Effects of Extrusion Process Variables on Quality Properties of Wheat-Ginseng Extrudates

Abstract

The present study investigated the effects of extrusion process variables (feed moisture, screw speed, and barrel temperature) on the physical [expansion ratio, water absorption index (WAI), and water solubility index (WSI)], pasting, and thermal properties of wheat-ginseng extrudates (WGE). A wheat flour-ginseng powder (GP) blend (10% GP, w/w) was extruded in a twin-screw extruder (L/D ratio of 25:1) with full factorial combinations of feed moisture (25, 30, and 35%), screw speed (200 and 300 rpm), and zone 5 barrel temperature (110, 120, 130, and 140°C). The expansion ratios of WGE were significantly increased with decreasing feed moisture, decreasing screw speed, and increasing barrel temperature. Increasing feed moisture significantly increased WAI values of WGE and significantly decreased WSI values of WGE. However, an increase in either screw speed or barrel temperature caused a significant decrease in WAI values of WGE and a significant increase in WSI values of WGE. Rapid visco analyzer peak viscosity values of WGE were significantly affected by changes in extrusion process variables studied, indicating that the degree of starch degradation and/or gelatinization in WGE is a very important factor associated with their peak viscosity. WAI values of WGE were positively correlated (r = 0.88, p ≤ 0.001) with peak viscosity values of WGE samples, whereas WSI values of WGE samples were negatively correlated (r = 0.82, p ≤ 0.001). Increasing feed moisture resulted in an increase in values of transition peak temperature (T_p) of WGE, whereas increasing screw speed and barrel temperature each led to a decrease in T_p values of WGE, determined by differential scanning calorimetry.

Keywords:

• Wheat flour
• Ginseng powder
• Extrusion processing
• Pasting properties
• Thermal properties

INTRODUCTION

The quality properties of final extruded products are considerably affected by extrusion process variables, such as feed rate, feed moisture, screw configuration, screw speed, diameter of exit die, barrel temperature, and the addition of other materials.[1–6] These variables can be optimized during the extrusion process to improve the quality properties of final extruded products, including physical (expansion ratio, color, bulk density, water absorption index [WAI], and water solubility index [WSI]), pasting, and thermal characteristics.[7–11]

It has been reported that texture is one of the most important factors in the quality properties of extruded snack products, and that texture is associated with an extrudate's expansion ratio.[12] Ryu and Ng[3] observed that the expansion ratio of wheat flour (WF) extrudates or cornmeal extrudates significantly increased with decreasing feed moisture, and Hagenimana et al.[6] reported that rice flour extrudates extruded at higher feed moisture levels are harder after cooling than those with lower feed moistures.

Pasting properties of extrudates have been studied using a Rapid Visco Analyzer (RVA). Ozcan and Jackson[11] reported that extruded corn starch shows lower RVA viscosity profiles than raw corn starch because of starch degradation during extrusion cooking. Bhattacharya et al.[13] evaluated the pasting properties of a potato and WF blend extruded in a twin-screw extruder at different moisture contents (16–21%) and screw speeds (200–400 rpm). They noted that the degree of starch gelatinization that occurred during extrusion cooking affected the viscosity of the extrudates.

Differential scanning calorimetry (DSC) has been applied to investigate the thermal properties of extruded corn starch,[9,11,14] corn flour,[7] and WF.[1] Anderson and Ng[1] investigated the transition peak temperature (T_p ), and transition enthalpy (ΔH) of non-extruded and extruded WF samples and they reported that the value of T_p is particularly associated with the heat stability of the non-extruded and extruded samples.

Ginseng is one of the most widely used herbal medicines in the world, and is known to exhibit various pharmaceutical effects, including antioxidation, antistress, immunostimulation, and anticancer effects.[15–18] In recent years, many consumers have become interested in nutraceutical foods. Ginseng, as a health supplement, has the potential to broaden its consumer base. In the world today, ginseng is usually consumed in the forms of raw plant material, dried plant material, extracts, and commercial products, such as capsules, tablets, drinks, and jellies. The addition of ginseng to common cereal flours (wheat, corn, and rice) for producing nutraceutical extruded products (snacks or instant powder) could be of interest to consumers.

The extrusion process is fundamentally extremely complicated due to its high degree of process variability, thus the product quality of any final extrudates can be considerably influenced by even small changes in extrusion processing conditions. However, no studies have been found evaluating the effects of extrusion process variables on the product quality of final extruded nutraceutical WF products containing ginseng root powder. Therefore, the present study was designed to investigate the quality properties [physical (expansion ratio, WAI, and WSI), pasting, and thermal characteristics] of wheat-ginseng extrudates (WGE) produced with a twin-screw extruder at different extrusion process conditions.

MATERIALS AND METHODS

Materials

Soft WF was obtained from the Mennel Milling Co. (Fostoria, OH, USA). The raw ginseng root (Panax ginseng C. A. Meyer) was cultivated in Geumsan, Korea, harvested after four years' growth, and purchased at a Korean ginseng market. The raw ginseng root was milled into powder (particle size of ≤167 μm) using a coffee grinder. WF and ginseng powder (GP) were analyzed for their moisture contents using AACCI Approved Method 44-15A,[19] and the moisture contents of WF and GP were 11.2% (wb) and 8.3% (wb), respectively.

Extrusion Cooking

Extrusion cooking and the 24 different extrusion process conditions studied were described in our previous study[20] and are listed in Table 1.

Table 1 Effects of extrusion process variables on the expansion ratio (ER), water absorption index (WAI), and water solubility index (WSI) of wheat-ginseng extrudates

Extrusion process variables
Feed moisture (%, wb)	Screw speed (rpm)	Temperature ^a (°C)	ER	WAI (g/g)	WSI (g/g)
25	200	110	2.23e	8.04cd	0.09ijk
30	200	110	2.11f	8.47b	0.08kl
35	200	110	1.99g	8.73a	0.06m
25	300	110	1.92ij	7.28gh	0.13e
30	300	110	1.89j	7.67f	0.12g
35	300	110	1.85k	7.86de	0.09ijk
25	200	120	2.45c	7.31gh	0.11gh
30	200	120	2.25e	7.92d	0.09ijk
35	200	120	1.97gh	8.21c	0.08kl
25	300	120	1.99g	6.91i	0.19c
30	300	120	1.99g	7.17h	0.13ef
35	300	120	1.94hi	7.38g	0.10hi
25	200	130	2.51b	6.70jk	0.12g
30	200	130	2.24e	7.31gh	0.09ij
35	200	130	1.98g	7.92d	0.07lm
25	300	130	1.98g	6.53k	0.23b
30	300	130	1.97gh	7.22gh	0.18d
35	300	130	1.97gh	7.21gh	0.11g
25	200	140	2.81a	6.70jk	0.12fg
30	200	140	2.30d	7.28gh	0.09jkl
35	200	140	2.13f	7.68ef	0.07lm
25	300	140	1.99g	6.56k	0.26a
30	300	140	1.99g	6.81ij	0.19cd
35	300	140	1.96gh	7.14h	0.13ef
Values with different letters within the same column differ significantly (P < 0.05).
^aTemperature: zone 5 (nearest exit die) barrel temperature.

Specific Mechanical Energy

The specific mechanical energy (SME) in Watt hours/kg was calculated according to the equation of Brent et al.[21]:

Expansion Ratio

After drying WGE samples extruded under different conditions in an air oven (45°C) overnight, a digital caliper (Model CD-6″CS; Mitutoyo Corp., Kawasaki, Japan) was used to measure their diameters. To calculate the expansion ratio, the average measurement of 10 WGE diameters was divided by the die diameter of 3 mm.

Water Absorption Index and Water Solubility Index

WAI and WSI of WGE samples extruded under different conditions were measured according to the procedure of Jin et al.[22] with some modifications. Two g of ground WGE samples were combined with 20 mL of distilled water in 30 mL round-bottom centrifuge tubes. The tubes were allowed to sit for 10 min and were inverted three times each at 5 and 10 min. After 10 min, the suspensions were centrifuged (Model J2-21M; Beckman Instruments Inc., Fullerton, CA, USA) at 1000 × g for 25 min. The supernatant was collected and WAI was calculated as:

WSI was measured by drying the supernatant in an air oven overnight at 60°C. WSI was calculated as:

Pasting Properties

Pasting properties (values of peak, trough, breakdown, final viscosity, and setback) of a non-extruded WF-ginseng powder (GP) blend (3.15 g WF + 0.35 g GP) and ground WGE samples extruded under different conditions were analyzed. Pasting properties were determined using a RVA (Model 4; Newport Scientific Inc., Warriewood, Australia). For each analysis, 3.5 g of a sample (14% wb) were mixed with 25 mL of distilled water. The profile for analysis was Standard Method 1 according to AACCI Approved Method 76-21,[19] and data from the RVA were processed by Thermocline version 1.2 software (Newport Scientific Inc., Warriewood, Australia).

Thermal Properties

Thermal properties of a non-extruded WF-GP blend (9 mg WF + 1 mg GP) and ground WGE samples extruded under different conditions were analyzed using a DSC (Model TA 2910; TA Instruments, Newcastle, DE, USA). Ten mg of a sample were weighed into a DSC aluminum pan (Model 0319-1525; PerkinElmer Inc., Shelton, CT, USA) and 20 μL of distilled water were added using a micro-syringe. The sample pan was hermetically sealed and allowed to equilibrate overnight at room temperature. Samples were heated from 30 to 200°C at the heating rate of 10°C/min. A sealed empty pan was used as a reference. T_p and ΔH were recorded and analyzed using TA Universal Analysis Software (version 3.6; TA Instruments, Newcastle, DE, USA).

Statistical Analysis

All statistical analyses were performed using SAS version 9.1 (SAS Institute Inc., Cary, NC, USA). Analysis of variance (ANOVA) was performed using the general linear models (GLM) procedure to determine significant differences among the samples. Means were compared by using Fisher's least significant difference (LSD) procedure. Significance was defined at the 5% level. In addition, Pearson correlation coefficients were determined among the samples. The CORR procedure was used to obtain correlation coefficients.

RESULTS AND DISCUSSION

Expansion Ratio

Increasing feed moisture from 25 to 35%, at constant screw speed and zone 5 barrel temperature (200 rpm and 140°C, respectively), caused a significant decrease in the expansion ratios of WGE samples from 2.81 to 2.13 (Table 1). The trend of decreasing expansion ratio of extrudates with increasing feed moisture was also noted by Ryu and Ng,[3] and Ding et al.[5,10] Ding et al.[10] observed that increasing feed moisture in the extrusion cooking of rice flour induced a significant decrease in the expansion ratio of the rice-based extrudates. Ryu and Ng[3] noted that higher feed moisture for the extrusion cooking of WF or whole cornmeal decreased the expansion ratios of those extrudates. In general, the expansion occurring in feed materials is mainly associated with a great difference in pressure between the exit die and atmosphere. The viscosity of the melt is higher during the lower feed moisture extrusion cooking, as compared to that during the higher feed moisture extrusion cooking. Therefore, the pressure differential would be greater for lower moisture feeds, resulting in a more expanded final product.[23] According to Chinnaswamy and Hanna,[24] the lower feed moisture during extrusion cooking can confine the flow of the melt and, therefore, increase residence time, thereby enhancing the degree of gelatinization and expansion. In studies conducted by Ding et al.,[5] feed moisture was found to be the main factor affecting the expansion ratio of WF extrudates. They reported that the expansion ratio of extrudates is attributed to the elasticity properties of the melt. They also reported that the elasticity of the melt at higher feed moisture extrusion conditions decreases due to plasticization of the melt, thus the SME is reduced and starch gelatinization is also considerably reduced, subsequently causing a decrease in the expansion ratio of the extrudates.

An increase in screw speed from 200 to 300 rpm, at constant feed moisture and zone 5 barrel temperature (25% and 140°C, respectively), led to a significant reduction in the expansion ratios of WGE samples from 2.81 to 1.99 (Table 1). The trend of decreasing expansion ratio at higher screw speed extrusion conditions has also been reported by Anderson and Ng[2] who observed that increasing screw speed in the extrusion cooking of WF caused a significant decrease in the expansion ratio of the extrudates. A change in screw speed can affect residence time, the network of starch and proteins, and the melt viscosity, thereby affecting the expansion ratio of extrudates.[25] Blanche and Sun[14] reported that melt viscosity decreased with increasing screw speed from 300 to 400 rpm during extrusion cooking of corn starch, causing a decrease in die pressure, subsequently lessening expansion ratio of the extrudates.

On the other hand, the expansion ratios of WGE samples significantly increased from 2.23 to 2.81 with an increasing zone 5 barrel temperature from 110 to 140°C, at constant feed moisture and screw speed (25% and 200 rpm, respectively, Table 1). This finding was consistent with Ding et al.[10] who reported that higher barrel temperatures led to increased expansion ratios of rice flour extrudates. In the present study, the increasingly higher barrel temperature extrusion cooking could have caused increasingly greater flashing-off of moisture upon die exit, inducing an increase in the expansion ratio of WGE samples.

Water Absorption Index and Water Solubility Index

WAI values of WGE samples produced under different extrusion process conditions ranged from 6.53 to 8.73 g/g, and WSI values of WGE samples ranged from 0.06 to 0.26 g/g (Table 1). The WAI of extrudates measures the amount of water held by the starch after dispersion of starch in excess water and may be related to the degree of starch damage due to gelatinization and fragmentation of starch during high temperature and shear extrusion cooking.[2] The WSI of extrudates measures the amount of soluble components released from starch upon extrusion and is considered an indicator of degradation of molecular components.[26]

In the present study, increasing feed moisture from 25 to 35%, at constant screw speed and zone 5 barrel temperature (200 rpm and 140°C, respectively), significantly increased WAI values of WGE samples from 6.70 to 7.68 g/g, whereas it significantly decreased WSI values of WGE samples from 0.12 to 0.07 g/g (Table 1). Higher WAI values and lower WSI values of WGE samples with increasing feed moisture may be associated with a decrease in the degradation of starch granules during the higher feed moisture extrusion cooking. During extrusion cooking at higher feed moisture levels, the excess water acts as a plasticizer, decreasing the degree of degradation of starch granules, leading to an increase in WAI values of WGE samples and a decrease in WSI values of WGE samples.[6]

Secondly, an increase in screw speed from 200 to 300 rpm, at constant feed moisture and zone 5 barrel temperature (25% and 110°C, respectively), resulted in a significant decrease in WAI values of WGE samples from 8.04 to 7.28 g/g, but caused a significant increase in WSI values of WGE samples from 0.09 to 0.13 g/g (Table 1). Anderson and Ng[2] noted that WAI values of WF extrudates are significantly decreased with increasing screw speed, because increasing screw speed leads to more damaged polymer chains, thereby decreasing the ability of starch molecules to bind more water molecules. This is also in agreement with Jin et al.[22] who reported that higher screw speeds resulted in lower WAI values of corn meal extrudates using a twin screw extruder.

Finally, WAI values of WGE samples were significantly reduced from 8.04 to 6.70 g/g, and WSI values of WGE samples were significantly increased from 0.09 to 0.12 g/g when zone 5 barrel temperature was increased from 110 to 140°C, at a constant feed moisture and screw speed (25% and 200 rpm, respectively, Table 1). Ding et al.[5] also observed the same result in the extrusion cooking of WF extrudates. They reported that lower WAI values and higher WSI values of the extrudates are the result of a higher degree of starch gelatinization occurring during higher temperature extrusion cooking. It is suggested in the present study that extrusion cooking under higher zone 5 barrel temperature conditions could increase the degree of wheat starch gelatinization, which causes lower WAI values and higher WSI values of WGE samples. Furthermore, it is plausible that at higher barrel temperatures, the combination of thermal and mechanical energies could have fully cooked the starch, resulting in an increased degree of starch gelatinization and/or degradation.

Pasting Properties

During the initial 4 min, RVA viscosity of the WGE sample was higher than that of WF-GP (Fig. 1), indicating that the gelatinized starch could hydrate faster than unmodified starch in the non-extruded blend. However, RVA viscosity of WGE samples remained low throughout the heating cycle (50–95°C), in contrast to that of WF-GP, which had substantial increases. The result can be related to the degradation and/or gelatinization of starch granules that occurred in WGE samples during extrusion cooking. This reduced the swelling capacity of amylose and amylopectin in extruded starches.

Figure 1 Rapid Visco Analyzer pasting curves of a non-extruded wheat flour (WF)-ginseng powder (GP) blend (10% GP, w/w), and a wheat-ginseng extrudate (WGE) extruded at 30% feed moisture, 200 rpm screw speed, and 110°C zone 5 barrel temperature. Viscosity reported in rapid visco units (RVU).

RVA peak viscosity values of ground WGE samples produced under different extrusion process conditions ranged from 36.3 to 206.1 Rapid Visco Units (RVU) (Table 2). Increasing feed moisture from 25 to 35%, at a constant screw speed and zone 5 barrel temperature (200 rpm and 120°C, respectively), significantly increased the peak viscosity values of WGE samples from 113.9 to 176.5 RVU. However, peak viscosity values of WGE samples were significantly decreased from 78.2 to 36.3 RVU with increasing screw speed from 200 to 300 rpm, at a constant feed moisture and zone 5 barrel temperature (25% and 140°C, respectively). Increasing zone 5 barrel temperature from 110 to 140°C, at constant feed moisture and screw speed (25% and 200 rpm, respectively), significantly decreased peak viscosity values of WGE samples from 118.9 to 78.2 RVU. The effects of extrusion process variables on peak viscosity values of WGE samples were generally consistent with other previous published results.[8,14]

Table 2 Effects of extrusion process variables on peak viscosity (PV), transition peak temperature (T _p ), and transition enthalpy (ΔH) of wheat-ginseng extrudates

Extrusion process variables
Feed moisture (%, wb)	Screw speed (rpm)	Temperature ^a (°C)	PV (RVU ^b )	Tp (°C)	ΔH (J/g)
25	200	110	118.9ef	167.8cd	2.65ijk
30	200	110	138.0d	170.3b	3.70fgh
35	200	110	206.1a	172.3a	1.75jk
25	300	110	56.3kl	158kl	6.00ab
30	300	110	62.8k	163.4ef	5.04cde
35	300	110	111.8fgh	164.0e	2.72ij
25	200	120	113.9efg	166.1d	3.94fgh
30	200	120	121.3e	167.9cd	4.56def
35	200	120	176.5b	168.7bc	3.43ghij
25	300	120	44.7mn	160.5hij	3.98fgh
30	300	120	55.6kl	161.8fghi	5.89abc
35	300	120	97.1i	162.3efgh	3.90fgh
25	200	130	112.5fg	162.4efg	3.76fgh
30	200	130	110.1gh	162.9efg	3.78fgh
35	200	130	159.4c	163.9e	3.81fgh
25	300	130	38.3no	156.2m	2.57jk
30	300	130	55.9kl	159.9j	5.39bcd
35	300	130	83.7j	160.2ij	3.53ghi
25	200	140	78.2j	161.5ghij	5.41bcd
30	200	140	103.9hi	162.6efg	3.49ghij
35	200	140	138.4d	163.6ef	4.37efg
25	300	140	36.3o	153.7n	3.42hij
30	300	140	49.0lm	156.2lm	6.60a
35	300	140	62.8k	159.7jk	4.55def
Values with different letters within the same column differ significantly (P < 0.05).
^aTemperature: zone 5 (nearest exit die) barrel temperature.
^bRVU: Rapid Visco Units.

In the present study, it is speculated that a decrease in the peak viscosity values of WGE samples resulted from an increase in severity of extrusion process conditions. It is possible that extrusion cooking at the higher screw speed led to more starch degradation, subsequently producing WGE samples with much lower pasting peak viscosity values. With increasing barrel temperature, the starches were likely gelatinized to a greater extent, resulting in reduced peak viscosity values of the ground WGE samples. Blanche and Sun[14] observed that higher feed moisture and lower screw speed each caused an increase in the peak viscosity values of corn starch extrudates. They reported that the increased peak viscosity of the corn starch extrudates is due to the presence of starch granules remaining ungelatinized upon extrusion cooking. According to Guha et al.,[8] on the other hand, who showed that the peak viscosity values of rice flour extrudates decreased with increasing barrel temperature and screw speed, the degree of breakdown of a starch granule is dependent on the type of starch, mechanical shear, temperature, and chemical agents present during the gelatinization of starch.

Thermal Properties

A gelatinization peak was observed on the DSC thermogram obtained for WF-GP (Fig. 2), and T_p and ΔH values (Table 2) of WF-GP were 69.4°C and 3.36 J/g, respectively. However, WGE samples did not exhibit a gelatinization peak on the DSC thermogram. T_p and ΔH values of WGE samples extruded under different conditions were observed in the range from 153.7 to 172.3°C and in the range from 1.75 to 6.60 J/g, respectively.

Figure 2 Differential scanning thermograms of a wheat-ginseng extrudate (WGE) extruded at 30% feed moisture, 300 rpm screw speed, and 140°C zone 5 barrel temperature, and a non-extruded wheat flour (WF) and ginseng powder (GP) blend (10% GP, w/w).

Ozcan and Jackson[11] suggested that a gelatinization peak was not found on the DSC thermogram obtained for corn starch extrudates, since the starch granules were already melted and gelatinized during extrusion cooking. Blanche and Sun[14] noted that high shear and pressure extrusion cooking can produce damaged and depolymerized starch molecules, so that no gelatinization peak is found on the DSC thermograms of these corn starch extrudates. Therefore, it seems consistent in the present study that no gelatinization peak was found on the DSC thermograms for the ground WGE samples due to the gelatinization and/or breakdown of starch granules during extrusion cooking.

In the present study, increasing feed moisture from 25 to 35%, at constant screw speed and zone 5 barrel temperature (300 rpm and 140°C, respectively), resulted in a significant increase in T_p values of WGE samples from 153.7 to 159.7°C (Table 2). On the other hand, T_p values of WGE samples were significantly decreased from 161.5 to 153.7°C with increasing screw speed from 200 to 300 rpm, at constant feed moisture and zone 5 barrel temperature (25% and 140°C, respectively). Increasing zone 5 barrel temperature from 110 to 140°C, at constant feed moisture and screw speed (30% and 200 rpm, respectively), significantly reduced T_p values of WGE samples from 170.3 to 162.6°C. Anderson and Ng[1] evaluated the effects of twin-screw extrusion processing on the thermal properties of WF-based extrudates. They observed that increasing screw speed from 240 to 320 rpm, and die temperature from 120 to 160°C, each decreased T_p values of WF extrudates, noting that extrusion cooking under lower screw speed or lower die temperature improved heat stability of the extrudates. In general, heat stability of an extrudate describes the resistance to breakdown of molecular aggregate structures formed during extrusion cooking by gelatinized starch and/or denatured proteins. It was hypothesized in the present study that higher T_p values of WGE samples would be associated with a higher melting temperature of WGE samples, subsequently increasing the heat stability of WGE samples. Based on the results obtained in the present study, it is suggested that lower feed moisture, higher screw speed, and higher barrel temperature extrusion conditions could each diminish the heat stability of WGE samples.

Correlations among Extrudate Quality Properties and System Variables

Product temperature represents the actual temperature of the dough at the exit die during extrusion cooking. For the WGE samples studied, product temperature was found to be negatively associated (r = −0.71, p ≤ 0.001) with WAI values and positively associated (r = 0.45, p ≤ 0.05) with WSI values (Table 3). SME values were negatively related (r = −0.45, p ≤ 0.05) to WAI values and negatively related (r = −0.79, p ≤ 0.001) to peak viscosity values of WGE samples. In contrast, SME values were positively related (r = 0.69, p ≤ 0.01) to WSI values of WGE samples (Table 3). Tanhehco and Ng[27] also found a negative relationship between SME values and WAI values of ground WF extrudates as well as a positive correlation between SME values and WSI values of ground WF extrudates. They attributed their findings to the fact that SME values reflect degree of starch fragmentation.

Table 3 Pearson's coefficients of correlation (r) among quality properties [expansion ratio (ER), water absorption index (WAI), water solubility index (WSI), peak viscosity (PV), transition peak temperature (T _p ), and transition enthalpy (ΔH)] of wheat-ginseng extrudates and system variables [specific mechanical energy (SME) and product temperature (PT)]

ER	WAI	WSI	PV	Tp	ΔH	SME	PT
ER	−0.22	−0.18	0.13	0.17	0.05	−0.63***	0.27
WAI		−0.76***	0.82***	0.88***	−0.38	−0.45*	−0.71***
WSI			−0.82***	−0.81***	0.23	0.69**	0.47*
PV				0.86***	−0.52**	−0.79***	−0.39
Tp					−0.42*	−0.67**	−0.67**
ΔH						0.31	0.24
SME							−0.02
PT
*, **, ***Significantly different at the 5%, 1%, and 0.1% levels, respectively.

As expected, WAI values of WGE samples were negatively correlated (r = −0.76, p ≤ 0.001) with the respective WSI values (Table 3). WAI values were positively correlated (r = 0.82, p ≤ 0.001) with peak viscosity values, whereas WSI values were negatively correlated (r = −0.82, p ≤ 0.001) with peak viscosity values for the WGE samples in the present study. The positive relationship between WAI and peak viscosity of the extrudates can probably be explained by the fact that both WAI and peak viscosity values describe the degree of starch degradation and/or gelatinization occurring during extrusion cooking.[2,8] For example, increasing barrel temperature increased the starch gelatinization during extrusion cooking, subsequently leading to a reduction in both WAI and peak viscosity values of WGE samples.[5,8]

T_p values of WGE samples were positively correlated (r = 0.88, p ≤ 0.001) with their respective WAI values as well as positively correlated (r = 0.86, p ≤ 0.001) with peak viscosity values. Based on these results, it is suggested that heat stability of WGE samples could be increased if the degree of starch gelatinization and/or degradation were decreased during extrusion cooking.

CONCLUSIONS

Effects of extrusion process variables (feed moisture, screw speed, and barrel temperature) on the quality properties (physical, pasting, and thermal) of WGE samples produced under different extrusion conditions were investigated. During extrusion cooking, the melt viscosity and the degree of gelatinization and/or degradation could influence the final quality of WGE samples. The data on quality properties obtained in the present study also suggested that the expansion ratio, WAI, WSI, peak viscosity, and T_p of WGE samples were affected by each of the extrusion process variables tested in the present study. Data obtained from the present extrusion study may be helpful in predicting the expected performance of extruded materials in investigations into the potential use of WF mixed with ginseng, or with other nutraceutical materials, for the improvement of nutritional quality of final extruded products.

ABBREVIATIONS

DSC	=	Differential scanning calorimetry
GP	=	Ginseng powder
ΔH	=	Transition enthalpy
RVA	=	Rapid Visco Analyzer
SME	=	Specific mechanical energy
Tp	=	Transition peak temperature
WF	=	Wheat flour
WAI	=	Water absorption index
WGE	=	Wheat-ginseng extrudates
WSI	=	Water solubility index

ACKNOWLEDGMENTS

This research was partially supported by the Michigan Agricultural Experiment Station. Soft wheat flour provided by Mennel Milling Company (Fostoria, OH, USA) was greatly appreciated.