Challenges to processing non-traditional pastas

Traditional pasta is made from semolina and water. Its simplicity in composition has made it an inexpensive meal that is familiar to many people worldwide. The milling of durum wheat into semolina removes the bran and germ which are rich in dietary fibre, vitamins and minerals. To offset the loss of these healthful components, many countries require pasta to be fortified with thiamine, niacin, riboflavin, folic acid and ferrous sulphate.

Non-traditional ingredients are sometimes added to improve the nutritional quality and healthfulness of pasta. Enhanced protein quality (semolina proteins contain relatively low levels of lysine, methionine, and threonine) and dietary fibre are two common objectives that have driven the development of non-traditional pastas for the health food market. For a more complete review of this topic, the reader is referred to the article written by Marconi and Carcea1.

Dietary fads have impacted the view of traditional pasta. For example, the Adkins diet viewed carbohydrate rich food like pasta negatively. In response, some pasta manufacturers began to substitute high protein ingredients for semolina. While these pastas had recognisable pasta shapes, they lacked agreeable visual and organoleptic appeal. In time, multigrain pastas have been formulated that have a pleasing appearance and taste. Although low carbohydrate diets have diminished in popularity, they did have two important impacts on the pasta industry. Firstly, these diets changed consumer attitudes, making health food more mainstream and secondly, they created a niche market for non-traditional pastas. These non-traditional pastas are more nutritious and definitely add variety to the culinary experience. Today, traditional pasta and non-traditional pastas are available at most grocery stores.

Research has shown that an addition of a non-wheat ingredient up to 10 per cent w/w usually will not greatly reduce the physical or cooking qualities of the pasta. Above 10 per cent, the quality of the pasta declines quickly due to the disruption and dilution by the particles of non-traditional ingredients of the gluten matrix.

The basic processes involved in manufacturing non-traditional pastas are the same as those used to make traditional pasta. These processes include: blending and hydrating ingredients, kneading and extruding the dough and drying the pasta product. Challenges occur at each step that can affect the quality of the end-product.

Blending and hydrating ingredients

A first challenge can occur while mixing and maintaining a uniform blend of ingredients. Differences between non-traditional ingredients and semolina in size, shape and density can result in particle stratification and uneven distribution of ingredients in the extruded pasta. Vibration after blending but before hydration can cause particle stratification. The longer the blends are exposed to vibration, the greater the chance of particle stratification. Matching size and density of particles in an ingredient mixture is best done by changing the particle size distribution of the semolina. Unlike semolina and wheat flour that lose their physical structure during dough formation, most non-traditional ingredients retain their structure during kneading and extrusion. Thus, large particles of non-traditional ingredients will be seen as large particles in the pasta product. Generally, small particles of non-traditional ingredients are more desirable than large.

A second challenge can occur while hydrating the ingredients. Wheat based pasta products rely on a gluten matrix for its structural integrity. Gluten matrix formation requires complete hydration of semolina protein. Without adequate hydration, regions in the dough and pasta will exist where no gluten has formed. These regions of discontinuity in the gluten matrix are areas of structural weakness and will appear as white starchy areas in extruded pasta. Mixing under vacuum assists hydration by removing air. Air increases the surface tension of particles which decreases wetting and hydration.

Hydration is dependent on the size and chemical composition of the particle. Uniformity of particle size is very important, particularly for semolina. Small semolina particles hydrate quickly and become over-hydrated in the time required to fully hydrate large semolina particles.

Chemical composition of non-traditional ingredients will affect their ability to compete with semolina for moisture. Determining the proper hydration level for a semolina-non-traditional ingredient blend requires some experimentation. In general, semolina blends containing non-traditional ingredients that are high in fibre content tend to require increased hydration, while those containing non-traditional ingredients that are high in lipid content tend to require less hydration2. Semolina-high lipid ingredient blends are easy to over-hydrate. Calculations based only on ingredient weight, without consideration of lipid content, tend to result in over-hydration of the mixture. The optimal hydration level for non-traditional ingredients is not known. Those ingredients that do not change physically during kneading and extrusion probably only need to be hydrated enough to be pliable.

Hydration rate can be quicker, similar or slower for non-traditional ingredients as compared to the hydration rate of semolina. Semolina will tend to over-hydrate if the non-traditional ingredients hydrate slower than the semolina. Over-hydration tends to result in the ingredient blend sticking to the paddles and bridging over the opening to extrusion barrel. This has been observed for semolina-soy flour, semolina-buckwheat bran and semolina-flaxseed flour. To minimise bridging, the mixture should be run as dry as is reasonably possible.

A third challenge can occur when determining the hydration time in the mixing chamber. Hydrated mixtures need to be in the mixing chamber long enough for moisture to migrate into the semolina particles. Ideally, the hydrated mixture will appear as small balls (0.5-1 centimetre diameter). Over-hydration and/or extended time in the mixing chamber can result in the formation of an agglomerated mass of ingredients that is difficult to convey forward towards the extrusion barrel.

Kneading and extruding the dough

The hydrated ingredient mixture is typically choke-fed onto the screw located inside the extrusion barrel. The hydrated mixture is compressed as it is conveyed forward. After compaction, the remaining length of the screw is involved in kneading and conveyance of the dough towards the die. Dough flows both forward and backward inside the screw flight channel. These opposing flows knead the dough and develop the gluten matrix. Forward flow is a result of the action of the turning screw and dough resistance at the barrel surface. Backflow is due to the pressure gradient that increases towards the die. Pressure pushes the dough backward along the flight channel and through the gap between the top of the screw flights and the inside barrel surface.

A challenge can occur during kneading when developing the dough consistency necessary for uniform dough development and extrusion through the die. Dough consistency can be adjusted by varying hydration level and time in the mixing chamber. If the dough is too strong or too weak, then the moisture content can be increased or decreased, respectively.

Non-traditional ingredients with high fibre or high lipid content often pose challenges to developing proper dough consistency. Fibre generally has a high water binding capacity and will compete with semolina for moisture. High fibre ingredients often increase the apparent dough strength due to insufficient moisture available for semolina protein. Under-hydration results in stiff dough which requires more energy for extrusion and generates more heat during processing. Dough temperature is typically maintained between 45-50°C. Dough temperature above 50°C will denature proteins and interfere with gluten matrix formation. Dough temperature can be regulated by adjusting the flow rate of the water circulating in the water jacket that surrounds the extrusion barrel. An increase in flow rate will remove more heat while a decrease in flow rate will remove less heat from the dough. Extrusion of under-hydrated dough can result in breakage problems for long goods hung on drying rods and cutting problems for short goods.

Ingredients with high lipid content often reduce dough strength. Semolina-high lipid ingredient blends are easy to over-hydrate. Over-hydration weakens the dough causing a reduction in extrusion rate. Pasta extruded from over-hydrated semolina requires more energy to dry and can stretch and stick together on the drying rods. Mixtures containing ingredients with high lipid content tend to form weak, sticky dough that accumulates on metal surfaces. Lipid can be expelled from the ingredients during compaction and kneading of the dough. The expelled lipid is susceptible to oxidation and must be minimised to prevent premature rancidity of the pasta product. Further, the expelled lipid can coat the surface of the screw and extrusion barrel wall and cause a reduction in the friction needed for forward movement of the dough.

Other challenges associated with extrusion of non-traditional pastas include the plugging of screens that are used to prevent blockage of die orifices and the abrasive wearing of surfaces. A decline in extrusion rate accompanied by an increase in extrusion pressure would indicate possible plugging of the screens. Fibrous ingredients can cause abrasive wear of extrusion barrel wall, the screw and the die orifices. Over time, wear of the extrusion barrel and screw will result in a slow decline in extrusion rate and wear of die orifice will cause increased thickness of pasta.

Drying non-traditional pasta

Extruded long goods, like spaghetti, must have the flexibility to be hung on rods, cut to size and moved to the dryer without breaking apart. Extruded short goods must have the physical strength to maintain their shape after cutting.

Compared to low temperature drying (<50°C), high temperature drying (>_70°C) often increases the physical strength, tolerance towards checking and cooked firmness for both traditional and non-traditional pastas.

Commercial non-traditional pastas

Table 1 shows data for the physical and cooking qualities of commercial spaghettis that contain non-traditional ingredients. These products were purchased at a local grocery store.

Non-traditional ingredients changed the appearance of the spaghetti products (Figure 1). For example, spinach powder resulted in dark green pasta. The whole wheat pasta had a reddish brown colour. Variation in colour contributes to the culinary appeal of these products. The surface of the whole wheat pastas was rough. While a rough surface might lack visual appeal, it does allow the pasta to hold more sauce, which can enhance the culinary experience.

Discontinuity of the gluten matrix by non-traditional ingredients generally results in low mechanical strength, short cooking time and low cooked firmness. Low mechanical strength is a concern as it determines the ability of the product to withstand handling and shipping without breakage. Broken spaghetti strands were found in the boxes of products with low mechanical strength. Cooking loss seemed to vary with product (Table 1). Ingredients with relatively high levels of soluble compounds such as amylose, sugars and soluble proteins would be expected to produce pastas with elevated cooking loss.

Conclusion

Non-traditional pastas are a growing niche market. Manufacturing non-traditional pastas comes with challenges. Awareness and understanding of these challenges are important in ensuring a high quality product. These challenges can be overcome by carefully monitoring and making corrective adjustments at each step of the process.

References

1. Marconi, E., and Carcea, M. 2001. Pasta from non-traditional raw materials. Cereal Foods World. 46:522-530.
2. Yalla, S.R., and Manthey, F.A. 2006. Effect of semolina and absorption level on extrusion of spaghetti containing non-traditional ingredients. J. Sci. Food Agric. 86:848