Thermal, textural, and physicochemical analysis of corn extrudates enriched with broccoli or olive paste

Abstract

The growing consumers’ demand for healthier snacks is of high importance and is driving research and industry towards developing novel ready to-eat expanded products. Thus, it would be interesting to enrich these products with additives that possibly lead to highly nutrient products. In the present study, extruded snacks based on corn flour enriched with broccoli or olive paste were developed and studied. Although the addition of broccoli or olive paste in extruded snacks can improve their nutritional properties, the final properties of these products must also be considered. Textural analysis (compression tests), thermal analysis (glass transition temperature), and moisture sorption measurements (sorption isotherms) were performed. A Guggenheim–Anderson–de Boer model has been applied to describe moisture sorption isotherms. Moreover, simple mathematical models were developed in order to correlate textural properties (maximum stress, maximum strain, elasticity parameter, number of cracks) with the process conditions during the extrusion (screw speed, temperature) and the raw materials’ characteristics (moisture content, concentration of broccoli or olive paste). Regression analysis was used to estimate mathematical models’ parameters. The influence of process conditions and raw materials’ characteristics on the measured properties of corn-based snacks enriched with broccoli or olive paste was presented based on the alterations of the estimated parameters.

Keywords:

• Broccoli
• Extrusion
• Glass transition temperature
• Olive paste
• Textural analysis

INTRODUCTION

Extruded snacks consist mainly of starch, which is responsible for most of their structural behavior. The raw ingredients most commonly used in extrusion cooking are rice, wheat, corn, and/or legume flour that lead to different physico-chemical characteristics in the final products and are related to the variations in the chemical composition of the grain.^[1–4] Corn starch is the most commonly used ingredient for the production of puffed food snacks through extrusion, which present low protein and other nutrients and are, however, far from satisfying the needs of health-conscious consumers.^[2,5–7]

Several attempts to improve the nutritional profile of corn-based extruded snacks using oat flour, milk proteins, legume flours, etc. have been reported.^[6–10] The addition of vegetables, fruits, and their by-products to extruded snacks, in order to increase their functionality and nutritional quality, is a growing trend in recent literature. Fruits and vegetables are good sources of beneficial compounds for the human diet, containing many different antioxidant components, which provide protection against harmful free radicals and have been strongly associated with reduced risk of chronic diseases, such as cardiovascular disease, cancer, diabetes, etc., along with other health benefits.^[11] The addition of fruit juice, paste, powders, pomace, peels, and seeds, such as lycopene, fresh carrot and carrot leaf, garlic, apple pomace, etc., have been reported in several studies.^[1,12–16] Although the addition of several ingredients increases the functionality of the snacks, the acceptability of the snacks depends not only on the high-nutritional characteristics, but also on the acceptable physico-chemical, textural, and thermal properties. These properties are significantly affected by the material characteristics and the processing conditions during extrusion cooking.^[2,8]

Broccoli is becoming increasingly popular as a fresh vegetable and is a significant source of nutritional antioxidants, such as vitamins and carotenoids, as well as biologically active dietary components, such as flavonol glycosides, hydroxycinnamic acids, and sulphur-containing compounds.^[11,17,18] Olive products, such as olive paste, are widely known for the variety of triterpenes they contain, including oleanolic acid, maslinic acid, uvaol, and erythrodiol. Thus, antioxidant, anti-inflammatory, and antitumoral action have been identified in olive products.^[19,20]

Raw materials, such as vegetables, fruits, etc., must be dehydrated prior to extrusion, in order to be added in the extrusion mixture as powders.^[21,22] Among the different drying methods, freeze-drying is considered one of the most advanced methods for drying high-value products sensitive to heat, since it produces materials with unchanged nutrition quality, superior taste, aroma, and flavors.^[23–25]

The textural properties of foods are related with their sensorial characteristics, and consequently with the quality and acceptance of the product by the consumers. The most common method in determining the texture of foods and especially of extruded products is compression.^[26–28] In addition, the knowledge of sorption characteristics and glass transition temperature in different water contents is useful to specify the storage stability of food products.^[29] The aim of this study was to develop extruded snacks based on corn flour enriched with broccoli or olive paste. Textural analysis (compression tests), thermal analysis (glass transition temperature), and moisture sorption measurements (sorption isotherms) were performed. Simple mathematical models were developed in order to correlate structural properties (maximum stress, maximum strain, elasticity parameter, number of cracks during compression tests) with the process conditions during the extrusion (screw speed, temperature), as well as the raw materials’ characteristics (moisture content, concentration of broccoli or olive paste), and regression analysis was used to estimate mathematical models’ parameters. Moreover, a Guggenheim–Anderson–de Boer (GAB) model has been applied to describe moisture sorption isotherms.

MATERIALS AND METHODS

Sample Preparation

Corn flour was provided from Marra Bros. Mills S.A. (Korinthos, Greece), and olive paste from Minerva S.A. (Viotia, Greece), while broccoli was purchased from the local market. Broccoli and olive paste were preliminary dehydrated in a laboratory freeze dryer (Leybold-Heraeus GT 2A, Koln, Germany) for 2 days, and were ground to powder. Corn flour and broccoli powder were mixed in different ratios, ranging from 4 to 10% (broccoli concentration: 4, 7, 10%), while corn flour and olive paste powder were mixed in ratios ranging from 4 to 8% (olive paste concentration: 4, 6, 8%). The mixtures were then equilibrated to three moisture levels: 14, 16.5, and 19%. The moisture content was determined using the oven method at 70°C^[30] under vacuum, until constant weight. Extrusion was performed in a co-rotating double screw extruder (Prism Eurolab, model KX-16HC, Staffordshire, U.K.). Extrusion temperature varied from 140 to 180°C and the screw speed ranged from 150 to 250 rpm. Extruded materials were stored in laminated bags until required for analysis. The effect of process conditions and materials characteristics was investigated using a central composite design (Table 1).

Table 1  Extrusion conditions and materials composition

Material	X (wb%)	C (%)	T (°C)	R (rpm)
Broccoli/corn	14	4	140	150
Broccoli/corn	14	4	140	250
Broccoli/corn	14	4	180	150
Broccoli/corn	14	4	180	250
Broccoli/corn	14	7	160	200
Broccoli/corn	14	10	140	150
Broccoli/corn	14	10	140	250
Broccoli/corn	14	10	180	150
Broccoli/corn	14	10	180	250
Broccoli/corn	16.5	4	160	200
Broccoli/corn	16.5	7	140	200
Broccoli/corn	16.5	7	160	150
Broccoli/corn	16.5	7	160	200
Broccoli/corn	16.5	7	160	250
Broccoli/corn	16.5	7	180	200
Broccoli/corn	16.5	10	160	200
Broccoli/corn	19	4	140	150
Broccoli/corn	19	4	140	250
Broccoli/corn	19	4	180	150
Broccoli/corn	19	4	180	250
Broccoli/corn	19	7	160	200
Broccoli/corn	19	10	140	150
Broccoli/corn	19	10	140	250
Broccoli/corn	19	10	180	150
Broccoli/corn	19	10	180	250
Olive/corn	14	4	140	150
Olive/corn	14	4	140	250
Olive/corn	14	4	180	150
Olive/corn	14	4	180	250
Olive/corn	14	6	160	200
Olive/corn	14	8	140	150
Olive/corn	14	8	140	250
Olive/corn	14	8	180	150
Olive/corn	14	8	180	250
Olive/corn	16.5	4	160	200
Olive/corn	16.5	6	140	200
Olive/corn	16.5	6	160	150
Olive/corn	16.5	6	160	200
Olive/corn	16.5	6	160	250
Olive/corn	16.5	6	180	200
Olive/corn	16.5	8	160	200
Olive/corn	19	4	140	150
Olive/corn	19	4	140	250
Olive/corn	19	4	180	150
Olive/corn	19	4	180	250
Olive/corn	19	6	160	200
Olive/corn	19	8	140	150
Olive/corn	19	8	140	250
Olive/corn	19	8	180	150

Compression Tests

Compression tests were conducted in a universal testing machine (Zwick model Z2.5/TN1S, Germany-Ulm). The uniaxial compression tests were performed at room temperature, using a 100 N load cell. Constant deformation rate of 5 mm/min was used for all the materials. Extrudates were cut in cylinders and their dimensions were measured prior to each experiment using a Vernier caliper. Force and deformation were recorded electronically and the resulting stress–strain compression curves were constructed according to the following equations:

(1)

(2)

where σ (Pa) is the stress, ϵ_n (m/m) is the strain, A (m²) is the cross section area, L₀ (m) is the initial thickness of the samples, while F (N) and ΔL (m) are the force and the deformation, respectively.

Moisture Sorption

Extruded samples were first stored in a desiccator at room temperature for 3 weeks prior to the beginning of the experiment. The samples were then placed in chambers and equilibrated over saturated salts solutions (LiCl, MgCl₂, Mg(NO₃)₂, NaCl, KNO₃) of constant water activities environments, ranging from 0.11 to 0.94 (0.11, 0.33, 0.53, 0.75, 0.94) at room temperature. Equilibrium was reached when three consecutive weight measurements did not differ more than 0.001 g. The required time for equilibrium was about 3 weeks. The moisture content of each sample was determined using the oven method at 70°C under vacuum until constant weight.

Measurement of Glass Transition Temperature

Glass transition temperature was determined in the samples of different moisture contents obtained after the equilibrium in the saturated salts solutions. Glass transition temperature (T_g) was estimated using a differential scanning calorimeter (Perkin Elmer DSC 6, CT, USA) with Pyris operation software. Each sample (5–10 mg) was sealed in aluminum pans and scanned from 0 to 100°C at a rate of 5°C/min. A sealed, empty aluminum pan was used as a reference. The glass transition temperature, T_g, was determined as the temperature midpoint of the heat capacity change.^[31]

Mathematical Modeling

Several mathematical models were developed in order to predict the values of maximum stress (σ_max), maximum strain (ϵ_max), elasticity parameter (E), total number of cracks during compression tests (N), with the process conditions during the extrusion (screw speed, temperature) and the raw materials’ characteristics (moisture content, concentration of broccoli or olive paste). The values of σ_max, ϵ_max, E, and N were correlated with screw speed, extrusion temperature, feed moisture content, and concentration of broccoli or olive paste through the following equation:

(3)

where , a_x, a_C, a_T, and a_R are constants; A: σ_max (kPa) or ϵ_max (mm/mm) or E (kPa) or N (-) and X (wb%) is the feed moisture content; C (%) is the concentration in broccoli or olive paste; T (°C) is the extrusion temperature; and R (rpm) is the screw speed.

Different models, such as Brunauer–Emmett–Teller (BET) and Guggenheim–Anderson–de Boer (GAB) models, semi-empirical models like Ferro–Fontan, Henderson, and Halsey models, as well as empirical models like Smith and Oswin, have been developed to describe moisture sorption isotherms. The GAB model is considered as the most versatile sorption isotherm available in the literature, as it describes the entire range of water activity (0.1 to 0.99).^[32] Moreover, the GAB model contains three constants that have physical meaning.^[33] The GAB model, applied to the sorption isotherms of extruded products, is expressed as:

(4)

where X (kg water/kg dry solids) is the moisture content of the material on a dry basis, a_w is the water activity, X_m is the moisture content corresponding to an adsorbed monolayer, and C and K are constants related to the temperature effect.

Statistical Analysis

Regression analysis, which is based on the minimization of the sum of square residuals between predicted and experimental values, was used to estimate the models’ parameters, as described by Maroulis et al.^[33]

RESULTS AND DISCUSSION

The knowledge of the equilibrium moisture content of extruded snacks is indicative of the storage conditions and stability of the products. The sorption isotherms of corn extrudates enriched with broccoli and olive paste are presented in Figs. 1 and 2, respectively, and showed that equilibrium moisture content increased with the increment of water activity. Both broccoli/corn and olive/corn samples present an increment of water sorption in higher feed moisture content and lower extrusion temperature, while slight differences are presented in the different content concentrations and screw speed values. The parameters X_m, C, and K after the appliance of GAB model and the regression analysis are shown in Table 2. For broccoli/corn samples, an increment was presented in the three parameters with the increment of feed moisture content and broccoli concentration, while the increment of screw speed and extrusion temperature caused a decrement in X_m and C and an increment in K. Olive/corn extrudates exhibit similar behavior concerning the GAB parameters. The increment of feed moisture content and olive paste concentration causes an increment in X_m and C and a decrement in K, while the opposite trend occurs with the increment of screw speed and extrusion temperature. The increment of X_m and C values present more sorption available sites for water molecules and, hence, the increment in the heat of sorption at the monolayer.^[34] Parameter C is related to the adsorption energies of the monolayer, thus the decrease in this value would suggest water molecules less strongly bound to the active sites of the matrix, while K is indicative of the multilayer heat sorption.^[34] The highest values of X_m and C were observed in lower extrusion temperatures and higher screw speeds, as far as process conditions during the extrusion is concerned. These differences in extrudates were primarily due to greater starch fragmentation at high screw speeds, which increased the number of free hydroxyl groups available to interact with water, thus making the samples more hydrophilic. Extrusion temperature also has an important effect on GAB parameters. Low extrusion temperatures tend to hinder starch gelatinization making hydrogen bonding difficult and increasing the amount of hydrophilic groups in samples.^[35] Considering raw materials’ characteristics, the highest values of X_m and C were observed with the increment of feed moisture content and broccoli or olive paste concentration. This observation could be associated with the higher gelatinization of starch as feed moisture content increased, which breaks down the intermolecular bonds of starch molecules, allowing the hydrogen bonding sites (the hydroxyl hydrogen and oxygen) to engage more water.^[36] Moreover, the increment in GAB parameters, as broccoli or olive paste concentration increased caused by the increment in fibers’ content that are hydrophilic due to the presence of free hydroxyl groups on their chemical structure, which generate polarity and attract naturally polarized water molecules. Moreover, fibers have a porous structure that allows for a free movement of water.^[37] In addition, olive paste consists not only from fibers but also from antioxidants

Table 2  GAB model parameters and regression analysis from the sorption isotherms

Material	Extrusion conditions and materials composition			Xm	C	K	SR	SE
Broccoli/corn	T (°C) = 160	C (%) = 7	R (rpm) = 200
			X (wb%) = 14.0	0.054	25.755	0.725	0.026	0.001
			X (wb%) = 16.5	0.056	29.677	0.756	0.051	0.001
			X (wb%) = 19.0	0.061	32.000	0.726	0.034	0.000
Broccoli/corn	T (°C) = 160	X (wb%) = 16.5	R (rpm) = 200
			C (%) = 4.0	0.056	29.677	0.756	0.051	0.001
			C (%) = 7.0	0.057	26.573	0.759	0.041	0.003
			C (%) = 10	0.057	30.001	0.764	0.041	0.003
Broccoli/corn	T (°C) = 160	X (wb%) = 16.5	C (%) = 7.
			R (rpm) = 150	0.056	29.677	0.756	0.051	0.001
			R (rpm) = 200	0.057	32.002	0.755	0.013	0.005
			R (rpm) = 250	0.054	29.000	0.773	0.033	0.009
Broccoli/corn	X (wb%) = 16.5	C (%) = 7	R (rpm) = 200
			T (°C) = 140	0.056	29.677	0.756	0.051	0.001
			T (°C) = 160	0.063	45.707	0.706	0.010	0.002
			T (°C) = 180	0.053	28.569	0.777	0.028	0.002
Olive/corn	T (°C) = 160	C (%) = 6	R (rpm) = 200
			X (wb%) = 14.0	0.054	18.179	0.771	0.023	0.003
			X (wb%) = 16.5	0.057	33.317	0.735	0.023	0.003
			X (wb%) = 19.0	0.060	35.567	0.716	0.010	0.003
Olive/corn	T (°C) = 160	X (wb%) = 16.5	R (rpm) = 200
			C (%) = 6	0.055	33.244	0.768	0.031	0.006
			C (%) = 4	0.057	33.317	0.735	0.023	0.003
			C (%) = 8	0.058	33.400	0.728	0.042	0.003
Olive/corn	T (°C) = 160	X (wb%) = 16.5	C (%) = 6
			R (rpm) = 200	0.055	42.084	0.760	0.029	0.008
			R (rpm) = 150	0.057	33.317	0.735	0.023	0.003
			R (rpm) = 250	0.054	35.148	0.757	0.032	0.003
Olive/corn	X (wb%) = 16.5	C (%) = 6	R (rpm) = 200
			T (°C) = 160	0.056	41.155	0.753	0.025	0.003
			T (°C) = 140	0.057	33.317	0.735	0.023	0.003
			T (°C) = 180	0.052	33.000	0.762	0.030	0.002

Figure 1 Sorption isotherms of extruded broccoli/corn snacks (GAB model) in the different: (a) broccoli or olive paste/corn ratio, C (%), (b) feed moisture, X (wb%), (c) screw speed (rpm), and (d) extrusion temperature, T (°C).

Figure 2 Sorption isotherms of extruded olive/corn snacks (GAB model) in the different: (a) broccoli or olive paste/corn ratio, C (%), (b) feed moisture, X (wb%), (c) screw speed (rpm), and (d) extrusion temperature, T (°C).

that are either lipophilic or hydrophilic phenols,^[38] thus the synergistic action of fibers and hydrophilic phenols dominated and gave a more hydrophilic character in samples as olive paste concentration increased.

Table 3  Glass transition temperatures of extruded broccoli/corn and olive/corn snacks in a_w = 0.53

Material	Extrusion conditions and materials composition			Tg	Stdv
Broccoli/corn	T (°C) = 160	C (%) = 7	R (rpm) = 200
			X (wb%) = 14.0	52.43	0.69
			X (wb%) = 16.5	48.19	0.29
			X (wb%) = 19.0	49.18	0.92
Broccoli/corn	T (°C) = 160	X (wb%) = 16.5	R (rpm) = 200
			C (%) = 4.0	49.03	1.03
			C (%) = 7.0	48.19	0.29
			C (%) = 10	50.22	0.14
Broccoli/corn	T (°C) = 160	X (wb%) = 16.5	C (%) = 7.
			R (rpm) = 150	52.76	0.41
			R (rpm) = 200	48.19	0.29
			R (rpm) = 250	50.30	0.14
Broccoli/corn	X (wb%) = 16.5	C (%) = 7	R (rpm) = 200
			T (°C) = 140	52.76	0.22
			T (°C) = 160	48.19	0.29
			T (°C) = 180	50.30	0.17
Olive/corn	T (°C) = 160	C (%) = 6	R (rpm) = 200
			X (wb%) = 14.0	42.15	0.02
			X (wb%) = 16.5	40.79	0.82
			X (wb%) = 19.0	41.21	0.91
Olive/corn	T (°C) = 160	X (wb%) = 16.5	R (rpm) = 200
			C (%) = 4	41.84	0.21
			C (%) = 6	40.79	0.82
			C (%) = 8	42.97	0.01
Olive/corn	T (°C) = 160	X (wb%) = 16.5	C (%) = 6.
			R (rpm) = 150	40.93	0.08
			R (rpm) = 200	40.79	0.82
			R (rpm) = 250	41.58	0.83
Olive/corn	X (wb%) = 16.5	C (%) = 6	R (rpm) = 200
			T (°C) = 140	40.83	0.87
			T (°C) = 160	40.79	0.82
			T (°C) = 180	43.75	0.30

Table 4  Regression analysis of the applied mathematical models for broccoli/corn and olive/corn snacks

Material		Model parameters					Standard deviation
Broccoli/Corn		A0	αX	αC	αT	αR	SR	SE
Properties	σmax	115.860	2.019	−6.025	1.0187	−0.962	193.932	30.328
	ϵmax	0.220	−0.755	1.634	0.4729	−0.378	0.244	0.033
	E	494.445	1.075	−8.606	1.5785	−0.694	708.160	140.160
	N	34.549	0.078	6.036	−1.8234	0.391	39.763	5.120
Olive/Corn		A0	αX	αC	αT	αR	SR	SE
Properties	σmax	296.417	4.532	2.054	−2.595	−0.421	359.756	179.755
	ϵmax	0.494	0.683	−9.322	0.420	−0.373	0.622	0.177
	E	208.225	3.263	−20.930	−4.008	2.400	553.517	88.820
	N	20.908	−2.460	10.397	0.294	−0.865	39.268	7.949

Glass transition temperatures of corn extrudates enriched with broccoli and olive paste are shown in Fig. 3 as a function of samples’ moisture content after the water sorption in the different constant water activities. Glass transition temperature decreased with the increment of moisture content as it was expected, since water acts as a plasticizer.^[39] The knowledge of glass transition temperature is very important because above the T_g, molecular mobility is higher and causes acceleration of the reaction rates while below T_g the food is rigid and stable.^[25] As presented in Table 3, for both types of extruded corn-based snacks, glass transition temperature increased as broccoli or olive paste concentration and temperature increased due to the rise of starch gelatinization, which occurs in higher extruder temperatures, and the increment in fibers content that lead to denser molecules and as a consequence to higher T_g values.^[40,41] A decrement was observed in the highest feed moisture contents, since the increase of water content causes increased segmental mobility of chains in amorphous regions of glassy and partially crystalline polymers.^[25] Glass transition temperature values did not reveal a clear trend with the changes of screw speed. Changes in T_g correlated with the alteration on the structure of the extrudates occurred due to the different conditions applied during extrusion and the raw materials’ characteristics. Thus, T_g values are also related to crispness and denseness of extrudates.^[42]

Figure 3 Glass transition temperatures, T_g (°C), correlated with samples’ moisture content.

Figure 4 Maximum stress for broccoli/corn and olive/corn snacks correlated with: (a) broccoli or olive paste/corn ratio, C (%), (b) feed moisture, X (wb%), (c) screw speed (rpm), and (d) extrusion temperature, T (°C).

Maximum stress, maximum strain, elasticity parameter, and number of cracks during compression have been correlated with feed moisture content, extrusion temperature, screw speed, and broccoli or olive paste concentration. The regression analysis of the applied mathematical models for the two materials is presented in Table 4. The applied mathematical models fit well to the experimental data. Figure 4 present the maximum stress of corn extrudates containing broccoli and olive paste. Maximum stress and strain are indicative of the breakage point during the compression test, while the elasticity parameter is related to the hardness of the extruded snacks. In addition, the number of cracks during compression is a measure of crispness of the material. The compression parameters are controlled by the density and the thickness of the external walls and the swelling.^[43,44] Maximum stress presented an increment for broccoli/corn samples, while broccoli concentration, extrusion temperature, and feed moisture content were increasing. Samples with olive paste exhibit an opposite behavior concerning olive paste concentration and extrusion temperature. Screw speed causes a decrement of maximum stress for both materials. Maximum strain of broccoli/corn extruded snacks increased as extruding temperature increases and decreased with the increment of broccoli concentration, screw speed and feed moisture content. Samples containing olive paste present the same behavior concerning temperature and screw speed and an opposite behavior concerning olive paste concentration and feed moisture content (Fig. 5). Maximum stress usually exhibits the same behavior with elasticity parameter,^[43] as it is obvious at the results except from the alterations in olive/corn samples with different olive concentrations and screw speed values (Fig. 6). The number of cracks during compression for both materials is presented in Fig. 7. Crispness is related with a rapid drop of force during the mastication process. Thus, number of cracks during compression analysis, which are a measure of the rapid drop of force during the test, is a measure of crispness.^[45] The increment of broccoli concentration and extrusion temperature causes a decrement in number of cracks. However, the increment of screw speed and feed moisture content causes an increment in number of cracks of broccoli/corn samples. Number of cracks of olive/corn samples presented in all cases a decrement, except an increment, which was caused from the increment of extrusion temperature.

Figure 5 Maximum strain for broccoli/corn and olive/corn snacks correlated with: (a) broccoli or olive paste/corn ratio, C (%), (b) feed moisture, X (wb%), (c) screw speed (rpm), and (d) extrusion temperature, T (°C).

Figure 6 Elasticity parameter for broccoli/corn and olive/corn snacks correlated with: (a) broccoli or olive paste/corn ratio, C (%), (b) feed moisture, X (wb%), (c) screw speed (rpm), and (d) extrusion temperature, T (°C).

Figure 7 Number of cracks during compression tests for broccoli/corn and olive/corn snacks correlated with: (a) broccoli or olive paste/corn ratio, C (%), (b) feed moisture, X (wb%), (c) screw speed (rpm), and (d) extrusion temperature, T (°C).

The increment in cell wall thickness of extruded snacks as extruder temperature increasing causes an increment in breakage point and elasticity parameter and a decrement in crispness and thus in the number of cracks during compression^[43,44] as it presented in samples enriched with broccoli as extruder temperature was increasing. Moreover, amylopectin plasticization and amylose deterioration that are commonly done by extrusion at 120–220°C affect the density of extruded snacks.^[46] The same behavior was observed for samples enriched with broccoli as broccoli concentration, and feed moisture content were increased. Concerning the feed moisture content, this behavior could be due to water, which acts as a plasticizer of the starch material inside the extruder, increasing raw material moisture content under low extrusion temperature conditions, reducing the viscosity of this mass. Thus, starch gelatinization is reduced and bubble growth is suppressed, resulting in a denser, harder, and less crunchy final product.^[47] Broccoli and olive paste consist, among others, of dietary fibers. Extruded products containing fibers, such as broccoli or olive paste, in high concentrations are usually compact, tough, not crisp, and with undesirable texture due to the low expansion ratio. Fiber particles cause its decrement by rupturing the cell walls before the gas bubbles could expand to their full potential.^[8,48] Moreover, the presence of lipids significantly affects and leads to a reduction of expansion ratio, which affects the total hardness of the samples.^[49] Thus, extruded snacks that contain lipids are more difficult to expand due to mixture slippage within the extruder barrel^[48,50] and consequently their hardness presented an increment as olive paste content increased. Expansion ratio, which affects the snacks’ hardness, is decreased with the increment of screw speed during extrusion^[44,51] and could be correlated with the decrement in breaking point and elasticity parameter and an increment in number of cracks since the samples became less hard and more crispy. The differences between the behavior of samples enriched with broccoli and olive paste concerning the increment in extrusion temperature could be due to the reduction of the viscosity of the materials containing olive paste within the extruder, thus favoring bubble growth in the molten material, consequently increasing extrudates’ expansion and reducing hardness.^[47]

CONCLUSION

Thermal, textural, and physicochemical analysis of extruded snacks based on corn flour enriched with broccoli or olive paste has been performed. Process conditions during the extrusion as well as the raw materials’ characteristics have been proved to affect the measured properties. Simple mathematical models were developed and correlated well the process conditions and the raw materials’ characteristics with the measured properties. In specific, extruded corn snacks enriched with broccoli become harder with the increase of broccoli concentration, extrusion temperature, and feed moisture content, while the increase in screw speed derived in softer snacks. In addition, the increase of feed moisture content and screw speed causes an increment in snacks’ crispness. Extruded corn snacks enriched with olive paste became less hard and crispier as the extrusion temperature increased and harder and less crispy with the increment of olive paste concentration, screw speed, and feed moisture content. Both broccoli/corn and olive/corn samples present an increment of water sorption in higher feed moisture content and lower extrusion temperature while slight differences were presented in the different content concentrations and screw speed values. To conclude within the limitation of this article and concerning the examined properties, the most appropriate product conditions and material characteristics, which produce more soft and crispy extrudates, are extruded snacks based on corn flour enriched with broccoli (broccoli concentration: 4%, extrusion temperature: 160°C, screw speed: 200, feed moisture content: 16.5%) and extruded snacks based on corn flour enriched with olive paste (olive paste concentration: 4%, extrusion temperature: 160°C, screw speed: 150, feed moisture content: 14%).

FUNDING

This work has been performed under the framework of Basic Research Project 2010, funded by the National Technical University of Athens.