Available online 29 July 2015
Profiles of phenolics, carotenoids and antioxidative capacities of thermal processed white, yellow, orange and purple sweet potatoes grown in Guilin, China
- Open Access funded by Beijing Academy of Food Sciences
- Under a Creative Commons license
The objectives of this study were to systematically compare phenolic profiles, carotenoids profiles and antioxidant activities of raw and cooked sweet potatoes of five varieties (white, yellow, orange, light purple and deep purple). Total phenolic content (TPC), monomeric anthocyanin content (MAC), total carotenoid content (TCC), 2-diohenyl-1-picryhydrazyl (DPPH) free radical scavenging capacities and ferric reducing antioxidant powder (FRAP) were determined by colorimetric methods. Higher anthocyanin contents and antioxidant capacities were detected in purple sweet potato species, while higher carotenoid contents were detected in yellow and orange sweet potato. All cooked sweet potato exhibited significantly (p < 0.05) lower TPC, MAC, TCC, DPPH and FRAP values as compared to the respective raw samples. Under the same cooking time, steaming was good for the retention of TPC, roasting was good for keeping anthocyanins, and boiling was beneficial to preserve carotenoids.
- sweet potato;
- antioxidant capacities
- Deep-purple sweet potato exhibited greater phenolics and antioxidants than others.
- Yellow and orange sweet potatoes possessed higher carotenoids than others.
- Different processing methods significantly degraded antioxidants.
- Cooked sweet potato exhibited lower bioactive substances and antioxidants.
- Consumer may obtain bioactive substances by choosing cooking ways.
Sweet potato is a crop with rich nutritional values including carbohydrates, dietary fibers, vitamins, and minerals . Currently, it is the sixth most popular and abundant staple food worldwide. It plays an important role in solving the issues of food, energy, natural resources and environment. Four commonly available colored sweet potato species in China are white, yellow, orange, and purple, which have completely different chemical compositions.
The major bioactive substances in purple sweet potato are phenolics and anthocyanins. Phenolics are the antioxidant molecules with at least one aromatic ring and one or more hydroxyl groups . Anthocyanins, are a group of water-soluble flavonoids. As the predominant pigments and functional phenolics in purple sweet potato, anthocyanins are the naturally strong free-radical scavengers, which provide many pharmaceutical values including anti-oxidation, anti-tumor capacities, and prevention and treatment of cardiovascular diseases. In yellow or orange sweet potato species, carotenoids (such as β-carotene) act as the primary pigment molecule  as well as the source of provitamin A, which shows vitamin A activity . Carotenoids have strong antioxidant capacity to scavenge free radicals because of their conjugated double bonds .
Generally, sweet potato is cooked, either by boiling, steaming or roasting, before consumption. Such thermal processing can cause impairment of the functional compounds of sweet potato. There have been reports of negative correlation between heat treatments (steaming and baking) and some bioactive substances, such as anthocyanins. Carvalho et al.  reported a dramatic decrease in both total carotenoid and β-carotene contents of sweet cassava after cooking.
Although the benefits of sweet potato are widely established through numerous studies, there is limited information about how their functional components (e.g., phenolic substances, carotenoids), and antioxidant capacities are affected by different home-cooking ways. In the present study, we investigated the changes in total phenolic content (TPC), monomeric anthocyanin content (MAC) and total carotenoid content (TCC), as well as antioxidant capacities (DPPH and FRAP) of five species of sweet potato after three types of ordinary thermal processing, such as boiling, steaming and roasting with a view to understand detail changes in the functional compositions of different chemical constituents.
2. Materials and methods
2.1. Chemicals and reagents
Folin-Ciocalteu reagent, 2-diohenyl-1-picryhydrazyl (DPPH), and 2, 4, 6-tri (2-pyridyl)-s-triazine (TPTZ) were purchased from Shanghai Yuanye Biological Technology Co., Ltd (Shanghai, China). The 6-hydroxy-2, 5, 7, 8-tetramethlchroman -2-carboxylic acid (Trolox) was obtained from Sigma-Aldrich Co. (Shanghai, China). Absolute ethanol was obtained from Tianjin Fuyu Fine Chemical Co., Ltd. Other chemical reagents were supplied by Tianjin Damao Chemical Reagent Co., Ltd. (Tianjin, China). All chemicals were analytical grade unless specially mentioned.
2.2. Sweet potato samples
Five species of sweet potatoes were sampled, including light purple, yellow, white, orange, and deep purple (shown in Fig. 1. and Table 1). All of them were cultivated in Guilin Agricultural Research Institute in Guilin of Guangxi Province (China) in 2013.
Code Species Color of flesh Geographical location 1 Gui 04-53 Light purple Guilin Agricultural Research Institute of Guangxi Province, China 2 Gui 09-75 Yellow 3 Guishu #2 White 4 Guineng 05-6 Orange 5 Guijingshu 09-7 Deep purple
2.3. Cooking approaches and cooking time
Three thermal processes were performed for sweet potatoes with five species (light purple, yellow, white, orange and deep purple). All sweet potato samples were not peeled before and during heat treatment. After cooking, they were peeled. Boiling, steaming and roasting processes imitated cooking methods at home as far as possible.
For the boiling treatment, about 130 g of sweet potato was added to 650 mL tap water (sample/water - 1:5, w/v). The water was heated to its boiling point before being added to the different kinds of sweet potatoes, and then cooked in the electric hot plate cooker for about 30 min. For the steaming process, approximately 130 g sweet potato was placed in a steam cooker, in which one liter tap water was filled. Steaming was conducted for about 30 min after the water generated steam. For the roasting process, an electric oven (Galanz, China) was applied to preheat to 230 °C. After that, about 130 grams of sweet potato was placed in the oven and roasted for 30 min at 230 °C.
All samples (including non-cooked and cooked) were lyophilized by freeze-dryer (Labconco Corporation, Kansas City, MO, U.S.A.), and then sweet potato samples were ground by a grinder (Beijing Zhongxing Weiye Instrument Co., LTD). Ultimately, sweet potato powder were passed through 80 # mesh and stored at 4 °C in a refrigerator (Dukers) for further studies.