The characteristics of the products obtained by extrusion cooking depend on the moisture content, extrusion temperature [ 38 ], residence time, pressure, and shear [ 33 , 39 ]. The extrusion cooking process is preferable to other food-processing techniques in terms of its high productivity and significant nutrient retention due to the high temperatures and short times that are required [ 4 ]. Likewise, this technology has several applications including increasing numbers of ready-to-eat cereals, salty and sweet snacks, coextruded snacks, indirect expanded products, croutons for soups and salads, an expanding array of dry pet food and fish foods, textured meat-like materials from defatted high-protein flours, nutritious precooked food mixtures for infant feeding, and instant flour to make tortillas [ 32 , 40 , 41 ].
Moreover, extrusion has been used in the development of expanded cereals that include the addition of other ingredients i. Interest has grown in the physicochemical, functional, and nutritionally relevant effects of extrusion processing. Prevention or reduction of nutrient destruction, together with improvements in starch or protein digestibility, is clearly of importance in most extrusion applications [ 1 ].
Another advantage of extrusion cooking is the destruction of antinutritional factors, especially trypsin inhibitors, hemagglutinis, tannins, and phytates, all of which inhibit protein digestibility [ 1 , 43 ]. Several extrusion-processing conditions are accounted for the quality of finished products. The control of feed rate, screw speed, barrel temperature, and barrel pressure, together with the abovementioned critical parameters, will determine the crispness, hardness, and various other characteristics that will influence the success of the final product [ 44 ].
Cereal thermal processes such as baking, roasting, and extrusion cause a number of physical and chemical changes due to starch gelatinization, protein denaturation, component interactions, and browning reactions. These changes result in improved organoleptic properties, increased nutrient availability, and inactivation of heat-labile toxic compounds and enzyme inhibitors [ 25 ].
The major role of these ingredients is to give structure, texture, mouth feel, and many other characteristics desired for specific finished products [ 33 ]. Functional properties of extruded foods play an important role in their acceptability including water absorption, water solubility, oil absorption indexes, expansion index, bulk density, and viscosity of the dough [ 33 ]. Several changes in the matrix food have been reported using the extrusion process, which are the result of the combination of moisture content of the starting materials, pressure-temperature, and screw speed, which are responsible of physical and chemical transformations in the final product and therefore affect product quality [ 42 ].
Extrusion cooking technology brings numerous chemical changes, such as gelatinization or starch, denaturation of protein, lipid modification, as well as inactivation of enzymes and microorganisms [ 45 ]. Denaturation of grain proteins during extrusion cooking allows the opening of loose structures for tannin-protein interactions causing the formation of tannin-protein complexes and retention of antioxidant activity. These complexes are broken down in the human gastrointestinal tract to release bound tannins and act as free radical scavengers [ 46 ]. Another important chemical effect occurring during extrusion is the browning or the formation of Maillard reaction products MRP , which contributes to the antioxidant properties of the final product [ 47 ].
However, several investigators have reported that during extrusion, there is the loss of bioactive compounds with antioxidant properties i. Thermal degradation of phenolic compounds may be due to complex formation with Maillard reaction by-products and high moisture content promoting phenolic polymerization [ 50 ] affecting their extractability and antioxidant activity [ 51 ]. In contrast, earlier research on cereal products has shown that thermal processing might contribute in releasing bound phenolic acids by breaking down cellular constituents and cell walls [ 52 ].
All of these chemical changes are associated with structural changes that occur in the materials subjected to extrusion increasing the release of the bioactive compounds in the cell wall matrix [ 54 ], thus making these materials more easily extractable [ 55 ].
All these physical changes are related to high shearing force in combination with high temperature and pressure that can efficiently disintegrate the rigid cell walls of the matrix food. Nutritional properties in cereals are provided by two main groups of nutrients: macronutrients comprising of lipids, proteins, and carbohydrates, and micronutrients that include vitamins and minerals. Many researchers have reported both the positive and negative effects of the extrusion process on the nutritional quality of food and feed mixtures.
These results are dependent on the different extruder conditions temperature, feed moisture, screw speed, and screw configuration and raw material characteristics composition, particle size [ 1 ]. As discussed above, extrusion cooking has been studied extensively to produce a wide variety of foods [ 33 , 34 , 35 ]. On the one hand, extrusion 1 induces starch gelatinization improving its digestibility, 2 promotes the destruction of antinutritional factors undesirable enzymes, trypsin inhibitors, and hemagglutinins , 3 increases the content of soluble fiber, 4 improves protein digestibility, and 5 reduces lipid peroxidation [ 1 ].
On the other hand, extrusion can also negatively affect the bioavailability of certain nutrients [ 56 ]. The heat-labile vitamins and some amino acids are lost, and the Maillard reaction that occurs during the process can reduce the nutritional value of the proteins [ 1 ]. In this section, we will discuss the various effects of extrusion cooking on the nutritional properties of cereals.
While lipids can act as lubricants during extrusion, the amount of lipid content can affect the properties of the extrudates. Extruded foods, particularly expanded products, are susceptible to lipid oxidation, one of the main causes of food deterioration [ 58 ]. Although there is not much research focused on the nutritional changes in lipids after extrusion, it has been reported that extrusion cooking can minimize lipid oxidation, thus increasing the nutritional and sensory quality and shelf life of foods [ 1 ].
Among the factors involved in the delay of oxidation in extruded foods are 1 denaturation of lipase and other enzymes that may contribute to oxidation [ 58 ]; 2 formation of lipid-amylose complexes, thereby reducing both starch and lipid availability and increasing oxidative stability and shelf life of extruded products [ 59 ]; 3 release of endogenous antioxidants in grains during extrusion that may provide protection against peroxidation [ 60 ]; and 4 creation of Maillard reaction products with antioxidant activity.
In this regard, Sproston et al.
Thus, food-processing alternatives that result in minimal loss and the lower degree of oxidation of these components are desirable. Suzuki et al.
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In contrast, Ramos Diaz et al. The authors attributed the considerable reduction in the content of fatty acids and tocopherols during extrusion to the formation of amylose-lipid complexes [ 59 , 65 ].
It is also important to note that in the above-discussed studies, the ingredients used were different; hence, the available amylose for lipid binding and the formation of complexes was also different [ 59 ]. Cereals maize, sorghum, rice, barley and pulses beans, peas, chickpeas, lentils, and other dry edible seeds have traditionally been the dominant dietary plant protein source [ 66 ]. Protein nutritional value depends on the content of essential amino acids and the digestibility and utilization of the protein [ 67 ].
Several factors can affect protein digestibility of cereals, among them are the grain structure and composition, the presence of disulfide bonds, surface functional groups, and protein hydrophobicity and conformation [ 68 ]. Several reports have shown that extrusion can improve protein digestibility by denaturing proteins and exposing of enzyme-susceptible sites.
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This phenomenon is attributed to the effects of high shear on protein structure and conformation that occur during extrusion, leading to the manufacture of products with highly digestible proteins [ 70 ]. Similarly, enzymes and enzyme inhibitors generally lose activity during extrusion unless they are stable to heat and shear. Reductions in protease inhibitors can contribute to better plant protein utilization [ 57 ]. In addition, extrusion has been proposed as a viable alternative to influence allergenic properties of food proteins. The potential reduction in antigenicity is due to degradation of protein structures that ultimately results in the reduction of IgE- and IgG-binding capacity during thermal processing of foodstuffs [ 71 ].
As it has been discussed, extrusion process under high pressure causes major chemical changes, thermal degradation, dehydration, depolarization, and recombination of fragments, all of which can promote glycoxidation [ 69 , 72 ]. The concentration of advanced glycation end products AGE in foods, which are formed by Maillard reaction, has been demonstrated as a risk factor associated with the etiology of age-related diseases in humans, such as atherosclerosis, nephropathy, retinopathy, osteoarthritis, neurodegenerative diseases, and diabetes mellitus [ 73 ].
In addition, AGE by binding to their receptors RAGE , which are found in a wide variety of cells, can lead to oxidative stress, vasoconstriction, and inflammatory responses. The AGE can covalently cross-link tissue proteins and, thereby, modify structural and functional properties of the proteins [ 74 , 75 ].
During extrusion, the Maillard reaction is sometimes induced to contribute to desired flavor and color and to enhance palatability [ 76 ]. However, excessive Maillard browning can result in losses of lysine, destruction of vitamins, and reduction of bioavailability of trace elements [ 77 ]. Retention of lysine in the breakfast cereals is considered most important since it is the limiting amino acid among most of cereal snacks [ 10 ].
Thus, the Maillard reaction can result in unfavorable consequences such as a decreased protein quality due to the loss of bioavailable essential amino acids and, as mentioned before, the production of AGE. Future studies should focus on the optimization of processing conditions in a way that the desired beneficial effects are promoted, and the undesired effects are minimized. One of the more widely researched aspects of extrusion on the nutritional content of products is the way extrusion technology can affect carbohydrate digestibility.
Starch is usually the major food constituent in extruded foods such as breakfast cereals, snacks, and weaning foods. Humans do not readily digest raw starch [ 78 ]. However, the digestibility of starch may be improved by the extrusion process due to its partial gelatinization and fragmentation attributed to the effect of shear and temperature.
The depolymerization of the starch allows it to be more readily available to digestive enzymes. Moreover, during extrusion the physical breakdown of starch molecules takes place, resulting in smaller and more digestible fragments [ 79 ]. The extrusion process can increase the available digestible carbohydrate in cereals by up to threefold compared to raw unextruded cereals [ 80 ]. In the literature, there are several examples illustrating that extrusion improves starch digestibility. Borejszo and Khan [ 81 ] found that sucrose, raffinose, and stachyose decreased significantly in extruded pinto bean starch fractions.
While Alonso et al. Similarly, Mahasukhonthachat et al. High starch digestibility is desirable in the food industry for the manufacture of specialized nutritional foods such as infant and weaning foods or to target particular consumer needs elderly requiring rapidly digestible forms of starch, people participating in athletic activities, and those looking to reduce the content of indigestible oligosaccharides that cause flatulence in foodstuffs. However, these products tend to induce a higher glycemic response than their unprocessed raw ingredients.
High blood levels of postprandial glucose and insulin have been implicated in the development of insulin insensitivity and chronic metabolic diseases such as Type II diabetes and cardiovascular disease [ 85 ]. By altering not only the digestibility of starch but also the conformation of starch, extrusion offers the ability to reduce the high glycemic index of some foods by converting starch to digestion-resistant starch. Hence, the formation of resistant starch by extrusion may have value to promote reduced calories in food products [ 86 ].
In this respect, an interesting observation is that extrusion can also increase the amount of resistant starch and soluble dietary fiber present in extrudates. In regard with the fiber content, Jing et al. Under these conditions, the soluble dietary fiber content residue increased by Similar results were found by Chen et al.
These researchers found that the content of soluble dietary fiber from soybean residues increased by more than tenfold and showed improved water solubility, water retention capacity, and swelling capacity compared to unprocessed soybean. Furthermore, they tested the physiological effects of their high dietary fiber product and observed that it was able to significantly reduce total cholesterol, low-density lipoprotein cholesterol, and triglyceride levels in vivo.
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The increase in soluble dietary fiber in extruded products could be explained by the formation of additional components by transglucosidation, whereby 1,4 carbon-oxygen bonds are cleaved, and new anhydroglucose linkages are formed, and the resulting novel bonds would be resistant to digestion by enzymes [ 91 ]. Another possibility is to increase insoluble dietary fiber with the formation of retrograded amylose, insoluble at room temperature [ 91 , 92 ]. This could also be attributed to the formation of covalent interactions between macronutrients leading to components that are insoluble and not hydrolyzed by digestive enzymes [ 92 , 93 ].
These indigestible glucans may be Maillard reaction products likely resulting from chemical reactions between starch and proteins present within the dietary fiber-containing matrix [ 93 ]. In general, vitamins differ greatly in chemical structure and composition. Their stability during thermal process is also variable. The extent of degradation depends on various parameters during food processing and storage, e.
Extrusion cooking has a significant effect on the stability of hydrosoluble vitamins.