Abstract :
[en] Nowadays, fresh sweet potato is greatly influenced by seasonality and unable to be consumed all year. The roast sweet potato in market is usually made by personal, so the quality, safety and nutrition cannot be guaranteed effectively. In addition, roasted sweet potato is hard to storage and has a short shelf-life, what’s more, the price of roast sweet potato is high due to its seasonality, thus reducing consumers’ desire to buy it. Therefore, a new iced roasted sweet potato product which has a long shelf-life, good taste, and high nutritional value should be developed to meet the needs of consumers.
Firstly, we studied on changes in nutritional and functional value of roasted sweet potato during roasting and selected the special cultivar of roasted sweet potato. There were significant differences in the proximate and nutritional composition of the fresh and roasted sweet potatoes from the different cultivars. The results showed that fresh and roasted sweet potatoes were both rich in protein and dietary fiber, with low gross energy and fat. After roasting, the starch and protein contents declined, while total phenolic content and antioxidant activity were increased, and most of vitamins are preserved. In addition, the comprehensive nutritional value of Yanshu No.25 ranked first.
Secondly, we optimized the processing technology of roasted sweet potatoes. The results showed that roasting time, roasting temperature and roasting speed affected the measured responses, and the optimal roasting conditions included a roasting time of 40 min, a roasting temperature of 235°C, and a roasting speed of 40 rad/min. Under the optimal conditions, the result of reducing sugar content is 47.79±0.43 g/100g DW, the result of vitamin C content is 60.25±0.67 mg/100 g DW. In addition, 38 volatile compounds were identified in roasted sweet potato by GC-MS, 2-methyl butanal was the main aromatic compound. Low-field NMR results showed that roasting changes the water distribution, the proportion of free water increased and relaxation times (T2) were decreased after roasting process.
Thirdly, the effects of different freezing temperature on structure, moisture distribution, sensory, and nutritional characteristics of iced roasted sweet potato was studied. The results of ice crystals formation and microstructure showed that the tissues were severely damaged at freezing temperature of -18 °C, which took the longest freezing time (5.09 h). What’s more, the samples of -18 °C showed softer texture (hardness 454.39 g) and darker color (L* value 86.50) than that of the -40 °C and -70 °C frozen samples. The analysis of NMR and drip loss demonstrated that the condition of -18 °C freezing maintained less the intracellular water content. Moreover, with the freezing temperature increased, vitamin C and TPC content was decreased significantly, while protein and dietary fiber content did not show significant change. In conclusion, quality of the roasted sweet potato is better under the quick-frozen condition.
According to this dissertation, these investigations about raw sweet potato and roasted sweet potato qualities lead to a better understanding of the influence of processing technology on roasted sweet potato and propose new insight in improving the quality of sweet potato products. In spite of the extensive available information about the effect of roasting in different sweet potato cultivars, much of the work is essentially descriptive. What’s more, Maillard and other reactions during roasting process of sweet potatoes are complicated, and the formation of characteristic flavor is more complex, which requires further research. More basic research is still needed to elucidate the underlying mechanisms of reaction during roasting and freezing.
