The influence of roasting conditions on the potato starch (PS) composition, solubility, crystallinity, gel-forming thermal profiles, and texture of the corresponding gels was studied. Thermorheological testing of roasted starches with (RPS) and without (RPSI) the soluble fraction was conducted on a stress-controlled rheometer. Texture profile analysis (TPA) was used to determine the RPS final gel texture. Solubility tests showed equivalent effects of starch roasting for samples treated at 190°C for 8 h (RPS190-8), 210°C for 6 h (RPS210-6), and 230°C for 4 h (RPS230-4). The apparent amylose content of RPS linearly increased with the decreasing degree of crystallinity. Elastic (G
Potato starch (PS) is one of the most used biopolymers for industrial starch production, since the extraction procedure is simple in comparison to cereal starches [
Starch modification can be made using physical, chemical, or biological methods [
The knowledge of the thermomechanical behaviour during the preparation of starchy-based gels offers essential information to select the optimum processing conditions to achieve well-defined and tailor-made final products, in particular, for gels from modified starches [
The effects of roasting of flour or isolated starches in physicochemical, mechanical, and morphological properties have been scarcely addressed. In this context, the main objective of this work is to study the effect of PS roasting conditions on its gel-forming thermal profiles (so-called gel-forming/maturation kinetics), final thermomechanical properties, and thermoreversible properties, which is critically relevant to the industrial processing. A comprehensive research on the thermorheology and texture of native PS and roasted PS (RPS) in a broad range of temperatures was carried out. Rheological testing at small amplitude oscillatory shear (SAOS) and texture profile analysis (TPA) was used to evaluate the system structure evolution and the final gel texture, respectively. Fundamental chemistry (i.e., moisture content, total starch content, apparent amylose ratio, or damage starch), cold solubility, crystallinity, and particle size features of tested starches were also undertaken for further insight.
A commercial potato starch (PS) (CAS 9005-84-9, Panreac Química, S.A., Spain) was selected as a raw material for this research as it is a commonly used reference starch. All reagents used for the different tests were of analytical grade.
Commercial PS (moisture content: 24.3 ± 0.3%, dry basis (d.b.)) was dried at 40°C in a vacuum oven before roasting to reduce the moisture content (around 6.1 ± 0.8%, d.b) to values commonly used in roasting starch processes [
The starch solubility index was obtained following a modified protocol [
Moisture content was determined according to the standard method (925.10) [
The determination of the crystalline structure of tested potato starches was conducted on a diffractometer (X-ray PW1710, Philips, Netherlands) by means of CuK
The particle size distribution and weight average particle diameter (
Starches were dispersed in distilled water (30%
For rheological measurements, aqueous starch dispersions (PS, RPS, and RPSI) were directly placed into the rheometer measuring system. This allows tracking of the starch gelatinisation in situ, avoiding further gel matrix disturbances. For texture measurements, stirred PS and RPS dispersions (3000 rpm for 15 min) prepared in cylindrical flasks (4.5 cm height and 2.5 cm diameter) were immediately heated up to 80°C as reported elsewhere [
Rheological measurements (in terms of storage (G
Potato starch dispersions were heated from 25 to 80°C at a rate of 1°C/min. In order to stabilise the samples, time sweep assays (0.1 Hz) were thereupon conducted at 80°C for 60 min. Samples were cooled down to 25°C (1°C/min). All the above-mentioned tests were performed at 0.1 Hz and 5 Pa. Gel maturation was monitored by time sweeps (1 Hz, 30 Pa) after cooling at 25°C for 60 min. Frequency sweeps (over the range of 0.01 to 100 Hz, 30 Pa) were thereupon conducted at 25°C without disturbing the gel. All these stages were repeated following an experimental protocol to study the thermal reversibility of obtained gels [
Texture profile analysis (TPA) measurements were conducted on a texture analyser (TA-XT plus, Stable Micro Systems, UK) with a load cell of 5 kg as reported elsewhere [
Starch gel samples, prepared as those used in textural tests, were stored in centrifuge tubes in the fridge (5°C) for 7 days. Each sample was centrifuged (2200
All tests were performed in triplicate, and data sets were evaluated by means of one-way analysis of variance (ANOVA), using PASW Statistics (IBM SPSS Statistics 22.0.0). A Scheffé test was performed to distinguish means with 95% confidence (
Figure
Influence of time-temperature roasting conditions on the solubility of potato starch (PS). Note here that in these subsequent plots, error bars are not included whether the uncertainty in data magnitudes is smaller than the symbol size.
Solubility curves of RPS190, RPS210, and RPS230 displayed an initial step increase, and after the maximum the solubility is achieved, the plots showed a downward tendency. This solubility loss was slightly promoted at high temperatures. All RPS solubility curves tend to reach a common solubility value after 24 h (long time) subjected to roasting. This could be explained assuming that larger molecular weight glucans are formed from the fragments produced in the early starch roasting stages [
Composition experiments were carried out in roasted starches after the corresponding times that led to the largest solubility values. Table
Proximate physicochemical features of tested potato starch (PS) (30%
PS | RPS120-8 | RPS150-8 | RPS170-8 | RPS190-8 | RPS210-6 | RPS230-4 | |
---|---|---|---|---|---|---|---|
Roasting temperature | — | 120 | 150 | 170 | 190 | 210 | 230 |
Roasting time, h | — | 8 | 8 | 8 | 8 | 6 | 4 |
Initial moisture content, % d.b. | 6.1 ± 0.8a | 7.1 ± 1.0a | 6.8 ± 1.3a | 6.6 ± 1.2a | 5.8 ± 0.6a | 5.5 ± 0.9a | 5.3 ± 1.2a |
Total starch content, % d.b. | 98.7 ± 0.5a | 97.4 ± 0.4b | 97.1 ± 0.5b | 96.7 ± 0.5b | 93.5 ± 0.2c | 92.8 ± 0.4c,d | 92.4 ± 0.3d |
Apparent amylose content, % | 21.6 ± 0.4d | 23.4 ± 0.3c | 23.6 ± 0.2c | 23.2 ± 0.4c | 29.3 ± 0.5b | 29.7 ± 0.3a,b | 30.6 ± 0.3a |
Damaged starch level, % | 2.9 ± 0.6a | 2.9 ± 0.5a | 2.9 ± 0.5a | 3.0 ± 0.7a | 3.5 ± 0.4a | 3.7 ± 0.6a | 3.8 ± 0.5a |
Damaged/total starch, % | 0.029 ± 0.003b | 0.030 ± 0.002b | 0.030 ± 0.002b | 0.031 ± 0.002b | 0.037 ± 0.001a | 0.040 ± 0.002a | 0.041 ± 0.002a |
Crystallinity level (30 days), % | 22.6 ± 0.9a | 15.1 ± 0.2b,C | 14.6 ± 0.3b | 14.9 ± 0.3b,C | 6.7 ± 0.3c,C | 5.4 ± 0.1c,d | 5.3 ± 0.4d,C |
0 day |
— | 17.7 ± 0.4a,B | — | 17.2 ± 0.2a,B | 10.4 ± 0.2b,B | — | 9.8 ± 0.1c,B |
1 day |
— | 16.2 ± 0.2a,A | — | 16.2 ± 0.4a,A | 9.5 ± 0.2b,A | — | 8.6 ± 0.3c,A |
3 days | 15.2 ± 0.3a,C | — | 15.1 ± 0.5a,C | 6.9 ± 0.3b,C | — | 6.1 ± 0.3c,C | |
56.2 ± 0.5a | 50.5 ± 0.4b | 50.1 ± 0.3b | 49.6 ± 0.3b | 40.1 ± 0.4c | 39.9 ± 0.1c | 39.7 ± 0.3c |
Data are presented as mean ± standard deviation. Data values in a row with different superscript letters are significantly different at the
The degree of crystallinity (%) of tested starches is presented in Table
In all cases, the average particle size of tested starches was around 61.2 ± 2.4
Roasted starch samples with the largest solubility values (i.e., RPS120-8, RPS190-8, and RPS230-4) for each roasting temperature were selected to the thermorheological assessment (Figure
Thermorheological features of aqueous dispersions of potato starch (30%
The thermal cooling profiles for RPS120-8, RPS190-8, and RPS230-4 gels are presented in Figure
The maturation profiles in terms of the viscous and elastic modulus evolution with time for above RPS gels (i.e., RPS120-8, RPS190-8, and RPS230-4) are shown in Figure
In G
Figure
Mechanical spectra at 25°C of selected gels prepared from RPS120-8, RPS190-8, and RPS230-4. Note here that grey symbols correspond with the mechanical spectra data collected from the second heating/cooling cycle.
Influence of roasting conditions on the mechanical behaviour of the final potato starch gels (G
Further insight on the behaviour of concentrated starch gels is critically relevant in order to improve the processing conditions of in multicomponent systems with high starch content as flour doughs. Figure
Figure
Table
Texture parameters of tested potato starch gels (30%
Gels | Hardness (kg) | Adhesiveness (kg s) | Springiness (−) | Cohesiveness (−) | Chewiness (kg) |
---|---|---|---|---|---|
PS | 2.74 ± 0.08c | 0.04 ± 0.01b | 0.69 ± 0.01c | 0.29 ± 0.03b | 1.70 ± 0.12e |
RPS120-8 | 2.78 ± 0.07b,c | 0.04 ± 0.01b | 0.71 ± 0.02b,c | 0.39 ± 0.02a | 2.32 ± 0.12d |
RPS150-8 | 2.83 ± 0.08b,c | 0.04 ± 0.01b | 0.73 ± 0.01b,c | 0.39 ± 0.02a | 2.43 ± 0.11c,d |
RPS170-8 | 2.94 ± 0.09b | 0.05 ± 0.01a,b | 0.74 ± 0.02b | 0.40 ± 0.01a | 2.62 ± 0.13c |
RPS190-8 | 3.25 ± 0.06a | 0.07 ± 0.01a | 0.84 ± 0.04a | 0.41 ± 0.02a | 3.38 ± 0.08b |
RPS210-6 | 3.32 ± 0.11a | 0.08 ± 0.01a | 0.88 ± 0.09a | 0.42 ± 0.03a | 3.67 ± 0.09a,b |
RPS230-4 | 3.38 ± 0.10a | 0.08 ± 0.01a | 0.93 ± 0.10a | 0.42 ± 0.02a | 3.98 ± 0.12a |
The outcomes found from textural tests are related to those obtained from small deformation rheology. Hardness exhibited a linear relationship (
The absence of water syneresis observed in the most RPS gels aged during one week at room temperature is a critically relevant feature in gels. Only gels prepared from RPS230-4 exhibited slight water syneresis (below 3% of the total gel). Negligible release of water was considered (<1%) for other concentrated starchy gelled systems as those formulated with sorghum starch [
To conclude, it should be indicated that physicochemical features, the forming profiles of the gels, and the rheological and textural characteristics of the final gels for potato starch systems strongly depend on the roasting starch conditions. The largest physicochemical, rheological, and textural differences were identified between RPS170 and RPS190 systems. The adequate and almost equivalent solubility effect on starch roasting occurs for RPS190-8, RPS210-6, and RPS230-4. Temperatures below 170°C led to RPS gels with medium strength and fully thermal reversible gel formation, while temperatures between 190 and 230°C promoted strong and fully thermal reversible gel formation, with no fully thermal reversible gels at 230°C. The critically relevant role of the starch soluble fraction was observed in the weakness of RPSI gels with increasing roasting temperature, intimately linked with the apparent amylose content reduction. Linear and exponential dependences were identified between G
All data generated or analysed during this study are included in this published article.
The authors declare that they have no conflicts of interest.
The authors acknowledge the financial support (ED431B) from the Consellería de Cultura, Educación e Ordenación Universitaria de la Xunta de Galicia (Spain). We also would like to thank the use of RIAIDT-USC analytical facilities for X-ray diffraction experimental assays.