Effect of multi cycle heating-cooling treatment on the physical and morphological properties of white sweet potato flour

Abstract

We investigated the effect of multi cycle heating-cooling treatment (HCT) (1, 2, and 3 cycles at 85, 95, and 105 °C) on the physical properties (color, pasting, and crystallinity) and morphological properties of sweet potato flour (SPF). The results showed that the lightness, pasting viscosity, and relative crystallinity of SPF decreased gradually as the heating temperature and numbers of cycles were increased. No significant differences on the crystalline pattern of native and HCT samples were observed. The granule surface of HCT samples exhibited cracking.

Introduction

Among tropical crops, sweet potato (Ipomoea batatas L.) is an important carbohydrate source after rice, maize, and cassava (Pranoto et al., 2014). Sweet potato flour (SPF) can also be used as a substitute for cereal flours and it is also beneficial food for people with celiac diseases (Shih et al., 2006). However, due to the limited physical and chemical qualities of native sweet potato flour, it needs to be modified prior to application in certain types of food processing.

Heating-cooling treatment (HCT) which is also known as heat-moisture treatment (HMT) is a set of physical treatments to alter the physicochemical properties of starch without destroying its granular structure. HMT is typically performed under conditions such as restricted moisture content (10–30%) and heating at high temperatures (90–120 °C) for a time period ranging from 15 min to 16 h (Chung et al., 2009; Maache-rezzouget al., 2008). HCT is a cost-effective method that can easily be applied, and it has a greater effect on the digestibility, physicochemical, and structural properties of tuber starches than those of legume or cereal starches (Hoover and Vasanthan, 1994). Therefore, the objective of the current research was to study the effect of multi cycle HCT on the physical and morphological properties of SPF.

Materials and Methods

Materials    White sweet potato tuber was supplied by local farmer.

Preparation of sweet potato flour    Sweet potato flour was prepared by the following procedure: fresh sweet potato tuber was washed, peeled, soaked in 0.1% sodium metabisulfite for 30 min, and then sliced. The slices were dried in a cabinet dryer at 50 °C until their moisture content was less than 10%. The dried slices were ground by a disc mill and were sieved with a 40-mesh screen.

Heating-cooling treatment (HCT)    Sample was treated by HCT method of Huang et al. (2016) with slight modifications. Water was gradually mixed with SPF (300 g) until its moisture content reached 30%. The wet flour then was layered on an aluminum tray, sealed, and kept for 24 h at 4 °C. Then, the sample was heated in a forced air convection drying oven (Memmert, Germany) at a certain temperature (85, 95, and 105 °C). For one cycle treatment, the samples were heated continuously for 6 h (S-85, S-95 and S-105). Meanwhile, the samples of two (D-85, D-95, and D-105) and three (T-85, T-95, and T-105) cycles treatments were subjected to heatingcooling treatments with the following sequence: heating 3 h→cooling 24 h→ heating 3 h; and heating 2 h→cooling 24 h→ heating 2 h→cooling 24 h→ heating 2 h, respectively. All cooling stages were performed at 4 °C. Subsequent to the treatment, the sample was dried in a drying cabinet at 45 °C for 18 h. Then, the sample was ground and sieved by a 40-mesh screen. Prior to analysis, sample was stored in a sealed plastic bag and kept in an air-conditioned room.

Color    The color of samples was determined by a color meter (NH3, China) according to Makinde and Akinoso (2014). The color parameters including L*, a*, and b* were reported. The L*, a*, and b* values indicate the measure of lightness, red-green, blue-yellow, respectively. The color measurement of samples was done in duplicate.

X-ray diffraction    X-ray diffraction analysis of samples were performed by using an X-ray diffractometer (Rigaku Smartlab) operated at 40 kV and 30 mÅ with Cu Ka radiation (wavelength of 1.54 Å). The sample was scanned from 3° to 40° at a rate of 2°/min. The peaks of crystalline and amorphous were fitted by using SAXSIT software developed by the Synchrotron Light Research Institute of Thailand. Then, the relative crystallinity of sample was reported (Sholichah et al., 2017).

Pasting properties    The pasting properties of samples were measured by using a Rapid Visco Analyzer (RVA-TecMaster, Macquarie Park, Australia). Sample (3.5 g, moisture content of 14%) was gradually mixed with distilled water (25 g) in a RVA canister. The instrument was operated with the following sequence: the sample was spun at 160 rpm and heated from room temperature to 50 °C and then held for 1 min. The heating was continued up to 95 °C within 7.5 min and held at that temperature for 5 min. Then, the sample was cooled down to 50 °C within 7.5 min and kept at 50 °C for 2 min (Pinto et al., 2015). Pasting properties of sample including peak viscosity, final viscosity, breakdown viscosity, setback viscosity, and pasting temperature were determined. The pasting properties of samples were measured in duplicate.

Scanning electron microscopy    The morphological properties of sample were observed by a scanning electron microscope (JSM-6510) at an acceleration potential of 10 kV and magnification of 3 000x. Samples were scattered on a double-sided adhesive tape attached to a circular aluminum stub and then coated with gold (Pinto et al., 2015).

Statistical analysis    Data was statistically analyzed by a SPSS ver. 16.0 software using one-way ANOVA and Tukey's test (Heinio et al., 2003).

Results and Discussion

Color and crystal structure of native and HCT-SPF    The color and relative crystallinities of native and HCT-SPF are presented in Table 1.

Table 1. Color parameters (L*, a* and b*) and relative crystallinities of native and heating-cooling treated sweet potato flour (HCT-SPF) at one cycle heating temperature of 85 (S-85), 95 (S-95), and 105 °C (S-105), two cycles heating temperature of 85 (D-85), 95 (D-95), and 105 °C (D-105), and three cycles heating temperature of 85 (T-85), 95 (T-95), and 105 °C (T-105).

Treatments	Color parameters			Relative crystallinities, %
	L*	a*	b*
Native	78.79 ± 0.59a	2.37 ± 0.11e	12.84 ± 0.05cd	27.44
One cycle HCT-SPF
S-85	69.07 ± 0.85b	3.02 ± 0.13de	12.21 ± 0.60de	26.60
S-95	68.78 ± 2.80b	4.52 ± 0.27bc	13.56 ± 0.27bc	27.30
S-105	69.21 ± 3.56b	5.51 ± 0.71ab	15.34 ± 0.45a	26.77
Two cycles HCT-SPF
D-85	66.89 ± 0.62b	3.40 ± 0.19d	11.82 ± 0.21e	26.26
D-95	66.12 ± 0.71b	4.43 ± 0.12c	12.79 ± 0.20cd	26.75
D-105	65.66 ± 1.13b	5.70 ± 0.44a	14.37 ± 0.14b	27.40
Three cycles HCT-SPF
T-85	67.12 ± 0.78b	3.36 ± 0.27de	11.38 ± 0.12e	27.59
T-95	65.84 ± 0.08b	4.46 ± 0.50c	11.83 ± 0.14e	25.08
T-105	65.50 ± 0.15b	5.54 ± 0.13a	13.51 ± 0.23c	27.23

Values within each column with different superscript letters (a–e) represent that they are statistically significant different (p < 0.05).

Table 1 indicates that HCT significantly decreased the lightness value (L*) and increased the redness value (a*) of SPF. It was noticed that the effect of heating treatment on the color of SPF was more pronounce than the heating-cooling cycle treatments. The color changed during the HCT could be the result of Maillard reaction between reducing sugars and amino groups which exist in SPF. Moreover, Lorlowhakarn and Naivikul (2006) also reported that a decreased in L* and increased in a* and b* values of rice flour were observed during heat treatment at a temperature of 110 °C and 120 °C for 1–5 h.

Both native and HCT-SPF exhibited X-ray diffraction pattern of A-type crystal. This result indicated that HCT didn't alter the crystalline structure of starch of SPF. Generally, the relative crystallinities of HCT-SPF were lower than that of native SPF (Table 1). This finding is in accordance with the report of Klein et al. (2013), Huang et al. (2016), and Chung et al. (2009) in that HCT of low moisture content starch preserves the crystalline structure of the parental starch but lowering its crystallinity which is caused by partial disruption of amylopectin crystallites.

Pasting properties of native and HCT-SPF    The pasting properties of native and HCT SPFs are summarized in Table 2.

Table 2. Pasting properties ofnative and heating-cooling treated sweet potato flour (HCT-SPF) at one cycle heating temperature of 85 (S-85), 95 (S-95), and 105 °C (S-105), two cycles heating temperature of 85 (D-85), 95 (D-95), and 105 °C (D-105), and three cycles heating temperature of 85 (T-85), 95 (T-95), and 105 °C (T-105).

Treatments	Peak viscosity (mPa.s)	Final viscosity (mPa.s)	Breakdown viscosity (mPa.s)	Setback viscosity (mPa.s)	Pasting temperature (°C)
Native	1 145 ± 46.09d	739 ± 30.99de	550 ± 29.57a	144 ± 14.74a	81 ± 0.03d
One cycle HCT-SPF
S-85	1 909 ± 53.78e	1 859 ± 51.03a	151 ± 56.47b	100 ± 20.50a	85 ± 1.80cd
S-95	885 ± 31.32f	879 ± 51.19fg	69 ± 11.06bcd	64 ± 16.52a	89 ± 1.07ab
S-105	482 ± 81.54g	548 ± 69.09i	19 ± 23.67d	85 ± 9.29a	90 ± 0.63a
Two cycles HCT-SPF
D-85	1 800 ± 51.01b	1 780 ± 62.48b	121 ± 52.72bc	101 ± 79.01a	85 ± 0.81c
D-95	822 ± 11.50f	862 ± 47.50d	33 ± 25.50d	73 ± 10.50a	89 ± 0.40ab
D-105	547 ± 13.20g	614 ± 6.24gh	30 ± 18.68d	98 ± 26.73a	91 ± 2.05a
Three cycles HCT-SPF
T-85	1 384 ± 50.74c	1 381 ± 33.38c	74 ± 21.28bcd	71 ± 41.36a	86 ± 0.03bc
T-95	572 ± 54.67a	629 ± 25.50ef	35 ± 11.85cd	91 ± 27.50a	89 ± 1.12ab
T-105	331 ± 20.43g	377 ± 17.04i	38 ± 9.81cd	84 ± 8.02a	91 ± 0.28a

Values within each column with different superscript letters (a–i) indicate that they are statistically significant different (p < 0.05).

The pasting temperatures of HCT-SPF were higher than that of native flour SPF. It is well known that starch components undergo partial structural disorder and molecules rearrangement at high temperature with condition of limited amount of water during HCT. This event induced the associations between starch chains, and eventually strengthened the forces of the intra-granular bonds. As a result, the process requires more heat to breakdown the structure and to form the starch paste (Zavareze and Dias, 2011; Pranoto et al., 2014).

Table 2 indicates that the peak, breakdown, and set-back viscosities of HCT-SPF were lower than those of native SPF. Moreover, the pasting parameters of HCT-SPF decreased as the heating temperature and HCT cycles treatments were increased except S-85. T-105 exhibited the lowest peak viscosity. This result suggested that the heating temperature treatment more influenced on the pasting properties than the heating-cooling cycle treatment. This finding was in agreement with the result of Klein et al. (2013) in which increasing heating temperature caused a gradual decrease in pasting viscosity. In addition, Huang et al. (2016) reported that increasing the cycling times caused more associations between starch chains, thus it promotes the reduced viscosities and eventually increase the pasting temperatures. Table 2 also shows that the final viscosities of HCT-SPF were higher than that of native flour. High final viscosity indicates the ability of starch to form a viscous paste which represents the gel hardness of starches (Klein et al., 2013).

Granule morphology of native and HCT-SPF    The morphologies of native and HCT-SPF are shown in Fig. 1. The granules of native and HCT-SPFwere polygonal in shape.

Fig. 1.

Scanning electron microscopy images of native and heating-cooling treated sweet potato flour (HCT-SPF) at one cycle heating (S-85, S-95 and S-105), two cycles heating (D-85, D-95 and D-105) and three cycles heating (T-85, T-95 and T-105). Red arrow () showed that the HCT granules underwent cracking on their surface.

The granule surface of native sweet potato starch was relatively smooth (Huang et al., 2016). Moreover, some HCT granules underwent cracking on their surface as directed red arrows in Fig. 1. It was also noticed that the number of agglomerated granules of HCT-SPF was higher than that of native SPF. HCT particularly at high moisture level promotes granule's agglomeration (Zavareze et al., 2010). Moreover, Huang et al. (2016) reported that higher heating-cooling cycle produced higher agglomerated granules which were caused by partial gelatinization of starch granules (Sun et al., 2014; Xiao et al., 2017).

Conclusions

Heating-cooling treatment (HCT) altered the physical properties of sweet potato flour (SPF). HCT-SPF tended to have a lower lightness value. SPF treated with HCT at higher heating temperature exhibited higher pasting temperature.

Acknowledgements    We thanks to the Ministry of Research, Technology, and Higher Education, Republic of Indonesia and R&D team of the Research Center for Appropriate Technology, Indonesian Institute of Sciences.

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