The evaluation of sweetness, starch and sugar concentration of Ipomoea batatas L. Rancing cultivar (Cilembu sweet potato) cultivated at specific location in the
villages of Cilembu and Cimaung
Erly Marwani1*, Risti Desiyanti1, and Yeyet Setiawati1
1School of life sciences & technology, Institut Teknologi Bandung, Indonesia
*Corresponding author, Email: [email protected] ABSTRACT
In order to examine the potency of a non-typical land for cultivation of Cilembu sweet potato (I. batatas of Rancing cultivar) that might generate a sweet taste similar to the yield planted in typical land, evaluation of sweetness, sugar and starch concentration of the cultivated sweet potato was carried out at Cimaung and Cilembu villages. Results indicated that concentration of starch in the freshly tuber that harvested at Cilembu and Cimaung were 37 % and 35%, and decreased to 19.6 % and 31.5
%, within 5 weeks after storage, respectively. High Pressure Liquid Chromatography analysis showed that freshly sweet potato consisted of soluble sugar of fructose, glucose and sucrose, while baked sweet potato showed the presence of maltose, fructose, glucose and sucrose. The total soluble sugar in the freshly harvested sweet potato from Cilembu was higher than that of Cimaung, 4.0 % compared to 2.6% and reach maximum to 9.4% and 6.0%, at 4 weeks after storage. Principle component analysis indicated that starch and sugar content significantly showed positive correlation with elevation, rainfall, soil nutrient content, C/N ratio and cation exchange capacity levels. The Cilembu sweet potato cultivated in Cilembu show sweet taste, however it show normal taste in Cimaung.
KEYWORDS
Cilembu sweet potato, Rancing cultivar, sugar, sweetness, PCA
INTRODUCTION
Sweet potato, (Ipomoea batatas L.) is an important staple food in many countries (Law-Ogbomo &
Osaigbovo, 2019). In Indonesia, there is a unique and popular sweet potato known locally as Cilembu sweet potato (Ipomoea batatas Rancing cultivar) that originated from the village of Cilembu (typical area) of West Java. This sweet potato is very popular because when it is baked, it has a distinctive taste similar to sweet honey taste. The sweetness in sweet potato is due to the presence of reducing sugar such as fructose, sucrose, glucose and maltose (Kwarteng et al., 2014) . The honey taste was driven by types of dominant sugar of fructose, sucrose, and glucose (Parker et al., 2010; Anda et al., 2018). While, in normal taste the order of sugar is sucrose, glucose, and fructose (Lewthwaite et al., 1997).
Previous study indicated that the level of reducing sugar in Cilembu sweet potato Rancing cultivar was 14.9 to 21.6 % using Luff Schoorl and Anthrone methods, respectively (Taufik & Guntarti, 2016).
However, according to local consumer, there was a reduction of the sweet taste when the sweet potato was cultivated outside of the area of origin (Cilembu village) and several studies also showed
that there was a variation in the sugar content of Cilembu sweet potato in the Nirkum variety when grown in different cultivation locations (Tohidin, 2006). This might be related to the reported studies that the difference in sweetness level of in sweet potato was influenced by geographical and environmental factors (Ravi & Saravanan, 2012; Zaidiyah, 2014). Similarly, Solihin et al. (2016) reported that soil and climate factors significantly effects the yield and sugar content of Cilembu sweet potato.
Furthermore, Solihin et al. (2017) reported that the sweetness level showed the difference when Cilembu sweet potato was planted in areas with different characteristics of soil (pH, cation exchange capacity, Ca, P, Mg), elevation, and microclimate (temperature, rainfall) . Thus, there was a linear correlation between the physical characteristics of the land (soil depth, clay texture, and monthly rainfall) with a total sugar content of sweet potato in typical and non-typical areas. While, Anda et al. (2018) reported that soil exchangeable K+ and temperature were responsible for the generation of honey taste in sweet potato Rancing cultivar. The required soil exchangeable K+ was 1.0 cmole kg-1 and monthly temperature of 21-22°C occurring at 870-917 m above sea level (asl) accompanied by monthly rainfall of 96-199 mm/month.
The production area of the sweet potato Rancing cultivar is concentrated in the districs of Pamulihan, Rancakalong, Tanjungsari and Sukasari, in the Sumedang regency of West Java province. Those production areas have received Certificate of Geographical Indication (GI) for Cilembu sweet potato (DJKI, 2013). Because the demand for Cilembu sweet potato increased from year to year significantly (Anda et al., 2018), it was necessary to expand areas for cultivation of Cilembu sweet potato. The expansion of the cultivation areas must be directed to non-typical areas (outside the area of origin) because the typical area was already fully planted with Cilembu sweet potato and the area was very limited. Therefore, in order to find out a new location for cultivation of the Cilembu sweet potato at non-typical area (outside of the district of Pamulihan, Rancakalong, Tanjungsari and Sukasari, in the Sumedang regency) that might give the sweet potato Rancing cultivar with taste similar to the yield planted in typical area, Cimaung village was chosen as a test site for cultivation of Cilembu sweet potato. According to local farmer and Dinas Pertanian Kabupaten Bandung (2018), Cimaung village has been used for cultivating sweet potato varieties of Arnet (a variety that shows qualities similar to sweet potato Cilembu) however there was no report about the cultivation of Cilembu sweet potato of Rancing cultivar in Cimaung village. Furthemore, in the field, the Cilembu sweet potato generally is stored for at least 20 days from harvest to get a sweet taste when consumed (Onggo, 2006) or 1-3 weeks before being sold for consumption (Solihin, 2017). Therefore, it is necessary to test the suitability of Cimaung land for Cilembu sweet potato cultivation with test indicators for sugar content and sweetness levels at harvest and storage for up to 5 weeks to be able to assess the quality of the Cilembu sweet potato produced.
The aim of this study was to evaluate the potential of Cimaung village as a non-typical area for the Cilembu sweet potato cultivation that might generate a sweet taste similar to the yield planted in Cilembu by examining concentration of sugar and starch, and the sweetness of the sweet potato. The experiment was carried out at Cimaung village and Cilembu village as a control for comparation. The
experimental was randomized block design with using 64 plant replicates at each location. The main variable observed was the location with parameters measured at each location were microclimate and soil characteristics (soil texture, soil pH, nutrient content, and cation exchange capacity). After being cultivated for four months, the level of the sugar, starch concentration and the sweetness of the freshly and also the stored sweet potato that cultivated at two different locations were analyzed and compared.
Sweetness was also evaluated on the baked Cilembu sweet potato that had showed relatively high total sugar content during storage. Evaluation of sugar in baked sweet potato is intended to test consumer preferences on the sweetness as described by Anda (2018). In this study, the sweetness was evaluated based on total sugar analysis and relatively to sucrose equivalent according to the method of Shallenberger (1993); Owusu-Mensah et al. (2016). It was expected that this research will give information of the possibility for using Cimaung village as an alternative area for Cilembu sweet potato Rancing cultivar cultivation.
MATERIALS AND METHODS Experimental procedure in the field
Cultivation areas
Cultivation of the sweet potato (Ipomoea batatas L.) was carried out from April to July 2018 in two different areas, i.e. the Cilembu village, Pamulihan district and the Cimaung village, Jagabaya district of West Java (Figure 1). The Cilembu village was categorized as the typical area where the origin of sweet potato of Rancing cultivar came from, whereas Cimaung area was categorized as the non-typical area that was not the origin area of sweet potatoof Rancing cultivar (Solihin et al., 2017). Hence, Cilembu village was called the typical land and Cimaung village was called the non-typical land. The Cilembu village was located at an elevation of 870 m with coordinates 06°54'9.74" South Latitude and 107°51 '14.77" East longitude. The Cimaung Village is located at an elevation of 821 m with coordinates 07°04'29.47" South Latitude and 107°33'59.10" East Longitude.
Cultivar of sweet potato
The cultivar of sweet potato (Ipomoea batatas L.) used in this experiment was known as the Rancing cultivar which has yellow to redish yellow flesh color of root tubers. The Rancing cultivar was derived from Nircum cultivar which was crossed with another sweet potato that has wide adaptation.
Morphologicaly, shape of leaves of Rancing cultivar used was deeply lobed. In this experiments, shoot cuttings from 5 months old of cultivar Rancing were obtained from the local farmers of typical area in Cilembu village.
Land preparation and plant cultivation
The study was carried out at two spesific locations in Cilembu and Cimaung villages. The land used for cultivating the sweet potato at each location is an area of 100 square meters (10m x10m). Before the sweet potato were planted, the land was dug until the soil becomes loose and about 20 Kg manure was added. The soil left to dry for 1 week, followed by making land mounds with a width of 60 cm, the
height of 30-40 cm and the distance between mounds was around 1m. After the land was ready, planting sweet potato was done by making holes in the prepared land with a distance of about 50 cm between the hole. Each hole was enriched with NPK fertilizer (16/16/16) as much as 30 g/hole. The shoot cuttings that have been prepared were then planted into the hole and buried in half of the soil. The experimental design was randomized block design with using 64 plant replicates in each location. The main variable observed was the characteristics of the lands with parameters measured at each location were microclimate and soil characteristics (soil texture, soil pH, nutrient content, and cation exchange capacity). After planting, the plants were watered for 15-30 minutes/day continuously until the plants were 1-2 months old. However, after 2 months, the watering frequency was reduced to every 2 weeks until harvesting time. After 4 months cultivation, the tubers were harvested as follow, firstly the leaves with their stem were cut off from the roots followed by loosen the soil around the plant. Subsequently, pulled up the plant and dig up the roots by hand. The sweet potato roots storage (tubers) were then collected for weighing and stored at storage room.
Measurement of microclimate and soil characteristics
Before cultivation of the sweet potato were begun in each location, soil samples were taken for analysis, in March 2018. For analysis of soil characteristics in Cilembu and Cimaung villages, the soil samples were collected diagonally from 5 spots at 20 Cm depth of each location (Cilembu and Cimaung villages). Separately, the collected samples from each location were mixed and 0.5 Kg was taken. The soil samples were then analyzed according to standard procedure for soil chemistry analysis Balai penelitian tanah (2012). The equipment used including Spectrophotometer Hitachi, Flame Photometer Corning, and Atomic Absorption Spectrophotometer GBC. Measurement of microclimate were done on air temperature and air humidity using Digital temperature humidity meter HTC-1; soil temperature using soil thermometer Weksler, soil humidity and soil pH using soil tester Takemura DM-5. Elevation were measured using Garmin GPSMap 78. Monthly Rainfall during growing of the plant was obtained from the nearest climatology station. At the beginning of the cultivation (April to May, 2018) in both typical and non-typical areas was the rainy season, but in June and July 2018 there was no rain at all.
The sweet potato storage
Storage of the sweet potato in this study was carried out based on how the storage of sweet potato do by farmers i.e. the tubers were harvested, cleaned from the soil, and dried on the floor in a wind-dried for 2-3 days. After being dried, the tubers were stored in the storage room located near each site where the sweet potato was cultivated. The storage room in Cilembu village as well as in Cimaung village were set with having sufficient ventilation, floor covered with wooden board, ambient temperature and humidity conditions. The average room temperature and humidity in the storage room of Cilembu village were 21.5°C and 68.0%, respectively. While the average temperature and humidity in the storage room of Cimaung were 23.8°C and 69.3%, respectively. Storage period of the sweet potato at each location was carried out for five weeks then every week (week 1,2,3,4,5) the sugar content was examined following the method of Onggo (2006); (Solihin et al., 2017) with modification.
Analysis of biochemical products Concentration of starch
Starch concentration in freshly tubers were measured at harvest and at storage room on week 1, 2.3,4 and 5 of each location. For this purpose, the tuber was extracted according Cepeda et al. (2016) with modification as follow, as much as 500 g of tuber from each location were peeled and shredded roughly (about 3mm wide and 1 mm thick). The sample was dried in an oven at 60°C until the weight was constant. The samples were then mashed using mortar and pestle and sieved with a 240 mesh sieve to obtain flour. The flour (3 g) was added with 50 ml of 60% alcohol and stirred for 1 hour to become suspension. The suspension was filtered with filter paper and washed with distilled water until the volume of the filtrate become 250 ml. The remaining flour in the filter paper was transferred into the condenser flask by washing with 200 ml of distilled water, then 20 ml of 25% HCL was added and heated in the heating mantle and waited to boil for 2.5 hours. After that, the solution was cooled and neutralized with 45% NaOH and diluted to a volume of 500 ml before it was filtered. Furthermore, 2 ml sample from the filtrate was taken, put into a test tube and added 1 ml of 5% phenol and shaken.The filtrate was added with 5 ml of concentrated sulfuric acid and allowed to stand for 10 minutes, then shaken and placed in a water bath for 15 minutes. After that, absorbance of glucose was measured using a spectrophotometer Bio-Rad SmartSpec Plus at λ=490 nm. Glucose concentration was determined based on the equation between the concentration of standard glucose solution and its absorbance.
The standard curve for starch analysis was done by making a standard glucose (Merck, Germany) solution with a concentration of 10, 20, 40, and 80 mg/l. Two ml each standard glucose solution was taken and put into a test tube. Five ml concentrated H2SO4 was added into the standard glucose solution and allowed to stand for 10 minutes, shaken and incubated in a water bath for 15 minutes.
Then the absorbance was measured at λ=490 nm. The value of the equation was determined between the concentration of a standard glucose solution and its absorbance. To determine starch concentration, the weight of glucose obtained was multiplied by a factor of 0.9 divided by sample weight by using the following equation:
𝑆𝑡𝑎𝑟𝑐ℎ 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 = 𝐺𝑙𝑢𝑐𝑜𝑠𝑒 𝑤𝑒𝑖𝑔ℎ𝑡 × 0.9
𝑆𝑎𝑚𝑝𝑙𝑒 𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔) × 100%
Determination of soluble sugar concentration
Analysis of free soluble sugar (glucose, fructose, sucrose, and maltose) were carried out by using HPLC (High-Performance Liquid Chromatography Pump waters 1515) to the flour of sweet potato from the typical and the non-typical areas at harvest time, during storage and after baking according to Tihomirova et al. (2016) and Montesano et al. (2016) methods. Before the analysis, each sample was extracted using ethanol as follow as much as 0.5 g of the the sweet potato flour was put into a test tube followed by the addition of 25 ml of 80% aqueous ethanol into the tube then tightly covered by aluminium foil and then incubated in a water bath for 1.5 hours until it boiled. To obtain dry sugar, the sugar extract was filtered and rinsed with 80% aqueous ethanol, and was dried in an oven at 80 ℃. The
dried sugar was added with 5 ml of DW (Deoxinated water) in the Falcon tube before it was centrifuged for 10 minutes at 1500 x g ( 4500 rpm).
Twenty μl of each sample of sugar extracts was applied to the HPLC using column Shodex sugar SP 0810 (5 μm particles, 4.6 mm internal diameter and 25 cm length) with the mobile phase consisting of H2O:H2SO4 0.005 M at flow rate of 1.0 ml/minute, temperature 60 °C and using Refractive index (RI) detector according to Tihomirova et al. (2016) method. For determination and quantification of the sugar in the samples, the individual sugar standard (glucose, fructose, maltose and sucrose) resolution and retention time were recorded. The sugar were identified based on the retention time of the samples compared to the retention time of the of sugar standards. The sugar standards (glucose, fructose, maltose and sucrose) were obtained from Merck, Germany. The standard solution of glucose, fructose, maltose and sucrose were prepared in separate flask by diluting each sugar in H2O to obtain a final concentration of 10, 20, 40, and 80, mg/ml. Each standard solution was injected into HPLC and each value of the peak area (Y) was plotted to corresponding concentration (x) to obtain linear calibration curves. From the linear curve, a linear equation (Y= ax+b, where a and b are constant) can be obtained.
The linear equation then used to calculate the concentration of the injected samples (x). The retention times obtained in this hplc analysis of glucose, fructose, sucrose and maltose were 9.9 minute, 10.8 minute, 8.2 minute, and 8.4 minute, consecutively.
Preparation of baked sweet potato
In order to examine sugar total and sweetness level in the baked sweet potato, samples were taken from the sweet potato which showed relatively high total sugar content during storage (the sweet potato that had been stored for 3 and 4 weeks in the storage room) at each location. The tubers of the sweet potato were washed thoroughly using tap water and dried with tissue paper. The tubers were wrapped with aluminum foil and placed in the oven to bake the tubers of the sweet potato. The oven temperature used for baking was 80-100° C for 45 minutes. After baking, samples were cooled to normal temperature and before extraction, the baked sweet potato was peeled and grounded to obtain potato flour according to Wei et al. (2017) method. The extraction method for sugar analysis in the baked sweet potato samples used the same method as the method for freshly sweet potato sugar analysis.
Calculation of sweetness
The order level of the sweetness of free sugar was fructose> sucrose> glucose> maltose. The sweetness of maltose is only 42% of the total sweetness of sucrose (Kwarteng, 2014). Thus, to estimate the level of sweetness of the total sugar, calculation of sweetness could be done by score of sweetness that was calculated relatively to sucrose equivalent by the following equation:
Sucrose equivalent (SE) = 1.2 fructose + 1 sucrose + 0.64 glucose+ 0.43 maltose
Sucrose equivalent was expressed as grams per 100 g dry mass (DM). For example, a 30g/100 g DM indicates that the combined sweetness of the sugars present is equal to that of 30g sucrose/100 g DM (Shallenberger, 1993; Owusu-Mensah et al., 2016). In this study, level of the sweetness was measured in the samples of freshly sweet potato at harvest and freshly sweet potato at storage room on week 1, 2.3,4,5, which were the same samples as samples that have been analyzed for total sugar content. For
total sugar content, freshly tubers were measured at harvest and at storage room on week 1, 2.3,4 and 5 of each location according to the methods described by Onggo (2006); Solihin (2017). Furthermore, the sweetness evaluation was performed to the baked sweet potato from storage room on week 3 and 4 of each location by using sucrose equivalent (SE) formulae to determine the level of sweetness.
Statistical data analysis
Of the data on the average of starch concentration, sugar concentration, and sweetness of the sweet potato Rancing cultivar from the typical and the non-typical areas were calculated statistically using Independent T-test with Significance of (2-tailed) ≥ 0.05 No difference. Followed by principal component analysis (PCA) using Minitab version 20 to evaluate the correlation between microclimate, land properties with starch and sugar concentration at harvest.
RESULTS AND DISCUSSION Microclimate and soil characteristics
Based on the comparation of microclimate and land characteristics of the two experimental locations (the specific location in Cilembu and Cimaung), it appeared that the avrerage air temperature during the experiment in Cilembu was 21.5°C, while the average temperature in the Cimaung was 23.8°C.
Furthermore, there was a trend that air humidity in Cilembu was lower than that of Cimaung, i.e. 78.0%
and 86.6%, respectively. Similar trend was detected that monthly rainfall in Cilembu was lower than that of Cimaung, i.e.145.8 mm/month compared to 187.5 mm/month. Meanwhile, soil characteristics in the two locations including soil texture and pH tend to be similar. Macronutriens (, Carbon ( C), Nitrogen (N), Potassium (K), Natrium (Na); C/N ratio; CEC (Cation Exchange Capacity); micronutriens, Iron (Fe), Boron (B) in Cilembu soil tend to be higher than that of Cimaung soil. Macronutriens Cacium (Ca), Magnesium (Mg), Phosphorus(P) and Micronutriens Copper (Cu), Zinc (Zn), Manganese (Mn) in Cilembu soil tend to be lower than that of Cimaung soil (Table 1.).
Biochemical parameters
The starch concentration at harvest and during storage
The concentration of starch in the freshly tuber that harvested at the typical and the non-typical area were different, 37 % and 35%, respectively. According to Badu et al. (2017); Ngailo et al. (2019), differences in yields of starch were influenced by genotype and environmental conditions when the plant was grown, such as soil, weather and growing conditions. In this study, the genotype of sweet potato used for experiments was the same i.e. Ipomoea batatas of Rancing cultivar. Therefore, the difference in starch concentration is probably due to differences in the environment where the sweet potato were cultivated.
In this study, principle component analysis (PCA) loading plots were applied to evaluate the correlation of environmental factors (elevation, microclimate and land characteristic) at the two locations to the concentration of starch at harvest. The loading score shows that vectors of starch concentration, elevation, and rainfall are in the same direction and have large positive loading on PC1 (principle component-1) with total variance value of PC1 was 100% (Figure 2a.), which means that the elevation,
rainfall and starch are associated with PC1 (axis-1) that contributed most to variations among the cultivated sweet potato (Jolliffe & Cadima, 2016). Furthermore, the angles between starch concentration with altitude and with rainfall are less than 90°. This shows that there is a positive correlation between starch concentration with elevation and rainfall. On the other hand, it seen that concentration of starch is at opposite direction with humidity, soil and air temperature, which means that starch concentration has negative correlation with humidity, soil and air temperature.
Moreover, the loading score shows that vectors of starch concentration, nutrient K, Na, Fe, B, C, N, C/N and CEC in the two locations are in the same direction and have large positive loading on PC1 with total variance value of PC1 was 100% (Figure 2b.), which indicates that starch concentration, K, Na, Fe, B, C, N, C/N and CEC associated with PC1 (axis-1) that contributed most to variations among the cultivated sweet potato (Jolliffe & Cadima, 2016). Furthermore, the angles below 90° are seen between the starch concentration with K, Na, Fe, B, C, N, C/N and CEC , which means there is a positive correlation between starch concentration with K, Na, Fe, B, C, N, C/N and CEC ( (Jolliffe &
Cadima, 2016). On other hand, starch concentration is negatively correlated with P, Mg, Ca, Mn, soil pH, Cu and Zn because their angles with starch concentration are greater than 90°. The results of these analysis demonstrated that starch concentration at harvest are positively correlated with elevation, rainfall, and soil characteristics which include the amount K, Na, Fe, B, C, N, C/N and CEC, but negatively correlated with humidity, soil and air temperature , amount of P, Mg, Ca, Mn, soil pH, Cu and Zn. The finding in this study that elevation, rainfall and land characteristics have an effect on sweet potato starch at harvest is in line with the previous results reported by Kathabwalika et al. (2016), that starch content in orange-fleshed sweet potato (Ipomoea batatas L.) was affected by elevation, soil characteristics, temperatures and amount of rainfall in Malawi.
Measurement on the starch concentration of the sweet potato during storage showed a decrease of starch concentration both in the typical and the non-typical areas (Figure3.) The starch concentration in sweet potato from the typical area was 37% at harvest and decreased gradually every week to 19.6
% in 5 weeks after storage, while the concentration the starch in sweet potato from the non-typical area was 35% at harvest and decreased to 31.5% in 5 weeks after storage. These results reflected that there have been a reduction in starch concentration during storage due to the hydrolysis of starch into simple sugar by endogenous enzymatic processes. As reported that the reduction of starch content in the sweet potato was caused by the process of endogenous α-amylase activities (Nabubuya et al., 2012). The starch during 5 weeks storage in Cilembu decreased from 37% to 19.9% ( 47%
reduction), while during storage in Cimaung it decreased from 35% to 31.5% (11% reduction). Similar degree reduction of starch, 47%, in typical land tuber of certain sweet potato was reported in sweet potato cultivar from Sri Lanka ( Senanayake, 2013).
The difference in the decrease in starch concentration indicated the difference degree of hydrolysis in the same cultivar of sweet potato which were cultivated in different location. The degree of starch hydrolysis in the sweet potato that cultivated in typical area showed much higher than that of non-typical area.Thus, it was suspected that there were other sources of enzymes, not those presence in the sweet potato that contribute to the hydrolysis of the starch. Most probably, there was microorganism activity
that contribute to starch hydrolysis in Cilembu sweet potato cultivated in Cilembu village. This assumption was based on the report that in the Cilembu village soil sample had found colonies of rhizosphere bacteria that entered the sweet potato plant tissues and become endophytic bacteria such as Bacillus subtilis and B. amyloliquefaciens that played role in starch hydrolysis which were also found in sweet potato tuber during harvest and storage (Tangapo et al., 2018). On the other hand, it was assumed that the sweet potato cultivated at the non-typical area (Cimaung village) did not contain hydrolytic bacteria like the one in the typical area. This assumption was based on the concept of specificity in the interaction of plants with rhizosphere bacteria, which the presence of bacteria depends on genotype of the host plant (Marques et al., 2014) and soil conditions (Neuman et al., 2014). However there was no research reported yet regarding rhizospheric and endophityc bacteria in Cimaung village.
The concentration of soluble sugar and the sweetness in raw sweet potato The concentration of soluble sugar at harvest
The soluble sugar of raw sweet potato from the typical and the non-typical area consisted of fructose, glucose and sucrose (Figure 4.). Based on HPLC analysis, total concentration of soluble sugar in raw sweet potato from the typical area was 4.0% at harvest and then it increased to 5.4%, 6.9%, 9.2.%, 9.4%, 8.5%, in 1, 2, 3, 4, 5 weeks after storage, respectively. Meanwhile, the total concentration of soluble sugar in sweet potato from the non-typical area was 2.6% at harvest, then it increased to 3.4%, 5.4%, 6.0%, 6.0 %, 4.2% respectively, in 1, 2, 3, 4, 5 weeks after storage (Table 2.). In average the concentration of total soluble sugar in the sweet potato were 7.2% from typical land and 4.6% from non-typical land. These result was significantly different from the result of Solihin et al. (2017) who reported by using Luff Schoorl method that the total sugar content was only 2.84% and 2.3% in typical and non-typical area, respectively. The difference may be due to different sugar analysis methods. On the other hand, Tangapo et al. (2018) reported almost the same results with this study, in which the average of total sugar in the sweet potato from Cilembu was 8.50 which had been analyzed by HPLC methods.
In this study, the total percentage of soluble sugar increased progressively along with the increasing length of storage period and reached a maximum percentage at 4 weeks after storage in the typical land and also in the non-typical land i.e. 9.4% and 6.0%, respectively. Similar results was reported that total soluble sugar in sweet potato was around 4.5% -10.8% within 3 weeks of storage (Kwarteng et al., 2014). Another similar result found that total sugar content in Cilembu sweet potato of Rancing variety showed maximum level at 4 weeks after storage for typical locations and 3 weeks after storage for non-typical locations (Solihin et al., 2017). Others study indicated that total soluble sugar concentration in Cilembu sweet potato was 4.5% at harvest then increase to reach maximum, 11.6%, at 2 week after storage (Tangapo et al., 2018).
The difference in total sugar concentration in sweet potato from tipycal and non-tipycal in this study may be due to the difference in starch concentration at harvest and the degree of hydrolysis of starch in each of these locations. As mentioned earlier perhaps there was exogenous enzyme come from
endophytic microbial (bacterial and fungi) that present and interacts with the tuber of the sweet potato in the typical land (Cilembu) that contribute to starch hydrolysis of the sweet potato. The endophytic bacteria in Rancing sweet potato cultivar planted in the typical land may come from a location-specific rhizosphere bacteria and were not found in land outside the typical land. The endophytic bacteria originally comes from rhizospheric bacteria that enter and permanently inhabit the host plant tissues (Reinhold-Hurek et al., 2015). Some rhizospheric bacteria that become endophityc bacteria play a role in hydrolyzing starch into simple sugar units in certain parts of the plant (Buchholz et al., 2018). In the context of hydrolitic bacteria, Tangapo et al. (2018) found that a large number of hydrolytic bacteria such as Bacillus subtilis and B. amyloliquefaciens that which showed amylolitic activity. These bacteria were believed to play a role in the breakdown of starch into sugar in the sweet potato of Rancing cultivar that cultivated in typical land of Cilembu.
To evaluate correlation of sugar concentration at harvest in the two locations with environment (microclimate, elevation and land characteristics), PCA loading score plots were applied. The loading score shows that vectors of sugar concentration elevation, and rainfall are in the same direction and have large positive loading on PC1 with total variance value of PC1 was 100% (Figure 5a.), which indicate that elevation, rainfall and sugar concentration are associated with PC1 (axis-1) that contributed most to variations among the cultivated sweet potato (Jolliffe & Cadima, 2016).
Furthermore, the angles between sugar concentration with elevation and with rainfall are less than 90°.
This shows that there is a positive correlation between sugar concentration with elevation and rai nfall (Jolliffe & Cadima, 2016). On the other hand, sugar concentration give opposite direction with humidity, soil and air temperature. This indicates that sugar concentration has negative correlation with humidity, soil and air temperature. Moreover, The loading score shows that vectors of sugar concentration, K, Fe, Na, C/N, C, B, CEC, N in both locations are in the same direction and have large positive loading on PC1 with total variance value of PC1 was 100% (Figure 5b), which indicates that sugar concentration, K, Fe, Na, C/N, C, B, CEC, N are associated with PC1 that contributed most to variations among the cultivated sweet potato. Furthermore, the line angles below 90° are seen between the starch concentration with K, Fe, Na, C/N, C, B, CEC, N, which shows that there is a positive correlation between starch concentration with K, Fe, Na, C/N, C, B, CEC, N, however negatively correlated with P, Mg, Ca, soil pH, Mn, Cu and Zn, because their angles with sugar concentration are greater than 90°.
The results of these analysis demonstrated that sugar concentration at harvest are positively correlated with elevation, rainfall, and soil characteristics which include the amount K, Fe, Na, C, B, N, C/N, and CEC. There is a tendency for higher amounts of K, Fe and B in Cilembu than in Cimaung, causing the total sugar in Cilembu to be higher. This is in agreement with previous report that K concentration showed a significant correlation with content of total sugar , in which the sugar content increases with increasing of K in the soil (Anda et al., 2018). In addition, there is a trend that the higher the elevation, the more sugar concentration produced. This trend was seen at elevation of 870 m in Cilembu, where the sweet potato generate higher total sugar concentration than in Cimaung which
has lower elevation, 821m. Similar results have been reported that total sugar concentration at 870- 916 m elevation was higher than in elevation below 870 m in Cilembu vilage (Anda et al.,2018). There is also a trend showed in rainfall, the lower the rainfall amount the higher sugar concentration i.e. 4%
total sugar was obtained at 145.8 mm/month of rainfall in Cilembu compared to 2.6% of total sugar at 187.5 mm/month in Cimaung. However, others finding (Anda et al., 2018) had some different results in which monthly rainfall between 96–199 mm were favourable for generating high sugar concentration in sweet potato of Rancing cultivar. This is presumably because the concentration of total sugar is influenced not only by rainfall amount but also by complex interaction of many environmental factors that influence each other as shown in the loading plot.
Moreover, according to the report of Anda et al., (2018), total sugar concentration is not only affected by K, rainfall and elevation, but also by temperature. The required temperature for generating honey taste of Cilembu sweet potato was 21.0°C-22.0°C. Furthermore, sweet potato require temperature as high 28.2°C during photosynthesis during the day and minimum temperature as low as 14.6°C during formation of tuber at night (Solihin et al., 2016). In this study, the averages temperatures in the typical land and the non-typical land were 21.5 °C and 23.8°C, respectively. It seen that the temperature in Cilembu village is suitable for producing sweet potato with a level of sweetness like the taste of honey, however the temperature in Cimaung is not suitable. Nevertheless, temperature at the two locations are favorable for photosynthesis and tuber formation.
The sweetness
The sweetness is derived from the composition of endogenous sugar. Concentration and type of free sugar contained in the sweet potato will determine the sweetness level of the sweet potato, therefore through measurement level of endogenous sugar (fructose, glucose, sucrose) present at harvest and additional sugar (maltose) present after baking, the level of sweet potato’s sweetness can be determined.
Counting the total amount of sugar does not necessarily represent sweetness, because the level of sweetness of each component of total sugars, fructose, sucrose, glucose, and maltose was different.
Owusu-Mensah et al. (2016) have suggested a sucrose equivalent (SE) equation to score the level of sweetness of sugars. Based on calculation of the sucrose equivalent (SE) the level of sweetness of the sweet potato from the typical land was 4.4 at harvest and 4.7, 6.2, 9.4, 9.0, and 5.5, respectively, in 1,2,3,4,5 weeks after storage. The level of sweetness of sweet potato from the non-typical land was 2.1 at harvest and 3.6, 4.3, 5.5, 4.8, and 3.5 , respectively, in 1,2,3,4,5 weeks after storage (Table 2.).
It showed that the sweetness of sweet potato in the typical and the non-typical land increased progressively in 1, 2, 3, 4 weeks after storage, and decreased afterward (Figure 6). This finding was in agreement with previous work that total sugar content in Cilembu sweet potato reached maximum at 4 weeks after storage in the sweet potato from typical land and at 3 weeks after storage in the sweet potato from non- typical land (Solihin et al., 2018).
The concentration of soluble sugar and sweetness in baked sweet potato
The concentration of soluble sugar
The results indicated that soluble sugar in baked sweet potato consisted of fructose, glucose, sucrose and additional sugar maltose which was formed through hydrolysis of starch when baked. The total concentration of soluble sugar in baked sweet potato from two storage places of the typical land and the non-typical land in week 3 were 14.0% and 6.5%, respectively. On the other hand, the total concentration of soluble sugar in the baked sweet potato planted in the typical land and the non-typical land that has been stored for 4 weeks was 29.7% and 12.9%, respectively. These results indicated that total sugar concentration of baked sweet potato after both 3 and 4 weeks storage in the typical land was significantly higher than that of the non-typical land (Figure 7a). This was in agreement with previous studies reported by Lai et al.(2013) that after the baking treatment, the total sugar content of baked sweet potatoes was dramatically increased due to the formation of additional sugar maltose. In principle, sugar in sweet potato is produced as a result of starch hydrolysis which include acid hydrolysis, and enzymatic hydrolysis (Betiku et al., 2013). In addition, annealing treatment sweet potato resulted increase in amylose content and starch hydrolysis (Shariffa et al., 2017). During baking treatment, the temperature increases gradually, thereby facilitating the breakdown of the hydrolytic bonds of the starch. Under this condition the activity of endogenous-amylase enzyme increase resulting starch degradation to produce more free sugar, sucrose, fructose, and glucose and also resulting additional sugar, maltose (Kwarteng et al., 2014 ).
The sweetness
The level of sweetness of baked sweet potato from the typical and the non-typical land that have been stored for 3 weeks were 11.6 and 5.4, respectively. Moreover, the level of sweetness of baked sweet potato from the typical land and the non-typical land that has been stored for 4 weeks reached 21.0 and 11.1, respectively. These results indicated that the level of sweetness of sweet potato after 3 and 4 weeks storage in typical land was significantly higher than that of non-typical land (Figure 7b) and the highest degree of sweetness was obtained in the baked potato from the typical land at 4 weeks after storage which were had sucrose equivalent (SE) of 21.1 and 11.1, in the typical and the non-typical land, respectively (Table 3.). The degree of sweetness classification was determined by sucrose equivalent (SE) value according to Shallenberger (1993); Owusu-Mensah et al. (2016) in which SE ≤ 12: non sweet , SE=13-20: low, SE= 21-28: moderate, SE=29-37:high, SE ≥38: very high. In this study, sweet potato from the typical land with the sucrose equivalent of 21.1 was categorized as moderate degree of sweetness, meanwhile sweet potato from the non-typical land with the sucrose equivalent of 11.1 was classified as non-sweet one.
CONCLUSIONS
The levels of starch and sugar in sweet potato of Rancing cultivar grown in two different locations show differences. Based on principle component analysis (PCA), starch and sugar content significantly show positive correlation with elevation, rainfall, soil nutrient content, C/N ratio and CEC levels. Comparison of microclimate and land characteristics in Cilembu and Cimaung shows higher Cilembu elevation (870 m compared to 821 m), lower Cilembu rainfall (145.8 mm/month compared to 187.5mm/month), tendency of K, Na, C, N, Fe, B, content, C/N ratios and CEC were higher in Cilembu. These conditions
resulting in higher starch content and significantly higher sugar content in Cilembu than in Cimaung village.
There is difference in the degree of reduction of starch in the sweet potato during 1 to 5 weeks after storage, 47% in Cilembu and 11 % di Cimaung. As a results, the total soluble sugar in the sweet potato from Cilembu village (typical land) is much higher than that of Cimaung village (non-typical land).
Furthermore, based on sucrose equivalent score, the Cilembu sweet potato cultivated in Cilembu village shows sweet taste, however it shows normal taste when the sweet potato cultivated in Cimaung village. These results may serve as information that Cimaung Village is less favourable for producing sweet potato with a level of sweetness as in Cimaung.
Acknowledgement
We would like to thank the Program in Research, Community Services & Innovation (P3MI) of Bandung Institute of Technology, Indonesia (2018) for financial support.
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Figures and Tables
Figure 1. (a.) The location of typical land in Cilembu village and non-typical land in Cimaung village of West Java, (b.) sweet potato cultivation in Cimaung village (non-typical land), (c.) sweet potato cultivation at Cilembu village (typical land)
Figure 2. PCA Loading score plot of starch concentration at harvest and parameters of (a). microclimate (rainfall, humidity, soil temperature, air temperature) and elevation; (b) Land characteristic (K, Na, Fe, B, C, N, C/N and CEC, P, Mg, Ca, Mn, soil pH, Cu, Zn)
a b
c
a b
Figure 3. The starch content at harvest and during storage times of the raw sweet potato tuber cultivated in typical land and non-typical land
Figure 4. The composition and concentration of the soluble sugars in sweet potato (a). from the typical land; (b). from the non-typical land at harvest time and 1 to 5 weeks after storage
0 10 20 30 40 50
at Harvest
W-1 W-2 W-3 W-4 W-5
Starch Concentration (%)
Storage period of the sweet potato Cilembu (Typical) Cimaung (Non-Typical)
0 2 4 6 8 10
Concentration of soluble sugars (%)
Weeks after storage Glucose Fructose Sucrose
0 1 2 3 4 5 6 7 8 9 10
Concentration of soluble sugars (%)
Weeks after storage Glucose Fructose Sucrose
a b
Figure 5. PCA Loading score plot of sugar concentration at harvest and and parameters of (a).
microclimate (Rainfall, elevation, humidity, soil temperature, air temperature); (b) Land characteristic ( K, Fe, Na, C/N, C, B, CEC, N , P, Mg, Ca, soil pH, Mn, Cu Zn)
Figure 6. The percentage of sweetness of sweet potato from typical land and non-typical land at harvest time and 1 to 5 weeks after storage
- 2.00 4.00 6.00 8.00 10.00
at harvest
1 2 3 4 5
Total of soluble sugars (%)
Weeks after storage Cilembu village (Typical Land)
a b
Figure 7. (a). The total soluble sugar; (b). The sweetness of baked sweet potato from typical land and non-typical land
Table 1. Description of the experimental locations:
Microclimate and land property Location
Cilembu Village Cimaung Village
Average air temperature (0C) 21.5 23.8
Average soil temperature (0C) 24.7 29.6
Air Humidity (%) 78 86.6
Rainfall mm/month 145.8 187.5
Elevation (m) 870 821
Soil texture Sandy clay Sandy clay
Soil pH (in H2O) 6,5 6,5
Soil pH (in KCl) 5.2 5.7
N organic (%) 0.23 0.18
C organic (%) 3.12 2.13
C/N ratio 13 12
CEC cmol (+)/kg 21,15 18,07
Total K (cmol(+)Kg) 1,82 1,73
Total Na (cmol(+)Kg) 0,39 0,10
Total Ca (cmol(+)Kg) 9,79 11,08
Total Mg (cmol(+)Kg) 1,91 2,74
Total P (cmol(+)Kg) 0.21 0.28
Total Cu (ppm) 0,6 1,0
Total Fe (ppm) 4,0 3,5
Total B (ppm) 0,10 0,09
Total Zn (ppm) 1,0 4,0
Total Mn (ppm) 10,9 48,4
Table 2. The concentration of the soluble sugar and the sweetness score (sucrose equivalent) in sweet potato Rancing cultivar at harvest and during storage
- 5.00 10.00 15.00 20.00 25.00 30.00 35.00
3 4
Concentration of Sugar in baked sweet potato (%)
Weeks after storage Cilembu village (Typical Land) Cimaung village (Non-Typical Land)
- 5.00 10.00 15.00 20.00 25.00
3 4
Score of Sweetness baked potato
weeks after storage Cilembu village (Typical Land) Cimaung village (Non-Typical Land)
a b
Type of Sugar Concentration of soluble sugars (%)
At harvest W 1 W 2 W 3 W 4 W 5
Typical land
Glucose 1.8 2.5 2.7 3.7 5.1 3.4
Fructose 2.1 2.2 2.7 4.3 2.4 2.2
Sucrose 0.05 0.7 0.5 1.2 1.9 2.9
Total 4 5.4 6.9 9.2 9.4 8.5
SE 4.372 4.74 6.168 9.428 9.044 5.516
Non-typical land
Glucose 0.5 0.8 2.6 2 2.4 1.6
Fructose 1.1 1.9 1.6 2.9 2.39 1.8
Sucrose 1 0.7 1.2 1.1 1.2 0.8
Total 2.6 3.4 5.4 6 6 4.2
SE 2.088 3.56 4.288 5.528 4.808 3.47072
W =weeks after storage SE=sucrose equivalent
Table 3. The concentration of total soluble sugars and their sweetness in baked sweet potato from typical land and from non-typical land
Type of Sugar
Concentration of soluble sugar (%) Cilembu village
(typical land)
Cimaung village (non-typical land)
W 3 W 4 W 3 W 4
Glucose 7.6 12.1 2.8 6.2
Fructose 5.2 7.1 2.6 5.5
Sucrose 0.02 0.02 0.02 0.02
Maltose 1.2 10.5 1.1 1.2
Total 14.02 29.72 6.52 12.9
SE 11.6 21.0 5.4 11.1
W3 = three weeks after storage; W4=four weeks after storage SE= sucrose equivalent
