PRODUCTION AND PROXIMATE COMPOSITION OF A HYDROPONIC SWEET POTATO FLOUR DURING EXTENDED STORAGE

PRODUCTION AND PROXIMATE COMPOSITION OF A HYDROPONIC SWEET POTATO FLOUR DURING

EXTENDED STORAGE

MONTREKA Y. DANSBY and ADELIA C. BOVELL-BENJAMIN'

NASAiTuskegee Center for Food and Environmental System for Human Exploration of Space

Department of Food and Nutritional Sciences Tuskegee University Tuskegee, AL. 36088

Accepted for Publication March 21, 2003

ABSTRACT

In developing countries, where limited transport infrastructure exists, processing the sweet potato (Ipomoea batatas (L.) Lam) into flour provides an alternative to the difficulties associated with storage and transport of the raw roots. The objectives of this study were: (1) to process hydroponic sweet potato roots into flour; and (2) to evaluate the nutritive composition and the color of the processed hydroponic sweet potato flour during storage. The TU-82-155 hydroponic sweet potatoes were processed into flour and stored for five months at room and refrigerated temperatures. The sweet potato flour contained 3.0%, 4.5%, 1.076, 1.0%, 90.6% moisture, ash, fat , protein, and carbohydrate, respectively, with no significant changes during storage. The *L values fo r the sweet potato flour increased as storage time increased, but the *a and *b values decreased. Hydroponic sweet potato roots could be processed into flour and stored at 4C or 21C to 25C for five months without deterioration in quality.

INTRODUCTION

Globally, the sweet potato (Ipornoea batatas (L.) Lam) ranks among the seven most important food crops (FA0 1990). In the developing world it is a common staple food. For example, the highlanders of Papua, New Guinea rely on sweet potatoes as a major source of protein, and for 60 to 90% of their energy requirements (Clark and Moyer 1988). More importantly, the sweet

Correspondence to: Adelia C. Bovell-Benjamin, 300-A Campbell Hall, Depamnent of Food and Nutritional Sciences, Tuskegee University, Tuskegee, AL 36088. TEL: (334) 727-8717; FAX: (334) 727-8493; EMAIL: [email protected]

Journal of Food Processing Preservation 27 (2003) 153-164. All Righrs Reserved. Vopyr igh t 2003 by Food & Nuiriiion Press, Inc.. Trumbull, Connecticut 153

154 M.Y. DANSBY and A.C. BOVELL-BENJAMIN

potato is a low fat food that contributes important nutrients such as complex carbohydrates, fiber, &carotene (a precursor to vitamin A), and vitamin C to the human diet. Additionally, its diversity (different cultivars, textural properties, nutritional components, different flesh and skin colors) lends extremely well to the development of novel products that will add variety to the diets of different consumer groups (Woolfe 1992).

In many developing countries, vitamin A deficiency is a public health problem (Hagenimana er al. 1998a). The International Potato Center (CIP) has selected orange-fleshed cultivars of sweet potatoes as a food source to provide a safe, cheap, and simple delivery system to increase dietary intake of vitamin A, and assist in the control of vitamin A deficiency (Hagenimana el al. 1998a). CIP is promoting the use of orange-fleshed sweet potato cultivars through product development studies (Hagenimana ef al. 1998a).

Additionally, the sweet potato is of interest because it was selected as a candidate crop by the National Aeronautics and Space Administration (NASA) to be grown hydroponically on future long-duration space missions. For well over a decade, researchers and scientists at Tuskegee University’s NASA Center for Food and Environmental Systems for Human Exploration of Space (CFESH) have been developing a unique food system based on the TU-82-155 hydroponically grown sweet potatoes and peanuts for long-duration space missions. One of the objectives of CFESH is to use innovative technologies to process sweet potato roots into a variety of value-added products that could be used on long- duration missions, and with applications for the food system on Earth.

Sweet potato roots are highly perishable and difficult to store. In developing countries, where limited transport infrastructure exists, processing the sweet potato into flour provides an alternative to the difficulties associated with storage and transport of the raw roots. The sweet potato can be processed into flour, which is less bulky and more stable than the highly perishable fresh root (Collado and Corke 1999). Processing the sweet potato into flour increases its storage ability and value (Dawkins and Lu 1991). In India, dried, ground sweet potatoes are used to supplement flours in bakery products, chappathis, and puddings while in the Philippines, dried sweet potato chips are pounded into flour for use in a gruel (Nair ef al. 1987). Furthermore, sweet potato flour can serve as a substitute for wheat and other cereal flours, especially for individuals diagnosed with celiac disease. Celiac disease is intolerance to certain cereals, including wheat and wheat starch, and the only effective treatment to date includes complete exclusion of wheat and wheat-based products from the diet (Caperuto ef al. 2000).

In product development, the final quality of the product is highly dependent on the quality of the raw ingredients used. If sweet potato flour is to be incorporated into products, it must be of high quality. Some of the most important quality characteristics of sweet potato flour include substantial starch

SWEET POTATO FLOUR DURING STORAGE 155

content, and small amounts of ash, dietary fiber, and moisture (Van Hal 2000). Although sweet potato flour is identified as one of the most promising sweet potato products, its quality and storage stability are not yet documented (Hagenimana er al. 1998b). Van Hal (2000) concluded that further research is needed regarding the influence of storage conditions on the quality of sweet potato flour. Therefore, the objectives of this study were: (1) to process hydroponic sweet potato roots into flour: and (2) to evaluate the proximate composition and the color of the processed hydroponic sweet potato flour during storage.

MATERIALS AND METHODS

Materials

TU-82- 155 hydroponically grown sweet potatoes (HSP) were provided by the Crop Production and Environmental Systems (CPES) team at the Tuskegee University Center for Food and Environmental Systems for Human Exploration of Space (CFESH).

Proximate Composition

The proximate composition of the raw sweet potato roots was evaluated. Protein was assayed using a modified Bradford method that utilized the Pierce Coomassie@ Plus Protein Assay Reagent k t (Pierce, Rockford, Ill.); fat on 5 g sample by petroleum ether extraction in a Soxhlet apparatus (AOAC 1984); ash on 5 g sample by the muffler furnace method (AOAC 1984); and moisture in the IR-200 Moisture Analyzer (Denver Instrument Co., Arvada, Col.). The carbohydrate content was calculated by difference (Akubor et al. 2000).

Flour Production

The hydroponic sweet potato roots were processed into flour in the Food Processing Laboratory, using a modified version of the procedure described by Van Hal (2000). As shown in Fig. 1, the sweet potatoes were prepared and shredded using a Hobart VS9 vegetable slicer (Troy, Ohio). The shredded sweet potatoes were dehydrated at 70C for 12 h in a Magic Mill-Magic Aire I1 Solid State Dehydrator (Salt Lake City, Utah). The dehydrated hydroponic sweet potatoes were ground in a Hamilton Beach 12-speed Blendmaster ultra blender (Washington, N.C.), and gradually fed into the hopper of a Crossbeater Mill (Glen Mills Inc., Maywood, N.J.) with a 0.25 mm diameter hole screen. The particle size of the sweet potato flour was < 100 microns. A pastry brush was used to remove as much flour as possible from the mill to account for total


sweet potato flour yield. The sweet potato flour was stored in airtight containers at -14C until used.

Sweetpotato Peeled

Dehydrated 70C: 12h

Sweetpotato piil __* 1 Flour 1 FIG. 1. PRODUCTION OF SWEET POTATO FLOUR

Mass Balance of Flour

A mass balance was performed to determine sweet potato flour yield from the raw sweet potato (Fig. 2).


FI F2

I F3

(I) Sweetpotato F4 - FI= Raw unpeeled sweetpotato

F2= Shredded sweetpotato

F3= Dehydrated sweetpotato

F4= Sweetpotato flour

Calculations for Sweetpotato flour (SPF) FI= 25318 (peeledlraw sweetpotato= 2125/2531* 100)=83.9%

F2= 2105/2125*100=99%

Fj= 484/2105*100=23%

F4= 384/484* 100=79.3°/o

Yield XSpF =SP flour / SP Root * 100%

Yield % s p F = 384/2531*100

Yield % s p ~ = 15%

FIG. 2. MASS BALANCE OF SWEET POTATO FLOUR


Storage of the Flour

A portion of the processed hydroponic sweet potato flour (HSPF) was stored in 1 L glass jars in the dark at 4C (refrigerated temperatures) and 21 to 25C (room temperature) for five months. Samples were randomly withdrawn at monthly intervals for proximate analyses. Initial and monthly storage values were not determined for 0-carotene, ascorbic acid, thiamin, and riboflavin because of laboratory and other constraints. However, at the end of the storage period, 0-carotene, ascorbic acid, thiamin, and riboflavin contents were determined by Woodson-Tenent Laboratories (Memphis, TN). Dietary fiber of the hydroponic sweet potato flour was determined using AOAC (1997) and Bennink (1998) methods. 0-carotene and ascorbic acid were analyzed using HPLC and 2,6-dichloroindophenol titrimetric methods as described by Bureau and Bushway (1986) and Pelletier (1985), respectively. Thiamin and riboflavin were analyzed using the Thiochrome Flourometric and Fluorometric methods, respectively (Woodson-Tenent Laboratories).

Color

A Minolta chroma meter CR-3 10 tristimulus color analyzer (Ramsey, New Jersey) was used to measure the color of the sweet potato. The L* represents the lightnesddarkness of the sample, and the a* and b* coordinates denote the quadrant position relative to the red/green and the yellow/blue axes, respectively. Before measuring, the meter was calibrated with a white tile and checked for recalibration in between measurements, although no modifications were required. For measurement, the samples were placed in a covered petri dish, and duplicate readings were taken from two different positions.

Statistical Analysis

the two samples were analyzed by use of a t test (Huck er al. 1974). Differences between the means for the proximate composition and color of

RESULTS AND DISCUSSION

One constraint of the study was the lack of an appropriate control such as the TU-82-155 variety grown under controlled field conditions. Although the data are not shown, flour was processed from field grown sweet potatoes (cv. Beauregard). Although the results for the field sweet potato flour cannot be compared because of the cultivar difference, the results were similar to results reported for the TU-82- 155 hydroponic sweet potato. The proximate composition of the hydroponic sweet potato (HSP) roots is presented in Table 1. Moisture

TABL

E 1.

ROO

TS A

ND

FLO

URS

PR

OX

IMA

TE C

OM

POSI

TIO

N, &

CA

RO

TEN

E A

ND

VIT

AM

IN C

OM

POSI

TIO

N O

F H

YD

ROPO

NIC

SW

EET

POTA

TO (H

SP)

Nut

rien

ts

Swee

tpot

ato

Flou

r

4c

21 to

25C

3 z

HSP

Roo

t M

onth

1

Mon

th 5

M

onth

1

Mon

th 5

Moi

stur

e (%

) 84

.7k0

.9

2.6+

0.1

3.3*

0.1

2.9k

0.02

4.

2+0.

1 23

Ash

(%)

0.9 1

+O. 2

4.

4hO

.l 4.

22~0

.1

4.8*

0.1

4.5k

0.1

Fat (YO)

0.89

kO.O

1 0.

8kO

.l 0.

9*0.

1 0.

9kO

.O 1

0.9+

0.01

Prot

ein

(%)

1.2k

0.05

0.

9*0.

1 0.

9*0.

1 0.

9kO

. 1

1.2*

0. 1

Car

bohy

drat

e (%

) 12

.6f0

.9

91.3

k1.6

90

.7kO

. 3

91.3

kl.9

89

.1k1

.3

.---_

10

.9f0

.14

._.___

Ana

lyse

s no

t per

form

ed.


content averaged 84.7 f 0.9%. These findings were consistent with the Bovell- Benjamin ef al. (2001) reported mean moisture content of 82 f 2% for HSP. Moisture contents ranging from 75% to 80% were reported for field-grown sweet potatoes (FSP) of different cultivars. We can only speculate that the moisture content in hydroponically grown sweet potatoes is higher than that in FSP.

According to Collins and Pangloli (1997), field-grown sweet potatoes generally contain 2 to 4% dietary fiber, and are considered an important source of fiber (Hill et al. 1992; Woolfe 1992). The mean dietary fiber for the HSP roots was 2.3%. The 2.3% dietary fiber in the hydroponic sweet potato root was consistent with that reported in the literature for field-grown sweet potato of different cultivars (Van Hal 2000).

Except for plantains, sweet potato is the only starch staple with a substantial amount of 0-carotene (Woolfe 1992). The 0-carotene levels usually vary with cultivars. Ranges of 0 to > 20,000 pg/100 g are reported (Woolfe 1992; Kays 1992; K’Osambo ef al. 1998). The HSP exhibited a mean 0-carotene value of 1,566 pg/lOO g (Table l), thus falling within the range reported for field-grown sweet potatoes of different cultivars. Ascorbic acid concentration is known to vary with root size, location within the root, cultivar, and cooking method (Kays 1992). In this study, the TU-82-155 HSP exhibited a mean ascorbic acid content of 6 mg/100 g. To the best of our knowledge, ascorbic acid content of the HSP is not reported in the literature. However, ascorbic acid contents ranging from 0.8 mg/100 g to 23 mg/100 g were reported for FSP of different cultivars.

The mass balance is a reliable test that accounts for the loss in the milling process and thus supports the conclusion that the percentage yield is constant when processing conditions are equivalent (Toerne 2001). Any variations in the yield and in the mass balance were most likely due to loss during processing in the mill. The results of the mass balance for the production of sweet potato flour showed a yield of 15% (Fig. 2). Limited information regarding the flour yields from hydroponic sweet potato is available. However, Dawkins and Lu (1991) reported a 13 to 18% yield in flour prepared from steam blanched and microwave blanched or unblanched field sweet potatoes, respectively. A flour yield ranging from 17% to 38% for different field sweet potato varieties was also reported by Gakonyo (1993). Two Philippine sweet potato cultivars are reported to yield 12 kg or 37 kg floudl00 kg fresh roots (Woolfe 1992).

The moisture content of sweet potato flour is especially important because high moisture can speed up chemical or microbial deterioration (Van Hal 2000). According to Woolfe (1992) and Collado and Corke (1999), the moisture content of sweet potato flour ranges from 4.4 to 13%. The hydroponic sweet potato flour in this study exhibited a lower mean initial moisture content (2.9 f 0.02) than the moisture content reported in the literature for sweet potato flour. The final moisture content is directly related to the drying method and time.


Possibly, the low moisture content seen was related to the drying method used. In a review on sweet potato flour quality, Van Hal (2000) reported that artificial drying could reduce moisture content by 2 to 3 % , implying the drying time in this study was longer than necessary. However, in the procedures utilized, the shredded sweet potatoes were considered dry when they became brittle. In earlier studies, cited by Van Hal (2000) indicated that brittleness corresponded to a constant moisture content. Sweet potato flour has a capacity for absorbing moisture, therefore some increase in moisture content during storage is inevitable. Moisture content increased at the end of the five-month storage period, but this increase was not significant (2.9 f 0.02 to 3.3 i- 0.1 %; at 2.9 i- 0.02 to 4.2 i- 0.1 %) for flour stored at 4C or 21 to 25C, respectively. Also, the level of moisture uptake is partly dependent on the packaging material utilized.

There were no significant differences (PCO.05) in ash or fat concentrations for the HSPF stored at both temperatures. A small ash content is a desirable characteristic for sweet potato flour (Van Hal 2000). The mean ash contents were 4.5 k 0.2% and 4.6 & 0.2% for the HSPF at 4C and 21 to 25C, respectively. During storage, the mean fat contents were 0.8 f 0.04% and 0.9 f 0.03 % for the HSPF at refrigerated or room temperature, respectively. Sweet potato is an important source of protein in many developing countries. However, the protein content in sweet potato flour is generally low, ranging from 1 .O to 14%, with most levels between 1.0 and 8.5% (Woolfe 1992). The protein contents for the HSPF stored at both temperatures was similar and did not change significantly during storage. Carbohydrates account for the bulk of field- grown sweet potato flour, ranging from 85% to 95% (Van Hal 2000). Throughout the storage period, our findings were consistent with these ranges for the HSPF stored at both temperatures (Table 1).

The 11 k 0.1 % dietary fiber content of the HSPF was not surprising, since fiber content can vary with the analytical procedures used. A small dietary fiber content is a desirable attribute for sweet potato flour (Van Hal 2000). Dietary fiber between 0.4 and 13.8% was cited by Van Hal (2000). P-carotene content in sweet potato flour decreases over time with processing and during heat treatment (Van Hal 2000). It is not known whether there was a decrease in p- carotene during storage because it was measured only at the end of the study. Carotenoid content, including P-carotene is highly variable in sweet potato flour possibly due to cultivar and/or processing method (Van Hal 2000). The hydroponic sweet potato flours exhibited a final mean P-carotene value of 16,800 pg/100 g. This value was consistent with those of 60 to 39,848 pg 8- carotene/l00 g reported for field-grown sweet potatoes of selected cultivars (Woolfe 1992). The ascorbic acid, thiamin, and riboflavin values are presented in Table 1 .


4C - (SPF stored at refrigeration temperature) Replication I Replication 2

21 to 2SC - (SPF stored at room temperature) Replication 1 Replication 2

One of the most important appearance attributes of sweet potato flour is its color. Sweet potato flour color ranges from whitish or cream colored to different hues of yellow, brown, light or pale orange to pale purple (Hagenimana 1998a). The color is due to a number of complex biochemical reactions such as carotenoids and anthocyanins. It is suggested that in sweet potato flour, the yellow pigments such as the anthocyanins form colors that affect the red-green chromaticity of the flour (Collado and Corke 1999). Whiteness, redness and yellowness/orangeness are described by the L*, a* and b* values.

Gurkin-Ulm (1988) reported that the b* value was the only color component affected by storage of the sweet potato flour. However, in this study the L* values for the HSPF increased as storage time increased (74.4 k 0.03 to 77.3 k 0.01 and 75.5 k 0.01 to 78.9 k 0.01 for flour stored at 4C or 21 to 25C, respectively). The mean L*, a*, and b* values are presented in Table 2. Both a* and b* values decreased as storage time increased for the sweet potato flour stored at 4C and 21 to 25C. Throughout the storage period, the sweet potato flour stored at 21 to 25C exhibited higher L* values than the L* values at 4C, indicating that the orange color faded more readily with exposure to more light and/or ambient temperatures. These findings were consistent with those of Collins and Hall (1992) for field grown sweet potatoes.

L* a* b*

76.M1.5 3.Oi2.0 13.952.4 76.151.5 3.M2.0 13.9*2.4

77.4k1.7 1.6k3.6 12.455.3 77.451.7 2.853.1 12.4k5.3

TABLE 2. MEAN COLOR VALUES FOR THE SPF STORED AT 4C AND 21 TO 25C

In general, the orange color decreased as storage time increased for the hydroponic sweet potato flours stored at refrigerated and room temperatures. Except for some loss of the yellow/orange color, no discoloration or browning of the hydroponic sweet potato flour was observed during processing and storage as is commonly reported. It is speculated that the HSPF contains small to large concentrations of phenolic compounds and natural inhibitors of polyphenol oxidase. These compounds play a role in discoloration of sweet potato flour (Van Hall 2000).

It was concluded that the storage temperature and time period (4C or 21 to 25C for five months) did not significantly affect the proximate composition and


color of the hydroponic sweet potato flour. However, hydroponic sweet potato flour stored at 21 to 25C lost more of the yellow/orange color than that stored at 4C. The processing procedures utilized resulted in a hydroponic sweet potato flour with desirable quality which did not deteriorate significantly when stored for five months.

ACKNOWLEDGMENTS

The authors wish to thank NASA for the financial support to conduct the study, Dr. Desmond Mortley for providing the hydroponic sweet potatoes, and Mr. Perris Fields for assisting with the proximate analysis.

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PRODUCTION AND PROXIMATE COMPOSITION OF A HYDROPONIC SWEET POTATO FLOUR DURING EXTENDED STORAGE