food contamination, which completely negates the use of vacuum storage.

There is also another rather important vacuum defect. In the machine's heat-sealing component (especially when liquid foods are used or used bags are used) an accumulation of food residues occurs as a critical point of the so-called cross-contamination. Once again, the risk is to totally nullify the use of vacuum storage.

Another very important clarification must then be made; the vacuum does not destroy the mixed bacterial load nor inhibit it completely. This happens because many microorganisms are able to live or survive even in promiscuous conditions; is the case of facultative aerobic / anaerobic bacteria . These microorganisms, which tolerate the environment with and without oxygen, while boasting greater proliferation in one or the other circumstance, are however always active. Then there are the obligate anaerobes, that is those that grow only in the absence of oxygen; it can therefore be deduced that vacuum packing can only facilitate its life cycle. It's not all; even taking into consideration only the obligatory aerobic bacteria, which should perish in the absence of oxygen, some of these still manage to survive. This ability to adapt is due to the relative sporogenic potential; they are able to protect themselves inside spores that act as real armor. The micro-organisms therefore fall "in hibernation" until the optimal conditions for opening / germinating come about.

"Cutting the head to the bull", in order to prevent one or more of the aforementioned circumstances from manifesting itself, vacuum cooking was invented. By applying the heat treatment to foodstuffs already vacuumed it is possible to induce food pasteurization and, sometimes, ALMOST to reach sterilization. Obviously, not all microorganisms are the same. Non-spore-like ones that grow at low or medium temperatures are the first to die; in parallel, others resist higher temperatures and, finally, the spores do not perish even at boiling temperature.

Vacuum cooking

As anticipated, the last frontier of vacuum packing is that which allows the application of cooking. This, originally applied only in a steam or mixed oven, for medium and large-sized foods (mainly meat), is now widely used also for boiling or in pressure cookers and also affects plant foods.

The process of vacuum cooking in the oven is simple: after creating the vacuum (sometimes with integration of modified atmosphere) a cooking (more or less prolonged depending on the product) is applied at programmed temperatures of 65-98 ° C.

As regards the vacuum cooking of small foods, perhaps of vegetable origin, it is instead sufficient to immerse the vacuum-packed bag in boiling water up to the desired cooking level. The BIG advantage of short-term vacuum cooking for vegetables is due to the fact that due to the degradation of cellular enzymes, they do NOT oxidize and totally maintain their original color, taste, aroma and consistency.

Another advantage

We conclude the article by mentioning a big advantage of vacuum cooking. As a matter of environmental pressure inside the vacuum-packed bag, which we remember to be NEGATIVE (ie <1 bar atmospheric at sea level), the boiling temperature drops considerably. This means that if I immerse a vacuum-packed food in cold water in a pan and then put it on fire, the food will start cooking more quickly and at lower temperatures. Obviously, as a matter of gas expansion, the atmosphere inside the envelope will also slowly become positive; however, if the heat-sealing is successful, this (once the original temperature has been restored) will again become negative and vacuum-packed.

The advantages of this system are different; in addition to the conservation, of which we have already spoken, we observe:

  • A greater freshness of food at the time of use, therefore an optimal maintenance of color, aroma and taste
  • An exceptional preservation of food liquids.

This last point therefore makes it possible to gain greater food yields and lower the costs of raw materials.