Extrusion is the most latest and commercialized food processing technology that involves high temperature pressures and high shear. Food extrusion is a high-temperature short-time method which brings changes in texture, fluid flow and size reduction of the raw materials used for extrusion processing. The unit operations involved during extrusion treatment include mixing, shearing, conveying, heating, puffing and partial drying and these operations depend on the extruder design and process conditions. This treatment brings changes by caramelizing, mixing, cooking, heat and mass transfer, plasticizing, shearing, kneading, shaping and forming of the final product. The food materials are maintained at high pressures in the extruder and suddenly dropped to normal pressure when forced through a die or an orifice (shaped hole) at predetermined rates to produce a variety of puffed products with porous structure (Wang et al., 2020). Extruded foods combined with good quality ingredients provide processed products with precise nutrient levels, enhanced protein digestibility, reduced anti-nutrients, improved mineral and vitamin availability (Ainebyona et al., 2021). The high temperatures developed in extrusion treatment can gelatinize, degrade and fragment the starchy material in cereal flours, thereby causes effective mixing of such (hot extruded) powder with hot water. Hot extrusion treatment for short time can inactivate enzymes like lipases and microbes, denatures protein, retard lipid oxidation, fat rancidity and maillard reactions that causes undesirable and off flavour development, thus hot extrusion improves the flavour development and potentially extends the shelf life of the cereal grain flours (Onyeoziri, et al., 2021). Extrusion cooking is the most suitable technology to make traditional products from ancient cereals like sorghum, maize, rice, rye, wheat, barley and other millets and such products to be more acceptable in the food market.
The porous extrudates (made out of cereals or whole grains) obtained from hot extrusion can be ground into powder form that can be further used to develop ready to drink/eat products like instant soup powders, instant beverage powders, instant porridges type cereal food powders. Instant powders are best choice for consumers with quickness and convenience in food preparation. The instant powders are those that dissolve instantly upon addition of hot water without forming large lumps and with minimum agitation. In general, instant powders get agglomerated when rehydrated in hot water and pose a major problem that seriously affects its instant properties. To improve the desired rehydration properties of instant powders, various latest techniques are being used that include spray drying, foam mat drying, spray-freeze drying and extrusion processes as well as combination of these processes. Instant powders prepared through extrusion technology can gelatinize the whole grain/cereal flours, increases amylose starch content, improved freeze thaw stability of starch and digestibility mechanism of starches. The extrusion treatment improves instant properties of powders like rapid water absorption and reduced retrogradation rate that make these powders more easily hydrated by water (Wang et al., 2020).
Mechanisms in Hot extrusion
1. The sharp pressure drop during extrusion process causes puffing/porous structure of the feed material that provides rapid absorption of water when used as instant powders.
2. The higher pressure, temperatures and shear force involved in extrusion treatment will destroy the granular structure of instant powder.
3. Cleavages the intramolecular and intermolecular hydrogen bonds and disintegrates helical structure, thereby decreases crystalline structure and increases amorphous structures of cereal flours used for instant powders.
Process conditions of hot extrusion
Important process parameters that affect the instant properties of whole grain based/cereal based powders are moisture in feed material, barrel temperature, screw speed of the extruder, configuration of the screw(s) and the type of extruder (i.e. single-screw versus twin-screw). Higher moisture levels of the feed materials used in extrusion process provides reduces the puffing effect and thereby increases bulk density and agglomeration rates of instant powders. Lower moisture levels (10 to 20%) of the feed materials used in extrusion treatment damages crystalline structure and reduces agglomeration of instant powders, thereby hot extrusion treatment improves water absorption and water solubility of instant powders.
Important physicochemical properties of instant powders
Bulk density: Extrusion causes puffing of the base materials that provide instant powders of reduced bulk density with high porosity and specific surface area, thereby causing rapid penetration of water. But bulk density will gradually increase with the increase of moisture content, implying that increasing the moisture content inhibits the puffing effect.
Agglomeration rate:
For better instantizing nature of powders, the agglomeration rate should be lower. The instant powders with low agglomeration rates provide homogeneous paste without visible granules. Hot extrusion can reduce the agglomeration rate of instant powders.
Water absorption Index (WAI) indicates the water absorption capacity of instant powder. WAI has been generally attributed to the dispersion of starch in excess water, which can be increased by the degree of starch damage, due to gelatinization and extrusion. Hot extrusion causes damage in the crystalline regions of the starch granules of the feed materials. Hot extrusion breaks the intermolecular and intramolecular hydrogen bonds of the starch materials and allows more hydroxyl groups to form hydrogen bonds with water (added to instant powders) and increases the WAI of instant powders.
Water Solubility Index (WSI) indicates complete starch gelatinization during the extrusion process. WSI is related to the quantity of soluble solids and increased solubility of starch in instant powders from cereal flours. Lower WSI indicates minor degradation of starch and such conditions lead to less number of soluble molecules in the instant powders. Higher moisture content in extrusion process can diminish protein denaturation, which subsequently lowers WSI values. Increasing temperatures in hot extrusion might increase starch degradation with increased soluble starch amount, thereby increasing WSI.
Wettability:
The wettability of instant powders of cereal flours are the most basic characteristics for the preparation of instant drink/beverages. Extrusion treatment could improve the wettability of instant powders.
Particle size distribution:
Particle size reduction is most desirable characteristic of whole-grain/cereal based instant powders to improve the product expansion. Extruded products containing fine grained bran particles showed greater radial expansion when compared to larger or coarse-grained bran. The use of coarse particles slows down the air bubbles formation and can also rupture air bubbles formed before reaching the maximum expansion. When particles of coarser size are in extrusion process, surface area for hydration (ability to absorb) is reduced and water absorption rate will be slowed down. When fine particles are used in extrusion treatment, surface area for hydration will increase and hydration rate (also increases.
Conclusion
Extrusion technology is cost effective that limits its application in producing instant powders from stable cereal type food products and wholegrain value-added products in developing countries. Extrusion technology using simple single-screw type extruders can be suitable for small and medium scale entrepreneurs and food processors for commercial manufacture of convenience or instant powders from conventional products like millets, cereal legume blends with improved nutrition and nutraceutical properties.
References
Ainebyona, P., Kigozi, J., & Mukisa, I. M. (2021). Optimization of Extrusion Conditions and Cooked Vegetable-Chicken Mixture for Instant Banana-Vegetable Soup Powder. European Journal of Agriculture and Food Sciences, 3(2), 15-21.
Onyeoziri, I. O., Torres‐Aguilar, P., Hamaker, B. R., Taylor, J. R., & de Kock, H. L. (2021). Descriptive sensory analysis of instant porridge from stored wholegrain and decorticated pearl millet flour cooked, stabilized and improved by using a low‐cost extruder. Journal of Food Science.
Wang, H., Wu, J., Luo, S., Zou, P., Guo, B., Liu, Y.,& Liu, C. (2020). Improving instant properties of kudzu powder by extrusion treatment and its related mechanism. Food Hydrocolloids, 101, 105475.
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