| In this study, the optimum condition of osmotic dehydration pretreatment was determined firstly. Parameters of pulsed sudden decompression flashed drying (PSDFD) and their effects on quality of dried guava were investigated, and the relationships between process parameters and product quality were discussed. Furthermore, the optimized drying parameters of PSDFD was obtained using response surface central composite test method. Based on results of previous work, the present work investigated the effect of drying methods on bioactive compounds and antioxidant capacity of guava. The present work also studied the techniques of drying and grinding in terms of physico-chemical qualities and other characteristic properties of powder. This study was aimed at providing theoretical references of drying and grinding technologies as well as quality control. The main conclusions were summarized as follows:1. Osmotic dehydration pretreatment could improve the hardness and thickness of guava slices, meanwhile effectively improved the color quality. Took into account of the quality of guava slices and the effect of osmotic dehydration, the optimum condition of osmotic dehydration pretreatment was determined:osmotic solution was fructose syrup with 25°Brix and treatment time was 90 min. The effect of PSDFD on product quality was studied, and the results showed that intermediate moisture content had certain impacts on hardness and rehydration ratio of products. The flashed drying treatment improved rehydration ratio of products, yet the effect was not significant. Multiple flashed drying contributed to release of polyphenols and higher antioxidant activities. However, when the vacuum drying temperature was over 80℃, the antioxidant activities significantly decreased. The formation of hardness occurred in vacuum drying phase.Based on reduction of dimensionality, two common factors (color-active factor and texture factor) were obtained. The comparatively better process parameters were determined, which were intermediate moisture content (moisture content reserved after pre-drying by hot air) 40%, flashed drying temperature 90℃, flashed drying treatment 3 times, vacuum drying temperature 70℃ and vacuum drying time 2 h. The results of regression analysis showed that the color-active factor was significantly affected by vacuum drying temperature and time. The texture factor was significantly affected by vacuum drying temperature.2. Based on the previous work, the Box-Behnken center design was applied to establish multivariate quadratic regression equation models. The coefficient R2 of models were above 0.8, indicating the models predicted the change of evaluation index well, except the hardness index. Response surface analysis results show that, vacuum drying temperature significantly affected moisture content, L value and total phenolic content, in addition, vacuum drying temperature affected rehydration ratio and hardness. Vacuum drying time significantly affected moisture content, hardness and total phenolic content, and affected L value. Moreover, the interaction effect between vacuum drying temperature and vacuum drying time significantly affected total phenolic content. The optimized drying parameters were obtained:flashed drying temperature 91.29-96.96℃, vacuum drying temperature 70.51-72.79℃ vacuum drying time 1.92-2.10 h.3. Different drying methods, including hot air drying (HAD), hot air combined with infrared radiation drying (HA-IRD), vacuum drying (VD), vacuum freeze drying (FD) and pulsed sudden decompression flashed drying (PSDFD), had significant influence on bioactive compounds (phenolics, flavonoids and ascorbic acids) and antioxidant capacities. The results showed that, total phenolic content increased significantly after drying process, while the content of flavonoids and ascorbic acids degraded significantly as well as the antioxidant capacities. VD and FD samples showed higher content of both total phenolics and ascorbic acids, but VD sample showed lower total flavonoids. Meanwhile, the high level of total flavonoid content were found in HAD, HA-IRD and FD. The experiment results showed that drying process contributed to high content of phenol, moreover, vacuum and non-thermal drying environment were beneficial to high levels of phenol and ascorbic acid. Drying process had a negative impact on antioxidant capacities of guava. FD sample showed highest radical scavenging activity and ferric reducing ability. Under the same drying temperature, HAD was more favorable for the retention of antioxidant capacities than HA-IRD. Generally, HA-IRD and VD imposed destruction on antioxidant capacities of guava with lower retention.The results of LC-MS analysis showed that the phenolic compounds of guava could be classified into four categories:flavanol compounds, hydrolyzable tannins, ellagic acid derivatives and coumaric acid derivatives. After thermal drying process, both benzoic acid and cinnamon acid compounds with small molecular weight emerged. Drying process facilitated the formation of ellagic acid and its derivatives, especially thermal drying process led to the formation of ellagic acid. The effect of drying process on flavanol compounds depended on the polymerization degree of procyanidins. Within detection range, the effects of drying process on hydrolyzable tannins with high molecular weight were not the same, whatsmore, drying process decreased the content of hydrolyzable tannins with low molecular weight, yet the exceptions were found in bis-galloyl-glucose content from FD and PSDFD. The analysis results also showed that drying process caused a content decrease in ellagic acid derivatives, in addition, there might existed conversion occurred in ellagic acid derivatives with similar molecular structures. The effects of drying process on cinnamic acids were decided by specific types of cinnamic acids, which included a dramatic increase in cinnamic acids (m/z 487) and ferulic acid derivatives (m/z 517), and the high levels were found in VD and HA-IRD. On the whole, drying process decreased the content of ferulic acid derivatives (m/z 355) and sinapic acid derivatives (m/z 385).4. Different drying methods significantly affected physicochemical indicators, powder characteristics, total soluble sugar content and its composition, titratable acidity. The experiment results showed that flashed drying treatment had an excellent dehydration behavior and the powder by which had lower water content and water activity, significant lower than other drying methods. FD and HA-IRD samples showed better color properties. The powder obtained from PSDFD had smaller size and uniform particles with good solubility and flowability. FD sample had higher solubility, but had larger size and nonuniform particles, and also lower flowability. High temperature-long time drying process significantly decreased the solubility and flowability of powder. The high temperature-long time drying method could cause lower dissolution and flowability of powder. FD and PSDFD samples showed higher total soluble sugar content as well as its main composition content. PSDFD sample showed higher comprehensive quality.Chosen PSDFD as drying method, the study also investigated the effect of fine grinding process on powder. The results showed that fine grinding significantly increase L value, decrease a value, b value and AE, which improved the color quality of powder. Fine grinding could effectively smash seeds of guava, resulting in lower size particles and better distribution. Moreover, fine grinding contributed to dissolution of carbohydrate, yet no effect on acidity. But fine grinding increased the water content of powder and decreased the flowability. Based on experiment results, fine grinding improved powder properties. PSDFD combined with low-temperature fine grinding technology was suitable for whole-fruit powder processing, and could produce fruit powder with high quality. |
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