Introduction

Sweet potato (Ipomoea batatas (L.) Lam.) is a cultivated species belonging to the convolvulaceae family that can form tuberous root. It is an annual or perennial grassy vine, originating in South America, and has been widely planted in 111 countries and regions in the world1. Sweet potato is an important food crop as well as an important industrial raw material, and it ranks fourth in total food crop production in China, after rice, wheat and corn2,3. The sweet potato boasts a delightful sweet taste and is rich in nutritional value. It is abundant in dietary fiber, which aids in promoting intestinal peristalsis and digestion. Additionally, it serves as a significant source of energy for human activities, making it a crucial component of food, vegetables, and forage crops, so some countries in Africa and Asia, such as Congo and Japan, eat sweet potato as a staple food4,5,6. Sweet potato was introduced into China from the Philippines and Vietnam in the 16th century7,8. At present, sweet potato is cultivated all over China, especially in Huaihai Plain, Yangtze River basin and southeast coastal provinces and regions, and China has become the world’s largest producer of sweet potato9,10.

At present, more and more people who lose weight take sweet potato as the staple food during weight loss, because sweet potato is rich in protein, starch, dietary fiber, carotene, vitamin A, B, C, E and calcium, potassium, iron and other trace elements11,12,13. In addition, sweet potato contains a lot of dietary fiber, which can effectively promote intestinal peristalsis and digestive fluid secretion, reduce the incidence of intestinal diseases and the constipation, so sweet potato has a high nutritional and medicinal value14,15,16. Sweet potato has a very low fat content and produces less heat in the human body. According to research reports17,18 every 100 g of fresh sweet potato only contains 0.2 g of fat and produces 85 kcal of heat energy, which is only 1/3 of rice. It is a high-quality low-fat and low-energy food. Consequently, sweet potato can serve as a high-quality staple food during weight loss and bodybuilding endeavors. Furthermore, numerous foods and beverages are crafted from sweet potato, offering a diverse range of culinary options19,20,21.

The analysis of quality characteristics of sweet potato is an important basis in the process of seed selection and utilization of sweet potato. Therefore, 20 varieties of sweet potatoes were selected to determine their sugar content, drying rate, amylose content, amylopectin content, anthocyanin content, texture and colorimetry. By comparing the differences of these indicators, the main characteristics and indicators that determine the processing quality of sweet potato were found, and different processing purposes and processing methods suitable for sweet potato were divided, which to provide data reference for sweet potato breeding and processing decisions.

Materials and methods

Supplied experimental fields and plants

The experiment was carried out from the experimental field of Xiangyang Academy of Agricultural Sciences, Xiangyang, Hubei Province, more than 100 m away from the main road, the coordinates were 112°8′46.163″–E112°9′12.843″ E; 32°5′43.594″ N–2°5′53.065″ N. The soil in the experimental field is yellow soil, and the topsoil texture is loam. The basic physical and chemical properties of the soil were shown in Table 1. The tested materials were 20 sweet potato varieties, whose numbers, growth characteristics and sweet potato traits were shown in Table 2.

Table 1 The basic physical and chemical properties of the soil.
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Table 2 Variety number and growth characteristics of sweet potato.
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Experimental design

The field experiment was conducted from June to October 2022 in Xiangyang Academy of Agricultural Sciences. A total of 80 land blocks were set up in the experiment, each with a length of 10.0 m, a width of 1.0 m, a plant spacing of 0.30 m, a total area of 800.0 m2and a planting density of 4 sweet potato plants per square meter, each sweet potato variety was planted 4 times and repeated at random. Cultivation management was carried out according to conventional field management methods. Fertilizer application: compound fertilizer 75.0 g per square meter land with the ratio of nitrogen phosphorus and potassium was 15 to 15 to 15, total application of compound fertilizer was 7200 g.

Sweet potato seedlings colonized as cave cuttings on June 11, 2022. Sweet potato (WR) are widely used in Hubei Province, provided and cultivated by the Xiangyang Academy of Agricultural Sciences have been adopted in Xiangyang, Hubei Province. Seedlings were cultivated according to covered with film of vines seedling (patent license CN 103999664 A). When the seedlings grew 5–6 main leaves, healthy seedlings with constant growth were selected and transplanted cutting into the field.

Sample determination

The drying rate of sweet potato was determined by drying method. The three sweet potato blocks were randomly selected for each variety, washed, dried the skin, sliced and mixed the middle part, weighed at 100 g, dried at 80 °C to constant weight, and the drying rate was determined. The contents of amylose and amylopectin in sweet potato were determined by dual wavelength colorimetry22,23.

Anthocyanins are water-soluble pigments whose color varies with the pH of plant cell fluid24. If the cell fluid is acidic, the color is red, and if the cell fluid is alkaline, the color is blue25. The anthocyanins were determined by ultraviolet spectrophotometry (Evolution 350, Thermo Scientific)26,27. The 2.0 g sweet potato samples were extracted by hydrochloric acid ethanol extraction solution. The optical density values of the extracted solution at 530 nm, 620 nm and 650 nm were determined by ultraviolet spectrophotometer, and the anthocyanin content was calculated. The sugar content was determined by a sugar meter. 10.0 g fresh sweet potato was ground into pulp, the sweet potato pulp was filtered, and the sugar content was measured in the sugar meter.

The chroma of sweet potato was determined by colorimeter (PFXi195, Lovibond). The fresh sweet potato was cut into 1.0 cm slices in the middle. The test mode was not to remove specular reflection, and the standard whiteboard was used as the standard sample. The L* value indicates that the brightness of the sample changes from 0 (black) to 100 (white); The a* value indicates the red-green degree of the sample, +a* indicates red, and −a* indicates green; The b* value indicates the yellowish blueness of the sample, +b* indicates yellow, and −b* indicates blue28.

The texture of sweet potato was determined by a texture analyzer (TA.XT ExpressC, Stable Micro Systems)29,30. The P/36R cylindrical probe (diameter 36 mm) was used to determine the hardness, elasticity, chewiness, cohesiveness and resilience. The hardness, elasticity, chewiness and cohesiveness were used a double compression test (TPA mode), and resilience was used a single compression test. The adhesiveness was determined by the P/2 needle probe (diameter 2 mm) and used a single compression test. The middle part of fresh sweet potato, about 2.0 cm thick, was placed on the instrument tray, the trigger force was set to 15 g, the thickness was set to 8.0 mm, the speed was set to 1.0 mm/s, 2.0 mm/s and 5.0 mm/s before, during and after the test, respectively. To ensure that the test points do not interfere with each other, the distance between the points is > 0.5 cm. Exponent Lite (Version 6, Stable Micro Systems) was used to extract TPA-related parameters and analyze the test data.

Data analysis

The Excel (Version 2016, Microsoft Office) and SPSS (Version 22.0, IBM) were used for all data analysis and Origin (Version 2020, OriginLab) for graph rendering. ANOVA and post-hoc tests were used for measurement data. Three parallel tests were performed for each test group, and the data were presented as the mean ± SD of three replicates. Statistical significance levels are denoted as * (P < 0.05) and ** (P < 0.01).

Results and discussion

Comparative analysis of dry weight rate and starch content of different sweet potato varieties

The dry weight rate of sweet potato is the percentage of dry weight and fresh weight of sweet potato slices after drying31. Under normal circumstances, the sweet potato varieties used for flour production require a higher dry weight rate, while the fresh varieties require a lower dry weight rate32. The dry weight rate value of sweet potato variety 197-4 was the largest with 36.2% shown in Table 3. The dry weight rate of 196-16 was 33.5%, and the difference was significant (P < 0.05). There was no significant difference between the dry weight rate of 197-4 and 196-16, but there was significant difference between the dry weight rate of 197-4 and the other 18 sweet potato varieties. The dry weight rate of 193-3 was 18.2%, which was significantly different from that of other 19 sweet potato species. From the above analysis, it was found that varieties 197-4 and 196-6 were more suitable for flour production, and 193-3 were more suitable for fresh food.

Table 3 Quality index analysis of sweet potato.
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The amylose content of 196-16 was the highest, which was 26.58% (Table 3). Through the analysis of variance, it found that the amylose content of 196-16 was significantly different from that of other 19 sweet potato species (P < 0.05). The amylose content of 193-4 was the lowest, 8.81%. The amylose content of 196-14 and 197-37 was similar but no significant difference (P > 0.05). The content of amylopectin in 196-14 was the highest, 4.51%, which was similar to that in 193-31, but no significant difference (P > 0.05). The content of amylopectin in 198-2 was the lowest (2.55%), and there was no significant difference between the content of amylopectin in 193-4, 195-1 and 196-16 (P > 0.05).

Comparative analysis of anthocyanins and sugar content of different sweet potato varieties

Anthocyanins are one of the main pigments that constitute the color of petals and fruit. As can be seen from Table 3, the contents of anthocyanins in 193-3, 193-4, 193-16, 193-20, 193-31, 198-1, 198-8 and 204-8 were relatively low compared to other varieties. The anthocyanin content of 197-37 was the highest, which was 38.28 µg/g. Variance analysis showed that anthocyanin content of 197-37 sweet potato was significantly different from 19 other sweet potato species (P < 0.05). According to Fig. 1; Table 2, the tuberous root color of sweet potato with high anthocyanin content is purple and dark purple, while the tuberous root color of sweet potato with low anthocyanin content or even without anthocyanin is yellow or white.

Fig. 1
Fig. 1
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Cross-section of 20 varieties of sweet potato.

The degree of sugar is a unit that expresses the concentration of solids in the sugar solution33 and refers to the number of dissolved grams of solids contained in a 100 g sugar solution. It showed from Table 3 that the sugar degree of 193-16 and 197-32 was the largest with 3.5, which indicated that these two varieties of sweet potato were sweeter than other varieties. The sugar contents of 193-20, 193-31, 197-4 and 197-37 were the lowest with 1.5.

Comparative analysis of chroma of different sweet potato varieties

The chroma of 20 varieties of sweet potato was measured, and the L*, a* and b* were analyzed. The data were shown in Table 4. Colors of tuberous roots in 20 varieties of sweet potato included purple, yellow, white and other colors, resulting in diffences among sweet potato L* value, a* value, b* value and other indicators. The difference of L* among 20 sweet potato species was significant (P < 0.05). The L* value of 193-16 was the largest, 88.25, which was corresponding to the whiteness and yellowish color of the sweet potato meat of this variety, and had no significant difference with the L* of 193-3, 193-20, 193-31, 198-1 and 204-18 varieties (P > 0.05). The L* value of 197-37 was the lowest, 37.55, corresponding to the deep purple color of its sweet potato meat, and there was no significant difference in L* between 193-21, 195-1, 197-4 and 197-32 varieties (P > 0.05). The L* value of 193-4 was 70.75, which was significantly different from that of other 19 varieties (P < 0.05).

Table 4 Chromaticity analysis of sweet potato.
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The a* value represents the reddish-green degree of the sample. The a* value of 193-4 was the largest with 27.91, and the a* value was significantly different from that of other 19 varieties of sweet potato (P < 0.05). The 193-16 had the lowest a* value of 0.11. Among them, the a* values of 191-1, 197-4, 197-37 and 198-8 were similar, and the difference was not significant (P > 0.05).

The b* value represents the yellow-blue degree of the sample, and the b* value of 193-31 was the largest (34.07), which had little difference from the b* value of 193-3, 198-8 and 193-4 (P > 0.05). 197-32 had the lowest b* value of −2.21. The b* values of 193-16 and 193-20 were similar, and the b* values were significantly different from those of other 18 varieties (P < 0.05).

Comparative analysis of texture of different sweet potato varieties

The texture indexes (hardness, elasticity, adhesiveness, cohesiveness chewiness and resilience) of 20 varieties of sweet potato were determined, and the results were shown in Table 5.

Table 5 Texture index analysis of sweet potato.
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Hardness is an index that most directly reflects the taste of the sample34. The hardness of 196 16 was the highest, and that of 22,447, 198-8 was the lowest, and that of 14,285. The hardness difference between the two varieties was significant (P < 0.05). The hardness of 196-16 was similar to that of 191-1, 196-8, 197-4 and 197-32 (P > 0.05).

Elasticity is the degree to which the sample can recover after the first compression35. The stopping time between two compression tests is important for the determination of elasticity. The longer the stopping time is, the greater the recovery height is. Elasticity is expressed as the ratio of the recovery height of the sample detected in the second compression to the amount of deformation in the first compression. As shown in Table 5, the elasticity of 204-18 sweet potato was the worst (0.62), and the elasticity of this sweet potato variety was significantly different from that of other 19 sweet potato varieties (P < 0.05). The elasticity of 193-3 was the best (0.78), which was similar to that of 191-1 and 198-2 (P > 0.05).

Cohesion indicates the relative resistance of the test sample to the second compression after the first compression deformation36. The cohesion of 198-2 was the best (0.71), which was similar to that of 191-1 and 193-31 (P > 0.05). The cohesion of 204-18 was the worst (0.57), which was similar to that of 195-1 and 198-1.

Adhesiveness is defined as the product of hardness and cohesion. One of the characteristics of semi-solid foods is that they have low hardness and high cohesion37. Therefore, this indicator is generally used to describe the taste of semi-solid foods. The adhesive property of 191-1 was the highest (15269), which was similar to that of 196-16 (P > 0.05). The adhesive property of 198-8 was 8948, which was significantly different from that of 191-1 (P < 0.05).

Chewiness is a term used to describe solid foods, and reflects the taste of the food38. The energy required for the steady state of chewing solid food when swallowed. The masticability of 191-1 was the best with 11,711, but 204-18 was the worst with 5851. Their difference was significant (P < 0.05). The chewability of 191-1 was significantly higher than that of 204-18, and the difference between the two varieties reached 5860. The chewability of 191-1 was similar to that of 196-16 and 197-4 (P > 0.05).

Resilience is defined as the ratio of the area before the deformation target to the area after the deformation target at the first compression39. During the measurement, it is necessary to pay attention to the recovery status of the sample. In general, a slower test speed will be used to achieve the condition that the sample has enough time to recover, also ensure the accuracy of this feature40. The recovery of 193-31 varieties was the best (0.40), but the recovery of 204-18 varieties was the worst (0.31). The recovery of these two varieties was significantly different (P < 0.05).

Analysis of single sweet potato weight and yield of different sweet potato varieties

The single sweet potato weights of 20 varieties were shown in Table 6. The single sweet potato weight of 20 varieties was different, and the top three single sweet potato weights were 193-16, 193-21 and 198-1. The single sweet potato weight of 193-16 was the largest with 674 g, the single sweet potato weight of 193-21 was 534 g, and the single sweet potato weight of 198-1 was 469 g. The single sweet potato weight of 198-2 was the lowest with 143 g. Environmental factors such as climate and soil type could affect the results of the experiment, and more experiments would be conducted to verify the results.

Table 6 Single weight and yield per hectare of 20 sweet potato varieties.
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As shown in Table 6, the top four per hectare yields were 193-16, 193-21, 197-25, and 198-1. The yield of 193-16 was the highest with 42 000 kg/hm2, 193-21 was 40 500 kg/hm2, 197-25 and 198-1 was 39 000 kg/hm2. 197-37 yield was the lowest with 25 500 kg/hm2. Combined with the single sweet potato weight, there were about 62 310 sweet potatoes with 193-16 varieties per hectare and about 167 000 sweet potatoes with 197-37 varieties per hectare. It showed that the number of sweet potatoes produced per hectare by 197-37 varieties was greater than 193-16. The yield of 42 000 kg fresh sweet potato per hectare can process 560 kg of starch, with an output value of about 3930 US dollars per hectare. This was similar to the findings of some studies41,42. The planting area of varieties with higher yield per hectare should be appropriately increased, its economic efficiency is much higher than that of food crops and more than many cash crops.

The final processed sweet potato products have different forms, and the nutritional quality requirements of sweet potato varieties are also different. Dry sweet potato with high drying rate, high sugar content as the screening target, 193-16, 197-25, 197-32 with high sugar, moderate drying rate suitable for dried sweet potato. Fresh sweet potato is mainly selected for its good taste, low drying rate, suitable starch content and high sugar content after steaming or baking, and 193-3 and 196-16 are suitable for fresh sweet potato. 198-2 is suitable for making fresh mini sweet potato due to its small weight, small shape, moderate yield per hectare, large number of sweet potatoes, moderate drying rate, starch content and sugar content. High anthocyanin content and high drying rate were the main screening indexes for all-powder processed varieties. Therefore, variety 197-37, with the third drying rate (31.2%) and the first anthocyanin content (38.28 µg/g), was suitable for the promotion of all-powder processed sweet potato varieties.

Conclusion

The growth, development and quality of sweet potato are jointly affected by various factors such as the growing environment and the genotype of sweet potato itself. The yield, traits and nutritional value of the same variety of sweet potato will vary greatly when planted in different regions43,44. Therefore, when introduced into planting, it is necessary to determine whether it is appropriate to plant in this region according to the local planting traits of the variety.

The sweet potato varieties in this study were divided into fresh and processed categories through statistical analysis. 191-1, 193-13, 193-16, 196-14, 197-32, 198-2, 198-8, 204-18 were classified in the fresh category, and 193-3, 193-4, 193-20, 193-21, 193-31, 195-1, 196-8, 196-16, 197-4, 197-25, 197-37, 198-1 were classified in the processed category.

The utilization rate in sweet potato processing is relatively low, and a large amount of sweet potato residue is wasted, causing environmental pollution and other problems, which have affected the sustainable development of the sweet potato industry. In the future, efforts should be made to enhance the utilization rate in the sweet potato processing process and explore suitable processing methods for sweet potato residue to promote the sustainable development of the sweet potato industry.