SOME PHYSICOCHEMICAL PROPERTIES OF COMMERCIAL MODIFIED STARCHES IRRADIATED WITH MICROWAVES

Teresa Fortuna¹, Izabela Przetaczek-Rożnowska¹, Krystyna Dyrek², Ewa Bidzińska³, Maria Łabanowska^2
1 Department of Analysis and Food Quality Evaluation, Agricultural University, Cracow, Poland
² Faculty of Chemistry, Jagiellonian University, Cracow, Poland
³ Regional Laboratory of Physicochemical Analyses and Structural Research, Jagiellonian University, Cracow, Poland

ABSTRACT

Commercial modified starches: gelling oxidized starch (E 1404), starch phosphate (E 1412), acetylated starch diphosphate (E 1414) and acetylated starch adypinate (E 1422), were irradiated with microwaves (440 and 800 W) and examined for water-binding capacity and water solubility at temperatures of 25°C and 60°C. Rheological properties of starch pastes were investigated by determination of flow curves using a rotational rheometer Rheolab MC1. It was found, that microwave irradiation increases the water-binding capacity and water solubility of starches. The pastes of starches irradiated with microwaves differed in the values of rheological parameters from those of non-irradiated ones. Investigation of the ability to form radicals, studied by EPR, confirmed that. irradiation with microwaves causes partial degradation of  the starch.

Key words: commercial modified starch, irradiation with microwaves, rheological properties, EPR spectroscopy, free radicals.

INTRODUCTION

Starch is one of the most functional components of food, but its low stability and low resistance to the mechanical, thermal or chemical factors involved in the technological processes of production, as well as the accompanying phenomena of syneresis or retrogradation, considerably limit the use of this polymer in all branches of the food industry [6,17,24].

In order to enhance particular properties of native starches or give them new, desirable features, starches are modified by chemical, physical or enzymatic methods (or their combinations). One of the physical methods of starch modification is irradiation with microwaves. Modified starches exhibit various properties and thus have a variety of uses as thickening, gelling and stabilizing agents. They are also added to foods intended for special groups of consumers, e.g. children, sportsmen and diabetics [5,8,9,17,22].

The qualitative properties of a food product depend both on the type and amount of starch addition, being determined by interactions between modified starch and water and/or other food components. So far, only few studies [12,13] have been focused on the influence of microwaves on the functional properties of starches. The present study was designed to investigate the effects of microwave irradiation on some physicochemical properties of commercial modified starches.

MATERIALS AND METHODS

Commercial modified starches: gelling oxidized starch (E 1404), starch phosphate (E 1412), acetylated starch diphosphate (E 1414) and acetylated starch adypinate (E 1422), were provided by Wielkopolskie Przedsiębiorstwo Przemysłu Ziemniaczanego S.A. (Luboń, Poland). The other materials used in the research were saccharose (Chempur, Piekary Śląskie, Poland), sorbitol (Dakart, Kielce, Poland), acesulfam K (Nutrinova, Germany) and aspartame (Nutra Sweet, Switzerland).

Starch samples weighing 20±0.5 g each were placed in Petri dishes (diameter 20 cm) and heated in a microwave oven Panasonic NN-K257W (frequency 2450 MHz) using a power of 440 or 800 W. All of the samples were irradiated for 5 min then the microwave oven was turned off for 5 min and after that the samples were irradiated again for 5 min. The heating time for microwave treatments was determined by trial and error. Care was taken to put the sample in the same place in the oven for each treatment. Following the treatments, the samples were cooled down to room temperature (2°C).

Both non-irradiated and treated with microwaves starches were examined for water-binding capacity (WBC), water solubility at temperatures 25°C and 60°C [20] and for reducibility [19].

The rheological experiments were performed using a rotational rheometer Rheolab MC1 with coaxial cylinders (gap 2.12 mm) as a measuring system (Physica Messtechnik GmbH, Germany). The rheometer was programmed using US 200 software via a networked PC. To determine flow curves, 5% (w/w) solutions of starches in water and in solutions of saccharose or its substitutes were prepared. The concentration of saccharose was chosen according to the recommendations of the manufacturers of blancmanges, while the solutions of other sweeteners were prepared in such a way that their sweetness was comparable with that of the saccharose solution. The samples were blended at room temperature for 5 min by using a mechanical stirrer (300 r.p.m.). Then, the solutions were heated in a water bath at 95±0.5°C for 30 min with continuous stirring (300 r.p.m.). The pastes obtained were immediatedly placed in the measuring element of the rheometer and thermostated for 15 min prior to determination. The flow curves were obtained at 25±0.5°C, 40±0.5°C and 60±0.5°C, at a shear rate increasing from 1 to 100 s^-1 during 10 min. The curves were described using the Herschel-Bulkley model:

where:
τ – shear stress (Pa),
τ₀ – yield stress (Pa),
– shear rate (s^-1),
K – consistency coefficient (Pa · sⁿ),
n – flow index (dimensionless).

The EPR measurements were carried out using an X-band (9.2 GHz) spectrometer Bruker ELEXSYS 500 (Karlsruhe, Germany) with 100 kHz field modulation. The EPR spectra were recorded at room temperature with 0.3 mT modulation amplitude and 3 mW power. The radicals were generated by irradiation with microwaves and by thermal treatment (at 150°C and 230°C). The number of radicals was determined by a comparison method with the VOSO₄ · 5H₂O/K₂SO₄ standard containing 5 × 10¹⁹ spins/gram. The procedure described in [4] was followed to ensure high precision of the quantitative measurements. The EPR parameters of radicals were found by the simulation procedure using program SIM 14 [16].

RESULTS AND DISCUSSION

The water-binding capacity of commercial starches non-irradiated and starches irradiated with microwaves increased with increasing temperature (Table 1), which agrees with the results of many studies [6,10,11]. Starches irradiated with microwaves had higher values of this parameter than non-irradiated ones. The increasing of microwave power from 440 W to 800 W had at temperature 25°C a significant influence on the water-binding capacity only for gelling oxidized starch and acetylated starch adypinate (Table 1). At temperature 60°C the effect of increasing power was visible only for gelling oxidized starch and starch phosphate (Table 1). Also the water solubility of starches increased with temperature and depended on irradiation power (Table 1). After irradiation of the starches at 400W only acetylated starch adypinate had significantly higher value of that parameter at temperature 25°C. All starches which were irradiated at 800 W shown visibly higher values of the water solubility at temperatures 25°C and 60°C (Table 1) than starches non-irradiated.

Table 1. Water-binding capacity and water solubility of commercial modified starches irradiated and non-irradiated with microwaves

Starch		Water-binding capacity [g·g-1 d.m.]		Water solubility [%]
		25°C	60°C	25°C	60°C
Gelling oxidized starch E 1404	non-irradiated	0.90 m	0.96 o	0.19 a	3.02
	irradiated  at 440 W	1.11	2.12	0.27 a	5.11
	irradiated at 800 W	1.19	4.22	0.67	10.21
	LSD 0.05	0.06	0.85	0.16	0.85
Starch phosphate E 1412	non-irradiated	0.92 m	1.85 o	0 c	0.88 e.s
	irradiated  at 440 W	1.14 b	3.93	0.04 c.d	1.50 e
	irradiated at 800 W	1.21 b	6.19	0.24 d	3.15
	LSD 0.05	0.15	1.39	0.21	1.95
Acetylated starch diphosphate E 1414	non-irradiated	0.82 m.n	2.00 o	0.14 h.p.r	1.51 s
	irradiated  at 440 W	1.04 f	7.74 g	0.20 h.i	4.14 j
	irradiated at 800 W	1.12 f	7.86 g	0.24 i	4.58 j
	LSD 0.05	0.14	0.91	0.07	1.53
Acetylated starch adypinate E 1422	non-irradiated	0.76 n	7.19 a	0.10 r	6.40
	irradiated  at 440 W	1.25	12.76 k	0.65	10.52
	irradiated at 800 W	1.40	13.28 k	0.83	11.56
	LSD 0.05	0.07	2.27	0.16	0.97

Values followed by letters a–k are not significantly different at α = 0.05.

Based on the obtained flow curves (Fig. 1) it was found that the pastes of all starches behaved as non-Newtonian fluids, were thinned by shear, and tended to exhibit yield stress, which is consistent with other studies [1,2,7,25]. The microwave power of 800 W markedly decreased the shear stress of starch pastes compared to non-irradiated samples, while the irradiation at 440 W produced an inconsiderable difference (Fig. 1).

Fig 1. Flow curves of commercial modified starch (acetylated starch diphosphate)

Fig 2. Flow curves of starch paste at temperatures 25°C, 40°C and 60°C

As the temperature of rheological measurements was raised, the shear stresses of all starches dropped (Fig. 2), which agrees with the findings of other authors [14,15,18,21].

Tables 2–5 show the values of the Herschel-Bulkley parameters describing the flow curves. High values of determination coefficients (R²) confirm that this model described well the experimental curves. The pastes prepared from non-irradiated starches exhibited higher values of yield stress (τ₀) at each temperature (25°C, 40°C and 60°C) than those irradiated with microwaves. This suggests that microwave irradiation considerably affects the structure of starch and reduces its flow resistance.

Table 2. Parameters of Herschel-Bulkley model for 5% pastes of gelling oxidized starch (E 1404)

Gelling oxidizing starch	25°C				40°C				60°C
	τ0 [Pa]	n [-]	K [Pa sn]	R2	τ0 [Pa]	n [-]	K [Pa sn]	R2	τ0 [Pa]	n [-]	K [Pa sn]	R2
non irradiated	2.27	0.51	3.01	0.9964	4.19	0.50	3.86	0.9981	6.38	0.45	3.72	0.9959
irradiated at 440 W	1.60	0.46	3.40	0.9965	3.65	0.46	3.14	0.9966	1.79	0.32	5.11	0.9905
irradiated at 800 W	2.02	0.51	2.48	0.9975	1.38	0.46	2.86	0.9971	1.22	0.42	2.89	0.9954

Table 3. Parameters of Herschel-Bulkley model for 5% pastes of starch phosphate (E 1412)

Starch phosphates	25°C				40°C				60°C
	τ0 [Pa]	n [-]	K [Pa sn]	R2	τ0 [Pa]	n [-]	K [Pa sn]	R2	τ0 [Pa]	n [-]	K [Pa sn]	R2
non irradiated	31.69	0.62	42.16	0.9954	39.02	0.66	17.94	0.9985	31.98	0.74	8.59	0.9989
irradiated at 440 W	22.70	0.68	19.04	0.9965	34.50	0.74	11.61	0.9983	28.27	0.79	6.33	0.9981
irradiated at 800 W	17.30	0.62	8.30	0.9982	15.58	0.64	7.40	0.9970	19.00	0.70	4.09	0.9987

Table 4. Parameters of Herschel-Bulkley model for 5% pastes of acetylated starch diphosphate (E 1414)

Acetylated starch diphosphate	25°C				40°C				60°C
	τ0 [Pa]	n [-]	K [Pa sn]	R2	τ0 [Pa]	n [-]	K [Pa sn]	R2	τ0 [Pa]	n [-]	K [Pa sn]	R2
non irradiated	17.06	0.65	13.21	0.9974	16.45	0.60	12.95	0.9963	17.18	0.63	8.35	0.9949
irradiated at 440 W	14.61	0.60	18.50	0.9996	15.63	0.63	11.31	0.9990	15.00	0.60	10.17	0.9973
irradiated at 800 W	12.06	0.69	6.44	0.9976	8.24	0.60	7.18	0.9957	9.82	0.60	5.55	0.9965

Table 5. Parameters of Herschel-Bulkley model for 5% pastes of acetylated starch adypinate (E 1422)

Acetylated starch adypinate	25°C				40°C				60°C
	τ0 [Pa]	n [-]	K [Pa sn]	R2	τ0 [Pa]	n [-]	K [Pa sn]	R2	τ0 [Pa]	n [-]	K [Pa sn]	R2
non irradiated	6.36	0.52	24.69	0.9999	8.08	0.53	18.58	0.9999	10.07	0.53	15.12	0.9998
irradiated at 440 W	6.88	0.54	21.61	0.9999	7.10	0.53	16.70	0.9999	9.04	0.54	12.04	0.9999
irradiated at 800 W	7.57	0.59	8.31	0.9990	6.16	0.53	8.82	0.9988	6.83	0.53	7.29	0.9995

As indicated by the values of flow index (n) (Tables 2–5), the extent of shear thinning depended both on the kind of starch and the power of microwave radiation. The shear thinning of starch pastes is due to the destruction of the existing intermolecular bonds of polysaccharide. When the rate of destruction is faster that the rate at which new bonds are formed, the shear resistance of starch decreases, which results in a drop of apparent viscosity [25].

In all three ranges of rheological measurements, the pastes of gelling oxidized starch irradiated with microwaves were shear-thinned to a greater degree than those of non-irradiated starch (Table 2). The pastes of acetylated starch adypinate showed an opposite pattern (Table 5). For starch phosphate, the degree of thinning depended on the power of microwaves: treating the paste with 800 W microwaves reduced its flow index (n) to the value that was lower than that of non-irradiated starch, while irradiating at 440 W caused it to increase over the latter level (Table 3). Acetylated starch diphosphate did not show any clear relationships between flow index and temperature or microwave power (Table 4).

Comparison of the values of consistency coefficient (K), which is a measure of fluid viscosity, demonstrates that it was considerably affected by microwave irradiation. The pastes of non-irradiated acetylated starch adypinate and starch phosphate showed apparent viscosities much higher than those of irradiated starches (Tables 3 and 5).

Gelling oxidized starch and acetylated starch diphosphate treated with 800 W microwaves had markedly lower consistency coefficients than their non-irradiated counterparts. Exposing them to 440 W did not produce any clear effect (Tables 2 and 4).

Mechanism of the interaction of starch with microwaves studied by EPR
As stated by us previously, irradiation with microwaves or conventional heating of the native potato and maize starch cause formation of two types of radicals, exhibiting EPR signals with similar g factor values (g = 2.006) [3]. One of them (signal I), revealing hyperfine structure (HFS), is attributed to the unpaired electron localized at C₍₁₎ atom of the glucose unit and interacting with nuclear spin of β-hydrogen at the neighboring carbon atom (Fig. 3).  Another signal, without HFS (signal II), is also assigned to unpaired electron localized at C₍₁₎ atom.

Fig 3. Schematic formulae of radicals generated in starch by irradiation with microwaves and/or by conventional heating

In the present work, influence of irradiation of commercial potato starch with microwaves at various power levels (440 W or 800 W) on the ability of radical generation was investigated during heating of the modified starch samples at 150°C or 230°C.

Fig. 4 shows the results of the EPR measurements for two reference samples of native starch (Sigma), potato and maize, and two commercial potato samples, gelling oxidized starch and acetylated starch adypinate. In all these cases increase of heating temperature causes generation of the bigger number of radicals. Influence of the power level of microwaves on the content of radicals is well seen for the samples heated at 150°C: the number of radicals increases with increasing power level (dashed line in Fig. 4).

Fig 4. Number of radicals generated thermally in starch upon irradiation with microwaves and heating

Heating of the samples at higher temperature (230°C) leads to the formation of greater amount of radicals in all the samples, but the influence of the different power level is observed only in the case of native potato starch and acetylated starch adypinate. Diminishing of the influence of microwaves on radical processes of starch degradation in other samples is caused by the fact that at 230°C the dominating role in destruction of the starch matrix plays thermal energy provided by heating, whereas the relative value of the previously supplied energy of microwaves became negligible.

CONCLUSIONS

1. Starch modified by irradiation with microwaves has a higher water-binding capacity and water solubility than non-irradiated starch. The values of both parameters increase with increasing irradiation power. Irradiation with microwaves causes starch reducibility to rise, with the increase being dependent on microwave power.

2. The pastes of non-irradiated commercial starches show higher yield stresses (in the temperature range used in this study) than the pastes of irradiated starches with the same concentration. Apparent viscosity is lowest for pastes obtained from starches treated with 800 W microwaves, and highest for non-irradiated starches.

3. The rheological results are consistent with the EPR data. The greater number of thermally generated radicals in irradiated samples than in non-irradiated ones indicates that microwave energy initiates degradation of the starch structure.

REFERENCES

1. Abu-Jdayil B., Azzam M.O.J., Al-Malah K.I.M., 2001. Effect of glucose and storage time on the viscosity of wheat starch dispersions. Carbohydr. Polym. 46, 207-215.

2. Acquarone V.M., Rao M.A., 2003. Influence of sucrose on the rheology and granule size of cross-linked waxy maize starch dispersions heated at two temperatures. Carbohydr. Polym 51, 4, 451-458.

3. Dyrek K., Bidzińska E., Łabanowska M., Fortuna T., Przetaczek I., Pietrzyk S., 2007. EPR study of radicals generated in starch by microwaves or by conventional heating. Starch/Stärke 59, 318-325.

4. Dyrek K., Rokosz A., Madej A., 1994. Spin dosimetry in catalysis research. Appl. Magn. Reson. 6, 309-332.

5. Fortuna T., 1995. Skrobie modyfikowane w produkcji żywności [Modified starch used In ford industry]. Żywn. Techno. Jakość 1(2), 3-7 [in Polish].

6. Fortuna T., Juszczak L., 2000. Struktura powierzchniowa ziaren skrobiowych [A starch's surfach structures]. Żywn. Techno. Jakość 4, 25, 36-47 [in Polish].

7. Genovese D.B., Acquarone V.M., Youn K.-S., Rao M.A., 2004. Influence of fructose and sucrose on small and large deformation rheological behaviour of heated amioca starch dispersions. Food Sci. Technol. Int. 10, 1, 51-57.

8. Golachowski A., 1998. Stosowanie skrobi i jej przetworów w przemyśle spożywczym [Usage starch and starch products In a ford industry]. Zesz. Nauk. AR Wrocł. Technol. Żywn. 326, 117-124 [in Polish].

9. Kuntz L.A., 1997. Making the most of maltodextrins. Food Prod. Design 8, 89-104.

10. Leszczyński W., 1992. Zmiany właściwości skrobi wywołane działaniem czynników fizycznych [The changes of starches properties which were effects of physical modifivation]. Mater. 4 Letniej Szkoły Skrobiowej – problemy modyfikacji skrobi. Zawoja 63-78 [in Polish].

11. Leszczyński W., 2001. Zróżnicowane właściwości skrobi. [Differential starche's properties]. Przem. Spoż. 55, 3, 38-39 [in Polish].

12. Lewandowicz G., 2001. Modyfikacja skrobi z użyciem pola mikrofalowego [Starch modification by microwave irradiation]. Zesz. Nauk. AR Krak. 276 [in Polish].

13. Lewandowicz G., Jankowski T., Fornal J., 2000. Effect of microwave radiation on physico-chemical properties and structure cereal starches. Carbohydr. Polym. 42, 193-199.

14. Li J.-Y., Yeh An-I., 2003. Effects of starch properties on rheological characteristics of starch/meat complexes. J. Food Eng. 57, 3, 287-294.

15. Lovedeep K., Narpinder S., Navadeep S.S., 2002. Some properties of potatoes and their starches. II Morphological, termal and rheological properties of starches. Food Chem. 79, 183-192.

16. Lozos G. P., Hoffman B. M., Franz C. G., 1974. SIM 14 Program. Chemistry Department, Northwestern University IL, QCPE, 265.

17. Nadison J., 1995. Skrobia modyfikowana. Rodzaje, właściwości, zastosowanie produktu [Starche's modificated. Kinds, proprties and using starch’s products]. Przem. Spoż. 49, 209–212 [in Polish].

18. Nayouf M., Loisel C., Doublier J.L., 2003. Effect of thermomechanical treatment on the rheological properties of crosslinked waxy corn starch. J. Food Eng. 59, 2, 209-219.

19. PN 78/A-74701 Hydrolizaty skrobiowe (krochmalowe). Metodyka badań [Starch hydrolizates. The methods]. [in Polish].

20. Richter M., Augustat S., Schierbaum F. 1968. Ausgewählte Methoden der Stärkechemie [Selected methods in starch analyses]. VFB Fachbuchverlag, Leipzig [in German].

21. Tattiyakul J., Roa M.A. 2000. Rheological behavior of cross-linked waxy maize starch dispersions during and after heating. Carbohydr. Polym. 43, 3, 215-222.

22. Tomasik P., 2000. Skrobie modyfikowane i ich zastosowania [The modified starches and way for use them]. Przem. Spoż. 4, 16-18 [in Polish].

23. Walkowski A., Lewandowicz G., 2004. Skrobie modyfikowane, właściwości technologiczne i zakres stosowania [The modified starches, their properties useful In technology and a range of interesting them]. Przem. Spoż. 5, 49-51 [in Polish].

24. Wurzburg O.B., 1987. Modified starches – properties and uses. Boca Raton Florida CRS Press Inc.

25. Yoo D., Yoo B., 2005. Rheology of rice starch – sucrose composites. Starch/Stärke, 57 (6), 254-261.

Accepted for print: 4.11.2008

---

Teresa Fortuna
Department of Analysis and Food Quality Evaluation,
Agricultural University, Cracow, Poland
Balicka 122, 30-149 Cracow, Poland
email: rrfortun@cyf-kr.edu.pl

Izabela Przetaczek-Rożnowska
Department of Analysis and Food Quality Evaluation,
Agricultural University, Cracow, Poland
Balicka 122, 30-149 Cracow, Poland

email: jeany19@tlen.pl

Krystyna Dyrek
Faculty of Chemistry, Jagiellonian University, Cracow, Poland


Ewa Bidzińska
Regional Laboratory of Physicochemical Analyses and Structural Research,
Jagiellonian University, Cracow, Poland


Maria Łabanowska
Faculty of Chemistry, Jagiellonian University, Cracow, Poland

---

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.

---

Main  -  Issues  -  How to Submit  -  From the Publisher  -  Search  -  Subscription