Physical Properties of Cooked Wheat Grains as Affected by Cooking Temperature and Duration

Abstract

The physical properties of soft and hard wheat grains, cooked with steam under pressure, were investigated. These included water uptake, rheological properties as observed by modulus of elasticity (E) and maximum compressive contact stress (S_max), color in CIE L*a*b* system and pasting properties by Rapid Visco Analyzer (RVA). Four cooking temperatures (110, 120, 130 and 1408 C) and six cooking durations (20, 40, 60, 80, 100 and 120 min) for each temperature were studied. Water uptake (WU) and rheological properties were analyzed immediately after cooking conditions were achieved. For color and pasting properties, cooked wheat grains were dried at low temperature and ground before analysis. It was found that WU increased proportionally with cooking temperatures and durations. Hard wheat grains tended to absorb more water than soft wheat grains. Both E and S_max decreased rapidly when wheat grains were cooked. Such decreases were found to be more rapid in hard wheat grains. Wheat grains became darker when cooked, resulting in the decrease of L* values and increase of a* and b* values. Cooked wheat grains lost their natural pasting properties compared to uncooked grains, resulting in the decrease of viscosity in their RVA profiles. The experimental data fitted well in second-order polynomial models (p # 0.0001). The built models were sufficiently accurate for most of the studied properties (R² ranging from 0.58–0.97).

Keywords:

• Wheat grain cooking
• Physical properties
• Water uptake
• Rheological properties
• Color
• RVA

Introduction

The cooking of whole wheat grains with steam under pressure is the initial process for some breakfast cereals. Generally, this process is a batch process where grains are cooked in a rotary pressure cooker consisting of a slowly rotating cylindrical vessel. Saturated steam is supplied continuously and is in direct contact with the grains providing extra water, in addition to other ingredients such as added water, salt, sugar and malt extract. Cooking temperatures and durations are the key control for this type of cooking process. Typical cooking temperature (pressure) and duration vary from one plant to another depending on the formulation used. The cooking temperatures range from 120–140° C and cooking durations range from 20–90 min.[1–5] Although, this type of cooking is old, slow and less economical than newer continuous process e.g., extrusion cooking, it is still required for some cereal products in which desired product characteristics cannot be obtained using other cooking techniques.[1]

The cooking step plays an important role in achieving the desired final product characteristics such as flavor, color and texture.[6] Hence, a thorough understanding of the cooking operation is necessary. However, the published data on wheat grain cooking are limited.[4] Most of the published work on wheat grain cooking has focused on water uptake during cooking under various conditions, using different monitoring techniques.[7–10] This study examines the changes of some physical properties of cooked wheat grains as a function of cooking temperature and duration. The properties studied include water uptake, color, rheological and pasting properties. These physical properties have been studied extensively for many years, however, mostly in flour or dough systems, but not in the whole grain system.

Materials and Methods

Wheat Samples and Preparation

Mixed varieties of soft and hard wheat grains, harvested in 2001–2002, were obtained from grain traders in Australia. Their properties are shown in Table 1. Samples (100 g) of sorted whole grains with fairly uniform sizes were placed on a wire mesh and then cooked in a pressure cooker, based on a static autoclave (45 L volume, 413 kPa maximum pressure). Before cooking, the initial moisture content of wheat grains was adjusted to be about 10% (dry basis), by vacuum drying at 40° C until desired moisture content was reached. Wheat samples were cooked immediately or, if needed, kept in desiccator with the saturated salt solution at the desired relative humidity to maintain the moisture content. Four cooking temperatures (110, 120, 130 and 140° C) and six cooking durations (20, 40, 60, 80, 100 and 120 min) for each temperature were studied. At least three replicates were conducted. The nominal cooking duration was defined as the duration for which the desired temperature was reached and held constantly. The total cooking duration was about 10 min longer due to the need to pressurize and vent to ensure saturated steam conditions (about 6 min) and to depressurize (about 4 min).

Table 1 The properties (mean values) of raw wheat used in this study ^a

Properties	Soft wheat samples	Hard wheat samples
Protein (%)	10.5	15.5
Color (L*)	90.1	85.6
Color (a*)	0.91	1.8
Color (b*)	11.1	13.4
Modulus of elasticity (Pa)	964	990
Maximum contact stress (Pa)	14,825	17,528

Once cooking conditions were achieved, cooked wheat grains were removed from the cooker and immediately cooled down using refrigerated air at 0° C to room temperature (about 20° C). Samples at room temperature were used for the measurement of water uptake and rheological properties as well as for further preparation for the measurement of color and pasting properties.

Determination of Water Uptake

Samples were accurately weighted before and after cooking. Percentage water uptake (WU) was calculated based on the weight gained after cooking (dry basis).

Determination of Rheological Properties

Cooked wheat samples re-equilibrated to room temperature were subject to the compression test. Wheat grains are convex in shape so it is not feasible to perform the classic compression test that is suitable for geometrically well-defined and uniform shaped test specimens such as sphere or cube. Previous work on the compression test of grains was conducted in two different ways: (1) machining of test specimens into well-defined shapes[11,12] or (2) applying the compression test to a single grain without machining.[13,14] The latter requires the estimation of rheological properties depending on the shape of test samples. This study followed the latter technique as described in the standard method S368.4,[15] because machining cooked wheat grains was impractical, especially samples exposed to high temperatures and long cooking durations. The texture analyzer (LLOYD TA 500, Lloyd Instruments Ltd, UK) equipped with the parallel plate contact, 100N load cell and speed setting at 0.1 mm/s was used. Samples of cooked wheat grains were placed on the most stable position (crease-side faced down) before testing. Force-deformation curves were obtained, and values of the force and deformation, at the instant when the force-deformation curve ceased to be linear, were used for calculation. The Poisson's ratio for a wheat grain (μ = 0.3) was obtained from the literature.[13] The apparent modulus of elasticity (E) was calculated as:

where F is applied force (N); D is deformation (m); μ is Poisson's ratio (dimensionless); R _min and R _max are the minimum and maximum radii of curvature of the convex body (m); and K is the constant (dimensionless) obtained from the standard.[15] For wheat grains, R _min and R _max may be estimated from the height (H) and the length (L) of a single grain as R _min = H/2 and R _max = (H² + (L²/4))/2H.

The maximum compressive contact stress (S _max), which is the maximum stress occurring at the center of the surface of contact (the first point of contact between the compression plate and the sample), is numerically equal to 1.5 times the average contact pressure and can be calculated from:

where a is the semi-major axis and b is the semi-minor axis (m) of the elliptical contact area. The values a and b are calculated as:

where m, n are the constants obtained from the standard, K ₁ and K ₂ are calculated from the material properties of two surfaces in contact given by K ₁ = (1 – μ ₁ ²)/E ₁ and K ₂ = (1 – μ ₂ ²)/E ₂ where E ₁ and μ₁ are the modulus of elasticity and Poisson's ratio for the test specimen (wheat), respectively, and E ₂ and μ₂ are from the compression plate, which is negligible (assumed to be zero) as E ₂ is much greater than E ₁.

Determination of Color and Pasting Properties

Samples after cooking and equilibration to room temperature were dried in a vacuum oven at 40° C for about 24 hr. This low-temperature drying was used to limit the effects of heat to the samples. Dried samples were ground using a laboratory mill (LM 3100, Perten Instruments, 0.8 mm sieve). Ground-dried-cooked wheat (cooked wheat flour) were analyzed for color and pasting properties. Color was measured by a color meter (Minolta CR-300, Japan) using the CIE L*a*b* system. It should be noted that this study did not take into account the color difference between surface and interior as studied by Horrobin et al.[4] They determined color in both interior and surface as it developed differently during cooking. Surface color determination can be easily measured by color meter (measuring small quantities of wheat kernels packed in the container). However, this cannot be used for interior color as a single wheat kernel is too small to be measured by color meter probe. Therefore, they obtained the microscopic images of sectioned wheat kernels and used for color measurement. This study measured the color as a combination of both surface and interior readings that enables quantitative comparison; in addition, the method is simple and practical. Previous studies have found when wheat grains were cooked, the color changes occurred uniformly.[4,9] The dried and milled technique used in this study was suitable for comparing wheat grains cooked under different conditions.

Pasting properties were determined by the Rapid Visco Analyzer (RVA) (RVA-3D, Newport Scientific, Australia) using standard 1 profile (13 min) as described in the standard method AACC 76–21.[16] Peak Viscosity (PV; maximum viscosity developed during or soon after the heating portion of the test), Hot Paste Viscosity (HPV; minimum viscosity after the peak, occurring around the commencement of cooling period) and Cold Paste Viscosity (CPV; viscosity at the end of the cooling period) were obtained from the RVA profiles. The Breakdown Viscosity (BV) was calculated as the difference between PV and HPV. The Setback Viscosity (SV) was also calculated as the difference between CPV and HPV. Recently, the RVA has become a popular tool for the determination of starch pasting properties.[17–19]

Statistical Analysis

The experimental data were fitted to a second-order polynomial model (Eq. 5). Statistical analysis was conducted using JMP version 5 (SAS Institute Inc., US).

where b ₀, b ₁, b ₂, b ₁₁, b ₁₂, b ₂₂ are the coefficients, ϵ is the error, cooking temperature (°C) and cooking duration (min) are denoted by x ₁ and x ₂ respectively.

Results and Discussion

Water Uptake

Fig. 1 shows the WU of both soft and hard wheat grains cooked under the studied conditions. WU increased rapidly at the beginning and slowly increased further when cooking for longer durations. Such increases are in proportion with cooking temperature and duration. When cooking at high temperatures for long durations (130° C for 100 min and 140° C for 60 min), WU started to decrease, as a consequence from the destruction of grain structure at these temperatures. When cooking for curtain durations (see Fig. 1), the mass of water absorbed was greater than the original mass of dry wheat used. Similar results were previously reported by Stapley et al.[5] Hard wheat grains absorbed more water than soft wheat grains due to the higher protein content. In flour or a dough system, it is well known that they absorb water more than their original weights due to some constituents e.g., protein (114–215%) damaged starch (200–430%) and pentosan (500–1,500%).[20–22] In this study, the results for whole grain system, which has a small amount of damaged starch, are similar to the results obtained for flour or dough systems which have much more damaged starch.

Figure 1 Percentage water uptake of cooked wheat grains, (a) soft wheat (b) hard wheat.

Figure 2 Modulus of elasticity (E) and maximum compressive contact stress (S _max) of cooked wheat grains, (a-b) soft wheat (c-d) hard wheat.

Rheological Properties

The E and S _max values, obtained from the force-deformation curves, of cooked wheat grains are presented in Fig. 2. It is clear that values decreased when wheat grains were cooked because wheat grains absorbed water and underwent associated physical changes. Bargale et al.[13] also reported the E and S _max values decreased when the moisture content of grains increased. Both soft and hard wheat grains showed the rapid decrease at the beginning of cooking period. In cooked wheat, both E and S _max values of hard wheat grains are generally lower than the corresponding values from soft wheat grains due to the rapid water absorption of hard wheat grains when cooked.

Color

It is well known that wheat grains become dark when cooked; the longer the cooking duration, the darker the grains. Grains exposed to higher temperature become darker than those exposed to lower temperature. Color changes in wheat grains exposed to heat are due to the Maillard reaction.[23] The results clearly showed that L* values decreased and a* values increased (Fig. 3).

Figure 3 Color (CIE L a* b*) of cooked wheat grains, (a-c) soft wheat (d-f) hard wheat.

Pasting Properties

RVA profiles from soft wheat cooked under different conditions are shown in Fig. 4. It can be seen from the RVA profiles that wheat grains lose their natural pasting properties when cooked. Considering PV, the value decreased as the cooking proceeded. The RVA profiles became almost straight lines in which there were no difference between PV, HPV and CPV when wheat grains were cooked for long durations e.g., starting from 80 min at 130° C, because all the starch content has been completely gelatinized. Thermal treatment of starches as occurs in the cooking process results in the changes of RVA profiles. These changes include the absence of gelatinization peak and a steady decrease in viscosity during the heating process and low setback during cooling in the RVA profile.[24]

Figure 4 RVA profiles of soft wheat samples cooked at different cooking temperatures for 20–120 min, (a) 110°C, (b) 120°C, (c) 130°C and (d) 140°C, including RVA profiles from uncooked wheat samples.

Statistical Analysis

The coefficients of the fitted models (reference Eq. (5)) and summary of fit (R ² and Root Mean Square Error (RMSE)) are given in Table 2. The analysis of variance (raw data are not shown here) suggested that the experimental data fitted well with the built models (p ≤ 0.0001 for all models). The values of R ² for most of the models, ranging from 0.58–0.97, indicated that they were sufficiently accurate for predicting the dependent variables (physical properties) from any combination of independent variables (cooking temperatures and durations) within the ranges studied. It appears that cooking temperatures and durations highly affect the physical properties of cooked wheat grains as most of the estimated coefficients in the models are significant (p ≤ 0.05). Notably that some of the estimated coefficients are not significant at this confidential level (see Table 2).

Table 2 Coefficients and summary of fit for the built models of all studied parameters ^a

Coefficients	WU	E	S max	L*	a*	b*	PV	HPV	CPV	BV	SV
Soft wheat
b 0	47.4	398.6	9,502	151.9	−17.36	−9.60	2,154	2,060	4,295	94.6	2,235
b 1	0.23 ^b	−2.31	−56.8	−0.55	0.15	0.23	−12.83	−12.38	−25.88	−0.45	−13.50
b 2	0.61	−0.80	−22.2	−0.14	0.03	0.03	−4.54	−4.10	−8.76	−0.44	−4.66
b 11	−0.12	0.05 ^b	1.04 ^b	−0.01	0.003	−0.00001 ^b	−0.25	−0.33	−0.63	0.08	−0.31
b 12	−0.03	0.06	1.33	−0.005	0.001	−0.0002 ^b	0.12	0.12	0.27	0.01 ^b	0.15
b 22	−0.01	0.01	0.35	0.0002 ^b	−0.0001	−0.0003	0.07	0.06	0.14	0.01	0.08
R^2	0.90	0.71	0.81	0.96	0.96	0.88	0.92	0.91	0.93	0.83	0.93
RMSE	9.40	30.8	587.2	1.76	0.43	1.08	70.98	73.18	132.9	8.99	68.52
Hard wheat
b 0	94.83	290.7	7,023	164.9	−21.40	0.06 ^b	1,173	1,070	2,272	103.4	1,203
b 1	0.01 ^b	−1.57	−41.21	−0.72	0.20	0.18	−7.21	−6.62	−14.08	−0.58	−7.46
b 2	0.56	−0.69	−16.32	−0.14	0.04	0.02	−1.86	−1.59	−3.48	−0.26	−1.88
b 11	−0.18	0.06	1.17	−0.004	0.001 ^b	−0.003 ^b	−0.04 ^b	−0.04 ^b	−0.19 ^b	−0.003 ^b	−0.15 ^b
b 12	−0.04	0.02	0.72	−0.004	0.001	−0.0002 ^b	0.01 ^b	0.01 ^b	0.01 ^b	0.004 ^b	0.001 ^b
b 22	−0.01	0.01	0.20	0.001	−0.0001 ^b	−0.0001 ^b	0.01 ^b	0.004 ^b	0.03	0.003	0.03
R^2	0.94	0.69	0.82	0.97	0.92	0.58	0.86	0.84	0.84	0.73	0.80
RMSE	7.02	22.4	384.3	1.67	0.79	1.94	43.73	42.89	94.76	7.58	59.15

Special considerations should be made on some physical properties e.g., WU, rheological properties and some RVA parameters (PV, HPV, CPV) for soft wheat as most of the estimated coefficients are significant (p ≤ 0.05). Therefore a measurement of these physical properties is useful for monitoring of the cooking process. On the other hand, the estimated coefficients of some independent factors in some properties e.g., color (particularly in a* and b* values) and BV for soft wheat, are close to zero (may be assumed as zero) and/or not significant (p > 0.05). These physical properties are not suitable for monitoring of the cooking process. However, the lack of effect (not significant) was found only in the combination of independent factors e.g., in the coefficients for b ₁₁, b ₁₂, or b ₂₂ not in the main factor alone e.g., in the coefficients for b ₁ or b ₂. Moreover, it appears that the estimated coefficients are more significant (p ≤ 0.05) in soft wheat compared to hard wheat which suggests that the models are more applicable to soft wheat samples (see Table 2).

Conclusion

The cooking of wheat grains with steam under pressure is an important process for the manufacturing of some cereal products as it plays an important role in achieving desirable end product characteristics. Cooking temperatures and durations significantly affect the physical properties of cooked wheat grains as investigated by water uptake, rheological property parameters (E and S _max), color in CIE L*a*b* system, and pasting properties (RVA). They induced changes in those properties such as increasing of water uptake, decreasing of some rheological characteristics, grains becoming darker resulting in lower L* values as well as losses of natural pasting properties. In this study, the experimental data fitted well in second-order polynomial models. Most of the models built were acceptable in terms of their robustness and applicability. The information on changes of the physical properties of cooked wheat grains provides useful information about the cooking process and may help in the control of process by means of cooking temperatures and durations.

Notes

^aAll properties were examined using the standard methods[16] or methods as described in “Materials and Methods.”

^a p ≤ 0.0001 for all models. All of the estimated coefficients are significant at p ≤ 0.05 unless specified otherwise.

^bCoefficients are not significant (p > 0.05).