Physical characteristics of extrudate from mixed corn grit – red bean flour with treatment of moisture content and extruder barrel temperature

. Extruded snacks, typically derived from low-protein corn, can significantly benefit from the incorporation of red beans to boost their protein content and enhance overall nutritional value. In this study, the influence of feed moisture content and extruder barrel temperature on various physical attributes of the extruded products, including moisture content, particle density, bulk density, expansion ratio, and hardness, were examined. The snacks were produced using a SYSLG-IV twin-screw extruder with a throughput capacity of 10-15 kg/h. The ingredient composition remained consistent, consisting of a 20% red bean flour and 80% corn grit blend. Three moisture content levels (14%-18%) and three different extruder barrel temperature settings (120°C-140°C) were used as variables. The results demonstrated that both variables significantly impacted the physical properties of the extruded snacks. Specifically, an increase in feed moisture content or a decrease in barrel temperature led to higher moisture content (1.01%-1.82%), greater particle density (0.17 g/cm 3 -0.39 g/cm 3 ), increased bulk density (0.12 g/cm 3 -0.32 g/cm 3 ), greater hardness (8.34 N-32.18 N), and reduced expansion ratio (2.17-3.0).


Introduction
Stunting is still a major issue in Indonesia, with prevalence of 21.6% [1].This number falls short of the WHO standard, which specifies a threshold of less than 20% [2].One of the causes of stunting in children is the lack of protein intake [3].Providing energy and protein supplements to children and pregnant women is one of the ways to prevent stunting [4].
Snack consumption by children in Indonesia is relatively high, reaching 81.6% in Bandung, West Java [5].Another study reported that children in Demak, Central Java, who were stunted (94.4%) consumed snacks more frequently than children who were not stunted (88.7%) [6].These studies suggest that snacks could potentially serve as a means of supplementing children's nutrition to mitigate the risk of stunting.
Corn is often used as a base ingredient for extruded snacks because of its high starch content [7].The high starch content in corn can produce extrudates that expand well [8].However, corn contains a low content of amino acids, vitamins, minerals and fibre [9].Hence, the addition of other ingredients could increase the nutritional value of extruded snacks.
Legumes are a food source known to contain high protein and low-fat content [10].One type of legume is red beans.Red beans (Phaseolus vulgaris L.) have quite high nutritional content.One hundred grams of red beans contain 23 grams of protein, 1.38 grams of fat, 70.48 grams of carbohydrates, 20.93 grams of dietary fibre, 0.50 mg of vitamin B1, 0.03 mg of vitamin B2, 0.51 mg of vitamin B3, 104.12 mg of calcium, 53.48 mg of sodium, 1517.36 mg of potassium, 118.95 mg of magnesium, 7 mg of iron, and 2.38 mg of zinc [11].The high protein content in red beans can be utilized in the form of extruded snacks to increase the nutrient of snacks.
Extrusion is one of the methods for producing ready-to-eat (RTE) snacks.It is a thermomechanical process which consists of the mixing process of materials driven by a screw so that they flow in the extruder barrel under high pressure, high temperature, and high shear conditions [12].The extrusion process involves various processes simultaneously, such as cooking, mixing, shearing, gelatinization of starch, and denaturation of protein [13].
Process factors, such as the extruder barrel temperature, can influence product characteristics produced by the extrusion.Apart from the process, material factors, such as moisture content and material composition, also play roles [14].Generally, at high barrel temperatures, there will be better expansion compared to low barrel temperatures.This expansion is also directly proportional to the starch content in the material [15].Apart from temperature, moisture content can also influence the expansion of the extrudate because it affects the melt viscosity, which is influenced by the gelatinization of the material during the extrusion process [16].
Several studies on red bean-based extrudate production have been carried out.One study focused on the effect of extrusion temperature and red bean flour ratio to rice on the physical properties of extrudate [17].Another study investigated the effects of red bean and navy bean addition on corn starch-based extrudate [18].However, there is still limited information about the use of corn grit and red beans as an ingredient to produce extrudate.Hence, this study aimed to find out the effect of barrel temperature and feed moisture content on the physical characteristics of extrudates based on corn grit-red bean, such as expansion ratio, particle density, bulk density, moisture content, and hardness.

Materials
Corn grit and red bean flour were used as primary ingredients in this study.Corn grit (Zea mays) was purchased from CV. Surya Grain Indonesia, Sidoarjo.Red beans (Phaseolus vulgaris L.) were obtained from Beringharjo Market, Yogyakarta, and were grounded with a disk mill (Shandong Jimo FFC-15, China) to obtain red bean flour.Corn grit and red bean flour were then sieved through mesh-24 and mesh-80 sieves, respectively.

Feed preparation
Red bean flour and corn grit were mixed at a 20% red bean flour and 80% corn grit ratio using a mixer (Ossel B7, Indonesia).The final moisture content of the mixture was fitted to the required values prior to the extrusion process by adding water (14, 16, and 18% moisture content).The mixture was then stored in polypropylene bags for 24 hours at room temperature to achieve equilibrium.

Extrusion processing
Extrusion process was accomplished utilizing a twin-screw extruder.The prepared mixture was put into extruder feeder which is operated at 15 rpm feeder speed.Extruder main screw and cutter were operated at 20 rpm and 6 rpm, respectively.Barrel temperatures were set at 40ºC, 50 ºC, and 100 ºC and varied at three different temperatures of 120, 130, and 140 ºC at the last zone.When a stable extrusion process was achieved, extrudates were collected, aircooled, and dried using an oven at 105ºC for 3 hr.Dry extrudates were packed in a polypropylene standing pouch until further analysis.Each extrusion condition was done in triplicate.

Expansion ratio
The expansion ratio (ER) of extrudates was concluded by measuring the diameter of extrudate's cross section area using a calliper.ER was computed by dividing the diameter of the extrudate to the diameter of the die orifice using Equation 1 [19].Ten random extrudates, with three locations for each extrudate, per extrusion condition were used to acquire ER values.

Particle density and bulk density
Particle density (PD) was calculated by dividing the mass of an extrudate piece (m) to its volume (V) [20].Volume of the extrudate piece was measured using its dimensions (length, l; diameter, d).Extrudate dimensions of ten randomly taken extrudates were measured using a vernier calliper.PD of extrudates was calculated using Equation 2. , Bulk density (BD) was evaluated according to the procedure of Kantrong et al. (2018) [21].A graduated cylinder with 1000 mL volume was filled with extrudates and weighed using a digital scale.BD of extrudates was computed using the equation below.BD measurement was done in three replication.

Moisture content
Moisture content of extrudates was gauged using thermogravimetric method by drying the samples inside a forced-air oven at 105ºC for 24 hours until constant mass was achieved [22].
Moisture content = mass of water mass of moist sample × 100% (4)

Hardness
Extrudate hardness was examined utilizing a Brookfield CT-3 texture analyzer.The texture analyzer was fitted with a 10 kg load cell and a TA-39 cylindrical stainless-steel probe, 2 mm diameter and 20 mm length, to measure the required force for breaking the samples, which indicates hardness.The test was conducted at pre-test, test, and return speeds of 2, 2, and 1 mm/s, respectively, and a target distance of 7 mm.

Experimental design and statistical analysis
Feed moisture content and extruder barrel temperature were considered as the independent variables in this study, with 3 x 3-barrel temperature (120, 130, and 140 ºC) and feed moisture content (14, 16, and 18%).A completely randomized design (CRD) was used to map the extrusion conditions.Each extrusion condition was done in triplicate.The effect of feed moisture content and extruder barrel temperature on the physical characteristics of extrudates was analyzed by two-way ANOVA using IBM SPSS Statistics 27.

Expansion ratio
The ER is one of the most crucial characteristics of extruded snacks.The size, number, and dispersion of air bubbles inside the extrudate is notably related to the ER, which affect the texture of the extruded snack.Barrel temperature, feed moisture content, and their interaction significantly influenced the ER of extrudates (p<0.05).The ER of extrudates were varying between from 2.17 to 3.00, with the lowest value obtained at 120ºC barrel temperature and 18% feed moisture content and the highest at 140ºC barrel temperature and 14% feed moisture content (Fig. 1).ER of extrudates was positively influenced by the temperature of the barrel, whereas the feed moisture content had an adverse impact on the ER.Increased moisture content, along with heat addition, promotes gelatinization of starch, yet increased moisture would also increase the lubricating effect inside the extruder barrel, causing the cooking degree of raw materials and die pressure to decrease, resulting in lower ER [23,24].Another study reported a similar phenomenon in which an increase in moisture content, from 12% to 18%, caused the ER of extrudate to decline (1.99-3.97)yam-rice-corn-based extrudate using a twin-screw extruder [23].ER of extrudates also significantly affected positively as the barrel temperature increased (p<0.05).High barrel temperature produces more heat to evaporate superheated water when exiting the die, resulting in more evaporation, therefore increasing the ER [25].Melt viscosity will also decrease as barrel temperature increases.Reduced melt viscosity would promote bubble growth during extrusion, leading to increased ER [26].A similar finding was reported for banana flour and corn-based extrudates as the barrel temperature increased from 130 to 170ºC, with ER values ranging from 2.19 to 2.58 [19].

Particle density and bulk density
Particle density (PD) and bulk density (BD) were inversely correlated with the expansion ratio.PD and BD were affected significantly by feed moisture content, barrel temperature, and their interaction (p<0.05).PD of extrudates was varying from 0.17 to 0.39 g/cm 3 (Fig. 2), whereas BD was varying from 0.12 to 0.33 g/cm 3 (Figure 3).The highest PD and BD value was obtained at 120ºC and 18%, while the lowest value for PD and BD was obtained at 130ºC and 14%.It was found that feed moisture content positively influenced both PD and BD, while higher barrel temperatures had the opposite effect.The increase in density (0.059-0.109 g/cm 3 ) at higher feed moisture content was also reported for corn and yam-based extrudate using a single screw extruder [27].Low moisture content materials tend to be more viscous than those with higher moisture content; thus, the pressure difference between the die and the atmosphere would be greater for materials with lower moisture, resulting in low-density products due to more expanded products [28].

Extrudate moisture content
Moisture content is (MC) one of the parameters that influence product shelf life and stability.Lower MC is more desirable for shelf stability and safety [29].Moisture content of extrudates ranged from 1.01 to 1.82%.The MC of extrudates varied between 1.01 to 1.82%.The lowest value of MC was reached when the barrel temperature was 120ºC, and the feed moisture content was 14%.In contrast, the maximum value of MC was reached when the barrel temperature was 130ºC, and the feed moisture content was 18% (Figure 4).MC was only affected significantly by feed moisture content at p<0.05.It is also shown in Figure 3 that higher value of feed moisture content produced extrudate with higher moisture content.The result of increased moisture content (4.11 -6.37%) by adding feed moisture content, as the feed moisture content increased from 15 to 18%, was also found for corn grit-based extrudate [30].

Hardness
Hardness measures the force required to break a product through penetration using a probe [31].Average hardness data is shown in Figure 5, ranging from 8.34-32.18N, with the lowest value obtained at 130ºC and 14% feed moisture content.Hardness was positively significantly influenced by feed moisture content at p<0.05.
A similar findings was also reported by Alam et al. ( 2016) [32] that extrudate hardness increased as feed moisture content increased on extrudates based of rice, defatted soybean flour, carrot pomace, and cauliflower trimmings, ranging from 47.68 to 209.71 N. Within the extruder barrel, water functions as a plasticizer, reducing the melt viscosity, energy dissipation, cooking degree, and vapour pressure of the material, thus inhibiting the air bubble formation, resulting in denser extrudate [28].Reduced cooking degree causes the starch conversion degree to lower, suppressing air bubble growth when the material exits through the die, resulting in a more compact extrudate.
Even though barrel temperature did not have effect on extrudate hardness significantly (p>0.05), the hardness of extrudates tended to decrease as barrel temperature increased.Another study reported reduced hardness (22.9-198.9N) when barrel temperature increased from 130 to 150ºC on rice starch-pea protein-based extrudate using a twin-screw extruder [33].Melt viscosity reduces as barrel temperature increases while simultaneously increasing the vapour pressure, increasing the bubble growth, lowering the extrudate density and decreasing the extrudate hardness [26].

Conclusions
Barrel temperature affected all physical characteristics of the extrudate except for its moisture content and hardness value.The expansion ratio increased as the barrel temperature increased while decreasing the hardness, particle density, bulk density, and moisture content.Feed moisture content affected all physical characteristics of the extrudate.A rise in feed moisture content causes a reduction in the expansion ratio and an increase in the hardness, particle density, bulk density, and moisture content.

Fig. 1 .
Fig. 1.Feed moisture content and barrel temperature effect on the expansion ratio.

,Fig. 2 .
Fig. 2. Feed moisture content and barrel temperature effect on the particle density.

Fig. 3 .
Fig. 3. Feed moisture content and barrel temperature effect on the bulk density.

,Fig. 4 .
Fig. 4. Feed moisture content and barrel temperature effect on moisture content of extrudates.

Fig. 5 .
Fig. 5. Feed moisture content and barrel temperature effect on hardness.