Improvement Of Suspension Freeze Concentration Technology And Effect Of Ripening Of Suspension Ice Particles On Freeze Drying

Low-temperature processing technology can produce high-quality food,because low temperature can avoid the loss of flavor substances.Suspension freeze concentration or freeze drying can reduce the water content of liquid food under low temperature conditions,thus both of them have broad application prospects in food processing.But the high operating cost limits the development of suspension freeze concentration and freeze drying in the food industry.Therefore,in order to improve the production efficiency of suspension freeze concentration and freeze drying,suspension freeze concentration equipment has been improved in this study and more efficient production methods of suspension freeze concentration and freeze drying have also been explored.Suspension freeze concentration prepares dispersed suspended ice particles in the solution.After ripening,the ice particles are separated from the mother liquor to obtain a concentrated solution.An agitation has been added to the existing suspension freeze concentration equipment in the laboratory to increase the appropriate turbulence during the ripening,successfully concentrated the sucrose solution from 10 wt.% to 17.6wt.%coordinating with centrifugation in a run of 2h.After that,a scanning electron microscope was used to observe the distribution position of sucrose attached to the surface of the ice particles for the first time,and the solute loss(concentration of sucrose in the melting liquid of ice particles)was reduced to 0.6wt.% by partly melting the ice particles after the first separation step,and the solute yield finally reaches to 97%.Experimental results show suspension freeze concentration is also suitable for liquid foods such as orange juice,skimmed milk and wine.The principle of the increasing volume of ice particles in suspension freeze concentration is Ostwald Ripening,ripening is called for short.The product at the end of ripening is a solid-liquid mixture of ice particles and concentrated liquid,called slurry.Freeze drying of liquid food is the combination of pre-freezing and transferring frozen material to low temperature and low pressure environment to sublime ice crystals.The ice crystals formed during the pre-freezing have a great influence on the drying process.Because the mass transfer resistance of the drying layer left by large ice crystals sublimed is smaller,thus frozen materials with larger ice crystals have a faster primary drying rate.The ice particles experienced ripening have a larger volume than the ice crystals formed by normal freeze.Regard ripening as a pretreatment of pre-freezing in the freeze-drying of liquid foods(slurry freeze)can shorten the freeze-drying time by the large volume of ice particles that have been formed.The experimental results prove that the freeze-drying time of orange juice pre-freezing with slurry is 45.2% shorter than that of normal freeze orange juice.Finally,maltodextrin was used as an additive to orange juice to produce freeze-dried orange juice powder with the help of slurry freeze.The effect of slurry freeze on the physical and chemical properties of freeze-dried orange juice powder was also discussed.