Effect of formulation and extrusion technique on technological properties of snack containing jujube flour

Extruded snacks were among the most commercially successful extruded foods. Extrusion cooking is the process extensively used for the production of snacks which are mainly produced from cereal flour or starches.  Extruded snacks are normally high in calories and fat with low content of protein, fiber, and perceived as unhealthy food to many consumers. Several attempts to improve the nutritional profile of extruded starch by using food by-products have been reported (Onwulata et al., 2001; Stojceska et al., 2008). Zizyphus jujube (Rhamnaceae) is widely distributed in Iran and the fruit of this plant has gained wide attention in native herbal medicine for the treatment of a broad range of disorders. Chemical analysis of the fruit has shown the presence of antioxidants. The high consumption of low-value meals and malnutrition among the population, especially children and adolescents, is one of the problems that exists today in society, Therefore, in this research, production of high-nutritional snack with jujube flour was studied.

Extrusion: In this study, a parallel twin-screw extruder (SAIXIN, model DS56, China) was applied, screw diameter of 15 mm, die diameter of 3 mm, and extrusion temperature of 180°C. Central composite statistical design was used to study the effect of feed moisture (12, 16, 18%), jujube flour addition (5, 10, 15%) and screw speed (120, 150, 180 rpm) on physicochemical properties, texture and color of expanded extrudates based on corn-wheat flour (50-50%). Bulk density was calculated using AACC (2000). The WSI of the dry solids regained through the evaporation of the supernatant obtained from the water absorption test was calculated. Textural measurement: The hardness of the extruded snacks was measured using Texture Analyzer (TAXT plus, England). The cylinder steel probe (2 mm diameter) was set to move at a speed of 1 mm/s. The samples were punctured by the probe to a distance of 8 mm. the color of samples was measured using an image processing technique (Lotfi Shirazi et al., 2020). The color parameters L* (lightness), a*(redness), b*(yellowness) values of the samples were obtained. Response surface methodology was applied for experimental data using a commercial statistical package, Design Expert (version 8.0) for the generation of response surface plot and statistical analysis of the experimental data. The second-order polynomial model was selected to predict the optimal point of the responses.

Results showed that the addition of jujube flour independently increased the density 0.33, hardness 35.73 N, solubility 42% and redness (a*) 3.41, and reduced the color brightness parameter (L*) 89.1 of the samples. Similar fiber effects for pineapple pulp (Salani et al., 2014), a mixture of barley flour and grape pulp (Altan et al., 2009) and beet fiber (Lue et al., 1994) has been observed. The reduction of bulk density in samples with increasing screw speed is probably due to the starch gelatinization. When gelatinization of the starch material increased, the volume of extruded product also increased giving low density to the samples (Hagenimana et al., 2006). The WSI is a parameter which indicates the degradation of starch granules (Sharma et al., 2015). WSI decreased with increasing feed moisture and increased with increasing jujube flour or screw speed. Increase in solubility of extruded products under mild extrusion condition could result from dispersion of amylose and amylopectin molecules during gelatinization; while, the formation of low molecular weight compounds under severe conditions (low moisture and high temperature) is the main reason for an increase in flour solubility (Colonna et al., 1984). Color changing during extrusion process can provide important information regarding the degree of browning and Maillard reaction, and also pigment degradation. Increasing the feed moisture increased flour L* while it decreased the a* and ∆E of the final product. This is probably due to the decrease in mass residence time inside the extruder which causes shorter residence time necessary for non-enzymatic darkening (Lazou and Krokida, 2011). Texture is one of the most important sensory properties of extruded products in the development of snack products. Increasing the feed moisture and jujube flour increased hardness of samples. This can be explained by the effect of fiber on cell wall thickness.  However, the simultaneous effect of two parameters of screw speed and jujube flour improved the properties of the snacks, by which the increase of the screw speed may cause a decrease of the starch viscosity and then lead to extrudates with reduced hardness 1.79 N. Optimum conditions for puffed snack production was found to be 13.19% feed moisture content, 6.30 % jujube flour, 122.2% screw speed.