Determination of citric acid pretreatment effect on nutrient content

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Received: 26 April 2018    Revised: 20 June 2018    Accepted: 25 June 2018 DOI: 10.1002/fsn3.747

ORIGINAL RESEARCH

Determination of citric acid pretreatment effect on nutrient content, bioactive components, and total antioxidant capacity of dried sweet potato flour Chala G. Kuyu1

 | Yetenayet B. Tola1

1

Department of Postharvest Management,  Jimma University College of Agriculture and Veterinary Medicine, Jimma, Ethiopia

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Food Science and Agricultural Chemistry,  McGill University, Ste. Anne de Bellevue, QC, Canada Correspondence Yetenayet B. Tola, Department of Postharvest Management, Jimma University College of Agriculture and Veterinary Medicine, P.O. Box 307, Jimma, Ethiopia. Emails: [email protected]; [email protected] Funding information Jimma University College of Agriculture and Veterinary Medicine

 | Ali Mohammed1 | Hosahalli S. Ramaswamy2

Abstract Orange flashed sweet potatoes are rich and inexpensive source of diet and antioxidants. The purpose of this study was to evaluate the effects of CA pretreatments and convective hot air drying temperature on proximate composition, bioactive components, and total antioxidant capacity of flour of five orange flashed sweet potato varieties. Moisture, protein, ether extract, ash, carbohydrate, fiber, β-­carotene, total phenolic compounds, and total antioxidant capacity in the dried flour samples were evaluated and reported in the range of 4.1–7.4%, 2.4–4.2%, 1.2–1.1.8%, 2.2–3.2%, 82.7–87.1%, 1.3–1.8%, 35.5–91.6 mg/100 g, 49.8–107.9 mg GAE/100 g, and 27.3– 85.4%, respectively. The interaction effects of varieties, drying temperature, and CA concentration were significant (p ˂ 0.05) except for fiber. Kulto and SPK006/6/6 performed better for most of the parameters studied followed by SPK00/06. For almost all varieties, samples dried at 55°C and after treated in 3% CA solution had the highest percentage in terms of proximate composition, bioactive components, and total antioxidant capacities. KEYWORDS

antioxidant, bioactive photochemical, drying, polyphenols, sweet potatoes

1 |  I NTRO D U C TI O N

& Shao, 2006). Recent research indicates that bioactive compounds such as polyphenols have many physiological benefits such as an-

Sweet potato [Ipomoea batatas (L.) Lam.] is a highly nutritious modi-

tioxidant, antiinflammation, blood vessel relaxation, and capillary

fied root crop rich in carbohydrate, only next to rice, corn, and cas-

wall-­ stabilizing activities (Hassellund et al., 2013). They improve

sava (Zuraida, 2003). It is a starch root, which contains high amounts

blood lipid profiles by increasing plasma high-­density lipoproteins

of β-­carotene and amino acid (especially lysine) which is deficient

(HDL) and decreasing the low-­density lipoproteins (LDL) (Qin et al.,

in other cereal products like rice (Hassellund et al., 2013). Besides

2009). Being rich in carotenoids, total polyphenol content, and

these, it also contains polyphones, which act as antioxidants to safe-

ascorbic acid, sweet potato is gaining importance as the least expen-

guard the human body from certain chronic diseases (Huang, Chang,

sive source of antioxidants (Alam, Rana, & Islam, 2016).

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2018 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc. Food Sci Nutr. 2018;1–10.

   www.foodscience-nutrition.com |  1

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KUYU et al.

2      

White fleshed sweet potato (WFSP) variety is the staple food for 13 million people in the Southern Regional State of Ethiopia

and two drying temperatures (55 and 65°C) in factorial arrangement, and replicated three times.

(Kurabachew, 2015). In contrast, the orange fleshed sweet potato (OFSP), known to be a good source of β-­carotene and energy (293 to 460 kJ/100 g), is easy to cultivate and fairly drought-­tolerant (Hagenimana et al., 2001). These characteristics make OFSP an excellent food and nutrition security crop to the region, but in terms

2.3 | Data collected 2.3.1 | Proximate composition analysis

of their nutrient content, bioactive components and antioxidant

For evaluating the effect of treatment on the nutritional quality

capacity are not characterized. Furthermore, despite its increasing

of the flour, all components of proximate composition were deter-

importance as a valuable crop for food security, so far value addition

mined using standard analytical methods of AOAC (2005) (methods

attempts have not been conducted in terms of production of de-

for moisture (925.09), dietary fiber (993.21), protein (960.52), fat

hydrated product and minimization of the associated after-­harvest

(920.85), and ash (923.03)).

losses (Tiruneh, 2017). The use of tuber crop in Ethiopia is limited, and it is consumed as an alternative carbohydrate source. This is generally performed with

2.3.2 | Total polyphenol content

fresh tuber as postharvest storage or processing technology is not

The total polyphenol contents were determined according to

yet well developed. Dehydration could be an inexpensive technol-

Blainski, Lopes, and De Mello (2013) which involved the reduction

ogy that can be easily adapted to reduce the losses and improve its

of Folin-­Ciocalteu reagent by phenolic compounds. Absorbances of

utilization in food formulations. Proper drying of OFSP can result in a

prepared samples were measured at 765 nm using UV–Vis spectro-

stable product with better quality (Utomo, Man, Yaakob, Rahman, &

photometer (T80 Jiangsu, China). Gallic acid was used as the stand-

Saad, 2008) when assisted with predrying treatments. Singh, Raina,

ard, and the total phenolic contents were expressed as mg of gallic

Bawa, and Saxena (2003) used potassium metabisulfite and sodium

acid equivalent (GAE) per g of sample (mg GAE/g sample).

chloride to improve the quality of chips from sweet potato. Ahmed, Akter, and Eun (2010) also used sodium hydrogen sulfite to improve the flour quality of OFSP. It has been shown that losses of scaveng-

2.3.3 | β-­Carotene determination

ing ability, total phenolic contents, and degree of oxidation increase

Extraction and determination of total β-­carotene were based on

with increasing processing temperature and decrease when tuber is

the method described in Park (1987). After extraction, absorbance

soaked in citric acid solution (Shih, Kuo, & Chiang, 2009). On the con-

was read at 450 nm using UV–Vis spectrophotometer (T80 Jiangsu,

trary, short heating reduces the activity of endogenous polyphenol

China) and estimated against with concentration of β-­carotene

oxidase which is responsible for oxidation of bioactive compounds

standard curve (Sigma-­Aldrich).

(Ahmed et al., 2010). Therefore, this study aimed at to evaluate the effects of citric acid (CA) pretreatment and drying temperature on nutrient content, bioactive components, and antioxidant capacity of OFSP flours produced from different varieties.

2.3.4 | Determination of total antioxidant capacity and IC50 value Antioxidant capacity was determined according to the method of Lu

2 |  M ATE R I A L S A N D M E TH O DS

and Foo (2000) which involved DPPH (2,2-­diphenyl-­1-­picryl-­hydraz yl) free radical scavenging assay. Briefly, 10 g of sweet potato flour was mixed with 100 ml methanol and the mixture was homogenized

2.1 | Sample collection and preparation

for 1 min in a homogenizer (POLYTRON® 2500E, Switzerland) and kept in a water bath at 20°C for 60 min. The samples were then

Five varieties (SPK00/06, SPK004/6/6, Guntute, Bucteca, and Kulto)

centrifuged at 748 g for 15 min, and the supernatant was taken for

of OFSP were collected from Jimma Agricultural Research Center.

analysis. The solvent extract of the sample was taken in 200, 400,

The roots were washed in tap water, and only those with uniform

600, 800, and 1,000 μl concentrations in a test tube, and the volume

overall appearance, size, and shape were selected for the study.

was made up to 1 ml with the solvent and 2 ml of 0.1 mM DPPH was

Tubers were then sliced into 1 mm size and dried for 8 hr (after pre-

added to each tube. The mixture was shaken well and incubated at

liminary work) after CA treatment (Ahmed, Akter, & Eun, 2011).

room temperature in the dark for 30 min. The decrease in absorbance of the resulting solution was then measured using UV–Vis spec-

2.2 | Experimental design and treatment combinations Experiments were carried out using a completely randomized design having five sweet potato varieties (SPK00/06, SPK004/6/6, Guntute, Bucteca, and Kulto), with two CA treatments (1 and 3%)

trophotometer (T80 Jiangsu, China) at 517 nm. Scavenging activity was calculated from absorbance values of samples and control sample using the following equation: ⎛ Ac − At As RSA (%) = ⎜ ⎜ Ac ⎝

⎞ ⎟ × 100 ⎟ ⎠

65°C

55°C

6.0 ± 0.3abcde

Guntute 6.7 ± 0.33ab

Kulto 4.6 ± 0.13ef

SPK004/6/6

2.4 ± 0.18 cd 3.0 ± 0.14abc

4.1 ± 0.17 4.1 ± 0.28f 4.2 ± 0.13f

Guntute Bucteca Kulto 2.217

abc

1.253

2.9 ± 0.17

2.4 ± 0.16 cd

f

SPK004/6/6

3.15 ± 0.14

4.2 ± 0.13f

4.2 ± 0.16

3.2 ± 0.12a

6.6 ± 0.35ab

Kulto SPK00/06

2.5 ± 0.16 

6.7 ± 0.27

Bucteca a

cd

ab

f

2.9 ± 0.13abc

6.4 ± 0.14abc

Guntute

2.4 ± 0.34 

6.1 ± 0.33

SPK004/6/6

cd

3.1 ± 0.32ab

abcde

3.1 ± 0.17

6.3 ± 0.13abcd

SPK00/06

5.0 ± 0.32

ab

cdef

Kulto

2.5 ± 0.16 cd

4.7 ± 0.24def

Bucteca

2.8 ± 0.14

5.4 ± 0.32

abcd

2.2 ± 0.32d

3.1 ± 0.12

ab

2.9 ± 0.33abc

2.55 ± 0.15

bcd

2.8 ± 0.15abcd

2.4 ± 0.14 

Guntute

bcdef

4.9 ± 0.23

SPK00/06

cdef

7.4 ± 0.21

Bucteca

a

7.2 ± 0.22

cd

3.15 ± 0.25a

7.4 ± 0.13a a

Ash

MC

SPK004/6/6

SPK00/06

Varieties

1.645

3.7 ± 0.33bcd

3.2 ± 0.33cdef

3.6 ± 0.33

bcde

3.7 ± 0.33bcd

3.7 ± 0.33

bcde

4.2 ± 0.09a

3.8 ± 0.16

abcd

4.1 ± 0.33ab

3.9 ± 0.11

abc

4.0 ± 0.12ab

3.3 ± 0.19

cdef

2.6 ± 0.14fgh

2.5 ± 0.32

gh

3.0 ± 0.22efh

2.4 ± 0.21 

h

3.7 ± 0.14bcd

3.3 ± 0.12

cdef

3.1 ± 0.18defh

3.2 ± 0.16

cdef

3.4 ± 0.32bcde

Protein

1.540

1.2 ± 0.07d

1.3 ± 0.12 cd

1.4 ± 0.11 

cd

1.4 ± 0.11 cd

1.2 ± 0.13

d

1.8 ± 0.15a

1.8 ± 0.16

a

1.8 ± 0.15a

1.7 ± 0.14

ab

1.8 ± 0.11a

1.2 ± 0.08

d

1.3 ± 0.07 cd

1.4 ± 0.06 

cd

1.5 ± 0.13bcd

1.2 ± 0.13

d

1.7 ± 0.15ab

1.6 ± 0.14

abc

1.7 ± 0.13ab

1.7 ± 0.14

ab

1.8 ± 0.12a

EE

1.860

86.0 ± 0.27ab

87.1 ± 0.28a

86.3 ± 0.30

a

86.5 ± 0.21a

86.2 ± 0.23

a

82.7 ± 0.24e

83.8 ± 0.29

de

83.4 ± 0.22de

84.5 ± 0.23

bcd

83.3 ± 0.23de

85.7 ± 0.22

abc

87.1 ± 0.27a

86.9 ± 0.24

a

86.6 ± 0.25a

86.6 ± 0.27

a

83.7 ± 0.25de

83.9 ± 0.26

de

84.2 ± 0.23cde

84.2 ± 0.23

cde

82.8 ± 0.24e

Total carbo.

1.020

370.4 ± 0.44abc

372.9 ± 0.40a

371.8 ± 0.37ab

373.6 ± 0.36a

370.8 ± 0.34abc

363.8 ± 0.33 fg

366.8 ± 0.41bcdef

366.4 ± 0.37cdef

368.9 ± 0.38abcde

365.4 ± 0.43defg

367.2 ± 0.33bcdef

370.6 ± 0.45abc

369.9 ± 0.37abcd

371.7 ± 0.33ab

367.2 ± 0.42bcdef

365.0 ± 0.33defg

363.3 ± 0.37 fg

364.5 ± 0.36efg

364.9 ± 0.43efg

360.9 ± 0.37 g

Energy (kcal/100 g)

Notes. CAC = CA concentration; EE = ether extract; Carbo = carbohydrate. Results are mean values of triplicate determination, and means with different letters across the column are significantly different (p