Physicochemical Characteristics of Starch Component of Wheat Flours Obtained from Fourteen Iranian Wheat Cultivars

Abstract

In this study, the differences in the physicochemical properties of starch component of fourteen Iranian wheat cultivars (Triticum aestivum L.) in terms of thermal, pasting and gel properties were studied. Positive correlations between the apparent amylose content and the gelatinization temperature (r = 0.8) and also the gel strength (r = 0.7) of different samples were established. There was no correlation between the amylose content and pasting properties. Moreover a positive relationship between the peak and final viscosities of different wheat flours (r = 0.8) was found. It was concluded that well selection of wheat cultivars is critical for production of high quality products. For this reason, all factors influencing the quality of wheat or flour should be studied carefully. Commonly yield, weight, protein content and quality have been mentioned as quality factors, however as this study indicated, the physicochemical properties of starch should also be considered as a quality criterion.

Keywords:

• Iranian wheat cultivars
• Wheat starch
• Thermal properties
• Pasting properties
• Gelling properties

INTRODUCTION

Wheat is the leading cereal crop in the world, consumed as food, and feed in many different ways throughout the globe. Wheat is one of the most important sources of calorie and protein in human nutrition. It also provides essential vitamins and minerals such as vitamins B and E, magnesium and phosphorous as well as fibre.[1] Wheat is mainly milled to flour before being used as a food component. However, whole-wheat grains or grits may also be used to make foods. Bread, pasta, noodles, biscuits, and cakes are the most common foods, made basically from wheat flour. Breakfast cereals, curries, soups and porridge are examples of the foods in which whole-wheat grains or grits may be used as an ingredient.[2,3] In many parts of the world the main application of wheat is to make bread as the major staple food. The need for production of economic and nutritive foods in developing countries such as Iran has made wheat and other cereals an important source of raw material in food industry.

Wheat grains or flour is used for starch production in many parts of the world including many European and Asian countries. Starch is commonly used as an ingredient in food industry, which acts as thickening, water absorbing, gelling and dusting agents. The non-food applications of starch are in paper, textile, oil, chemical and pharmaceutical industries.[3,4] The chemical composition of wheat or wheat flour is an imperative factor determining the quality of the wheat-based products, which is affected by wheat variety, climate and growth conditions.[5–7] Amongst the different components of wheat grains, starch is the main constituent (70–80% dry weight basis), which has been proven to have a great impact on the quality of wheat-based products.[8–10] For instance the crumb structure of bread, its crust color, staling kinetics, stickiness of pasta and noodles during cooking, the quality of wheat malt and many other similar products are governed by the starch component.[2,4,11]

In order to produce high quality products from wheat, an important step is to select the appropriate wheat variety in accordance with the characteristics of the final product. Therefore, it is necessary to determine the quality of the wheat in advance. Wheat quality criteria are mainly based on the grain weight, flour yield and protein content and quality. Although starch constituent of wheat is the major part of the grain and has a great influence on the quality of the final products, it has received less attention as a quality parameter. Therefore, it is important to study the physicochemical properties of starch along with other quality parameters of wheat if production of high quality products is meant. Accordingly, this research was carried out in order to characterize the physicochemical properties of the starch component of different Iranian wheat cultivars and to provide basic information in this area. The results of this fundamental study can help explaining many physicochemical properties of the final products and also may be applicable by many starch producers and consumers in order to select the correct type of starch to match its application. Having similar view, many researches have been devoted to characterize starches from different sources such as wheat, rice, potato and cassava grown in different parts of the world indicating the importance of the issue.[12–18] Unfortunately, little information is reported on the physicochemical properties of starch components of the flours obtained from Iranian wheat cultivars. Therefore, the results of this study can provide the basis for selecting a suitable wheat cultivar depending upon the end use, particularly for the users of Iranian wheat cultivars.

MATERIALS AND METHODS

Materials

Wheat samples

Fourteen samples of the most widely cultivated Iranian wheat (Triticum aestivum L.) in 2005 were collected from a number of Agricultural Research Centres in Iran. The wheat cultivars were Alvand, Atrak, Chamran, Falat, Mahdavi, Marvdahst, Omid, Roshan, Sardari, Shahriar, Shiroodi, Tajan, Toos and Zarrin. Amylose from potato and amylopectin from waxy maize were purchased from ICN, Biomedicals Inc. (Aurora, OH). All other chemicals used in this study were purchased from Merck Company (Germany) and were of analytical grade, unless otherwise mentioned.

Methods

Chemical analysis

Prior to the experiments, wheat grains were milled using a Cyclotec model 1093 sample mill (0.5 mm) (Foss Tecator Amersfoort, the Netherlands). Moisture content of the samples were determined as weight loss of 5.0 g accurately weighted wheat flour dried in an oven at 130°C until a constant weight of samples was obtained (approximately 165 min).[19] The protein content of the samples was determined using the micro-Kjeldahl method (N × 5.7).[19] To determine the fat content of the samples, Soxhlet extraction method was used with petroleum ether (boiling point of 40–60°C) as the solvent for fat extraction.[19] To determine the ash content, an accurately weighted sample (about 5 g) was incinerated in a muffle oven at 600°C until a constant weight was achieved. Ash content was calculated as the weight of the residue divided by the original weight of the sample, expressed as percentage.[19] Total carbohydrate content was obtained from the subtraction of protein, fat, and ash contents from 100.

Amylose content

The apparent amylose content (AAC) of the defatted wheat flours was determined using the iodine method according to Morrison and Laignelet.[20] Apparent amylose content was evaluated from absorbance at 635 nm. The recorded values were converted into percent of amylose by reference to a standard curve prepared with amylose from potato and amylopectin from waxy maize. In this study, the starch was not separated from the wheat flour since the aim was to study the starch component of wheat as it is naturally present along with other components in wheat flour.

PHYSICOCHEMICAL PROPERTIES OF THE FLOURS OBTAINED FROM DIFFERENT WHEAT CULTIVARS

Thermal Properties Using Differential Scanning Calorimetry (DSC)

Starch gelatinization is an irreversible process during which several changes occur at the granular and molecular scales including water absorption, swelling, loss of birefringence, and crystallinity.[7,26,32] Many food processing involve heating of starchy materials in water, therefore determination of the temperature at which gelatinization occurs; i.e. gelatinization temperature (GT), is an important factor since at this point starch is cooked completely. DSC is one of the principle techniques available for determination of GT of starches. It determines the temperature range during which gelatinization (structural transition) occurs. Thermal properties typically reported using DSC include gelatinization onset (T_o), peak (T_p), conclusion temperatures (T_c) and enthalpy (ΔH) of the process.

In this study a DSC-7 (Perkin-Elmer, Beaconsfield, UK), calibrated with indium and cyclohexane, was used to measure the gelatinization temperature of starch samples in excess water. The samples were weighed into stainless steel pans (high-pressure pans); 1 part of sample was added to 2–3 parts of distilled water. The sealed pans were scanned at a heating rate of 10°C.min⁻¹, from 0–180°C, after overnight storage for equilibration. An empty pan was used as a reference. The DSC traces were then analyzed using Pyris software (Perkin-Elmer, USA) and the gelatinization temperature was differentiated into T_o (onset temperature), T_p (peak temperature), and T_c (conclusion temperature).[21]

Pasting Properties Using Rapid Visco Analyser (RVA)

It has been shown that starch pasting properties could have a great impact on the quality of the final products.[8,34,35] Therefore the pasting properties of the samples were studied using an RVA. An RVA (Newport Scientific Pty. Ltd., Warriewood, Australia) was used to study the pasting properties and to measure the apparent viscosity of the samples as a function of temperature in excess water.[22] For this purpose wheat flour (4.0 g, 14% moisture basis.) and 2 mM of AgNO₃ to inhibit endogenous α-amylase activity were added to a disposable aluminium canister[23] containing 25.0 g accurately weighted distilled water at 20°C. A disposable plastic stirrer was placed into the canister to mix the suspension continuously. The stirring speed was 960 rpm for the first 10 s and then 160 rpm for the rest of the experiment. The heating profile was adjusted so that it heated the sample at 50°C for 1 min to stabilise the temperature and to ensure uniform dispersion and wetting. Then the temperature increased to 95°C at a constant heating rate of 12°C/ min, held at this temperature for 3 min and then cooled down to 50°C with a constant rate of 12°C/ min and remained at this temperature for one minute. The Thermocline software was used to control the heating and cooling cycles of the instrument. The RVA parameters including pasting temperature, peak, breakdown, setback and final viscosities were recorded and expressed in centi-Poise units (cP). These parameters along with a typical RVA heating profile are shown in Fig. 1.

Figure 1 RVA heating profile along with the pasting parameters used for determination of pasting properties of wheat flours.

Gel Texture

After RVA testing, the paddle was removed and the starch paste in the canister was covered by Parafilm to avoid moisture loss and stored at room temperature (18°C) overnight. The gels were then subjected to a puncture test using a Texture Analyser TA plus (Stable Micro Systems, Godalming, Surrey, England) and compressed to a distance of 10 mm with a 6 mm cylindrical metal probe at a speed of 0.5 mm/s at room temperature. Slopes of the force (N) versus time (s) graphs were determined and used as an indication of the hardness of the starch gels.[24,25]

Statistical Analysis

All experiments were carried out in triplicates and the data were analyzed using the ANOVA procedure of SPSS followed by the comparison of means using the Duncan multiple range test (DMRT, p ≤ 0.05).

RESULTS AND DISCUSSION

Chemical Composition of Wheat Flours

Table 1 shows the chemical composition of Iranian wheat cultivars. The protein content varied significantly; Sardari had the highest protein content (14.98%), while Zarrin had the lowest (7.83%). The fat content of the samples varied from 0.98% (for Omid) to 2.95% (for Shiroodi). The ash content of the samples was in the range of 0.53% (e.g., Sardari and Shiroodi) to 2.96% (Mahdavi). The total carbohydrate contents of the samples were significantly different and in the range of 82.11% for Sardari to 88.41% for Zarrin.

Table 1 Chemical composition (dry basis) of different Iranian wheat cultivars.*

Wheat cultivars	Protein (%)	Fat (%)	Ash (%)	AAC ^† (%)	TCC ^♣ (%)
Alvand	12.73 ± 0.11 ^c	1.41 ± 0.24 ^i	1.33 ± 0.35 ^ab	24.11 ± 0.09 ^f	84.53
Atrak	11.68 ± 0.11 ^f	1.91 ± 0.09 ^e	1.82 ± 0.29 ^b	23.60 ± 0.38 ^f	84.59
Chamran	11.77 ± 0.11 ^e	1.84 ± 0.35 ^ef	1.58 ± 0.14 ^b	24.63 ± 0.66 ^e	84.81
Falat	12.04 ± 0.12 ^d	2.03 ± 0.12 ^d	1.86 ± 0.35 ^b	21.92 ± 0.19 ^g	84.07
Mahdavi	9.74 ± 0.11 ^h	2.24 ± 0.12 ^ab	2.96 ± 0.35 ^a	22.78 ± 0.76 ^g	85.06
Marvdasht	11.69 ± 0.11 ^f	1.86 ± 0.11 ^f	1.33 ± 0.35 ^ab	21.10 ± 1.56 ^g	85.12
Omid	10.41 ± 0.11 ^g	0.98 ± 0.12 ^k	1.07 ± 0.13 ^c	29.32 ± 0.38 ^b	87.54
Roshan	13.39 ± 0.11 ^b	1.85 ± 0.11 ^f	1.32 ± 0.35 ^ab	24.26 ± 0.38 ^f	83.44
Sardari	14.98 ± 0.45 ^a	2.38 ± 0.09 ^b	0.53 ± 0.05 ^d	27.31 ± 0.09 ^c	82.11
Shahriar	10.59 ± 0.11 ^g	1.61 ± 0.24 ^g	1.88 ± 0.35 ^b	27.31 ± 0.19 ^c	85.92
Shiroodi	13.39 ± 0.12 ^b	2.95 ± 0.35 ^a	0.54 ± 0.15 ^d	24.66 ± 0.09 ^e	83.12
Tajan	13.09 ± 0.23 ^c	1.52 ± 0.14 ^h	1.63 ± 0.10 ^b	24.61 ± 0.10 ^e	83.76
Toos	9.84 ± 0.12 ^h	1.17 ± 0.17 ^j	1.87 ± 0.35 ^b	26.38 ± 0.09 ^d	87.12
Zarrin	7.83 ± 0.06 ^i	1.88 ± 0.35 ^ef	1.88 ± 0.35 ^b	29.82 ± 0.19 ^c	88.41
*Mean ± SD; different letters within each column show significant difference at p < 0.05.
^†Apparent amylose content.
^♣Total carbohydrate content, determined by subtraction of protein, fat and ash contents from 100.

The AAC of the samples varies amongst different wheat cultivars ranging from 21.10% (Marvdasht) to 29.82% (Zarrin). Variety, climate and soil conditions during grain development and the method for determination of amylose content can influence the amylose content of the grains.[7,26] The amylose content of Japanese wheat cultivars (measured using Con A method) was in the range of 22.9–24.9%.[16] Using iodine-binding capacity the AAC of some wheat cultivars (Triticum aestivum L.) grown in eastern Australia was reported to be in the range of 26–35%.[27] For European wheat cultivated in 2002–2004, the AAC measured by iodine binding capacity method varied from 25.2 to 28.4%.[18] It has been proven that amylose content is higher in larger starch granules (A-types).[28] Therefore, it may be concluded that the cultivars with higher amylose content may have higher proportion of A-granules.

The amylose content is an important characteristic of cereals since it can affect some physicochemical properties of the final products including susceptibility to enzyme hydrolysis, swelling, pasting, gelling, and retrogradation. It has been shown that high amylose starches are used as thickening and strong gelling agents in many products. Moreover, texture, stability, and uniformity of gelatinised starch may be improved by introduction of amylopectin.[29] It has been reported that wheat starches with high amylose content show lower peak, breakdown, and final viscosities measured by the RVA. Moreover, it has been found that with increasing the amylose content, the swelling power of wheat flours increased.[20,27,30,31]

THERMAL PROPERTIES

The values for each DSC parameter extracted from the DSC diagrams of all samples are summarized in Table 2. The results show that Marvdasht had the highest ΔH value (8.60 J/g), whilst Shiroodi had the lowest value (6.58 J/g). At present there is no clear understanding of the correlation between starch structure and thermal properties. Tester and Morrison suggested that the ΔH reflects the overall crystallinity of amylopectin (quality and the amount of starch crystallites).[30] However, Cooke and Gidley have shown that the ΔH values of gelatinization may reflect the loss of double helical structure rather than the loss of crystallinity.[32] Noda et al. indicated that DSC parameters are influenced by the molecular design of crystalline region of starch which corresponds to the distribution of short chain amylopectin.[33] Table 2 indicates that Tajan had the lowest value of T_p (65.20°C), while Roshan had the highest values (68.77°C). The T_c–T_o of different cultivars was significantly different; Shiroodi (13.00°C) had the widest range, whereas Roshan (8.65°C) had the narrowest range. The wider the gelatinization temperature range, the greater is the distribution of starch granules, in terms of size and thermal properties.

Table 2 Differential scanning calorimetry (DSC) results of gelatinization of the flours of different Iranian wheat cultivars.*

Wheat cultivars	Δ H (J/g)	To(°C)	Tp(°C)	Tc(°C)	Tc-To(°C)
Alvand	6.79 ± 0.17^a	60.54 ± 0.01^a	66.37 ± 0.08^a	73.13 ± 0.11^a	12.59^a
Atrak	6.67 ± 0.06^a	60.56 ± 0.23^a	66.54 ± 0.19^a	73.37 ± 0.18^a	12.81^a
Chamran	6.89 ± 0.33^a	60.93 ± 0.35^a	66.45 ± 0.02^a	72.17 ± 0.35^b	11.24^b
Falat	6.73 ± 0.08^a	60.58 ± 0.02^a	66.54 ± 0.04^a	73.20 ± 0.08^a	12.62^a
Mahdavi	7.46 ± 0.07^b	61.87 ± 0.24^b	67.20 ± 0.07^a	73.40 ± 0.08^a	11.53^b
Marvdasht	8.60 ± 0.12^c	61.90 ± 0.01^b	66.87 ± 0.07^a	72.46 ± 0.01^b	10.56^c
Omid	6.91 ± 0.10^a	63.64 ± 0.05^c	68.70 ± 0.03^b	74.57 ± 0.01^c	10.93^c
Roshan	7.35 ± 0.87^b	63.63 ± 0.15^c	68.77 ± 0.66^b	72.28 ± 0.83^b	8.65^d
Sardari	7.07 ± 0.66^a	62.61 ± 0.35^d	67.70 ± 0.35^b	73.37 ± 0.69^a	10.76^c
Shahriar	7.42 ± 0.34^b	60.97 ± 0.12^a	67.45 ± 0.10^b	73.57 ± 0.02^a	12.60^a
Shiroodi	6.58 ± 0.21^a	60.60 ± 0.49^a	66.68 ± 0.13^a	73.60 ± 0.71^a	13.00^d
Tajan	6.84 ± 0.45^a	59.63 ± 0.24^e	65.20 ± 0.33^c	71.24 ± 0.17^d	11.61^b
Toos	7.57 ± 0.31^b	60.43 ± 0.47^a	67.20 ± 0.66^a	72.32 ± 0.83^b	11.89^b
Zarrin	8.10 ± 0.05^c	62.76 ± 0.02^d	68.03 ± 0.11^b	73.59 ± 0.07^a	10.83^c
*Mean ± SD; different letters within each column show significant differences at p < 0.05.

It has been suggested that GT is affected by the crystalline perfection.[30] The crystalline perfection is controlled by the molecular structure of amylopectin (unit chain length, extent of branching, molecular weight and polydispersity), starch composition (amylose to amylopectin ratio, phosphorous content), granular structure (crystalline to amorphous proportion), and growth condition.[30,34,35] For instance the T_p values of Japanese, Australian and North American wheat cultivars were reported to be in the range of 60.9–61.7°C, 58.8–60.5°C, and 60.7–61.0°C, respectively.[16] These values are lower than those of Iranian wheat cultivars tested in this study. Using a simple linear regression analysis, a positive relationship (r = 0.8) between T_p of the starch component of different Iranian wheat cultivars and AAC was found (Fig. 2), while no relationship between T_o, T_c, ΔH, and AAC was observed. No correlation between amylose content and thermal properties of wheat[16] was also observed. Similar results have been reported by Wickramasinghe et al.[16] and Singh et al.[10] for wheat and rice starches, respectively.

Figure 2 Relationship between peak temperature (T_p) and apparent amylose content (AAC) of the flours of different Iranian wheat cultivars.

PASTING PROPERTIES

The analyzed RVA pasting properties of the flours of different Iranian wheat cultivars are given in Table 3. The pasting temperature indicates the temperature at which the increase in viscosity can be observed. The results show that the pasting temperature ranged from 59.50°C to 66.20°C, Sardari had the highest pasting temperature while Roshan had the lowest one. The peak viscosity indicates the maximum consistency obtained when the flour is heated in excess water and it varied from 2345 to 3725 cP. Amongst the samples, Shahriar had the highest peak viscosity, while Alvand had the lowest. The trough viscosity value which is the lowest viscosity of the flour during mixing at high temperature was the highest for Sardari (2258 cP), while it was the lowest was for Alvand (1806 cP). The final viscosity value for Roshan (3904 cP) was the highest while Chamran (3291 cP) had the lowest value.

Table 3 Pasting properties of Iranian wheat flours measured by the RVA.*

Wheat cultivars	Pasting temperature (°C)	Peak viscosity (cP)	Through viscosity (cP)	Break down (cP)	Set back (cP)	Final viscosity (cP)
Alvand	61.40 ± 0.40	2345 ± 47.2	1806 ± 36.1	539 ± 10.8	1688 ± 33.8	3494 ± 70.0
Atrak	64.65 ± 0.50	2988 ± 60.1	2009 ± 42.3	979 ± 30.1	1709 ± 34.0	3718 ± 72.8
Chamran	63.05 ± 0.45	2912 ± 58.2	1891 ± 38.4	1021 ± 20.4	1400 ± 27.3	3291 ± 66.0
Falat	63.05 ± 0.60	2853 ± 48.0	1940 ± 35.8	913 ± 17.9	1559 ± 31.2	3499 ± 68.9
Mahdavi	64.65 ± 0.40	3232 ± 96.5	2126 ± 43.0	1106 ± 22.5	1740 ± 34.5	3866 ± 77.3
Marvdasht	64.65 ± 0.50	2914 ± 56.3	1915 ± 35.8	999 ± 25.0	1626 ± 35.2	3541 ± 70.5
Omid	64.65 ± 0.35	2925 ± 58.2	2061 ± 41.0	864 ± 17.5	1742 ± 33.8	3803 ± 76.0
Roshan	59.50 ± 0.50	3642 ± 98.4	2131 ± 41.3	1511 ± 30.2	1773 ± 35.5	3904 ± 78.0
Sardari	66.20 ± 0.45	3216 ± 64.3	2258 ± 67.7	958 ± 19.1	1603 ± 32.0	3861 ± 77.2
Shahriar	63.75 ± 0.50	3725 ± 72.5	2251 ± 45.0	1474 ± 30.0	1381 ± 28.0	3632 ± 71.8
Shiroodi	62.10 ± 0.40	3270 ± 64.5	2250 ± 67.5	1020 ± 20.0	1640 ± 31.8	3890 ± 77.5
Tajan	64.60 ± 0.55	2688 ± 53.5	1929 ± 39.1	759 ± 15.0	1676 ± 33.5	3605 ± 72.0
Toos	62.10 ± 0.52	3377 ± 100.3	2242 ± 44.5	1135 ± 23.1	1525 ± 45.5	3767 ± 75.3
Zarin	65.50 ± 0.47	3066 ± 61.3	2033 ± 40.0	1033 ± 20.3	1687 ± 33.7	3720 ± 74.0
*Values are the average of duplicates ± standard deviation.

Some reports indicate that there is a negative correlation between amylose content and the RVA parameters while others report no relationship between these parameters; for example, Zeng et al.[34] found a reverse relationship between the amylose content and the peak viscosity, breakdown and final viscosity of wheat starch. Black et al. found a weak and negative relationship between peak and breakdown viscosities (r = −0.45 and −0.57, respectively) of wheat flour and its total amylose content.[36] Wickramasinghe et al. found a reverse correlation between most RAV parameters and total amylose content of wheat starch.[16] In this study the existence of any correlation between the AAC and the RVA parameters was tested, however no relationship between these parameters was found. This finding is in agreement with Blazek and Copeland who did not find any relationship between RVA parameters of wheat flour and the amylose content.[27] Moreover in this research a strong relationship (r = 0.80) between the peak and final viscosities of the samples was found (Fig. 3). Similarly Srichuwong et al. found a strong correlation (r = 0.83) between peak and final viscosities of starches from different tuber sources.[15] However, Singh et al. found a weak correlation between the two parameters for starches from different rice cultivars (r = 0.24).[10]

Figure 3 The correlation between the peak and final viscosities of wheat flours obtained from different Iranian wheat cultivars measured by the RVA.

It has been shown that the RVA parameters of wheat flour and the starch extracted from the same wheat are closely correlated. These parameters for the flour are slightly higher than starch. For example, peak viscosity of the starch pastes was in the range of 172 to 259 Rapid Visco Analyzer Units (RVU), while for wheat flour it was in the range of 159–272 RVU.[36] Therefore, it seems that other components in wheat flour such as proteins, lipids, and non-starch polysaccharides can contribute to flour pasting.

GEL TEXTURE

Figure 4 shows the slope of force against time curve in puncture test for each gel obtained from different Iranian wheat flours, which is an indication of gel strength of the starch component. The results indicate that there are significant differences between the gel strength of the flours of different wheat cultivars; Alvand had the highest gel strength while Shahriar and Toos had the lowest. When starch paste is cooled below the coil to helix transformation temperature, amylose polymers form aggregates through hydrogen bonding and consequently a gel network is created. The side chains of amylopectin molecules also take part in gel formation, however it takes longer for these molecules to align and produce stronger junction zones. Gel formation is an important characteristic of the starch, which is used in many food and non-food formulations.

Figure 4 The slope of force against time curve in puncture test measured for the gels of the flours of different Iranian wheat cultivars.

There are many factors influencing the behaviour of a starch gel including starch concentration, the ratio of amylose:amylopectin and the presence of lipids and proteins.[30,37] The wheat flours used in this study consisted of different components which can be the reason for different gel strengths of the samples. However, starch and its molecular structure seem to play an important role since starch is the most abundant component of the flour and has the gelling ability. Flour proteins (particularly gluten) also have the potential to absorb water; however under the condition used for gelation, most of them could be denatured by heating and hence lose their gelling ability. Other components in the flour may have less influence on the flour gel strength. Finding any correlation between the gel strength of the wheat flour and its constituents requires further studies.

The importance of amylose content in gelling properties of starch has been previously reported.[10,38] Therefore, the correlation between the AAC and the slope of force against time of each sample was studied. The results (Fig. 5) showed that there is a positive correlation (r = 0.70) between these two parameters. These results are in good agreement with Gidley who reported that high amylose starches produce firm, strong and non sticky gels.[38]

Figure 5 Relationship between the apparent amylose content (AAC) and the slope of force against time of the gels obtained from the flour of different Iranian wheat cultivars.

CONCLUSION

The results of this study indicated that the starch component of wheat flours obtained from different wheat cultivars showed different physicochemical characteristics in terms of composition, thermal, pasting and gelling properties. These physicochemical properties of starch can have a great impact on the quality of the final products. Therefore, correct selection of the wheat for specific applications seems to be crucial. For example the results showed that wheat cultivars such as Zarrin and Mahdavi had low protein content (<10%) and regarded as weak flours. Therefore, these flours are inappropriate for bread making. However, starch attributed properties such as peak and final viscosities and gel strength of these two cultivars are very high. Therefore, they can be used for starch extraction or as an ingredient in production of high viscosity products. Toos on the other hand, had low protein content (9.84%) and low starch pasting and gelling properties. This cultivar may be used for production of acid thinned starch, glucose and fructose syrups. Therefore, it may be concluded that for better application of wheat, the physicochemical properties of starch component of the flour should be considered as a quality criterion. In many countries, however, wheat grains are mainly ranked based on the specific weight, protein content and gluten quality, but starch is never considered as a standard factor. Therefore, depending on the characteristics of the end products, a thorough knowledge of all aspects of the wheat or flour including starch properties is required. The information given in this study on the thermal, pasting, and gelling properties of the starch constituent of Iranian wheats may be useful in further studies on the correct selection and utilization of wheat as a raw material in many food and non-food products.

ACKNOWLEDGMENTS

This project was funded by Iran National Science Foundation (INSF: 87041155).