Effect of feed mixture and process variables on physicochemical properties of solid foams made from corn starch and sesame seed by extrusion

Extrusion is one of the technologies used for solid foams production. In this process, pressure is the most important parameter and the most important variables affecting pressure are feed mixture, die diameter, barrel temperature and screw speed. A reduction of die diameter or plasticizer contents such as moisture and fats in the feed mixture or an increase in the screw speed or barrel temperature can increase the extruder barrel pressure. Also, the increased barrel temperature, in addition to changing cooking properties, escalates the temperature difference inside and outside of the die, raising the rate and amount of evaporation from melted mixtures, therefor affecting the solid foam structure and characteristics. On the other hand, the type and amount of feed mixture components are key factors affecting the extrudate properties (Moraru et al., 2003; Plews et al., 2009; O’Shea et al., 2014).
Sesame seed is one of the ancient edible oil seeds used in many food products. In addition to oil, it contains carbohydrate, protein and fiber (Namiki et al., 2001) which can provide a variety of compounds in the feed mixture to change the properties of foam. This study investigates the effect of partial replacement of corn starch with edible oil seed containing a mixture of various compounds and the effect of the extrusion process on the changes in the physicochemical properties of the produced solid foam relative to the foam produced from corn starch. In this regard, different proportions of sesame seeds were added to the corn starch with specific moisture contents, and following the application of the extrusion process, the effect of feed mixture, operation temperature, screw speed and die diameter on physicochemical properties of solid foams was evaluated. Solid foams made from corn starch with 0, 10, 20 and 30 percent of sesame seed in the formulation and 15 percent of moisture content were processed in a co-current twin-screw extruder at a screw speed of 120, 150 and 180 rpm, a barrel temperature of 120, 145 and 170°C, a die diameter of 2.5 and 4 mm, and a constant feed rate of 40 kg per hour. A completely randomized design was employed to investigate the effect of these variables on chemical and physical properties of extruded products. The moisture content of samples was measured using oven method at 105°C (AOAC, 1990). Water absorption and water solubility indices were measured through solving the sample powder in distilled water, which was followed by centrifuging, weighing gel, drying supernatant and weighing dried matter (Singh et al., 2015; Huang et al., 2014). In addition, particle density was measured using the rapeseed displacement method (Singh et al., 2015) and solid density was calculated by the weight /volume ratio of the sample powder, as measured by the gradient cylinder (Ushakumari et al., 2004; Yagci et al., 2008). The porosity of samples was measured in terms of the ratio of particle density to solid density (Plews et al., 2009; O’Shea et al., 2014) and the radial expansion ratio was calculated in terms of the ratio of sample diameter, as measured by the caliper, to die diameter (Chanlat et al., 2011; Huang et al., 2014). Results showed that adding 10% sesame seed, due to the variety of compounds and their increased interactions, produced foams of maximum expansion and porosity, and minimum particle density. Adding 30% sesame seed had an opposite effect due to increased fat content and reduced pressure effect on the melted mixture in the barrel. Moreover, increased die diameter demonstrated augmented residual moisture content, water absorption index, density and porosity, as well as decreased water solubility index and expansion ratio of solid foams caused by pressure reduction on the melted mixture in the barrel. The increased barrel temperature was associated with greater changes in cooking, escalated temperature difference between inside and outside of the die and production of foams with higher water solubility index and expansion ratio, as well as lower residual moisture content, water absorption index and solid density. The higher screw speed increased the applied energy, and despite decreasing the time of temperature effect, produced foams with properties comparable to those caused by temperature increase.
In general, it can be stated that the process factors that raise the energy applied to the extrusion material leads to the increased water solubility index and the expansion ratio of the solid foams and decreased residual moisture, water absorption index and density. Consequently, by selecting the right type and amount of feed mixture to create proportions in various compounds and determine the appropriate process conditions, solid foams with desired properties can be produced by means of extrusion using available raw materials.