Free sugar composition of sweetpotato cultivars after storage

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1 New Zealand Journal of Crop and Horticultural Science ISSN: (Print) (Online) Journal homepage: Free sugar composition of sweetpotato cultivars after storage S. L. Lewthwaite, K. H. Sutton & C. M. Triggs To cite this article: S. L. Lewthwaite, K. H. Sutton & C. M. Triggs (1997) Free sugar composition of sweetpotato cultivars after storage, New Zealand Journal of Crop and Horticultural Science, 25:1, 33-41, DOI: / To link to this article: Published online: 22 Mar Submit your article to this journal Article views: 533 View related articles Citing articles: 10 View citing articles Full Terms & Conditions of access and use can be found at

2 New Zealand Journal of Crop and Horticultural Science, 1997, Vol. 25: /97/ $2.50/0 The Royal Society of New Zealand Free sugar composition of sweetpotato cultivars after storage S. L. LEWTHWAITE New Zealand Institute for Crop & Food Research Limited Pukekohe Research Centre Cronin Road, RD 1 Pukekohe, New Zealand K. H. SUTTON New Zealand Institute for Crop & Food Research Limited Canterbury Agriculture & Science Centre Private Bag 4704 Christchurch, New Zealand C. M. TRIGGS Department of Statistics University of Auckland Private Bag Auckland, New Zealand was significant (P < 0.001) excluding cultivars 'Jewel' and "Toka Toka Gold'. All of the clones produced considerable amounts of maltose during cooking, which was significantly (P < ) related to % dry weight. Using canonical variates to group the clones, 'Owairaka Red', the New Zealand standard cultivar, was shown to cluster with the Japanese cultivar, 'Beniazuma', and the Taiwanese breeding line, '93N12/1', on the basis of high dry matter and low fructose + glucose levels. The minor New Zealand cultivar, 'Toka Toka Gold', clustered near the North American cultivar, 'Jewel', on the basis of low maltose, high sucrose, and medium dry weight. 'Toka Toka Gold' had a higher fructose + glucose than its sucrose concentration would predict, as well as a lower maltose level than estimated from its dry matter content. Keywords sweetpotato; Ipomoea batatas; cultivar; sugar; storage; microwave Abstract The concentrations of fructose, glucose, sucrose, and maltose in sweetpotato {Ipomoea batatas (L.) Lam.) roots, following an 8-month storage period, were assessed by high performance liquid chromatography (HPLC). Imported cultivars and breeding lines were compared in both the raw and cooked state against New Zealand standards. Strong linear relationships were demonstrated between concentrations of the sugars (fructose, glucose, and sucrose) in cooked roots, and the corresponding sugars in raw roots (R 2 of 93.5, 93.8, and 88.8% respectively). The relative proportion of fructose to glucose 0.44 : 0.56 was very stable across all cultivars, and independent of the total concentration of the three sugars fructose, glucose, and sucrose. The negative relationship between sucrose and the levels of fructose + glucose H96036 Received 14 June 1996; accepted 2 December 1996 INTRODUCTION Sweetpotato (Ipomoea batatas (L.) Lam.) is a traditional crop in New Zealand where it is known as "kumara". The crop is composed primarily of one cultivar, 'Owairaka Red', (Lewthwaite 1991) which is generally marketed for table consumption. Over recent years commercial interest in processed sweetpotato products has developed, but has been impeded by the lack of suitable cultivars. A comparison of the established cultivar, 'Owairaka Red', against imported germplasm will guide development of process specific cultivars by highlighting common and novel characteristics. In particular, cultivars are required which retain the eating qualities of 'Owairaka Red' but have a uniform root shape. The sugar composition of sweetpotatoes is a fundamental component of their eating quality. Although generally considered sweet by definition, there is potentially a large range in perceived sweetness amongst cultivars, depending on sugar components and starch conversion at cooking

3 34 New Zealand Journal of Crop and Horticultural Science, 1997, Vol. 25 (Takahata et al. 1992). It has been suggested (Collins 1988) that sugar levels for staple and supplementary staple sweetpotato cultivars should be up to 2 and 5% respectively, with associated dry matters (DM) of > 35% and 30-35%. Sweetpotato as a luxury food item may have a lower DM of 24 28% and variable sugars. In raw tissue, sucrose, glucose, and fructose are present, whereas maltose is only produced during cooking (Picha 1985). During heating, much of the starch is converted into dextrins and maltose by a-amylase and P-amylase (Walter et al. 1975). However, there are cultivar differences in the degree of starch conversion (Babu 1993). It is well established that different sugars, at the same concentrations, have differing perceived sweetness levels. Fructose is c. 5 times as sweet as maltose, sucrose is 3 times sweeter than maltose, whereas glucose is twice as sweet as maltose (Biester et al. 1925). However, simply converting the various sugar components by a sweetness factor to express them in sucrose equivalents does not explain all the sensory response. When individual sugars were compared at the same level of sweetness in sweetpotato puree, sensory panellists consistently ranked the sugars in order of preference, maltose > sucrose > fructose (Koehler & Kays 1991). Therefore, the type of sugar has an effect on flavour in addition to sweetness. Apart from the organoleptic qualities attributable to the sugar components and starch conversion through cooking, sugar composition may significantly affect the appearance of processed products. For example, the presence of high levels of reducing sugars in fried products can cause excessive darkening (Kays 1985). In this paper we report on the concentration of fructose, glucose, sucrose, and maltose in raw and cooked roots from sweetpotato breeding lines and cultivars, grown and stored under New Zealand conditions. MATERIALS AND METHODS Root production The sweetpotato clones were planted as rooted cuttings on 24 November 1993 and harvested on 18 April All clones were planted in the same experimental field at Pukekohe (Lat 'S), New Zealand, in Patumahoe clay loam soil. A base fertiliser of 30% potassic superphosphate was broadcast at 1 t/ha. General cultural practice followed commercial recommendations (Coleman 1972). Following mechanical harvest, uncured roots of marketable quality with a diameter > 2.5 cm, were stored in multi-walled paper bags at 13 C and 90% relative humidity for 8 months. A duplicate sample of the cultivar, 'Beauregard', was stored in multi-walled paper sacks at ambient temperature. Root sampling Twenty randomly selected roots of each clone were divided into four replicates. Each replicate was processed to produce three separate samples: one for determination of % dry weight, and the remainder for high performance liquid chromatography (HPLC) of raw and cooked tissue. To compensate for any bias in carbohydrate distribution within the root, each unpeeled root was cut into quarters along its main axis. Each quarter was then cut into strips at right angles to the main axis. The strips were square in cross-section with sides of c. 7 mm and an overall length of c. 50 mm. For each replicate, the strips of tissue from equivalent quarters of the root were pooled into groups, random strips were then combined from each of the four groups to give a sample with an average weight of 35 g. Cooking consisted of heating each sample in a sealed container, using a microwave oven at 750 W with a frequency of 2450 MHz for 3 min. The samples reached 100 C in min, and after cooking the samples were allowed to cool for 30 min at ambient temperature, before freezing at -30 C. Percentage dry weight was determined by drying samples at 80 C for 5 days. Preparation of root extracts for HPLC The HPLC method used was a modification ofthat of Picha (1985). Exactly 5.00 g was randomly selected from each sample, and the tissue was homogenised in 80% (v/v) ethanol for 1 min at high speed using a Waring 801/G blender (model ). The resulting slurry was agitated gently for 18 h and filtered through Whatman #4 paper. The residue and original container were washed with additional 80% (v/v) ethanol and filtered through Whatman #4 paper. The first and second filtrates were combined and made to a final volume of 50 ml with 80% (v/v) ethanol. Approximately 5 ml of the combined filtrate was clarified by centrifugation (10 OOOg, 10 min) prior to injection into the HPLC.

4 Lewthwaite et al. Free sugar composition of sweetpotato Preparation of sugar standards Sugar standards were prepared by dissolving appropriate amounts of the sugar in 80% (v/v) ethanol and making up the solution to an appropriate total volume in a standard flask. Standard sugar solutions were clarified by centrifugation (10 OOOg, 10 min) prior to injection into the HPLC. HPLC analyses A Waters liquid chromatograph, consisting of a model 600 pump and controller, model 712 autosampler and a model 410 refractive index detector, was used. The detector signal (output at attenuation setting 64) was stored, integrated, and manipulated using an IBM-compatible personal computer running Waters "Maxima" software. Sugars were separated with a 220 x 4.6 mm Applied Biosystems Brownlee AMINO column fitted with a 15 x 3.2 mm Applied Biosystems Brownlee NewGuard AMINO guard column thermostatted to 30 C using a Waters column heater. The mobile phase was degassed HPLC-grade acetonitrile: water (80:20 v/v). Solvent flow rate was 1.5 ml/min. Injection volume for both sugar standards and root extracts was 10 ul. Identification of each sugar was based on HPLC retention times. Detector response to all sugars was linear over the concentration range 0 2% (w/v). Standard sugars exhibited less than 2% variability in individual sugar concentrations between triplicate injections of the same sample. RESULTS AND DISCUSSION The cultivars in this investigation (Table 1) include 'Owairaka Red' and 'Toka Toka Gold' (Lewthwaite 1991) from New Zealand, 'Beauregard' (Rolston etal. 1987) and 'Jewel' (Pope et al. 1971) from the United States, and 'Beniazuma' (Shiga et al. 1985) from Japan. The remaining clones were breeding lines supplied by the Asian Vegetable Research and Development Center, Taiwan (Anon. 1991). The relationships among % dry weight, fructose, glucose, sucrose, and maltose in both raw (R) and cooked (C) tissue were examined (Fig. 1). All of the sugar components were expressed on the common basis of g/100 g raw fresh weight. Examination of the plot indicates strong linear relationships between the sugars (fructose, glucose, and sucrose) from cooked roots, with those from raw roots. Regressing the concentration of fructose, glucose, and sucrose in the cooked sample on the concentration of the corresponding sugar in the raw sample gives adjusted multiple correlation coefficients (R 2 ) of 93.5, 93.8, and 88.8% respectively. In each instance the intercept was not significantly different from 0.0, so regressions were fitted without intercept. The regression coefficients (with standard errors) were 0.94 (0.018), 0.95 (0.017), and 0.97 (0.017) for fructose, glucose, and sucrose respectively. For all three sugars the coefficients are consistent with a 5% reduction during the cooking process. The coefficients for fructose and glucose are significantly < Table 1 Dry matter (DM) content and free sugar composition of raw and cooked roots of stored sweetpotato (Ipomoea batatas (L.) Lam.) genotypes. Sugar contents are expressed as g/100 g raw fresh weight. (Fru, Fructose; Glu, Glucose; Sue, Sucrose; Mai, Maltose.) Genotype Beniazuma 93N12/1 Owairaka Red 93N2/2 Toka Toka gold 93N9/2 Jewel 93N8/2 Beauregard a Beauregard LSD ^Z %DM Total Sugars in raw root Fru 'Beauregard' stored at ambient temperature. Glu Sue Total () () () () () () ().2O Sugars in cooked root Fru. Glu Sue Mai

5 36 New Zealand Journal of Crop and Horticultural Science, 1997, Vol. 25 Dry.wt \.l # *. I <. V '.' '" % *. A' C.fructose A" r" C.glucose.. ^ '.V. % ' " * :, 'A ::. v *» C.sucrose y...: } C. maltose t* /* : s'' A' /-/** "*.* > * * '+ ' '. * * R.fructose * R.glucose * * * :-J * * > 4. / '. r <i V k *. >% R.sucrose Fig. 1 Scatterplot matrix of all pairs of variables: % dry matter (DM) and sugar components (g/100 g fresh weight), from cooked (C) and raw (R) sweetpotato (Ipomoea batatas (L.) Lam.) samples. (P < 0.05). Although some workers have found increased concentrations of fructose, glucose, and sucrose after cooking (Picha 1985; Takahata et al. 1992; Babu 1994) others have shown a reduction in some components (Truong et al. 1986). Our method of root preparation was similar to that of Collins & Walter ( 1985), who also found a reduction in fructose, glucose, and sucrose on cooking. In this experiment there is only a small change in these sugars with cooking. A linear relationship is also seen between fructose and glucose concentrations (Fig. 1). The relative proportion of fructose to glucose of 0.44 : 0.56 is very stable across all cultivars (Fig. 2) and is independent of the total concentration of the three sugars, fructose, glucose, and sucrose. The concentration of fructose and glucose in cooked roots decreases by 1.1 g/100 g of fresh weight with an increase in sucrose concentration of 1 g/100 g (Fig. 3) for all cultivars except for two: 'Jewel' which is high in both sucrose and fructose + glucose, and 'Toka Toka Gold' with a somewhat higher fructose + glucose than its sucrose concentration would predict. For the eight cultivar samples considered, excluding 'Jewel' and 'Toka Toka Gold', the regression line for the cooked sugars glucose + fructose on sucrose is: glucose + fructose = 4.30(0.31)-1.1(0.13) sucrose, (R 2 = 70.3%, PO.001). This relationship can only hold for concentrations of sucrose < 4 g/100 g as the concentration of glucose + fructose cannot fall below 0. The regression coefficient is not

6 Lewthwaite et al. Free sugar composition of sweetpotato 37 Fig. 2 Ternary diagram of fructose, glucose, and sucrose concentrations in raw tissue from various sweetpotato (Ipomoea batatas (L.) Lam.) clones. 1 Beniazuma 2 93N12/1 3 Owairaka Red 93N2/2 Toka Toka Gold 6 93N9/2 7 Jewel 8 93N8/2 9 Beauregard - Ambient 10 Beauregard Fructose 0% Sucrose, 0% Glucose. 100% Fructose Fructose:Glucose ratio 0.44:0.56 Sucrose 100% Sucrose, 0% Glucose, 0% Fructose Sucrose 0% Sucrose. 100% Glucose. 0% Fructose oi o O 3.5 o _ ri LO _ (\Í O - m _ 2" ö 10 6 ix, 8 * \ 6 \ 6 9 \ ^ \ \ \ X 2, Beniazuma 2 93N12/1 3 Owairaka Red 4 93N2/2 5 Toka Toka Gold 6 93N9/2 7 Jewel 8 93N8/2 9 Beauregard - Ambient \ 10 Beauregard A * Sucrose concentration (g/100 g fresh wt) Fig. 3 Sum of fructose and glucose concentrations versus sucrose concentration in cooked samples from various sweetpotato (Ipomoea batatas (L.) Lam.) clones. significantly different from 1.0, suggesting that except for 'Jewel' and 'Toka Toka Gold', the concentration of fructose + glucose + sucrose is effectively constant in all cultivars at a level of 4.11 (0.052) g/100 g fresh weight. N 5 5 The principal change in sugar composition with cooking is the production of maltose from starch. Both cooked and raw samples were tested for maltose. As expected, no maltose was found in the raw sweetpotato roots (Table 1). Maltose is only produced when sweetpotato roots are heated and may contribute substantially to product sweetness. The general principles of maltose synthesis are known. Sweetpotato starch is gelatinised at C, and is then available for hydrolysis by enzymes (Walter et al. 1976). From the onset of gelatinisation, a-amylase rapidly degrades starch, whereas 3-amylase hydrolyses the starch and starch fragments into maltose (Kiribuchi & Kubota 1976). In this study, fructose, glucose and sucrose levels (Table 1) in cultivars from Japan and the United States were of similar magnitude to published data (Picha 1986a; Takahata et al. 1992); however, maltose levels were lower. The samples were prepared by cutting the tissue into strips and the cooked samples were then heated in a microwave oven. Other researchers have shown that if sweetpotato roots are cut into strips and cooked rapidly, significant amounts of starch remain, whereas the cooking of whole roots allows a more complete conversion of starch into dextrins and sugars (Collins & Walter 1985). As processing of raw sweetpotato into pieces before cooking is the standard method used in both household and

7 38 New Zealand Journal of Crop and Horticultural Science, 1997, Vol. 25 O) 1 Beniazuma 2 93N12/1 3 Owairaka Bed 4 93N2/2 5 Toka Toka Gold 6 93N9/2 7 Jewel 8 93N8/2 9 Beauregard - Ambient 10Beauregard 10 UP i i 1 ' ^ S % dry matter CD rial CD > "ÖS o 1 canoi UÎ - O - 8 CD * r" 2 1 I 3 ' '4 6 \ ', Í 3"--.v Ï /' i Beniazuma 2 93N12/1 3 Owairaka Red 4 93N2/2 5 Toka Toka Gold 6 93N9/2 7 Jewel 8 93N8/2 9 Beauregard Ambient 10 Beauregard a 4 / 2 5 IV *? 5 / First canonical variate i * ' I'B / / : 10 III / 8 88'" /' t 1.9, / \ : 1tl io; / 10 Fig. 4 Plot of maltose concentration versus % dry weight for cooked samples from various sweetpotato (Ipomoea batatas (L.) Lam.) clones. Fig. 5 Sweetpotato (Ipomoea batatas (L.) Lam.) clones grouped into clusters by canonical variates analysis, based on % dry matter and sugar concentrations in cooked samples. industrial sweetpotato preparation, it was used in this study. It has been shown that sugar concentrations in microwaved whole roots are similar to those of roots cooked in a convection oven (Picha 1985). As maltose is produced from starch in the cooking process, and starch makes up a large component of sweetpotato dry weight, the relationship between maltose and % dry weight was examined. There are a number of factors which may obscure the relationship, as researchers have shown that there are cultivar differences in maltose production because of the various levels of a- amylase and p-amylase (Takahata et al. 1992; Babu 1994). Also, with our system of sample preparation and heating, the reaction would not be expected to go to completion. A plot of maltose concentration versus % dry weight (Fig. 4) shows that, with the exception of 'Toka Toka Gold', as % dry weight increases by 5%, the concentration of maltose increases by 1.45(0.14) g/100 g (R 2 = 75.5%, P < 0.001). Assuming that starch is a large component of the DM content of the clones examined, it would appear that maltose production was not primarily limited at an enzyme level, but at a substrate level. The maltose level of 'Toka Toka Gold' was low relative to the % dry weight, but whether that represents a lower starch content or a lower enzyme level cannot be assessed from the data. It would be of interest to examine the predictive value of dry weight for maltose levels under preparation and cooking systems that favour starch conversion. Of the 54 clones investigated by Takahata et al. (1992), 89% were classified in the high maltose production group (average 10.7 g/100 g fresh weight). This category appears to describe the clones examined here, as none showed exceptionally low maltose levels even with an unfavourable cooking method. The similarities between clones can be assessed by carrying out a canonical variates analysis (Mardia et al. 1979), using the variables; % dry weight and the concentrations of fructose, glucose, sucrose, and maltose in the cooked samples. A canonical variate is a weighted average of the variables in the study, the weights being chosen to maximise differences between the cultivars. The first two canonical variates explained a total of 84.8% of the variation between cultivars (53.3 and 31.5% respectively). The distribution of cultivars is shown in Fig. 5, and can be seen to fall into four clusters. Cluster I 'Owairaka Red', 'Beniazuma', and '93N12/1'; Cluster II '93N2/2' and '93N9/2'; Cluster III 'Beauregard', '93N8/2', and

8 Lewthwaite et al. Free sugar composition of sweetpotato 39 'Beauregard' ambient; and Cluster IV 'Toka Toka Gold' and 'Jewel'. Variables that distinguish these clusters can be inferred from the previous discussion and from the variable loadings of the first two canonical variates. Cluster I contains cultivars with high dry weight (>26%) and low fructose + glucose levels, Cluster II comprises cultivars with high maltose and low sucrose concentrations, with medium dry weight. The third cluster contains cultivars with low dry weight (<20%), whereas the fourth cluster includes cultivars with low maltose and high sucrose, with medium dry weight. In this study, the sugar composition was examined after only one storage period. Carbohydrate metabolism is not static during storage. Nevertheless, there appears to be stability in rates of change and a degree of conformity across cultivars after several months of storage (Picha 1986a, 1987a). As the sweetpotato season is restricted in New Zealand, most of the fresh market produce is sold from storage. Processing requirements also generally dictate a storage period so that the supply of raw material corresponds to production windows in the factory and market. Unlike the roots used in this study, most commercial sweetpotato roots are cured for a short period at high temperature and humidity before storage. However, if roots are stored for a sufficient length of time, cured and uncured roots have similar sugar levels (Walter 1987). Sweetpotato roots exhibit a gradual weight loss throughout their storage life. Transpiration is the major source of weight loss in storage and the contribution of respiratory carbon loss to total weight loss is slight (Picha 1986b). The % dry weight following storage is higher than at harvest. However, weight loss is not determined solely by root moisture content at harvest as there is a cultivar effect on the rate of transpiratory loss. The % dry weights shown here (Table 1 ) are the result of the cultivar by storage interaction. However, their relative order appears to follow that of freshly harvested roots. Approximately 70% of sweetpotato DM is starch (Kohyama & Nishinari 1992) and, in general, starch content decreases in storage (Picha 1987a). Picha (1987a) demonstrated that in stored raw roots, carbohydrate metabolism after curing and the first 4 weeks of storage led to a stabilisation or a slight increase in the monosaccharides, fructose and glucose. The sucrose content, after 4 weeks of storage in some cultivars and 14 weeks in others, showed a general increase, although the rate of change slowed in later stages of long-term storage. In raw roots sucrose was the major sugar, and the glucose concentration tended to be higher than fructose, which is also demonstrated in this experiment. When baked roots were examined (Picha 1986a) after various storage periods in the raw state, maltose was the primary sugar and sucrose the secondary sugar. However, over longer storage periods, sucrose became the primary sugar, as maltose declined after 4 weeks of storage and sucrose content increased. The decline in maltose may be because of less available starch for enzyme hydrolysis and lower P-amylase levels. The level of a-amylase activity increases during storage (Deobald et al. 1968), whereas p-amylase decreases slightly (Walter et al. 1975). The largest sugar changes in baked roots occur during curing and the first 4 weeks of storage (Picha 1986a). One 'Beauregard' sample was stored at ambient temperature while a similar sample was stored under controlled conditions at 13 C. In New Zealand, most sweetpotato storage, after curing, is conducted without temperature and humidity control. 'Beauregard' was selected to demonstrate sugar changes under ambient conditions as it rarely sprouts in storage, is robust to storage rots and was recently made available to New Zealand commercial growers. Sweetpotatoes are known to be susceptible to chilling injury when stored at temperatures below 12 C. Low temperature sweetening may occur, generally by accumulation of sucrose, but there are considerable cultivar differences in the extent of sweetening and in specific sugar changes (Picha 1987b). In this experiment, 'Beauregard' at ambient storage, was subject to cool conditions for some months followed by warmer temperatures (Table 2). There was no significant difference in % dry weight (P = 0.51) Table 2 Mean monthly air temperatures ( C) at Pukekohe, New Zealand during 1994, over the sweetpotato (Ipomoea batatas (L.) Lam.) storage period (Pukekohe Research Centre meterological data). Month: Temperature Apr 15.2 May 13.9 Jun 9.7 Jul 9.9 Aug 10.5 Sep 11.3 Oct 12.7 Nov 14.7 Dec 17.3

9 40 New Zealand Journal of Crop and Horticultural Science, 1997, Vol. 25 or maltose production (P = 0.46) between the two storage regimes. Storage in ambient conditions did causea significant reduction of fructose (P < 0.01) and glucose (P < 0.01) levels. It has been reported for 'Jewel' that transferring roots from 7 to 15.6 C storage either maintained or increased sucrose levels above those produced during the cooler storage (Picha 1987b). Sucrose concentration for both raw and cooked samples increased in this investigation (P < 0.001, Table 1 ). This corroborates the inference of Picha ( 1987b) that sucrose synthesising enzymes are more active at lower temperatures with concentration gains maintained during warmer storage. Fructose and glucose levels declined in ambient conditions, however the total sugar change was not significant. The decline in reducing sugars, although retaining product sweetness, may be advantageous for some processed goods such as sweetpotato crisps where it has been shown that the dark colour is positively correlated with reducing sugar content (Picha 1986c). CONCLUSIONS The empirical relationships between fructose, glucose, sucrose, maltose, and % dry weight were examined across clones. The main effect of cooking was to add maltose to the sugar composition, as levels of fructose, glucose, and sucrose remained largely unchanged. Other researchers have identified a small number of clones with low maltose production, probably because of low p1- amylase levels, but all clones examined here produced considerable amounts of maltose and therefore appear to have conventional amylase activity. An examination of the sugar levels in raw tissue demonstrated some interrelationships between individual sugars that may be of use in screening germplasm destined for specific end uses. The stability of the fructose to glucose ratio across clones, despite varying total raw sugars, was particularly striking. The established New Zealand cultivar, 'Owairaka Red', appeared to develop DM content and sugar components similar to those of other clones in this study, particularly the Japanese cultivar, 'Beniazuma', and the Taiwanese breeding line, '93N12/1 '. The minor New Zealand cultivar, 'Toka Toka Gold', exhibited characteristics comparable to those of the North American cultivar, 'Jewel'. As 'Owairaka Red' was not exceptional in terms of the attributes measured here, it would seem imported germplasm could allow development of cultivars suited for particular end uses while retaining traditional organoleptic qualities. REFERENCES Anonymous 1991: Elite sweet potato germplasm now available for distribution. Centerpoint 9(2): 12. Babu, L. 1994: Changes in carbohydrate fractions of sweet potato tubers on processing. Tropical agriculture 71(1): Biester, A.; Wood, M. W.; Wahlin, C. S. 1925: Carbohydrate studies. 1. The relative sweetness of pure sugars. American journal of physiology 73: Coleman, B. P. 1972: Kumara growing. Wellington, New Zealand, A. R. Shearer, Government Printer. 42 p. Collins, W. W. 1988: Improvement of nutritional and edible qualities of sweetpotato for human consumption. Pp in: Exploration, maintenance, and utilization of sweet potato genetic resources. Report of the First Sweet Potato Planning Conference Peru, International Potato Center (CIP). Collins, W. W.; Walter, Jr, W. M. 1985: Fresh roots for human consumption. Pp in: Sweet potato products: a natural resource for the tropics. Bouwkamp, J. C. ed. Florida, CRC Press. Deobald, H. J.; McLemore, T. A.; Hasling, V. C; Catalano, E. A. 1968: Control of sweet potato alpha-amylase for producing optimum quality precooked, dehydrated flakes. Food technology 22: Kays, S. J. 1985: Formulated sweet potato products. Pp in: Sweet potato products: a natural resource for the tropics. Bouwkamp, J. C. ed. Florida, CRC Press. Kiribuchi, T.; Kubota, K. 1976: Studies on cooking of sweet potato (Part 1). Changes in sugar content and (3-amylase activity during cooking. Kaseigaku Zassi 27: Koehler, P. E.; Kays, S. J. 1991: Sweet potato flavour: quantitative and qualitative assessment of optimum sweetness. Journal of food quality 14: Kohyama, K.; Nishinari, K. 1992: Cellulose derivatives effects on gelatinization and retrogradation of sweet potato starch. Journal of food science 57(1): Lewthwaite, S. L. 1991: Potential for introduced sweet potato cultivars in New Zealand. Proceedings of the Agronomy Society of New Zealand 21: 1-5.

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