EFFECT OF DRYING TEMPERATURE ON FUNCTIONAL AND DIGESTIVE PROPERTIES OF SWEET POTATO FLOUR

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1 EFFECT OF DRYING TEMPERATURE ON FUNCTIONAL AND DIGESTIVE PROPERTIES OF SWEET POTATO FLOUR 1 FESSEHAYE HDREMARIAM DESALE, 2 RUNGARUN SASANATAYART 1 MSc. Student in Technology Management of Agricultural Produces, Mae Fah Luang University (MFU), Thailand, 2 PhD. Lecturer in Technology Management of Agricultural Produces, MFU, Thailand 1 dessale4@gmail.com, 2 rungarun.s@mfu.ac.th Abstract: Proper drying of food products helps in extending shelf life and either maintaining or improving their quality. In this study, the effect of hot air drying temperature (at 55, 70 and 85 0 C) in comparison to freeze drying on the functional and digestive properties of flours from sweet potato roots with orange flesh (OFSP) and purple flesh (PFSP) were investigated. The proximate compositions and amylose content (AC) were also analyzed. Results showed that flours obtained from hot air dried sweet potatoes had higher AC (12.36 to 18.95%db for OFSP and to 19.31%db for PFSP) than that from freeze dried ones (11.44%db for OFSP and 13.09%db for PFSP). Drying possibly modified the structure of starch granules and consequently changes in pasting viscosity based on Rapid Visco Analyzer (RVA). Overall pasting viscosities ranges include Peak viscosity (PV) (freeze OFSP) to 2996 (85 PFSP), Trough viscosity (TV) (55 OFSP) to 1398 (85 PFSP), Breakdown viscosity (BDV) (freeze OFSP) to 1598 (85 PFSP), Final viscosity (FV) (55 OFSP) to 1825 (85 PFSP) and Setbak viscosity (SBV) (55 OFSP) to 427 (85 PFSP) all increased with increasing drying temperature. The exception was for freeze dried OFSP since it showed high swelling index, resulting in the high pasting viscosities. Drying showed less apparent effect on gelatinization temperatures at onset (To) range C (70 OFSP) to C (freeze OFSP), peak (Tp) range C (70 OFSP) to C (freeze PFSP) and conclusion (Tc) range C (70.OFSP) to C (freeze PFSP) based on Differential Scanning Calorimetry (DSC) test results. However, positive correlations were observed between gelatinization enthalpy and AC (p<0.05). Drying under the temperature range of this study affected starch fractions including resistant starch (RS), non-resistant starch (NRS) and total starch (TS) only slightly. Results of this study suggested that high temperature drying could be applicable in modifying paste viscosities of sweet potato for use as thickening or stabilizing in food product. Keywords: Flour functional properties, Freeze drying, Hot air drying, Starch, Sweet potato I. INTRODUCTION Root and tuber crops are plants grown all over the world in hot and humid regions, providing starchy roots, corms, rhizomes, stems and tubers. Sweet potato (Ipomeabatatas) (SP) is one among them which is cultivated for edible purposes [1]. It is a staple food, serves as animal feed and raw material for several food and feed based industries [2, 3]. It requires less amount of fertilizers and other agricultural chemical inputs during cultivation; highly tolerant to environmental stresses and has very high productivity [4, 5]. Though SP is a very important staple crop in many developing countries, the high perishability of the storage roots limits its postharvest shelf life [6, 7]. To extend the uses and to reduce loses due to its perishability, fresh roots can be dried and converted in to flour [7]. Drying is an easiest and the most common method, playing an important role in preserving a variety of food products [8]. It is based on the removal of moisture from the fresh produce which is susceptible to microbial growth (at water activity levels of 0.9 to 1.0) to a safe water activity levels near to 0.6 [9]. However, the quality of plant tissues are also affected in many ways by drying[10], but comparatively freeze drying better retains the quality of plant tissues during drying [11]. In comparison to solar dried flour, morphologically freeze dried flours are the least affected[12]. It has been established that drying affected amount of some bioactive compounds such as total carotenoid and consequently antioxidant property [7, 13, 14] etc. Furthermore, the application of heat on tothe starch based product could also alter its starch digestibility [15]. Researches related on the effect of drying on starch digestibility and other functional properties are very rare. This study investigated the effect of heated air cabinet drying at different temperatures on the starch functional and digestive properties of two varieties of sweet potato; orange and purple fleshed varieties in comparison to freeze drying. II. MATERIALS AND METHODS 2.1. Sweet potato roots The two Sweet Potato roots (SPR), OFSP and PFSP were bought from Buriram province, Thailand; and transported to Mae Fah Luang University, Chiang Rai, Thailand. Roots were cleaned with tap water, surface dried and stored at 10 0 C in a refrigerator before preparing for drying Drying operation and flour preparation For drying operation roots were selected manually and separated it to two groups. One group for hot air drying and the other one for freeze drying. Roots for hot air drying were peeled manually with dual blade peeler, then sliced in to around 3mm thickness using slicing machine (Officine Savioli, serial No. A55/2266, Italy). To minimize browning due to heat 25

2 treatment, slices were dipped in Potassium metabisulfite solution (0.4gL -1 ) for two minutes at room temperature. Then slices were loaded in to trays for drying operation at three temperature levels using lab scale hot air tray dryer (KluayNamThaiTowop, Thailand) to a moisture content less than 8% (w/w). Roots for freeze drying were peeled in the same way, cut into small pieces followed by grinding in to thick puree and subsequently freeze dried using freeze dryer (Mertin Christ, Delta 2-24 LSCplus, serial No , Germany). For flour preparation, samples were ground in to powder. The collected flours were then sieved with 60 mesh screen using sieving machine (Retsch, AS200 digit, serial No , Germany). The fine powder was then packed in plastic bags till analysis Determination of proximate composition The proximate composition in terms of moisture, protein, fat, ash, carbohydrate and dietary fiber contents were determined using AOAC methods [16] from the fresh sweet potato roots Determination of flour functional properties Flour moisture content was determined by hot air oven (memmert, E07086, Germany) using AOAC method [16]. Water activity of sweet potato flour (SPF) was analyzed using aw meter (aw-center novasina, RS-232, Switzerland). Flour colour was determined based on L *, a * and b * colour values using colorimeter (HunterLab /ColorQuestXE,serial No.cox2339, USA) as described by [17]. The hue angle was calculated from a * and b * colour values using the formulae; o Hue = tan ( b/ a)... 1 The percentage swelling index (SI) was determined following to the method used by [18]. Flour sample (10g) in triplicate of each sample were put in 100 ml graduated cylinders. After measuring the initial flour volume, distilled water (80ml) was added to the cylinders. For removing air voids, the cylinders were taped gently and allowed to stand in room temperature for one hour. Just at the end of one hour, the rise in volume of the flour was recorded and the swelling index was determined as the ratio of the final volume Determination of flour Amylose content Flour amylose content (AC) was determined following the method [19]. About 0.1g flour sample in duplicate based on 12% moisture content was boiled for 20 minutes with 1 ml of absolute Ethyl alcohol. Samples were cooled and diluted with distilled water to 100ml. Five milliliter of this supernatant was treated with 1ml of 1N Glacial acetic acid solution and 2ml of Iodine solution (containing 0.2% I 2 and 2% KI), then left for 20 minutes to develop dark colour after the volume was adjusted to 100 ml with distilled water. Finally the absorbance was measured at 620nm. Standard curve with R 2 = was developed using standard potato amylose Determination of starch fractions Resistant starch (RS), non-resistant starch (NRS) and total starch (TS) were determined in duplicate using resistant starch assay (K-RSTAR, Megazyme International, Ireland) Determination of pasting properties of flours Based up on the initial moisture content of flours recommended amounts of flours were mixed to a 25g distilled water to make a flour suspension of 90% mc in sample holding cup to determine the viscosity using rapid visco analyzer ((RVA 4500, Perten Instruments pty limited, Australia). A 13 minute heating and cooling cycle was used in which the sample was held at 50 0 C for 1 minute, heated from 50 0 C to 95 0 C in 3.5 minutes, held at 95 0 C for 3 minutes and then cooled to 50 0 C in 3.5 minutes, held for 2 minutes at 50 0 C. All pasting properties; peak viscosity (PV), hot paste viscosity (HPV), break down viscosity (BDV), cold paste viscosity (CPV), set back viscosity (SBV) and pasting temperature (PT) were determined using the software ThermoCline for windows version 3 (TCW3) [20, 21] Determination of thermal properties A total weight of around 25mg (water to flour ratio of 4:1) in a stainless steel pan with the use of differential scanning calorimeter (DSC 8000, PerkinElmer Ltd, United Kingdom). The pans were equilibrated at room temperature overnight after being sealed tightly. With the use of empty stainless steel pan as a reference, the pans were given isothermal treatment for 5 minutes at 30 0 C followed by heating phase at the rate of 10 0 C/minute from 30 0 C to C. Using the Pyris software gelatinization temperatures; onset (To), peak (Tp) and conclusion (Tc) and enthalpy of gelatinization were calculated [21, 22] Statistical analysis SPSS for windows version 20 (IBM SPSS statistics) was used to analyze one way statistical analysis of variance (ANOVA). The comparison of means was done using Duncan method at 95% level of confidence. Pearson correlation coefficient was used to study the relationship among different properties. III. RESULTS AND DISCUSSION 3.1. Proximate composition of sweet potato roots Table 1: shows the proximate composition of fresh sweet potato roots. Dry matter content 33.47% in OFSP and 39.49% in PFSP was observed. Dry matter content in sweet potato was found in range of 30.2% 26

Overall, OFSP had higher protein and fat content than PFSP, but lower in ash (mineral content). 3.2.

3 to 36.2% depending on variety [23]. High carbohydrate content (around 93% db) was found in both varieties. Protein, fat, ash and crude fiber contents found in this study are in agreement with literature values eg. [7]. Overall, OFSP had higher protein and fat content than PFSP, but lower in ash (mineral content) Flour physical and functional properties Moisture content flour decreased with increasing drying temperature (Table 2). The lowest moisture content was obtained from sample dried at 85 0 C in both OFSP (4.25% wb) and PFSP (3.11% wb). Similar effect was reported by [7]. Similarly, water activity (aw) of flour decreased with increasing drying temperature. It has been well established that aw below 0.6 can inhibit the susceptibility of food products to microbial spoilage [9]. In terms of colour, Lightness (L * ) of flour from hot air drying was higher than those from freeze drying. L * value increased with the increasing of drying temperature which may be due to inhibition of Poly-phenol oxidase (PPO); an enzyme that can cause browning effect to food products [7]. In both cultivars, flours from hot air drying showed higher hue angle values than freeze dried flours. A significant difference Table 1: Proximate composition of sweet potato roots (p<0.05) of hue colour among treatments and between varieties (Table 2) was also observed. Flour swelling index (SI) which is an indication of noncovalent bonding between starch molecules, tended to increase with increasing drying temperature. However, with in OFSP, SI of freeze dried was higher than the other drying temperatures and this contributed to high pasting viscosity). Greater degree of association between flour granules possibly lowers swelling power[24]. Presence of higher amylose content, non-starch carbohydrates, lipid-starch complexes etc. can also reduce swelling of flours [25]. Moreover, among different drying methods, different ways of water evaporation from crystals could cause to variation in swelling power of flours produced [26] Amylose content and starch digestibility Between freeze dried samples, OFSP had lower AC (11.44%) than PFSP (13.09%). AC in sweet potato as percentage of starch was reported in range of 17.9 to 23.4 by [23]. Hot air dried samples showed greater AC and increased with increasing drying temperature. The highest AC was found in samples dried at 85 0 C for both OFSP (18.95% db) and PFSP (19.31% db) (Table 2). Values are means of triplicate determinations ± standard deviation; in a column values followed by different subscripts are significantly different (p/0.05); wb and db indicates wet and dry basis respectively Table 2: Physical properties and composition of flours obtained from different drying treatments In a column values followed by different subscripts are significantly different (p<0.05); values are duplicate ( ** ) and triplicate determinations ± standard deviation. DT, Mc, aw, L *, SI, AC, RS, NRS and TS indicates drying temperature, moisture content (w/w of flour), water activity, colour lightness, swelling index, amylose content, resistant starch, non-resistant starch and total starch respectively. The increased AC as a result of drying could be attributed to the stimulating of starch degrading enzymes like α-amylase, β-amylase pullulanase and glucoamylase which degrade starch crystallinity, giving a free amylose chain [27]. Similar effect was observed in a study on chestnut flours by [28]. The increase of resistant starch 27

(RS) was expected regarding the increased amount of AC. However, RS in hot air dried samples increased only slightly. A slight increase in NRS and TS were also observed (Table 2). 3.4.

4 (RS) was expected regarding the increased amount of AC. However, RS in hot air dried samples increased only slightly. A slight increase in NRS and TS were also observed (Table 2) Pasting properties Figure 1: shows the pasting characteristics of sweet potato flour as a result of different drying temperatures. In both varieties, higher peak viscosity (PV) due to hot air drying than freeze drying was recorded with progressive increase (p<0.05) with increasing drying temperature (Table 3a). The effect of drying temperature was more apparent in PFSP (from cP in freeze drying to 2996cP in 85 0 C drying) than OFSP (from cP for freeze drying to only cP in 85 0 C drying). Similar effects were reported in [28]. PV of sweet potato flour for different varieties ranges from 826cP to 3039cp [23]. Effect of drying to the other pasting characteristics (TV, BDV, FV and SBV) was seen to increase with the increase in drying temperature. The exception was for freeze dried OFSP which showed high pasting viscosities (Fig. 1 and table 3a), indicating that hot air dried sweet potato gets more thinning after peak viscosity due to the shearing during the gelatinization process. However, this was not observed in PFSP sample. The pasting properties of flour depend up on the difference in components of flours such as protein, fat, sugar, fiber, amylose:amylopectin ratio etc. [7]. Up on cooling, higher values of final viscosity and break down viscosity are indications for greater levels of retrogradation potential and elevated viscosity [28]. The sweet potato flour obtained from high drying temperature which gave high viscose paste could be served as a thickening and stabilizing agent in some food products. It was also found thatpasting temperature slightly decreased with increasing drying temperature (Table 3a) due to modification of starch granules[24] Gelatinization properties of flour Gelatinization temperature of flour: onset temperature (To), peak temperature (Tp) and conclusion temperature (Tc) were not significantly different (P>0.05) between those prepared by freeze drying and hot air drying except in some cases example OFSP dried at 70 0 C (Table 3a). These phase transition temperatures are influenced by molecular architecture of the crystalline region[1]. Gelatinization temperature is not only depend on water content but also on existence of other extraneous chemicals, sugars, salts and other small molecules [27]. Positive correlation (P<0.05) was observed between AC and enthalpy of gelatinization (ΔH) (table 3b). The range of ΔH observed in this study was 6.98 to J/g dry flour and this was between the range 5.5 to 14.0 J/g dry sweet potato as reported by [29] (Table: 3a). Other than AC, factors that affect gelatinization enthalpy of sweet potato starch are intermolecular bonding, degree of crystallinity etc. [30] Fig. 1: RVA pasting profiles of OFSP (A) and PFSP (B) flours due to freeze drying and hot air drying at 55, 70 and 85 0 C temperatures. Table 3a: Changes in pasting and gelatinization properties of sweet potato flour due to drying temperature 28

Table 3b: Pearson correlation coefficients between different compositional and functional properties of flours as affected by drying temperature Table 3 indicates the effect of drying temperature on

5 Table 3b: Pearson correlation coefficients between different compositional and functional properties of flours as affected by drying temperature Table 3 indicates the effect of drying temperature on pasting & gelatinization properties of sweet potato flour (a) and the Pearson correlation coefficients (p<0.05) among different compositional and functional properties of sweet potato flours. In a column (Table 3a) values were means of duplicate determinations ± standard deviation and values followed with different subscripts are significantly different (p<0.05). To, Tp, Tc, ΔH, AC, RS, NRS, TS, SI, PV, TV, BDV, FV, SBV and PT indicates onset temperature, peak temperature, conclusion temperature, enthalpy of gelatinization, amylose content, resistant starch, non-resistant starch total starch, swelling index, peak viscosity trough viscosity, break down viscosity, final viscosity, set back viscosity and pasting temperature respectively. ** p<0.01; * p<0.05 CONCLUSION Based on hot air drying in this study, pasting characteristics of sweet potato flour in both varieties have been significantly altered and this could be applicable in food industry. The effect on starch fractions was only minimal, indicating the less effect on starch digestibility. ACKNOWLEDGEMENTS The author would like to thank Thailand International Cooperation Agency (TICA) for financial support. REFERENCES 1. Hoover, R., Composition, molecular structure, and physicochemical properties of tuber and root starches: a review. Carbohydrate Polymers, (3): p Woolfe, J.A., Sweet potato: An Untapped Food Resource. 1992: Cambridge University Press. 3. Tekwu, E.M., V.P. Beng, and V. Kuete, 22 - African Medicinal Plants Acting on the Reproductive, Cardiovascular, and Central Nervous Systems, in Medicinal Plant Research in Africa. 2013, Elsevier: Oxford. p Tang, Y., W. Cai, and B. Xu, Profiles of phenolics, carotenoids and antioxidative capacities of thermal processed white, yellow, orange and purple sweet potatoes grown in Guilin, China. Food Science and Human Wellness, (3): p Ishida, H., et al., Nutritive evaluation on chemical components of leaves, stalks and stems of sweet potatoes (Ipomoea batataspoir). Food Chemistry, : p Ahmed, M., M.S. Akter, and J.-B. Eun, Peeling, drying temperatures, and sulphite-treatment affect physicochemical properties and nutritional quality of sweet potato flour. Food Chemistry, (1): p Ruttarattanamongkol, K., et al., Effect of drying conditions on properties, pigments and antioxidant activity retentions of pretreated orange and purple-fleshed sweet potato flours. J Food SciTechnol, Chemkhi, S., F. Zagrouba, and A. Bellagi, Modelling and Simulation of Drying Phenomena with Rheological Behaviour. Brazilian Journal of Chemical Engineering, (2): p Oliveira, S.M., T.R.S. Branda o, and C.L.M. Silva, Influence of Drying Processes and Pretreatments on Nutritional and Bioactive Characteristics of Dried Vegetables: A Review. Food Eng Rev, Ruttarattanamongkol, K., et al., Effects of Pretreatments on Colour and Antioxidant Activities of Dehtdrated Purple and White-fleshed Sweet Potatoes, in The 16th Food Innovation Asia Conference : BitecBangna, Bangkok, Thailand. 11. Jeng, T.L., et al., Effects of drying on caffeoylquinic acid derivative content and antioxidant capacity of sweet potato leaves. Journal of Food and Drug Analysis, (4): p Suriya, M., et al., Influence of blanching and drying methods on molecular structure and functional properties of elephant foot yam (Amorphophalluspaeoniifolius) flour. LWT - Food Science and Technology, : p Bechoff, A., et al., Effect of hot air, solar and sun drying treatments on provitamin A retention in orange-fleshed sweetpotato. Journal of Food Engineering, (2): p Yang, J., et al., Effects of Drying Processes on the Antioxidant Properties in Sweet Potatoes. Agricultural Sciences in China, (10): p Donlao, N. and Y. Ogawa, Impact of postharvest drying conditions on in vitro starch digestibility and estimated glycemic index of cooked non-waxy long grain rice (Oryza sativa L.). Journal of the Science of Food and Agriculture, AOAC International in Official methods of analysis (17thed.). 2000: Maryland, USA. 17. Bengtsson, A., et al., Effects of various traditional processing methods on the all-trans-b-carotene content of orange-fleshed sweet potato. Journal of Food Composition and Analysis, : p Amajor, J.U., et al., Studies on the Characteristic Properties of Fermented, Sun-Dried Orange-Fleshed Sweet Potato Flour. Nigerian Food Journal, (1): p AACC. (2000), in Approved method of the American Association of Cereal Chemists, 10th ed, Method and : St. Paul, Minnesota. 29

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