Effects of high hydrostatic pressure and soaking solution on proximate composition,polyphenols,anthocyanins,β-carotene,and antioxidant activity of white,orange,and purple fleshed sweet potato flour

Abstract

Effects of high hydrostatic pressure (100, 200, and 400 MPa) and soaking solution (citric acid, calcium chloride, ascorbic acid, and distilled water) on proximate composition, polyphenols, anthocyanins, β-carotene, and antioxidant activity of white, orange, and purple fleshed sweet potato flour were investigated. Total polyphenol content was increased in sweet potato flour of Jishu 98 (white) at 200 MPa with ascorbic acid and Pushu 32 (orange) at 0.1 MPa with ascorbic acid treatment (0.51 and 0.83 mg gallic acid equivalent/g dry weight, respectively), but was decreased in Xuzishu No. 3 (purple) in both high hydrostatic pressure and soaking solution treatments. Total anthocyanin content was declined in all treated sweet potato flour. Nevertheless, high hydrostatic pressure with citric acid, calcium chloride, and distilled water significantly increased the β-carotene content in Pushu 32. Correlation analysis between total polyphenol content, total anthocyanin content, and antioxidant activity suggested that polyphenols are the most pivotal antioxidant in sweet potato flour. High hydrostatic pressure and soaking solution treated sweet potato flour could be potentially utilized in food with acceptable nutritional values.

Keywords

Sweet potato flour high hydrostatic pressure soaking solution polyphenols anthocyanins β-carotene antioxidant activity

Introduction

Most developing countries are facing the problem of increasing population and huge food supply within limited cultivated land. This increasing demand encouraged the scientists to consider new underutilized plant for human, especially the areas where staple crops are difficult to cultivate due to insufficient supply of water (Motsa et al., 2015). Sweet potato shows positive attributes, such as short production cycle, adaptability to marginal conditions, versatility in term of flesh texture, color, and taste (Bovell-Benjamin, 2007). China is the largest sweet potato grower that produced approximately 72 million metric tons (FAOSTAT, 2017). Four types of sweet potatoes with purple, yellow, orange, and white flesh are commonly grown in China, which reveals completely different chemical composition (Tang et al., 2015). Depending on the flesh color, sweet potato is rich in anthocyanins, β-carotene, polyphenols, dietary fiber, and minerals.

Sweet potato has been widely used in the food industry in different product formulation, particularly starchy and other allied products. The major processing problem of sweet potato is browning that resulted from enzymatic and non-enzymatic reactions. Non-enzymatic browning mainly occurred due to the reducing sugars and amino acids at high temperature during Maillard reaction, while enzymatic browning is attributed to the oxidation of phenols by polyphenol oxidases and peroxidases which are associated at low temperature (Graham-Acquaah et al., 2014). In order to overcome the problem of changes on physicochemical properties and nutritional quality which occur during the sweet potato flour (SPF) processing, some soaking solution has already been adopted, such as salt, citric acid, sodium metabisulfite, etc. as well as different dehydration methods, e.g. fluidized bed drying, solar drying, sun drying, and drum drying (Haile et al., 2015; Kuyu et al., 2018; Ruttarattanamongkol et al., 2016). Most of the previously used methods significantly affect the quality of SPF. Therefore, it becomes necessary to investigate appropriate processing techniques with minimal effect on quality degradation of SPF.

In recent years, high hydrostatic pressure (HHP), a non-thermal processing technology, has gained enormous attention in the food industry, due to that it could alter the structure of biopolymers, avoid color loss, inactivate endogenous enzymes, etc., and there were no harmful effects on the foods (Liu et al., 2013; Peng et al., 2016; Stolt et al., 2001). HHP has been already utilized for the production of numerous commercial foods, such as fish, seafood, cooked meats, fruit juices, and vegetables (Rastogi et al., 2007). Some studies have demonstrated that HHP could increase the antioxidant activity in purple waxy corn kernels (Saikaew et al., 2017), smoothies (Andrés et al., 2016), strawberry and blackberry purées (Patras et al., 2009b), sweet potato nectar (Wang et al., 2012), tomato and carrot purées (Patras et al., 2009a). Similarly, numerous researches have been conducted to elucidate that the HHP could improve the nutritional composition in Prosopis chilensis seed (Briones-Labarca et al., 2011) and Cape gooseberry pulp (Vega-Gálvez et al., 2014). However the effectiveness of HHP application depends upon the processing condition (pressure, time, temperature, and pH) and particular food form (complete, small pieces, juices). However, limited information about the effects of HHP combination with different soaking solutions on the nutritional composition and antioxidant activity of SPF is currently available.

In the present study, the effects of HHP (100, 200 and 400MPa) and soaking solutions (citric acid, calcium chloride, ascorbic acid) on proximate composition, total polyphenol content (TPC), anthocyanins, β-carotene and antioxidant activity of SPF were investigated, thus to provide useful information on effective utilization of HHP and its combination with a soaking solution in the quality enhancement of SPF.

Materials and methods

Materials

Sweet potato with varieties of Pushu 32 (orange) and Jishu 98 (white) were supplied by Institute of Grain and Oil Crops, Hebei Academy of Agricultural and Forestry Sciences (Hebei, China). Sweet potato with variety of Xuzishu No. 3 (purple) was supplied by Baoji Academy of Agricultural Sciences (Shanxi, China). The citric acid, calcium chloride, ascorbic acid, petroleum ether, sulfuric acid, potassium sulfate, sodium carbonate, ethanol, sodium acetate, potassium chloride, acetone, sodium acetate, ferric chloride, and hydrochloric acid were analytical grade and were purchased from Chinese Pharmaceutical Group (Beijing, China). Folin–Ciocalteu reagent, gallic acid, β-carotene, methanol–acetonitrile, 2,2-diphenyl-1-picrylhydrazyl, l-ascorbic acid, 2,4,6-tripyridyl-s-triazine, 6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid, and 2,2′-azinobis(3-ethylbenzothiazolin-6-sulfonate) were HPLC grade and were purchased from Sigma-Aldrich, Inc. (St Louis, MO, USA).

HHP treatment with soaking solution

Sweet potatoes were washed with tap water to remove soil and dirt, peeled, and cut into slices. Fresh sweet potato slices (250 g) were packed in a vacuum sealed flexible Nylon/LLDPE pouch (16 cm diameter and 25 cm length). HHP was performed using HHP machine (model HHP.L3-600/0.6; Huatai Senmiao Engr. & Tech. Ltd Co., Tianjin, China). The water was utilized for the temperature control as well as pressure transfer medium. The vacuum-packed sample was subjected to 100, 200, and 400 MPa for 6 min at 25 ℃, and the compression rate was 5 MPa/s. Subsequently, these slices were dipped into distilled water, citric acid (1%, w/v), calcium chloride (1%, w/v), and ascorbic acid (1%, w/v) for 15 min at room temperature, respectively, drained, and then dried in hot air oven (LJ-101, Guangdong LIK Industry Co., Ltd, Guangdong, China) at 60 ℃. The dried slices were milled using a blender (GRT-06B, Yongkang Tiange Electric Co., Ltd, Zhejiang, China) and sieved through a 0.45 mm sieve mesh size for further use.

Proximate composition

Moisture (method 925.09), crude fiber (method 991.43), crude fat (method 920.39) and ash (method 942.05), and crude protein (method 955.04) contents were checked according to AOAC official methods (2000). Starch contents were analyzed using Megazyme Assay Kits (Megazyme Int., Wicklow, Ireland). Minerals analysis was performed by the procedure previously described by Sun et al. (2011). Carbohydrate contents were calculated according to the following equation = 100 − (crude protein + crude fiber + crude fat + ash).

TPC

TPC was analyzed using the Folin–Ciocalteu method reported by Sun et al. (2014a). Extraction of SPF (1 g) was carried out with 20 ml of 70% ethanol subjected to ultrasonic (SK2200H, Shanghai Kudos Ultrasonic Instrument Co., Ltd, Shanghai, China) wave treatment for 25 min at 50 ℃. Afterward, all samples were centrifuged (GL-21M, Shanghai Lu Xiangyi centrifuge instrument Co., Ltd, Shanghai, China) at 5000 × g for 10 min to collect the supernatant. Then the above procedure was repeated twice. Sample extract 0.5 ml (10-fold diluted) and 1.0 ml of Folin–Ciocalteu reagent were mixed and kept in dark place for 25 min. Subsequently, 2.0 ml of Na₂CO₃ (10%, w/v) was mixed and kept again for 25 min. Absorbance was monitored at 736 nm in a UV–Vis spectrophotometer (Hitachi, Japan). A gallic acid equivalent (GAE) calibration curve was used for TPC quantification. TPC was presented as milligram GAE equivalents per gram on a dry weight (mg GAE/g DW).

Total anthocyanin content (TAC)

TAC was quantified using a spectrophotometric pH-differential method illustrated by Lee et al. (2005). Briefly, 1 g SPF was extracted with 15 ml of 90% (v/v) ethanol through ultrasonic wave treatment for 1 h at 50 ℃. Subsequently, the sample was centrifuged at 7000 × g for 10 min to collect the supernatant. An aliquot of 0.5 ml extract was diluted with 10 ml of 0.4 M sodium acetate buffer (pH 4.5) and 10 ml of 0.025 M KCl buffer (pH 1.0), respectively. Absorbance of each sample was monitored at 520 and 700 nm, respectively, in a UV–Vis spectrophotometer (Hitachi, Japan). TAC was presented as cyanidin-3-glucoside equivalents (mg/g DW).

β-carotene content

β-carotene was analyzed according to the method illustrated by Trancoso-Reyes et al. (2016) and performed by using HPLC system (Shimadzu, Kyoto, Japan) with column C18 (150 mm × 4.6 mm; 5 µm particle size) and UV detector (SPD-10A). The mobile phase was methanol–acetonitrile (90:10, v/v) with a flow rate of 1 ml/min at 25 ℃. The injection volume was 20 µl and the detection was accomplished at 450 nm. The quantification was carried out using β-carotene standard curve created from different concentrations. The results were presented as microgram of β-carotene per gram of sample on a dry weight (µg/g DW).

Antioxidant capacity

DPPH radical scavenging activity. DPPH radical scavenging activity of SPF was analyzed according to the method elaborated by Brand-Williams et al. (1995) with minor modifications. The 0.1 mmol/ DPPH stock solution was prepared in 70% ethanol, and the working solution was obtained by diluting the stock solution with ethanol to achieve an absorbance of 0.70 ± 0.02 using the spectrophotometer. The extract that was used for TPC (150 µl) and 3.0 ml of working solution was mixed and kept in dark place for 25 min. Absorbance was monitored at 517 nm through a UV–Vis spectrophotometer (Hitachi, Japan). The results were presented as milligram ascorbic acid equivalents per gram of sample on a dry weight (mg AAE/g DW).

Ferric reducing antioxidant power (FRAP). FRAP assay was carried out using the procedure illustrated by Benzie and Strain (1996) with minor modifications. The working solution was obtained by mixing 25 ml of 300 mM acetate buffer, 2.5 ml of 20 mM FeCl₃·6H₂O, and 2.5 ml of 10 mM TPTZ (in 40 mM HCl) solution, and then warmed at 37 ℃ for 30 min prior to analysis. The extract that was used for TPC (150 µl) and 3 ml of working solution was mixed, and left in the dark place for 30 min. Absorbance was monitored at 593 nm, and results were presented as microgram Trolox equivalents per gram of sample on a dry weight (µg TE/g DW).

ABTS assay. ABTS assay was carried out according to the procedure ascribed by Arnao et al. (2001) with minor modification. A stock solution of 7 mM ABTS was mixed with 2.45 mM potassium sulfate and kept in the dark place at 4 ℃ for 24 h before the analysis. The stock solution was diluted with ethanol to attain the absorbance 1.0 ± 0.02 at 734 nm. An aliquot of extract that was used for TPF (150 µl) was mixed with 3.0 ml of working solution and left for 25 min. Absorbance was monitored at 734 nm in a UV–Vis spectrophotometer (Hitachi, Japan). Results were presented as mg AAE/g DW.

Statistical analysis

All treatments were carried out in triplicate. Results were presented as mean ± standard deviation. Data were analyzed through SAS (v. 5.1 SAS Institute Inc., Cary, NC, USA). P < 0.05 was declared as a statistical significance. Pearson correlation coefficient was applied to investigate the relationship between total polyphenol, anthocyanins, and antioxidant activities.

Results and discussion

Changes in proximate composition

Proximate composition of SPF as affected by HHP and soaking solution is shown in Table 1. Carbohydrate contents in Jishu 98 and Xuzishu No. 3 increased significantly with a successive application of HHP and soaking solution, while that in Pushu 32 showed an increase at 200–400 MPa with a soaking solution of distilled water (P < 0.05). Similarly, carbohydrate contents of Cape gooseberry pulp were also increased after HHP treatment (Vega-Gálvez et al., 2014).

Table 1.

Proximate composition in Jishu 98, Pushu 32, and Xuzishu No. 3 sweet potato flour influenced by high hydrostatic pressure (HHP) and soaking solution (%, DW).

Variety	HHP (MPa)	Soaking solution	Carbohydrate	Starch	Crude protein	Crude fiber	Crude fat	Ash
Jishu 98	0.1	Untreated	76.09 ± 0.49m	65.31 ± 0.32i	8.25 ± 0.05b	12.59 ± 0.01b	0.61 ± 0.05h	2.44 ± 0.05e
		Citric acid	76.09 ± 0.22m	74.24 ± 0.80gh	7.75 ± 0.18d	13.45 ± 0.22a	0.73 ± 0.03e	2.13 ± 0.06h
		Cal. chloride	74.72 ± 0.43o	77.23 ± 0.74defg	7.99 ± 0.02c	13.75 ± 0.12a	0.81 ± 0.04c	2.83 ± 0.01a
		Ascorbic acid	74.55 ± 1.66p	73.96 ± 0.32gh	8.75 ± 0.17a	13.42 ± 0.15a	0.67 ± 0.06f	2.59 ± 0.02c
		Distilled water	75.84 ± 0.67n	72.61 ± 1.18gh	7.45 ± 0.03e	13.18 ± 0.24ab	0.74 ± 0.02de	2.77 ± 0.07b
	100	Citric acid	80.61 ± 0.89k	74.45 ± 1.87fgh	6.42 ± 0.04f	10.65 ± 0.27de	0.65 ± 0.08fg	1.68 ± 0.03k
		Cal. chloride	81.25 ± 0.29i	71.45 ± 0.78h	5.27 ± 0.04h	10.67 ± 0.87de	0.63 ± 0.01gh	2.16 ± 0.01g
		Ascorbic acid	80.72 ± 1.51j	77.30 ± 1.44defg	4.32 ± 0.06kl	11.53 ± 0.15c	0.93 ± 0.05a	2.52 ± 0.01d
		Distilled water	80.39 ± 0.98l	76.06 ± 1.21efgh	5.96 ± 0.02g	11.06 ± 0.84cd	0.88 ± 0.03b	1.69 ± 0.02k
	200	Citric acid	84.11 ± 0.58b	80.90 ± 0.93bcde	4.83 ± 0.03j	8.95 ± 0.36hi	0.75 ± 0.01d	1.34 ± 0.09n
		Cal. chloride	83.06 ± 0.12f	86.61 ± 1.12a	5.25 ± 0.01h	9.22 ± 0.25gh	0.56 ± 0.09i	1.90 ± 0.06i
		Ascorbic acid	82.61 ± 0.73h	80.66 ± 0.30bcde	4.03 ± 0.01m	10.54 ± 0.12def	0.42 ± 0.02j	2.38 ± 0.01f
		Distilled water	83.69 ± 0.38d	79.57 ± 0.54cdef	4.21 ± 0.06l	10.02 ± 0.16ef	0.42 ± 0.02j	1.64 ± 0.01l
	400	Citric acid	85.42 ± 0.84a	81.68 ± 1.14abcd	4.39 ± 0.03k	8.49 ± 0.19hi	0.65 ± 0.03fg	1.03 ± 0.05o
		Cal. chloride	83.39 ± 0.66e	85.83 ± 1.16ab	5.01 ± 0.01i	9.09 ± 0.15h	0.55 ± 0.06i	1.68 ± 0.03k
		Ascorbic acid	82.72 ± 0.08g	85.27 ± 0.19ab	5.33 ± 0.09h	9.86 ± 0.17fg	0.27 ± 0.05k	1.84 ± 0.014j
		Distilled water	84.05 ± 0.28c	83.14 ± 0.04abc	3.91 ± 0.04m	8.26 ± 0.201i	0.24 ± 0.07l	1.56 ± 0.06m
Pushu 32	0.1	Untreated	78.66 ± 0.30c	45.76 ± 0.63def	5.44 ± 0.05i	11.69 ± 0.21cd	1.10 ± 0.01cd	3.09 ± 0.02cd
		Citric acid	74.05 ± 1.57n	40.40 ± 1.03g	6.40 ± 0.03fgh	15.46 ± 0.23a	1.19 ± 0.05b	2.88 ± 0.11cdefg
		Cal. chloride	71.47 ± 0.48p	44.38 ± 0.04ef	7.10 ± 0.01c	16.20 ± 0.49a	1.29 ± 0.04a	3.92 ± 0.02a
		Ascorbic acid	75.95 ± 2.81k	45.37 ± 0.20def	6.32 ± 0.14gh	13.43 ± 0.02b	1.13 ± 0.01c	3.15 ± 0.06c
		Distilled water	76.02 ± 1.36j	43.40 ± 0.37fg	6.16 ± 0.14h	13.64 ± 0.81b	1.07 ± 0.03de	3.13 ± 0.08c
	100	Citric acid	76.51 ± 2.14i	50.09 ± 1.87bc	7.59 ± 0.25b	11.84 ± 0.53cd	1.09 ± 0.02cde	2.96 ± 0.27cdef
		Cal. chloride	73.55 ± 0.28o	47.99 ± 1.51bcde	8.21 ± 0.28a	13.52 ± 0.11b	1.10 ± 0.01cd	3.60 ± 0.03b
		Ascorbic acid	75.59 ± 1.7l	44.68 ± 0.23ef	8.14 ± 0.20a	12.01 ± 0.18cd	1.13 ± 0.01c	3.11 ± 0.06cd
		Distilled water	77.53 ± 2.03f	46.56 ± 0.25cdef	6.74 ± 0.04def	11.50 ± 0.19cd	1.11 ± 0.02cd	3.10 ± 0.01cd
	200	Citric acid	77.19 ± 0.53g	48.39 ± 1.74bcde	7.55 ± 0.13b	11.10 ± 0.92cde	1.02 ± 0.01fg	3.12 ± 0.10cd
		Cal. chloride	76.67 ± 0.63h	51.06 ± 0.55ab	7.09 ± 0.04cd	11.61 ± 0.73cd	1.05 ± 0.03ef	3.56 ± 0.03b
		Ascorbic acid	75.40 ± 0.73m	54.23 ± 0.13a	7.04 ± 0.14cd	13.55 ± 0.61b	0.95 ± 0.01hi	3.04 ± 0.03cde
		Distilled water	79.01 ± 0.73b	53.98 ± 0.61a	6.55 ± 0.23fg	10.63 ± 0.01de	0.99 ± 0.01gh	2.81 ± 0.04efg
	400	Citric acid	78.21 ± 0.08d	45.09 ± 2.81def	6.52 ± 0.02fg	11.47 ± 0.35cd	0.93 ± 0.01i	2.85 ± 0.01defg
		Cal. chloride	77.52 ± 0.18f	49.05 ± 0.26bcd	6.99 ± 0.14cde	11.01 ± 0.85cde	0.94 ± 0.03i	3.52 ± 0.04b
		Ascorbic acid	77.63 ± 1.03e	47.97 ± 2.10bcde	6.69 ± 0.26ef	12.26 ± 0.35bc	0.80 ± 0.02j	2.65 ± 0.32g
		Distilled water	79.93 ± 0.73a	47.75 ± 1.78bcde	6.55 ± 0.05fg	9.83 ± 0.71e	0.97 ± 0.02hi	2.70 ± 0.05fg
Xuzishu No. 3	0.1	Untreated	73.11 ± 0.22p	57.23 ± 0.48cdef	6.61 ± 0.03ab	16.73 ± 0.41abc	0.67 ± 0.015c	2.86 ± 0.07cdef
		Citric acid	73.33 ± 0.16o	56.85 ± 0.64cdef	5.46 ± 0.03hi	17.68 ± 0.97ab	0.73 ± 0.07b	2.78 ± 0.01cdef
		Cal. chloride	71.89 ± 0.09q	59.20 ± 1.20bc	5.93 ± 0.13de	18.24 ± 0.76a	0.74 ± 0.08ab	3.18 ± 0.03bc
		Ascorbic acid	75.25 ± 0.32k	58.85 ± 0.62bcd	5.77 ± 0.11ef	15.57 ± 0.65bcd	0.76 ± 0.01a	2.83 ± 0.01cdef
		Distilled water	76.75 ± 0.17a	57.29 ± 0.94cdef	5.31 ± 0.06i	14.23 ± 0.06cde	0.56 ± 0.02fgh	2.93 ± 0.04cde
	100	Citric acid	75.94 ± 0.31g	59.38 ± 0.34bc	6.80 ± 0.06a	14.25 ± 0.36cde	0.41 ± 0.09i	2.58 ± 0.01efg
		Cal. chloride	73.9 ± 0.13n	55.37 ± 1.08ef	5.47 ± 0.07hi	16.18 ± 0.66abc	0.74 ± 0.02ab	3.69 ± 0.08a
		Ascorbic acid	76.29 ± 0.05c	61.30 ± 1.85ab	5.11 ± 0.01j	15.11 ± 0.7bcde	0.60 ± 0.08d	2.88 ± 0.05cdef
		Distilled water	74.86 ± 0.15m	57.25 ± 0.55cdef	6.45 ± 0.09b	13.04 ± 0.20de	0.56 ± 0.08h	3.00 ± 0.01bcd
	200	Citric acid	76.11 ± 0.09e	55.72 ± 1.3def	5.67 ± 0.08fgh	14.86 ± 0.59cde	0.58 ± 0.09ef	2.51 ± 0.01fg
		Cal. chloride	75.40 ± 0.33j	55.47 ± 1.42ef	5.54 ± 0.02gh	14.89 ± 0.39cde	0.58 ± 0.02efg	3.35 ± 0.64ab
		Ascorbic acid	76.62 ± 0.43b	62.42 ± 0.48a	4.79 ± 0.03k	14.59 ± 0.08cde	0.60 ± 0.001de	2.86 ± 0.08cdef
		Distilled water	75.18 ± 0.21l	58.10 ± 0.41cde	6.17 ± 0.22c	12.52 ± 0.66e	0.57 ± 0.06fgh	2.95 ± 0.13cde
	400	Citric acid	76.05 ± 0.11f	54.92 ± 1.40f	5.62 ± 0.1fgh	14.85 ± 0.06cde	0.33 ± 0.09j	2.87 ± 0.05cdef
		Cal. chloride	75.82 ± 0.09h	56.44 ± 0.79cdef	5.94 ± 0.11de	14.62 ± 0.22cde	0.56 ± 0.02gh	2.59 ± 0.05efg
		Ascorbic acid	76.20 ± 0.10d	63.13 ± 0.90a	5.72 ± 0.02fg	13.42 ± 0.36de	0.58 ± 0.02ef	2.37 ± 0.10g
		Distilled water	75.76 ± 0.13i	56.48 ± 0.67cdef	6.02 ± 0.11cd	12.47 ± 0.82e	0.39 ± 0.03i	2.70 ± 0.04defg

Different letters in the same column for each specific variety indicate significant differences (P < 0.05).

Starch content of untreated Jishu 98 was 65.31 g/100 g DW, which exhibited a slight decrease with soaking solution of calcium chloride, ascorbic acid, and distilled water, but was significantly increased after HHP and soaking solution treatment (P < 0.05). In the case of Pushu 32, starch content of untreated SPF was 45.76 g/100 g DW, which presented a high value of 54.23 g/100 g DW at 200 MPa with ascorbic acid. For Xuzishu No. 3, starch content of untreated SPF was 57.23 g/100 g DW, which increased up to 63.13 g/100 g DW at 400 MPa with ascorbic acid. The results range obtained in this study exhibited resemblances with the findings of Olatunde et al. (2016). Briones-Labarca et al. (2011) also found that starch in Granny Smith apple was increased corresponding to progressive pressure.

Crude protein content in Jishu 98 decreased significantly compared to the untreated one, except that at 0.1 MPa with ascorbic acid (P < 0.05). Crude protein content in Pushu 32 after HHP and soaking solution treatment increased significantly compared to the untreated one, while those in Xuzishu No. 3 decreased except that at 100 MPa with citric acid. The decrease in crude protein contents might be attributed to an increase in moisture content that exhibited dilution effect on other components of HHP treated samples (Vega-Gálvez et al., 2014).

Crude fiber content in Jishu 98, Pushu 32, and Xuzishu No. 3 increased by the application of four kinds of soaking solutions at 0.1 MPa, while exhibited a decreasing trend after HHP treatment with soaking solutions compared to the untreated ones (freeze-dried SPF) (P < 0.05, Table 1). It might be due to that HHP solubilized some polysaccharides, such as hemicellulose and pectin (Wennberg and Nyman, 2004). As observed from the above-mentioned parameters, the proximate composition in all these three varieties exhibited a non-linear trend which might be attributed to different sensitivity of each nutritional constituent corresponding to various treatment applications. Stability and instability of the bioactive compound varying might be due to distinct molecular structure changes induced by chemical and enzymatic reaction corresponding to different treatment (Mahadevan and Karwe, 2016).

Crude fat contents in Jishu 98 and Pushu 32 were increased at 100 MPa but decreased at 200 and 400 MPa as compared to the untreated ones, except that in Pushu 32 after 0.1 MPa with distilled water and 100 MPa with citric acid treatment. For Xuzishu No. 3, treatments of 0.1 MPa with citric acid, 0.1 MPa with calcium chloride, 0.1 MPa with ascorbic acid, and 100 MPa with calcium chloride increased crude fat content as compared to the untreated ones, while other treatments decreased it. Crude fat contents range obtained in this study was in coincidence with the previous findings (Kuyu et al., 2018). Ash content in Jishu 98, Pushu 32, and Xuzishu No. 3 declared irregular changes after HHP treatment with soaking solution when compared to the untreated ones (Table 1). Vega-Gálvez et al. (2014) indicated that ash content in Cape gooseberry pulp decreased after HHP treatment. In addition, ash was found more predominant in calcium chloride soaking samples (Table 1). Current outcomes were in agreement with the findings reported by Osim et al. (2010). Lyimo et al. (2010) reported that loss of nutrient composition in sweet potato during processing depended on varieties, as some varieties are more tolerant to certain processing techniques than others.

Changes in mineral contents

Mineral contents of SPF as influenced by HHP and soaking solution are shown in Table 2. K was the most abundant macroelement that was varied between 2.33 and 15.65 mg/g DW. K contents in Jishu 98 were increased by the application of calcium chloride, ascorbic acid, and distilled water at 0.1 MPa as compared to the untreated one, which was drastically decreased after HHP with soaking solution treatment (P < 0.05). In Pushu 32, K values were increased by ascorbic acid treatment at 0.1 MPa, as well as at 200 MPa with citric acid and ascorbic acid. In the case of Xuzishu No. 3, K contents were increased with ascorbic acid and distilled water treatment at 0.1 MPa as compared to the untreated one, as well as at 100 MPa with calcium chloride, ascorbic acid, and distilled water; 200 MPa with calcium chloride and ascorbic acid; 400 MPa with citric acid and distilled water, respectively. Ca contents in three varieties varied between 0.51 and 4.89 mg/g DW that was observed more predominant in the samples treated with calcium chloride. Na, Mg, P, Mn, and Zn contents varied from 0.011 to 2.41, 0.38 to 1.46 and 0.64 to 2.28 mg/g, 0.11 to 6.92 and 0.43 to 1.00 mg/100g DW, respectively. Olatunde et al. (2016) evaluated the influence of different pretreatment, variety, and drying methods on SPF, and found similar changes. Mineral contents show an unclear trend of increasing and decreasing by the application of HHP in the current study. Torres-Ossandón et al. (2015) indicated that pressurization significantly increased the minerals with the non-systematic trend. The finding in this study indicated that sweet potato is a good source of minerals, particularly K, Ca, Na, Mg, P, Mn, and Zn.

Table 2.

Minerals contents in Jishu 98, Pushu 32, and Xuzishu No. 3 sweet potato flour influenced by high hydrostatic pressure (HHP) and soaking solution (mg/g, DW).

Variety	HHP (MPa)	Soaking solution	K	Ca	Na	Mg	P	Mn*	Zn*
Jishu 98	0.1	Untreated	8.30 ± 0.11d	1.05 ± 0.023e	1.50 ± 0.021b	1.45 ± 0.024a	1.77 ± 0.027b	0.39 ± 0.019a	0.72 ± 0.027a
		Citric acid	7.90 ± 0.17e	0.75 ± 0.012gh	1.19 ± 0.025e	1.10 ± 0.015d	1.63 ± 0.028c	0.26 ± 0.003b	0.67 ± 0.02b
		Cal. chloride	8.55 ± 0.09c	1.13 ± 0.002d	1.75 ± 0.018a	1.23 ± 0.008c	1.58 ± 0.01d	0.35 ± 0.003a	0.74 ± 0.06a
		Ascorbic acid	9.58 ± 0.14a	0.95 ± 0.02f	1.42 ± 0.018c	1.40 ± 0.019b	1.87 ± 0.027a	0.25 ± 0.003b	0.66 ± 0.038b
		Distilled water	8.96 ± 0.03b	0.51 ± 0.003l	1.39 ± 0.004d	1.44 ± 0.001a	1.78 ± 0.004b	0.35 ± 0.003a	0.66 ± 0.027b
	100	Citric acid	3.69 ± 0.03k	0.72 ± 0.007hi	0.60 ± 0.004j	0.47 ± 0.001i	0.89 ± 0.001i	0.12 ± 0.001e	0.51 ± 0.006gh
		Cal. chloride	4.20 ± 0.04j	2.51 ± 0.011c	0.61 ± 0.01j	0.44 ± 0.005j	0.92 ± 0.011h	0.13 ± 0.001e	0.65 ± 0.009bc
		Ascorbic acid	2.89 ± 0.10n	0.71 ± 0.018i	0.58 ± 0.022k	0.58 ± 0.022g	0.73 ± 0.026l	0.14 ± 0.005e	0.58 ± 0.014de
		Distilled water	5.16 ± 0.03h	0.71 ± 0.018i	1.19 ± 0.011e	0.84 ± 0.006e	1.33 ± 0.007e	0.24 ± 0.038bc	0.57 ± 0.037def
	200	Citric acid	4.55 ± 0.03i	0.61 ± 0.024jk	0.83 ± 0.003g	0.63 ± 0.003f	1.04 ± 0.002g	0.14 ± 0.004de	0.47 ± 0.005hi
		Cal. chloride	2.52 ± 0.01o	4.59 ± 0.028a	0.49 ± 0.001m	0.40 ± 0.001k	0.74 ± 0.001l	0.12 ± 0.002e	0.47 ± 0.015hi
		Ascorbic acid	2.33 ± 0.01p	0.78 ± 0.013g	0.52 ± 0.002l	0.46 ± 0.002ij	0.64 ± 0.001m	0.21 ± 0.16bcd	0.72 ± 0.025a
		Distilled water	5.78 ± 0.06g	0.60 ± 0.009k	0.95 ± 0.007f	0.64 ± 0.005f	1.19 ± 0.007f	0.17 ± 0.02cde	0.51 ± 0.009gh
	400	Citric acid	3.52 ± 0.02l	0.64 ± 0.009j	0.65 ± 0.004i	0.45 ± 0.003j	0.93 ± 0.004h	0.11 ± 0.006e	0.55 ± 0.029efg
		Cal. chloride	3.04 ± 0.03m	4.18 ± 0.046b	0.55 ± 0.006l	0.38 ± 0.003l	0.81 ± 0.003j	0.11 ± 0.002e	0.52 ± 0.014fg
		Ascorbic acid	2.87 ± 0.12n	0.77 ± 0.031g	0.54 ± 0.022l	0.55 ± 0.024h	0.78 ± 0.029k	0.13 ± 0.012e	0.61 ± 0.035cd
		Distilled water	5.92 ± 0.01f	0.51 ± 0.015l	0.71 ± 0.006h	0.45 ± 0.015j	1.05 ± 0.015g	0.12 ± 0.01e	0.43 ± 0.027i
Pushu 32	0.1	Untreated	9.05 ± 0.18cd	1.64 ± 0.01fg	2.32 ± 0.04b	1.13 ± 0.02cd	1.89 ± 0.04f	4.05 ± 0.08c	0.87 ± 0.01de
		Citric acid	8.07 ± 0.07f	1.71 ± 0.03f	2.12 ± 0.01d	1.15 ± 0.01c	1.97 ± 0.01e	3.09 ± 0.01g	0.91 ± 0.01cd
		Cal. chloride	9.24 ± 0.3bc	4.89 ± 0.13a	2.08 ± 0.07d	1.46 ± 0.05a	2.28 ± 0.07a	6.92 ± 0.26a	0.95 ± 0.03abc
		Ascorbic acid	10.18 ± 0.08a	1.65 ± 0.01fg	2.12 ± 0.04d	1.15 ± 0.02c	2.00 ± 0.03de	4.09 ± 0.05c	0.98 ± 0.03a
		Distilled water	8.80 ± 0.12d	1.91 ± 0.02cde	2.32 ± 0.05b	1.25 ± 0.01b	2.10 ± 0.05b	4.30 ± 0.08b	1.00 ± 0.04a
	100	Citric acid	8.17 ± 0.26ef	1.61 ± 0.07fg	2.41 ± 0.08a	1.10 ± 0.03de	2.06 ± 0.07bcd	2.15 ± 0.07j	0.98 ± 0.05a
		Cal. chloride	8.86 ± 0.23d	4.36 ± 0.13b	2.27 ± 0.07bc	1.05 ± 0.03fg	2.01 ± 0.06cde	2.13 ± 0.06j	0.99 ± 0.01a
		Ascorbic acid	9.00 ± 0.08cd	2.01 ± 0.03c	2.29 ± 0.01b	1.25 ± 0.01b	2.07 ± 0.01bc	3.37 ± 0.02f	0.97 ± 0.05ab
		Distilled water	8.94 ± 0.04d	1.68 ± 0.01fg	2.31 ± 0.02b	1.09 ± 0.01de	1.96 ± 0.02e	2.43 ± 0.02i	0.90 ± 0.01cd
	200	Citric acid	9.51 ± 0.09b	1.45 ± 0.02h	1.43 ± 0.02h	0.93 ± 0.02h	1.81 ± 0.02gh	3.12 ± 0.04g	0.83 ± 0.04e
		Cal. chloride	9.08 ± 0.06cd	4.88 ± 0.1a	1.65 ± 0.01g	1.03 ± 0.01g	1.85 ± 0.01fg	3.06 ± 0.02g	0.94 ± 0.03abc
		Ascorbic acid	9.42 ± 0.05b	1.73 ± 0.04f	1.63 ± 0.01g	1.04 ± 0.02fg	1.89 ± 0.02f	3.66 ± 0.03e	0.95 ± 0.01abc
		Distilled water	8.42 ± 0.33e	1.87 ± 0.07de	1.72 ± 0.07f	1.09 ± 0.04de	1.72 ± 0.08ij	3.89 ± 0.07d	0.91 ± 0.03bcd
	400	Citric acid	6.97 ± 0.04g	1.58 ± 0.01g	2.01 ± 0.01e	0.92 ± 0.01h	1.67 ± 0.01j	2.88 ± 0.02h	0.83 ± 0.01e
		Cal. chloride	6.69 ± 0.08h	4.82 ± 0.06a	1.94 ± 0.03e	0.87 ± 0.01i	1.70 ± 0.02j	2.48 ± 0.02i	0.84 ± 0.01de
		Ascorbic acid	6.58 ± 0.04h	1.84 ± 0.02e	1.97 ± 0.02e	1.06 ± 0.1efg	1.65 ± 0.02j	3.36 ± 0.02f	0.98 ± 0.09a
		Distilled water	6.68 ± 0.09h	1.97 ± 0.05cd	2.21 ± 0.01c	1.07 ± 0.01ef	1.78 ± 0.02hi	3.15 ± 0.04g	0.86 ± 0.02de
Xuzishu No. 3	0.1	Untreated	11.79 ± 0.12e	1.36 ± 0.01h	0.024 ± 0.003a	1.15 ± 0.015c	1.05 ± 0.012f	1.96 ± 0.03d	0.72 ± 0.02bcd
		Citric acid	11.96 ± 0.08e	1.23 ± 0.01i	0.011 ± 0.002i	1.18 ± 0.005b	0.84 ± 0.004j	2.64 ± 0.01b	0.74 ± 0.055bc
		Cal. chloride	12.08 ± 0.22e	2.91 ± 0.04d	0.011 ± 0.002hi	1.18 ± 0.008b	0.94 ± 0.017i	3.29 ± 0.04a	0.88 ± 0.013a
		Ascorbic acid	12.40 ± 0.31d	1.17 ± 0.02ij	0.012 ± 0.0ghi	1.11 ± 0.02d	0.97 ± 0.024h	3.25 ± 0.08a	0.84 ± 0.102a
		Distilled water	12.64 ± 0.25d	1.22 ± 0.02i	0.013 ± 0.0efg	1.16 ± 0.01bc	0.93 ± 0.014i	2.12 ± 0.03c	0.75 ± 0.043bc
	100	Citric acid	11.26 ± 0.04f	1.37 ± 0.01h	0.011 ± 0.002hi	1.05 ± 0.005e	1.12 ± 0.001e	1.48 ± 0.01h	0.67 ± 0.008de
		Cal. chloride	13.30 ± 0.11c	4.30 ± 0.04a	0.013 ± 0.03fgh	1.29 ± 0.013a	0.81 ± 0.009k	1.84 ± 0.02f	0.66 ± 0.005def
		Ascorbic acid	13.42 ± 0.05c	0.82 ± 0.01k	0.02 ± 0.002b	0.87 ± 0.03gh	1.01 ± 0.007g	1.29 ± 0.01jk	0.65 ± 0.014ef
		Distilled water	14.26 ± 0.14b	1.10 ± 0.03j	0.015 ± 0.02def	0.85 ± 0.008h	1.13 ± 0.009e	1.31 ± 0.01j	0.66 ± 0.015def
	200	Citric acid	10.31 ± 0.22g	1.74 ± 0.01e	0.015 ± 0.01cde	1.16 ± 0.01bc	1.25 ± 0.027c	1.36 ± 0.02i	0.69 ± 0.012cde
		Cal. chloride	15.65 ± 0.09a	3.43 ± 0.04c	0.012 ± 0.02ghi	0.87 ± 0.05gh	0.92 ± 0.001i	1.12 ± 0.06l	0.68 ± 0.012cde
		Ascorbic acid	12.72 ± 0.05d	1.48 ± 0.03g	0.012 ± 0.06ghi	0.85 ± 0.007h	1.73 ± 0.01a	0.87 ± 0.01m	0.75 ± 0.001bc
		Distilled water	11.41 ± 0.25f	1.64 ± 0.02f	0.014 ± 0.06def	1.30 ± 0.022a	0.70 ± 0.012l	1.91 ± 0.03e	0.60 ± 0.016f
	400	Citric acid	12.59 ± 0.18d	1.39 ± 0.04h	0.011 ± 0.05ghi	1.09 ± 0.021d	1.28 ± 0.019b	1.25 ± 0.02k	0.65 ± 0.006ef
		Cal. chloride	8.37 ± 0.17i	3.92 ± 0.14b	0.011 ± 0.006hi	0.89 ± 0.021g	1.25 ± 0.032c	1.32 ± 0.03ij	0.72 ± 0.02bcd
		Ascorbic acid	9.13 ± 0.25h	1.75 ± 0.03e	0.017 ± 0.002c	1.12 ± 0.034d	1.22 ± 0.034d	1.64 ± 0.05g	0.77 ± 0.051b
		Distilled water	12.68 ± 0.15d	1.43 ± 0.02gh	0.016 ± 0.003cd	0.98 ± 0.011f	1.21 ± 0.011d	1.66 ± 0.01g	0.71 ± 0.03bcde

Different letters in the same column for each specific variety indicate significant differences (P < 0.05). *Mn and Zn values were presented as mg/100g.

TPC

TPC of SPF as influenced by HHP and soaking solution is summarized in Table 3. TPC in untreated Jishu 98, Pushu 32, and Xuzishu No. 3 was 0.35, 0.19, and 1.98 mg GAE/g, respectively (Table 3). HHP and citric acid, calcium chloride, and distilled water treatment significantly decreased TPC in Jishu 98, while that with ascorbic acid increased it (P < 0.05). TPC in Pushu 32 increased by the application of citric acid, calcium chloride, and ascorbic acid at 0.1 MPa, which were also increased by HHP with different soaking solution, except at 100 MPa with citric acid and 400 MPa with citric acid and calcium chloride as compared to the untreated one (P < 0.05). While in Xuzishu No. 3, TPC was reduced significantly in all samples as compared to untreated ones. Barba et al. (2012) reported that TPC of orange juice-milk beverage was significantly increased at 100 MPa, whereas a non-significant decrease was presented at 400 MPa. However, ascorbic acid addition resulted in a positive effect with preservation of TPC after processing in this study. The possible reason for the loss of TPC after HHP treatment might be attributed to the oxidation and polymerization of phenolic compounds with protein (Zhao et al., 2016).

Table 3.

Total polyphenol, anthocyanins in Jishu 98, Pushu 32, and Xuzishu No. 3 sweet potato flour influenced by high hydrostatic pressure (HHP) and soaking solution.

Variety	HHP (MPa)	Soaking solution	Total polyphenol (mg GAE/g DW)	Anthocyanins (mg/g DW)
Jishu 98	0.1	Untreated	0.35 ± 0.004e	0.19 ± 0.009a
		Citric acid	0.20 ± 0.01h	0.16 ± 0.019b
		Cal. chloride	0.17 ± 0.01i	0.10 ± 0.003cd
		Ascorbic acid	0.37 ± 0.004d	0.12 ± 0.018c
		Distilled water	0.25 ± 0.005f	0.09 ± 0.007de
	100	Citric acid	0.24 ± 0.012f	0.10 ± 0.001d
		Cal. chloride	0.23 ± 0.002g	0.06 ± 0.002ef
		Ascorbic acid	0.40 ± 0.003c	0.07 ± 0.019ef
		Distilled water	0.17 ± 0.008i	0.05 ± 0.001fg
	200	Citric acid	0.14 ± 0.019j	0.05 ± 0.003fg
		Cal. chloride	0.17 ± 0.012i	0.06 ± 0.002ef
		Ascorbic acid	0.51 ± 0.002a	0.04 ± 0.019gh
		Distilled water	0.12 ± 0.011j	0.03 ± 0.001h
	400	Citric acid	0.13 ± 0.006j	0.09 ± 0.003d
		Cal. chloride	0.17 ± 0.01i	0.05 ± 0.002fg
		Ascorbic acid	0.44 ± 0.008b	0.06 ± 0.009fg
		Distilled water	0.11 ± 0.006k	0.02 ± 0.001h
Pushu 32	0.1	Untreated	0.19 ± 0.012g	0.39 ± 0.004a
		Citric acid	0.34 ± 0.006f	0.18 ± 0.008g
		Cal. chloride	0.21 ± 0.003g	0.22 ± 0.015f
		Ascorbic acid	0.83 ± 0.028a	0.24 ± 0.023ef
		Distilled water	0.22 ± 0.012g	0.14 ± 0.009h
	100	Citric acid	0.18 ± 0.012g	0.28 ± 0.015de
		Cal. chloride	0.69 ± 0.034b	0.15 ± 0.014gh
		Ascorbic acid	0.53 ± 0.024c	0.26 ± 0.015de
		Distilled water	0.47 ± 0.024d	0.25 ± 0.023ef
	200	Citric acid	0.30 ± 0.006f	0.33 ± 0.008b
		Cal. chloride	0.41 ± 0.012e	0.15 ± 0.03gh
		Ascorbic acid	0.52 ± 0.038c	0.32 ± 0.016bc
		Distilled water	0.32 ± 0.052f	0.34 ± 0.004b
	400	Citric acid	0.18 ± 0.012g	0.28 ± 0.037de
		Cal. chloride	0.22 ± 0.018g	0.25 ± 0.012ef
		Ascorbic acid	0.45 ± 0.083de	0.29 ± 0.001cd
		Distilled water	0.21 ± 0.022g	0.38 ± 0.015a
Xuzishu No. 3	0.1	Untreated	1.98 ± 0.01a	3.08 ± 0.047a
		Citric acid	1.84 ± 0.02bc	0.60 ± 0.007fg
		Cal. chloride	1.79 ± 0.05cde	0.86 ± 0.029d
		Ascorbic acid	1.74 ± 0.04ef	0.49 ± 0.012h
		Distilled water	1.77 ± 0.03de	1.29 ± 0.053b
	100	Citric acid	1.83 ± 0.04bcd	0.54 ± 0.005gh
		Cal. chloride	1.87 ± 0.03bcd	0.69 ± 0.002e
		Ascorbic acid	1.77 ± 0.03de	0.61 ± 0.007fg
		Distilled water	1.87 ± 0.04bc	1.06 ± 0.007c
	200	Citric acid	1.91 ± 0.04b	0.49 ± 0.009h
		Cal. chloride	1.83 ± 0.04bcd	0.59 ± 0.003fg
		Ascorbic acid	1.85 ± 0.09bcd	0.33 ± 0.044i
		Distilled water	1.80 ± 0.08cde	0.65 ± 0.073ef
	400	Citric acid	1.78 ± 0.03de	0.48 ± 0.06h
		Cal. chloride	1.84 ± 0.06bcd	0.31 ± 0.058i
		Ascorbic acid	1.83 ± 0.02bcd	0.65 ± 0.002ef
		Distilled water	1.68 ± 0.04f	0.55 ± 0.014gh

DW: dry weight; GAE: gallic acid equivalent.

Different letters in the same column for each specific variety indicate significant differences (P < 0.05).

TAC

TAC of SPF as influenced by HHP and soaking solution is presented in Table 3. TAC in untreated Jishu 98, Pushu 32, and Xuzishu No. 3 was 0.19, 0.39, and 3.08 mg/g, respectively. A significant decrease in TAC in three varieties was observed after all processes as compared to the untreated ones (P < 0.05). In fact, TAC is a kind of naturally water-soluble pigment, so that its concentration was drastically decreased during soaking solutions and HHP with soaking solutions (Table 3). Current study findings were consistent with the report by Saikaew et al. (2017). Reque et al. (2014) indicated that abrupt decline of TAC might be attributed to oxidation and condensation of anthocyanin pigments with phenolic compounds. Similarly, loss of TAC might be explained that a temperature more than refrigeration caused oxidative degradation of ascorbic acid that creates many reactive components, which may reduce the anthocyanin contents (Movahed et al., 2016).

β-carotene content

β-carotene content of SPF with variety of Pushu 32 as influenced by HHP and soaking solution is shown in Figure 1. β-carotene of the untreated sample was 82.33 µg/g DW. β-carotene was increased by soaking solution of ascorbic acid but was decreased with citric acid and calcium chloride as compared to the untreated one. HHP with soaking solution increased β-carotene content significantly as compared to the untreated one (P < 0.05). In addition, HHP at 100, 200, and 400 MPa with citric acid, calcium chloride, and distilled water increased β-carotene content as compared to those at 0.1 MPa with the same soaking solution, but HHP with ascorbic acid decreased it significantly (P < 0.05). The increase of β-carotene might be attributed to HHP that induced denaturation of the carotenoid-binding protein, thus inducing the efficient release of carotenoids. HHP might also induce membrane disruption of chromoplast where carotenoids are located, thus enhancing its extraction (Zuluaga et al., 2016). Current results range is consistent with our previous study about the impact of different cooking methods on orange fleshed sweet potato, which presented that β-carotene was in the range of 19.15–152.09 µg/g DW (Kourouma et al., 2019).

Figure 1.

β-carotene of orange SPF (Pushu 32) treated by different HHP and soaking solutions. Different letters above the bars indicate significant differences (P < 0.05).

Antioxidant activities

Antioxidant activities of SPF as influenced by HHP and soaking solution are presented in Table 4. DPPH radical scavenging activity, FRAP, and ABTS of SPF with variety of Jishu 98 were 5.92 mg AAE/g, 10.36 µg TE/g, and 5.63 mg AAE/g DW, respectively, which was increased to the highest value at 200 MPa with ascorbic acid (6.05 mg AAE/g DW, 18.10 µg TE/g DW, and 6.02 mg AAE/g DW, respectively), and presented the lowest value at 400 MPa with calcium chloride (1.61 mg AAE/g DW, 1.98 µg TE/g DW, and 3.99 mg AAE/g DW, respectively). SPF with variety of Pushu 32 with soaking solution of ascorbic acid showed the highest antioxidant activity, of which DPPH radical scavenging activity, FRAP, and ABTS were 10.62 mg AAE/g DW, 25.19 µg TE/g DW, and 6.46 mg AAE/g DW, respectively. When treated at 400 MPa with calcium chloride, SPF with variety of Pushu 32 presented the lowest antioxidant activity. In the case of Xuzishu No. 3, the untreated sample presented the highest antioxidant activity with DPPH radical scavenging activity being 12.39 mg AAE/g DW, FRAP being 117.39 µg TE/g DW, and ABTS being 7.65 mg AAE/g DW, respectively. While both soaking solution and HHP with soaking solution treatment decreased the antioxidant activity of SPF with variety of Xuzishu No. 3. In the current study, SPF with soaking solution of ascorbic acid showed higher antioxidant activity as compared to others (Table 4). Burguieres et al. (2007) reported that ascorbic acid helped to maintain the active state of many bioactive compounds due to the excellent antioxidant activity in the food system.

Table 4.

Antioxidant activities in Jishu 98 (white), Pushu 32 (yellow), and Xuzishu No. 3 (purple) sweet potato flour influenced by high hydrostatic pressure (HHP) and soaking solution.

Variety	HHP (MPa)	Soaking solution	DPPH radical scavenging activity (mg AAE/g DW)	FRAP (µg TE/g DW)	ABTS radical scavenging activity (mg AAE/g DW)
Jishu 98	0.1	Untreated	5.92 ± 0.18a	10.36 ± 0.34d	5.63 ± 0.008b
		Citric acid	2.17 ± 0.11f	5.02 ± 0.425f	4.63 ± 0.039f
		Cal. chloride	2.22 ± 0.23f	3.96 ± 0.403fg	4.24 ± 0.028i
		Ascorbic acid	3.74 ± 0.40d	8.43 ± 0.326e	5.26 ± 0.008c
		Distilled water	5.55 ± 0.17b	10.63 ± 0.31cd	5.59 ± 0.003b
	100	Citric acid	2.79 ± 0.06e	4.06 ± 0.094fg	4.92 ± 0.008d
		Cal. chloride	2.26 ± 0.07f	3.67 ± 2.045fg	4.89 ± 0.009d
		Ascorbic acid	4.78 ± 0.33c	12.02 ± 0.169c	4.79 ± 0.01e
		Distilled water	2.02 ± 0.08fg	2.86 ± 0.102gh	4.05 ± 0.054j
	200	Citric acid	1.96 ± 0.01fgh	3.29 ± 0.409gh	4.62 ± 0.01f
		Cal. chloride	1.75 ± 0.08gh	3.33 ± 0.904gh	4.52 ± 0.002g
		Ascorbic acid	6.05 ± 0.17a	18.10 ± 0.604a	6.02 ± 0.018a
		Distilled water	2.07 ± 0.08fg	3.27 ± 0.463gh	4.02 ± 0.014jk
	400	Citric acid	1.93 ± 0.10fgh	4.28 ± 1.031fg	4.34 ± 0.02h
		Cal. chloride	1.61 ± 0.03h	1.98 ± 0.183h	4.49 ± 0.012g
		Ascorbic acid	5.03 ± 0.04c	13.55 ± 0.919b	4.80 ± 0.048e
		Distilled water	1.95 ± 0.02fgh	4.13 ± 0.007fg	3.99 ± 0.007k
Pushu 32	0.1	Untreated	4.88 ± 0.21de	7.01 ± 0.39i	5.61 ± 0.165e
		Citric acid	4.21 ± 0.01f	7.47 ± 0.43hi	5.88 ± 0.014d
		Cal. chloride	5.53 ± 0.34c	11.13 ± 0.22e	5.33 ± 0.135gh
		Ascorbic acid	10.62 ± 0.11a	25.19 ± 0.36a	6.46 ± 0.013a
		Distilled water	3.65 ± 0.18g	6.02 ± 0.16jk	5.26 ± 0.014ghi
	100	Citric acid	3.40 ± 0.20g	6.31 ± 0.31j	4.60 ± 0.141k
		Cal. chloride	5.71 ± 0.37c	18.54 ± 0.01b	6.06 ± 0.03c
		Ascorbic acid	7.35 ± 0.06b	15.08 ± 0.29c	5.87 ± 0.017d
		Distilled water	5.33 ± 0.06cd	9.06 ± 0.41f	5.52 ± 0.014ef
	200	Citric acid	3.62 ± 0.23g	11.27 ± 0.01e	5.22 ± 0.009hi
		Cal. chloride	2.83 ± 0.13h	11.20 ± 0.01e	5.14 ± 0.003i
		Ascorbic acid	5.71 ± 0.20c	12.50 ± 0.12d	6.52 ± 0.026a
		Distilled water	4.51 ± 0.27ef	8.21 ± 0.19g	4.96 ± 0.007j
	400	Citric acid	2.69 ± 0.10h	5.47 ± 0.20k	5.40 ± 0.017fg
		Cal. chloride	2.66 ± 0.17h	7.76 ± 0.01gh	4.21 ± 0.037l
		Ascorbic acid	5.23 ± 0.25cd	11.35 ± 0.47e	6.22 ± 0.064b
		Distilled water	5.54 ± 0.41c	7.49 ± 0.01hi	4.32 ± 0.014l
Xuzishu No. 3	0.1	Untreated	12.39 ± 0.06a	117.39 ± 1.35a	7.65 ± 0.01a
		Citric acid	12.09 ± 0.03f	78.50 ± 1.48c	7.39 ± 0.01c
		Cal. chloride	12.10 ± 0.07f	55.39 ± 0.64j	6.92 ± 0.04hi
		Ascorbic acid	11.41 ± 0.04l	45.24 ± 0.28k	6.99 ± 0.05gh
		Distilled water	11.85 ± 0.05i	58.64 ± 1.70hi	6.84 ± 0.02ij
	100	Citric acid	12.26 ± 0.02c	74.67 ± 0.01d	7.32 ± 0.03cd
		Cal. chloride	12.29 ± 0.01c	66.12 ± 0.93f	7.35 ± 0.02c
		Ascorbic acid	11.59 ± 0.01j	56.08 ± 0.02ij	7.07 ± 0.01fg
		Distilled water	12.22 ± 0.04d	78.59 ± 1.51c	7.36 ± 0.01c
	200	Citric acid	12.22 ± 0.06d	88.66 ± 0.01b	7.53 ± 0.01b
		Cal. chloride	12.34 ± 0.05b	59.39 ± 1.48h	7.24 ± 0.01de
		Ascorbic acid	12.15 ± 0.01e	58.39 ± 0.55hi	7.35 ± 0.02c
		Distilled water	12.19 ± 0.01d	63.64 ± 1.71g	7.20 ± 0.17e
	400	Citric acid	12.04 ± 0.07g	69.44 ± 1.26e	7.21 ± 0.01e
		Cal. chloride	11.61 ± 0.07j	62.49 ± 1.49g	7.07 ± 0.02fg
		Ascorbic acid	11.99 ± 0.01h	57.07 ± 1.43hij	7.17 ± 0.01ef
		Distilled water	11.48 ± 0.02k	33.32 ± 0.01l	6.81 ± 0.04j

AAE: ascorbic acid equivalent; ABTS: 2,2′-azinobis(3-ethylbenzothiazolin-6-sulfonate); DPPH: 2,2-diphenyl-1-picrylhydrazyl; DW: dry weight; FRAP: ferric reducing antioxidant power.

Different letters in the same column for each specific variety indicates significant differences (P < 0.05).

The correlations between antioxidant activity (DPPH radical scavenging activity, FRAP, and ABTS) and TPC and TAC are shown in Figure 2(a) to (f), respectively. The linear correlation coefficient between TPC with DPPH radical scavenging activity, FRAP, and ABTS in Jishu 98, Pushu 32, and Xuzishu No. 3 was 0.9654, 0.9475, and 0.9207, respectively (p < 0.0001). There were positive correlation coefficients between antioxidant activity and TPC. Similarly, Sun et al. (2014b) also demonstrated that TPC was mainly responsible for antioxidant capacity in sweet potato leaves. In addition, the linear correlation coefficient between TAC with DPPH radical scavenging activity, FRAP, and ABTS in Jishu 98, Pushu 32, and Xuzishu No. 3 was 0.6055, 0.7568, and 0.5929, respectively (p < 0.0001). It was obvious that anthocyanins also exhibited a positive correlation with antioxidant activity. Similarly, comparable results were described by the previous author who evaluated the effects of HHP on strawberry and blackberry purées and concluded that anthocyanin content and antiradical power declared a positive significant correlation (R = 0.64; P < 0.05) (Patras et al., 2009b). Therefore, TPC could be mainly responsible for the antioxidant capacity in SPF.

Figure 2.

(a) The linear correlation coefficient between DPPH and total polyphenol of Jishu 98, Pushu 32, and Xuzishu No. 3 SPF (R = 0.9654; p < 0.0001). (b) The linear correlation coefficient between FRAP and total polyphenol of Jishu 98, Pushu 32, and Xuzishu No. 3 SPF (R = 0.9475; p < 0.0001). (c) The linear correlation coefficient between ABTS and total polyphenol of Jishu 98, Pushu 32, and Xuzishu No. 3 SPF (R = 0.9207; p < 0.0001). (d) The linear correlation coefficient between DPPH and anthocyanins of Jishu 98, Pushu 32, and Xuzishu No. 3 SPF (R = 0.6055; p < 0.0001). (e) The linear correlation coefficient between FRAP and anthocyanins of Jishu 98, Pushu 32, and Xuzishu No. 3 SPF (R = 0.7568; p < 0.0001). (f) The linear correlation coefficient between ABTS and anthocyanins of Jishu 98, Pushu 32, and Xuzishu No. 3 SPF (R = 0.5929; p < 0.0001).

Conclusion

In conclusion, white, orange, and purple fleshed SPFs with varieties of Jishu 98, Pushu 32, and Xuzishu No. 3 are excellent sources of carbohydrate, protein, fiber, minerals, and polyphenols, with orange fleshed ones rich in β-carotene and purple fleshed ones high in anthocyanins. Understanding SPF can help human beings to enhance nutrition intake, while the processing would affect the quality of SPF, thus this work must consider appropriate processing techniques for SPF. The results presented here clearly demonstrated that HHP and soaking solution could make the components of SPF quite different from that of the untreated ones. In this study, TPC was increased in SPF of Jishu 98 at 200 MPa with ascorbic acid and Pushu 32 at 0.1 MPa with ascorbic acid but was decreased in Xuzishu No. 3 (purple). TAC was decreased in all treated SPFs. HHP with citric acid, calcium chloride, and distilled water significantly increased β-carotene content in Pushu 32. Correlation analysis between TPC, TAC, and antioxidant activity indicated suggested that antioxidant activity of SPF can be mainly attributed to polyphenols.

Footnotes

DECLARATION OF CONFLICTING INTERESTS

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: The authors gratefully acknowledge the earmarked fund for China Agriculture Research System (CARS-10-B21). We also thank the National Key R&D Program of China (2016YFE0133600).

ORCID iDs

Tai-Hua Mu https://orcid.org/0000-0002-1308-0121 Miao Zhang

References

Andrés

Villanueva

Tenorio

(2016) The effect of high-pressure processing on colour, bioactive compounds, and antioxidant activity in smoothies during refrigerated storage. Food Chemistry 192: 328–335.

Arnao

Cano

Acosta

(2001) Analytical, nutritional and clinical methods section the hydrophilic and lipophilic contribution to total antioxidant activity. Food Chemistry 73: 239–244.

Barba

Cortés

Esteve

Frígola

(2012) Study of antioxidant capacity and quality parameters in an orange juice-milk beverage after high-pressure processing treatment. Food and Bioprocess Technology 5: 2222–2232.

Benzie

IFF

Strain

(1996) The ferric reducing ability of plasma (FRAP) as a measure of “antioxidant power”: The FRAP assay. Analytical Biochemistry 239: 70–76.

Bovell-Benjamin

(2007) Sweet potato: A review of its past, present, and future role in human nutrition. Advances in Food and Nutrition Research 52: 1–59.

Brand-Williams

Cuvelier

Berset

(1995) Use of a free radical method to evaluate antioxidant activity. LWT – Food Science and Technology 28: 25–30.

Briones-Labarca

Muñoz

Maureira

(2011) Effect of high hydrostatic pressure on antioxidant capacity, mineral and starch bioaccessibility of a non conventional food: Prosopis chilensis seed. Food Research International 44: 875–883.

Briones-Labarca

Venegas-Cubillos

Ortiz-Portilla

Chacana-Ojeda

Maureira

(2011) Effects of high hydrostatic pressure (HHP) on bioaccessibility, as well as antioxidant activity, mineral and starch contents in Granny Smith apple. Food Chemistry 128: 520–529.

Burguieres

McCue

Kwon

Shetty

(2007) Effect of vitamin C and folic acid on seed vigour response and phenolic-linked antioxidant activity. Bioresource Technology 98: 1393–1404.

10.

FAOSTAT. (2017). FAO, statistical database. Food and agriculture organization of United Nations. Available at: http://www.fao.org/faostat/en/#data/QC (accessed 6 April 2019).

11.

Graham-Acquaah

Ayernor

Bediako-Amoa

Saalia

Afoakwa

(2014) Spatial distribution of total phenolic content, enzymatic activities and browning in white yam (Dioscorea rotundata) tubers. Journal of Food Science and Technology 51: 2833–2838.

12.

Haile

Admassu

Fisseha

(2015) Effects of pre-treatments and drying methods on chemical composition, microbial and sensory qualities of orange-fleshed sweet potato flour and porridge. American Journal of Food Science and Technology 3: 82–88.

13.

Kourouma

Zhang

Sun

(2019) Effects of cooking process on carotenoids and antioxidant activity of orange-fleshed sweet potato. LWT – Food Science and Technology 104: 134–141.

14.

Kuyu

Tola

Mohammed

Ramaswamy

(2018) Determination of citric acid pretreatment effect on nutrient content, bioactive components, and total antioxidant capacity of dried sweet potato flour. Food Science and Nutrition 6: 1724–1733.

15.

Lee

Durst

Wrolstad

(2005) Determination of total monomeric anthocyanin pigment content of fruit juices, beverages, natural colorants, and wines by the pH differential method: Collaborative study. Journal of AOAC International 88: 1269–1278.

16.

Liu

Wang

Liao

(2013) Effects of high hydrostatic pressure and high temperature short time on antioxidant activity, antioxidant compounds and color of mango nectars. Innovative Food Science and Emerging Technologies 1–9.

17.

Lyimo

Gimbi

Kihinga

(2010) Effect of processing methods on nutrient contents of six sweet potato varieties grown in Lake Zone of Tanzania. Tanzania Journal of Agricultural Sciences 10: 55–61.

18.

Mahadevan

Karwe

(2016) Effect of high-pressure processing on bioactive compounds. Food Engineering Series 479–507.

19.

Motsa

Modi

Mabhaudhi

(2015) Sweet potato (Ipomoea batatas L.) as a drought tolerant and food security crop. South African Journal of Science 111: 1–8.

20.

Movahed

Pastore

Cellini

Allegro

Valentini

Zenoni

, et al.(2016) The grapevine VviPrx31 peroxidase as a candidate gene involved in anthocyanin degradation in ripening berries under high temperature. Journal of Plant Research 129: 513–526.

21.

Olatunde

Henshaw

Idowu

Tomlins

(2016) Quality attributes of sweet potato flour as influenced by variety, pretreatment and drying method. Food Science and Nutrition 4: 623–635.

22.

Osim

Odoemena

Etukudo

Okonwu

Eremrena

(2010) Evaluation of proximate composition of fruits of Lycopersicon esculentum (Roma VF) under stress and staking. Global Journal of Pure and Applied Sciences 16: 277–279.

23.

Patras

Brunton

Pieve

Butler

(2009b) Impact of high pressure processing on total antioxidant activity, phenolic, ascorbic acid, anthocyanin content and colour of strawberry and blackberry purées. Innovative Food Science and Emerging Technologies 10: 308–313.

24.

Patras

Brunton

Pieve

Butler

Downey

(2009a) Effect of thermal and high pressure processing on antioxidant activity and instrumental colour of tomato and carrot purées. Innovative Food Science and Emerging Technologies 10: 16–22.

25.

Peng

Zhang

Sun

Chen

(2016) Effects of pH and high hydrostatic pressure on the structural and rheological properties of sugar beet pectin. Food Hydrocolloids 60: 161–169.

26.

Rastogi

Raghavarao

KSMS

Balasubramaniam

Niranjan

Knorr

(2007) Opportunities and challenges in high pressure processing of foods. Critical Reviews in Food Science and Nutrition 47: 69–112.

27.

Reque

Steffens

Jablonski

FlÔres

Rios

Jong E V

(2014) Cold storage of blueberry (Vaccinium spp.) fruits and juice: Anthocyanin stability and antioxidant activity. Journal of Food Composition and Analysis 33: 111–116.

28.

Ruttarattanamongkol

Chittrakorn

Weerawatanakorn

Dangpium

(2016) Effect of drying conditions on properties, pigments and antioxidant activity retentions of pretreated orange and purple-fleshed sweet potato flours. Journal of Food Science and Technology 53: 1811–1822.

29.

Saikaew

Lertrat

Meenune

Tangwongchai

(2017) Effect of high-pressure processing on colour, phytochemical contents and antioxidant activities of purple waxy corn (Zea mays L. var. ceratina) kernels. Food Chemistry 243: 328–337.

30.

Stolt

Oinonen

Autio

(2001) Effect of high pressure on the dielectric properties of relaxor. Innovative Food Science & Emerging Technologies 1: 167–175.

31.

Sun

Song

(2014a) Effects of domestic cooking methods on polyphenols and antioxidant activity of sweet potato leaves. Journal of Agricultural and Food Chemistry 62: 8982–8989.

32.

Sun

Zhang

Chen

(2014b) Sweet potato (Ipomoea batatas L.) leaves as nutritional and functional foods. Food Chemistry 156: 380–389.

33.

Sun

Guo

Wei

Fan

(2011) Multi-element analysis for determining the geographical origin of mutton from different regions of China. Food Chemistry 124: 1151–1156.

34.

Tang

Cai

(2015) 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 4: 123–132.

35.

Torres-Ossandón

López

Vega-Gálvez

Galotto

Perez-Won

Scala K

(2015) Impact of high hydrostatic pressure on physicochemical characteristics, nutritional content and functional properties of Cape gooseberry pulp (Physalis peruviana L.). Journal of Food Processing and Preservation 39: 2844–2855.

36.

Trancoso-Reyes

Ochoa-Martínez

Bello-Pérez

Morales-Castro

Estévez-Santiago

Olmedilla-Alonso

(2016) Effect of pre-treatment on physicochemical and structural properties, and the bioaccessibility of β-carotene in sweet potato flour. Food Chemistry 200: 199–205.

37.

Vega-Gálvez

López

Torres-Ossandón

José Galotto

Puente-Díaz

Quispe-Fuentes

, et al.(2014) High hydrostatic pressure effect on chemical composition, color, phenolic acids and antioxidant capacity of Cape gooseberry pulp (Physalis peruviana L.). LWT – Food Science and Technology 58: 519–526.

38.

Wang

Liu

Cao

Chen

Liao

(2012) Comparison of high hydrostatic pressure and high temperature short time processing on quality of purple sweet potato nectar. Innovative Food Science and Emerging Technologies 16: 326–334.

39.

Wennberg

Nyman

(2004) On the possibility of using high pressure treatment to modify physico-chemical properties of dietary fibre in white cabbage (Brassica oleracea var. capitata). Innovative Food Science and Emerging Technologies 5: 171–177.

40.

Zhao

Zhang

(2016) Effects of high hydrostatic pressure processing and subsequent storage on phenolic contents and antioxidant activity in fruit and vegetable products. International Journal of Food Science and Technology 52: 1–10.

41.

Zuluaga

Martínez

Fernández

López-Baldó

Quiles

Rodrigo

(2016) Effect of high pressure processing on carotenoid and phenolic compounds, antioxidant capacity, and microbial counts of bee-pollen paste and bee-pollen-based beverage. Innovative Food Science and Emerging Technologies 37: 10–17.