Thai Purple Sweet Potato Flours: Characteristic and Application on Puffed Starch-Based Snacks

Characteristics and properties of 4 Thai purple sweet potato flours, Maejo 343, Phichit 65-3, Phichit 290-9 and Torperk were determined in terms of native and pre-gelatinized flours. Color, physicochemical properties and antioxidant activity of native and pre-gelatinized flours depend on their varieties. All native flours showed low redness (a*) and blueness (-b*) values but high pasting properties. Pre-gelatinized flours had a unique purple color and high antioxidant activity. Flours produced by Phichit 65-3 showed a deep intense purple color, high anthocyanins and good antioxidant activity. Both native and pre-gelatinized Phichit 65-3 flours were used as the main raw materials of air-puffed pre-gelatinized flours with different ratios at 10%, 30% and 50%. Increasing the content of pre-gelatinized flours improved color, expansion and antioxidant activity of snacks. Low hardness (9.55-11.65 kg) was presented in all snacks prepared from sweet potato flours. Purple sweet potato snacks prepared from 50% pre-gelatinized flour had good appearance, light texture, high anthocyanins and high antioxidant activity. Results showed that Thai purple sweet potato flours can be used to produce healthy snacks with improved appearance and texture. They also have potential as sources of natural colorants and antioxidants in food products.


Introduction
Starch-based snacks are well-known global food products with a unique appearance, taste and texture. Normally, starch-based snacks are puffed to create a porous structure and crispy texture (Mariotti et al., 2006;Nemś et al., 2015). Expansion of the puffed snacks depends on several factors such as raw materials used, amylose-amylopectin ratio, degree of gelatinization, moisture content and puffing methods (Norton et al., 2011;Wang et al., 2012;Jiamjariyatam et al., 2017). Starch-based snacks have been traditionally puffed by deep-frying; however, high oil content in these products can cause health problems and short shelf life (Chen et al., 2016). Air-puffing or baking were applied to snacks for health-conscious consumers for convenient operation. Addition of dietary fiber, protein from plant sources and natural antioxidants were also used to improve the nutritional and health benefits of the snacks (Nath & Chattopadhyay, 2007;Reddy et al., 2014;Nemś et al., 2015). Moreover, root and tuber crops such as potato, sweet potato, yam, taro and beet root were selected to develop the appearance and nutritional properties of snacks from corn and black gram flour (Reddy et al., 2014). Snacks produced with colored potato flour had 2-3 times higher antioxidant activity, high content of polyphenols, attractive color and better expansion. Losses of anthocyanins during snack processing depended on potato varieties (Nemś et al., 2015). The utilization of starchy tubers and air-puffing has become an alternative method of producing healthy snacks with good expansion, increasing acceptance and source of phytochemicals. 2008). Purple sweet potato anthocyanins can be used as natural food-safe edible colorants (Suda et al., 2003;Terahare et al., 2004). Sweet potatoes have great potential to contribute to the human diet and food production as starch, bakery and snacks. Moreover, they can be used as functional ingredients in various food products, especially in flour form. Flour production is a common technique for root preservation with many advantages including longer shelf life, ready usage and easy delivery, thus increasing the economic value of sweet potatoes.
In recent years, purple sweet potatoes have been widely grown in several areas of Thailand. The four selected varieties had outstanding characteristics. Phichit 290-2 is the base for new purple-fleshed breeding while Torperk is widely consumed in Thai markets. The two new breeding varieties, Maejo 343 and Phichit 65-3, are fast-growing and have higher productivity. Moreover, Phichit 65-3 variety has a noticeable deep purple color. The properties of Thai purple sweet potato flours are sparsely reported in the literature; however, the characteristics and physicochemical properties of flour-based plants play important roles in the commercial application of starch in various food product categories.
This study investigated and compared characteristics, physicochemical properties and antioxidant activity of native and pre-gelatinized Thai purple sweet potato flours. Applications of purple sweet potato flour on air-puffed snacks were formulated from selected varieties that exhibited deep intense purple color, high anthocyanin content and high antioxidant activity. Physical properties of purple sweet potato snacks as expansion ratio, hardness, crispiness, macrostructure and antioxidant activity were observed.

Samples
Four varieties of purple sweet potato (Maejo 343, Phichit 65-3, Phichit 290-9 and Torperk) were provided by the Agriculture Development and Research Center, Phichit, Thailand. Fresh roots (50 kg per each variety) were stored at 10 °C prior to use (Figure 1).

Native Flour
Purple sweet potato flours were prepared following the method of Aina et al. (2009) with some modifications. Briefly, all the roots were cleaned, hand-peeled and cut into 2 mm thick slices using a food processor (CombiMax 600, Braun, Hungary). The thin slices were dried in a hot air oven at 50 °C (Memmert, Germany) until the moisture content was below 12%. They were then ground and sieved through a 100 mesh screen. The native flours were stored in polyethylene bags at 4 °C until required for use.

Pre-gelatinized Flour
The peeled purple sweet potatoes were cut into 4 cm pieces crosswise, then steamed at 100 °C for 30 min (Tefal steam cooker, France), cooled and cut into 2 mm thick slices with a sharp knife. The steamed samples were dried in a hot air oven at 50 °C (Memmert, Germany) until the moisture content was below 12%, then ground to a powder using a laboratory grinder and sieved through a 100 mesh screen. The pre-gelatinized flours were stored in polyethylene bags at 4 °C until required for use.

Chemical Compositions and Color Measurement
Purple sweet potato flours were analyzed for proximate composition (AOAC, 2000). Color attributes as Hunter L*, a*, b* values (Lightness, redness and yellowness) were determined using a Datacolor Spectrophotometer Spectraflash SF 600 plus (Datacolor International, USA).

Antioxidant Activity
Purple sweet potato flours were extracted using modified methods of Bridgers et al. (2010). Briefly, a mixture of 10 g flour sample and 100 ml 80% ethanol (ratio 1:10) was placed in a shaking water bath at 80 °C for 2 h and then filtered through Whatman No.1 filter paper. Extracts were kept at -20 °C until required for analysis. Total phenolic contents (TPC) of extracts were determined following the Folin-Ciocalteu method described by Singleton et al. (1999) and TPC was calculated and expressed as mg gallic acid equivalent (GAE) per g db. Total anthocyanin content (TAC) was determined following the procedure of Huang et al. (2006) and TAC was calculated from Equation (8): Where, A = (A520 -A700) pH1.0 -(A520 -A700) pH4.5 , M w = molecular weight of cyanidin-3-glucoside (449.2), DF = dilution factor,  = molar absorptivity (26,900), and W = sample weight (g).
Antioxidant activities of extract were investigated by 3 different assays. The DPPH free radical scavenging activity assay followed a modified method described by Brand-Williams et al. (1995). Briefly, 1 ml of extract was mixed with 3 ml of 0.2 mM DPPH. After 30 min of incubation in the dark, the absorbance was measured at 517 nm. The %inhibition was calculated using the equation: Trolox equivalent antioxidant capacity (TEAC) assay was performed as described by Zhou and Yu (2004) and ferric-reducing antioxidant power (FRAP) assay was tested according to Benzie and Strain (1999). The calibration curve of Trolox was constructed and antioxidant activities (DPPH, TEAC and FRAP values) were expressed as mg of Trolox equivalent (TE) per g db.

Preparation of Purple Sweet Potato Snacks
Purple sweet potato snacks were prepared using purple sweet potato flour, composite flour (tapioca, corn and wheat flour) and water at ratio 1:1:1.5 (w/w). Salt at 0.75% was added and the mixture was kneaded for 15 min.
The dough was rolled out to a thickness of 40 mm and steamed for gelatinization. After cooling, the dough was cut into strips (40 × 20 × 450 mm) and dried at 50 °C for 1 h. The dough was formulated with 10, 30 and 50% pre-gelatinized flour (w/w). For preparing the control snack, sweet potato flours were replaced by steamed purple sweet potato (db) at the same ratio. All dried dough was puffed in an oven (Healsio AX-1500X, Sharp, Japan) at 230 °C for 80 seconds. The purple sweet potato snacks were kept in polyethylene bags until required for physical analysis.

Color Measurement
Surface color of snacks was measured by a Datacolor spectrophotometer Spectraflash SF 600 plus (Datacolor International, USA) and expressed as L*, a*, b* values (Lightness, redness and yellowness).

Expansion Ratio
Expansion ratio of snacks was determined using the modified method of Segnini et al. (2004). A ratio of the difference between final volume and initial volume was expressed.

Bulk Density
Bulk density of snacks was determined using the sesame seed displacement method described by Sahin and Sumnu (2006). Bulk density (p) was calculated as: Bulk density = Mass of snacks/Volume of snacks (10)

Microstructure of Purple Sweet Potato Snacks
Cross sections of snacks were mounted on a specimen holder and then coated with gold. Fixed specimens were examined using SEM (Jeol JSM-6390, Korea).

Textural Properties
Hardness and crispiness were examined on a Texture Analyzer (TA-XT2, Macro stable systems, UK) with 5-blade Kramer shear cell and 5.0 kg load cell. Hardness value was considered as maximum peak force (kg) and jas.ccsenet. crispiness test speed 2.5.6 Antio Extraction analysis. T were appli described.

Statisti
All experi ANOVA a estimate d using Pear Color values: L*, a* and b* mean lightness, redness and yellowness, respectively. a-g Mean values within the same column with different letters are significantly different at p < 0.05.

Flour C
Native and pre-gelatinized sweet potato flours were evaluated for hydration properties as presented in Table 2. The WBC, WHC and SV values of pre-gelatinized flours were 2 times higher than native flours. Highest hydration properties were observed in pre-gelatinized Torperk flours. The WAI and WSI of native Torperk flours were 8.18 g/g and 33.53 g/g, higher than WAI and WSI (2.57 g/g and 16.58 g/g) reported by Soison et al. (2014). Native flours had higher WAI and SP values than pre-gelatinized flours at 90 C, while high WSI was presented in pre-gelatinized flour. Soison et al. (2014) reported that pre-gelatinized flour showed 3 times higher WSI than native flour. Swelling behavior of Phichit 290-9 flours gave the lowest WSI and SP values, possibly due to the high fat and protein contents. Oil absorption capacity (OAC) of flours ranged between 2.19 and 2.60 g/g with no significant differences (p < 0.05) among varieties or types. The oil absorption implied the nonpolar sited of protein or lipid in starch granules (Fiorda et al., 2015). Hydration properties and gelling behavior of flours depended on their characteristic of starch granules. Water-binding capacity constituted the ability of hydrophilic part in starch molecules, water was absorbed in the amorphous zone of the starch and become swollen. The change in small crystallites caused the striping of chains and melting of crystallites. Pre-gelatinized flours showed the high hydration properties due to the structural rearrangement or degradation of starch granules during thermal processing (Neelam et al., 2012). Moreover, the variation in solubility and swelling power could be caused by differing degrees of engagement of the hydroxyl groups, forming hydrogen and covalent bonds between the starch chains (Gunarate & Hoover, 2002). Results indicated that thermal pre-treatment improved both the hydration properties and gelling behavior of flours of Thai purple sweet potato flours and pre-gelatinized flours showed the cold water-soluble ability. Pasting properties of Thai purple sweet potato flours were determined by a Rapid Visco Analyzer (RVA) ( Table  3). Sweet potato varieties and flour preparation both significantly affected the pasting properties (p < 0.05). Pasting temperature (PT) was only observed in native Phichit 65-3, Maejo 343 and 290-9 flours (83.32, 85.02 and 86.53 °C, respectively). Results indicated that starch granules of Torperk and pre-gelatinized flours swell more rapidly beyond the gelatinization temperature. Phichit 290-9 flours exhibited the highest values of final viscosity and set back, indicating the starch granule had high capacity to form gel with high retrogradation tendency. Breakdown reflects the shear stability of hot paste, all pre-gelatinized flours presented lower breakdown viscosity than native flour. Differences in breakdown have been related to disparate rigidities of swollen granules. Pre-gelatinized Torperk flours showed the lowest breakdown (1.17 RVU) and setback (8.11 RVU), indicating the high sensitivity to thermal processing and the low rate of retrogradation. Viscosity profiles of Maejo 343 and Phichit 65-3 flours showed more similar patterns than other varieties. Peak time ranged between 4.13 and 6.98 min. The short peak time supported that all Thai purple sweet potato flours were easy to cook. Different pasting properties might be due to the genetic factor, the accumulation of starch granules and their structural properties (Aina et al., 2009;Soison et al., 2014). Antioxidant activity of Thai sweet potato flours is dependent on the phytochemical content and several in vitro mechanisms. Total phenolic content (TPC) and total anthocyanin content (TAC) were measured (Table 4). TPC and TAC values ranged between 22.7-226.9 mg GAE/100g db and 10.3-182.8 mg/100g db, respectively. TPC and TAC values of flours differed between varieties and increased in pre-gelatinized flours. The pre-gelatinized 290-9 flour showed the higher TPC at almost 10 times than native. Among the pre-gelatinized flours, Phichit 65-3 showed the highest TPC and TAC values, while Torperk presented the lowest values. In this study, Torperk flours recorded lower TPC and TAC values than previously reported (Soison et al., 2014) which might be related to the cultivation area, growing conditions and environment of Torperk root production. High TPC and low TAC were found in both Phichit 290-9 and Torperk varieties. Results suggested that anthocyanin quantities as the large group of phenolic compounds in purple sweet potatoes varied depending on the cultivar. Here, well-known in vitro methods such as DPPH, TEAC and FRAP assays were used to measure antioxidant activities of Thai purple sweet potato flours. DPPH and TEAC assays determined the free radical scavenging capacity in different conditions while FRAP assays assessed the chelating metal ion. Antioxidant activity differed depending on the sweet potato variety and preparation method. An increase in antioxidant activity of pre-gelatinized flours was observed in all analyses. DPPH and TEAC values ranged between 0.556-9.094 mg TE/g db and 4.966-74.836 mg TE/g db, respectively. Different flour preparation directly affected DPPH and TEAC values of Phichit 290-9 flours; native flour had the lowest values while pre-gelatinized flour showed the highest. The FRAP assays followed the same trend as DPPH and TEAC assays. All flours had FRAP values ranging between 1.150-14.847 mg TE/g db with pre-gelatinized Phichit 290-9 flour giving the highest FRAP values. The Phichit 65-3 variety showed high levels of anthocyanins and antioxidant activity for both native and pre-gelatinized flours, while pre-gelatinized Phichit 290-9 flour had the highest phenolic content and antioxidant activity.
Changes in the phenolic compounds after thermal processing related to the loss of heat-sensitive constituents. Degradation of phenolic compounds occurs through enzyme activity as a result of thermal processing. Degradation rate of phenolic compounds depends on many factors including the number of substituents, the molecular structure and the accumulation of phenolic compounds in the plant. However, an increase in phenolic compounds after thermal processing was reported by several authors as the degraded polyphenols resulted in free and smaller molecules with easy extraction of constituents (Huang et al., 2006;Tokusoglu & Yildirim, 2012). Many studies reported that thermal treatment enhanced anthocyanins content in several purple sweet potato varieties including Stokes purple, NC415, Okinawa, Phichit 65-3 and Torperk Soison et al., 2014;Ruttarattanamongkol et al., 2016). Phenolic compounds and anthocyanins in purple sweet potatoes show powerful antioxidant activity. The main anthocyanins in purple sweet potatoes are 3,5-diglucoside derivatives from cyanidin and peonidin, acylated with p-hydroxybenzoic acid, ferulic acid, or caffeic acid, respectively (Kim et al., 2012). High concentrations of anthocyanins were observed in Phichit 65-3 and Maejo 343 flours. Their pre-gelatinized flours also showed strong antioxidant activity which might be due to higher amounts of acylated anthocyanidins which are more stable during processing and storage. Acylation of anthocyanin improves its stability through intermolecular co-pigmentation (Giustri & Wrolstad, 2003;Cevallos-casals & Cisneros-Zevallos, 2004). The high antioxidant activity of Phichit 290-9 and Torperk flours related to their high phenolic compounds. Results also confirmed that Thai purple sweet potatoes are good sources of natural antioxidant constituents and very stable under thermal processing. Flours produced from Thai purple sweet potatoes show potential as good sources of natural colorants, and dietary phytochemicals and antioxidants. Moreover, steam pre-treatment enhanced the deep purple color and improved the pasting properties of Thai sweet potato flours as beneficial for use in food products.

Physical Properties of Air-Puffed Purple Sweet Potato Snacks
From results in 3.1, native and pre-gelatinized flours produced from Phichit 65-3 variety showed good characteristics for color, pasting properties, high anthocyanin and antioxidant activity. Thus, Phichit 65-3 flours were used for purple sweet potato snack production. Figure 3 presents air-puffed purple sweet potato snacks with jas.ccsenet. different ra from swee control an the conten (6.65 and expansion snacks dep gelatinizat ratio of n air-puffed Color of s of pre-gela in G50 pr pre-gelatin low a* va supporting Purple swe ratio and lo