Study On Osmotic Dehydration Of Asparagus Lettuce Based On Sucrose System

Osmotic dehydration (OD) as a pretreatment prior to drying can not only shorten drying time of dehydrated vegetables, but may also save energy cost. However, OD is quite a time-consuming mass transfer process, so it requires some auxiliary methods for improving efficiency. In this work, OD and its auxiliary methods of asparagus lettuce immersed in sucrose solution have been investigated, and the influence of different OD treatments on quality of the final products has been discussed.The effect of different factors on OD was studied. On the basis of the single factor experiment, response surface methodology was used to analyze the influence of solution concentration, osmotic time and temperature on OD efficiency. The results obtained Mathematical Models about water loss and solid gain respectively, indicating that time contributed to water loss most while temperature was most remarkable to solid gain. The final optimum OD process conditions for maximum water loss and minimum solute gain were:55%sucrose solution,4h process duration and30℃temperature. In order to determine blanching condition, thermal inactivation of peroxidase (POD) in asparagus lettuce was kinetically modeled. The results showed Weibull distribution model provided a better predictability, considering the VC content simultaneously, chose blanching at90℃hot water for3min as suggested condition. Then discussed ultrasound and agitation treatment conditions, according to water loss and solid gain indexes,45kHz frequency and100W power were applied to ultrasound treatment while160r/min was taken as shaking speed.The kinetic characteristics and diffusion coefficient of mass transfer during static OD and its auxiliary methods (ultrasound, blanching and agitation) were compared. It was found that applying auxiliary methods could promote dehydration process effectively. Among them, ultrasound was significant in the initial period, and then gradually weakened with the increasing of time. However, agitation showed the best dewatering effect with the greatest diffusion coefficient.Having studied different OD treatments on quality of asparagus lettuce, the results indicated that the hardness decreased after static OD while combined with ultrasound and blanching could prevent hardness changing. OD provided color protection to asparagus lettuce, especially with blanching pretreatment. Further drying the osmotic dehydrated asparagus lettuce indicated that OD could shorten the equilibrium time of diying process, and auxiliary agitation was more significant. OD lowered rehydration ratio, but was beneficial to recover the dehydrated asparagus lettuce. According to sensory evaluation, OD improved final product quality, particularly showed higher acceptability in combination with ultrasound and blanching.