Extrusion processing, its parameters and physical effects during extrusion

Extrusion Processing:

Extrusion is a thermos-mechanical process in which we added raw material and mixed it in pre-conditioner then transfer to extruder for transformation from glassy to rubbery state and further pass-through die to cut in different size and shapes.

Parameters:

• Ø  Pressure
• Ø  Temperature
• Ø  Moisture

Physical effects during extrusion:

·      Density and expansion

Ø  Expansion and density have inverse relation. More the expansion, less will be the density. In cold extrusion, there will be less expansion and more density because due to less expansion, volume of product will be low. Volume and density have inverse relation. Cooking temperature is below 100°C and further cooking is required. In hot extrusion, there will be more expansion and less density because due to more expansion, volume of product will be high. Cooking temperature is above 100 degree and no further cooking is required. In pre-conditioner, the temperature should be below 100°C, because if we provide 100°C temperature then the liquid converts into vapors that effect on expansion.

Ø  When the temperature and pressure is high in extruder, water will be same in liquid phase because when pressure and temperature increases, boiling point also increases. Behind the die, temperature and pressure are the parameters that are used to control expansion. If temperature and pressure is high behind the die then water can easily vaporize and expansion of product will be more. If temperature and pressure is low behind the die then water difficult to vaporize and expansion of product will be less. When product leave the die, the temperature and pressure lower sudden and water convert into vapors cause expansion. After expansion, they collapse due to viscoelastic properties. For example, gluten has more viscoelastic properties and less expansion                                                                                                                                                       Porosity

    Porosity tells about the internal structure and measured by density. If pores connected with each other and surfaces called open pores. If pores do not connect with surfaces called close pores.  In cold extrusion, there is less expansion, high density and less porosity. In hot extrusion, there is more expansion, low density and more porosity. X-rays microtomography is used to check size, number and regularity of pores. It gives 2D image and we convert it into 3D by using software and see the structure. Resolution should be high.

·      Texture

   Texture is main factor that impact on product. Texture should be according to market standards and consumer preference it not then re-process the product otherwise customer will claim. Texture should maintain during storage and transportation. There are different textural properties such as hardness, fracturability, softness etc. these properties are measured through texture analyzer. Texture analyzer tells about the force required to measure textural properties. Texture analyzer tells about final product quality. We maintain good texture by applying proper parameters in range.

·      Oil addition

     Oil provide lubrication that’s why add in last. If we add in extrusion, it will reduce frictional force and lesser the mechanical energy, no proper cooking, pressure and temperature will be low behind the die that will reduce expansion. Open pores used for oil addition but close pores do not used for oil addition because it does not connect with surfaces. If you want to use oil, then use that type of extruder that provide efficient mechanical energy in this condition. Such as double screw extruder handle the oil addition because it has more mechanical energy but single screw cannot because it has less mechanical energy that cause less expansion.

·      Diameter of screw

     Diameter of screw is more at initial and then decrease because at start, we feed the raw material for easy transport and decrease diameter because we want to more stay of raw material in extruder for more kneading, mechanical energy that will increase pressure and friction that will produce heat that will increase temperature. By this, the dough makes properly and more expansion. The heat produces due to kneading called viscous heat dissipation

·      Shape and size

     Shape and size of product should be according to product and consumer preference. Size and shape are adjusted through die that is set at the end of extruder. Size of raw material in more industries is 1.5mm. Smaller the size, larger the surface area and less will be the friction and vice versa. If raw material properties are same then grind and mix. If raw material properties are different then grind separately and then mix because every raw material require different mechanical energy to reduce their size. Size reduction equipment choose on the basis of properties of raw materials such as grinding for wheat and cutting for carrot.

·      Water absorption

     Water absorption depends on the properties of raw materials and particle size. If raw material has more intermolecular interaction, need more water and heat to cook. It has more viscosity. If particle size is very small the water absorption capacity is more and has good hydration power but due small size particle, surface area will be more that cause less friction.

·      Color

   Color is an important characteristic. Due to temperature in extrusion, cause Millard reactions and caramelization during extrusion that gives color. Determine through photometer that is depend on wavelength.

·      Starch gelatinization

     During extrusion, starch will partially gelatinize in pre-conditioner and fully gelatinized in extruder that will increase digestibility. Native starch converts into gelatinized starch by heat and moisture. Starch has more expansion because

• Ø  Viscoelastic properties of starch are low
• Ø  Glass transition and flow transition temperature of starch is low
• Ø  Expansion potential of starch is more because less intermolecular forces in starch because of more hydrophilic structure
• Ø  Less will be loss modulus in starch
• Ø  Less water requires for starch

·      Protein denaturation

      During extrusion, protein will partially denature in pre-conditioner and fully denatured in extruder that will increase digestibility. Protein has less expansion because

•        Protein has more binding potential
•        More intermolecular forces

·      Melting point and expansion

     Melting point and expansion have inverse relation. High melting point have less expansion because strong interactions that require more energy and temperature to melt.