QUALITY PROPERTIES OF CORN-BASED EXTRUDATES ENRICHED WITH DIATERY FIBERS

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1 QUALITY PROPERTIES CORN-BASED EXTRUDATES ENRICHED WITH DIATERY FIBERS A. N. Giannini a, M. K. Krokida a, N. P. Zogzas b a Laboratory of Process Analysis and Design, School of Chemical Engineering, National Technical University of Athens, 5 Iroon Polytechniou St., Zografou Campus, 1578 Athens, Greece, mkrok@ chemeng.ntua.gr b Laboratory of Food Engineering, Department of Food Technology, Technological Educational Institute (TEI) of Athens, Agiou Spyridonos St., 122 1, Egaleo, Athens, Greece, nzogzas@ teiath.gr ABSTRACT The purpose of this study was the investigation of the effect of process conditions (temperature, feed rate) and raw material characteristics (moisture content, fiber to corn ratio) on structural and mechanical properties (apparent/true density, porosity, expansion ratio and stress of compression) of corn based extrudates enriched with apple and oat fibers. Simple power models were developed to predict porosity and compression stress as functions of process conditions and material composition. The extrusion conditions regulated were the feed rate (.5-2. g/s) and the extrusion temperature (15-23 C), at a speed of screw rotation of 2 rpm. The feed material moisture content was regulated from 13 to 19% (wet basis) and the fiber to corn ratio was ranged from 1 to 3% for both types of extrudates. of extrudates was found to decrease with temperature, feed moisture content and fiber to corn ratio and to increase with feed rate for both the examined fibers. The same tendency was also found for the expansion ratio of extrudates, while the stress of compression revealed the opposite behavior. The true density of apple fiber/corn extrudates ranged at 155±387 (kg/m 3 ), while that of oat fiber/corn ranged at 145±27 (kg/m 3 ). Comparatively, the use of apple fibers led to products with higher porosity, compared to those with oat fibers. In general, the addition of fibers resulted to more dense products. Keywords: density; porosity; expansion ratio; apple/oat fiber; extrusion cooking; INTRODUCTION Dietary fiber is the indigestible component of foods that includes cellulose, hemicellulose, lignins, pectins, gums, and mucilages [1]. Its beneficial effects on human health have received much attention. Lack of adequate dietary fiber in the diet is associated with constipation, diverticulosis, cardiovascular disease, and cancer [2], and increased consumption of dietary fiber has been advocated. Dietary fiber-rich materials have gained popularity as food ingredients for health benefits in recent years. However, relatively little is known about the effects of processing conditions on structural and mechanical properties of fiber containing foods. Extrusion is a unique and versatile thermal process, where the raw materials are subjected to direct mixing and cooking, resulting to ready to eat expanded snacks. Moistened starchy or proteinaceous foods are worked together into viscous plastic-like dough and cooked before being forced through a die. Some results of the extrusion cooking are, gelatinization of starch, denaturation of proteins, inactivation of many native enzymes that cause food deterioration during storage, inactivation of antinutrient factors [3], [4] and reduction of microbial counts in the final product [5]. Product quality can vary considerably depending on the extrusion processing conditions such as the extruder type, the screw configuration, the feed moisture content, the temperature profile in the barrel sections, the screw speed and the feed rate. The influence of processing conditions on product quality by extrusion of starch based legume blends has been widely investigated [6], [7], [8], [9]. The degree of expansion determines the extrudate s structure and consequently its texture. Extrudate expansion has been reported to be most depended on material moisture content and extrusion temperature. Several theoretical considerations have been published on this subject [1], [11], [12]. During extrusion cooking of biopolymers the viscoelastic material is forced through the die so that the sudden pressure drop causes the water to evaporate, producing an expanded porous structure. The degree of expansion is affected from both the elastic swell and bubble growth effects [13]. A general model of extrudate expansion has been developed including the radial, longitudinal and volumetric expansion [1].

2 The incorporation of dietary fibers into extruded products plays an important role on their structural characteristics. High levels of fiber have often resulted into a compact, tough, non-crisp and undesirable texture [14]. Many studies on the effects of extrusion cooking using different fiber sources, like corn bran [15], soy fiber [16] and sugar beet fiber [14] have been reported. The purpose of this study was the investigation of the effect of process conditions (temperature, feed rate) and raw material characteristics (moisture content, fiber to corn ratio) on apparent/true density, porosity, expansion ratio and mechanical stress of corn based extrudates enriched with apple and oat fibers. Simple power models were developed to predict porosity and mechanical stress as functions of process conditions and material characteristics. MATERIALS & METHODS Oat and apple fibers were mixed with corn flour, with fiber to corn ratio ranging from 1 to 3%. The moisture content of the feed was adjusted from 13 to 19% wet basis, by adding the necessary distilled water. A prism Eurolab conical, counter-rotating twin screw extruder, model KX-16HC, with a process rotation speed of 2 rpm was used. The screw geometry was: length 4 cm, diameter 16 mm and die diameter 3 mm. Extrusion temperatures and mass rates of feed were regulated from 15 to 23 ºC and.5 to 2. g/s respectively. Apparent density, true density and porosity of extrudates were determined by measuring the actual dimensions of the samples using a Vernier caliper, along with their true volume, using a Quantacrome stereopycnometer (model SPV-3) with ±.1 cm3 accuracy. Expansion ratio was calculated by the extrudate s over the die s diameter and mechanical properties were evaluated through compression tests using a Zwick Universal Testing Machine 112. RESULTS & DISCUSSION The experimental values of porosity were calculated by the apparent to true density ratio, according to the suggestions of reference [17], using the following equation: ρ ε = 1 a (1) ρ t Where, ρ a, ρ t are the apparent and true density respectively in (gr/cm 3 ). The apparent and true densities were calculated by the weight, the geometric volume and the true volume of the sample measured by the stereopycnometer. It was assumed that the values of both densities do not vary significantly from those at zero moisture content, since the moisture content of extrudates is significantly low. The following parametric model was proposed to predict porosity as a function of process conditions and material composition: n T n F n X n C ε T F X C ε o T r F r X r C = (2) r Where, ε is the porosity of the extrudate, T is the extrusion temperature (ºC), F is the feed rate (gr/s), X is the feed moisture content (kg/1kg wb) and C is the materials ratio (kg fiber/kg corn). T r, F r, X r and C r are reference values and ε ο, n T, n F, n X, n C, are the respective parameters. The mechanical stress of compression was modeled by the following equation: p σ = E e + ( σ max E e max )(e / e max ) (3) Where, σ is the stress of compression and σ max its maximum value (stress of rapture) in (kpa), e is the strain (Δl/lo) and e max its value at σ max, Ε is the elasticity modulus in (kpa) and p is the viscoelastic exponent. The values of E, σ max and e max were related to process conditions and material composition by using parametric models similar to that of equation (2). The above parameters are evaluated by regression analysis that is based on the minimization of the sum of square residuals between predicted and experimental values. The proposed model can be considered acceptable if the standard deviation between experimental and predicted values (S R ) is close enough to the experimental error (S E ). Following to regression analysis procedure, all five parameters of equation (2) can be determined simultaneously. However, not all of these parameters affect the sum of square residuals to the same degree. In order to distinguish between the ones that are necessary to accurately predict porosity, the following procedure was adopted. Firstly the minimum change in sum of squares (SST) was estimated for all

3 five parameters and the mean deviation (S R ) was evaluated. Secondly, omitting one parameter at a time, the values of S R was evaluated for all combinations of the five remaining parameters. In this way, the parameter chosen to be eliminated was the one whose elimination produced the minimum S R. Continuing the former procedure, the minimum values of S R were evaluated for 4, 3, 2 and 1 parameters respectively. A typical diagram of S R versus the number of parameters is shown in Figure 1. As it can be seen, there must be used five parameters to accurately predict porosity. The values of these parameters are given in table 1, where S R and S E are the corresponded values of standard deviation and standard error.,25 Standard deviation S R,2,15,1,5 Lack of fit Experimental error S E Number of parameters Figure 1. The lack of fit (SR) and standard experimental error (SE) as a function of the number of parameters Table 1. Results of Parameters Estimation Extrudate ε o n T n F n X n C S R S E Corn/apple fiber Corn/oat fiber According to the above table and equation (2), the influence of feed rate, as it is expressed by the exponent n F, is positive for both types of extrudates. That is, the extrudates porosity increases with the increase of feed rate. On the contrary, the negative values of n T, n X and n C indicate the opposite behavior. In other words the increase of temperature, initial material moisture content and fiber to corn ratio, leads to lower porosity values. Lower porosity values can also be observed for the case of oat fiber to corn extrudates, something which is justified by the lower value found for ε ο. In figure 2, the porosity of extrudates is represented as a function of extrusion temperature, fiber to corn ratio, feed rate, and feed moisture content. Solid lines are used for the predicted values of porosity using the proposed model and the estimated parameters of Table 1. As it can be seen for both types of extrudates, the increase of feed rate leads to a slight increase of porosity. In fact, increased feed rate would influence the degree of filling, inducing the degradation of amylopectin networks and changing the melt rheological characteristics thus leading to a greater elastic effect and changes in product porosity [3], [11], [18], [19]. In the same figure, porosity decreases with extrusion temperature. Fiber particles usually decrease the product expansion by rupturing the cell walls before the gas bubbles could expand to their full potential. This phenomenon is more intense as temperature increases [2]. The increase of feed moisture content also seems to decrease porosity. Moisture content during extrusion provides the driving force for the expansion and also contributes to the rheological properties of the melt, which in turn affect expansion. Moisture is the main plasticizer of flours, which enables them to undergo a glass transition during the extrusion process and thus facilitates the deformation of the matrix and its expansion [21]. Similar results have been observed by [11], [19], [2] and [22]. It can also be seen that increasing the amount of fiber to corn ratio, porosity decreases. Generally, the addition of fibers leads to products which are usually compact, tough and not crisp, due to their fat and fiber content [14], [23].

4 1, X=13% F=1g/s fiber/corn=1% 1, X=13% F=1g/s T=17 o C,9,9,8,8, Temperature ( o C), Fiber/Corn (%) 1, X=16% C=1% T=17 o C 1, T=2 o C F=1 g/s C=1%,9,9,8,8,7,5 1 1,5 2 Feed rate (g/s), Feed moisture content (% wb) Figure 2. Comparative diagrams of porosity for both types of extrudates It is clear from figure2, that the extrudates with oat fibers have lower values of porosity than the ones with apple fibers. This might be due to their different lipid and fiber content. As far as the expansion ratio is concerned, it was found that in all cases the expansion ratio revealed the same behavior to that of porosity and thus it could be modeled in the same way. In contrast, maximum stress (σ max ) revealed the opposite tendency. That is, the maximum stress of compression was found to decrease with feed rate and to increase with temperature, feed moisture content and the fiber to corn ratio, as it can be seen from figure 3. In all cases, the values of maximum stress for oat fiber extrudates were higher than the ones for apple fiber, indicating a more rigid structure. CONCLUSION Density, porosity, expansion ratio and mechanical stress of expanded corn-fiber snacks, produced on a twin screw extruder, depend on several process conditions (extrusion temperature, feed rate, feed moisture content and corn to fiber ratio).

5 (a) T=17 C X=16% F=,6g/s Fiber/Corn (%) 1 (b) T=17 C 9 C=2% F=,6g/s Feed moisture content (%) 4 35 (c) C=1% X=13% F=.8g/s (d) T=17 C C=2% X=16% Temperature (ºC) 1,2,4,6,8 1 Feed rate (g/s) Figure 3. Comparative diagrams of maximum stress for both types of extrudates (: oat fiber, : apple fiber) of extrudates was found to decrease with temperature, feed moisture content and the fiber to corn ratio and to increase with feed rate. The addition of fibers produced more dense products. The influence of process conditions and material composition on mechanical stress had the opposite effects compared to those of porosity, in contrast with the expansion ratio that revealed the same behavior. Comparatively, the use of apple fibers in mixtures for the production of snacks, led to products with higher porosity and lower mechanical stress of compression than those with oat fibers. It is possible to predict the variation of extrudates porosity and mechanical stress us functions of the above process and material composition variables. REFERENCES [1] Goni, I., Díaz-Rubio, M.E., Pérez-Jiménez, J. & Saura-Calixto, F. 29. Towards an updated methodology for measurement of dietary fiber, including associated polyphenols, in food and beverages. Food Research International, 42, [2] Trowell, H., Burkitt, D. & Heaton, K. eds Dietary fiber, fiber-depleted foods and disease. Academic Press, New York. [3] Edwards, R.H., Becker, R., Mossman, A.P., Gray, G.M. & Whiteland, L.C., Twin Extrusion cooking of small white beans (Phaseolus vulgaris). Lebesm.-Wiss.-Technol., 27, [4] Steel, C.J., Sgarbieri, V.C. & Jackis, M.H., Use of extrusion technology to overcome undesirable properties of Hard-to-cook dry beans (Phasseolus vulgaris L.). J. Agric. Food. Chem., 43,

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