Changes in wheat and potato starches induced by gamma irradiation: A comparative macro and microscopic study

ABSTRACT

Wheat and potato starches were treated by gamma irradiation (0, 3, 5, 10, 20, 35, and 50 kGy). Apparent amylose content, gelatinization maximum consistency, swelling power, viscosity, and textural parameters decreased in potato and wheat starch pastes as irradiation dose increased. Nevertheless, the decrease of apparent amylose content and swelling power was greater in potato starch than in wheat one. High gamma irradiation doses made potato starch granules more sensitive to shear. On the other hand, no modification in the granule shape was observed by scanning electron microscopy. However, through heat-treatment, starch granules destroyed as irradiation dose increased. Consequently, the effect of irradiation on granular structure appeared to be greater in potato starch than in wheat starch.

KEYWORDS:

• Wheat starch
• Potato starch
• Gamma irradiation
• Rheology
• Scanning electron microscopy (SEM).

Introduction

Starch is an abundant and cheap raw material widely applied in various foods, pharmaceuticals, and non-edible purposes. It contributes 50–70% of the energy in the human diet providing a direct source of glucose, which is an essential substrate for generating metabolic energy.^[1] Furthermore, starch is composed mainly of two polymers of D-glucose: amylose which is a linear macromolecule and amylopectin with its highly branched structure. Minor components associated with starch are made up of lipids representing the most important fraction,^[2] minerals and phosphorus in the form of phosphates esterified to glucose hydroxyls.^[1] Natural starches are present in different polymorphic forms as classified by X-ray diffraction pattern (A-, B-, and C-types).^[3] Most cereal starches tend to have the A-type pattern,^[1] whereas tuber and root starches exhibit the typical B-type X-ray pattern.^[4] Several legume starches belong to the C-type^[3] which is believed to be a superposition of the A and B patterns.^[2]

Consequently, starches from different botanical sources are known to differ in their chemical structures and physicochemical properties and thus, find a wide range of applications in food and non-food industries. But limitations, such as low solubility, high viscosity, low shear and thermal resistance, thermal decomposition, and high affinity for retrogradation limit their use in some food and non-food systems.^[5,6] Therefore, starches may need to be modified by improving these physicochemical and functional characteristics in order to deliver the desired quality for any specific application.

Today, there is an increasing interest in the application of physical procedures for starch modifications, which are considered safer than chemical modifications. In fact, gamma radiation has been considered as an alternative perspective method that might substitute chemical and enzymatic methods applied, until now, at industrial scale. The basic advantages of degradation of polymers by radiation include the ability to promote changes reproducibly and quantitatively without the introduction of chemical reagents and without the need for special equipment/set-up to control for temperature, environment, and additives.^[7] Starch modification by gamma radiation may change the physicochemical and rheological properties of irradiated starch-rich products, resulting in decreased swelling power (SP)^[8] and decreased viscosity and consistency of starch pastes.^[9]

There have been a number of publications reporting on the effect of gamma radiation on the properties of starch from different botanical sources. But just a few of them^[3] have compared gamma irradiated starches with A- and B-type structures. Apart from corn, wheat and potato have become the main source of starch particularly in the European Union which produces 68% of wheat starch and 69% potato starch in the world. A comparative examination of the properties of wheat (A-type) and potato (B-type) irradiated starches is, therefore, important in understanding their behavior in food and non-food industrial applications. In fact, the starches’ physicochemical properties, which are a direct result of their structure, depend mainly on the botanical origin of the native starches, since the diversities in macromolecular constituents and form of starch granules influence their functionalities.^[1] In our previous study,^[10] we reported that irradiation treatment may generate free radicals, which are capable of inducing depolymerization and molecular changes of wheat starch. But the crystalline regions seemed to be unaffected by gamma radiation. Besides, some physicochemical properties changes were attributed to the weakening of the starch granule membrane.^[11] However, little information was available, until recently, on the nature of the starch molecules (amylose and/or amylopectin) as well as the granule zone (crystalline and/or amorphous) involved in starch degradation, and on the mechanisms of cleavage. Therefore, it seems indispensable to take out more studies dealing with gamma radiation effects on the physicochemical and structural properties of wheat and potato starches, since a correct understanding of starch properties and its interactions with other constituents are of interest to the industrial processing operations.

In this study, wheat and potato starches were irradiated at 3, 5, 10, 20, 35, and 50 kGy. Then macroscopic (rheological, pasting, and textural) and microscopic (morphological) properties of irradiated starches were examined. The effects of gamma radiation according to starch botanical origin were compared.

Materials and methods

Starches

Commercial potato and wheat starches were purchased from Roquette (Lestrem, France). The initial moisture content was determined according to ISO 712 (1998). Samples (18.3 and 13.4% moisture content for potato and wheat starches, respectively) were packed in two layers of polyethylene bags of 200 g. The starch specimens were grouped in lots of two bags each prior to irradiation.

Gamma radiations

Potato and wheat starches were treated with gamma radiations at 3, 5, 10, 20, 35, and 50 kGy with a dose rate of 13.84 Gy/min. The irradiation treatments were performed at room temperature (25 ± 0.5ºC) using a semi-industrial cobalt-60 irradiator in the National Center for Nuclear Sciences and Technology (CNSTN, Technopole of Sidi Thabet, Tunisia). Dosimetry was performed using Amber Perspex dosimeters (Harwell dosimeters). The dosimeters were calibrated against an international standard set by the Aerial Laboratory. All samples were stored in a dry and ventilated medium to avoid any humidification before analysis.

Physicochemical analyses

Apparent amylose content (AAC) of wheat and potato starches was estimated by a colorimetric method based on the iodine-binding-spectrophotometry according to ISO 6647-1 (2007).^[12] Amylose content was read from a standard curve developed using pure amylose solutions of known concentrations. All analyses were conducted in triplicate and the average values were reported.

The water solubility index (WSI) and SP were determined at 80°C according to the method of Tsai et al.,^[13] with modification as previously described.^[10] WSI and SP were determined in triplicate and calculated as follows:

(1)

(2)

where W₁ is the weight of dried supernatant and W_S is the weight of the sediment.

Rheological characterizations

The pasting properties of starches with different radiation doses were determined using a Brabender viscoamylograph (OHG DUISBURG Brabender Germany Instruments). At the same concentration of 10% (g/mL), the pasting properties of native wheat and potato starches showed an important shift in the gelatinization maximum consistency (C_max). This shift made impossible the study of the irradiation effect on C_max for both starches. Thus, starch dispersions with a concentration of 16% (g/mL) for wheat starch and 8% (g/mL) for potato starch in distilled water were prepared. The mixture was stirred and heated at temperatures ranging from 25 to 90ºC at a rate of 1.5ºC/min. Starch slurry was allowed to remain at 90ºC for 5 min before the end of each analysis. The obtained amylogram presented the evolution of consistency (BU) during heating cycle and the maximum consistency (C_max) of the gelatinization peak was recorded. Triplicate measurements were carried out for each irradiation dose (ID).

Viscosity measurements were carried out with the use of a rotary viscometer Rheometric RM180 (Rheomat, Caluire, France), equipped with the coaxial cylinders geometry. The bob and the cup used had 15.18 (R1) and 21 mm (R2) radius, respectively, giving a ratio R1/R2 = 0.72. The viscometer was controlled by RSI Orchestrator v6.5.8 software. Starch suspensions were prepared with the same concentrations as for pasting measurements. The suspensions were heated under stirring for 10 min at 58.5 and 61.3°C for wheat and potato starches, respectively. These temperatures corresponded to 95% of gelatinization peak temperature (T_p) of each starch as determined previously by differential scanning calorimeter (DSC). Viscosity profiles at increasing and decreasing strain rates were done between 10 and 1000 s⁻¹. Rheological measurements were evaluated at the sample preparation temperature (58.5°C for wheat starch and 61.3°C for potato starch). The regulation of temperature was obtained using a circulator bath (Julabo GmbH, Germany). Three measurements were carried out for each ID.

The flow behavior of starch pastes was also assessed by back extrusion test. Textural profile analysis was performed using TA-XT2i texture analyzer (Stable Micro Systems, Ltd., UK) equipped with a force sensor of 50 N at room temperature and monitored by the Texture Expert v1.22 software. Starch suspensions of 10% solids (dry basis, g/g) were prepared as for viscosity measurement. Then the heated suspensions were immediately transferred into a glass cell (70-mm diameter, 80-mm height) and were subjected to compression using a 50-mm diameter aluminum cylindrical probe (Cyd pilot stab P/50) at a test speed of 5 mm/s, for a distance of 15 mm. Two parameters of the back extrusion test were calculated from the resulting force versus displacement curve: maximum extrusion force (F_max) and extrusion surface (S_E). Three measurements were carried out for each ID.

Morphological observations

Scanning electron micrographs (SEM) of native and irradiated starch samples were carried out at ordinary temperature using a scanning electron microscope (Model JEOL JSM-5400, France). Starch samples were examined in two different states: non-heated granules (dry state) and heated granules suspensions (starch pastes). In the case of starch in the dry state, SEM examinations were carried out directly on starch granules powder. The samples were covered with a thin gold layer. The SEM images were taken with a magnification of 500× and an accelerating potential of 5.0 kV. In the case of starch pastes, heated suspensions were prepared as for viscosity measurements (10 min stirring and heating at 58.5°C for wheat starch and 61.3°C for potato starch). After heat treatment, samples were rapidly frozen at –80°C for 24 h and afterward, lyophilized. The micrographs were obtained with a magnification of 500× at an acceleration potential of 5.0 kV.

Statistical analysis

Analysis of variance (ANOVA) followed by the least significant difference (LSD) test was performed with the statistical software SPSS 17.0 (on PC; SPSS Inc., Chicago, IL). The level of significance used was 95%. A t-test, standard deviation at the 95% confidence level was performed on the pasting parameters measurements to compare the differences in the mean values. The repeatability of the results was evaluated by calculating the coefficient of variation (CV% = standard deviation/mean value).

Results and discussion

Physicochemical properties

The amylose content is a major quality attribute of starch and determines the diverse properties of starch and ultimately the end-use purposes. There are various methods for the quantification of amylose content in starch, based on DSC, iodine-binding, and chromatography, but the iodine-binding spectrophotometry remains the basic method. The ability of the iodine inclusion formation reflects the size and structure of the amylose.^[14] In our study, AAC determined by the colorimetric method was higher in native wheat starch (34.5% ± 0.2) than in potato (30.1% ± 0.1; Table 1). This result is in agreement with the literature data^[15] which showed that cereal starches (A-type) were richer in amylose than tuber ones (B-type).

Table 1. Linear, exponential, and binomial regressions for AAC, WSI, and SP, respectively. X = ID (irradiation dose, kGy).

Variables	Coefficients			Significance
Y	a	b	c	R^Citation2
AAC (%)	Linear law: Y = a + b X
Wheat starch	34.24	–0.42	—	0.98
Potato starch	30.55	–0.44	—	0.97
WSI (%)	Exponential law: Y = a. e^b.X
Wheat starch	3.31	0.031	—	0.99
Potato starch	2.68	0.017	—	0.98
SP (g/g)	Binomial law: Y = a + b X + c X^Citation2
Wheat starch	12.92	0.58	–0.014	0.92
Potato starch	39.88	1.17	–0.035	0.93

After gamma radiation at 3, 5, 10, 20, 35, and 50 kGy, the AAC decreased in both starches (Fig. 1). A significant linear regression model (R² 0.97) was observed between AAC and ID, with a smaller slope for potato starch (30.55) than for wheat starch (34.24). The decrease of AAC could be attributed to the production of short-chain amylose with reduced ability to form the blue iodine complex. Ionizing radiation is known to cause random scission in the glycosidic chains, producing short amylose chains and short linear chains from the branches or from the small-branched fraction of the amylopectin.^[16] According to Othman et al.,^[17] a minimum degree of polymerization (DP) of 15 is required to form a helix which will complex with iodine. Thus, AAC decrease could be associated with the depolymerization of existing amylose chains to shorter oligomers of DP ˂ 15. This result is closely consistent with the finding of Chung et al., ^[3] who reported that the proportions of short chain (DP 6-12) increased, whereas the proportion of DP 13-24 decreased with increasing ID causing a large decrease in iodine binding ability. Similar results have been reported by Chung et al.,^[3] Ashwar et al.,^[18] and Othman et al.^[17] for starches from different botanical sources (potato, bean, corn, rice, and sago).

Figure 1. Ratio of apparent amylose content (AAC) of irradiated starch (IS) to AAC of native starch (NS) vs. irradiation dose (▲: wheat starch, ■: potato starch). AAC of native starches were: 34.5% ± 0.2 for wheat starch and 30.1% ± 0.1 for potato starch.

In this comparative study, it should be mentioned that after irradiating wheat and potato starches until 50 kGy, the decrease in AAC was more pronounced for potato starch than for wheat starch. Indeed, AAC was reduced by 0.9, 8.1, 14.8, 29.0, 40.9, and 61.8% in wheat starch and by 3.3, 17.9, 33.2, 36.2, 48.7, and 68.6% in potato starch irradiated at 3, 5, 10, 20, 35, and 50 kGy, respectively. The greater decrease in AAC of potato starch compared with that of wheat starch could be attributed to the greater decrease in iodine binding ability of partly branched long chains in amylopectin caused by gamma radiation since potato starch had greater proportions of long chains than wheat starch. In this context, Jane et al.^[19] reported that A-type starches (wheat starch) had larger proportions of short chain (DP 6-12) and smaller proportions of long chain (DP ≥ 37) than B-type starches (potato starch) and the decreasing effect of gamma radiation on AAC was associated with the structure of starch.^[20] Taking these results into consideration, it should be noted here, that the effect of gamma radiation on molecular structural characteristics as reflected by the AAC values depends on starch botanical origin. Potato starch granules integrity would be more affected by gamma radiation treatment than wheat starch. It is interesting for food processing to observe that gamma radiation induced changes in amylose content because of the potential to modify the texture and quality of the end-used products.

ID significantly (p < 0.05) increased the WSI of both starches when increased from 0, 10, 20, 35, to 50 kGy (3 and 5 kGy gave significantly similar results than the native starch; Fig. 2). Significant exponential regressions were observed between WSI and ID (Table 1). It should be noted that the WSI was higher in wheat starches than in potato ones (native and irradiated). WSI increased from 3.3 to 15.7% for wheat starch and from 2.6 to 6.6% for potato starch as the IDs were increased from 0 to 50 kGy. Since the increase of WSI with gamma radiation may in part be due to the depolymerization of the polysaccharides of starch to simpler molecules such as dextrins or sugars that have higher affinity for water than starch, the greater increase showed for wheat starch means greater amounts of soluble fragments (such as dextrins) are present outside the wheat granules. As ID increase, the granules rupture with the disordering of the organization of the chains. Kang et al.^[21] reported that gamma radiation is capable of hydrolyzing chemical bonds, thereby cleaving large molecules of starch into smaller fragments of dextrin that may be either electrically charged or uncharged as free radicals. These changes result in an increase in starch solubility.^[22] The increase in WSI confirmed the decrease of AAC showed above after gamma radiation treatment. This result can be explained by the increase, during starch pasting, of soluble polysaccharide fragments within the aqueous phase. Thus, the hypothetic breakdown and depolymerisation of the polysaccharides by radiolysis may induce the production of fragments with low molecular weight, due probably to their ability to diffuse easier from starch granules. This hypothesis is in accordance with the results of Fig. 1 showing the decrease in AAC after gamma radiation treatment. Otherwise, easier diffusion of polysaccharide and/or polysaccharide fragments can happen due probably to the weakening of the starch granules membranes by irradiation, as suggested by Ben Bettaïeb et al.,^[11] leading to an increase in WSI. Sokhey et al.^[23] found similar results and reported that the water soluble fraction of 70% amylose starch increased from 0.03 to 0.18%, when the ID was increased from 0 to 30 kGy. De Kerf et al.^[24] also showed that the soluble fraction of corn starch was increased from 24% (non-irradiated) to 30, 70, and 75%, respectively, after IDs of 10, 50, and 100 kGy. Othman et al.,^[17] also observed that the solubility of sago starch irradiated at 25 kGy increased by 123%.

Figure 2. Ratio of water solubility index (WSI) of irradiated starch (IS) to WSI of native starch (NS) vs. irradiation dose (▲: wheat starch, ■: potato starch). WSI of native starches were: 3.3% for wheat starch and 2.6% for potato starch.

The extent of granular swelling can be quantified as SP. The SP of starch results from the capacity of starch molecules to hold water through hydrogen bonding. According to Gani et al.,^[25] these hydrogen bonds between the starch molecules are broken after gelatinization and replaced by hydrogen bonds with water. Thus, rapid swelling of the starches is due to the breaking of intermolecular hydrogen bonds in the amorphous areas.

The ability of the irradiated wheat and potato starches to swell in excess water at 80°C is presented in Fig. 3. A binomial relationship between SP and gamma radiation doses was shown with a maximum at 20 kGy for both starches (Table 1). This result is consistent with the finding of Ezekiel et al.^[8] who reported that in starch extracted from corn irradiated with 10, 20, 50, and 100 kGy, the SP increased up to 20 kGy and then decreased rapidly.

Figure 3. Ratio of swelling power (SP) of irradiated starch (IS) to SP of native starch (NS) versus irradiation dose (▲: wheat starch, ■: potato starch). SP of native starches were: 11.5 ± 0.1 (g/g) for wheat starch and 33.7% ± 1.1 (g/g) for potato starch.

Unlike the WSI, the SP was higher in potato starch than in wheat one (native and irradiated). The differences in the SP could indicate structural differences among starches. The SP is, in fact, influenced by amylopectin molecular structure, amylose content,^[26] as well as the presence of minor components (e.g., lipids, protein).^[14–27] Kaur et al.^[28] reported that a higher swelling ability for potato starch could be due to its large granules. Otherwise, the greater the granules are, the higher their water absorption capacity. SP also increases with increasing long chains of amylopectin and decreasing amylose content.^[27–29] Thus, SP was higher in potato starch (33.7% ± 1.1 [g/g]) than in wheat one (11.5 ± 0.1 [g/g]) since potato starch had larger proportions of long chain (DP ≥ 37)^[19] and lower AAC than wheat starch as already demonstrated in the above session about amylose. Tester et al.^[26] reported also that amylopectin contributes to swelling, whereas amylose and lipids inhibit swelling. Compared with potato starch, wheat starch contains much greater amount of lipids (1.12 and 0.009% for wheat and potato starches, respectively, according to Buléon et al.)^[2] which inhibit the ability of wheat starch granules to swell.

In this study, wheat and potato starches exhibited a rapid rise in SP from 0 to 20 kGy (Fig. 3). Up to this dose, irradiation would fissure the starch granules membranes leading to an easier penetration of water in the dispersed phase, causing a greater SP. At higher doses (35 and 50 kGy), cracking of the granules would be accentuated in particular for potato starch. Granules would become unable to retain water during gelatinization resulting in a reduction of the SP, a greater disruption of macromolecules (see Fig. 1) and a greater WSI. Furthermore, Zhu^[14] reported that the granule associated proteins played an important role in the granular swelling process. Heat treatment in excess water induced the redistribution of the granule-associated proteins and the formation of protein envelope encasing the starch content within the deformed granules. Thus, the decrease in SP from 20 to 50 kGy may suggest that high IDs had a great influence on the proteins envelope, which should retain the granular content during swelling.

The decrease in SP of starch irradiated at 50 kGy as compared to non-irradiated starch was 46.4 and 72.7% for wheat and potato starches, respectively. Thus, potato starch showed a greater reduction in SP than wheat starch. Since the amylopectin fraction of starch is considered to be primarily responsible for swelling^[26] and, therefore, a decrease in the swelling index may be related to a high reduction in amylopectin with irradiation treatments,^[30] which is in accordance with the hypothesis previously advanced on the breakdown of branched long chains of amylopectin during irradiation.^[10] It was, therefore, likely that depolymerization or shortening of amylose and amylopectin chains by irradiation was responsible for the reduction in water binding capacity and consequently for a decrease of the SP in particular for potato starch.

Rheological properties

Evaluating the pasting properties of native wheat and potato starches dispersed in water at the same concentration (10%, g/mL) showed an important shift in the gelatinization maximum consistency (C_max) between both starches. Whereas native wheat starch gave a C_max of 104.66 UB (±11.71), potato gave a C_max greater than 1000 UB (results not shown) at the concentration of 10% (g/mL). This shift made impossible the study of the irradiation effect on C_max for both starches at the same dispersion concentration. This latter were thus modified as follows: 16% (g/mL) for wheat starch and 8% (g/mL) for potato.

The new C_max of native and irradiated starches are given in Table 2. ID decreased significantly (p < 0.05) the C_max until 35 kGy when it became equal to zero for wheat starch and until 50 kGy for potato one. The viscosity of the heated dispersion may decrease with an increase of ID as a result of the volume fraction depression. Until 20 kGy, wheat starch lost 96.5% of its maximum consistency, whereas potato starch which had the half concentration during gelatinization in the Brabender bowl (that means a greater quantity of water was available in the continuous phase) lost only 69.0%. The pasting behavior is sensitive to the starch solid content in the system and the maximum consistency is mainly related to the swelling of starch granules. Thus, this result is consistent with previous observations in the above session about SP (see Fig. 3): Potato starch had greater ability to swell and granule integrity more affected by gamma radiations than wheat starch.

Table 2. Gelatinization maximum consistency (C_max, UB) of native and irradiated wheat and potato starches prepared at different concentrations: 16% (w/v) for wheat starch and 8% (w/v) for potato starch.

Starch	Irradiation dose (kGy)
	0	3	5	10	20	35	50
Wheat	999.7 ± 0.6 (a)	997.7 ± 0.6 (a)	721.7 ± 18.9 (b)	226.7 ± 11.5 (c)	35.0 ± 5.0 (d)	0.0 ± 0.0 (e)	0.0 ± 0.0 (e)
Potato	702.0 ± 19.7 (a)	413.3 ± 11.5 (b)	373.3 ± 11.5 (c)	334.0 ± 12.7 (d)	217.7 ± 2.5 (e)	78.3 ± 2.9 (f)	0.0 ± 0.0 (g)

Values are means of three replicates ± standard deviation.

Values with different letters in the same line are significantly different (p < 0.05).

Shear stress tests have been employed to describe the rheological properties of starch. The flow curves determined for native and irradiated starches until 50 kGy are shown in Fig. 4. Hysteresis loops were observed for both starch pastes (native and irradiated). The hysteresis observed indicates a time dependency of the starch pastes rheological properties. This behavior is frequently observed with concentrated suspensions and/or macromolecular solutions due to structural breakdown occurred in the specimen during the rheological measurement. These results are in agreement with trends found in previous works showing shear-thinning thixotropic behavior of several starch pastes.^[31] The potato starch which had the half concentration exhibited apparent viscosity (shear stress/shear rate) greater than that of the wheat starch between 10 and 1000 s⁻¹ (Fig. 5). At a shear rate of 1000 s⁻¹, potato starch viscosity was 0.51 Pa.s, whereas that of wheat starch was 0.20 Pa.s. The high viscosity observed for potato starch compared to that of wheat starch could be explained by the larger size of its granules and their greater ability to swell (SP of native potato starch was almost three times larger than SP of native wheat starch as already observed in the above session about SP). Srichuwong et al.,^[29] showed that large granules provide more consistent and viscous pastes on heating. In fact, the greater the granules are, the higher their water capacity absorption, giving rise to greater volume fraction and therefore to greater viscosity when heated. Amylose content also influenced the rheological properties of the starch pastes.^[32] Lii et al.^[9] suggested that the amylose leached out from the starch granule during heating did not contribute much to the rheological properties of the starch pastes. Hence, they concluded that the starch granular properties and characteristics were the major factors controlling the starch rheological behavior, followed by amylose. Amylopectin molecular structure-property correlation analysis showed that the proportion of longer unit chains (DP > 35) was positively related to resistance to shearing and frequency independence of rheological properties.^[33] Consequently, pasting properties of starch are affected by several factors, such as granule morphology (granule size is variable and ranges from 1 to 110 µm)^[4] and amylose/amylopectin ratio.^[34]

Figure 4. Shear stress versus shear rate curves of native and irradiated A: wheat; and B: potato starch pastes obtained at different irradiation doses: (●) 0 kGy; (■) 3 kGy; (▲) 5 kGy; (□) 10 kGy; () 20 kGy; (x) 35 kGy; and (+) 50 kGy.

Figure 5. Shear stress versus shear rate curves of native wheat (▲) and potato (■) starch pastes prepared at different concentrations: 16% (w/v) for wheat starch and 8% (w/v) for potato starch.

The flow curves profiles were apparently modified by gamma radiation (Fig. 4). Viscosity of irradiated starches showed a significant decrease with an increase of the gamma radiation level for both starches. The decrease in viscosity was intensified from 20 kGy, especially for potato starch paste, as well as the decrease in the loop area (degree of paste structural breakdown caused by shear, see Table 3). Wheat starch pastes viscosity at 300 s⁻¹ (first run) was reduced by 23.8, 26.5, 41.3, 75.6, 77.9, and 90.0%, respectively, with 3, 5, 10, 20, 35, and 50 kGy doses of irradiation compared with the non-irradiated sample. For potato starch pastes, viscosity at 300 s⁻¹ (first run) was reduced by 21.9, 35.1, 41.4, 79.5, 87.2, and 97.8% with the same doses of irradiation treatment. In other words, at high doses of irradiation (35 and 50 kGy), the decrease in viscosity of potato starch pastes was greater than that of wheat ones. These results would suggest that potato starch granule integrity would be more affected by high doses of gamma radiations. The decrease in viscosity could be attributed to the free radical formation and chain depolymerization as a result of glycosidic bond cleavage induced by gamma radiations.^[11,35,36] It was, therefore, likely that the decrease in viscosity had a close relation with the decrease of AAC previously observed which means a decrease in the hydrodynamic volumes of the macromolecules leached outside the granules during and after gelatinization leading to reduce the flow resistance and thus the viscosity. The decrease of starch pastes viscosity when the gamma radiation level increased had also been reported by other researchers.^[3,20,21] They reported, in fact, a decrease in peak viscosity determined by rapid visco-analyzer on irradiated starch, but no comparison was made at the same experimental conditions between A- and B-type structures.

Table 3. Hysteresis buckles surface S (cm²) and parameters of the power law model shear stress-shear rate curves of native and irradiated wheat and potato starches.

Starch	Parameter	Irradiation dose (kGy)
		0	3	5	10	20	35	50
Wheat	S (cm^Citation2)	2.26 ± 0.01a	1.68 ± 0.03b	1.48 ± 0.02 c	1.00 ± 0.01d	0.00 ± 0.00e	0.00 ± 0.00e	0.00 ± 0.00e
	n	0.32 ± 0.01c	0.33 ± 0.01c	0.32 ± 0.02c	0.41 ± 0.01c	0.82 ± 0.23b	0.88 ± 0.07b	1.18 ± 0.08a
	K (Pa.sⁿ)	22.81 ± 0.65a	17.04 ± 0.27b	18.58 ± 1.49b	11.48 ± 1.79c	3.97 ± 0.58d	4.57 ± 1.69d	0.03 ± 0.01e
Potato	S (cm^Citation2)	2.32 ± 0.01a	1.93 ± 0.03b	1.88 ± 0.02c	1.84 ± 0.01d	0.00 ± 0.00e	0.00 ± 0.00e	0.00 ± 0.00e
	n	0.53 ± 0.01c	0.45 ± 0.02e	0.45 ± 0.01e	0.42 ± 0.02e	0.49 ± 0.02d	0.62 ± 0.02b	1.16 ± 0.07a
	K (Pa.sⁿ)	21.24 ± 0.58a	18.21 ± 1.81b	14.82 ± 0.65c	16.08 ± 1.19c	4.47 ± 0.53d	1.27 ± 0.09e	0.01 ± 0.00e

Values are means of three replicates ± standard deviation.

Values with different letters in the same line are significantly different (p < 0.05).

The flow behaviors of the starch suspensions determined at increasing strain rates were described by the power-law model (Eq. 3; R² > 0.97). Consequently, n (flow behavior index) and K (consistency index) were determined (Table 3). This model is convenient for analysis of rheological behavior of a wide range of fluids that are either Newtonian (n = 1), shear-thinning (0 ˂ n ˂ 1) or shear-thickening (n ˃ 1).

(3)

The flow behavior index of the native and irradiated wheat starch pastes at 3 and 5 kGy was lower than that of the native and irradiated potato starch at the same doses, showing a greater shear-thinning behavior for wheat starch pastes. Shear-thinning behavior, which emphasizes the sensitivity of the solution to shear, indicates the presence of physical interactions and/or entanglements between structural units (i.e., macromolecular chains) that disappears progressively when shear rate increases. Jane et al.^[37] assumed that the long-branch chains of the amylopectin interact to greater extent with amylose via entanglement than do the other amylopectins. The amylose leached out from the starch granule during heating contributes to the rheological properties of the starch pastes. Consequently, entanglements concentration in the continuous phase of wheat starch pastes (between leached linear macromolecules: amylose and long-branch chains of amylopectin) was greater than in potato ones. It should be noted that until 10 kGy, the flow behavior index of wheat starch paste did not differ statistically compared with that of the native starch paste (Table 3). Nevertheless, a significant decrease (p < 0.05) of the flow behavior index was observed in potato starch pastes. This would suggest that entanglements concentration would increase with ID, which would decrease the flow behavior index. As the ID increase, depolymerization or shortening of the polysaccharides may induce the disordering of the organization of the chains. Most likely, more branched long chains of amylopectin are leached with amylose in the continuous phase. But at high doses of irradiation (from 20 kGy), the flow behavior index (n) increased rapidly for wheat starch paste and became close to unity and greater than that of potato one. Consequently, from 20 kGy, potato starch paste became more sensitive to shear than wheat starch paste. It is, therefore, likely that the greater decrease in AAC of potato starch at high doses of irradiation compared with that of wheat starch as already observed above could result in greater reduction of the flow resistance of potato starch inducing a smaller shear-thinning behavior during the test. At 50 kGy, wheat and potato starch pastes appeared to have shear-thickening behavior (with n ˃ 1). This result could indicate that structural reorganization took place by shear in both starch pastes.

ID significantly (p < 0.05) decreased the consistency index (value of the viscosity at 1 s⁻¹) for both starches (Table 3). Wheat starch consistency index was significantly reduced by 82.6% at 35 kGy compared with the non-irradiated sample, whereas potato starch consistency index was significantly reduced by 94.0% with the same radiation dose. Zero values of consistency index K were obtained at 50 kGy. These results could indicate that high radiation doses makes potato starch more sensitive to shear and suggest once again that starch potato granules integrity would be more affected by high level of gamma radiations than wheat ones.

Rheological properties of starch pastes were also assessed by back-extrusion test via texture analyzer equipment. The back extrusion test is frequently used in the food industry because it is a quick and simple test to identify the flow behavior of complex fluids.^[38] Even in the food science, the back extrusion test has had numerous applications.^[39–41] In this comparative study, the maximum extrusion force (F_max) and the extrusion surface (S_E) of potato starch pastes were greater than that of wheat starch pastes (Table 4). These results are consistent with those of the flow measurements obtained between 10 and 1000 s⁻¹. Potato starch pastes which had the half concentration were thicker than wheat ones at the same ID. This observation may be due to the greater sizes of the potato starch granules, compared to the wheat starch since pasting properties of starch are affected by several factors, such as granule morphology and amylose/amylopectin ratio.^[34] It is likely that the entanglements that occur between the leached linear macromolecules (amylose and long-branch chains of amylopectin), as previously stated in this section, could result in greater thickness of potato starch paste.

Table 4. Maximum extrusion force (F_max) and extrusion surface (S_E) of native and irradiated starches.

Starch	Parameter	Irradiation dose (kGy)
		0	3	5	10	20
Wheat	Fmax (N)	1.67 ± 0,14a	1.58 ± 0.05a	1.51 ± 0.04a	1.31 ± 0.01b	0.89 ± 0.00c
	SE (N.s)	2.93 ± 0.25a	2.79 ± 0.10a	2.66 ± 0.08a	2.35 ± 0.02b	2.12 ± 0.01c
Potato	Fmax (N)	4.37 ± 0.38a	2.98 ± 0.28b	2.55 ± 0.24b	1.98 ± 0.16c	1.75 ± 0.11c
	SE (N.s)	7.44 ± 0.60a	5.34 ± 0.49b	4.86 ± 0.32b	3.65 ± 0.31c	3.25 ± 0.23c

Values are means of three replicates ± standard deviation.

Values with different letters in the same line are significantly different (p < 0.05).

Gamma radiation treatment caused a significant decrease (p < 0.05) in F_max and S_E for both starches (Table 4). The result of ANOVA showed an ID effect on F_max and S_E from 10 kGy for wheat starch and from 3 kGy for potato starch. At 20 kGy, F_max was reduced by 46.7 and 60.0% and S_E was reduced by 27.6 and 56.3% compared with the native samples for wheat and potato starches, respectively. Back-extrusion test showed a greater reduction in F_max and S_E for potato starch than for wheat starch when subjected to gamma radiations. The decrease in extrusion resistance with increasing irradiation would be obviously a direct consequence of reduced starch viscosity, as discussed previously. Figure 1 shows smaller amounts of amylose chains for potato starches than for wheat starches (native and irradiated) especially at high doses of irradiation (from 20 kGy), which would predispose irradiated potato starches to greater reduction in consistency, making the flow of starch pastes through the annulus formed between the inner cylinder (piston) and the cylindrical container increasingly easy. The decrease in rheological properties resulted primarily from irradiation would induce starch degradation and may present opportunities such as ease of cooking and reduced starch retrogradation as shown by several researchers.^[42]

Morphological properties

The morphological study by SEM proved to be essential in view of the results obtained by the rheological methods employed. Because of the high reduction in starch viscosity, coupled with the starch degradation by gamma irradiation, the SEM micrographs were undertaking on starch granules (native and irradiated) in the dry state and on starch pastes (native and irradiated) for evidence of the starch granular damage.

Native and irradiated wheat starch in the dry state appeared to be a mixture of small spherical granules and a majority of large round granules (results not shown). Native and irradiated potato starch in the dry state showed large oval and smaller spherical granular shapes (Fig. 6A–D). Wheat and potato starches granules had a smooth surface, indicating that the granules surfaces were apparently unaffected by irradiation. It is therefore likely that gamma radiation did not significantly alter the granular morphology observed in the dry state before cooking and that wheat and potato starches retained the original shapes and sizes after gamma radiation treatment. Abu et al.^[30] and Sokhey et al.^[23] also reported the absence of physical damage to cowpea starch granules and waxy maize starch granules, respectively. Consequently, gamma radiation damage caused to starch granules seems to be only of a structural level.

Figure 6. Scanning electron micrographs of: potato starch in the dry state (A–D); potato starch pastes heated to 61.3°C (E–H); wheat starch pastes heated to 58.5°C (I–L; 500×; 5.0 kV). Irradiation doses: (A, E, I) 0 kGy, (B, F, J) 5 kGy, (C, G, K) 20 kGy, and (D, H, L) 50 kGy.

When heated in excess of water, the native wheat and potato starches reveal a honey comb structure (Fig. 6E and 6I). Starches irradiated at 5 kGy present the same structure but with the difference that the cavities became bigger as compared to those obtained with the non-irradiated starches (Fig. 6F and 6J). Furthermore, after heat treatment, some small granules of potato starch remained intact whereas a complete disappearance of large and small wheat starch granules can be noted. Gamma radiation has not probably the same effect on small and great granules. The small granules would be more resistant to irradiation treatment than the large granules. Chiotelli et al.^[43] reported, in this context, that small granules gelatinize at high temperatures compared to large granules.

From 20 kGy, flat and smooth structural elements appeared accompanied by the appearance of oriented lamellar structure (Fig. 6G and 6K). It is, therefore, likely that the flattening of the honeycomb cages induce the formation of smooth flat walls. However, the oriented lamellar structure was more developed in potato starch than in wheat starch indicating that potato starch paste could be more uniform. Potato starch irradiated at 50 kGy reveals the increase in lamellas leading to the formation of larger and more flattened blocs (Fig. 6H). The SEM of paste of wheat starch irradiated at 50 kGy (Fig. 6L) shows the disappearance of the veining structure on the surfaces of lamellas and the formation of flattened blocs. This result can be explained by the gradual decrease of the SP of starch when irradiated up to 20 kGy (Fig. 3).

The combined effect of irradiation and heat treatment would thus cause the disappearance of the granular structure. This change could be attributed to the free radicals generated by gamma radiation inducing the weakening of the starch granules membranes by irradiation, as suggested by Ben Bettaïeb et al.,^[11] and cleaving large starch molecules into smaller fragments.^[44] Moreover we confirmed these observations in a previous work^[10] by electron paramagnetic resonance (EPR) spectrometry.

The effect of gamma radiation on granular morphology appeared to be greater in B-type starch (potato starch) than in A-type starch (wheat starch), which would confirm the observations made previously showing that irradiated potato starch had greater ability to swell, a granule integrity more affected by gamma radiations, greater viscosities, greater consistencies, and greater extrusion resistance than wheat starches (native and irradiated).

Conclusions

The results of this study revealed the influence of botanical source of starch on physicochemical, rheological, pasting, textural, and morphological properties of starch treated by gamma radiation. The AAC decreased in both starches, but potato starch showed a greater reduction than wheat starch when ID increased. Thus, the degradation of the amorphous part of the starch structure induced by gamma radiation would be more important in B-type starch (potato starch) than in A-type (wheat starch) one. In both starches, the SP increased up to 20 kGy and then decreased rapidly. However, potato starch showed a greater reduction in SP than wheat starch. Consequently, consistency, which was directly related to the granular swelling and the integrity of the swollen granules, decreased during pasting. This decrease was attributed to the weakening of the starch granule membrane and the depolymerization or shortening of amylose and/or amylopectin chains of starch after irradiation due to the breakage of glycosidic linkages. On the other hand, gamma radiation increased significantly the solubility of both starches. The effect of gamma radiation on starch microstructure was also dependent on starch botanical source. As the gamma radiation dose increased, potato starch granules microstructure appeared more affected by gamma radiations than wheat one. Consequently, gamma radiation treatment altered molecular and physicochemical properties of A- and B-type starch.

Acknowledgments

The authors gratefully acknowledge Mr Mohamed Taïeb Jerbi and Mr Mokhtar Kraïem from the Centre National des Sciences et Technologies Nucléaires (CNSTN) for their encouragements and help. The authors are thankful to Mr. Mohamed Sghaier from the Electronic Microscope Laboratory of The Entreprise Tunisienne des Activités Pétrolières (ETAP) for his precious collaboration.