African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 4, Num. 2, 1996, pp. 233-242

M. A. Ameny and P. Wilson ^1

Department of Food Science and Technology, Makerere University, P.O. Box 7062, Kampala, Uganda
^1 Department of Horticulture, Louisiana Agricultural Experimental Station, Louisiana State University, Agricultural Center, Baton Rouge, LA. 70803, USA

(Received 10 April, 1996; accepted 30 June, 1996)

Code Number: CS96061
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ABSTRACT

White flesh sweetpotatoes (Ipomoea batatas (L.) Lam) are a major food crop in developing countries. The purpose of this study was to evaluate the protein and amino acid content of four white fleshed African sweetpotato roots and Apios americana tubers (American groundnuts). Although the sweetpotato and Apios are not normally considered sources of proteins, the wide use of sweetpotatoes as a source of carbohydrate and the renewed interest in Apios warrant the study of proteins in these crops. Protein content ranged from 4.26 to 8.23% of fresh weight even after pureeing and freeze-drying. One sweetpotato cultivar T1702 had a higher protein content and quality than the others based on amino acid profile evaluation. Processing into puree reduced most of the amino acids to varying degrees. There was a difference in amino acids among sweetpotato cultivars (P<0.05) and between sweetpotatoes and Apios. Apios had a higher level of protein than the sweetpotatoes before and after processing into puree. Two fractions of proteins were identified, namely a "whitle" and a "chomoplast" protein... white and chromoplast protein fractions. The white protein had a higher percentage of both protein and amino acids.

Key Words: Apios americana, Ipomoea batatas

RESUME

Les patates douces e chaire blanche (Ipomoea batatas (L.) Lam) constituent un aliment de base dans les pays en voie de developpement. Le but de cette recherche est d'evaluer la teneur en proteine et en amino acide de quatre patates douces africaines e chaires blanches et les gousses d'Apios americana. Bien que la patate douce et l'Apios ne sont pas normalement consideres comme sources des proteines, l'utilisation considerable de patate douce comme source de carbohydrate et l'interet renouvele dans le mandat en Apios justifient l'evaluation de proteines dans ces cultures. La patate douce et Apios ont des teneur en proteines variables entre 4,26 et 8,23% de poids frais meme apres avoir ete transforme en puree ou seche. Les donnees indiquent qu' un cultivar de patate douce (T 1702) avait une haute teneur et qualite de proteines, par rapport aux autres patates douces comme indique par le profil d'acides amines. Le fait de transformer la patate douce en puree reduisait le plupart des acides amines avec des degres variables et il y avait une difference significative entre les acides amines dans les cultivars de patate douce (P=0.05)/ et entre la patate douce et l'Apios. L'Apios avait un niveau eleve de 7,7% de proteines que la patate douce quant il est encore brute et de 8,28% lorsque transforme en puree. Deux fractions de proteines ont ete trouve et nomme fractions de proteines chromoplasmes et blanches. La proteine blanche avait un pourcentage eleve de proteines et d'acides amines.

Mots Cles: Acides amines, Apios americana, proteines, fractions de proteines, Ipomoea batatas

INTRODUCTION

The sweetpotato (Ipomoea batatas (L.) Lam) is a major food crop in developing and developed countries. Approximately 90% are produced in Asia, just under 5% in Africa and only about 5% in all the rest of the world. Although the protein content of the typical sweetpotato is not high, that which is present contains several of the essential amino acids (Walter et al., 1984). Little dietary information is known about the white fleshed varieties grown more commonly in developing countries, where nearly all the world's sweetpotatoes are produced and consumed.

The crude protein content of sweetpotato (Kjeldahl nitrogen X 6.25) has been reported to range from 1.3% to greater than 10% (dry basis) (Purcell et al., 1972; Splittstoesser et al., 1973; Li, 1974; Splittstoesser, 1977; Dickey et al., 1985; Picha, 1985).

A limited number of reports is available concerning the nutritional quality of isolated sweetpotato protein. Amino acid analyses that are available suggest that total sulphur is the most limiting and lysine is the second limiting amino acid in sweetpotato protein (Nagase, 1957; FAO/WHO, 1973; USDA, 1980; Walter and Catignani, 1981). Data available suggest that there is some amino acid variability both between cultivars and within the same cultivar. Complete amino acid analyses of sweetpotatoes have been carried out by several researchers and others have been reviewed (Walter et al., 1984). Wide ranges found in sweetpotato essential amino acids are believed to be due to cultivar, environment, cultural effects and postharvest treatment of the roots. Lysine contents have been found to range from 2.2g/16gN (Yang et al., 1975) to as much as 7.2g/16gN (Purcell et al., 1982). The S-containing amino acid cystine, has been found in some cultivars and completely absent in others. Total S-containing amino acids are also variable, and can be as high as 3.9g/16gN, as in one cultivar grown in Taiwan (Li, 1982). Tryptophan content has been reported to be very low by some authors, 0.1g 100g-1 protein (Splittstoesser and Martin, 1975) and 0.28g 100g-1 protein (Dickey et al., 1984), but adequate by others (Goodbody, 1984).

American groundnut (Apios americana) has been suggested as a plant that offers tremendous potential for domestication as a food source (Blackmon and Reynolds, 1986). Its sweet, starchy tubers were once highly esteemed by the native Americans (Beardsley, 1939). Apios americana is a nitrogen-fixing perennial legume which produces tubers on the rhizomes and seeds on the above ground part. It has a twining, climbing vine which may be 0.9 to 4.6 m long. The vine is vulnerable to freezing temperatures and, being succulent , will deteriorate during the winter. The plant is most often found in moist areas and along streams where it can get full sunlight, at least part of the day (Blackmon and Reynolds, 1986). The crop is widely distributed in the eastern North America from Canada to Southern Florida and has shown the capacity to grow in waterlogged and acidic soils (Reed and Blackmon, 1985). Early American settlers and explorers ate the tubers (which were called groundnuts, potato beans, or Indian potatoes) boiled, fried, or roasted. In spite of the importance of this plant for the North American Indians and early settlers from Europe, the composition and nutritive quality of its seeds and tubers have not been reported until recently. Researchers have reported a high crude protein content (17% dry weight) of the tubers (Yanowsky and Kingsbury, 1938; Sanchez and Duke, 1984; Wilson et al., 1987).

The objective of this study was to determine the protein and amino acid contents of sweetpotatoes and Apios and the effect of processing on these nutrients. Four cultivars of white fleshed sweetpotatoes of African origin and one variety of Apios were studied. The sweetpotato cultivars were T3013, T1702, T3002, T3006, and the Apios was No 784. The protein and amino acids were analysed in the processed and unprocessed samples.

MATERIALS AND METHODS

Sweetpotatoes were obtained from the International Institute of Tropical Agriculture, Nigeria, through the USDA-ARS Southern Regional Plant Introduction Station, and the Apios was obtained from the Louisiana State University. All sweetpotato, and Apios seedlings were grown at Hill Farm, Louisiana State University, Baton Rouge, LA. U.S.A., following common cultural practices. After harvest the sweetpotatoes were cured at 30 C, 80-90% relative humidity and stored at 15 C, 80-90% relative humidity for 1-6 months until needed for the experiment. Apios tubers were stored in plastic bags at 5 C. The sweetpotato and Apios were used for protein and amino acid content determination on raw samples, and for processing into puree.

Processing. Sweetpotato roots and Apios tubers were sprayed with warm water to remove surface soil and lye peeled for 3-5 minutes in a 15% sodium hydroxide solution at 80-90 C. They were rinsed in water to remove sodium hydroxide and coarsely chopped in a food cutter (Hobart Manufacturing Co, Troy, Ohio) to a 5-10mm particle diameter. Fifteen percent of the chopped portion was removed to be added back later as a source of enzyme for the conversion of starch. The remaining chopped material was cooked for 40 min. in a steam jacketed kettle at 900C and ground through a comminuting mill with a 5mm screen (Fitzpatrick Co., Chicago, Illinois) to form a puree. The chopped raw portion was added and the mixture held at 72-74 C for 20 min. after which the temperature was raised to 90 C, to inactivate enzymes. The puree was milled using 0.8mm screen. Consistency was adjusted by adding water to achieve 6-8 Bostwick at 90 C. Cans (401 x 411) were filled with the puree and processed in a steam retort at 115 C for 100 min. The canned purees were stored under ambient conditions. The puree samples were dried in a freeze drier (The Virtis Co. Inc., Gardiner, New York). Protein in freeze dried samples was determined by Kjeldahl method (AOAC, 1985).

Protein extraction. The protein concentrations and isolates from sweetpotatoes and Apios were extracted as described by Purcell et al. (1978), with the following modifications: The samples were washed in 15% lye for 3-5 minutes and rinsed in water to remove excess sodium hydroxide and coarse chopped. The fibre was then removed by passing through 1.3mm screen (Bader, 694; Bock, Toledo, OH). Equal parts of defibred sample were mixed with water except in the case of Apios where two parts were required. The water-sample mix were mixed in a Butcher Boy mixer (Bock, Toledo, OH) for 10 minutes.

The mixtures were then spun in a centrifuge (Bock, Toledo,OH) lined with cheese cloth mesh size (0.08mm) at 800 x g for 10 min. until all the extract was collected in plastic buckets. The extract was heated while stirring in a scraped surface steam kettle to 65 +/- 1C. Upon addition of 0.1% calcium chloride, the chromoplast protein precipitated and was compacted by centrifugation at 10,000 x g for 10 min. Heating the supernatant to 95 C coagulated the "white protein". This was compacted for 10 min. in a centrifuge at 10,000 x g. The protein pellets were purified by solvent extraction with ethanol, hexane-acetone (1:1) and ethyl ether. The resulting powders were dried in a forced air oven at 70 C overnight. The protein contents in the extracted sweetpotatoes and Apios were determined by Kjeldahl method (AOAC, 1985).

Hydrolysis procedure for quantifying Amino acid content. Approximately 200 mg of finely ground sample was placed in a 20 x 125 mm culture tube, with Teflon lined screw cap (Spitz, 1973). Six N HCl was purged with nitrogen for 15 min. The culture tube was then flushed with nitrogen and 5 ml of 6 N HCl was added. The sample was allowed to become thoroughly saturated in HCl, and the culture tube flushed with nitrogen again. Ten ml of 6 N HCl was further added to the culture tube and the tube was again flushed with nitrogen. The sample was then hydrolysed by heating for 22 hr. at 110 C in forced air oven. Samples were cooled in a fume hood, then vortexed for approximately 30 sec. and filtered through a Whatman No. 42 filter paper to remove large particulate matter. Each sample was again filtered with 0.45um disposable syringe filter into storage scintillation vials. Two ml of the aliquot was pipetted into a 12 x 75 mm test tube. This was evaporated to dryness on the Savant SpeedVac SVC 100 at 387 x g and 573 x g (Savant Instrument Inc, Farmingdale, NY). The dry sample was solubilised in 2 ml of Na-S that contained 0.1 uM Norluecine/ml(NLE)(buffer). Further dilution of sample was necessary, especially for the protein extracts. These were diluted 11:1 with Na-S (Buffer, 95% water, 2% sodium citrate, 0.5% thiodiglycol, 0.1% benzoic acid, 1% HCl, pH 2) (Beckman, Palo Alto, Ca). The dilution was necessary to avoid exceeding the linear range of the analyzer. Five ul of the final sample was injected for analysis in the analyser (Beckman System 6300, High Performance Analyser, Beckman Instruments, Palo Alto, Ca).

Hydrolysis for determination of sulfur amino acids. One hundred ml of performic acid was made by mixing 90ml formic acid to 10ml hydrogen peroxide (30%). This was cooled in a freezer for at least 1 hr, and used the same day (Gehrke et al., 1987). Ten mg crude protein of the sample was weighed into glass tubes with screw caps. Performic acid was added to the tubes held in an ice bath. These samples were placed on ice in a refrigerator overnight. The SpeedVac evaporator was brought to -92 C, and 1 ml 48% hydrobromic acid was added to samples under a hood. The tubes were swirled in the ice bath. After the initial reaction subsided, about 30-40 minutes, the tubes were allowed to warm to room temperature. The samples were then evaporated at 307 x g in Savant SpeedVac SVC 100 about 5-6 hr. They were then removed and 5ml of 6N-HCl added with 10% phenol, covered and placed in 1100C oven for 22 hr.

The samples were removed from the oven, cooled to room temperature under a hood, and evaporated at 307 x g (overnight). Samples were redissolved in 5 ml NLE buffer and spun at 840 x g for 5 min., filtered through with 0 .45u 25 mm filter, and then frozen. Before analyses, samples were diluted 1:5 with NLE (Beckman System 6300, High Performance Analyser).

Alkaline hydrolysis method for determination of tryptophan content. Nitrogen was used to deaerate 4.2 N NaOH prior to hydrolysis. The samples were weighed between 0.1 to 0.2 g into a pollyallomer tube, to which was added 0.1 ml thiodiglycol as antioxidant. Ten ml of nitrogen- deaerated 4.2 N NaOH was pipetted into the tubes without mixing. One-tenth ml of octanol was added and air-space was purged with nitrogen for a minimum of 30 sec. Tubes were sealed with Teflon lined cap, and then mixed thoroughly on a mixer. Hydrolysis was accomplished by heating in a forced-air oven at 110 C for 20 hr. Tubes were then removed and cooled without opening. After additional cooling in an ice bath (4 C), 5.0 ml of citrate buffer (pH 4.2) was pipetted into the tube in which a small stirring bar was placed and the pH was adjusted to 4.2 with cold 6 N HCl. The samples were then quantitatively transferred to a 50 ml volumetric flask and diluted to volume with the pH 4.2 citrate buffer. The tube was capped and mixed thoroughly, allowed to settle, and approximately 5 ml was transferred to a high speed centrifuge tube and frozen. As needed, samples were then thawed and centrifuged at 4200 x g for 1 hr and the supernatant was applied to the column of a High Performance Liquid Chromatography analyser.

Statistical analyses. One-way analysis of variance was carried out by general linear model (GLM) procedure using The Statistical Analysis System (SAS Institute, Inc., Cary, NC, 1994). The difference between the means were determined by Duncan's Multiple Range test.

RESULTS AND DISCUSSION

Apios had the highest protein content on a dry weight basis and sweetpotato T3013 had the lowest value as shown in Table 1. There were no measurable changes in the protein content of sweetpotatoes and Apios with processing. This contrasts with earlier reports of protein changes during heat processing (Juritz, 1921). Reports by Watts et al. (1975) and Englyst et al. (1988) indicate that processing may not reduce protein content. The protein content in this study ranged from 4.3% to 7.7% (dwb) when unprocessed and from 6.26% to 8.18% for processed samples. This falls within the range of 1.3% to 10% (dwb) (Purcell et al., 1972; Li, 1974; Splittoesser, 1977; Dickey et al., 1984; Picha, 1985). There was, however, variation in the protein content between cultivars as reported earlier by Bradbury et al. (1985). This implies that it should be possible to breed for high protein sweetpotatoes.

Table 1

Table 2

Amino acid profile of sweetpotatoes and Apios.

Tables 3 and 4 show that processing reduced lysine and cystine amounts in the sweetpotatoes and Apios but did not appear to affect the other amino acids severely. Lysine was reduced in Apios by 20%, in cultivar T1702 by 49%, in T3002 by 38%, in T3006 by 47% and in T3013 by 18%. For cystine the reduction was by 52% in Apios , 12% in T1702, 17% in T3002, 52% in T3006 and 19% in T3013.

Amino acid analysis of the protein fractions showed that the white protein had a substantially greater percentage of some essential amino acids than did the chromoplast fraction (Table 5 and 6). Both fractions, however, contained less than the FAO/WHO (1973) reference protein (methionine + cystine 29mg and lysine 52 mg). The chromoplast fraction was slightly deficient in lysine, while the white protein had a slightly higher percentage of lysine. The white fraction had higher amino acid levels for both the essential and nonessential amino acids as shown in Tables 5 and 6. Therefore, the white fraction would be a better source of protein and amino acid than the chromoplast fraction.

Only Apios had nearly as high values of protein as T1702. Processing reduced all the amino acids, but reduced lysine much more. The protein fractions had higher amino acids than the nonextracted portions, the white fraction having higher amino acids in all cases. There was a significant difference (P<0.05) between the cultivars of sweetpotatoes. The protein content and amino acid profile varied according to cultivar and this agrees with findings of Yang et al. (1975), and Purcell et al. (1982).

Table 3

Table 4

Table 5

Table 6

Protein content differed among the cultivars ranging from 4-8% on dry weight basis. Attempts to obtain protein fractions showed that there were two fractions, white and chromoplast fractions. The white fraction had more protein than the chromoplast fraction ranging from 9-74%.

There were also significant differences (P <0.05) between the amino acid percentages found in the different cultivars. It can therefore be concluded that American groundnut and white fleshed sweetpotatoes may be able to supply protein where needed. American groundnut, however, had higher values of essential amino acids like leucine, lysine, phenylalanine, valine tryptophan and threonine but not sulfur amino acids.

ACKNOWLEDGMENT

The authors would like to thank Louisiana Methodist Church, Gerber Foods, Louisiana and Agricultural Experimental Station and Lousiana Agricultural Center for making the funds available for this study.

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Copyright 1996 The African Crop Science Society

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