Optimization of hot-air drying conditions on functional properties of flour from dried South African sweet potato cultivars (<i>Impilo</i> and <i>Bophelo</i>) using the response surface methodology

MOLOTO, Phuti Itumeleng; MOSALA, Mmathaha; OMOLOLA, Adewale Olusegun; JIDEANI, Afam Israel Obiefuna; LAURIE, Sunette Marlize

doi:10.1590/fst.28019

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Food Science and Technology

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Original Article • Food Sci. Technol (Campinas) 41 (1) • Jan-Mar 2021 • https://doi.org/10.1590/fst.28019 copy

Optimization of hot-air drying conditions on functional properties of flour from dried South African sweet potato cultivars (Impilo and Bophelo) using the response surface methodology

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

The response surface method (RSM) has an important application in formulation of new products. This study determined the effect of electric hot-air drying on the functional properties of flour made from orange-fleshed sweet potato (OFSP). Flour from the cultivars Bophelo (B) and Impilo (IM) were treated with 13 temperature-time combinations designed by STAT EASE software version 8.01.0. Bophelo flour dried at 70 °C for 8.6 hrs and Impilo at 70 °C for 14.4 hrs showed the lowest gelatinization temperatures and enthalpy of gelatinization (ΔH), indicating ease of cooking. The highest swelling power of 12.45 g/g for Bophelo and 8.16 g/g for Impilo were significantly higher than the remaining samples. Flours for Bophelo and Impilo at drying temperatures and times at 70 °C for 4.4 hrs, 56 °C for 6.5 hrs and 60 °C for 6.5 hrs indicated favourable gelling properties and cost of drying, implying that these can be used at household level and in food industries as thickeners from both cultivars.

Keywords:
orange fleshed sweet potato flour; cabinet drier; pasting properties; gelatinization; swelling and solubility

S/N	Independent variables		Response variables
S/N	Temperature (°C)	Time (hrs)	Peak Viscosity (cP)	Trough (cP)	Breakdown (cP)	Final Viscosity (cP)	Setback (cP)	Peak time (min)	Peak Temperature (°C)
1	70	6.5	1275.67 ± 74.77^a	1186.67 ± 4.93^a	29.13 ± 0.12^cd	1408.33 ± 36.47^a	474.00 ± 1.00^a	5.81 ± 0.18^bcd	75.28 ± 0.53^de
2	70	8.6	398.33 ± 14.22^e	388.33 ± 14.84^e	30.67 ± 0.58^cd	610.00 ± 34.39^e	221.67 ± 19.55^d	5.44 ± 0.32^cde	84.72 ± 0.42^bc
3	80	5	379.00 ± 1.00e	373.67 ± 2.31^e	55.00 ± 1.00^bcd	604.00 ± 6.56^e	228.33 ± 1.53^d	6.67 ± 0.52^a	86.87 ± 0.48^ab
4	70	6.5	1239.04 ± 1.07^a	1166.18 ± 0.85^a	27.33 ± 0.51^cd	1172.41 ± 2.11^b	407.33 ± 8.08^b	5.95 ± 0.76^abc	75.09 ± 0.19^de
5	70	4.4	606.67 ± 172.39^bc	540.33 ± 127.42^cd	66.33 ± 45.65^abc	744.00 ± 129.90^d	203.67 ± 51.59^d	4.98 ± 0.59^ef	52.15 ± 2.84^f
6	80	8	466.33 ± 137.56^de	443.67 ± 118.73^de	22.67 ± 19.76^d	671.67 ± 146.72^de	228.00 ± 29.44^d	6.40 ± 0.93^ab	77.54 ± 7.70^cd
7	60	8	594.00 ± 55.02^bcd	496.00 ± 10.61^cd	98.00 ± 50.32^a	703.67 ± 64.14^de	207.67 ± 24.58^d	4.53 ± 0.40^f	67.42 ± 16.33^e
8	70	6.5	1301.16 ± 1.03^a	1186.06 ± 0.69^a	29.03 ± 0.13^cd	1415.00±38.11^a	442.99 ± 0.78^ab	5.87 ± 0.07^bcd	75.80 ± 0.78^de
9	70	6.5	1298.12 ± 0.12^a	1103.99 ± 0.22^a	29.00 ± 15.72^cd	1361.67 ± 77.52^a	457.67 ± 38.48^a	5.71 ± 0.32^bcde	75.55 ± 0.95^de
10	56	6.5	716.33 ± 142.06^b	638.67 ± 103.78^b	77.67 ± 42.44^a	968.00 ± 159.55^c	329.33 ± 60.68^c	4.50 ± 0.43^f	73.94 ± 0.52d^e
11	60	5	575.33 ± 52.01^cd	52.01± 32.79^cd	49.33 ± 20.26^bcd	772.33 ± 26.31^d	246.33 ± 11.72^d	5.20 ± 0.18^cdef	95.15 ± 0.81^a
12	70	6.5	1286.08 ± 0.34^a	1171.32 ± 0.34^a	28.67 ± 8.96^dc	1316.00 ± 1.00a	344.33 ± 26.08^c	5.49 ± 0.08^cde	76.83 ± 0.45^cd
13	84	6.5	606.33± 5.13^bc	584.67 ± 0.58^bc	18.67 ± 1.53^d	912.67 ± 9.45^c	324.00 ± 7.00^c	5.17 ± 0.10d^ef	75.46 ± 0.50^de
	Model		<0.0001^* * Significant at p < 0.05. A: direct effect of drying temperature; B: direct effect of drying time; AB: effect of drying temperature and drying time; A2: quadratical effect of drying temperature; B2: quadratical effect of drying time; S/N: Sample number.	<0.0001*	0.0021*	0.0010*	0.0038*
	A-Temp		0.0558	0.1540	0.0010*	0.3999	0.9690
	B-Time		0.3976	0.3529	0.2700	0.5593	0.9197
	A²		<0.0001*	<0.0001*	0.0167*	0.0018*	0.0108*
	B²		<0.0001*	<0.0001*	0.0157*	<0.0001*	0.0003*
	AB		0.6569	0.4477	0.0050*	0.5547	0.6881

S/N	Independent variables		Response variables
S/N	Temperature (°C)	Time (hrs)	Peak Viscosity (cP)	Trough (cP)	Breakdown (cP)	Final Viscosity (cP)	Setback (cP)	Peak Time (min)	Peak Temperature (°C)
1	70	6.5	812.00 ± 5.00^f	767.67 ± 12.34^d	34.00 ± 2.00^g	1119.33 ± 0.58^e	349.33 ± 1.15^d	5.41 ± 0.03^c	77.13 ± 0.55^g
2	70	8.6	452.00 ± 7.21^k	371.67 ± 6.66^l	77.00 ± 1.00^d	547.00 ± 18.06^k	163.67 ± 5.56^j	3.94 ± 0.04^h	83.74 ± 0.40^b
3	80	5	837.67 ± 3.21^e	786.00 ± 5.29^c	54.33 ± 0.58^e	1093.00 ± 5.20^f	310.33 ± 0.58^e	5.52 ± 0.13^bc	81.83 ± 0.29^cd
4	70	6.5	1042.67 ± 2.52^a	1004.67 ± 1.53^a	42.00 ± 2.65^fg	1438.67 ± 3.21^b	434.67 ± 4.04^a	5.55 ± 0.17^b	77.34 ± 0.21^fg
5	70	4.4	630.67 ± 7.02^g	481.33 ± 8.74^j	144.33 ± 11.59^a	660.33 ± 7.57ⁱ	184.33 ± 3.51i	4.33 ± 0.09^g	79.16 ±0.31^e
6	80	8	608.67 ± 2.52^h	517.33 ± 4.93ⁱ	92.33 ± 0.58^c	735.33 ± 3.51^g	219.67 ± 2.08^g	4.57 ± 0.03^ef	79.60 ±0.53^e
7	60	8	510.00 ± 5.00^j	491.67 ± 3.06^j	115.67 ± 1.53^b	712.00 ± 3.00^h	231.67 ± 16.07^f	5.41 ± 0.03^c	89.46 ±0.28^a
8	70	6.5	866.67 ± 4.93^d	706.33 ± 5.51^g	47.81 ± 0.30^ef	1476.72 ± 0.41^a	376.67 ± 2.52^b	4.69 ± 0.10^e	82.65 ±0.38^c
9	70	6.5	887.67 ± 3.21^g	737.00 ± 4.36^e	52.00 ± 1.00^e	1379.14 ± 0.11^c	347.67 ± 1.53^d	4.48 ± 0.10^f	81.80 ± 0.27^d
10	56	6.5	623.79 ± 0.20^g	548.33 ± 0.38^h	120.00 ± 16.64^b	657.32 ± 0.60ⁱ	197.00 ± 2.65^h	4.52 ± 0.05^f	70.74 ± 0.47ⁱ
11	60	5	528.00 ± 2.65ⁱ	389.67 ± 6.43^k	142.00 ± 2.00^a	580.00 ± 3.00^j	193.33 ± 1.53^hi	5.04 ± 0.04^d	74.95 ± 0.00^h
12	70	6.5	910.33 ± 5.51^b	882.67 ± 7.37^b	46.33 ± 1.53^ef	1241.00 ± 6.93^d	362.33 ± 4.04^c	5.97 ± 0.03^a	78.10 ± 1.26^f
13	84	6.5	840.33 ± 11.24^e	717.00 ± 5.29^f	54.00 ± 1.00^e	482.33 ± 13.87^l	155.67 ± 3.21^j	4.94 ± 0.06^d	77.13 ± 0.37^j
	Model		0.0126	0.0074*	0.0014*	0.0081*	0.0092*		0.0755
	A-Temperature		0.1132	0.0463*	0.0022*	0.5985	0.7455		0.2404
	B-Time		0.4964	0.2775	0.0969	0.4852	0.5726		0.2366
	A²		03314	0.0448*	0.0073*	0.0021*	0.0028*		0.0351*
	B²		0.0009^* * Significant at p < 0.05. A: direct effect of drying temperature; B: direct effect of drying time; AB: effect of drying temperature and drying time; A2: quadratical effect of drying temperature; B2: quadratical effect of drying time; S/N: Sample number.	0.0012*	0.0007*	0.0028*	0.0026*		0.1600
	AB		0.4743	0.0966	0.0754	0.2278	0.2272		1.1461

S/N	Independent variables		Response variables
S/N	Temperature (°C)	Time (hrs)	Swelling (g/g)	Solubility (g/g)	T_o (°C)	T_p (°C)	T_e (°C)	∆H
1	70	6.5	8.15 ± 0.08^d	21.35 ± 0.11^a	18.68 ± 0.47^bc	30.05 ± 0.20^bc	75.95 ± 0.36^c	168.7 ± 0.43^d
2	70	8.6	9.29 ± 0.32^c	25.06 ± 0.08^c	18.44 ± 0.27^bc	29.34 ± 0.30^cd	55.92 ± 0.10^h	61.79 ± 1.93^h
3	80	5	5.68 ± 0.23^e	18.64 ± 0.26^e	18.54 ± 0.02^bc	31.03 ± 0.32^ab	94.96 ± 0.10^b	268.09 ± 0.21^a
4	70	6.5	8.18 ± 0.37^d	21.63 ± 0.36^d	18.45 ± 0.31^bc	30.85 ± 0.90^ab	74.64 ± 0.38^d	168.72 ± 1.81^d
5	70	4.4	3.48 ± 0.34^h	18.28 ± 0.42^ef	18.17 ± 0.01^c	31.70 ± 0.01^a	73.01 ± 0.83^e	251.30 ± 0.51^b
6	80	8	11.07 ± 0.11^b	35.97 ± 0.15^a	19.65 ± 0.09^a	28.12 ± 0.25^e	95.13 ± 0.19^b	89.09 ± 0.25^g
7	60	8	4.85 ± 0.20^f	17.98 ± 0.05^f	18.78 ± 0.08^b	29.00 ± 0.74^ed	60.77 ± 0.61^g	129.08 ± 0.19^f
8	70	6.5	8.42 ± 0.27^d	21.41 ± 0.35^d	18.60 ± 0.22^bc	29.43 ± 0.54^cd	73.39 ± 0.35^e	169.04 ± 0.28^d
9	70	6.5	8.33 ± 0.24^d	21.49 ± 0.31^d	18.68 ± 0.44^bc	30.62 ± 0.24^b	73.64 ± 0.55^e	168.61 ± 8.31^d
10	56	6.5	3.03 ± 0.08ⁱ	15.98 ± 0.05^h	18.73 ± 0.18^bc	29.32 ± 0.27^cd	96.70 ± 0.88^a	240.87 ± 0.09^b
11	60	5	3.94 ± 0.17^g	17.02 ± 0.21^g	18.63 ± 0.27^bc	31.84 ± 0.40^a	95.83 ± 0.16^ab	229.44 ± 0.46^c
12	70	6.5	8.14 ± 0.15^d	21.3 ± 0.41^d	18.49 ± 0.09^bc	30.55 ± 0.57^b	75.84 ± 0.14^c	227.15 ± 17.79^c
13	84	6.5	12.46 ± 0.38^a	29.47 ± 0.42^b	19.47 ± 0.48^a	31.79 ± 0.93^a	64.47 ± 0.40^f	153.01 ± 0.99^e
	Model		0.0002^*	< 0.0001	0.0031*	0.0162*	0.3184	< 0.0001*
	A-Temperature		< 0.0001*	< 0.0001*	0.0081*	0.4475	0.7627	0.1187
	B-Time		0.0005*	< 0.0001*	0.0144*	0.0058*	0.1627	< 0.0001*
	A²		0.2343		0.0024*
	B²		0.0113*		0.2197
	AB		0.0337*	< 0.0001*	0.0329*		0.2317

S/N	Independent variables		Response variables
S/N	Temperature (°C)	Time (hrs)	Swelling (g/g)	Solubility (g/g)	T_o (°C)	T_p (°C)	T_e (°C)	∆H
1	70	6.5	5.31 ± 0.23^d	8.39 ± 0.34^fg	18.64 ± 0.46^cde	30.12 ± 0.56^ef	95.19 ± 0.24^bc	207.09 ± 0.04^d
2	70	8.6	6.81 ± 0.24^c	10.82 ± 0.26^c	19.32 ± 0.33^ab	31.80 ± 0.48^bc	95.46 ± 0.37^b	243.72 ± 0.22^a
3	80	5	4.2 ± 0.14^e	9.49 ± 0.33^d	18.27 ± 0.36^e	30.03 ± 0.40^efg	67.25 ± 0.27^e	201.68 ± 0.27^g
4	70	6.5	5.18 ± 0.23^d	8.1 ± 0.15^g	19.41 ± 0.27^a	31.81 ± 0.34^bc	94.07 ± 0.36^d	205.97 ± 0.02^e
5	70	4.4	3.62 ± 0.17^f	6.9 ± 0.19^h	18.70 ± 0.26^bcde	28.20 ± 0.51^h	57.88 ± 0.10^f	55.56 ± 0.04^j
6	80	8	7.18 ± 0.16^b	11.66 ± 0.17^b	19.23 ± 0.39^abcd	32.09 ± 0.31^ba	97.38 ± 0.37^a	242.53 ± 0.73^b
7	60	8	2.46 ± 0.23^g	6.22 ± 0.22ⁱ	19.29 ± 0.30^abc	29.50 ± 0.44^fg	94.23 ± 0.32^d	56.20 ± 0.07^j
8	70	6.5	5.01 ± 0.11^d	8.55 ± 0.40^ef	18.53 ± 0.15^e	33.13 ± 1.10^a	94.48 ± 0.29^cd	201.86 ± 0.22^g
9	70	6.5	5.06 ± 0.06^d	8.89 ± 0.17^e	18.87 ± 0.49^abcde	31.39 ± 0.54^bcd	93.67±0.69^d	203.81 ± 0.35^f
10	56	6.5	1.15 ± 0.08ⁱ	3.04 ± 0.08^k	18.42 ± 0.45^e	28.05 ± 0.77^h	56.64±0.39^g	59.20 ± 0.27^h
11	60	5	1.85 ± 0.23^h	4.57 ± 0.37^j	18.34 ± 0.13^e	28.90 ± 0.74^hg	55.57±0.41^h	57.31 ± 1.23i
12	70	6.5	5.01 ± 0.25^d	8.85 ± 0.26^e	18.57 ± 0.14^de	30.90 ± 0.29^cde	94.08 ± 0.09^d	208.35 ±0.87^c
13	84	6.5	8.16 ± 0.31^a	13.06 ± 0.27^a	18.60 ± 0.10^de	30.43 ± 0.01^def	66.47 ± 0.76^e	201.45 ± 0.39^g
	Model		< 0.0001*	< 0.0001*	0.0282*	0.0280^* * Significant at p < 0.05. A: direct effect of drying temperature; B: direct effect of drying time; AB: effect of drying temperature and drying time; A2: quadratical effect of drying temperature; B2: quadratical effect of drying time; S/N: Sample number.	0.0003	0.0043
	A-Temperature		< 0.0001*	< 0.0001*	0.8883	0.0334*	0.1076	0.0010
	B-Time		0.0050*	0.0004*	0.0091*	0.0234*	0.0001	0.0174
	A²					0.0287*	0.0003	0.0248
	B²					0.1351	0.0177	0.0720
	AB					0.4668	0.4640	0.5668
	Model					0.0280*	0.0003	0.0043

Variables	Models	R²
Peak Viscosity (cP)	-21461.00579 + 447.02972 * A + 2263.34584 * B -3.28902 * A² - 181.47507 * B² + 1.14444 * A * B	0.98
Trough (cP)	-19124.76725 + 398.83716 * A + 1999.58452 * B -2.95131 * A² - 163.91009 * B² + 1.66667 * A * B	0.98
Breakdown (cP)	+378.03266 - 9.83778 * A + 21.77843 * B + 0.11928 * A² + 5.37533 * B² - 1.35000 * A * B	0.90
Final Viscosity (cP)	-13615.14184 + 267.02556 * A + 1763.14493 * B - 2.03770 * A² - 149.08276 * B² + 2.27222 * A * B	0.92
Setback (cP)	-4396.91395 + 79.52943 * A + 629.10618 * B - 0.59820 * A² - 51.91977 * B² + 0.63889 * A * B	0.88
Peak Time (cP)	+5.51615	0.00
Peak Temperature(cP)	+76.29205	0.00
T_o (°C)	+36.80452 -0.51552 * A - 0.43790 B+ 3.11875E-003 A2 -0.040278 * B2 +0.015667* A * B	0.89
Tp (°C)	+33.51119 + 0.025721 * A - 0.75730 * B	0.56
Te (°C)	+386.57069 - 3.96771 * A - 46.02324 * B + 0.58717 * A * B	0.31
∆H (J/g)	+578.01045 - 1.56991 * A - 45.59275 * B	0.85
Swelling	-25.48151 + 0.36817* A + 2.33572* B - 4.19500E - 003* A2 -0.48867 * B2 + 0.074667* A * B	0.95
Solubility	+97.15430 - 1.28982 * A - 16.77514* B + 0.27283* A * B	0.96

Variables	Models	R²
Peak Viscosity (cP)	9606.12162 + 109.08483 * A + 1939.29540 * B - 0.55210 * A² - 128.42566 * B² - 3.51667 * A * B	0.83
Trough (cP)	10386.83667 + 173.50901 * A + 1517.49137 * B - 0.89355 * A² - 85.52770 * B² - 6.17778 * A * B	0.86
Breakdown (cP)	+2459.36943 - 40.11111 * A - 276.62279 * B + 0.21848 * A² + 14.96955 * B² + 1.07222 * A * B	0.91
Final Viscosity (cP)	-24626.67447 + 521.79102 * A + 2360.95725 * B - 3.32238 * A² - 140.14165 * B² - 8.16111 * A * B	0.85
Setback (cP)	-6248.41121 + 130.76265 * A + 630.02372 * B - 0.83004 * A² - 37.40926 * B² - 2.15000 * A * B	0.85
Peak Time (cP)	+4.95231	0.00
Peak temperature (cP)	-232.19116 + 8.66783 * A + 3.46946 * B - 0.050611 * A² + 1.35654 * B² - 0.27917 * A * B	0.70
T_o (°C)	+17.16533 + 1.55698E - 003 * A + 0.23223 * B	0.51
T_p (°C)	-27.65198 + 1.31291 * A + 2.45260 * B - 9.87500E - 003 * A^² - 0.27000 * B^² + 0.024333 * A * B	0.78
T_e (°C)	-868.40017 + 20.88008 * A + 57.32335 * B - 0.13998 * A^² - 2.86233 * B^² -0.14217 * A * B	0.94
∆H (J/g)	-2480.20368 + 54.85981 * A + 138.46192 * B - 0.37684 * A^² - 12.45606 * B^² + 0.69933 * A * B	0.88
Swelling	-14.55660 + 0.21230 * A + 0.67511 * B	0.87
Solubility	-18.18690 + 0.30663 * A + 0.78031 * B	0.95

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[1] *Corresponding author: pimoloto@gmail.com