Effect of hot processing on taste quality, starch structure, and pasting properties of areca taro

ABSTRACT

Areca taro was processed using microwaving, frying, boiling, and steaming. Electron microscopy, color, texture, X-ray diffraction, Fourier transform infrared spectroscopy, pasting properties, water mobility, and flavor quality analyses were used to observe the areca taro. The experimental results showed that areca taro hardness and chewiness decreased after processing (P < .05). The peak viscosity of areca taro after boiling and steaming increased, whereas it decreased after microwaving and frying. Processing also decreased the setback, improved the stability of the cold paste, and made aging more difficult (P < .05). After different heat treatments, the bound water content of areca taro decreased, whereas the semi-bound and free water contents increased. Additionally, the relative crystallinity and short-range-ordered structures reduced under all treatments. Furthermore, fried and steamed areca taro had higher sensory scores, owing to their moderate aroma, hardness, and viscosity.

KEYWORDS:

• Areca taro
• starch structure
• X-ray diffraction
• Fourier transform infrared spectrum
• viscosity profile

1. Introduction

Areca taro is a taro crop, also known as Lipu taro, which is a characteristic Chinese agricultural prduct (Sun et al., 2020). Areca taro is delicate, soft, waxy, fragrant, and rich in nutrients, with high levels of starch, protein, vitamins, and various trace elements. In addition to its edible value, areca taro possesses medicinal properties; the traditional Chinese medicine book, Compendium of Materia Medica, shows that areca taro can improve spleen and stomach function. Currently, domestic and international research on the thermal processing of subterranean bulb crops has focused on potatoes, sweet potatoes, and yams, whereas negligible research has been conducted on the processing and food quality of areca taro. According to the studies on potatoes, sweet potatoes, carrots, and other crops, pasting properties of starch has an important effect on its taste, viscosity, and hardness (Syafutri et al., 2016). Agrawal et al. (2019) found that instant rice dried using vacuum-microwave assisted hot air had a much lower hardness than rice dried using microwave hot air. Peng et al. (2012) studied the effects of blanching, steaming, microwaving, and frying on the sensory and nutritional qualities of parsley. Results showed that blanching had an excellent color protection effect, steaming preserved the color and brittleness to the maximum extent, and microwaving and frying increased the total phenol content of parsley and improved its antioxidant effect. Pan (2022) found that boiling, roasting, and frying, had different degrees of influence on the digestive characteristics and nutrient composition of potatoes. Li et al. (2012) found that the antioxidant activity and polyphenol content of potatoes decreased after regular pressure boiling, high-pressure boiling, and frying, while microwave treatment maintained high antioxidant activity and polyphenol content. However, few studies have investigated the effects of processing on the structure, pasting properties, and flavor quality of areca taro starch. Therefore, this study investigated the thermal processing of areca taro using four heating methods: microwaving, frying, boiling, and steaming. Changes in starch structure, pasting properties, texture, and eating quality of areca taro after heat treatment were studied to provide a scientific basis for food processing and the use of areca taro.

2. Materials and methods

2.1. Materials

Areca taro was purchased from Taixing Life Supermarket in Hezhou, Guangxi, and porcine pancreatic alpha-amylase and glycosidase were purchased from Shanghai Yuanye Biotechnology Co., Ltd. (Shanghai, China). Glucose kit (glucose oxidase method) was purchased from Nanjing Jiancheng Bioengineering Institute (Nanjing, China).

Equipment used included: Gemini300 thermal field emission scanning electron microscope (Carl Zeiss Group, Germany), Chroma meter CR-400 chromometer (Konica Minolta Holdings Ltd., Japan), TA-XT plus texture analyzer (Stable Micro Systems, UK), JC-MB36 enzyme-tagging instrument (Qingdao Juchuang Huanye Analytical Instrument Co., Ltd. Qingdao, China), RVA-TecMaster rapid viscosity analyzer (Perten, Sweden), D8 ADVANCE X-ray diffractometer (Bruker GMBH, Germany), Nicolet iS50 Fourier transform infrared spectrometer (Thermo Fisher Technology, Ltd., U.S.A.), NMI20 MRI analyzer (Shanghai Niumai Electronic Technology Co., Ltd., Shanghai, China). and FW100 Ultra-Micro Pulverizer (Tianjin Test Instrument Co., Ltd. Tianjin, China)

2.2. Treatment of raw materials

The areca taro was washed, peeled, and cut it into “cube” pieces of 2 cm thickness. Based on preliminary experimental results, four thermal processing conditions were identified. Microwaving: The areca taro chunks were heated in a microwave (700 W) for 2 min. Frying: Heat the oil to above 220°C before adding the areca taro chunks and frying for 2 min. Boiling: Water was heated in a wok until it boiled. Areca taro cubes were then added and heated for 6 min. Steaming: Water was heated until it boiled in an air steamer and then areca taro cubes were added and steamed for 6 min. The color, texture, water mobility, and sensory characteristics of fresh and processed areca taro were measured. Areca taro samples were dried in a constant temperature oven at 48°C to a constant weight and pulverized by an ultra-micro pulverizer, and sifted through a 200 mesh screen for scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and pasting properties analyses.

2.3. Scanning electron microscopy (SEM)

Observations were made using a thermal field-emission scanning electron microscope (Kong et al., 2009). Freshly and thermally processed taro powder samples were weighed and placed on copper sample tables. After electroplating and gold spraying, the surfaces were observed using SEM. The apparent morphology of the taro powder particles was analyzed at a magnification of 100–2000 at an acceleration voltage of 2 kV.

2.4. Color

A slightly modified version of the method described by Trinh et al (Trinh & Nguyen, 2020). was used to assess sample color. Using a Chroma meter CR-400 color difference meter, the L*, a*, and b* values of fresh and processed areca taro blocks were determined. A black and white ceramic plate was used to calibrate the chromometer in transmission mode. L* indicates brightness, with values from 0 to 100 indicating the color from black to white, respectively. a* represents red and green values, with -a* to +a* representing green to red and 0 being neutral. b* indicates yellow and blue values, with -b* to +b* indicating blue to yellow and 0 being neutral. Each group of samples was analyzed three times (Wang et al., 2023).

2.5. Texture profile analysis (TPA)

A slightly modified version of the method described by Peng et al. (2021) was used for TPA. A P2 probe was used with a pre-test rate of 5.0 mm/s, a mid-test rate of 1.0 mm/s, and a post-test rate of 5.0 mm/s in the TPA mode. The first compressive strain was 10% and the second was 50%, with a time interval of 6 s (Santos et al., 2016).

2.6. Pasting properties (RVA)

Referring to Hu’s method (Hu, 2018), 0.5 g of areca taro powder (particle size < 200 mesh, modified for a 14% wet basis) was placed into an aluminum bucket and 25 mL distilled water was added. The aluminum bucket was placed into the instrument after mixing with a stirring paddle. The measured values included peak viscosity, Trough viscosity, final viscosity, breakdown, and setback.

2.7. X-ray diffraction (XRD) and relative crystallinity

The crystal structure of the samples was determined using a D8 ADVANCE X-ray diffractometer. A slightly modified version of the method described by Chen et al. (2019) was used. The areca taro powder (particle size < 200 mesh) was pressed under the 2θ range of 5° to 80°. The scanning rate voltage was 40 kV, current was 40 mA, scanning rate was 0.02°, and testing speed was 0.1 s/step. A copper target was used and the incoming ray wavelength was 0.15 nm. Jade 6.0 software was used to calculate the relative crystallinity of the samples using the following formula (Chen et al., 2018):

(1) $\mathrm{Relative}\mathrm{crystallinity}=\frac{A_c}{A_{\mathit{c}}+A_a}$(1)

Here, A_c is the total area of the crystal region (%) and A_a is the total area of the amorphous region (%).

2.8. Fourier transform infrared spectroscopy (FT-IR)

Infrared spectroscopy was performed on a sample of areca taro powder using a Fourier transform infrared spectrometer. The areca taro powder sample was was ground, mixed with KBr in the ratio of 1:100, pressed and scanned for IR spectra. The scanning wave number range was 400–4000 cm-1 with a resolution of 4 cm, and the infrared spectrum of the sample was obtained by scanning 64 times (Yan et al., 2022).

2.9. Characteristics of water mobility

The water mobility characteristics of the areca taro were determined using an NMI20 MRI analyzer. Areca taro clumps were cut into strips 5 mm wide and 2 cm long and placed in sample vials for testing. The parameters of the test were as follows: waiting time − 4500 ms, echo time − 0.2 ms, cumulative − 4 times, inversion relaxation time − 500000 ms (Cheng et al., 2018; Xv, 2019). The T₂ (transverse relaxation time) is calculated using the NM120 MRI analyzer system software. NMR relaxation signals were expressed mathematically as the sum of exponential decays according to the following equation:

(2) $\mathit{Y}=\left(y_0\right)+\sum _{\mathit{i}=1}^n\left(A_{2\mathit{i}}\right)\left(e^{-\mathit{t}/{\mathit{T}}_{2\mathit{i}}}\right)$(2)

Here, Y represents the NMR signal intensity at time t and n the number of uni-exponentials. The time constant associated with uni-exponential decay or relaxation time T_2i corresponds to the mobility of protons in the water fraction i, and A_2i reflects the ratio of peaks corresponding to apparent water content.

2.10. Simulated in vitro digestibility

The in vitro determination of starch digestion was performed by slightly modifying the in vitro determination methods of Englyst et al. (1992), Zhang (2021), and Wang (2016). Areca taro powder samples (500 mg) were weighed and dispersed evenly in 10 mL 0.2 mol/L CH₃COONA buffer (pH 5.2). Samples were heated to gelatinize in a boiling water bath for 30 min. Following the addition of 10 mL (300 U/mL) α-amylase and 40 µL (10000 U/mL) glycosidase of swine pancreas, samples were mixed and incubated in an oscillating water bath at 37°C and 170 r/min. Samples of 1 mL were collected after 20 and 120 min of incubation, enzymes were inactivated in a boiling water bath for 5 min, centrifuged at 2000×g 5 min, and supernatant collected. The glucose content was determined using the Glucose kit. The formulas for calculating the rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) in the sample are as follows:

(3) $\mathit{RDS}=\frac{{\mathit{G}}_{20}-G_0\times 0.9}{\mathit{TS}}$(3)

(4) $\mathit{SDS}=\frac{{\mathit{G}}_{120}-G_{20}\times 0.9}{\mathit{TS}}$(4)

(5) $\mathit{RS}=100\mathrm{\%}-\mathit{RDS}-\mathit{SDS}$(5)

Here, G₀ is the amount of glucose before enzymolysis of the sample (mg), G₂₀ is the amount of glucose after 20 min of enzymolysis (mg), G₁₂₀ is the amount of glucose after 120 min of enzymolysis (mg), 0.9 is a conversion factor, and TS is the total sample mass (mg).

2.11. Sensory evaluation

Ten researchers specializing in food assessed the sensory properties of processed areca taro (five males and five females). They evaluated the color, aroma, hardness, and taste of microwaved, fried, boiled, and steamed areca taro. Sensory assessors were required to be healthy, have a normal sense of smell and taste, and avoid eating for one hour before the sensory assessment, especially foods with a strong taste (Xiao et al., 2020). The scoring criteria are as follows:

Table

Targets	marking scheme
Color (10 points)	Bright color and attractive luster. (8~10 points)	Normal color without dark yellow or grayish white. (5~7 points)	Dull color, dark yellow or grayish white. (0~4 points)
Aroma(20 points)	The taro has a rich aroma. (16~20 points)	The aroma of taro is generally. (8~15 points)	The aroma of taro is not evident. (0~7 points)
Hardness(10 points)	The hardness of the taro head is relatively high. (8~10 points)	Taro has a moderate hardness. (5~7 points)	Taro with low hardness. (0~4 points)
Viscosity(10 points)	high viscosity. (8~10 points)	Moderate viscosity. (5~7 points)	low viscosity. (0~4 points)
Taste(50 points)	Good texture and rich taro flavor (35~50 points)	Average texture with taro flavor. (17~35points)	Poor taste, no taro flavor. (0~16 points)
Overall rating	Color+Aroma+Hardness+viscosity+taste

2.12. Statistical analysis

The measurements were repeated three times for each group of samples. All values were shown in the average ± standard deviation. Mean values and differences were analyzed using the Duncan’s multiple range test. The means were tested for least significant differences using the SPSS statistical software for Windows (version 6.0).

3. Results and discussion

3.1. Scanning electron microscope analysis

The SEM images showed clear differences in the microstructure of the areca taro after the different methods of thermal processing (Figure 1). Because areca taro contains a sticky, slippery mucopolysaccharide that surrounds small starch grains (Fan & Chen, 2004), starch particles are difficult to isolate. Therefore, the starch particles in the fresh ground powder showed a high degree of attachment, with irregular polyhedral and smooth surfaces. This was similar to the structure of areca taro starch observed by Yan et al. (2022). After microwaving and frying, most of the starch granules were gelatinized; however, a few intact starch granules were still observed. In contrast, the areca taro starch was completely gelatinized after boiling and steaming and no intact starch granules were observed. Microwaving and frying are considered dry heat processes, during which the water content of the areca taro samples rapidly decreases. Insufficient water content limits the gelatinization of starch in these methods; therefore, a small amount of intact starch particles could be observed in the SEM images. Boiling and steaming are hot and humid processes that provide sufficient water for starch gelatinization. Consequently, the gelatinization of areca taro starch by boiling and steaming was greater than that by microwaving and frying.

Figure 1. Scanning electron microscope image of areca taro treated by hot processing a. Fresh sample; b. microwaving; c. frying; d. boiling; e. steaming.

3.2 Color analysis

Table 1 lists the results of the tests on the different processing methods for areca taro using a chroma aberration meter. Color is an important factor in the subjective evaluation of food (Guo et al., 2008). After scanning with a chromatic difference meter, the brightness value of the fresh sample was the highest, followed by microwaved and fried areca taro and that of the boiled and steamed areca taro were the lowest. This indicated that hot and humid processing had a greater influence on areca taro whiteness (Wei et al., 2019). The a* values of microwaved, fried, and boiled areca taro decreased slightly compared with that of the fresh samples; there was no significant change in the a* value of the steamed areca taro. The b* values of microwaved, boiled, and steamed areca taro decreased slightly compared with that of the fresh sample, whereas that of fried areca taro increased significantly. This is because the Maillard reaction that occurs during the frying process gave the samples an attractive golden color, which increased the b* value (Qi et al., 2018).

Table 1. Color of areca taro.

Processing mode	L*	a*	b*
fresh sample	84.73 ± 0.16^a	6.18 ± 0.05^a	7.68 ± 0.06^b
microwaving	74.89 ± 0.07^b	4.93 ± 0.02^b	4.41 ± 0.13^d
frying	72.81 ± 0.21^c	4.21 ± 0.01^c	17.74 ± 0.01^a
boiling	60.85 ± 0.54^e	4.64 ± 0.18^b	4.09 ± 0.10^e
steaming	63.07 ± 0.27^d	6.31 ± 0.03^a	6.54 ± 0.08^c

All values were shown in the average (± standard deviation). Different superscript letters in each column indicate significant differences (P < .05).

3.3 Texture profile analysis

Texture is an important indicator of the eating quality of areca taro. The results of the TPA of the different methods of processing areca taro are shown in Table 2. Hardness was defined as the force required to deform the areca taro to a certain extent. The hardness of the processed areca taro was significantly lower than that of the fresh sample, indicating that all methods reduced the hardness of areca taro. Among the four processing methods, fried areca taro was the hardest. This could be attributed to the crusting of the areca taro surface after frying, increasing its hardness compared with that in other methods (Zhou et al., 2023). Furthermore, elasticity was defined as the degree to which the areca taro could recover after the first deformation (Jang et al., 2019). Elasticity in the processed areca taro was lower than that of fresh areca taro, with no significant differences across the four processing methods. This indicated that all hot processing methods effectively reduced the elasticity of the areca taro. Cohesion was defined as the relative resistance of the areca taro to a second compression after the first. This reflects the strength of the intermolecular or structural elements within the sample and the ability of the areca taro to resist damage and maintain integrity (He et al., 2010). The cohesiveness of areca taro decreased after all processing methods; the decrease was greater after boiling and steaming than after microwaving and frying. This suggests that the internal structure of the areca taro is less damaged by dry heat processing, and it is able to better resist external forces during the second compression. Boiling and steaming rendered a less cohesive areca taro, leading to a difficulty in maintaining the full form while chewing. Additionally, the texture was softer and waxier, which agrees with the measured hardness values. Chewiness was defined as the amount of work it takes to chew the areca taro to be swallowed, and reflects the resistance of areca taro to human chewing (Wang, 2013). Lower chewiness is associated with easier eating and improved taste. The chewiness y of areca taro decreased after processing, with microwaved and fried areca taro being more rigid than boiled and steamed samples. This is because microwaving and frying lose water at a higher rate, limiting the expansion and rupture of starch particles and results in increased chewiness (Fan et al., 2012; Fu et al., 2020). Our results are consistent with the results of Bao et al. (2020) and Yang et al. (2016), who reported a higher chewiness after hot frying and microwaving than that after steaming and boiling.

Table 2. Texture profile of areca taro.

Processing mode	Hardness/N	Springiness	Cohesiveness	Chewiness/N
fresh sample	405.32 ± 3.26^a	0.71 ± 0.01^b	0.65 ± 0.01^a	119.77 ± 1.03^a
microwaving	33.45 ± 0.77^c	0.57 ± 0.01^c	0.43 ± 0.01^b	6.88 ± 0.10^c
frying	61.73 ± 1.36^b	0.58 ± 0.01^c	0.45 ± 0.01^b	16.68 ± 0.64^b
boiling	24.62 ± 0.21^d	0.51 ± 0.01^d	0.23 ± 0.01^d	1.19 ± 0.03^e
steaming	28.44 ± 0.31^cd	0.59 ± 0.02^a	0.25 ± 0.01^c	4.13 ± 0.08^d

All values were shown in the average (± standard deviation). Different superscript letters in each column indicate significant differences (P < .05).

3.4 Pasting properties analysis

The results of the rapid viscometer analysis are shown in Table 3. Peak viscosity was defined as the maximum viscosity that could be reached during the pasting process, indicating the hydration and thickening capacity of the starch particles (Liu et al., 2016). The peak viscosities of microwaved and fried areca taro were lower than those of fresh areca taro, whereas those of boiled and steamed areca taro were higher than fresh samples. The decrease in peak viscosities after microwaving and frying could be linked to the pasting results observed in the SEM images; during pasting, part of the molecular chains become smaller, and some amylopectin degrades to form amylose, increasing the content of amylose and causing a decrease in peak viscosity (Tang et al., 2020). The observed results are consistent with the peak viscosity measured by Wang (2022) after frying starch. The increased viscosity of areca taro after boiling and steaming may be because the starch structure of the areca taro powder is altered by these methods, and the reaction between the starch and non-starch components improves its ability to bind water. At the same time, pasting is accompanied by a high proportion of water in the environment. This stable environment prolongs the time required for starch to absorb water during pasting, resulting in increased viscosity. The trends of trough and final viscosities and peak viscosities of thermal treatment areca taro were basically the same. The viscosity of boiled and steamed areca taro was higher than that of microwaved and fried areca taro.

Table 3. Pasting properties of areca taro.

Processing mode	Peak viscosity /cP	Trough viscosity /cP	Final viscosity /cP	Breakdown /cP	Setback /cP
fresh sample	1024.00 ± 6.25^c	736.33 ± 2.33^b	1078.33 ± 2.96^a	287.67 ± 5.24^b	340.02 ± 1.15^a
microwaving	848.33 ± 2.33^d	622.33 ± 4.37^c	843.33 ± 5.46^b	226.00 ± 2.65^c	221.00 ± 1.15^c
frying	864.00 ± 2.08^d	571.67 ± 2.19^d	778.67 ± 2.60^c	290.67 ± 1.33^b	209.67 ± 1.20^d
boiling	1529.00 ± 7.51^a	733.00 ± 5.03^b	783.33 ± 2.60^c	290.67 ± 1.33^b	209.67 ± 1.20^d
steaming	1491.67 ± 0.33^b	837.33 ± 1.86^a	1084.33 ± 5.49^a	661.00 ± 7.21^a	260.00 ± 1.53^b

All values were shown in the average (± standard deviation). Different superscript letters in each column indicate significant differences (P < .05).

Falade et al (Falade & Oluwatayo, 2015). proposed that during processing, the sample may be pre-pasted to a certain extent, resulting in a decrease in peak and trough viscosities. The breakdown is the difference between the peak and trough viscosities, and a smaller breakdown is associated with better thermal stability of the starch paste (Yuan et al., 2009). Steamed areca taro had the highest breakdown, which was approximately twice that of fresh areca taro, indicating that the thermal stability of the starch paste had deteriorated. The breakdown of fried and boiled areca taro were generally similar to those of fresh samples. In contrast, the breakdown of microwaved areca taro was significantly lower than that of fresh samples, indicating that the thermal paste stability of areca taro improved after microwave processing.

The setback is the difference between the final and trough viscosities, representing the stability of the starch cold paste viscosity against aging, with the setback being inversely proportional to aging difficulty (Bai et al., 2018; Wei et al., 2020). The setback of processed areca taro was lower than that of fresh samples, indicating that the cold paste of processed areca taro is stable and does not age as easily as fresh areca taro.

3.5 Water mobility

Magnetic Resonance Imaging was used to detect the water mobility properties of areca taro during thermal processing (Table 4). The transverse relaxation time (T₂) reflects the difference in the degrees of freedom of water (Yang et al., 2021) and is denoted as bound, semi-bound, and free water (from small to large); the T₂ value is larger for higher water degrees of freedom and vice versa (Shi et al., 2021). The A₂ value represents the relative content of each species of water and is denoted as bound, semi-bound, and free water (from small to large) (Wang et al., 2023). During the heating process, the T₂₁ value and peak area of the areca taro continuously decreased. The trend of T₂₂ was consistent with that of T_21, but its peak fraction was higher than that of T₂₁, indicating that bound water decreased as the heating process proceeded, while semi-bound and free water increased (Yu, 2017). Another phenomenon during the heating process was that T₂ stabilized at a certain value at the beginning of heating, followed by a decrease in T₂₁ and increase in T₂₃. This may be because the water and starch are tightly bound in the initial stage of heating and have low degrees of freedom. As heating continued, the starch folded and the freedom of the water increased. Finally, the starch particles break down due to pasting, which stabilizes the water-starch interaction. This was consistent with the results reported by Fan et al. (2013). Notably, the ratio of semi-bound water to free water in fried areca taro was significantly higher than that in fresh areca taro, while no difference was observed with microwaving, boiling, and steaming. This was probably due to the reduction in the proportion of bound and free water caused by frying and the consequent increase in the proportion of semi-bound water (Huang et al., 2019).

Table 4. Characteristics of water mobility.

Processing mode	Heating time	Total relaxation time/ms	Relaxation time			Peak area
			T21 /ms	T22 /ms	T23 /ms	A21(%)	A22(%)	A23(%)
microwaving	0s	533.76	1.75 ± 0.03^a	12.31 ± 0.01^a	304.79 ± 0.25^a	20.22 ± 0.30^a	5.24 ± 0.044^c	74.75 ± 0.12^c
	40s	49.77	0.84 ± 0.02^c	3.54 ± 0.02^d	32.78 ± 0.12^b	3.68 ± 0.05^d	11.53 ± 0.40^a	85.05 ± 0.16^b
	80s	49.77	1.04 ± 0.04^b	4.06 ± 0.04^c	28.48 ± 0.04^c	5.22 ± 0.06^c	9.37 ± 0.04^b	85.42 ± 0.38^b
	120s	43.29	0.67 ± 0.01^d	6.13 ± 0.02^b	21.53 ± 0.02^d	7.45 ± 0.10^b	4.55 ± 0.06^c	89.05 ± 0.04^a
frying	0s	613.59	1.75 ± 0.02^a	10.71 ± 0.03^a	350.86 ± 0.34^a	16.09 ± 0.23^a	4.41 ± 0.05^c	79.62 ± 0.29^a
	40s	132.19	1.14 ± 0.02^b	7.05 ± 0.03^b	65.87 ± 0.46^b	4.61 ± 0.12^d	18.82 ± 0.09^a	76.53 ± 0.33^b
	80s	100.00	1.14 ± 0.02^b	7.06 ± 0.02^b	65.53 ± 0.19^b	5.95 ± 0.10^c	18.28 ± 0.07^b	75.83 ± 0.38^b
	120s	114.98	1.00 ± 0.020^c	6.12 ± 0.01^c	57.88 ± 0.67^c	7.78 ± 0.06^b	18.73 ± 0.05^a	73.47 ± 0.58^c
boiling	0min	613.59	1.75 ± 0.01^a	12.31 ± 0.01^a	350.86 ± 0.34^a	17.50 ± 0.04^a	5.27 ± 0.04^d	77.22 ± 0.14^c
	2min	114.98	1.30 ± 0.01^b	7.06 ± 0.02^b	65.57 ± 0.19^c	3.01 ± 0.07^b	9.30 ± 0.05^a	87.66 ± 0.17^b
	4min	100.00	1.31 ± 0.01^b	7.06 ± 0.02^b	65.53 ± 0.45^c	3.13 ± 0.03^b	7.54 ± 0.08^c	89.28 ± 0.16^a
	6min	86.97	1.14 ± 0.02^c	6.12 ± 0.01^c	75.62 ± 0.19^b	2.68 ± 0.10^c	7.75 ± 0.04^b	89.53 ± 0.11^a
steaming	0min	613.59	2.04 ± 0.092^a	14.18 ± 0.04^a	350.73 ± 0.47^a	16.08 ± 0.13^a	3.84 ± 0.03^c	80.21 ± 0.72^b
	2min	151.99	1.53 ± 0.04^b	9.30 ± 0.02^b	86.51 ± 0.62^b	3.18 ± 0.06^c	9.34 ± 0.07^a	87.38 ± 0.05^a
	4min	114.98	1.14 ± 0.02^c	6.12 ± 0.01^c	65.59 ± 0.15^c	3.64 ± 0.07^b	8.17 ± 0.06^b	88.39 ± 0.38^a
	6min	100.00	1.14 ± 0.02^c	5.33 ± 0.03^d	57.54 ± 0.34^d	3.26 ± 0.05^c	8.32 ± 0.04^b	88.44 ± 0.31^a

All values were shown in the average (± standard deviation). Different superscript letters in each column indicate significant differences (P < .05).

3.6 X-ray diffraction analysis

XRD is the most widely used method for determining the crystallinity of starch (Yang et al., 2007). Starches are classified into types A, B, and C according to their different crystalline structures (Chi et al., 2022). The XRD pattern of processed areca taro is shown in (Figure 2). Referring to the method used by Chen et al. (2011), the MDI Jade software was used to calculate the relative crystallinity of fresh and processed areca taro. Fresh areca taro had strong diffraction peaks at 2θ = 15.23° and 23.17°, unresolved double peaks at 17.19° and 18.02°, and weak diffraction peaks at 20.15°. These peaks indicated that it was a typical A-type starch. After processing, the peak intensity of starch in areca taro changed significantly; the characteristic peaks of starch in the samples disappeared upon processing, with steaming and boiling showing the most pronounced changes. Following processing, the peaks tended to flatten, indicating that the original grain structure of starch was altered, and the XRD pattern exhibited a dispersive state. The observed results were the same as that in the study by Zhang et al. (2019), where rice protein and wheat starch were mixed and gelatinized, then dried and ground into a powder for XRD analysis.

Figure 2. XRD spectrogram of areca taro.

The relative crystallinity of the processed starch decreased, and the trend of the crystallinity change was the same as that observed in the XRD pattern (Figure 2). The relative crystallinity of microwaved and fried areca taro decreased from 10.55% in fresh samples to 2.74% and 4.18%, respectively. After boiling and steaming, the relative crystallinity of the areca taro starch decreased to 0.41% and 0.37%, respectively. The results showed that the crystalline structure of the starch in areca taro was damaged after processing; a larger decrease in relative crystallinity is associated with more severe damage to the crystalline structure of the starch and higher starch pasting. This change is consistent with the SEM observations.

3.7 Fourier transform infrared spectrum analysis

FT-IR can detect changes in the short-range ordered structure of starch. Figure 3 shows the FT-IR spectra of areca taro after thermal processing. The 800–1200 cm⁻¹ FT-IR spectra range of starch mainly corresponds to C-C and C-O bonds, which are extremely sensitive to changes in starch polymers. There were three characteristic absorption peaks in this region, which were located at approximately 995, 1022, and 1045 cm⁻¹, corresponding to the control of hydration carbohydrate helix, amorphous, and crystalline structures (Demiate et al., 2000). Therefore, R₁ (1045/1022 cm⁻¹ absorbance ratio) and R₂ (1022/995 cm⁻¹) reflect the ratio of short-range ordered and disordered structures of starch molecules, respectively (Falade et al., 2014; Frederick et al., 2016).

Figure 3. FT-IR spectrogram of areca taro.

The FT-IR spectra of processed areca taro were in general agreement with those of fresh areca taro, with differences in the positions and intensities of some absorption peaks. Compared to the other three treatments, the peak intensity of fried areca taro increased at approximately 2850 and 2930 cm⁻¹, the peak intensity increased significantly at approximately 1745 cm⁻¹, and a new peak appeared at approximately 3023 cm⁻¹. This was likely because the edible oil used for frying contained a large amount of methylene (CH₂; 2927 and 2850 cm⁻¹) and hydroxyl (C=O; 1745 cm⁻¹) groups, which is consistent with the infrared spectrum of hot-fried Huai Shan observed by Peng et al. (2021). The new peak near 3023 cm⁻¹ is attributed to the stretching vibration of C-H on the unsaturated carbon.

The R₁ value of areca taro after processing was lower than that of fresh samples, indicating that the short-range ordered structure of areca taro after processing was destroyed, which corresponds to the relative crystallinity observed in SEM and XRD analysis (Table 5). The R₂ value of areca taro after processing was higher than that of the fresh sample, which also confirms the destruction of short-range ordered structure, leading to an increase in the short-range disordered structure. Cui (2019) also found that the R₁ value decreased and the R₂ value increased with continuous pasting of areca taro starch when studying dragon and lotus areca taros.

Table 5. Absorption peak intensity ratio of areca taro.

Processing mode	R1 (1045/1022)	R2 (1022/995)
fresh sample	1.19 ± 0.0020^a	0.79 ± 0.0048^d
microwaving	1.14 ± 0.0009^b	0.83 ± 0.0024^c
frying	1.09 ± 0.0001^e	0.96 ± 0.0009^a
boiling	1.13 ± 0.0009^c	0.89 ± 0.0018^b
steaming	1.11 ± 0.0005^d	0.88 ± 0.0020^b

All values were shown in the average (± standard deviation). Different superscript letters in each column indicate significant differences (P < .05).

3.8 Simulated in vitro digestibility analysis

In vitro experiments were used to model the digestion of areca taro under different processing conditions (Table 6). RDS refers to the starch that can be digested in the small intestine within 20 min, which is problematic for middle-aged and elderly people with diseases such as hypertension and diabetes mellitus. SDS refers to the starch that cannot be fully digested within 20–120 min in the small intestine. The relatively prolonged period of moderate hydrolysis can continuously produce glucose to maintain a feeling of fullness and blood sugar regulation. RS refers to the starch that is not digestible within 120 min in the small intestine, allowing it to reach the colon and be hydrolyzed by microbial fermentation to produce short-chain fatty acids, which are considered a dietary fiber (Wang, 2016).

Table 6. Simulated in vitro digestibility of areca taro.

Processing mode	RDS/%	SDS/%	RS/%
fresh sample	62 ± 0.5^a	25 ± 0.8^ab	13 ± 1.5^bc
microwaving	61 ± 0.3^a	24 ± 1.2^b	15 ± 1.5^b
frying	60 ± 0.7^a	17 ± 1.1^c	23 ± 1.3^a
boiling	61 ± 1.1^a	24 ± 1.2^b	15 ± 0.3^b
steaming	61 ± 1.3^a	28 ± 0.9^a	10 ± 1.0^c

All values were shown in the average (± standard deviation). Different superscript letters in each column indicate significant differences (P < .05).

The RDS content of the processed areca taro did not change significantly compared with that of fresh areca taro, remaining above 60%. The SDS content of fried areca taro was lower than that of fresh areca taro, whereas no significant differences were observed in terms of the SDS content, between the remaining three processing methods and fresh samples. The RS content of areca taro was generally unchanged after processing; however, the RS content of fried areca taro increased significantly. Some researchers have shown that starchy foods with elevated relative crystal content have increased resistant starch content, which is consistent with the relative crystal content calculated using XRD (Jian et al., 2003). It is also possible that the formation of starch-lipid complexes during frying leads to the inhibition of starch digestion, and thus an increase in RS content (Farooq et al., 2018; Ye et al., 2018). Yang et al., (2020) also found that with an increase in the frying temperature and duration of wheat starch, the relative crystallinity of fried starch increased and the resistant starch content increased from 8.54–12.72%.

3.9 Sensory evaluation analysis

The tissues of processed areca taro are shown in Figure 4a. After microwaving, the areca taro surface was dehydrated and hardened, with little to no aroma. After frying, the areca taro head occurs Maillard reaction, the surface is golden, the exterior is crusty, and the aroma is everywhere. After boiling, areca taro was observed to absorb water and swell, causing peeling and cracking of the surface, with a slight taro aroma. After steaming, we observed that the purple fibers in the areca taro were more absorbent and expanded, and that the taro had a strong aroma. The sensory evaluation results are shown in Figure 4b. Sensory evaluation showed that microwaved areca taro had the lowest scores for aroma, color, hardness, viscosity, and overall taste. Fried and steamed areca taro had the highest overall sensory scores, and boiled areca taro had slightly lower scores. The high scoring for fried samples is because of the Maillard reaction, which occurs when areca taro is fried, emitting an attractive aroma and color. Both boiled and steamed areca taro have high viscosities, however, the increased aroma emitted from steamed areca taro made it more attractive than boiled samples.

Figure 4. (a). The hot processed of areca taro.

Figure 4. (b). Sensory evaluation of areca taro.

4. Conclusion

The effects of thermal processing on the microstructure, color, texture, pasting properties, water mobility, relative crystallinity, short-range structure, digestive properties, and sensory properties of areca taro were investigated. The relationships between the texture, pasting, short-range ordered structure, and relative crystallinity of areca taro were analyzed based on the starch structure. Through the experiment, it was found that different processing treatments would have different effects on areca taro. In conclusion, this study contributes to the understanding of the mechanisms by which the textural quality of areca taro changes during processing and provides a theoretical basis for further research on the development of the areca taro food industry and scientific processing of areca taro.

Author contributions

Yuqiao Zhang: Conceptualization, Methodology, Software, Investigation, Formal Analysis, Writing Original Draft. Feifei Shang: Conceptualization, Funding Acquisition, Resources, Supervision, Writing – Review & Editing. Tetyana Kryzhska: Data curation, Visualization, Writing – Review & Editing. Linlin Fan: Conceptualization, Methodology, Software, Investigation, Formal Analysis. Lili Li: Conceptualization, Methodology, Software. Qianwei Cheng: Visualization, Writing – Review & Editing. Zhenhua Duan: Resources, Supervision. Shanshan Chen: Visualization, Investigation.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Additional information

Funding

This research was funded by ‘The National Natural Science Foundation of China’ [No. 31860458], ‘The Natural Science Foundation of Guangxi Province’ (No. 2017JJA130645Y) and ‘Guangxi Key Research and Development Program’ [No. 2022AB20149].