A batch-type microwave vacuum dryer was used to study the effects of different microwave power densities on the drying characteristics and quality of whole lychee (Litchi chinensis Sonn.) fruits. Lychees were treated at different power levels and it was observed that the drying process included two drying stages when power density was above 0.3 W/g, and drying process was consisted of three drying stages when power density was 0.1 W/g. The optimal power density was found to be 0.6 W/g. A typical pulp temperature profile of lychees was found during microwave vacuum drying (MVD), including four stages: increasing, decreasing, gently increasing and sharply increasing. Moreover, the decrease in quality was negatively correlated with an increase in temperature at every stage in the temperature profile. Temperature control was applied to improve the quality of MVD lychees. The optimal temperature range value in terms of drying time, vitamin C content, bulk density and sensory evaluation was found to be 60-65C. PRACTICAL APPLICATIONSAir-dried (AD) lychees are popular as traditional food in South China and its sales volume was about 80% of all processed lychee foods. AD lychees have deep brown pulp with much shrinkage and poor flavor. However, microwave vacuum drying (MVD) could produce high-quality dried lychees with light golden yellow color, less shrinkage, better taste, shorter drying time and lower energy. The information provided in this work will sever as the basis for further mass production. MVD lychees are much more promising as a snack food than AD lychees.
Three heating patterns, constant temperature, temperature programmed and programmed cooling process were applied to freeze‐drying (FD) the litchi pulp in order to obtain the optimal FD heating technology. The results show that the three patterns all could be used to FD litchi pulp while the appearance was maintained well. The drying time of FD litchi pulp using programmed cooling process was the shortest among the three optimal heating patterns and the drying time was reduced by 15.8 and 11.6% comparing that of FD using constant temperature and temperature programmed. Higher temperature can be applied at the early primary drying stage when the temperature of litchi pulp is under the eutectic temperature and at secondary drying stage but should not be used at the later primary drying stage. PRACTICAL APPLICATIONS Vacuum FD is widely used in food, protein and drug drying. It is still the best drying method to obtain high‐quality biological products. The objective of this paper was to optimize the FD process of litchi pulp. The significance of this research can improve the efficiency of FD in litchi pulp drying. Throughout the trial, the production‐scale vacuum freeze‐dryer was used, which means that the FD process can provide technological support to produce freeze‐dried litchi pulp. In addition, it can also provide a reference for other fruits and vegetables' FD process optimization.
The quality of freeze drying (FD)4.5 h-microwave vacuum drying (MVD) iron yam slices was almost the same with FD samples in terms of bulk density, color, etc., in the previous experiments. This work verified it by comparing the microstructure of the samples. In addition, the microstructure and quality of samples with different conversion point during combined FD and MVD were tested in order to illuminate the change mechanism for cell structure and the relationship between microstructure and corresponding quality. Moisture conversion point and melting significantly affected the microstructure of samples. The cell structure of samples dried by FD3.5 h-MVD with melting samples was maintained very well. However, obvious cell wall breakage and even collapse was observed in FD2 h-MVD samples. Quality indices such as bulk density and color are positively related to microstructure change. FD3.5 h-MVD with melting was better than FD4.5 h-MVD because of almost the same quality and shorter drying time. PRACTICAL APPLICATIONSIn most previous researches, the optimal moisture conversion point was usually the end of sublimation stage during freeze drying (FD)-microwave vacuum drying (MVD). There was no obvious ice crystal in sample at this point, which showed that the materials absorbed the microwave energy uniformly and caused high quality of final products. However, for vegetables and fruits materials that are rich in starch, such as iron yam, moisture conversion point was chosen in advance. Ice crystal in the samples melted slowly in low temperature and well-distributed moisture again after FD. The structure of the materials was maintained very well because of the support from raw starch granules. Therefore, high-quality dried products were also obtained under this technique conditions. In general, energy consumption for FD-MVD drying was decreased further while the quality of the materials was maintained well. 2012), polysaccharides (Ju et al. 2014) and steroidal saponins (Yang and Lin 2008). Therefore, it is also processed into various kinds of products except when it is used as a vegetable . Iron yam slices dried by hot-air drying (AD) were the common products in markets and Chinese medicine shops.Combined drying methods, including tandem or parallel way, can be described as two or more drying methods combined in order to synthesize the advantages for every drying method and obtain high-quality products. Tandem drying
Compared to other cultivar lychees, volume density of Feizixiao lychee was higher due to serious shrinkage during freeze-drying (FD). Guiwei lychee and Nuomici lychee were used for comparison in order to illuminate the reason of the aforementioned phenomenon. Lower prefreezing temperature could not improve the volume density of Feizixiao lychee. Microstructure results show that pulp cell of Feizixiao lychee (tail) was smaller and more compact than Guiwei and Nuomici lychee pulp cell. In addition, there is a membrane around the surface of Feizixiao lychee pulp. And the microstructure of Feizixiao lychee tip pulp cell is different from tail pulp cell. Membrane and tip pulp cell are both smaller and more compact than tail pulp cell. These structure differences hinder the moisture removing of Feizixiao lychee during FD. Removing the membrane and tip pulp could not improve the volume density of Feizixiao lychee. Ultrasound treatment for 30 min could significantly enhance the volume density of Feizixiao lychee.
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