Introduction

Sorghum (Sorghum bicolor (L.) Moench) belongs to the group of grasses native to the tropical and subtropical regions of East Africa. It has the agronomic advantages of being resistant to pests and diseases, flexible in planting time, adaptable to arid areas, and has low-cost seeds that can forego fertilizers and agrochemicals [1, 2]. In addition, it is environmentally friendly, being one of the most efficient crops in solar energy conversion and water use [3]. These characteristics have encouraged its growth worldwide since the second half of the 20th century, making sorghum the fifth most produced cereal in the world, behind maize, wheat, rice and barley [4].

In Africa and Asia, sorghum is a staple food. Half of the sorghum consumed in Asia comes from abroad, because local production cannot cover its regional demand. Africa consumes everything it produces, 70% of which is destined for human consumption. America is the continent with the greatest production and exportation, with the United States, Mexico and Argentina as protagonists. Its consumption is about 67% of what is produced but only 3% is destined for human food. Europe and Oceania are small producers, with the former using most of what is produced, while latter mainly exports its production [5].

The concept of sorghum economy denotes two sectors: a traditional, subsistence, small-scale agricultural sector, mainly in Africa and Asia, in which most of the production is consumed directly as human food; and a modern, highly mechanized and large-scale sector, in which the product is mostly used as animal feed. This occurs mainly in Europe and America, where the potential of sorghum grain as an ingredient of human diet has not yet been fully exploited [5, 6].

Noerhartati et al. [7] described the potential of sorghum as a raw material, and how creative and innovative ideas could develop products towards industry 4.0. They showed how the automation of raw material preparation, the creation of new communication channels and the developing of new digital sorghum-based markets could be achieved.

Sorghum grain is a spheroid of approximately 3 mm in diameter and comprises three parts: the germ and the endosperm inside the grain, and the outer layers. The colors (red, brown, white or black) are given by the nature of its outer layers and, in many cases, this determines the use given to the grain [3]. Sorghum is an excellent source of complex carbohydrates (about 70% of the grain mass, with approximately 25% amylose), iron, zinc, B vitamins, polyphenols and anthocyanins, and can supply the needs of people with celiac disease or gluten intolerance [8,9,10].

In general, sorghum contains antioxidant compounds and, regardless of color, all types can have phenolic acids and flavonoids, although only sorghum with pigmented testa can develop condensed tannins [11]. Some of these tannins retard the hydrolysis of food by reducing the digestibility of proteins and starch, thus increasing dietary fiber levels. The presence of condensed tannins can therefore be considered as an antinutritional factor for monogastric animals [10, 12]. The intensity of the pericarp color does not correlate with the content of condensed tannins, and one of the erroneous ideas about sorghum is that all red and brown varieties contain condensed tannins. There were numerous reports that this cereal cannot be used as an ingredient in human or animal diet. However, even sorghum high in tannins is already used to prepare many foods and beverages, from beer to popped sorghum. Sorghum offers the opportunity to produce foods with various natural colors, high levels of dietary fibers and antioxidants [12,13,14].

Celiac disease is characterized by intestinal chronic villous inflammation and atrophy, which damage absorption processes [15]. Celiac disease, gluten sensitivity and wheat allergy are common conditions with different pathogenesis but similar symptoms, caused by the ingestion of wheat or other gluten-containing cereal. Regular pasta and most noodle formulations contain wheat, and thus gluten, and 1% of the global population with these conditions cannot consume them [16]. However, sorghum is a potential alternative to develop pasta with nutritional benefits and that is gluten-free, thus enlarging the diet diversity for celiacs [17, 18].

Pasta is a traditional Italian food made from durum wheat semolina which, thanks to European emigration, has become popular around the world and is consumed almost daily in many cultures [19]. Semolina is the primary product of a durum mill, used almost exclusively to make pasta. Its particles are similar in size to middlings, with no more than 10% passing through a 180-μm sieve. Cooked pasta has a complex, compact network through which starch digestive enzymes act more slowly than in other matrixes [20]. It is also a carrier of bioactive compounds, which allows sensory and nutritional properties to be enhanced [21]. There is worldwide interest in the production and availability of convenience foods that provide quality nutrition, and pasta can meet this requirement with its long shelf-life and need for only minimal in-home preparation. Sorghum, a sustainable and underexploited crop, can sometimes substitute wheat flour, which could improve product variety, decrease costs, and reduce dependency on wheat importation in countries like Nigeria that lack wheat production [22]. It also improves the nutritional properties of pasta and noodles and is a high value-added ingredient for the formulation of health foods.

The purpose of this review is to highlight scientific and technological information on the production of pasta and noodles with sorghum. We discuss the attributes of sorghum flour and its effect on the properties of pasta and noodles, its influence on cooking behavior, processing parameters, and textural, sensory and nutritional properties. We divide pasta as a function of the relative amount of sorghum flour into sorghum-based pasta and pasta with sorghum as a secondary ingredient. In addition, we suggest guidelines for processing and ingredient selection for the industrial sectors to encourage sorghum pasta production and further research.

Sorghum Flour

The properties of sorghum flour are central to the quality of pasta and noodle products and depend on the sorghum grain variety selected and the flour production process [23]. Polyphenols are concentrated mainly in the outer layers but, for sensory and technological reasons, low proportions of whole grain sorghum flour are incorporated or the grain is subjected to a dehulling process prior to milling, resulting in a refined flour. As reported by Pineli et al. [24], gluten-free breads produced with sorghum have been mainly developed with commercial sorghum flours and their composition was not fully determined in the studies reviewed. It is fundamental to analyze the raw material of pasta and noodle with sorghum.

Hager et al. [25] investigate several commercial gluten-free flours and compare their composition with wheat flour. The sorghum flour selected shows low protein content (4.6%), high unsaturated fatty acid (83.8% of total lipids) and medium content of polyphenols (103 mg/100 g) compared with the other types of flour. Its nutritional quality is better than that of the other commonly used gluten-free flours, such as rice and maize.

The reported raw materials for producing noodles and pasta ranged from unspecific commercial sorghum flour to cultivars obtained after years of work (Table 1). Nonetheless, it was possible to associate some characteristics of flour with good quality products. Several studies reported that grains with high hardness and small particle size and consequent considerable amounts of damaged starch are associated with higher firmness and tensile strength [33,34,35]. Grain hardness is determined by sorghum cultivar and by edaphoclimatic conditions. To obtain a finer particle size, these grains need to be subjected to a more extensive milling process i.e., higher energy input [34]. Particle size uniformity and low ash content are also important for making acceptable sorghum noodles and pasta [17, 34]; these provide higher gelatinization and greater retrogradation during noodle processing due to the lower content of fiber which normally impairs this process [35]. The sorghum cultivar also determines the amylose/amylopectin ratio, which influences the extent of gelatinization and retrogradation. Higher amylose content promotes retrogradation; however, as starch gelatinization is impaired as from 40% amylose, lower amounts are recommended in order to obtain acceptable noodles [35, 36].

Table 1 Formulation, processing conditions and cooking properties of sorghum-based pasta and pasta with sorghum as additive
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Effect of the Ingredients on Cooking Properties

Generally, pasta is made of tetraploid wheat (durum wheat) by extrusion or sheeting and cutting, and this is characterized by high firmness, low adhesiveness, low cooking loss, diversity of shapes and tolerance to overcooking [19]. In this product, the gluten network enables the formation of different shapes, imparts its particular texture and prevents or reduces starch granule lixiviation during cooking process [37].

There are many types of noodles, generally characterized by high stickiness, springiness and hardness, and long string shape, and they are usually made of hexaploid wheat (bread wheat) by sheeting and cutting [38, 39]. Starch noodles are usually made with rice (or other starchy flours) by pregelatinization of the starch through several methods, including extrusion of strands into boiling water and high temperature extrusion. In general, the structure in cooked starch noodles is formed by a network of retrograded starch created during production (Fig. 1).

Fig. 1
figure 1

Schematics of wheat pasta, sorghum gluten-free pasta and noodle structure

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Sorghum-Based Pasta

In sorghum-based pasta, sorghum flour is the main ingredient and the use of wheat is often avoided to produce gluten-free products [18]. Sorghum proteins cannot develop a tridimensional network as can gluten proteins, so in pasta with low or little wheat the gluten network functionalities are performed by other ingredients, such as starch [16]. Gelatinized starch is one of the most important structuring agents in gluten-free pasta (Fig. 1). It can be produced during processing or used as an ingredient. Palavecino et al. [18] found that the use of pregelatinized corn starch reduced cooking loss and its absence in the formulation significantly increased this parameter. Starchy flours can also be employed (Table 1): potato flour (10–40%) was used in sorghum pasta with good results in such quality indicators as cooking time and cooking loss [17], and adding maize flour (50%) to a sorghum formulation increased water absorption, producing a significant change in the pasta structure [32].

Another approach to replacing the gluten network function involves the use of egg or egg albumen [40], which also reduces cooking loss (CL) by up to 24% [17, 18, 30]. Other authors chose to enhance the cooking behavior of pasta with gums (Table 1), as these reduce CL due to their gel formation properties and increase water absorption through their ability to bind water [4, 18, 26, 27, 30].

Various ingredients have been added to sorghum pasta to enhance its nutritional properties. Pasta prepared with 45–60% sorghum grits (sieved with 1 mm mesh) and wheat semolina (40%) was mixed with ground Gingelly seeds. Gingelly-enriched pasta showed significantly more protein (10.4%), fat (2.3%), ash (2.6%) and carbohydrate (85.2%) than control pasta, and very low solid loss [31]. Adding high protein flour from legumes like soy, locust bean or chana to sorghum pasta, along with gums, produced lower starch digestibility and high protein digestibility but similar cooking quality parameters to durum pasta [27].

Sorghum as a Secondary Ingredient in Pasta

It is common practice to use pasta as a carrier of health-promoting compounds since its nutritional properties can be improved by adding several types of proteins, fibers and plant phytochemicals [20, 21]. Its high levels of functional compounds like fiber and polyphenols make sorghum an attractive ingredient to incorporate into food products, enhancing their nutritional properties and positively impacting human health [23, 25]. Not only sorghum flour but also sorghum bran layers (resulting from the dehulling process) can be used in this way [41].

Susanna and Prabhasankar [26] developed a hypoallergenic pasta, mixing 60%Triticum durum semolina with sorghum flour, other non-wheat flours and additives (HPMC, xanthan gum or locust bean). This showed a similar CL, microstructure and texture profile to that of control pasta (only wheat), and low antigenic activity, and therefore could be consumed by celiacs.

Up to 40% of red or white sorghum flour has been substituted for durum wheat semolina and the starch fractions, phenol profile and antioxidant capacity evaluated [42]. Incorporating whole grain sorghum flour increased resistant starch and total phenolic content, resulting in higher antioxidant capacity than that of durum wheat pasta. Particularly, the addition of red sorghum increased free phenolic acids and anthocyanins. However, this pasta exhibited higher CL than wheat pasta, due to the lack of gluten in sorghum flour.

Benhur et al. [28] mixed sorghum flour with up to 50% wheat semolina to produce pasta by cold extrusion (not extrusion cooking). Cooking time, water absorption and CL increased with the sorghum proportion, with lower amounts of wheat leading to weaker structure, allowing greater water intake and material lixiviation.

Rice pasta has been enriched with different types of sorghum flour to improve its textural and nutritional properties, using sorghum flour fermented by lactic acid bacteria, prepared from sprouted grains, and untreated flour [29]. Fermentation and sprouting affected proteins and starch. Sprouting significantly reduced large proteins, decreased starch and broke down its structure, and therefore enrichment with sprouted sorghum flour was impaired. On the other hand, adding fermented sorghum protein (mainly thiol-rich products of kafirins) enhanced crosslinking capacity, with the formation of an inter-protein network through disulfide bonds which retained starch granules during cooking.

Wheat semolina has been replaced by sorghum flour (between 20 and 30%), finger millet flour and gluten to produce a multigrain pasta [4]. Pasta with a medium level of sorghum flour (24.61%) had cooking properties similar to those of wheat pasta, showing that this enrichment does not affect product quality.

Noodles

Starch noodles production usually includes several thermal cycles in which the heating promotes amylose leaching during gelatinization, and cooling produces the formation of the retrograded continuous phase [16]. Higher extent of gelatinization and amount of amylose are thus important attributes to increase noodle quality but more than 40% of amylose impairs gelatinization [43]. The use of waxy sorghum flour produced noodles with high CL and stickiness [35]. To gelatinize the starch, Suhendro et al. [35] treated the flour-water mixture with microwave oven or hot plate heating, and the former resulted in firmer noodles with lower water uptake and dry matter loss, although the hot-plate method produced greater gelatinization and more retrogradation.

Another approach is to use pregelatinized starch as an ingredient. Here, different methods have been evaluated (Table 2). Starch was extracted from 10 sorghum genotypes and starch noodles were prepared by extrusion and then cooked in boiling water [39]. The genotypes were non-waxy with amylose content of 23.8–30.8% and most exhibited pink coloration, which was enhanced through cooking. Cooking loss was low (average 2.4%). Positive correlations were found between noodle elasticity and rehydration, and starch pasting properties (hot paste and cold paste viscosity). Nevertheless, a relationship was not always established between starch and noodle properties, as was found between starch and cooking loss [34, 45].

Table 2 Formulation, processing conditions and cooking properties of sorghum noodles
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Rani et al. [44] replaced wheat flour with sorghum flour (10–50%), soy flour and gluten. The addition of sorghum flour increased cooking time because of higher gelatinization temperature and enthalpy. Also, noodle water absorption decreased and cooking loss increased, due to weakened amylose and protein networks, ascribed to the fiber content of sorghum flour.

On the other hand, Chinese wheat-free egg noodles were also developed employing white and red sorghum hybrids [34]. The structure of these noodles is mainly formed by egg proteins and gums, but sorghum starch also helps: higher damaged starch from finer particle size flours resulted in noodles with greater firmness and tensile strength. This underlines the importance of evaluating the raw material viscosity profile rather than its protein properties [34].

Production Technology

Pasta production consists of a few simple steps: mixing, shaping and drying (for dry products). The mixing stage aims at hydrating the different dry ingredients to reach approximately 30% water content and homogenize the material. Then, the mix is shaped through one of the three main methods: sheeting, extrusion and extrusion-cooking. During this stage, the ingredients are chemically and physically transformed as a function of the treatment conditions. Finally, the product is dried through several stages to extend its shelf-life, keeping its technological and nutritional characteristics. Noodles have most of the same production steps as pasta but, between formation and drying, at least one cooking and cooling cycle is carried out.

Sheeting

The most ancient way to produce pasta and wheat noodles is through sheeting, the successive compression of the dough between two rollers, progressively reducing the thickness. This creates a dough sheet which is later cut into different lengths and widths for the different shapes [16, 46]. In wheat-based products, sheeting inputs energy that promotes intra- and intermolecular disulfide bonds and the formation of the tridimensional gluten network, which imparts its particular rheological properties and encloses starch granules. However, in gluten-free (GF) formulations, the difficulties in forming an elastic and malleable dough reduce the possibilities of developing pastas with this technology. Despite this, some GF pasta products were successfully elaborated by sheeting at laboratory scale [47, 48], but we have not found sheeting used to produce sorghum-based pasta in the literature.

Extrusion

Extrusion is one of the most widely used technologies for producing pasta at industrial and laboratory scales [19]. Pasta extrusion forces the hydrated mix through a die, with a screw controlling its speed, motor torque, the dough temperature and pressure in the die. One of the major advantages of extrusion is its versatility to produce pasta in a continuous process. Under low temperature (<50 °C) and shear force conditions, extrusion produces minimal transformation of the ingredients and mainly gives pasta its shape. Wheat pasta is usually produced with an extruder in which semolina is mixed with water and/or eggs and then pushed through the die, simultaneously transforming shear stress into gluten network bonds and shaping the pasta.

One of the approaches to obtain good quality GF pasta is the use of structuring ingredients/additives. Pregelatinized starch and flour, combined with the extrusion procedure, gives good results [18, 49]. The moisture of the dough is one of the most critical parameters for producing good quality extrudate since this determines dough viscosity. High moisture would produce very sticky dough and result in low noodle firmness, and low dough moisture may produce cracked surface in the noodles [30]. The moisture levels normally employed in pasta extrusion are around 30 and 40% (Table 1).

Sorghum-based pasta has been successfully produced at industrial scale through extrusion both in a wheat pasta facility [18] and in a gluten-free factory [32]. Neither formulation presented problems during the different production stages, resulting in adequate cooking parameters.

Extrusion-Cooking Process

The extrusion-cooking process applies high temperature (>100 °C) and shear forces to the mix, producing severe modification in the ingredients. This technology involves the formation step, producing starch gelatinization, protein denaturation, and carbohydrate–protein–lipid interactions [21]. The extrusion-cooking process develops the retrograded starch three-dimensional network, which gives pasta its good textural properties and excellent cooking attributes. This is the principal noodle production method used nowadays and is similar to the ancient noodle-making procedure, since both treatments induce starch gelatinization and retrogradation [38]. The extrusion-cooking process is one of most widely used technologies in the gluten-free pasta industry due to its speed and low-cost characteristics [21].

Drying

Pasta and noodle drying extends their shelf-life and can also enhance product cooking and textural attributes. In general, longer and wetter drying processes promote amylose mobility and enhance starch retrogradation, improving cooking behavior and avoiding the tension inside the product structure which causes defective shapes. Noodles dried in one stage (low relative humidity: 30%) were curved and chewier, while two-stage drying (first high and then low relative humidity: 100 and 30%, respectively) produced straight noodles [35]. In the literature (Table 1), there was a large range of time periods for drying sorghum-based pasta (1.5 to 24 h) and temperatures (40 to 70 °C) with a relative humidity of 75%.

Texture

Pasta and noodles are defined by their texture; however, several studies lack measurement or reported only sensory or instrumental determinations. Firmness is one of the essential attributes of a pasta as it represents structure strength, which is provided by the gluten network in wheat pasta. Adding sorghum reduces the firmness of wheat pastas because its proteins are non-structuring and its fibers interrupt the gluten network [34]. For that reason, when Kamble et al. [4] replaced wheat with sorghum and millet they added gluten to reinforce the structure. In sorghum-based pasta, this role was successfully performed by egg proteins [18]; Paux and Rosentrater [30] also reported maximal firmness with a minimum level of egg white. Taking advantage of their gel formation properties, gums have also been added to increase firmness and make gluten-free pasta comparable to durum pasta [18, 27, 30].

As mentioned above, noodle structure is mainly provided by the starch amylose network, which depends on the extent of retrogradation. Harder sorghum kernels and smaller flour particles thus resulted in firmer noodles because highly damaged starch allowed higher initial viscosity and extent of gelatinization [34].

Sensory Evaluation

The ultimate goal of pasta and noodle research is to make products for the consumers, and thus it is necessary to study the sensory properties of the samples. Sensory parameters were assessed by different methods and compared with commercial products and/or samples with widely different formulations. Texture is one of the main characteristics of wheat pasta, therefore, low levels or lack of gluten content have a sensory impact. Pasta may have low acceptance because of granular texture and crumbly characteristics [30] or wide acceptance because of its low stickiness and high firmness [18].

In many cases in the laboratory/pilot plant, it is quite difficult to ensure that the products developed are gluten-free because there may be cross-contamination. It is therefore common to select judges for the sensory panels who are not gluten-sensitive, but this could lead to misleading results. Paiva et al. [32] tested sorghum pasta with celiac and with non-celiac individuals, and the two groups reported different preferences, probably because the latter are used to consume only wheat pasta. In that case, the acceptability of sorghum pasta was satisfactory for the celiac group, compared to corn samples.

Pasta color and superficial aspects are major sensory attributes since these are the first elements of the product that the consumer perceives [20]. Sorghum flour provides color to pasta mostly due to the fiber from the outer layers of the grain, and thus grain color differentiates pasta color [18]. Sorghum pasta had a similar color to that of whole-wheat pasta, which consumers could interpret as healthiness [30, 50].

Flavor is probably the parameter through which sorghum most often reduced pasta and noodle acceptability. [30]. Pasta with higher sorghum content was perceived as bitter because of phenolic acids [17]. Acceptability increased with the decrease of sorghum in noodle formulations [22]. Pasta prepared with equal amounts of sorghum and wheat scored better in terms of texture, color, taste and acceptability than those with a higher sorghum content [28]. However, samples from white hybrids scored better on flavor than those from brown hybrids [18], and sorghum-based pasta had higher overall acceptability than finger millet samples [4]. It should be noted that incorporating other ingredients such as gingelly can improve not only the nutritional but also the sensory profile [31].

Nutritional Properties of Sorghum Pasta and Noodles

A fair proportion of gluten-free products currently on the market are nutritionally inadequate [25] mainly due to an unbalanced content of carbohydrates, protein and fat, as well as limited essential nutrients. As a result, sorghum flour could become an alternative to improve the nutritional profile of pasta products. Sorghum pasta was also used as a vehicle to overcome zinc deficiency [31]. Incorporating up to 20% gingelly seed gave a zinc content of 2.55 mg/100 g (15% of the FDA daily value).

Protein and Starch Digestion

Tannins in sorghum are an antinutritional factor as they reduce protein and starch digestibility [51]. Adding 30% sorghum flour to gluten-free pasta had no significant effect on protein digestibility [27], but fermented sorghum flour added to rice pasta showed a modified protein aggregate that could increase protein susceptibility to enzymes [29]. In a study with gluten-free pasta made from white and brown sorghum flour, the latter presented 23% lower protein digestibility, related to its higher tannin content [23, 52]. Noodles with 10% sorghum flour showed slightly less protein digestibility than maize and rice noodles in a soybean-based formulation, related to the greater level of antinutrients (phytic acid) in cereal and soy flour used in the study [53].

It is well-known that wheat pasta products have a low glycemic index (GI) since the gluten network restricts the passage of enzymes through the structure [20]. Some authors have reported lower GI of sorghum flour pasta compared to wheat pasta, probably due to the presence of polyphenols which reduce starch digestion [54]. A moderate GI was estimated after in vitro digestion of gluten-free pasta made from white or brown sorghum flour, and it was lower than that estimated for commercial pasta made from rice and corn [18], probably due to the polyphenol content and egg and albumin used as ingredients. Other authors reported that only 20% of starch was digested in gluten-free pasta with 30% sorghum flour and different types and proportion of gums [27]. Incorporating 10% rice or sorghum flour in soy-based noodles decreased starch digestibility around 15% compared to the control sample with no added cereal flour [53]. Thus, not only tannin content in sorghum flour affect starch and protein digestion in pasta and noodle products, but other additives included in the formulation could also alter the structure and, consequently, the accessibility of enzymes.

Resistant Starch and Dietary Fiber

Englyst and Hudson [55] identified three starch fractions according to their digestibility: rapidly digested starch (RDS), slowly digested starch (SDS), and resistant starch (RS) [55]. Resistant starch, which is a part of dietary fiber in conjunction with soluble fibers, increases viscosity in the gastrointestinal tract and slows down sugar absorption, related to decreased LDL-cholesterol and moderate glucose levels and to reduced risk of several types of cancer [56]. Some studies considered starch fractions and found an increase in resistant starch when white or red sorghum flour was added to durum pasta [42]. In a comparison of commercial gluten-free pastas, sorghum presented a higher RS content than rice- or corn-based pasta [52].

It should be noted that in these studies, less than 2% of starch in sorghum-enriched wheat-pasta was resistant to digestion, while around 45% of RS was observed in gluten-free pasta. Other studies focused on the long gastric emptying times observed in low or tannin-free sorghum traditional food in Africa, associated with the high viscosity developed during digestion in comparison to other starchy foods (white rice, potato, wheat pasta) [57]. In consequence, sorghum added to a gluten-free formulation had a marked effect on starch and protein digestibility compared to sorghum-enriched wheat-based pasta, indicating that both food microstructure and antinutrient content play an important role in enzyme accessibility.

Phenolic Compounds

Phenolic compounds are a heterogenic group of compounds extensively studied over the last 10 years, with special emphasis on the polyphenol composition of food, and on polyphenol bioavailability, metabolism and biological effects [58]. Phenolic compounds present in sorghum are typically higher than those in rice, wheat, barley, maize, rye and oats [42]. Although all sorghum varieties contain phenolic compounds, the specific profile and proportion of each one depend on the pericarp color and presence of a pigmented testa or red and brown sorghum grains [59]. In addition, when considering pasta and noodles, part of those phenolic compounds as well as free phenolic acids and anthocyanins may be lost during cooking, thus reducing the antioxidant potential of adding sorghum flour [42].

Many clinical studies have shown an association between enriched phenolic food and a decreased risk of chronic diet-related diseases due to its antioxidant potential reducing oxidative stress [59]. An increase in plasma polyphenols and improved antioxidant status were observed after the consumption of red sorghum-enriched pasta [60], though the same study found no significant effects from white sorghum-enriched pasta, probably due to the different polyphenol contents typical of each variety [23].

The phenol profile and in vitro fecal fermentation after gastrointestinal digestion were assessed in several commercial gluten-free pastas and compared with a product enriched with 20% white sorghum. The study showed decreased total polyphenols and reduced power with a considerable change in their profile after 24 h fermentation, with sorghum-based pasta being the most effective in delivering phenols to the large intestine [61].

Polyphenol content depends not only on the variety of sorghum, but also on the food processing, which influences the amount of ingested phenols released from the food matrix, determining the compounds absorbed and available for metabolism [61, 62]. Cooking sorghum-enriched pasta released total phenolic content and anthocyanins from the matrix into the cooking water but increased the bound phenolic acids, although all the cooked pasta samples showed a significant increase in antioxidant activity compared to pasta without added sorghum [4, 18, 42]. These results could derive from depolymerization of conjugated polyphenols occurring during boiling, hence increasing their apparent amount in pasta while thermal degradation of anthocyanins takes place [42].

Polyphenols are known to interact with food macronutrients and digestive enzymes, affecting the bioavailability of polyphenols, although the extent to which proteins and carbohydrates compete to bind with digestive enzymes in pasta products is not well understood [21, 61]. Depolymerization of polyphenols and increased antioxidant activity were observed after in vitro digestion of white and brown sorghum-enriched gluten-free pasta, yet only 8% of polyphenols was dialyzable [18].

The low proportion of studies considering either the bioavailability of sorghum polyphenols or their real contribution to target cells clearly shows the need for well-designed studies to better understand their characteristics and the matrix in which they must be included to provide health benefits.

Antinutrients

Sorghum has some nutritional limitations, due to the presence of antinutritional factors, such as trypsin and amylase inhibitors, phytic acid and tannins. The presence of these compounds interferes with protein and carbohydrate digestion and mineral bioavailability [63]. Sorghum contains lower amounts of phytate than millet and wheat, but generally has more tannins [4]. The tannins content in sorghum varies greatly with cultivars and environmental factors, ranging from 0.2 to 48.0 mg/g [59].

Sorghum addition significantly increased phytic acid content in noodles compared to that in wheat flour noodles, but the thermal process then reduced its content by about 40%. These aspects are particularly important since phytic acid, ranging from 875.1 to 2211.9 mg/100 g [5] in sorghum flour, is known to reduce the absorption of zinc and iron.

Digestibility can also be affected by processing: extruded sorghum-soy noodles increased in vitro protein digestibility by 35% compared to unprocessed flour, in which extrusion produced protein depolymerization and destruction of antinutritional factors [53]. The same authors also found that processing increased starch digestibility by around 50% after pasta extrusion.

Conclusions

Pastas and noodles from wheat and rice have been deeply investigated and reviewed, but sorghum products offer consumers an important new alternative. Sorghum pasta, produced with 5 to 100% sorghum showed low cooking loss and high water absorption and it could be obtained through different extrusion technologies. Different types of noodles with adequate cooking properties could be produced by extrusion and sheeting methods. The sensory attributes of these products were adversely affected, yet these can be improved by adequate flour selection and/or a proper dehulling process. To allow sorghum to perform a structuring role, the most suitable sorghum flour to produce pasta and noodles should have high amylose content (up to 40%), small particle size and low fiber content, even though the latter is detrimental to its nutritional properties.

In general, sorghum pasta and noodles showed an improved nutritional profile compared to wheat and non-gluten products without sorghum. Although sorghum tannins decrease the digestibility of proteins (commonly around 10–25%), they also decrease the digestibility of starch and can generate low glycemic index (GI) pasta (lower than 65). In addition, sorghum-enriched pasta exhibits high antioxidant activity. Gluten-free sorghum pasta and noodles are thus a great alternative for celiac people.

Future research should focus on the bioavailability of sorghum polyphenols and their interaction with digestive enzymes to better understand their health benefits. Authors should take care to specify grain type and improve the characterization of the sorghum flour. The study and development of sorghum varieties to improve pasta quality should be encouraged, taking into account the effects of amylose content on product quality.