Two complex technologies
That is a very good question, Monsieur Nappì! A good and also challenging question, I would say. In fact, even though the preparation of loose and unpasteurized fresh pasta is simple and essential and its shelf life is limited, this question deals with two of the most sophisticated and complex technologies as far as the transformation of perishable products into unperishable products is concerned. However, this occurs without mistreating them and affecting their genuineness; therefore their original organoleptic and nutritional characteristics are not jeopardized.
These two technologies (freeze-drying and drying by atomization) are both based on the principle of dehydration to preserve a product, although their methods are different.
If these two processes are performed properly and if the products are “naturally” suitable for these advanced technologies, the answer to the above question is positive. I say this with a quiet conscience. However, in order to have the same clear approach to these ingredients for high-quality pasta production, I think it is necessary to illustrate, briefly but quite precisely, what food freeze-drying and atomization are and which are the physical process both technologies are based upon. I would rather remind the reader – for it is important – that these two technologies only share the final result, i.e. a high level of product dehydration, so that no microorganism contained in the product before this process could ever survive.
First of all, I will describe drying by atomization. Through this process, the product is atomized in liquid suspension and has to go through diffusion nozzles that will divide it into millions of fine droplets (that is where the word “atomization” comes from). The product is diffused into the air while warm air is simultaneously introduced. In fact, the mass to surface ratio of liquid suspension fine droplets ensures extremely favourable heat exchange and therefore when surface water evaporates, the process is extremely quick, almost immediate. Each fine droplet immediately becomes a very small solid particle (typically about 150 microns, or even less, as small as 20-25 microns).
Dehydrated particles are collected, as an agglomeration of dried powders, in the bottom of the atomizer – typically a cylindrical and cone-bottom atomizer. The water removed from fine droplets in a gaseous state (vapour) is extracted from the atomizer by exploiting the specific physical properties of vapour, so as not to interfere in any way with the final agglomerate of the atomized product.
An essential point: the “atomized” product is thermally processed, even though extremely quickly and efficiently in the heat exchange process; therefore thermal damage is certainly very slight, unimportant really, but it still exists.
Atomized products can be preserved at room temperature, under safe conditions in terms of their residual hygroscopicity (packaging). In a nutshell, they have excellent possibilities to regenerate their original organoleptic (and nutritional) properties once they are rehydrated. Powder milk is a typical example. However, the range of food products that could be “atomized” is much larger and includes both animal and vegetal products – meat, fish, eggs, cheese, vegetables and aromatic plants.
Freeze-drying process: unlike atomization, which is essential for a continuous process, the (duly prepared) product is dehydrated in three steps.
In the first step, the product is deep-frozen according to specific conditions (temperature, type of container) that are suitable for its nature and perfect to maintain its structure and to obtain a substrate that is more suitable for the second step of the process.
The second step of the process takes place under vacuum conditions. When the temperature reaches -20° C, ice crystals start sublimating, the water they contain turns from solid to gaseous state (vapour) without going through the liquid stage. In fact, ice evaporates directly. In order to speed up sublimation, the product is heated, under vacuum conditions again, at a pressure lower than 1.33 mbar, according to a heat transfer system that is suitable for the type of product (conduction or irradiation). In practice, the essential result that is achieved here is that the original structural matrix of the product remains intact, with all the advantages that this implies (colour, smell, taste, organoleptic properties and nutritional value).
The water vapour that is obtained through ice crystal sublimation is “condensed” on refrigerated coils, and then removed from vacuum. Sublimation is the second stage of the freeze-drying process and it is also called “cryogenic drying process”, i.e. drying by (quick) freezing.
Eventually, there is a third step, the so-called “secondary drying process”. The little quantity of water that remains on the product surface is removed by heating at a temperature lower than 60° C and at a very low pressure (vacuum). This is possible by using microwaves that allow heating to be modulated in the best way possible, thus avoiding darkening of product surface, since its structure has become “porous” through the sublimation of internal ice crystals.
In order to maintain this essential structural porosity, normal atmospheric pressure is restored (“vacuum breaking”) by introducing inert gas (nitrogen) that will fill inner free spaces of the product, thus maintaining its structural porosity which is perfect for its rehydration in case of use.
Time and equipment
The duration of the freeze-drying process can vary – from a few hours to a few days – based on the processed product. Equipment is particularly sophisticated. Moreover, the quantity of required energy is quite high and so is the cost. The price for the quality obtained in freeze-dried food products (included benefits like storability, lightness, quickness and complete rehydration) will be consistent with the cost.
Freeze-dried products can not be cheap, therefore this parameter shall also be taken into account.
We suggest that Monsieur Nappì should surf the internet searching for useful contacts and advice for his needs. There are firms that freeze-dry products on behalf of a third party: it could be extremely useful and even necessary to get in touch with these firms in order to deeply analyze his problem and make a choice.
A very last point: water used for rehydration – both for freeze-dried and atomized products – is always “adsorbed water“; only in few cases and in a very slight way, it can become “structured” water . Therefore, in a rehydrated product, water is always “free water” and it shall be handled and considered as such.