Ultrastructure of potato starch granules as affected by microwave treatment

ABSTRACT

The effect of microwave heating on the lamellar ultrastructure of potato starch granules was studied using small-angle X-ray scattering (SAXS), and the linear correlation function was generated from SAXS data to obtain the parameters of the lamellar repeats architecture. To investigate how the molecular vibration of the microwaves influences the lamellar structure during heating, rapid convection heating was applied through an oil bath for comparison. The results showed that the samples subjected to the microwave and the rapid convection heating displayed similar data, which could demonstrate that the effect of microwave heating on submicroscopic structure of potato starch may be mainly due to its rapid heating effect.

KEYWORDS:

• Microwave
• Potato starch
• Small-angle X-ray scattering

Introduction

The structure of a native starch granule consists of alternating amorphous and semi-crystalline growth rings of 120–400 nm thickness, and the semi-crystalline growth rings are made of alternating amorphous and crystalline lamellae with a repeat distance around 9 nm.^[1] Starch granules are often heated in water for application, when the granules are observed to swell, absorb water, lose crystallinity, and to leach amylose known as gelatinisation.^[2,3] Exhaustive techniques, including DSC, X-ray scattering, microscopy, and NMR spectroscopy, have been employed to study this event in the last two decades.^[4] However, the detailed mechanisms during the gelatinisation prove to be hard to elucidate, since a large number of independent variables involved species, growth conditions, extraction procedures, water content, and heating rate.^[5]

Microwave heating (MV) differs from traditional convection heating: Materials absorb microwave energy, and this energy is subsequently converted into heat through the molecular vibration and friction.^[6] A large amount of heat can be generated within a very short time, and thus more modified starches have been produced by the microwave treatment.^[7] However, it remains unclear the mechanisms underlying how microwave heating affects starch granules, which might be related to the fast heating rate^[8] or to the strong vibrational motion of the polar molecules.^[9] Therefore, in order to develop a better understanding of such issue, an oil bath was used to imitate the high heating rate for comparison in this study.

In the initial stage of heating, the general physicochemical properties are not dramatically altered. However, the submicroscopic lamellar structure has been reported to be obviously changed in the early stage of heating treatment.^[1,4,5] Therefore, some techniques such as DSC could not detect the initially altered structure. In this work, small-angle X-ray scattering (SAXS) was applied to monitor the changes of amorphous and crystalline lamellae in the potato starch during microwave heating with rapid heating method as control, and we try to obtain detailed characteristic parameters of starch lamellae through the linear correlation function derived from SAXS data, such as long period (LP), thickness of amorphous layers (da), and thickness of crystalline layers (dc). This study might provide more details of how the microwave heating affects starch granules.

Materials and methods

Materials

Potato starch (protein content 0.23%, starch content 94.22%, amylose content of 13.62%, water content 5.5%, particle size of 38.55 ± 2.48 µm, weight-average molar mass of 5.048 × 10⁷ g/mol) was from Fengning Shuangxin Agricultural Development Co., Ltd. The onset gelatinisation temperature (To) of potato starch was 57.16 ± 0.11°C. The water content would affect the dielectric properties of starch samples and further enhance the effect of microwave on starc.^[10] The permittivity (ε’) and dielectric loss factor (ε”) of native starch with 5.5% water content is 3.04 and 0.23, respectively, and the ε’ and ε” of 3% concentration of starch suspensions (97% water content) is 70.76 and 9.75, respectively, which indicates that the water significantly improves the dielectric properties of the system.

Rapid convection heating

Heating models were based on previously reported methods.^[5] An oil bath was conducted using a thermostat (Thermo Fisher Scientific Co., Ltd., Massachusetts, USA), and the set temperature was 200°C. After the oil bath reached 200°C, the starch suspensions (100 g) with a concentration of 3% (w/w) were heated to the desired temperature (40°C, 45°C, 50°C, 55°C, and 60°C, respectively) in quartz reactor under mechanical agitation with a stirrer to ensure uniform heating. The temperature of starch sample was monitored online using an optical fibre probe (FISO Technologies Inc., Québec, Canada). The suspensions were then immediately placed into an ice bath. Subsequently, the treated samples were lyophilised.

Microwave heating

The MV experiment was conducted in the same manner as the rapid convection heating (RCV) condition described above, except that a microwave oven was used for heating. A 2450 MHz oven (Xianou Instrument Manufacturing Co., Ltd., Nanjing, China) with an output power of 1.2 kW was used to match the heating rate obtained using an oil bath. The following temperature programme was applied: (1) 1000 W heating for 70 s; (2) 350 W heating for 50 s; and (3) 650 W heating for 25 s. The time-temperature curves for MV and RCV completely coincide, and the heating rates of starch samples heated by MV and RCV are both observed to be 27.2°C/min.

Small-angle x-ray scattering

SAXS experiments were carried out by following the procedures reported previously with slight modifications.^[11] Briefly, SAXS tests were performed on a SAXSess mc2 nanostructure analyser (Anton Paar, Austria), with an incident X-ray monochromatic beam (λCuKα = 1.54 Å). Scattering in the range (scattering vector q (q = 4πsin(θ)/λ, with λ as the wavelength and θ as half the scattering angle)), 0.04–2.70 nm⁻¹ was detected. Dried starch samples were weighted (11.0 ± 0.3 mg) and placed on the aluminium foil. Water was carefully added (11 μL) to the starch, and the sample was sealed to prevent any change in the water amount during experimentation. Experimental SAXS curves were then desmeared with the SAXS Quant software (Anton Paar, Austria) for collimation distortions. The obtained SAXS data are analysed via the linear correlation function γ(x) to obtain the morphological parameters of starch samples. The correlation function was calculated using a cosine transformation of the processed intensity curves, I(q)^[12]):

(1)

where x represents the distance in real space. The further analysis of γ(x) was constructed by methods essentially as described previously.^[13]

Results and discussion

In this work, the effect of microwave heating on the lamellar ultrastructure of potato starch granules is studied at sub-gelatinisation levels, since the lamellar repeats architecture would not exist when the heating temperature is above the gelatinisation level. Before starch samples were heated to 60°C, typical SAXS patterns of the treated starch could be observed in Fig. 1, which exhibits broad scattering peaks in the range with q 0.60–0.80 nm⁻¹. The peak reveals a position related to the average total thickness of the crystalline and amorphous regions in the lamellar arrangement in potato starches, called Bragg spacing, DB = 2π/qp with qp being the modulus of the scattering vector at peak intensity.^[1] The value of DB for the native potato starch is 9.98 nm, which is consistent with the previous literature.^[14] It could be seen from the changes of DB shown in Table 1 that there is only a slight shift of the peak position after the heating treatment. The peaks of starch samples respectively subjected to MV and to RCV become less pronounced with the increase of heating temperature. This decrease of peak intensity implies the amorphisation of the crystalline lamellae.^[15] However, above 55°C, the rate of decrease in peak intensity is much higher for MV, suggesting that in the later stage of heating, the vibrational motion of polar molecules applied by the microwave might speed up the rate of the dissociation of semi-crystalline structure inside the starch granule.^[13]

Table 1. The feature dimensions of the lamellar structures of starch samples conducted by microwave (MV) and rapid convection (RCV) heating.

Parameters^a	Native starch	MV 40°C	MV 45°C	MV 50°C	MV 55°C	MV 60°C
DB (nm)^d	9.98	9.93	9.53	9.44	9.35	ND^b
LP (nm)^e	9.60	9.64	9.73	9.76	9.80	CNC^c
ΔL (nm)^f	0.38	0.29	−0.20	−0.32	−0.45	CNC
da (nm)^g	1.94	1.94	1.96	1.96	1.97	CNC
dc (nm)^h	7.66	7.70	7.77	7.80	7.83	CNC
Φ^i	0.1982	0.2018	0.2013	0.2005	0.2796	CNC
Parameters^a	Native starch	RCV 40°C	RCV 45°C	RCV 50°C	RCV 55°C	RCV 60°C
DB (nm)	9.98	10.03	9.53	9.44	9.35	ND^b
LP (nm)	9.60	9.63	9.71	9.74	9.80	CNC^c
ΔL (nm)	0.38	0.40	−0.18	−0.30	−0.45	CNC
da (nm)	1.94	1.96	1.97	1.97	1.98	CNC
dc (nm)	7.66	7.67	7.74	7.76	7.82	CNC
Φ	0.1982	0.2006	0.2109	0.1999	0.2581	CNC

^aThe standard deviations of each parameter value were all smaller than 0.08.

^bNo scattering maximum detectable in the desmeared SAXS pattern.

^cCannot be calculated. ^dDB, Bragg lamellar repeat distance.

^eLP, long period, i.e., the lamellar repeat distance.

^fThe difference between DB and LP (the value of DB subtracted by LP).

^gda, the thickness of amorphous layers.

^hdc, the thickness of crystalline layers.

ⁱΦ, the volume fractions of the amorphous phase in the semi-crystalline stacks of starch.

Figure 1. Small-angle X-ray scattering (SAXS) patterns of starch samples conducted by microwave (MV) and rapid conventional (RCV) heating.

To obtain more details about the lamellar structure of potato starch, here we apply the pseudo-two-phase model in lamellar semi-crystalline polymers (sharp boundaries at the crystal/amorphous interface with an electron density gradient transition layer) by use of one dimension correlation function, γ(x), based on SAXS data to analyse the SAXS patterns of treated potato starch samples. Table 1 shows that the changes of the lamellar structure (DB, dc, and da) of samples treated by MV and RCV have similar trends during heating, suggesting that the effect of microwave heating on submicroscopic structure of starch may be mainly due to its rapid heating effect.

DB could only accurately describe the average repeat distance when all of the lamellae within the semi-crystalline growth rings are identical. Thus, we introduce LP, the average repeat distance of the amorphous and crystalline material (i.e., da + dc). The difference between LP and DB (ΔL) has positive relation with long-range disorder.^[16] It can be seen from Table 1 that for both heating manners, the smallest differences occur in the starch samples with a final temperature of 45°C, indicating that rapid heating treatment would reduce the long-range disorder within the semi-crystalline growth rings of potato starch in the very initial stage of heating. It is noteworthy that the molecular vibration of the microwaves seems to enhance this reduction of the long-range disorder since the value of ΔL at 45°C for the MV treated samples is smaller than its RCV counterparts.

The fraction of the amorphous phase in the semi-crystalline stacks of potato starch (Φ) is increased by both heating methods, compared with the native starch samples, which is different with our previous study on the rice starch.^[13] This might be because the potato starch particle has a relatively loose lamellar structure, that is, its semi-crystalline could be more easily affected by heating. After 50°C, a sharp increase of the value of Φ can be observed, identifying the disordering of the crystalline lamellae.

Table 2 reports the electron density differences between the crystalline and amorphous regions of starch samples subjected to MV and RCV. It is found that there is a slight increase of Δρ before starch samples are heated to 45°C, which reveals that the rapid heating treatment increases the electron density of the crystalline phase or decreases that of the amorphous phase in the early stage of heating. Obviously, 45°C is far lower than the onset temperature of potato starch gelatinisation; therefore, insufficient time is available for the starch granules treated by MV and RCV to absorb adequate water, that is, the electron density of the amorphous lamellae shall remain unaffected. Hence, we demonstrate that the electron density of the crystalline phase has been increased by the rapid heating treatment, which is consistent with the ΔL results shown above. It indicates that the rapid heating effect of microwaves would render the crystalline lamellae in potato starch more ordered in the initial stage of heating. However, such rapid heating could not maintain the sharp surface area of the phase boundary since the thickness of transition layer decreases with the increase of heating temperature.

Table 2. The electron density difference and the transition layer thickness between the crystalline and amorphous phases of starch samples conducted by microwave (MV) and rapid convection (RCV) heating.

Parameters^a	Native starch	MV 40°C	MV 45°C	MV 50°C	MV 55°C	MV 60°C
Δρ (a.u.)^c	2.38	2.69	2.79	2.60	2.39	CNC^b
σ (nm)^d	0.56	0.33	0.27	0.29	0.30	CNC
Parameters^a	Native starch	RCV 40°C	RCV 45°C	RCV 50°C	RCV 55°C	RCV 60°C
Δρ (a.u.)	2.38	2.71	2.82	2.62	2.50	CNC
σ (nm)	0.56	0.29	0.27	0.27	0.26	CNC

^aStandard deviations of each parameter value were all smaller than 0.03.

^bCannot be calculated.

^cΔρ, the electron density difference between the crystalline and amorphous material.

^dσ, the thickness of transition layer.

Conclusion

In this study, we established heating models by comparison microwave heating to rapid heating method to investigate the effect of microwave on lamellar structure in the semi-crystalline growth rings of potato starch based on the lamellar parameters obtained from the linear correlation function γ(x). The fraction of the amorphous phase in the semi-crystalline stacks of potato starch (Φ) is increased by both heating methods. It could be seen that due to the rapid heating effect of microwaves, ordered lamellar arrangements within starch semi-crystalline region are destroyed before the onset of gelatinisation. The behaviour of lamellar structure in the early phase would be different from that in the late phase of microwave heating process. In the further study on the process of gelatinisation, the heating rate should be attached more importance.

Funding

This research was supported by National Natural Science Foundation of China (Grant No. 31571879, 31301504) and Doctoral Program of Higher Education Research Fund (Grant No. 20130093120011).

Additional information

Funding

This research was supported by National Natural Science Foundation of China (Grant No. 31571879, 31301504) and Doctoral Program of Higher Education Research Fund (Grant No. 20130093120011).