Sweet potato staple foods

CHAPTER 10

Sweet potato staple foods Tai-Hua Mu1, Miao Zhang1, Hong-Nan Sun1 and Isela Carballo Pérez1,2 1

Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China Institute of Food Research, Havana, Cuba

2

Overview of sweet potato staple foods Definition and types of sweet potato staple foods The sweet potato is an economical and healthful food crop and is the fifth most important food crop in the world after rice, wheat, maize, and potato (FAOSTAT, 2016). The sweet potato is rich in protein, dietary fiber, vitamins, minerals, and many other healthy ingredients (Woolfe, 1992), and sweet potato protein has a high content of essential amino acids and a balanced amino acid composition that is superior to those of cereal proteins. Based on its good nutritional value, sweet potato has been processed into flour, flakes, granules, paste, purees, chips, canned products, beverages, and various snack foods. Nowadays, sweet potato is also used as an important supplement for different staple products in the food industry, such as sweet potato steamed breads, baked breads, noodles, and pancakes, etc. Thus sweet potato staple foods refer to the staple foods made from sweet potato or supplemented with sweet potato components. Sweet potato steamed breads As a traditional Chinese staple food, steamed bread has been consumed for at least 2000 years in China. Nowadays steamed bread is gaining considerably more popularity in many countries due to its lower acrylamide content and the lower loss of soluble amino acids compared with baked bread (Sui et al., 2016). Mu et al. (2014a) prepared gluten-free sweet potato steamed bread with sweet potato flour (SPF), sweet potato starch and modified starch with the addition of protein, pectin, gum arabic, sugars, and yeast. Mu et al. (2014b) also prepared gluten-free sweet potato steamed bread rich in dietary fiber with sweet potato dietary fiber, sweet potato starch, different hydrocolloids, protein, sugars, salt, and yeast. Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00010-7

© 2019 Elsevier Inc. All rights reserved.

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Sweet potato breads Bakery products, especially breads, are one of excellent sources of energy foods, which can be used in vehicle, planetary and ground-based food systems (Greene and Bovell-Benjamin, 2004). To improve the nutritive value of bakery products, studies have been conducted on the use of composite flours by blending wheat flour (WF) with flours from other cereals, oilseeds, legumes, or tubers (Trejo-González et al., 2014). Breads supplemented with 50%
65% SPF showed higher β-carotene contents, lower protein contents, and different appearance, texture, and flavor compared with whole wheat breads (Greene and Bovell-Benjamin, 2004). TrejoGonzález et al. (2014) replaced WF with 5%
20% SPF prepared from sun-dried slices of an orange-fleshed Mexican cultivar, and indicated that SPF addition to the extent of 5% yielded acceptable doughs and breads from the perspective of physical dough and bread properties, and WF replacement with 10% SPF yielded good quality breads based on sensory properties. Sweet potato noodles The popularity of noodles is increasing, particularly in Asian countries, due to their simple preparation, long shelf life, desirable sensory attributes, product diversity and nutritive value. With the expansion of the world market, research in the development and improvement of the quality of noodles in order to satisfy consumer’s demands is of immense importance. WF is the main ingredient for the manufacturing of noodles, and the demand to use novel sources as substitutes for WF has increased in recent years. To add variety and functionality to the noodle products, flours from alternative sources such as sweet potato, colocasia, water chestnut, and other tubers are being used as potential WF substitutes for noodle making (Yadav et al., 2014). Collado and Corke (1996) prepared Chinese-style yellow alkaline noodles and Japanese-style white salted noodles from a standard brand of hard red winter WF and from composite flours containing 25% SPF, and indicated that the addition of ascorbic acid tended to increase the firmness of noodles with wheat
sweet potato composite flours, while inducing a higher degree of browning after storage. Collins and Pangloli (1997) studied the chemical, physical, and sensory attributes of noodles supplemented with sweet potato and soy flour, and found that SPF increased color acceptability with no change in flavor or overall acceptability. Pangloli et al. (2000) indicated that noodles supplemented with 10% defatted soy flour and 10% SPF or 15% sweet potato

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puree could be stored successfully under air at 4.4°C, which showed greater quality retention than WF noodles. Other types of sweet potato staple foods Cake is a kind of bread or bread-like food, typically a baked sweet dessert, which has enjoyed a relatively constant place in our diet for a long time and its continuing popularity has encouraged the development of more attractive cake products that are available in the market today (EkeEjiofor, 2013). Cake produced from sweet potato
wheat flour presents improved nutritional and sensory properties when 20% of SPF is substituted, while in the case of biscuit, up to 30% of SPF is substituted (EkeEjiofor, 2013). Okorie and Onyeneke (2012) indicated that sweet potato cakes were at their best for volume increase, softness, and overall acceptability at 20% sweet potato substitution. In addition, Shih et al. (2006) prepared gluten-free pancakes using rice flour and rice flour replaced with 10%
40% of SPF, and indicated that the nutritional properties of the rice
sweet potato pancakes, including protein, dietary fiber, total carbohydrate contents, and calories, were generally comparable with those of their wheat counterpart. Saeed et al. (2012) indicated that the addition of 10%
20% of SPF lowered the width and thickness of cookies, but improved their flavor, taste, and overall acceptability.

Main raw ingredients for sweet potato staple foods Many different types of sweet potato raw ingredients can be used to produce sweet potato staple foods, and the main raw ingredients used for sweet potato staple foods now include fresh sweet potato, mashed sweet potato, and SPF, etc. Fresh sweet potato The fresh sweet potato can be directly used as a processing ingredient for sweet potato staple foods. For example, the fresh sweet potato can be peeled, steamed, pounded, and then mixed with WF to prepare steamed bread, noodles, and other foods. The direct use of fresh sweet potato to make sweet potato staple foods has low processing costs, while sweet potato has high moisture content, short shelf life, and difficulty in storage and transportation, and the processing process is not easy to handle. Therefore the production of sweet potato staple foods using fresh potatoes has certain limitations, and it is more suitable for home-made processing

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and not suitable for large-scale industrial production. In addition, the final quality of staple foods is highly dependent on the quality of the raw ingredients used. Thus if sweet potato is to be incorporated into staple foods, it must be of high quality. Mashed sweet potato Mashed sweet potato is a kind of puree-like product made from fresh sweet potato after being processed and matured. Generally, it can be eaten directly, eaten after being adjusted with different flavors, or made into various kinds of snack foods. However, the high starch gelatinization degree of mashed sweet potato limits the proportion of mashed sweet potato in staple foods. Wu et al. (2009) investigated the effects of 5%
30% sweet potato pastes from different varieties on the physicochemical properties of dough and bread, and found that toast of bread supplemented with sweet potato paste was more favorable than that of the control, while loaf volume slightly decreased with the addition of more than 20% of sweet potato paste. Bhosale et al. (2011) added 0%
15% of mashed sweet potato to chicken meat nuggets to improve their nutritional value and present some beneficial effects due to the presence of dietary fibers and β-carotene, and indicated that chicken meat nuggets with 10% of mashed sweet potato sustained the desired cooking yield and emulsion stability, and showed higher overall acceptability scores. Sweet potato flour Flour is the powder made from cereals—most commonly wheat—which is the key ingredient in bread production and constitutes a staple diet in many countries. Therefore the availability of an adequate supply of flour has often been a major economic and political issue. Flour can be also made from legumes and nuts, roots, and tubers such as sweet potato, yam, cassava, etc. Flour produced from nonwheat sources is known as composite flour (Adeleke and Odedeji, 2010). Sweet potato can be processed into raw flour by being peeled, cut into slices, and dried in an oven at different temperatures (55°C
65°C). The flour can be used as a thickener in soup, gravy, fabricated snacks, and bakery products (Ahmed et al., 2010). SPF can also be processed into cooked flour, in a process where sweet potatoes are peeled, cut into slices, color protected, steamed, and air-dried. Nowadays, to improve the nutrition value of stable food, SPF is partly substituted for cereals flours, for example, WF, which shows benefits for individuals diagnosed with celiac

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disease (Sukhcharn et al., 2008). A study of Ahmed et al. (2007) showed that the dough and bread characteristics were essentially unchanged when 5%
10% of SPF is substituted for WF bread. In addition, SPF and flakes can partly substitute for wheat and other cereals flours in order to enrich the β-carotene content in bakery products and pancakes (Woolfe, 1992). It was found that the incorporation of SPF in the formulation of rice pancakes improved the flow behavior of the batter and the physicochemical properties of the product (Shih et al., 2006).

Development of sweet potato steamed bread As a main staple food in China, steamed bread comprises almost 40% of wheat consumption (Wu et al., 2010). There is less acrylamide content and lower soluble amino acids loss in steamed bread when compared with baked bread (Becalski et al., 2003). However, for only-wheat steamed bread, there is not enough lysine, vitamins, and mineral elements for human nutritional balance, so it is necessary to supplement some functional components or other flours to improve its nutritional values and provide choices for consumers, such as steamed breads with sweet potato, potato, yam, oat, barely, buckwheat, corn, and wheat germ flours, as well as those with fiber and polyphenols (Liu et al., 2016). Some research on the development of sweet potato steamed bread is introduced here.

Sweet potato steamed bread prepared with sweet potato flour and other food components Mu et al. (2014a) prepared sweet potato steamed bread with 20%
40% raw SPF, 30%
50% cooked SPF, 10%
18% sweet potato starch, 6%
13% sweet potato modified starch, 3%
8% protein, 0.5%
2.5% pectin, 0.1%
0.6% gum arabic, 0.1%
2% sugar, and 2%
3% yeast. When producing the sweet potato steamed bread prepared with SPF and other food components, the ratio of the mixed powder to water is 100:70
90 (w/w). The sweet potato steamed bread produced by the method above is suitable for people with allergies caused by gluten and presents a relatively comprehensive nutrient composition.

Sweet potato steamed bread prepared with sweet potato fiber and other food components Mu et al. (2014b) prepared sweet potato steamed bread with 10%
40% sweet potato crude dietary fiber, 10%
40% sweet potato ultrafine dietary

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fiber, 10%
20% sweet potato nano dietary fiber, 4%
12% sweet potato modified starch, 5%
12% extruded puffed sweet potato dietary fiber, 5%
13%, microwave-treated sweet potato crude dietary fiber, 2%
6% sweet potato starch, 0.5%
3% hydrocolloids, 0.5%
3% protein, 0.1%
3% sugar, 0.1%
1% salt, and 1%
2% yeast. When producing the sweet potato steamed bread prepared with sweet potato fiber and other food components, the ratio of the mix powder to water is 100:50
70 (w/w). The sweet potato steamed bread produced by the method above is yellowish, presents the unique flavor of sweet potato, and has good nutritional values.

Development of sweet potato
wheat bread Different food processing methods could have different effects on the quality of food. As the main ingredients of sweet potato staple food, the properties of SPF are important in order to develop sweet potato staple foods with acceptable quality. Take sweet potato
wheat bread as an example: the effects of heat and high hydrostatic pressure treatment of SPF on dough properties and bread characteristics were researched by the author team and are introduced here.

Effect of heat treatment of sweet potato flour on dough properties and bread characteristics The heat treatment (HT) of flour can be used in many applications in food processing and has been suggested to be a viable method of improving bread quality, particularly for weak and substandard flour (Marston et al., 2016). The mechanism by which HT improves the flour is well known. During the HT process, protein denaturation and the partial gelatinization of starch granules occurs, as well as an increase in batter viscosity (Neill et al., 2012). HT of WF at 100°C for 12 min enhanced the dough stability of bread (Bucsella et al., 2016). HT of wheat and the resulting changes in rheological properties were of considerable importance to the characteristics of the final baked products (Lagrain et al., 2005). Nakamura et al. (2008) observed an increase in the volume of Kasutera cake by the heating of WF at 120°C for 30 min. Marston et al. (2016) developed the bread from sorghum flour treated at 125°C for 30 min with good characteristics in the structure of bread. In addition, Puncha-Arnon and Uttapap (2013) found that a significant increase in paste and greater effects on

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thermal parameters of gelatinization and gel hardness of flours were observed when rice flour was treated at 100°C for 16 h. To develop sweet potato
wheat bread with acceptable quality, the author team investigated the effect of HT on color, particle size, thermal properties, and microstructure of SPF, as well as the effect of HT of SPF on dough rheology, fermentation, and texture of sweet potato
wheat bread, which had a partial substitution of WF by SPF. Characteristics of wheat flour and sweet potato flour Table 10.1 shows the characteristics of WF and SPF. WF showed higher moisture content (9.01%) than SPF (5.76%). WF had higher protein (11.41%) than SPF (5.62%), which was in accordance with the results of Vallons and Arendt (2010) and Adeyeye et al. (2014), who reported that SPF had 7.4% and 5.8% of protein, respectively, while Dewaest et al. (2017) found that protein content was 8.6% in WF. Starch content of WF (70.41%) was greater than SPF (59.02%), which was in accordance with the results reported before that starch values ranged from 60% to 75% in WF and from 38.6% to 62.29% in SPF (Sukhcharn et al., 2008). Fat content analysis showed that WF contains higher percentages of fat (1.06%) than SPF (0.90%). The ash contents of WF and SPF were 0.26% and 2.91%, separately. Sukhcharn et al. (2008) found the ash content of WF to be 0.65%. Akonor et al. (2017) found the ash content of SPF to be in Table 10.1 Characteristics of wheat and sweet potato flour. Basic components (%)

Wheat flour (WF)

Sweet potato flour (SPF)

Moisture Protein Starch fiber Fat Ash Vitamins (mg/100 g) B1 B2 B3 B6 B9 C

9.01 6 2.16a 11.41 6 0.06a 70.41 6 0.12a 0.54b 1.06 6 0.12a 0.26 6 0.05b

5.76 6 0.01b 5.62 6 0.01b 59.01 6 0.01b 1.88a 0.90 6 0.04b 2.91 6 0.04a

0.08 6 0.01b 0.075 6 0.001b 0.82 6 0.01b 0.06 6 0.01b 11.90 6 0.14b 7.48 6 0.12b

0.14 6 0.04a 0.28 6 0.01a 4.85 6 0.02a 0.37 6 0.01a 12.75 6 0.01a 38.15 6 0.15a

The values denoted by different letters in the same column are significantly different (P , .05).

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the range of 2.25%
2.54%, which was possibly due to a higher content of minerals and inorganic salts. In addition, SPF contained higher proportions of vitamins than WF, especially vitamins B3 and C, which were about six and five times than those in WF, respectively. Many researchers have studied the importance of vitamins in adult and child nutrition as alternatives to food fortification (Prasad and Kochhar, 2015; Low et al., 2015; Laurie et al., 2015), and SPF could be a good source of vitamins. Characteristic of sweet potato flour after heat treatment Color The color results of SPF after HT showed an increase in terms of lightness (L ), with respect to the control (83.9), except for at 120°C (83.71) (Table 10.2). Significant differences appeared in all treatments. There was an increase in a with HT and the highest value was observed at 110°C. In the case of b , there was an increase with HT and the highest value was 16.57 at 120°C. Particle size The middle particle size of SPF without HT (Control) was 33.52 μm, which decreased to 26.23 μm after HT at 90°C (Table 10.2). Volume mean diameter and area mean diameter also decreased significantly after HT compared to the control. Particle size, including size distribution, was one of the characteristics that most markedly affected the functional properties. Kim and Yao (2014) indicated that as the temperature increased from 30°C to 90°C, the peak particle size of waxy WF increased in different ways. Scanning electron microscopy After HT a slight change in the structure was shown, compared to the control, where the rupture of the granule occurred, which increased as the temperature increased from 90°C to 120°C (Fig. 10.1). Depending on the presence of moisture, HT can change the granular and molecular structure of starch (Chung et al., 2007). Sun et al. (2014) performed 130° C of HT to proso millet flour for 4 h and observed that the structure of the gel became more compact compared with the control, which could be due to the nonstarch compositions interacting with the starch granules and adhering to the surface of the granule during HT.

Table 10.2 Color, particle size, volume mean diameter, and area mean diameter of SPF after HT. Samples

L

a

b

Middle particle size (µm)

Volume mean diameter (µm)

Area mean diameter (µm)

Control 90°C 100°C 110°C 120°C

83.9 6 0.0b 84.13 6 0.01a 84.14 6 0.01a 83.96 6 0.03b 83.71 6 0.00c

0.3 6 0.0e 0.35 6 0.01d 0.41 6 0.00c 0.60 6 0.02a 0.46 6 0.05b

15.1 6 0.0e 15.58 6 0.00d 16.13 6 0.00c 16.31 6 0.01b 16.57 6 0.05a

33.52 6 0.39a 26.23 6 0.99b 26.98 6 0.52b 26.68 6 0.63b 27.46 6 0.98b

38.97 6 0.61a 29.98 6 1.19b 30.83 6 0.77b 30.43 6 0.79b 31.31 6 1.19b

20.13 6 0.97a 16.06 6 0.46b 16.54 6 0.34b 16.85 6 0.30b 17.16 6 0.28b

The values denoted by different letters in the same column are significantly different (P , .05).

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Figure 10.1 Scanning electron micrograph (magnification: 3 1000) of SPF after HT. (A) SPF without HT (control); (B) SPF at 90°C; (C) SPF at 100°C; (D) SPF at 110°C; (E) SPF at 120°C.

Differential scanning calorimetry for sweet potato flour It is important to understand thermal properties, such as gelatinization behavior temperature and enthalpy changes, during the baking process. Significant differences were observed with respect to endothermic peak temperatures (TP) of SPF after HT (Table 10.3). The lowest value was observed at 110°C, while there was a slight increase in values of TP in the flour treated at 90°C, 100°C, and 120°C with respect to control. The gelatinization enthalpy change (ΔH) of SPF without HT was 5.64 J/g, which decreased after HT, and the lowest value was observed at 120°C (3.68 J/g). Sun et al. (2014) found slightly higher values of TP and reduction on ΔH in HT millet flour, which might be related to the presence of nonstarch components such as proteins. Dough properties Dough fermentation Different factors could influence dough fermentation and modify the development of the final product, such as yeast, temperature, and the use of new materials. Dough height (Hm) was influenced by HT (Table 10.4). The greatest height of the dough was observed in the control. With regard to dought formation time (T1), there were significant differences in all treatments compared with the control. The longest time was observed in the treatment at 90°C and the shortest was at 110°C. Rosell et al. (2001) found different values of T1 on WF dough which could be attributed to the treatments of HT and the type of yeast. Regarding the gas 0 behavior, the time of maximum gas formation (T1 ) was the shortest in the treatment at 110°C and was much higher at 90°C. After HT the gas retention of the dough with SPF increased significantly from 1199 mL without HT to 1214 mL at 90°C. Liu et al. (2016) studied the steamed breads with potato flour and WF, and showed values of Hm from 17 to 36 mm and gas volume from 1572 to 2100 mL, respectively.

Table 10.3 Differential scanning calorimetry (DSC) of SPF and dough after HT. Samples

SPF ΔH (J/g)

TP (°C)

Control 90°C 100°C 110°C 120°C

Dough

78.59 6 0.08 78.68 6 0.02ba 79.00 6 0.15a 77.94 6 0.34c 78.66 6 0.1ba b

ΔH1 (J/g)

TP1 (°C)

5.64 6 0.00 4.94 6 0.35b 5.08 6 0.04b 3.67 6 0.05c 3.68 6 0.22c a

69.63 6 0.44 69.62 6 0.41a 69.44 6 0.01a 69.43 6 0.03a 69.21 6 0.42a a

The values denoted by different letters in the same column are significantly different (P , .05).

ΔH2 (J/g)

TP2 (°C)

0.79 6 0.56 0.56 6 0.14b 0.68 6 0.06ba 0.81 6 0.24a 0.55 6 0.06b a

98.33 6 0.11 98.71 6 0.02b 98.87 6 0.43ba 97.74 6 0.12d 99.17 6 0.28a c

0.32 6 0.02b 0.28 6 0.02c 0.28 6 0.01c 0.34 6 0.02ba 0.36 6 0.01a

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Table 10.4 Analysis of fermentation on dough treated with HT by rheofermentometer. Samples

Dough Development Hm (mm)

Control 90°C 100°C 110°C 120°C

Gas behavior 0

T1 (min)

39.4 6 0.4 37.5 6 0.0b 31.9 6 0.0c 16.1 6 0.1e 29.3 6 0.0d a

Gas retention (mL)

T1 (min)

103 6 0.0 110 6 0.0a 78.5 6 0.0b 65 6 0.03c 91 6 0.03d a

174 6 1.41 176 6 1.42a 76.5 6 0.7c 67 6 2.82d 155.5 6 2.1b a

1199 6 4.94b 1214 6 2.12a 487 6 1.41e 984 6 2.12d 1136 6 2.82c 0

Hm, maximum dough height; T1, time at which dough reaches the maximum height; T1 , time of maximum gas formation; gas retention, volume of the gas retained in the dough at the end of the assay. The values denoted by different letters in the same column are significantly different (P , .05).

Differential scanning calorimetry for dough For DSC two peaks were shown in all the dough (Table 10.3). TP1 was from 69.21°C to 69.63°C and TP2 was from 97.74°C to 99.17°C. It was observed that as the temperature increased, TP1 of SPF after HT decreased compared to the control, but with no significant differences. The values of ΔH1 were 0.55 to 0.81 J/g. In the second phase, the ΔH2 was 0.28 to 0.36 J/g, and the lower values were at 90°C and 100°C. Zaidul et al. (2008) suggested that TP2 of the mixtures of WF and sweet potato starch were lower compared to WF. Sun et al. (2014) explained that the differences of thermal properties might be attributed to the different varieties of flour and the influence of nonstarch components of the flour such as proteins.

Bread-making process and quality evaluation Crust and crumb color The greatest brightness (L ) of crust was observed in sweet potato
wheat bread with SPF at 100°C, which showed lower a and b (Table 10.5). The color of the sweet potato
wheat bread crust that exceeded 100°C was caused by Maillard reactions and the caramelization of sugars, which depended on the distribution of water and the presence of reducing sugars as well as amino acids and their types (Purlis, 2010). The crumb of sweet potato
wheat bread with SPF at 120°C showed the lowest brightness, while the crumb at 90°C showed the lowest a and the control had the lowest b followed by that at 90°C.

Table 10.5 Color of crust and crumb and specific volume (cm3/g) of sweet potato
wheat breads. Samples

Control 90 °C 100 °C 110 °C 120 °C

Crust

Crumb

L

a

b

L

a

b

54.94 6 0.04a 56.44 6 0.33d 58.38 6 0.40b 57.68 6 0.31c 56.41 6 0.38d

15.5 6 0.1a 15.68 6 0.16a 11.96 6 0.03c 12.60 6 0.24b 12.37 6 0.21b

34.9 6 0.1a 35.31 6 0.09a 35.29 6 0.30a 35.72 6 0.08a 32.72 6 0.98b

67.53 6 0.16a 67.77 6 0.27a 64.80 6 0.26b 63.49 6 0.39c 62.71 6 0.26d

2.04 6 0.04c 1.92 6 0.00c 2.77 6 0.01a 2.55 6 0.28b 2.85 6 0.19a

18.48 6 0.31c 18.68 6 0.10c 19.47 6 0.17a 18.73 6 0.06c 20.72 6 0.24a

The values denoted by different letters in the same column are significantly different (P , .05).

Specific volume (cm3/g)

2.3 6 0.0c 2.53 6 0.03a 2.35 6 0.03c 2.47 6 0.04b 2.43 6 0.02b

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Specific volume The highest volume was obtained in the sweet potato
wheat breads with SPF at 90°C, which was 2.53 cm3/g (Table 10.5). Infinite loaf volume is not so desirable, but consumers associate a certain amount of lightness and high loaf volume with certain breads, and low loaf volumes with others (Hathorn et al., 2008). The results of the specific volume in breads might be related to dough development, gas retention, middle particle size, volume mean diameter, and area mean diameter. Texture analysis Texture analysis results of springiness, hardness, cohesiveness, and chewiness of crust and crumb of sweet potato
wheat breads are shown in Fig. 10.2. Springiness was defined as the speed at which a deformed material returned to the initial condition after the force causing the deformation was removed. This was greatly affected by moisture content, moisture redistribution, and retrogradation of starch (Osella et al., 2005; Lazaridou and Biliaderis, 2009). The springiness of the crust and crumb

Figure 10.2 Springiness, hardness, cohesiveness, and chewiness of crust and crumb of sweet potato
wheat bread.

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were high in the control, followed by sweet potato
wheat bread with SPF at 90°C. High-quality bread with a good degree of freshness was related to high springiness values, while a loaf with low springiness values tended to crumble when it was sliced (McCarthy et al., 2005). The force required to achieve a given deformation (hardness) was the highest in the crumb of sweet potato
wheat bread with SPF at 110°C, while that of the control was the lowest. In composite breads the lower volume and harder texture had been mainly attributed to the “dilution” of the gluten matrix in the mixture and to protein network disruption (Pérez et al., 2008). Chewiness was expressed as the intensity of chewing needed before swallowing (Pasqualone et al., 2017), and cohesiveness was an indicator of the internal cohesion of the material: generally, breads with low cohesiveness were susceptible to fracture and crumble (Onyango et al., 2010) and were not desirable. In the crust the cohesiveness increased as the temperature increased, while there was a slight decrease in crumb. In addition, chewiness showed significant differences in different sweet potato
wheat breads, and the lowest value was found for the sweet potato
wheat crumb with SPF at 100°C.

Effect of high hydrostatic pressure to sweet potato flour on dough properties and bread characteristics In the last decades the development of nonconventional methods for food processing, like high hydrostatic pressure (HHP), has attracted much attention. HHP is applicable to food and raw material processing for obtaining innovative sensorial and functional properties (Huang et al., 2017). During HHP processing, different pressure and temperature combinations can be utilized to achieve the desired effects on texture, color, and flavor of foods. The quality of HHP processed food can, however, change during storage due to coexisting chemical reactions, such as oxidation and biochemical reactions (Nunes et al., 2017). HHP modifies the microstructure and rheological properties in a different way than thermal treatment (Cappa et al., 2016a), and it is highly dependent on the type of pressure level and time of treatment. Cappa et al. (2016b) indicated that breads with rice flour treated with HHP (600 MPa, 5 min, 40°C) showed high specific volumes and good crumb softness. To develop novel raw material for sweet potato
wheat bread, the author team investigated the effect of HHP on color, particle size, thermal properties, and microstructure of SPF, as well as the effect of HHP to

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SPF on dough fermentation, specific volume, texture, and flavor of sweet potato
wheat bread, which had a partial substitution of WF by SPF. Characteristic of sweet potato flour after high hydrostatic pressure Color After HHP, the L value of SPF significantly decreased from 83.9 at 0.1 MPa to 80.01 at 400 MPa, while the a value increased from 0.3 at 0.1 MPa to 1.12 at 400 MPa (Table 10.6). In the case of b , the smallest value was 12.38 at 100 MPa, while the highest value was 16.73 at 300 MPa. Ahmed et al. (2017a, b) indicated that the L value of the rice flour treated with HHP at 300
400 MPa with a flour to water ratio of 1:4 decreased significantly compared to that without HHP treatment. Particle size Compared to SPF at 0.1 MPa, the median particle size, volume mean diameter, and area mean diameter of SPF after HHP at 100
400 MPa were decreased significantly (Table 10.6). The lower values were observed in SPF at 100, 200, and 400 MPa, followed by that at 300 MPa. Ahmed et al. (2017a) proved the effect of HHP (300
600 MPa) on whole WF and observed a significant decrease in particle size at 10% (Dv10), 50% (Dv50), and 90% (Dv90) of the volume distribution. Ahmed et al. (2017a, b) indicated that the rice flour showed a bimodal particle size distribution, of which the particle size significantly decreased after HHP treatment. Scanning electron microscopy Compared to SPF without HHP (0.1 MPa), no significant differences were observed in the granules of SPF treated at 100 and 400 MPa (Fig. 10.3 A, B, and C), while the rupture of the granules occurred at 300 and 400 MPa (Fig. 10.1D and E). The surfaces of starch granules in HHP-treated chestnut flour dispersions were smooth and showed a minor crack when pressure was raised up to 600 MPa (Ahmed and Al-Attar, 2017). Zhu et al. (2016) found that the majority of starch granules in brown rice flours after HHP treatment kept their integrity, while granule size increased slightly for the flours treated at 400 and 500 MPa. Differential scanning calorimetry Compared with SPF at 0.1 MPa, the TP of SPF after HHP treatment at 100
300 MPa decreased significantly, while that at 400 MPa showed no significant difference (Table 10.7). Among all the treatments, the ΔH of

Table 10.6 Color, median particle size, volume mean diameter, and area mean diameter of sweet potato flour after high hydrostatic pressure (HHP). HHP (MPa)

L

a

0.1 100 200 300 400

83.93 6 0.11 80.35 6 0.01c 80.92 6 0.02b 80.13 6 0.02d 80.01 6 0.00e a

b

0.30 6 0.00 0.65 6 0.07c 0.61 6 0.04c 0.84 6 0.05b 1.12 6 0.05a d

Size (μm)

15.11 6 0.01 12.38 6 0.01e 12.86 6 0.02d 16.73 6 1.70a 15.90 6 0.02b c

The values denoted by different letters in the same column are significantly different (P , .05).

Volume (μm)

34.86 6 2.56 18.79 6 3.69bc 14.95 6 0.80c 21.65 6 2.19b 18.71 6 1.53bc a

Area (μm)

40.63 6 3.27 20.93 6 2.28bc 16.66 6 0.96c 24.36 6 0.87b 21.09 6 1.85bc a

20.13 6 0.97a 12.37 6 2.19b 9.77 6 0.45c 13.80 6 1.30b 11.90 6 0.86bc

290

Sweet Potato

Figure 10.3 Scanning electron micrograph of sweet potato flour (SPF) after high hydrostatic pressure (HHP) (magnification: 3 1000). (A) 0.1 MPa; (B) 100 MPa; (C) 200 MPa; (D) 300 MPa; (E) 400 MPa.

SPF after HHP treatment at 100 MPa was the highest, followed by that at 400 MPa (Table 10.7). Ahmed and Al-Attar (2017) found that there was no significant increase in the TP of chestnut flour after HHP treatment. McCann et al. (2013) indicated that the TP of WF with 56% moisture increased with an increase in the pressure level, particularly at 500 and 600 MPa. Dough properties Thermal properties TP of dough with SPF after HHP was shown by two peaks, of which TP1 ranged from 69.01°C to 70.16°C and TP2 ranged from 97.85°C to 98.66° C (Table 10.7). There were no significant differences in TP1 and TP2 of dough with SPF after HHP with respect to that with SPF at 0.1 MPa. Zaidul et al. (2008) suggested that the interaction between wheat and sweet potato starch could bring anomalies in the gelatinization temperature and enthalpy. The values of the enthalpy of gelatinization (ΔH) in the first phase were from 0.76 to 0.88 J/g. In the second phase, ΔH2 was from 0.26 to 0.38 J/g, and the lower values were shown in dough with SPF at 0.1, 100, and 400 MPa. Ahmed et al. (2017a) noted that whole WF dough treated by HHP exhibited two endothermic peaks: TP1 : (63° C
66°C) and TP2 (107°C
117°C), which decreased with the pressure increased. The TP1 was ascribed to the gelatinization of starch, whereas the TP2 could be the fusion of amylose
lipid complexes formed in the

Table 10.7 Differential scanning calorimetric (DSC) of SPF and dough after HHP. HHP (MPa)

0.1 100 200 300 400

SPF

Dough

TP (°C)

ΔH (J/g)

TP1 (°C)

ΔH1 (J/g)

TP2 (°C)

ΔH2 (J/g)

78.59 6 0.08a 78.16 6 0.41b 77.32 6 0.18c 78.00 6 0.12b 78.60 6 0.26a

5.64 6 0.00c 11.86 6 0.48a 4.11 6 0.12d 5.85 6 1.56c 8.66 6 0.90b

69.35 6 0.44ab 69.01 6 0.16b 69.01 6 0.69b 69.30 6 0.51b 70.16 6 0.79a

0.79 6 0.56a 0.76 6 0.14a 0.84 6 0.01a 0.88 6 0.02a 0.82 6 0.43a

98.33 6 0.12a 97.85 6 0.38a 98.66 6 1.54a 98.22 6 0.25a 98.03 6 0.43a

0.32 6 0.02ab 0.31 6 0.00ab 0.38 6 0.00a 0.36 6 0.00a 0.26 6 0.05b

The values denoted by different letters in the same column are significantly different (P , .05).

292

Sweet Potato

course of the starch gelatinization or due to protein denaturation. Liu et al. (2009) explained that the pressurization treatment caused the destruction of the granular structure of starch followed by hydration of the amorphous phase, resulting in the decrease of the enthalpy of gelatinization. Zaidul et al. (2008) reported that the apparent shifting of slightly higher temperatures resulted in a more prominent biphasic gelatinization behavior of the mixture due to the influence of the wheat gluten. Dough fermentation Dough height (Hm) was in the range from 36.74 to 38.85 mm when treated with HHP at 100
400 MPa (Table 10.8). With regard to dough formation time (T1), there were significant differences in all treatments compared with the 0.1 MPa, which significantly decreased after HHP at 100
400 MPa. Regarding the gas behavior, the time to reach the maxi0 mum gas formation rate (T1 ) of SPF was less in the treatment of 100 MPa, while there were no significant differences at 200 and 300 MPa compared with 0.1 MPa. Gas retention of dough with SPF at 100 MPa was the highest, at 1246.5 mL. The relation between gas production and retention is given as a percentage of gas retained in the dough. Gas retention was related to dough’s ability to be stretched in a thin membrane, which was attributed to the quality of the protein network (Renzetti and Rosell, 2016). Favorable gas production and retention were essential to obtain good product quality. Thus dough with SPF after proper HHP treatment could be used in the production of bread (Table 10.8). Table 10.8 The effect of HHP on dough fermentation by rheofermentometer. HHP (MPa)

0.1 100 200 300 400

Dough development

Gas behavior 0

Hm (mm)

T1 (min)

T1 (min)

Gas retention (mL)

39.40 6 0.41a 38.85 6 0.07b 36.74 6 0.06d 36.85 6 0.07d 38.05 6 0.07c

108.00 6 4.24a 97.50 6 3.53b 95.50 6 4.94b 97.50 6 3.53b 97.50 6 2.10b

174.0 6 1.7a 148.5 6 1.4c 173.5 6 2.1a 174.02 6 1.44a 164.6 6 0.7b

1199.0 6 4.9c 1246.5 6 3.5a 1189.5 6 0.7d 1221.5 6 2.1b 1133.0 6 2.8e

Hm, maximum dough height; T1, dough formation time at which dough reaches the maximum 0 height; T1 , time to reach the maximum gas formation rate; gas retention, volume of the gas retained in the dough at the end of dough fermentation. The values denoted by different letters in the same column are significantly different (P , .05).

Sweet potato staple foods

293

Bread-making process and quality evaluation Crust and crumb color Compared with that at 0.1 MPa (59.93), the lightness of crust significantly decreased from 55.38 to 44.96 as the pressure increased from 100 to 400 MPa (Table 10.9). After HHP the a value of the crust increased and was the highest at 400 MPa. The b values decreased with the increase of HHP, which decreased from 34.88 (0.1 MPa) to 30.49 (400 MPa). It was observed that the L value of the crumb decreased from 67.53 (0.1 MPa) to 57.32 (400 MPa). The a value of the crumb at 100 MPa was the highest, which was 3.06. The b of the crumb showed the highest value in SPF at 0.1 MPa (18.48) and the lowest value at 100 MPa (16.57). Bárcenas et al. (2010) indicated that the L , a , and b values in the crust and crumb of wheat bread decreased as HHP increased from 50 to 500 MPa. Cappa et al. (2016b) found that no significant differences were observed for L and b values, whereas an increase of a value was evidenced in gluten-free bread with rice flour treated at 600 MPa. Specific loaf volume The highest specific loaf volume was obtained in bread with SPF at 400 MPa, which was 2.62 cm3/g (Table 10.9). The lowest specific loaf volume was presented in the bread with SPF at 100 MPa, which might be due to the disruption of the protein network in the dough during baking. Hüttner et al. (2010) researched the bread made with oat flour treated at 200, 350, and 500 MPa, and indicated that the specific loaf volume of bread significantly increased when 10% of oat flour treated at 200 MPa was added, while incorporation of oat batters treated at 350 or 500 MPa resulted in reduced bread quality with low specific loaf volumes and uneven gas cell distribution. Appropriate HHP treatment could provide proper dough elasticity and protein network formation to obtain a high specific loaf volume (Hüttner et al., 2009). In the case of SPF, HHP at 400 MPa might be suitable to reach specific loaf volume for sweet potato
wheat bread. Texture analysis Springiness is defined as the speed at which a deformed material returns to its initial condition after the force causing deformation is removed. There were significant differences in springiness of crust and crumb at 300 and 400 MPa with respect to that at 0.1 MPa (Table 10.10). Springiness is greatly affected by moisture content, moisture redistribution, and starch

Table 10.9 Color of crust and crumb and specific loaf volume of bread with different treatments. HHP (MPa)

Crust L

0.1 100 200 300 400



59.93 6 0.35a 55.38 6 0.21b 54.94 6 0.20b 52.53 6 0.49c 44.96 6 0.70d

a



15.46 6 0.33d 16.70 6 0.30b 15.50 6 0.10c 15.63 6 0.35c 18.40 6 0.25a

Crumb b



34.88 6 0.11a 34.80 6 0.16a 33.42 6 0.20b 33.94 6 0.69b 30.49 6 0.21c

L



67.53 6 0.16a 63.50 6 0.29b 61.26 6 1.96c 58.67 6 0.26d 57.32 6 0.33d

The values denoted by different letters in the same column are significantly different (P , .05).

a



2.04 6 0.04c 3.06 6 0.02a 2.23 6 0.19c 2.58 6 0.08b 2.77 6 0.12b

b



18.48 6 0.31a 16.57 6 0.03d 17.57 6 0.07bc 17.72 6 0.13b 17.29 6 0.03c

Specific loaf volume (cm3/g)

2.30 6 0.00c 1.46 6 0.03e 2.00 6 0.02d 2.51 6 0.03b 2.62 6 0.02a

Table 10.10 Springiness, hardness, cohesiveness, and chewiness of the crust and crumbs of sweet potato
wheat bread with sweet potato flour (SPF) after high hydrostatic pressure (HHP). HHP (MPa)

0.1 100 200 300 400

Bread samples

Springness

Hardness (N)

Crust Crumb Crust Crumb Crust Crumb Crust Crumb Crust Crumb

0.92 6 0.01 0.93 6 0.01a 0.90 6 0.01a 0.93 6 0.01a 0.78 6 0.01c 0.92 6 0.01a 0.85 6 0.02bc 0.85 6 0.02bc 0.86 6 0.01b 0.90 6 0.01b a

2278.16 6 1.96 1840.30 6 0.05e 3134.08 6 1.58a 3295.95 6 6.16b 2350.83 6 1.54b 3326.62 6 2.95a 1977.40 6 1.09d 2540.66 6 0.73c 1356.14 6 1.45e 2071.41 6 0.63d

The values denoted by different letters in the same column are significantly different (P , .05).

c

Cohesiveness

Chewiness (N)

0.48 6 0.01 0.65 6 0.01a 0.53 6 0.01a 0.58 6 0.01c 0.53 6 0.01a 0.59 6 0.01c 0.53 6 0.02a 0.62 6 0.01b 0.52 6 0.02a 0.64 6 0.01a

968.21 6 0.19b 1120.15 6 0.04e 1485.65 6 0.57a 1779.82 6 0.27a 920.19 6 1.01c 1752.71 6 0.89b 903.28 6 0.28d 1406.28 6 0.31c 600.07 6 0.99e 1193.32 6 0.97d

b

296

Sweet Potato

retrogradation (Osella et al., 2005). The hardness was the force required to achieve a given deformation, which was the highest in the crust at 100 MPa and crumb at 200 MPa, and the lowest in the crust at 400 MPa and in crumb of SPF at 0.1 MPa. The hardness of gluten-free bread with rice flour treated with HHP at 600 MPa decreased significantly (Cappa et al., 2016b). However, HHP treatment on doughs at 50
200 MPa increased the hardness of wheat bread, which might be due to the protein network modification induced by HHP (Bárcenas et al., 2010). Cohesiveness is the extent to which a material can be deformed before breaking. In the crust, no significant differences were observed among all treatments, while there were significant differences in the crumb, being firstly decreased and then increased as the pressure increased. The changes in cohesiveness of sweet potato
wheat bread were not consistent with the report by Angioloni and Collar (2012), who demonstrated that the incorporation of pressured flours from wheat, oat, millet, and sorghum into bread formulations provoked a significant general decrease in crumb cohesiveness. Considering that dough cohesiveness has been reported as a good predictive parameter of fresh bread quality, the maximization of dough cohesiveness is a recommended trend for providing good breadmaking performance. Chewiness is the energy required to chew solid food until it is in the appropriate state to be swallowed. The lowest value was at 400 MPa in crust, and the chewiness in the crust and crumb decreased as the pressure increased. It was reported that the bread quality could be significantly improved when the specific volume was increased and hardness and chewiness was reduced (Renzetti et al., 2010). Thus SPF treated at 400 MPa could be potentially used in the production of breads with acceptable texture.

Development of sweet potato noodles Noodles are one of the Chinese’s favorite foods due to their acceptable taste to almost all age groups, availability at affordable prices, and the fact they can be produced by small-, medium-, or large-scale industries. The main ingredient of noodles is WF (Krishnan et al., 2012). In recent years, more and more studies have been carried out to improve the nutritional properties of noodles by adding other flours or functional components, such as sweet potato, yam, oat, corn, potato, wheat germ, barely, buckwheat, dietary fiber, polyphenols, etc. (Chillo et al., 2008; Gelencsér et al., 2008; Iafelice et al., 2008). Sweet potatoes have been

Sweet potato staple foods

297

reported to contain high amounts of dietary fiber, β-carotene, phenolic compounds, etc., and possess high antioxidant activity (Bovellbenjamin, 2007; Teow et al., 2007). Several researchers have tried the addition of sweet potato in making noodles (including traditional Chinese noodles, pasta, etc.) (Ginting and Yulifianti, 2015; Krishnan et al., 2012). Some of them are introduced here.

Traditional Chinese noodles prepared with sweet potato mash and wheat flour Ginting and Yulifianti (2015) prepared traditional Chinese noodles with sweet potato mash (40%) and WF. The processing method of sweet potato mash was that the fresh roots of sweet potatoes were steamed, the skins were removed, and then they were mashed to obtain a paste/mash. The physicochemical and sensory properties of the abovementioned noodle samples were analyzed. Results showed that a blend of 60% domestic WF with 40% sweet potato mash could improve the noodle color acceptance. The noodles prepared from 100% WFs and the blend with 40% sweet potato mash both met the national standard quality for moisture and protein content, which suggested that sweet potato mash shows promise as a WF substitute in noodles.

Pasta prepared with sweet potato flour and wheat flour Pasta has its origin in Italy and has gained wide popularity as a convenient and nutritionally palatable food (Petitot et al., 2009). Although traditionally pasta is made from durum wheat semolina which provides the desired texture and cooking quality to the product, wheat semolina proteins are deficient in lysine and threonine leading to low biological value for the product (Stephenson, 1983). Gopalakrishnan et al. (2011) prepared pasta with SPF and WF. The processing method of SPF was as follows: sweet potato roots were peeled and sliced to 0.5 cm thickness. The slices were soaked in acetic acid (1.0% w/v; 1.0 kg sweet potato slices per 5.0 L water) for 1 h to eliminate the browning problem, after which they were washed in running water and dried in sunlight for 36 h. Dried chips were powdered in a blender and sieved (mesh: 355 μm) to obtain fine SPF. All the formulations had 27% refined WF and 3% gelatinized cassava starch. The content of SPF was 50%
70%, and the rest was whey protein concentrate, defatted soy flour, or fish powder. Pasta was extruded at room temperature (30°C 6 1°C)

298

Sweet Potato

using the round die (No. 62) and cut to short pieces of length 3.0 cm. The freshly extruded pasta tubes had an internal diameter of 0.5 cm and were dried at 50°C in an air oven for 18 h to get a product with ,12.0% moisture content. The hydration level, swelling index, cooking loss, protein nutritional quality, in vitro starch digestibility, and the size, shape, and arrangement of particles in the pasta matrix were analyzed. Results showed that all samples exhibited high swelling index and significantly high lysine and threonine contents. Whey protein concentrate-fortified sweet potato pasta had high values for essential amino acid index, biological value, nutritional index, and protein efficiency ratio. In vitro starch digestibility progressed slowly over a period of 2 h for all samples, with the lowest values for the whey protein concentrate-fortified pasta.

Prospect of sweet potato staple foods Sweet potato staple foods have the special flavor of sweet potato, and could be a good source of dietary fiber, minerals, and vitamins. However, the current processing technologies of staple foods are not suitable for making sweet potato staple foods, and sweet potato lacks the gluten protein, thus making it difficult to form into a stable dough structure. Thus a series of key production techniques and technical problems during sweet potato staple foods processing is one of the important means to successfully prepare sweet potato staple foods. In addition, the addition of different food components, such as starch, protein, polysaccharide, and hydrocolloid from other food sources, is necessary to form a stable dough structure and to obtain an acceptable quality of sweet potato staple foods. Meanwhile, the storage condition also has a significant effect on the quality of sweet potato staple foods. Thus studying the effects of sweet potato type/amount, packaging atmosphere, storage temperature, and storage period on color, β-carotene concentration, and sensory attributes of sweet potato staple foods is also necessary.

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