Sweet potato snack foods Tai-Hua Mu, Hong-Nan Sun and Meng-Mei Ma 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
Introduction Sweet potatoes have become a research focus in recent years due to their unique nutritional and functional properties (Wang et al., 2016). Bioactive carbohydrates, proteins, lipids, carotenoids, anthocyanins, conjugated phenolic acids, and minerals represent versatile nutrients in different parts (tubers, leaves, stems, and stalks) of the sweet potato. Sweet potato root starch with its unique physicochemical properties is particularly valued as a functional food ingredient (Zhu and Wang, 2014). Yellowand orange-fleshed sweet potatoes contain a blend of phenolic acids (i.e., hydroxycinnamic acids) and have relatively high levels of carotenoids (i.e., β-carotene). Purple-fleshed sweet potato has high levels of acylated anthocyanins and other phenolics with antioxidant and antiinflammatory activities (Grace et al., 2014). Anthocyanins of purple sweet potatoes possess aromatic acylated glycosyl groups, and exhibit relatively high pH tolerance and thermostability (Kim et al., 2012). The unique composition of sweet potato contributes to their various health benefits, such as antioxidative, hepatoprotective, antimicrobial, antiobesity, antiinflammatory, antidiabetic, antitumor, and antiaging effects (Wang et al., 2016). Considering the many varieties of sweet potato processing products, this chapter will mainly introduce a series of common sweet potato snacks, for example, sweet potato chips, roasted sweet potatoes, sweet potato biscuit, dried sweet potato slices, sweet potato cakes, doughnuts, and extruded sweet potato snacks.
Roast sweet potato Roast sweet potatoes have become a famous street snack in China, and some other Asian countries, such as Japan and Korea, because of their sweet and delicious taste and the tender texture in the outer coke. Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00011-9
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Recently, yellow-, orange-, and purple-fleshed varieties have been used to make roast sweet potatoes owing to their healthful components, and many researchers have studied how these components change during the roasting process.
Effects of home-processed methods on the content of carotenoid and anthocyanin from different sweet potato varieties Kim et al. (2015) collected nine freshly harvested sweet potato cultivars, and studied the effects of different cooking methods, such as baking, boiling, frying, steaming, and pressure-cooking, on the different kinds of carotenoids, anthocyanins, and antioxidant capacity of sweet potatoes. For the yellow-fleshed sweet potatoes, Juhwangmi and Sinhwangmi showed the highest levels of total carotenoids, which were 665 and 500 mg/g dry weight (DW), respectively. For the purple-fleshed sweet potato, Sinjami and Yeonjami exhibited the highest anthocyanin contents, which were 7.14 and 2.98 mg/g DW, respectively. The total carotenoids, β-cryptoxanthin, and all-trans-β-carotene of Sinhwangmi were decreased after all of the different cooking methods, and the contents of these components in roasted, boiled, and steamed sweet potatoes were higher than that of other cooking methods. However, the contents of 13Z-β-carotene and 9Z-β-carotene were increased after cooking by all methods. The anthocyanin content of sweet potatoes was decreased after cooking by all the home-processed methods, however, the anthocyanin content in roast, boiled, and steamed bread was higher than that of the other methods. The color results showed that the lightness (L ), redness (a ), and yellowness (b ) values were decreased after different cooking methods. Kim et al. (2012) compared the effects of roasting and steaming on the anthocyanin changes in the purple-fleshed sweet potato (cultivar Shinzami) using liquid chromatography-diode array detector-electrospray ionization/mass spectrometry (LC-DAD-ESI/MS). Before roasting and steaming, the total anthocyanin content was 1342 mg/100 g DW in the raw sweet potatoes, and 15 kinds of individual anthocyanin were detected. The amounts of peonidin 3-caffeoyl-p-hydroxybenzoyl sophoroside-5-glucoside and di-acylated cyanidin 3-caffeoyl-p-hydroxybenzoyl sophoroside-5-glucoside were the highest, at 566 and 137 mg/100 g DW, respectively. After roasting and steaming, the main two individual anthocyanins were same as that of the raw materials, however, the contents of each individual and total anthocyanin were significantly decreased
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after steaming, while roasting could protect from the loss of anthocyanin. For example, the contents of total anthocyanin were 751 and 1086 mg/100 g DW after steaming and roasting.
Influence of roasting treatment on the sugar composition and starch morphology of sweet potatoes Apart from nutritional and functional components, the physicochemical and sensory characteristics are also related to the quality of roast sweet potatoes, and any change of these properties has to be carefully monitored during the food processing. The sugar content is the key factor for determining the sensory evaluation of roast sweet potatoes, however, some previous studies only focused on the carbohydrates. In addition, some previous studies showed that thermal treatment could change the textural properties of sweet potatoes, for example, the starch granules, which gelatinize at high temperatures, have a significant influence on textural properties (Koehler and Kays, 1991; Lindeboom et al., 2010). Therefore the study of starch morphology will be useful for understanding the changes of the textural properties of roast sweet potatoes. Lai et al. (2013) investigated the changes of sugars, such as fructose, sucrose, maltose, glucose, and total sugars during the process of roasting, and observed the electronic micrographs. Results showed that the total sugar content in the raw fresh sweet potatoes ranged from 5.08% to 8.41%, and the main sugars were sucrose (2.52% 7.77%) and glucose (0.38% 2.02%), followed by fructose (0.24% 1.06%). In the roasted sweet potatoes, the content of sucrose and glucose was significantly decreased, in the range of 1.53% 7.45%, and 0.31% 1.37%, respectively. However, the maltose content was increased, and was between 8.81% and 13.97%. Electronic micrographs of fresh sweet potato samples showed that the starch granules were oval-shaped, and the size of them was generally less than 20 μm. After the roasting treatment, starch granules completely gelatinized.
Aroma components of roast sweet potatoes Roast sweet potatoes are a snack food with a long history in China, and their rich and attractive flavor is greatly popular with large customers. Many researchers have focused on the nutritional and functional composition changes during the process of roasting. Besides this, the aroma components and their mechanisms of formation should also be known. Some researchers have analyzed the aroma components of roast sweet
potato by gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), gas chromatography olfactometry (GCO), and aroma extract dilution method combined with GCO method. The Maillard reaction and caramel reaction occur during the process of sweet potato roasting, which play an important roles in the aroma of sweet potatoes. In addition, the synthesis of the sweet potatoes’ characteristic aroma also involves enzymatic reactions. Different kinds of aroma components were detected, and the variety of aroma substances depended on the testing instruments and the varieties of sweet potatoes. For example, 8, 48, and 75 kinds of aroma components were detected from roast sweet potatoes using gas chromatography-headspace (GC-HS), gas chromatography-flame ionization detector (GC-FID), and GC-MS (Tiu et al., 1985; Nakamura et al., 2013). Furthermore, the most important aroma component is maltol, which can be endowed with the sweet smell of roast sweet potatoes, and maltose is the main precursor of many volatile compounds in the roast sweet potato. The flavors of cooked sweet potatoes were significantly different as a result of roasting, boiling, and microwave cooking. Results determined by GCO showed that the kinds of aroma components were 37, 20, and 32 from roasted, boiled, and microwave-cooked sweet potatoes, respectively. The main reasons for the faint scent of boiled and microwaved sweet potato might be as follows: (1) lower heating temperature could inhibit Maillard reaction; (2) the leaching of water-soluble substances and the loss of polar compounds near the surface of sweet potato could lead to the loss of water phase, thus leading to the decrease of the volatile compounds in the roasted sweet potatoes; (3) because of the high thermal conductivity of water, boiling water can easily inactivate the key enzymes involved in the release of aroma precursors, such as maltose, and the release of bound aromatic substances, such as glycoside release; (4) the lack of terpene aroma compounds, a series of microwave baking and sweet potato aroma terpenes’ great contribution (such as linalool, geraniol, copaene) does not appear in the cooked sweet potato; (5) low temperature and isothermal treatment conditions are not conducive to the caramelization reaction and Maillard reaction, and thus under the condition of microwave radiation are very unlikely to happen; and (6) the rapid heating system inactivates the amylase, maltose is one of the main precursors of many baked sweet potato volatile compounds, the rapid heating and microwave treatment of the passivation system is essential to produce amylase hydrolysis of starch and maltose, hampering the production of maltose (Sun et al., 1994; Wang and Kays, 2001).
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Pollution analysis and safety evaluation of roasted sweet potatoes Nowadays, there are many problems in the food safety of cooked food, and many more people pay attention to them. In recent years, many reports of polycyclic aromatic hydrocarbons, such as polycyclic aromatic hydrocarbons, have been reported in the roasted food. Roast sweet potato is one of the most common products in sweet potato processing products and for the analysis of the food safety of roast sweet potato, the acrylamide content needed to be analyzed and detected. Zhang et al. (2012) determined the acrylamide content of roast sweet potato bought from supermarket and roasted in the oven. The results showed that the acrylamide content in the skin of roast sweet potato was 4.73 μg/kg, and there was no determination in the central layer of flesh. Lu et al. (2011) collected roast sweet potato from the supermarket, and determined the content of S, Cu, Pb, Hg, and As, and found that the skin, subcutaneous flesh, and central layer flesh of roasted sweet potato were highly polluted by S and heavy metals with comprehensive pollutant indexes of 209.1, 24.0, and 3.9, respectively, which indicated that roast sweet potato made by coal is very dangerous, and it should be forbidden in terms of food safety.
Frozen roasted sweet potatoes The traditional roasted sweet potato has short shelf life, is not easy to store, has a small circulation radius, and is easily restricted by season, so it cannot meet the daily needs of consumers. Therefore it is urgent to develop a product with a long shelf life, good taste, high nutritional value, and open bag (or heated) ready-to-eat roasted sweet potato, so as to improve the fresh food proportion of sweet potato and promote the healthy development of the sweet potato industry. Houqin (2013) made a kind of quick-frozen sweet potato conditioning product by adding many other ingredients besides roasted sweet potato flesh, such as sugar, milk powder, custard powder. Xianyao (2014) and some other researchers from South Korea and Japan studied a method for processing frozen roasted sweet potato without any food additives, the technological flow was as follows: sweet potato was collected, washed, and roasted for 200°C 280°C for 60 120 min (depending on the size of sweet potato), and then was frozen for 2 4 h under 16 to 21°C.
However, there are many sweet potato cultivars in the world, and the quality of frozen roasted sweet potato cultivars may not all be excellent under the condition of slow freezing. Thus further studies might focus on the screening of special varieties of roasted sweet potato, and explore the optimum freezing conditions.
Sweet potato chips or French fries The classification of sweet potato chips or French fries In general, the term “chips” or “French fries” means fried slender pieces of potatoes in the United States and most of Canada. While in Australia, Ireland, New Zealand, and the United Kingdom, the fried potatoes which are cut thinly are called skinny fries or shoestring fries to distinguish from chips, which are cut thicker. French fries sometimes can be made from sweet potatoes instead of potatoes, and are usually eaten together with salt, tomato sauce, vinegar, mayonnaise, etc. There are two groups of chips, that is, traditional or general chips and simulation chips. General chips are made from the processes of cutting and cleaning, thin cutting, and frying; while simulation chips are made from flour which undergoes the process of mixing, thin layer forming, molding, and frying. Compared to general chips, the simulation chips have some advantages such as (1) form and size can be molded as preferred and in a uniform manner; (2) seasoning can be applied easily; and (3) higher yield (Elisabeth, 2015).
The traditional or general sweet potato chips or French fries Bovell-Benjamin (2007) prepared a French fries-type product from Jewel and Centennial sweet potatoes. The sweet potato roots were washed, lye peeled, sliced into strips, and blanched in hot water containing 1% sodium acid pyrophosphate. The blanched strips were partially dried at 121°C. The dehydrated strips were frozen until fried at 175°C. Panelists evaluated the color, flavor, and texture of the products on a five-point scale. The flavor and texture results indicated that a good product could be prepared from both cultivars of sweet potato. French fries primarily contain carbohydrates (mostly in the form of starch) and protein from the sweet potato, fat absorbed during the deepfrying process, and some of the highest levels of acrylamides of any foodstuff. According to the American Cancer Society, it is not clear as of 2013
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whether acrylamide consumption affects people's risk of getting cancer. A meta-analysis indicated that dietary acrylamide is not related to the risk of most common cancers, but could not exclude a modest association for kidney, endometrial, or ovarian cancers (Pelucchi et al., 2014). Vacuum frying is an alternative method to produce fried foods with superior product quality attributes (Da Silva and Moreira, 2008; Nunes and Moreira, 2009) and low acrylamide content (Granda and Moreira, 2005). Ravli et al. (2013) carried out a study on a two-stage (TS) frying process for high-quality sweet potato chips. In their study, vacuum frying (1.33 kPa), with the aid of a deoiling mechanism, was used to produce low-fat sweet potato chips. The kinetics of oil absorption and oil distribution in the chips (total, internal, and surface oil content) was studied so that effectiveness of the deoiling system could be established. Product quality attributes (PQAs) such as moisture content, oil content, diameter shrinkage, and thickness expansion, as well as, color, texture, bulk density, true density, and porosity of chips fried at different temperatures (120°C, 130°C, and 140°C) was performed to evaluate the effect of process temperature on the product. The final oil content of the vacuum fried chips was 60% lower than those found in traditionally fried sweet potato chip, which indicates that the deoiling mechanism is crucial in the vacuum frying process. The rate of change in PQAs is greatly affected by temperature; however, the final values of bulk density, true density, porosity, diameter shrinkage, and thickness expansion were not affected by temperature. The structure of the chips settled faster when fried at 130°C 140°C. Color b values were not affected by the range of temperature used in this study. The product fried in a TS frying process (1 min fried at atmospheric pressure and 2 min under vacuum) had better appearance and texture compared to the ones that were only fried under vacuum or single-stage (SS) conditions. The samples were lighter and more yellow (less compact) than the chips fried under the SS process. The atmospheric frying prior to vacuum frying helped the starch to gelatinize, thus producing a better product in terms of texture, oil content, and flavor. The final oil content of the TS fried chips was 15% lower than those fried by the SS process, showing that the structure of the chips formed during the process affected the oil absorption during frying.
The simulation sweet potato chips Sweet potato chips are deeply loved by consumers, and different consumers have different requirements for the flavor of sweet potato chips, so
the simulation sweet potato chips came into being. More recently consumers are increasingly demanding products that are additive-free (Oke and Workneh, 2013). Oh et al. (2017) studied the effects of pulsed infrared (IR) radiation followed by hot-press (HP) drying on the properties of mashed sweet potato chips to develop an additive-free dried sweet potato snack. To provide a crispy texture, sweet potatoes were dried by a TS process. At the first drying stage, steamed sweet potatoes were semidried for 6 h using hot-air convection or pulsed IR radiation, and the drying rate was compared under varying sample thicknesses and drying temperatures. The IR exhibited enhanced drying speed, in particular the IR radiation at 60°C was favorable for application to the drying of sweet potatoes with large thickness. For the secondary drying, the IR-dried sweet potatoes with varying moisture content were applied to HP drying at 180°C for 2 s. The quality of final products indicated that the crispy texture of the products was generated when the semidried sample had a moisture content lower than 0.5 kg/kg dry base (d.b.). When the moisture content of samples prior to the HP process was lower than 0.3 kg/kg d.b., the final product was easily broken and had a discoloration to dark-brown. Considering the entire processing procedure, they demonstrated that IR radiation at 60°C for 5 h followed by HP was an effective combination for the mass production of a crispy sweet potato snack.
Dried sweet potato slices Besides roast sweet potato, sweet potato bread, biscuits, and chips, and dried sweet potato slices can also be regarded as a kind of healthy food. What is more, the dried sweet potato slices can provide various advantages, such as a long shelf life, convenient application, and increased concentration of nutrients (Kim and Chin, 2016). The conventional processing method of dried sweet potato slices is with sunshine, which takes more than 50 hours and the obtained products may exhibit inferior quality with high microbial contamination (Silayo et al., 2003). In order to reduce the drying time, to obtain better quality products, to avoid the dependency on the weather, and to reduce microbial contamination of the product, some mechanical dryers have been applied to process dried sweet potato slices (Jayaraman and Gupta, 2006). Besides the above advantages, much more water could be removed by mechanical dryers, thus reducing the mass and volume, and improving the efficiency of packaging, storing, and transportation
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(Jayaraman and Gupta, 2006). Therefore some researchers investigated some kinds of mechanical dryers, such as high-velocity cross-flow hot-air dryer, pulsed IR radiation followed by HP drying, osmotic dewatering combined with oven drying, etc., on the qualities properties of dried sweet potato slices.
High-velocity cross-flow hot-air dryer The high-velocity cross-flow hot-air dryer is one of the most popular dryers at present. It is characterized by the vertical flow of hot air and the flow of samples. During the process of a traditional hot-air dryer, the sample near the side of the hot-air chamber is always in contact with the high-temperature drying medium, and the water loss is faster. The sample near the exhaust chamber is always in contact with the medium with low temperature and humidity, and the water loss is slower, so the sample drying is very uneven. However, the high-velocity cross-flow hot-air dryer could resolve the problem of drying inhomogeneity. Singh et al. (2012) investigated the effect of drying temperature (50°C 90°C), air-flow rate (1.5 55 m/s), and sample thickness (5 12 mm) on the moisture ratio, drying rate, and diffusion coefficients of sweet potato slices, and confirmed model equations. The results showed that during the initial period of drying, the drying rate was similar in all the drying conditions. After the removal of surface moisture, increasing the drying temperature caused an important increase in the drying rate, thus the drying time was decreased. For the sweet potato thickness, as the thickness of sample increased, the drying time increased due to an increased diffusion path. During the process of drying, the activation energy for moisture diffusion was 11.38 kJ/mol.
Pulsed infrared radiation followed by hot-press drying As we know that, for some ready-to-eat dried sweet potato slices, sweet potato should be steamed completely prior to drying, leading to higher starch gelatinization and increased sweetness generated during the steaming process. A higher content of sugar in the steamed sweet potato can lower the drying rate and make a hard texture when the drying process is completely finished (Oke and Workneh, 2013). Therefore some alternative techniques should be applied to accelerate the drying rate and improve the final quality of dried sweet potato slices.
IR drying is an effective method compared to hot-air drying, and some researchers reported that IR drying could decrease the drying time of vegetables and fruits and improve their quality (Nowak and Lewicki, 2004; Sharma et al., 2005). However, this method with intensive heat may cause some adverse influence, such as surface overheating, lipid oxidation, and some nutrients damage (Doymaz, 2012; Oh et al., 2017). Therefore IR combined with HP is a good method to produce high quality, and crispy textured dried sweet potato slices. Oh et al. (2017) showed that IR radiation at 60°C for 5 h was favorable for application for the drying of sweet potatoes with large thickness, and 180°C for 2 s was applied for HP drying; the above combination was effective for the mass production of dried sweet potato slices.
Osmotic dewatering combined with oven drying In order to improve nutritional, sensorial, and functional properties of dried sweet potato slices without changing their integrity, different concentrations of NaCl (10% 30%, w/v) were used to pretreat sweet potato slices for different durations (20 100 min) and then the sweet potato slices were dried by oven drying. Results showed that the amount of water removed by pretreatment with NaCl solution decreased significantly as the concentration and the time soaking increased, NaCl solution pretreatment had no effect on the content of β-carotene, but subsequent oven drying degraded the β-carotene content (Clifford et al., 2014).
Effect of different drying methods on the quality properties of dried sweet potato slices Zhao et al. (2013) investigated the effects of different drying methods, such as oven drying, microwave drying, and oven combined with microwave drying, on the drying time, water retention capacity, oil holding capacity, and sensory evaluation of dried sweet potato slices. Microwave drying exhibited the shortest time for drying the same quality of sweet potato slices, while oven drying exhibited the longest time, this might be because sweet potato is a bad conductor, the heat transfer is slow, and the temperature gradient is not consistent with the direction of the humidity gradient, so the drying time is longer. The dried sweet potato slices obtained by microwave drying had the lowest water retention capacity and oil retention capacity, followed by oven drying and oven combined with microwave drying. This might be
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because the temperature gradient of wet material is consistent with the moisture gradient, which is conducive to the migration of moisture in the material, thus increasing the diffusion rate of moisture and speeding up the drying process during the process of microwave drying, so the structure of sweet potato was damaged to some extent, and decreased the immersion of water and oil. However, the combined drying by hot air and microwave can avoid the rapid escape of water during the drying process, causing little damage to the material structure and is conducive to the immersion in external moisture and grease. Sensory evaluation consists of color, surface shrinkage, and sense of taste. Results showed that oven combined with microwave drying had the best sensory evaluation, followed by microwave drying and oven drying. For example, the color of dried sweet potato had become yellowish brown, and with a small amount of brown spots after being dried by the oven. Microwave drying can inhibit the enzyme activity of sweet potato, which can prevent the occurrence of browning. In addition, microwave drying can penetrate sweet potato slices, accelerating the mass transfer rate, thereby reducing the drying shrinkage of the slices, however, it is very difficult to control the drying rate and time, thus affecting the sense of taste and color. Oven combined with microwave drying can combine the advantages of oven and microwave drying.
Sweet potato cakes The concept and classification of cakes Cake is a kind of sweet dessert which is typically baked. A long time ago, cakes were modifications of breads, but now cakes cover a lot of preparation methods which can be simple or complicated, and share features with other snacks such as meringues, custards, pastries, and pies. The ingredients in typical cakes include flour, eggs, butter, oil, sugar, baking soda, etc. Sometimes additional ingredients including nuts, fruits, cocoa, and so on will also be used. Cakes are broadly divided into several categories, based primarily on their ingredients and mixing techniques. Butter cakes are made from creamed butter, sugar, eggs, and flour. They rely on the combination of butter and sugar beaten for an extended time to incorporate air into the batter (Robbins, 2018). A classic pound cake is made with a pound each of butter, sugar, eggs, and flour. Baking powder is in many butter cakes, such as a Victoria sponge (Cloake, 2018). The ingredients are sometimes
mixed without creaming the butter, using recipes for simple and quick cakes. Sponge cakes (or foam cakes) are made from whipped eggs, sugar, and flour. They rely primarily on trapped air in a protein matrix (generally of beaten eggs) to provide leavening, sometimes with a bit of baking powder or other chemical leaven added as insurance. Sponge cakes are thought to be the oldest cakes made without yeast. An angel food cake is a white sponge cake that uses only the whites of the eggs and is traditionally baked in a tube pan. The French Génoise is a sponge cake that includes clarified butter. Highly decorated sponge cakes with lavish toppings are sometimes called gateau, the French word for cake. Chiffon cakes are sponge cakes with vegetable oil, which adds moistness (Medrich, 1997). Chocolate cakes are butter cakes, sponge cakes, or other cakes flavored with melted chocolate or cocoa powder (e.g., brownies) (Berry, 2018). German chocolate cake is a variety of chocolate cake. Fudge cakes are chocolate cakes which contain fudge. Coffee cake is generally thought of as a cake to serve with coffee or tea at breakfast or at a coffee break. Some types use yeast as a leavening agent while others use baking soda or baking powder. These cakes often have a crumb topping called streusel or a light glaze drizzle. Baked flourless cakes include baked cheesecakes and flourless chocolate cakes. Cheesecakes, despite their name, are not really cakes at all. Cheesecakes are in fact custard pies, with a filling made mostly of some form of cheese (often cream cheese, mascarpone, ricotta, or the like), and have very little flour added, although a flour-based or graham cracker crust may be used. Cheesecakes are also very old, with evidence of honey-sweetened cakes dating back to ancient Greece. Butter or oil layer cakes include most of the traditional cakes used as birthday cakes, etc., and those sold as packaged cakes. Baking powder or bicarbonate of soda is used to provide both lift and a moist texture. Many flavorings and ingredients may be added, examples include devil's food cake, carrot cake, and banana bread. Yeast cakes are the oldest and are very similar to yeast breads. Such cakes are often very traditional in form, and include such pastries as babka and stollen.
Effect of adding different proportions of sweet potato on the quality of cakes Selvakumaran et al. (2017) determined the physicochemical properties of orange-fleshed sweet potato-enriched brownies and investigated the consumer’s acceptance level on different formulations of orange-fleshed sweet potato-enriched brownies. The raw orange-fleshed sweet potatoes were
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washed with tap water, manually peeled, and sliced to 10 mm thickness. Then they were steamed with a commercial steamer for 20 min and mashed into puree. A total of four brownie formulations were prepared. Wheat flour was substituted by 25%, 50%, and 75% orange-fleshed sweet potato puree. First, butter and chocolate chips were melted using a double boiler method and cooled to room temperature. Then, eggs, oil, brown sugar, orange-fleshed sweet potato puree, and vanilla extract were mixed in a bowl. The mixture was then combined with the cooled chocolate mixture. Wheat flour, cocoa powder, and baking powder were then added into the mixture. The final mixture was placed in a baking tin and was oven-baked at 180°C for 23 min. After baking, it was removed from the baking tin and left to cool for 1 h at room temperature. Results showed that the substitution of wheat flour with orange-fleshed sweet potato puree in brownies formulations significantly increased total dietary fiber in the order: 75% orange-fleshed sweet potato (6.41%) . 50% orange-fleshed sweet potato (5.13%) . 25% orange-fleshed sweet potato (3.24%) . 0% (1.70%). Additionally, moisture, fat, and specific volume were elevated with increased amount of orange-fleshed sweet potato puree. The color of brownies was affected with decreases of L , a , and most prominently b . Hardness, adhesiveness, gumminess, cohesiveness, and chewiness of brownies reduced significantly with the incorporation of up to 50% orange-fleshed sweet potato puree, but 75% orange-fleshed sweet potato puree brownies were not significantly different to 50% orange-fleshed sweet potato puree brownies. Springiness and resilience had more prominent impacts with the increased amount of orangefleshed sweet potato puree. Sensory scores of appearance and color of enriched fiber did not differ significantly with control brownies. However, texture, flavor, and overall acceptability of the brownies were most preferred for 50% and 75% orange-fleshed sweet potato puree substitution. The study suggests that orange-fleshed sweet potato substitutions at 50% and 75% are suitable to increase the fiber in brownies with little effects on the appearance and were able to improve sensory attributes.
Sweet potato biscuits The etymology of the biscuit is “the bread which is toasted two times,” from French bis (again) and cuit (bake). Biscuits are baked with some kind of flour, water, or milk, and some kinds of biscuits are also processed by adding yeast. It can be used as a storage food for travel, navigation, and
mountaineering. It is also very convenient for military personnel’s spare food during wars. At the beginning the biscuit industry was mainly concerned with long-term sea travel and war, in order to provide nutritional and emergency food with a long shelf life. The production of biscuits was based on handmade-type (manual transmission form), but after the industrial revolution due to mechanical technology, the rapid development of production equipment and technology of biscuits, spread across the whole world. Biscuits include hard biscuits, crisp biscuits, and fermented biscuits. Traditionally, biscuits are made of wheat flour, corn flour, and some other cereal flours. As we already know, sweet potato is rich in protein, dietary fiber, vitamins, minerals, and some other nutrients—it is an allround nutritious food which is recognized worldwide. In recent years, with the rapid development of China’s economy and the improvement of the living standard of its residents, the demand for nutritious and healthy food is increasing. Under this background, using sweet potato as a raw material for making biscuits can not only enrich the types of sweet potato food and increase the consumption of sweet potato, but can also improve on the disadvantages of single nutritional components of the existing biscuit products. This could be of great significance for improving the dietary nutrition and structure of Chinese residents.
The concept and classification of sweet potato biscuit Sweet potato biscuit is a kind of delicious food, of which the main ingredients are sweet potato, wheat flour, and sugar. To obtain the sweet potato product which is used in biscuit-making, there are mainly four types or process: (1) sweet potato is made into mash after being steamed completely; (2) sweet potato is made into paste directly; (3) sweet potato can be made into flour after steaming (or not steaming), drying, and smashing; and (4) sweet potato residue obtained after starch extraction can also be used in biscuit-making. According to the different processing methods, sweet potato biscuits can be divided into the following categories: 1. Hard sweet potato biscuit: less oil and sugar are used during the processing of hard biscuit; when making the dough, it is easy to form gluten; the dough is rolled and extended to form a thin slice, which is then baked in the roaster. 2. Crisp sweet potato biscuit: much more oil and sugar, less water are used; not much more gluten is formed during the processing of the crisp biscuit.
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3. Fermented sweet potato biscuit: with sugar, oil, yeast, and some other auxiliary material, yeast as a loose agent; then processed by fermentation, molding, and baking.
Dietary fiber-enriched biscuits Sweet potato residue is generated in the processing of starch, and is rich in dietary fiber. Dietary fiber cannot be digested by the enzymes in the intestine, but can promote intestinal peristalsis, chelate cholesterol, and inhibit the hypertension, hyperlipidemia, hyperglycemia, and obesity, thus it is regarded as “the seventh nutrient.” Therefore sweet potato dietary fiber can be used to process biscuit, so as to increase the variety of high dietary fiber food, and meet the demands of the market. Ni et al. (2011) and Wang et al. (2015) extracted dietary fiber from sweet potato residue, and determined the best formula and processing technology of biscuits. Results showed that the hardness increased with the increase of dietary fiber addition (10% 40%), sensory evaluation and crispness was increased when the dietary fiber was increased from 10% to 30%, and then decreased when the dietary fiber addition further increased to 40%, thus 30% dietary fiber addition was chosen as the best addition amount. The best formula and baking conditions were obtained by single factor and orthogonal test, the results were as follows: shortening 30%, ammonium bicarbonate 0.6%, sweet potato dietary fiber 30%, sugar 30%, baking soda 0.4%, eggs 5%, salt 0.6%. The best baking condition is 180°C for 8 min.
Some other kinds of biscuits enriched with sweet potato Iron deficiency (anemia) and vitamin A deficiency are the most common nutritional disorders worldwide, and mainly affect preschool children and pregnant woman, contributing to 20% of all maternal mortality, and are considered public health conditions of epidemic proportions (Infante et al., 2017). Infante et al. (2017) prepared sorghum biscuits with biofortified sweet potato carotenoids, and evaluated their acceptance, nutritional composition, and iron bioavailability. They prepared four kinds of biscuits: (1) sorghum biscuit with 100% dry sorghum flour; (2) extruded sorghum biscuit with 100% extruded sorghum flour; (3) enriched biscuit with 50% dry sorghum flour and 50% sweet potato flour; and (4) enriched extruded sorghum biscuit with 50% extruded sorghum flour and 50% sweet potato
flour. Nutritional analysis showed that the carotenoids (from 0 to 28.09 mg/100 g) and total polyphenol content (from 0.19 to 33.48 mg EGA/100 g) were significantly increased with the addition of sweet potato flour. Animal experiments showed that the iron bioavailability of biscuits which were made of extruded sorghum and sweet potato flour groups were similar to that of the ferrous sulfate control. In addition, high total antioxidant capacity and adequate expression of the intestinal proteins were related to the absorption of iron. The above results showed that biofortified sweet potato carotenoids can increase nutritional and sensory quality of the biscuits, allowing their potential as a functional food to reduce the risk of iron deficiency.
Sweet potato doughnuts The concept and classification of doughnuts Doughnuts are a kind of ring-shaped snack food popular in many countries, which are usually deep fried from flour doughs. After being fried, doughnuts can be spread with chocolate or icing on top, covered with powdered sugar or fruit, or glazed with sugar icing. Generally, people like to enjoy doughnuts together with a cup of coffee or milk.
Effect of adding different proportions of sweet potato on the quality of doughnuts Collins and Aziz (1982) used the Jewel cultivar of sweet potato as raw material. This has orange-colored flesh and is classified as a “moist” type of sweet potato. Firstly, they processed the sweet potato roots to flour, obtained puree from baked roots, and obtained puree from steam-cooked flesh. The following ingredients were used in the recipe for preparing yeast-raised doughnuts: wheat flour (all purpose) 425.4 g, cane sugar 56.8 g, salt 7.1 g, nonfat milk solids 21.3 g, margarine 42.6 g, water 180.0 g, whole egg 46.4 g, vanilla extract 2.0 g, mace 1.8 g, active dry yeast (suspended in water) 28.4 g, and sodium stearoyl-2-lactylate 3.8 g. Doughnuts were prepared according to the recipe presented above, but a portion of the flour was replaced with an equivalent portion of sweet potato ingredient (0, 7%, 14%, and 21% of the wheat flour), and water was adjusted to give the desired dough consistency. Then the cut pieces of dough were fried in soybean oil at 191°C using a small commercialtype fryer. The doughnuts were heated on each side for 50 s. The proximate composition, color, firmness, volume, and sensory of the doughnuts
Sweet potato snack foods
samples were evaluated. Results showed that, sweet potato as flour and puree can be used as an ingredient of yeast-raised doughnuts. Measurements and analyses of samples which contained sweet potato up to 21% showed that certain attributes and their components were altered by the addition of sweet potato. From a quality standpoint baked sweet potato puree was probably the most desirable form; doughnuts did not undergo any adverse changes and, in fact, some of the changes may be desirable. From the standpoint of composition, amount of fat, and caloric content, steamed sweet potato would seem to be the least desirable form to use. Comparatively, doughs with steamed sweet potato required a greater amount of moisture to develop a consistency comparable to that of the control of dough. The higher level of moisture resulted in a greater uptake of fat when the doughnuts were fried. Concomitant to the increase in fat uptake was an increase in caloric content. Generally, the use of EEC and baked sweet potato resulted in a calorie reduction in the doughnuts. Sweet potato flour might be most desirable from the production standpoint since it is a dry material and could be handled and stored more easily and inexpensively than the other forms.
Extruded sweet potato snacks Extrusion cooking is a high-temperature, short-time process in which starchy and/or proteinaceous food materials are plasticized and cooked in a tube by a combination of moisture, pressure, temperature, and mechanical shear (Singh et al., 2007). Extrusion is a relatively easy process that is widely used to produce a variety of textured and shaped convenience products including breakfast cereals, baby foods, soups, and ready-to-eat snacks (Brennan et al., 2013; Singh et al., 2007). Extruded snacks are, however, predominantly prepared from high carbohydrate-containing ingredients such as corn, rice, wheat, potato, and oats (Brennan et al., 2013). The snacks are thus energy dense with a limited content of protein and other nutrients, such as vitamin A (Brennan et al., 2013; Riaz et al., 2009). Consumers are, however, increasingly demanding more nutritious snacks that are low in fat but rich in protein, fiber, minerals, and vitamins (Brennan et al., 2013). Honi et al. (2018) investigated the proximate composition, provitamin A retention, and shelf life of extruded orange-fleshed sweet potato and bambara groundnut-based snacks. In their study, six formulations of orange-fleshed sweet potato and bambara groundnut were extruded at a
feed rate of 10.15 kg/h, screw speed of 30 rpm, and at 100°C and 130°C in the first and second zones, respectively. Proximate composition was determined using standard methods. Provitamin A was determined using high-performance liquid chromatography. An untrained panel (n 5 73) was used to determine consumer acceptability. Shelf life was predicted by using peroxide values. Results showed that the concentration of orangefleshed sweet potato or bambara groundnut significantly (P , .05) affected proximate composition of the snacks. Moisture (4.79 8.34 g/100 g), carbohydrates (55.53 78.99 g/100 g), and provitamin (0.54 17.33 mg/100 g) contents increased with the increasing proportion of orange-fleshed sweet potato. Extrusion significantly reduced provitamin A content from 0.90 20.73 to 0.54 17.33 mg/100 g (P , .05). The presence of orangefleshed sweet potato improved provitamin A retention and consumer acceptability. Predicted shelf life (ranging from 118 to 150 days at room temperature) was inversely proportional to the concentration of bambara groundnut.
Packaging of sweet potato snack food Generally, sweet potato snack foods are carotenoids-rich products. The carotenoids are susceptible to degradation during packaging, which is influenced by oxygen availability in the head space of the package, oxygen dissolved in the product, oxygen permeability through the packaging material, light transmission, faults in the hermiticity of the seal, and the storage time and temperature (Lesková et al., 2006; RodriguesAmaya, 1999). Thus care must be taken during packaging. Júnior et al. (2018) evaluated the influence of the packaging material and packaging system on the stability of dehydrated carotenoid-rich sweet potato chips, and evaluated which packaging system provided the longest shelf life. In their study, the sweet potato chips were processed and packaged with nitrogen in polyester (PET)/aluminum foil (Al)/low density polyethylene (LDPE), metallized PET/LDPE, biaxially oriented polypropylene (BOPP)/metallized BOPP, and BOPP/metallized BOPP with an oxygen scavenger; and also without nitrogen in BOPP/metallized BOPP, and stored at 25°C and 75% relative humidity. The shelf life of the chips packed in BOPP/met BOPP without nitrogen was 153 days, losing 61% of the β-carotene, and leading to sensory alterations in the flavor, odor, and color. The shelf life of the chips packaged with nitrogen in PET
Sweet potato snack foods
met/LDPE was defined as 184 days due to sensory alterations involving the loss of crispness. The chips packaged with nitrogen in PET/Al/LDPE, BOPP/met BOPP, and BOPP/met BOPP with an oxygen scavenger retained 90%, 83%, and 80% of the β-carotene, respectively, and showed no significant sensory alterations during 207 days of storage. One can use the packaging systems with nitrogen with the structures of PET/Al/ LDPE, BOPP/met BOPP, or BOPP/met BOPP with oxygen scavenger to obtain a shelf life of 7 months at 25°C/75% relative humidity, the packaging system of BOPP/met BOPP with nitrogen showing the greatest cost benefit due to the lower cost of the packaging material, which is also the material most used on the market for chips in general.
Trends and prospects Snack foods occupy an extremely important position in the food consumption market. They meet the consumption habits and consumption fashion of modern people for their good color, fragrance, and taste, they are convenient for eating, and offer a variety of nutrition. Therefore the snack foods can be seen in supermarkets, large food stores, hotels, restaurants, and so on, and the market space is huge. According to the longterm development plan of the food industry in China, the output value of the instant food manufacturing industry will have grown at an average annual rate of 30% by 2020, and the value of the snack food industry will reach hundreds of billions of Chinese yuan. As a kind of snack food, sweet potato snack foods have huge market potential and broad development space. In recent years, sweet potato processing enterprises have introduced new technology and equipment to improve the quality of their products. At the same time, new ideas in raw materials, ingredients, flavors, texture, production, and packaging have been brought forth, and many new type sweet potato snack foods with various shapes and tastes have developed to meet the diversified demand of market consumption. Meanwhile, sweet potato snack foods are developing toward health, nutrition, and safety. For example, crisp fruit and vegetable chips made from dozens of fresh fruits and vegetables, such as sweet potato, compound apple, carrot, and edible fungus, are a kind of pure natural snack food with a good taste and rich in nutrition, which sell well in the market.
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Further reading Cureton, P., Fasano, A., 2009. The increasing incidence of celiac disease and the range of gluten-free products in the marketplace. In: Gallagher, E. (Ed.), Glutenfree Food Science and Technology. Wiley-Blackwell, Oxford, pp. 1 15. Hill, I.D., Dirks, M.H., Liptak, G.S., Colletti, R.B., Fasano, A., Guandalini, S., 2005. Guideline for the diagnosis and treatment of coeliac disease in children: recommendations of the North American Society for pediatric gastroenterology, hepatology and nutrition. J. Pediatr. Gastroenterol. Nutr. 40, 1 19. Okorie, S., Onyeneke, E., 2012. Production and quality evaluation of baked cake from blend of sweet potatoes and wheat flour. Acad. Res. Int. 3, 171 177. Rosell, C.M., Foegeding, A.E., 2007. Interaction of HPMC with gluten proteins: small deformation properties during thermal treatment. Food Hydrocoll. 21, 1092 1100. Stokes, A.M., Tidwell, D.K., Briley, C.A., Burney, S.L., Schilling, M.W., 2014. Consumer acceptability of gluten-free sweet potato cookies. J. Acad. Nutr. Diet. 114, A49-A49.