Theoretical And Experimental Research On The Vacuum Spray Freezing Process For Lactic Acid Bacteria

Lactic acid bacteria are very good health care food and have reveived more and more attention recently.There are many methods of preparing lactic acid bacteria,such as spray drying and freeze drying.These are arguably the two most common approaches for producing lactic acid bacteria products.But lactic acid bacteria are very sensitive,its survival rate of spray drying is low and the cost of freeze drying is high.The results of former research showed that the droplet vacuum freezing effect could be applied to spray freeze drying process.It would simplify the procedures,reduce the cost of drying.The spray freezing process of the spray freezing drying method was revealed by using lactic acid bacteria as the research object in this paper.Numerical simulations and experiment methods were used to explore the operating mechanism of the droplet vacuum freezing effect in spray freezing drying process.The heat and mass transfer process and crotrol mechanism of the droplet vacuum freezing effect were studied experimentally and theoretically.This project aims to provide a new method for spray freezing drying of lactic acid bacteria.In this paper,the main research work is as follows:A freezing phase transition model was developed to investigate the heat and mass transfer of a single water droplet during the vacuum freezing process.The model was based on the diffusion-controlled evaporation mechanism and phase transition characteristics.The numerical calculations of three stages of the droplet vacuum freezing process were performed and the results were analysed.The effects of the vacuum chamber pressure,initial droplet diameter and initial droplet temperature on the heat and mass transfer characteristics at each stage were studied.The droplet experienced supercooling breakdown at the end of the evaporation freezing stage before the isothermal freezing stage began.The temperature of the droplet had transiently risen as the result of the supercooling breakdown phenomenon,whose effects on the freezing process and freezing parameters were considered.The complete thermal dynamic process of liquid vacuum freezing process was studied based on the energy and mass conservation principle and the thermodynamic.The vaporization process of the solution and its transportion process from the vacuum chamber to the condensation chamber and the phenomenon of its coagulation on the condenser were modeled.The model was calculated by using pure water as the research material.The changes of the vacuum chamber total pressures,the condensation chamber total pressures,the liquid evaporation rates and the condensation rates with different effective volumes of vacuum chamber,effective volumes of condensation chamber,flow conductances between vacuum chamber and condensation chamber and effective pumping speeds were explored.The change laws of pressures in the chambers and the rules of liquid evaporation and condensation were revealed by analyzing the contrast curves.A vacuum spray freezing experiment equipment,including vacuum chamber,pressure atomizer equipment,gas extraction device and GLZ-0.4 type vacuum freezing drier,was designed and established.This experiment was usd to performe the liquid vacuum spray freezing experiment and two kinds of nozzles were used in the experiment.The flow rates and the size distributions of these two nozzles were clear.The correctness of the freezing effect of a liquid droplet in vacuum and the feasibility of the vacuum spray freezing experiment equipment were validated by carrying out the liquid vacuum spray freezing experiment,which can be used as reference to guide the actual production.