Pasta production, mixing and extrusion

Across many Countries around the World, pasta is an appreciated staple food. The European Union area is the main producer worldwide (see Figure 1), with Italy being the Country with both the biggest production (over 3 million tonnes in 2011) and the biggest consumption (26 kg per capita/year) of pasta. Pasta is generally made from an unleavened dough of durum wheat (Triticum turgidum) or from flour of other grains and cereals, simply mixed with water. In egg pasta, flour is mixed with eggs instead of water, and flattened into thin layers to prepare various traditional pasta shapes. In recent years, pasta made with other ingredients or fortified with beneficial substances is on the rise: among this category the most popular products are whole grain pasta (with high fibre content) and gluten-free pasta (made from corn or rice flours).

Figure 1 – Geographic distribution of the 13.1 million tonnes of pasta produced worldwide. October 2011, http://www.pasta-unafpa.org/ingstatistics5.htm

The two main step of pasta production before drying are dough mixing and dough extrusion.

Dough mixing: flour (usually durum wheat semolina) is mixed with water, which is absorbed by flour components, in particular starch and gluten proteins. Most mixers (kneaders) operate under vacuum to limit oxygen presence and its oxidative effects on dough lipid fraction, and to limit the englobation of small bubbles into the dough, detrimental for final pasta structure. Furthermore, without atmospheric air the flour granules can be hydrated more quickly and thoroughly. Also the action of hydrolytic enzymes (amylase, protease) can play a role on dough characteristics. It is important that water is evenly distributed into the dough, to be well absorbed by starch granules and allow proper gluten net formation, which will give pasta its structure. Durum wheat semolina is composed of granules of different shape and size, so with a different ratio between their mass (weight) and their outside surface area. When wet, each of these granules has a specific behaviour: the smaller granules absorb water faster than those with a larger diameter. Additionally, they require less water to reach the correct level of wetting. If the needs of all flour components are not satisfied, there will be an initial condition of a water imbalance in the formation of the dough which will be hard to rebalance along the kneading. The amount of water added to the semolina is in relation to the percentage of moisture required for the final pasta shape: for the long shapes (e.g. spaghetti) destined to be spread on sticks, the dough will have to be less humid, to prevent pasta collapsing or excessively lengthening as it suffers the effects of its gravitational weight. For short pasta, on the contrary, dough can be moister. In addition, dough moisture affects the colour of the cutting surface, which must be taken into account, in particular for pasta shapes in which the cutting surface (at the die outfeed) is equal or even superior to the rest (for example in tiny pasta). In any case, dough with a moisture of 33-34% in the forming tank is the normal standard for short pasta, while the moisture is normally ca. 2% points lower for long shapes. Also water temperature is an important factor: opinions are many and discordant on this point. The choice water temperature is also important, and related to many other variables: semolina’s temperature, its particle size (fine or coarse), mixing time required by the type of kneader, presence of vacuum, shape/speed of the blades, final pasta shape, extruder and die kinds, type of drying, etc. Cold water is recommended in pierced long pasta to prevent the product peeling where it comes into contact with the supporting stick, and also in fine and tiny short pasta to prevent the cutting surface from turning white. Hot water (40-60°C) is instead recommended for short pasta with a big diameter, long non-pierced and thin pasta (spaghetti and spaghetti of reduced diameter), and in the production of nests of egg pasta (e.g. tagliatelle). A limit to this rule is however reserved for dough with the addition of eggs, which should never exceed a temperature of 30-33°C or even less if the time of formation of the dough exceeds 12-15 minutes.

Dough extrusion: pasta dough formed in the kneader is then driven by the extrusion screw towards the head of the press, and forced through a die whose characteristics strongly affect the appearance of pasta surface. Different incisions are present in the die depending on desired pasta. Shape and dimensions of pasta will also depend on cutter’s speed at the outfeed end of the die. Most common materials for dies are nowadays resilient plastics such as polyamides (Nylon®) or Teflon®. In the past, bronze dies were used, and still are in some traditional pasta production plants. Dies entirely made of bronze make the pasta surface rough, which helps to capture the sauce, whereas the Teflon® insert gives the product an even surface and a smoother texture. In fact, pasta extruded through bronze dies also has higher porosity: this characteristic causes reduction in breaking strength and a potentially higher insects infestation levels as the rough surface of pasta makes the deposition of eggs easier. At the extrusion stage, pasta is still in a “plastic” status but a quite strong mechanical energy is necessary to force the dough through the die orifices: therefore the dies are sometimes automatically cooled in order to avoid dough excessive heating. Regarding the factors that influence pasta quality, raw materials are certainly one of the most important. In Italy it is required by law that products named “pasta” are made exclusively with flours from durum wheat (semolina). In other Countries the use of common “soft” wheat (Triticum aestivum) is also permitted. The main difference of the two kernels is that the appearance is “glassy” in the case of durum wheat and “floury” in that of soft wheat, due to different protein composition. This aspect has strong implications on the final product quality: when mixed with water, semolina flour proteins give rise to stronger water bonding, firmer dough and more resilient final dry products, which don’t tend to lose mass and firmness during cooking. On the contrary, dry pasta obtained from soft wheat flour becomes softer in a shorter time during boiling, tends to release mass into the cooking media and to be stickier. Nevertheless, some populations prefer such texture. Anyhow, obtaining the right texture for both the dry and cooked pasta is necessary in order to meet the market demands, since cooked pasta textural properties will determine the overall quality of the pasta and dominate in influencing consumer acceptance. The storage proteins of the wheat endosperm (namely glutenin and gliadin) are the main determinants of the dough properties. Glutenin is a polymeric protein responsible for the elasticity of the dough whereas gliadin is a monomeric protein responsible for the extensibility-related properties of it. Furthermore, the ratio of glutenin to gliadin is directly related to dough strength and extensibility as the interaction between them via the generation of disulphide bonds, forms a gluten network that gives rise to the cohesive and visco-elastic matrix of the dough. These intermolecular bonds trap starch granules in the pasta during cooking, thus reducing the loss of solids into water. The result is a reduced surface stickiness and the pasta is able to hold its shape (firmness) while cooking. There are other components such as starches, non-starch polysaccharides, and non-gluten proteins that also play a role to some extent. The texture of cooked pasta can be measured using instruments such as texture analysers with different probes depending on the characteristic to be evaluated. Gluten strength of the semolina greatly determines the break strength of dry pasta, which in turn is useful to predict how well the product will tolerate transportation and cooking.

References

Sissons M., 2004. Pasta. Encyclopedia of Grain Science;

Lucisano M., Pagani M.A., Mariotti M., Locatelli D.P., 2008. Influence of die material on pasta characteristics. Food Research International, 41(6):646-652;

by Rita Lorenzini