Study On Mechanism Of Microwave Vacuum Puffing Berry Snacks

Microwave vacuum puffing (MVP) is a novel method for the berry fruit processing. Thedielectric properties of berry are the key index to influence energy consumption and quality offinal product. This research focuses on the variations of dielectric properties of berry leather underMVP conditions. The optimal mode and parameters of MVP are deteriminated to get quality berrysnack.Raspberry fruit was selected as the material, the effect of microwave vacuum parameters onthe dielectric properties in terms of dielectric constant and dielectric loss factor of raspberryleather were studied as follows: First, according to the puffing rate of berry leather in the processof vacuum puffing, the puffing characteristics of material processed were studied. Second, half-theory and half-experience of MVP dynamics model was built. Thirdly, dielectric properties ofslices in microwave vacuum process were studied considering the absorebed microwave energyand attenuation factor to construct the dynamic dielectric model of slices in MVP process. Fourth,based on heat and mass transfer process considering the dynamic dielectric properties under MVP,the temperature and moisture distribution inside berry leather were simulated by using matlabprogramme, and volume expansion of berry leather was simulated by using Visual C#.Netsoftware. At last, through the genetic algorithm combined with the neural network method, MVPparameters were optimized and the comparisons with the response surface methods were done.The main conclusions about this topic research are as follows:1.Based on the existing formula, the berry MVP dynamics models were built, and the effect ofvacuum pressure and initial moisture content on the puffing rate were deterimined. In the processof berry MVP process, puffing rate function is presented by using sigmoid model. At the initialstage of the expansion, puffing rate of berry leather follow the slow increase of limited growthLogistic model, and in later stage, the puffing rate follows the modified Logistic model. In theconditions of MVP berry leather, puffing kinetic results from the difference between internal vaporpressure caused by microwave energy absorption and vacuum pressure in environment. Vacuumpressure and initial moisture content have significant influence on the average rate of puffing.2.Berry’s physical condition,the process parameters of microwave processing will affect itsdielectric properties, affect the incident wave attenuation factor, heat volume, the absorption anduse of microwave energy, and then influence heat and mass transfer inside the material, at lastaffect the distribution of the internal temperature and moisture; meanwhile, internal temperatureand moisture distribution will react on the material to absorb microwave energy andtransformation, ultimately affect the dielectric properties of materials.Response surface center combination experiment with four factors and five levels wereemployed to calculate the dielectric constant and dielectric loss factor and to analyze the effect ofMVP conditions on the dielectric properties. Influencing factors are vacuum pressure, expansiontime, initial moisture content and processing capacity, and the response indicators are dielectricconstant and dielectric loss factor of berry leather. The initial moisture content of berry leather has the significant influence on the dielectric constant and dielectric loss factor.At the initial stage ofthe expansion, the dielectric constant decreases with the increase of initial moisture content; Atlater stage of expansion, dielectric properties increase with initial moisture content and thendecrease. And dielectric loss factor increase with the initial moisture content of berry leather. Theimprotance of factors influencing the dielectric constant and dielectric loss factor is follows: initialmoisture content, time, vacuum pressure and mass. When mass was96g and initial moisturecontent was22%, at the early stage of the expansion, the berry leather of the dielectric constantincreases with the increase of processing capacity, in later expansion, dielectric property decreaseswith the increasing of mass. When moisture content is in low level, dielectric constant has negativerelationship with temperature, and in high moisture content, no significant effect of puffing timeon the dielectric constant and dielectric loss factor was found.3.According to the process of MVP berries chip, heat transfer and mass transfer mathematicalmodel is deduced. According to the dynamic dielectric model deduced, heat and mass transfermathematical model related to MVP process was developed. Temperature and moisturedistribution inside berry leather puffed is obtained by using computer simulation method. Higherinternal temperature has a more obvious effect on heat transfer. With the increase of leather radiusand thickness, temperature of berry leather decreases. the increase both in the direction of fastestdecrease, Highest temperature gradient occur at the location of both decrease of temperature andthickness. Highest temperature is in the centre of berry leather, and lowest point is at edge.4.Optimal parameters of MVP berries were obtained by integrating genetic algorithm (GA) andBP neural network (BPNN) optimization method with lowest crisp brittle consumption energy.According to the actual experimental results, optimal parameters from GA are superior to thatrespone curves. Therefore, genetic algorithm, BP neural network to berry MVP optimum processparameter optimization is completely feasible.In the research, the absorption of microwave energy within the fresh slice of raspberry and heatconversion was studied. The models of dielectric properties of berry leather were established asfunction of temperature, moisture and porosity of berry leather. The optimal paremeters and modesof MVP raspberry leather were developed, which is expected to promote deeply processingtechnology level of berry fruit in China.