General comment on and investigation into drying pasta
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The great advantage of dry pasta: self-preservability |
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In normal ambient conditions dried pasta keeps a long time without needing any preservatives or particular storage requirements. Moisture and insects are the main concerns, although good packaging and a suitable storage place are sufficient to ward of these dangers. The essential prerequisite is, however, for the pasta not only to be dry, but also to have been dried properly.
Let's now see the essential principles of pasta drying technology.
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What is drying? |
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It is wise to have a clear idea of some fundamental concepts of drying in general, so as to understand how pasta is dried and what differences there are between dried and moist pasta.
Drying is the elimination of a liquid, normally water, from a substance or a solid body. The water to be eliminated may be just on the surface, or inside the solid body to be dried. If it is only on the surface, drying does not depend on the properties of the body at issue. Whereas, if the water is inside it, the methods of drying will depend on the body's physical and chemical properties. This is because the water particles need to move from the inside onto the surface in order to then be removed (normally by evaporation). Clearly, how this takes place depends on the characteristics of the body at issue (structure, nature, dimensions, type of chemical bond of the water molecules with the other components, etc.).
And it is this last case that concerns drying pasta.
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The states of pasta: plastic state and elastic state |
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On leaving the die, pasta normally has a moisture content of approximately 31-32% (depending on the type of dough and the shapes made). It is considered dry when its internal moisture content is equal to or less than 12.5% and balanced with the surrounding environment. This means that, to keep well, besides being dry, pasta needs to be "stable": in other words, within certain environmental climatic limits (air temperature and humidity) it must keep its remaining internal moisture content uniform.
On leaving the die, with a moisture content of approximately 30%, pasta is in a "plastic state". This condition has specific physical properties: a body in a plastic state can deform under the action of external forces without any particular tension forming inside it and, moreover, it can permanently keep the shape acquired as a result of these forces.
This property is perfectly clear in drawing: the dough in its plastic state is deformed by the action of the die and the shape obtained will not be altered at all after the pressure of the die has stopped. Pasta in its plastic state can then undergo even powerful drying without this causing any internal tension and the risk of damage; also the deformation (contraction) suffered due to extraction of the water will be maintained. In the "plastic state" the contraction of the pasta is generally in proportion to the amount of water subtracted from it.
When, proceeding with drying, the product's moisture content falls further (22-18%), the state of the pasta changes from plastic to elastic.
In this new state the product's behaviour is totally different: an elastic body subjected to stress deforms, but tends to recover its original shape as soon as the stress stops. Besides causing deformation, stresses can then bring about tension inside the product. If the tension comes within the product's specific limit of elasticity, it can be absorbed precisely by its own elasticity; whereas, if it exceeds this limit it will inevitably be damaged.
This is exactly what happens when drying pasta: when the moisture inside the product falls to approximately 20% its physical state passes from being "plastic" to "elastic". Clearly, the moisture level marking the change in state is not fixed, it can change according to the temperature of the product and be for example 18% (the higher the temperature of the pasta, the lower the level of moisture its state changes at). In addition, this change is never sudden, it happens progressively: close to the above-mentioned moisture level, both states, plastic and elastic, initially coexist in proportions continually varying until the change of state is complete.
In practice, the change in the state of the pasta from "plastic" to "elastic", starting from 22 - 18% moisture, has the following consequences:
From this point of the process onwards, drying generates tension inside the product.
The pasta tends to "recover" the even minimal deformation caused by eliminating the water inside it.
The water extracted from the pasta produces a contraction that however can no longer be recovered from the product except by reabsorbing water, which is precisely what must not happen, since the goal is that of drying.
The water must therefore be extracted so that the tension generated does not exceed the product's limit of elasticity. If this occurs, the pasta will be damaged to a greater or lesser extent (cracks, splits, veining, etc.).
Since water is extracted from the surface, during the drying process the internal part close to the surface will inevitably have a lower moisture content than the central portion. This unbalance also generates tension that needs to be able to be reabsorbed to prevent damaging the product.
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Drying phases |
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The most significant physical states for pasta drying technology are the moisture and temperature of the air and the humidity and temperature of the product. The laws governing the phenomenon of these physical states must therefore be applied for all drying operations. Very briefly, drying pasta means modulating and appropriately controlling the evaporation of water from the product, using heat and ventilation.
The surfaces of liquids or moist bodies give off water molecules into the surrounding air if this is not saturated. Evaporation is that much more intense the higher the temperature. In order to evaporate, water needs heat, i.e. the energy necessary for the molecules to break away from its surface. Evaporation is that much greater the larger the surface and the more agitated the air moving over this surface. The air close to the surface, as saturation point is approached, slows down evaporation, which is then facilitated by the air continually being changed.
In drying pasta, ventilation plays a fundamental role since, besides removing the water given off by the product due to evaporation, it is used as a vehicle to convey heat. The heat energy conveyed by the ventilation air is used to heat the product and the water it contains, making it evaporate. Therefore, knowing the volume of air required for a certain phase of the drying process and controlling its intensity and flow is then an essential condition for drying pasta correctly.
Since drying pasta brings about a reduction in its moisture content from 30% to 12%, it is done technologically in two distinct phases that correspond to the plastic and elastic states of the product.
During pre-drying (first phase) the moisture content of the product falls from 30-32% to 18-17%. In practice, this means that it is necessary to eliminate approximately 22 kg of water for every 100 kg of final dry product. The time this phase takes depends on a few variables, the main one of which is temperature. Using temperatures that enable moist product to reach and/or exceed 75°C speeds up this phase of the process and at the same time determines a number of advantages.
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Rapidly heating the product causes drastic evaporation of the water on the surface of the pasta and therefore an equally drastic migration of water particles from the inside towards the surface. The first transfer of water takes place at the cost of the starch, which during preparation of the dough has absorbed approximately 1/5 as much of it as the gluten. Afterwards, by osmosis, the water moves from the gluten to the starch. Since gluten is elastic it tends to follow the water particles, moving from the innermost parts of the product, where it is more highly concentrated because there is more moisture there, towards the outside. This redistribution of gluten can take place at up to approximately 26% moisture of the pasta. If the pre-drier chamber is very hot and damp, the conditions are ideal for this complex phase of migration and redistribution, this being decisive for the end result of the entire drying process.
In short, then, this pre-drying technology makes it possible to accomplish:
Partial blockage of some enzyme activity and virtually total blockage of any product fermentation, helping to sanitize it, since there are relatively few micro-organisms that at 75°C are capable of surviving, and also any insect eggs are easily destroyed.
Uniform gluten distribution making full use of the capacity of gluten to hold back the starch particles (so better cooking capacity and less stickiness of the product).
A decrease in oxidation of the yellow pigments contained in the semolina and therefore a brighter colour of the dried product.
Better shape stability.
Maintenance of the product's capillarity, essential to redistribute the particles of water during the following phases of the process.
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The following phase of drying must envisage alternating phases of water evaporation from the surface and redistribution inside. In this phase ambient temperature and humidity normally decrease, clearly complying with the current temperature and moisture of the product. The speed of this phase is inevitably less than that of pre-drying because the structure of the product (passed on to the elastic state) has become more rigid, capillary action has decreased and so the migration of the remaining particles of water from the inside to the outside of the product is slower.
The drying phase is a delicate one because on the one hand it is necessary to prevent drying that is too fast from completely blocking the capillary action of the pasta (with disastrous consequences for the product...), on the other hand it is always a good rule for drying to be completed relatively quickly, compatibly with the technology used. Drying normally takes approximately 6-8 times longer than the time required for pre-drying, including the phases of the internal redistribution of water particles. This figure changes in relation to the formats: longer ones take much longer to dry than short ones, especially if they are of medium-high thickness.
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