System Optimization And Experimental Study On The Vacuum Cooling Device Of Steamed Bread

Vacuum cooling technology is a kind of cooling method that based on the basic principle when the water evaporates it absorbs heat, and with the vacuum pump extracts gas to decrase the pressure in the vacuum chamber. At the same time,since the boiling point of water decrases with the decrase of pressure,and the latent heat of evaporation is increasing with decrasing pressure, Therefore, while the pressure is reduced, the moisture inside the product due to the evaporation will take away a lot of heat, so that the temperature of the product will decrease too.As the cooling technology is not only fast,but also clean and healthy,easy to operate.The temperature of products is uniform after cooling,and the vacuum cooling equipment makes small pollution to the environment.So in recent years,it has been widely used by people.The paper mainly makes the theoretical analysis and experimental research of the vacuum cooling process of steamed bread,and also makes numerical simulation of the pressure distribution in the vacuum chamber,at the same time compares the effects of different parameters on the flow field in the vacuum chamber.There are four different cooling methods are used to do experimental research on steamed bread, including natural cooling,air cooling,vacuum cooling and hybrid cooling.And it also makes comparison on the quality,mass loss,cracking rate and energy consumption of steamed bread after the four cooling methods.After different cooling methods,the steamed bread were storaged at the same condition,and measured the quality regularly,by this way to determine the best cooling method of steamed bread.By the simulation and experimental study,the paper has made the following conclusion:1) Simulation results:From the influence of different parameters on pressure field we can know that the vacuum pumping rate has a big influence on the rate of pressure’s decline in the vacuum chamber,while the free volume of the vacuum chamber has little influence.And the vacuum pumping rate is more high,the ultimate pressure that the vacuum chamber can reach is more low,while the free volume doesn’t affact the ultimate pressure.2) Experimental results:The fastest cooling method to derease the temperature of steamed bread is vacuum cooling,and then is the N&V hybrid cooling method;The cooling method which causes the greatest mass loss of steamed bread is vacuum cooling,while the nature cooling method causes the least,and the N&V hybrid cooling method is next to the nature cooling;On the quality of steamed bread’s hardness,chewiness and viscosity,the best method is N&V hybrid cooling;The method which causes steamed bread at the latest time to crack is N&V hybrid cooling,and also the method has the smallest cracking rate;In the four cooling methods that cuases the biggest energy consumption is vacuuming cooling,while the lastest is natural cooling,its energy consumption is zero,and the N&V hybrid cooling method causes energy consumption is moderate.The conclusion can be drawn from the comprehensive analysis of the steamed bread’s cooling is that the best cooling method for steamed bread is N&V hybrid cooling.3) The biggest drawback of the vacuum cooling technology is the big mass loss,and the error of mass loss between theoretical value and experimental value is 16.42%.