In the current century, consumers are more concerned about the quality and safety of food. They are focusing more on high nutritional value, tasty, and visually appealing food items which are available at low cost. Food industries are making a consistent effort in developing such food products from nutrition-rich ingredients such as millet using the new technology of food processing such as extrusion. Millet is rich sources of protein, fiber, antioxidants, polyphenols, and micronutrients which make it an excellent raw material for developing a variety of nutritious and value-added extruded food products. In food industries extrusion technology is being used to develop a wide range of food products with better product quality. The study of the effects of extrusion processing parameters, namely feed moisture (FM), extruder screw speed (FM), and barrel temperature (BT), and co-ingredients on product quality characteristics is an important concern for developing a food product with better quality. The review focuses on various studies that have been reported on the development of millet-based extruded products. Emphasis has been given on effects of co-ingredients and extrusion processing parameters on extruder operation parameters, namely mass flow rate, specific mechanical energy, average residence time, torque, and die pressure, and the cooking characteristics, physicochemical, pasting, textural, thermal, nutritional properties, and overall acceptability of the millet-based extruded products. The extrusion process parameters widely influence the product characteristics. During extrusion cooking, starch gelatinization, protein denaturation, modification of lipid, Maillard reactions, and complex formation among starch, protein, and lipids lead to significant changes in chemical and physical characteristics of the extruded products. Extrusion cooking has a significant effect on the inactivation of enzymes, and reduction in anti-nutritional factors. Extrusion processing at high BT with high FM increases the degree of starch gelatinization which leads to an increase in water absorption index and expansion of the product. An increased degree of starch gelatinization decreases the melt viscosity and thereby decreases the torque and specific mechanical energy. Proper control of extrusion conditions and proportion of co-ingredients give a product with higher nutritional value, better texture, and better sensory characteristics with high overall consumers' acceptability. Practical Applications Ready-to-eat (RTE) snacks are in high demand among consumers in recent years, owing to their better texture quality, good sensory characteristics, and easy availability at low cost. Extruded snacks are valuable food products in terms of production, packaging, handling, transportation, storage, and shelf-life. Food industries are highly capable of improving consumers' acceptability and nutritional value by altering the extrusion process parameters and co-ingredients. Proper monitoring of extrusion process parameters and the proportion of co-ingredients plays an important role in obtaining desirable characteristic properties and improving the overall acceptability of extruded snacks. The development of extruded products by the use of locally and easily available raw materials is a good practice for improving the nutritional value of the final products. Millets are highly nutritious crops containing significantly high contents of protein, vitamins, minerals, and fiber. They are also rich sources of polyphenols and antioxidants which have additional health benefits. The information provided in this review can be helpful in analyzing the effects of extrusion process parameters and co-ingredients on characteristic properties of millet-based food products and improving their consumers' acceptability.
