Characteristics of Different Types of Starch in Starch Noodles and Their Effect on Eating Quality

Abstract

Several commercial starch noodles made from legume, tuber, geshu (kudzu and sweet potato) and fernery starches were used to study the characteristics of starch in starch noodles and their effect on eating quality of starch noodles. Scanning electron microscopy observation found that the special inner structure of starch noodles was composed of some broken starch granules and some gel-like substances. Tuber and legume starches had the highest and lowest solubility, swelling power, swelling factor, setback, breakdown, peak viscosity, and final viscosity, respectively. Legume and tuber starches had the highest and lowest gelatinization temperature, respectively. Tuber and geshu starches had the highest amylose leaching rate, while legume starches owned the lowest value (p < 0.05). Tuber starches had the highest conclusion temperature of gelatinization (151.12~158.86°C). Fernery starches had the lowest value of retrogradation enthalpy (967.33 J/g dry starch). Legume starch noodles had the lowest broken rate (0.00~1.67%), swelling ratio (332.64~343.57%), and cooking loss (2.40~2.74%), and the highest hardness (87.47~93.29 g/mm2), shear deformation (0.49~0.52), and elasticity (0.58~0.62), However, tuber and fernery starch noodles did the opposite, tuber and legume starch noodles had the highest and lowest cohesiveness, respectively. All the above cooking and starch properties test results of starch noodles demonstrated that, compared with others, legume starch noodles are relatively well in eating quality. The correlation analysis showed that the cooking and physical quality of starch noodles could be perfected significantly by improving the swelling and pasting properties for starch of starch noodles, while thermal properties had no obvious influence on them.

Keywords:

• Starch noodles
• Starch properties of starch noodles
• Eating quality
• Scanning electron microscopy

INTRODUCTION

Starch noodles, produced from purified starch from various plant sources, are a favorable traditional food in Asian countries. Traditionally, there are three kinds of starch noodles according to the type of raw materials used in manufacture, including legume starches such as mung bean, pea, broad bean, red bean, various tubers, or root starches such as potato, cassava, sweet potato, and a variety of grain starches such as maize and sorghum. But the qualities of mung bean starch noodles are the best for its high amylose content.^[1] Excellent starch noodles would have clear or transparent and fine threads, high tensile strength, and low cooking loss even with prolonged cooking.^[2]

Starch noodles are made by the processing of forming the starch dough, dropping, hanging, freezing, thawing, drying, and packing. Starch is the main component of noodles. Thus, starch itself plays an essential role in both the production of starch noodles and the final quality of starch noodles.^[1] Recently, the research in starch noodles is mainly focused on processing technology,^[3–5] the development of new nutrition starch noodles,^[6] improving the quality by founding and controlling factors that affect the quality of starch noodles,^{[3–5,7–16]} safety testing for final product and it’s improvement.^[17,18]

It was stated that as the amylose content increased, the swelling tended to be restricted, the shear strength and elasticity tended to be increased, and the rate of rupture and cooking loss tended to be decreased.^[7,8] The qualities of sweet potato starch noodles were crucially affected by the physicochemical properties. The parameters affecting the qualities exhibited the order as follows: swelling power > solubility > amylose content > protein content > granule size. Furthermore, there was a significant correlation between the quality of starch noodles and the rapid visco analyzer (RVA) parameters, which was considered as an important way of predicting the qualities of starch noodles.^[9]

Many measures were taken to perfect the quality of starch noodles. Some researchers used other materials such as pigeonpea starch, potato starch, and rice starch or their mixture to substitute totally or partly mung bean starch.^[10,11] It is considered that better quality of starch noodles can be obtained by chemical or physical modification of native starch. For instance, Muhammad, Kusnandar, Hashim, and Rahman^[12] reported that substitute potato starch with up to 17% tapioca starch phosphate could increase transparency, reduce the stickiness and cooking loss, and give the product a moderate elastic. According to Kasemsuwan, Bailey, and Jane,^[13] the use of mixtures of crosslinked tapioca and high-amylose maize starches could develop acceptable product, and the sensory evaluation indicated that consumers preferred the noodles made from mixtures of tapioca and high-amylose starch rather than mung bean noodles. In addition to these measures, biologically treating starch, such as fermentation,^[4] and using additives such as glycerol monostearate (GMS) could improve the quality of starch noodles as well. It was reported that starch noodles containing GMS showed lower values for texture profile analysis parameters such as hardness, cohesiveness, gumminess, chewiness, springiness, cooked weight as well as cooking loss for corn and potato starch noodles.^[15]

As native starch processed into starch noodles, quality of the native starch would be changed, which will influence the eating quality of starch noodles directly.^[19] Many studies have been made on the effect of native starch properties on eating quality of starch noodles.^[7–9] However, no systematic study has been carried out on the starch properties of starch noodles and their effect on eating quality of starch noodles. The present study was designed to provide basic research information on improving the eating quality of starch noodles by improving processing technology so as to promote the starch properties of starch noodles. Thus, the starch properties were studied (swelling power and solubility, amylose leaching rate, swelling factor, pasting and thermal properties, scanning electron microscopy [SEM] observation) of starch noodles made from legume starches (pea, mung bean), tuber or root starches (sweet potato, potato), fernery starches and geshu starches that made by kudzu and sweet potato starches and their effect on eating quality (broken rate, cooking quality, physical properties) of starch noodles.

MATERIALS AND METHODS

Materials

Commercial mung bean starch noodles, pea starch noodles, and sweet potato starch noodles were purchased from Yantai Shuangta Food Co., Ltd. (Shandong, China) (Production License No.: QS 3706 2301 0021); commercial potato starch noodles were purchased from Longkou Longxu Vermicelli Co., Ltd. (Shandong, China) (Product standard code: GB/T 23587-2009); commercial geshu starch noodles that made by kudzu and sweet potato starches were purchased from Pengan Yifengyuan Bio Technology Co., Ltd. (Sichuan, China; Production License No.: QS 5113 2301 0184); commercial fernery starch noodles were purchased from Jiuhuan Wild Food Co., Ltd. (Sichuan, China; Production License No.: QS 5107 2301 0051). All these starch noodles are made from single native starch except geshu starch noodles. Starch noodles were grounded followed by passed through a 100-mesh sieve before the starch properties of starch noodles were studied.

Cooking Test of Starch Noodles

Broken rate

Truncate 20 starch noodles that have no technical damages and cut into 10 cm lengths, and then cooked them in 500 mL boiling distilled water for 30 min. Filtering water and counting the total number of starch noodles after cooking. The experiments were replicated three times and the mean values were reported. The broken rate of starch noodles^[20] was calculated as follows:

(1)

where, X is the number of starch noodles after cooking.

Cooking quality

Three grams (W₀) of starch noodles were cut into lengths of about 3 cm, and then were dried at 105°C and atmospheric pressure for 4 h. Determine the dry matter mass (W₁), and were cooked in 100 mL boiling distilled water for 15 min and stirred. And then fish for starch noodles, cool them off rapidly, water that clinging to the surfaces of starch noodles were soaked up by filter, determine the matter mass (W₂) of aquiferous starch noodles. Drying them again at 105°C for 4 h, determine the dry matter mass (W₃). The experiments were replicated at least three times and the mean values were reported. Swelling ratio, cooking loss, and dry matter content of starch noodles made from different starches^[8,21] were calculated as follows:

(2)

(3)

(4)

where, W₀ and W₁ are the initial weight (g) of starch noodles and the weight of dry matter (g) after the first drying, respectively; W₂ is the weight of dry matter (g) of aquiferous starch noodles; W₃ is the weight of dry matter (g) after the second drying.

Physical properties of starch noodles

Truncate ten even thickness starch noodles that have no cracks and bending, and were cut into 10 cm lengths, steeped for 10 min in 200 mL hot distilled water (95°C), and then fish out starch noodles, cool them off in cold distilled water. Water that was clinging to the surface of the starch noodles was soaked up by the filter, and the diameter of starch noodles in three different parts were measured by venier caliper, take the average and reported as d_i.

A TA.XT2i texture analyzer (Stable Micro Systems Ltd., UK) equipped with a A/LKB-probe was used to determine the physical properties under the following conditions: pre-test speed, 2.0 mm/s; post-test speed, 2.0 mm/s; test speed, 0.8 mm/s; induction force, 20 g; test deformation, 100%; load cell capacity, 5 kg. The value of maximum shear force (F_max), shear deformation, elasticity coefficient and cohesiveness of starch noodles made from different starches will be obtained. One starch noodle would spend at each measure, repeat the determination ten times and take the average. Shear strength and shear deformation^[22,23] were calculated as follows:

(5)

(6)

where, F_max is the maximum shear force (g), which is the force up to shear starch noodles that results in the fracture (the test deformation is 100%); S_i is the cross sectional area (mm²) of each starch noodles; Distance 1 is the thickness (mm) of starch noodles that have been fractured with F_max; d_i (mm) is the diameter of each starch noodles.

Starch Properties of Starch Noodles

Swelling power and solubility

Swelling power and solubility of starches at different temperatures (60, 70, 80, 90°C) were determined according to the method described by Crosbie^[24] with slight modifications. A 2% (wt/v) of starch solution (50 mL) was prepared, and heated in a water bath with continuous mixing at different temperatures (60, 70, 80, 90°C) for 30 min. The resulting slurries were cooled to room temperature and centrifuged at 3000 rpm (TDL80-2B, Shanghai Anting Scientific Instrumental Factory, China) for 20 min. The supernatant was decanted into an aluminum specimen box that had been dried to constant weight. The supernatant then was dried at 105°C until constant weight achieved. The dried supernatant and the sediment were weighed. The solubility (%) and swelling power (%, dry basis) was calculated as follows:

(7)

(8)

Amylose leaching rate

One hundred milligram starches (dry basis) and 10.0 mL deionized water were added into a centrifuge tube, and heated in a thermostat water bath at different temperatures (60, 70, 80, 90°C) for 30 min, with continuous mixing. The resulting slurries were cooled to room temperature and centrifuged at 2000 rpm (TDL80-2B, Shanghai Anting Scientific Instrumental Factory, China) for 10 min. A 722 visible spectrophotometer (Shanghai Jinghua Technology Co. Ltd., China) was used to determine the absorbance value of the supernatant at 620 nm. Amylose leaching rate (%) is expressed as the percentage of dissolving amylose mass in 100 g starches.^[25]

Swelling factor

The swelling factor of starches was measured using the blue dextran dye exclusion method.^[26] The swelling factor is reported as a ratio of the volume of swollen granules to the volume of dry starch. Four hundred milligram starches (dry basis) and 20.0 mL deionized water were added into centrifuge tube, and heated in a thermostat water bath at different temperatures (60, 70, 80, 90°C) for 30 min, with continuous shaking. The resulting slurries were cooled to 20°C. Two milliliters of blue dextran (Pharmacia, Mr 2 × 106, 5 mg/mL) was added into the centrifuge tube that contained the slurries. The mixture was mixed vigorously and centrifuged at 2000 rpm (TDL80-2B, Shanghai Anting Scientific Instrumental Factory, China) for 10 min. A 722 visible spectrophotometer (Shanghai Jinghua Technology Co. Ltd., China) was used to determine the absorbance value of the supernatant at 620 nm. The absorbance of reference tubes (A_R) that contained no starch was also measured. Calculation of swelling factor was based on starch weight corrected to 12% moisture, assuming a density of 1.4 g/mL. Free or interstitial-plus-supernatant water is given by:

(9)

The initial volume of the starch (V₀) of weight W (in mg) is calculated as follows:

(10)

The volume of absorbed intragranular water (V₁) is:

(11)

The volume of the swollen starch granules (V₂) is:

(12)

The swelling factor can be calculated as:

(13)

Pasting properties

Pasting properties of various starches were determined by a Brabender Visco-graph-E (Brabender Co., Germany). Five grams of starches were dissolved in distilled water to prepare starch solution with mass fraction of 5%. Starch solution was heated at a rate of 7.5°C /min from 30 to 92°C, maintained at 92°C for 5 min, cooled at the same rate to 50°C, and finally maintained at 50°C for 1 min. Viscosity data during the whole period was recorded and the brabender viscosity curves of starch pastes were obtained.

Thermal properties

Thermal properties were analyzed using Differential Scanning Calorimeter (DSC-200F3, NETZSCH Co., Germany) according to the method of Singh, Sodhi, and Singh^[27] with slight modifications. Twenty-five milligrams starches was accurately weighed into an aluminum pan and distilled water was added by using a microsyringe to prepare starch-water suspension with mass fraction of 40%. The pans were hermetically sealed and equilibrated at room temperature for 1 h prior to the analysis. DSC was calibrated using indium and an empty pan was used as the reference. Samples were heated at 10°C/min over a temperature range of 30°C~180°C. These gelatinization parameters: Peak temperature (T_p); Onset gelatinization temperature (T₀); Conclusion temperature (T_c) and retrogradation enthalpy (ΔH) were recorded.

SEM

The starch granules of starch noodles were observed using a SEM (Hitachi SN-3400, Japan). The starch samples were sprinkled on a double-sided tape mounted on a SEM stub, coated with gold, and then placed in the SEM chamber immediately after 20 min, photomicrographs were taken using a SEM apparatus at an accelerating voltage of 20 kV and a magnification of ×150.00.

Statistical Analysis

The data obtained from this study were analyzed by Data Processing System (DPS). The comparison between the internal mean value of index were tested by Least Significant Difference (LSD) test, and the level of significance was p < 0.05. Significance analysis was performed using SPSS 18.0 software.

RESULTS AND DISCUSSION

The Starch Properties of Starch Noodles

SEM

Starches exist naturally in the form of discrete granules within plant cells. These granules may be viewed as partially crystalline and partially amorphous polymeric systems.^[28,29] Inside the starch noodles, the starch granules were broken. Some gel-like substances can be observed in tuber starches and fernery starches. During cooling, aggregation of the amylose phase with linear segments of amylopectin will result in a strong gel.^[30] Based on the observation of the SEM micrographs of mung bean starch noodles’ surface and cross-section, Kasemsuwan, Bailey, and Jane^[13] reported that the starch granules were gelatinized and ruptured, and the starch formed networks resulting from the retrogradation process. Mestres, Colonna, and Buleon^[31] and Xu and Seib^[32] found that the structure of mung bean starch noodles were a ramified three-dimensional network held together by short segments of strongly retrograded amylose that melts at temperatures above the boiling point of water. Therefore, it can be inferred that the special inner structure of the starch noodles were composed of some broken starch granules and some gel-like substances, which were formed during the process of gelatinization and retrogradation in the production process (Fig. 1).

Figure. 1 The images for starch of starch noodles by scanning electron microscopy. (a) pea; (b) mung bean; (c) geshu (made by kudzu and sweet potato starches); (d) fernery; (e) sweet potato; (f) potato.

Swelling power and solubility

The solubility and swelling power for starch of starch noodles were correlated with the temperature directly. With the temperature increasing, solubility and swelling power increased (Table 1). The highest and lowest values of solubility and swelling power were in tuber and legume starches, respectively. In the temperature rise process, tuber starches owned the greatest growth in solubility (p < 0.05), which rose from 16.18%~23.12% at 60°C to 25.5%~30.93% at 90°C. On the contrary, legume starches owned the lowest growth (p < 0.05), which rose from 5.98%~7.23% at 60°C to 8.46%~9.1% at 90°C. As for swelling power, tuber starches had the biggest growth (p < 0.05), which rose from 17%~20.88% at 60°C to 24.22%~32.13% at 90°C. While geshu and fernery starches owned the lowest growth (p < 0.05). In a word, starch of tuber starch noodles was significantly superior to geshu, fernery, and legume starches in both the value and the growth degree of solubility and swelling power (p < 0.05), it showed that tuber starches could easily swell and leach when heated in water.

TABLE 1 The solubility, swelling power, swelling factor, and amylose leaching rate for starch of starch noodles

		Pea	Mung bean	Sweet potato	Potato	Geshu	Fernery
Solubility/%	60°C	7.23 ± 0.43^e	5.98 ± 0.53^f	16.18 ± 0.39^b	23.12 ± 0.37^a	11.91 ± 0.65^c	9.14 ± 0.34^d
	70°C	7.89 ± 0.27^e	6.61 ± 0.48^f	22.76 ± 0.75^b	24.33 ± 0.28^a	14.46 ± 0.39^c	10.45 ± 0.65^d
	80°C	8.54 ± 0.74^e	7.32 ± 0.26^f	23.85 ± 1.04^b	25.84 ± 0.48^a	15.26 ± 0.48^c	13.09 ± 0.39^d
	90°C	9.1 ± 0.31^d	8.46 ± 0.64^d	25.5 ± 0.51^b	30.93 ± 0.42^a	15.92 ± 0.52^c	15.43 ± 0.53^c
	Growth degree	1.87 ± 0.16^f	2.48 ± 0.23^e	9.32 ± 0.18^a	7.81 ± 0.19^b	4.01 ± 0.13^d	6.29 ± 0.25^c
Swelling power/%	60°C	7.16 ± 0.63^e	7.78 ± 0.28^e	17 ± 0.75^b	20.88 ± 0.67^a	13.12 ± 0.28^d	14.53 ± 0.34^c
	70°C	7.43 ± 0.47^e	7.95 ± 0.76^e	18.78 ± 0.55^b	23.43 ± 0.59^a	13.27 ± 0.39^d	14.57 ± 0.56^c
	80°C	7.88 ± 0.38^e	8.1 ± 0.42^e	22.68 ± 0.4^b	28.39 ± 0.33^a	13.48 ± 0.48^d	14.68 ± 0.45^c
	90°C	8.84 ± 0.53^e	8.95 ± 0.59^e	24.22 ± 0.86^b	32.13 ± 0.37^a	13.73 ± 0.36^d	14.83 ± 0.38^c
	Growth degree	1.68 ± 0.04^c	1.17 ± 0.29^d	7.22 ± 0.21^b	11.25 ± 0.18^a	0.61 ± 0.03^e	0.3 ± 0.08^f
Swelling factor	60°C	4.26 ± 0.53^f	5.5 ± 0.43^e	14.34 ± 0.39^b	17.67 ± 0.37^a	11.21 ± 0.46^c	10.34 ± 0.35^d
	70°C	4.79 ± 0.94^d	5.56 ± 0.39^d	17.78 ± 0.57^b	19.95 ± 0.58^a	12.58 ± 0.32^c	11.73 ± 0.28^c
	80°C	5.32 ± 0.48^d	5.62 ± 0.52^d	19.25 ± 0.64^b	21.07 ± 0.62^a	13.47 ± 0.56^c	13.28 ± 0.4^c
	90°C	5.65 ± 0.37^d	5.69 ± 0.57^d	21.48 ± 0.49^b	24.47 ± 0.48^a	14.24 ± 0.42^c	14.9 ± 0.33^c
	Growth degree	1.39 ± 0.07^e	0.19 ± 0.19^f	7.14 ± 0.23^a	6.8 ± 0.07^b	3.03 ± 0.08^d	4.56 ± 0.06^c
Amylose leaching rate/%	60°C	1.89 ± 0.23^d	2.7 ± 0.39^c	5.73 ± 0.57^b	6.38 ± 0.27^ab	6.65 ± 0.24^a	2.93 ± 0.32^c
	70°C	2.67 ± 0.31^d	3.45 ± 0.42^c	8.2 ± 0.42^a	6.68 ± 0.36^b	7.36 ± 0.33^b	3.97 ± 0.29^c
	80°C	3.48 ± 0.46^d	4.1 ± 0.18^d	9.38 ± 0.38^b	10.16 ± 0.47^a	8.66 ± 0.35^b	5.7 ± 0.37^c
	90°C	4.95 ± 0.37^e	4.75 ± 0.46^e	12.44 ± 0.33^b	13.29 ± 0.51^a	9.35 ± 0.28^c	7.59 ± 0.4^d
	Growth degree	3.06 ± 0.24^c	2.05 ± 0.03^e	6.71 ± 0.11^a	6.91 ± 0.2^a	2.7 ± 0.15^d	4.66 ± 0.12^b

The comparison between the internal mean value of index were tested by using the least significant difference (LSD) test;

Sample means with different letters in the same row are significantly different at p < 0.05;

Standard deviations are given within parenthesis;

Growth degree, which is the difference between the maximum and the minimum value of each index.

TABLE 2 The pasting properties for starch of starch noodles

Pasting characteristics	Pea	Mung bean	Sweet potato	Potato	Geshu	Fernery
A	72.4	76.0	49.6	44.8	56.6	54.4
B	76	49	672	885	218	355
C	52	40	574	463	157	278
D	76	49	302	298	218	348
E	91	66	522	521	308	496
F	93	66	578	591	322	523
G	91.9	91.9	85.9	77.6	91.8	92.1
B–D	0	0	370	587	0	7
E–D	15	17	220	223	90	148

(A) gelatinization temperature (oC); (B) peak viscosity (BU), which is the maximum viscosity after the beginning of gelatinization; (C) viscosity at the beginning of the first constant phase (BU); (D) viscosity at the beginning of the cooling phase (BU); (E) viscosity at the end of the cooling phase (BU); (F) viscosity at the final constant phase (BU); (G) peak temperature, which is the temperature at the end of gelatinization (oC); B–D, breakdown (BU), which could reflect the thermal stability of starch paste; E–D, setback (BU), which could reflect the cold stability of starch paste.

Amylose leaching rate and swelling factor

As shown in Table 1, the values of amylose leaching rate and swelling factor increased with increasing temperature. The values of amylose leaching rate and swelling factor of legume starches were both significantly lower than those of all the others (p < 0.05), indicating that legume starches were hard to swell and leached less amylose when heated in water.

Pasting properties

The viscosity curve for starch of starch noodles can reflect the ease or complexity of pasting, the retrogradation property, thermal stability and cold stability for different kinds of starch. Table 2 shows the pasting properties for starch of starch noodles. During the temperature rising phase, constant phase and cooling phase, the viscosity of legume starches increased, but increased with a low growth degree, and the maximum value was less than 100 BU, the peak viscosity was not obvious. On the contrary, the value of viscosity changed significantly for starch of tuber starch noodles, and increased quickly with increasing initial temperature, and the peak viscosity was up to 672~885 BU. Higher peak viscosity implied that more starch has been gelatinized during processing. Thus, water binding capacity is higher and the starch granules will be disintegrated easily.^[10]

The highest (72.4~76°C) and lowest (44.8~49.6°C) gelatinization temperatures were obtained in legume starches and tuber starches, respectively. Legume starches and tuber starches had the lowest (15~17 BU) and highest (220~223 BU) setback, respectively; Tuber starches had the highest breakdown (370~587 BU). According to the results above, legume starches were hard to swell when heated in water, a higher temperature was needed for their gelatinization. Meanwhile, the viscosity increased with a low rate, and the lowest peak viscosity (49~76 BU) and final viscosity (66~91 BU) were obtained. Both the thermal stability and cold stability were good for starch in legume starch noodles, compared with starch in other kinds of starch noodles. Generally, starch noodles made by starches with good thermal stability would have lower cooking loss and great taste, and starch noodles with good cold stability would have lower broken rate.^[8]

Thermal properties

The thermal property parameters for starch of various starch noodles are presented in Table 3. The gelatinizing process for starch of starch noodles was an endothermic reaction. The onset temperatures of several starches were higher than 70°C. Endothermic peak temperatures were higher than 100°C, which showed that these were crystallization peaks of amylase. Tuber starches had the highest conclusion temperature, followed by geshu, legume, and fernery starches in descending order. For fernery starches, the values of the above temperatures were lowest, according to the study of Varavinita et al.,^[33] and Moorthy, Andersson, and Eliasson,^[34] possibly due to the lower content of amylose.^[35] Retrogradation enthalpy (ΔH) was in the range of 967.33 J/g~1166.82 J/g for all samples tested, The lowest ΔH was found in fernery starches, showing that fernery starches had the smallest trend of retrogradation.^[19]

TABLE 3 The thermal parameters for starch of starch noodles

Sample	T0 (°C)	TP (°C)	TC (°C)	ΔH (J/g dry starch)
Pea	88.48	116.47	139.61	1128.87
Mung bean	82.35	111.19	139.91	1092.04
Sweet potato	83.28	115.00	151.12	1162.94
Potato	86.08	115.20	158.86	1096.20
Geshu	88.54	119.31	143.21	1166.82
Fernery	72.74	101.58	133.92	967.33

T_o, T_p, and T_c indicate the onset, peak, and conclusion temperature of gelatinization, respectively. ΔH indicates retrogradation enthalpy.

Eating Quality of Starch Noodles

Cooking test

The cooking test of various starch noodles are presented in Table 4. The broken rate of starch noodles were less than 3.33%, which meets the related national standards regulations.^[20] Legume starch noodles had the lowest broken rate, while fernery and potato starch noodles were higher. The swelling ratio and cooking loss of tuber starch noodles were the highest, which reached up to 442.77%~487.17% and 7.41%~8.85%, respectively, followed in descending order by fernery, geshu, and legume starch noodles. There were significant differences among various starch noodles for swelling ratio and cooking loss (p < 0.05). Cooking loss of starch noodles were less than 10 g/100 g, which is in the acceptable range.^[10] The dry matter contents of all starch noodles were higher than 87.15% and geshu starch noodles had the highest value (90.41%).

TABLE 4 The cooking test and mechanical properties of starch noodles

	Pea	Mung bean	Sweet potato	Potato	Geshu	Fernery
Cooking test
Broken rate/%	1.67 ± 2.89^a	0.00 ± 0.00^a	1.67 ± 2.89^a	3.33 ± 2.89^a	1.67 ± 2.89^a	3.33 ± 5.77^a
Swelling ratio/%	343.57 ± 5.76^e	332.64 ± 3.66^f	442.77 ± 4.32^b	487.17 ± 6.54^a	372.07 ± 5.15^d	410.47 ± 6.92^c
Cooking loss/%	2.74 ± 0.06^e	2.40 ± 0.05^f	7.41 ± 0.06^b	7.41 ± 0.06^b	5.1 ± 0.15^d	6.83 ± 0.07^c
Dry matter content/%	88.81 ± 0.08^b	87.15 ± 0.09^f	88.49 ± 0.09^c	88.15 ± 0.13^d	90.41 ± 0.07^a	87.35 ± 0.12^e
Mechanical properties
Hardness/g/mm^2	87.47 ± 1.07^b	93.29 ± 1.21^a	67.17 ± 1.09^d	63.87 ± 1.14^e	70.32 ± 1.38^c	49.7 ± 0.97^f
Shear deformation	0.52 ± 0.03^a	0.49 ± 0.03^ab	0.42 ± 0.02^c	0.43 ± 0.04^c	0.46 ± 0.03^b	0.5 ± 0.04^ab
Elasticity	0.58 ± 0.05^ab	0.62 ± 0.04^a	0.43 ± 0.03^d	0.46 ± 0.03^cd	0.51 ± 0.03^bc	0.33 ± 0.06^e
Cohesiveness	0.29 ± 0.03^c	0.31 ± 0.03^c	0.56 ± 0.04^a	0.54 ± 0.03^a	0.45 ± 0.05^b	0.4 ± 0.03^b

The comparison between the internal mean value of index were tested by using the least significant difference (LSD) test; Sample means with different letters in the same row are significantly different at p < 0.05; Standard deviations are given within parenthesis.

Physical properties

The mechanical properties of various starch noodles are presented in Table 4. The legume starch noodles had the maximum hardness, shear deformation, and elasticity, while geshu and tuber starch noodles had lower hardness, shear deformation, and elasticity (p < 0.05), indicating that legume starch noodles were not easy to be broken and had better chewiness quality.^[21] Tuber and legume starch noodles had the maximum and minimum cohesiveness, respectively (p < 0.05), showing that legume starch noodles were not easy to bond together, they were smoother and non-sticking to teeth while eating.^[1]

Correlation Analysis between Eating Quality and Starch Properties of Starch Noodles

Correlation between eating quality and swelling properties for starch of starch noodles

Table 5 shows the correlation between eating quality and swelling properties for starch of starch noodles. The solubility, swelling power, amylose leaching rate, swelling factor for starch of starch noodles were positively significant (p < 0.01) or positively correlated (p < 0.05) with swelling ratio, cooking loss, and cohesiveness of starch noodles, and were negatively significant (p < 0.01) or negatively correlated (p < 0.05) with shear deformation of starch noodles. The result above demonstrated that swelling properties for starch of starch noodles can significantly influence the cooking and physical characteristics of starch noodles.

TABLE 5 The correlation between the eating quality and swelling, pasting, thermal properties for starch of starch noodles

Correlation coefficient (r)	Broken rate/%	Swelling ratio/%	Cooking loss/%	Hardness (g/mm^2)	Shear deformation	Elasticity	Cohesiveness
Swelling properties
Solubility/%	0.602	0.971**	0.933**	–0.586	–0.882*	–0.531	0.946**
swelling power/%	0.600	0.973**	0.915*	–0.542	–0.847*	–0.490	0.900*
Amylose leaching rate/%	0.540	0.922**	0.913*	–0.596	–0.929**	–0.533	0.987**
Swelling factor	0.657	0.970**	0.975**	–0.717	–0.867*	–0.664	0.969**
Pasting properties
A	–0.758	–0.936**	–0.981**	0.834*	0.772	0.772	–0.931**
B	0.642	0.989**	0.943**	–0.609	–0.826*	–0.576	0.906*
F	0.761	0.934**	0.983**	–0.881*	–0.712	–0.865*	0.891*
B–D	0.450	0.898*	0.789	–0.327	–0.817*	–0.292	0.810
E–D	0.667	0.968**	0.978**	–0.761	–0.816*	–0.750	0.940*
Thermal properties
T0 (°C)	–0.319	–0.136	–0.238	0.559	–0.203	0.662	0.037
TP (°C)	–0.320	–0.049	–0.130	0.464	–0.366	0.562	0.200
TC (°C)	0.248	0.754	0.636	–0.116	–0.855*	–0.042	0.775
ΔH (J/g dry starch)	–0.473	–0.089	–0.158	0.485	–0.433	0.530	0.242

Using two-tailed t-test for correlation analysis; (A) gelatinization temperature (°C); (B) peak viscosity (BU), which is the maximum viscosity after the beginning of gelatinization; (F) viscosity at the final constant phase (BU); B–D, breakdown (BU), which could reflect the thermal stability of starch paste; E–D, setback (BU), which could reflect the cold stability of starch paste; T_o, T_p, and T_c indicate the onset, peak, and conclusion temperature of gelatinization, respectively; ΔH indicates retrogradation enthalpy (J/g dry starch);

*significant at 0.01 < p < 0.05; **significant at p < 0.01.

The solubility, swelling power, and swelling factor for starch of starch noodles can reflect the swelling extent of starch when heated in water. The higher values of those three parameters indicated the higher swelling extent of starch. The amylose leaching rate indicates that how much the amylose will leach out when starch was heated in water. The gel structure of starch noodles is easier to collapse due to a higher amylose leaching rate, thus water swelling occurred. So the swelling ratio, cooking loss, and cohesiveness of starch noodles increased and shear deformation of starch noodles decreased with increasing solubility, swelling power, swelling factor, and amylose leaching rate for starch of starch noodles. The eating quality of starch noodles are manifested as more likely to burnt soup, poorer in boil-resisting ability, and easier to be broken. Besides, the eating quality needs to be developed by decreasing viscosity and increasing the smoothness level accordingly.^{[1,5,8,11,21]}

Correlation between the eating quality and pasting properties for starch of starch noodles

Table 5 shows the correlation between the eating quality and pasting properties for starch of starch noodles. Gelatinization temperature for starch of starch noodles were positively correlated (p < 0.05) with hardness of starch noodles, and were significant negatively correlated (p < 0.01) with swelling ratio, cooking loss, and cohesiveness of starch noodles. Peak viscosity, final viscosity, and setback were positively significant (p < 0.01) or positively correlated (p < 0.05) with swelling ratio, cooking loss, and cohesiveness of starch noodles. Peak viscosity, breakdown, and set back were negatively correlated (p < 0.05) with shear deformation of starch noodles; Final viscosity were negatively correlated (p < 0.05) with hardness and elasticity of starch noodles. Break down were positively correlated (p < 0.05) with swelling ratio of starch noodles. The previous results demonstrated that pasting properties for starch of starch noodles can significantly influence the cooking and physical characteristics of starch noodles.

The higher the gelatinization temperature for starch of starch noodles, the harder in pasting, indicating that starch noodles has lower water swelling extent, and the gel structure retained very well, thereby making starch noodles with higher hardness, swelling ratio, cooking loss and lower cohesiveness, and thus, make starch noodles harder to burn in soup, higher in boil-resisting ability, smoother and non-sticking to teeth while eating. Peak and final viscosity are the maximum viscosity after the beginning of gelatinization and at the final constant phase, respectively. The higher value of these two parameters, the higher the swelling extent of starch is, thus the swelling ratio, cooking loss, and cohesiveness increase and the hardness, shear deformation, and elasticity decrease, which makes starch noodles more likely to burn in soup, poorer in boil-resisting ability, and easier to be broken. The eating quality should be improved by decreasing viscosity, increasing smoothness level, and chewiness quality accordingly. The break down and setback can reflect the thermal and cold stability, respectively, which states that the starch paste can’t retain the viscosity well in the constant and cooling phase, and often change in a wide range. Therefore, starch noodles have a higher swelling ratio and the gel structure is difficult to be maintained, and then owns lower fear deformation, swelling ratio, cooking loss, and cohesiveness, which make starch noodles more likely to burn in soup, poorer in boil-resisting ability, easier to be broken and the eating quality should be improved by decreasing viscosity and increasing the smoothness level accordingly.^{[1,5,8,11,21]}

Correlation between the eating quality and thermal properties for starch of starch noodles

The correlation between the eating quality and thermal properties for starch of starch noodles are shown in Table 5. Only the final pasting temperature for starch of starch noodles was negatively correlated (p < 0.05) with the shear deformation of starch noodles, indicating that thermal properties for starch of starch noodles had no obvious influence on the cooking and physical characteristics of starch noodles.

The final pasting temperature is strongly correlated to the crystal structure of starch. Generally, the higher the amylose content, the higher extent of recrystallization in retrogradation, more heat are required to melt crystal as starch crystal structure is more compact, thus the final pasting temperature will be higher.^[36] Meanwhile, the higher extent of recrystallization, the denser the gel structure will be formed, and the shear deformation will be higher. However, the correlation analysis shows that the final pasting temperature was negatively correlated with the shear deformation. One likely reason is that different amylose have different crystal melting rates and different temperatures are needed for complete melting.

CONCLUSIONS

The main component of starch noodles is starch, and starch from different plant sources have different characteristics, thus the eating qualities are very different in various starch noodles. The experimental results indicate that tuber and legume starches have the highest and lowest values of solubility, swelling power, swelling factor, setback, breakdown, peak viscosity, and final viscosity, respectively. Legume and tuber starches have the highest and lowest gelatinization temperature, respectively. Tuber and geshu starches have the highest amylose leaching rate, while legume starches owns the lowest value. The previous results demonstrated that, compared with others, legume starch is harder to swell when heated in water, a higher temperature was needed for its gelatinization, only a few amylose will leach out, and also has better thermal and cold stability. However, tuber starches do exactly the opposite. As to thermal properties, tuber starches had the highest conclusion temperature of gelatinization. Fernery starches have the lowest value of retrogradation enthalpy (ΔH), which shows that fernery starches had the smallest trend of retrogradation. According to the research in eating quality of starch noodles, the highest swelling ratio and cooking loss were found in tuber starches, followed in descending order by fernery, geshu, and legume starch noodles, indicating that tuber starch is more likely to burn in soup, and are poor in boil-resisting ability. More work needs to be done on the improvement of the eating quality of starch noodles by decreasing viscosity and increasing smoothness level.^[5,8,11] Among the six kinds of starch noodles, legume starch noodles have the highest hardness, shear deformation and elasticity, and the lowest cohesiveness, which indicates that legume starch noodles have a good chewiness quality. In addition, legume starch noodles are more elastic and strong, smoother, and non-sticking to teeth while eating. All the previous results showed that legume starch noodles are good in eating quality. The correlation analysis between the eating quality and starch properties of starch noodles shows that the swelling and pasting properties for starch of starch noodles can significantly influence the cooking and physical characteristics of starch noodles. However, thermal properties had no obvious influence on eating quality of starch noodles. This demonstrated that the judging and perfection of the cooking and physical quality of starch noodles can be achieved by testing and improving the swelling and pasting properties for starch of starch noodles. Thus, the eating quality of starch noodles was improved.