真空冷冻干燥技术及其在火龙果冻干中的应用

介绍真空冷冻干燥的机理和工艺以及真空冷冻干燥食品的特点,探索火龙果的真空冷冻干燥工艺.试验表明,在综合考虑干燥时间和产品质量的前提下,火龙果样品在厚度为11 mm,一次干燥加热板温度为25℃,二次干燥加热板温度为50℃时真空冷冻干燥效果较好,冻干后火龙果的含水量为4%,冻干后放入60℃的水中时,5 min左右可复水成新鲜时的形状.     

OPTIMIZATION OF DEHYDRATED EGG QUALITY BY MICROWAVE ASSISTED FREEZE-DRYING AND HYDROCOLLOID INCORPORATION

Enhancement of the physical and sensory properties of rehydrated egg products as a consequence of microwave assisted freeze-drying and addition of hydrocolloids to the formulation was assessed. When compared to conventional freeze-drying (CFD), microwave assisted freeze-drying (MFD) led to products with greater water-holding capacity and reduced elastic modulus. Comparisons were also made of moisture sorption rates. The addition of xanthan gum and, to a lesser extent, microcrystalline cellulose increased both water-holding capacity and elastic modulus. Magnitude estimation by a trained texture panel of critical sensory properties (chewiness, rubberiness, moistness, moisture release, and roughness) revealed that MFD samples were superior to CFD samples and that MFD samples containing xanthan were closest to the "optimal" for the egg products. It was concluded that the more uniform and less heat stressing process of microwave assisted freeze-drying coupled with the presence of xanthan gum leads to a rehydrated product that best retains its structural     

EXPLORING AIRBORNE SOUND IN A NONVACUUM FREEZE-DRYING PROCESS

SUMMARY– A study was made to determine if sound energy could be used to enhance freeze-drying rates of foods and to analyze the kinetics in such a system. A prototype pilot freeze-dryer with airborne sound was designed and built to freeze-dry liquid foods, e.g., tea and coffee extracts. The sound source was a stem-jet whistle producing 10.8–12.2 kcps with a maximum sound intensity level of 149 db. Desiccated compressed air at 70° to 90°F served as the driving force of the whistle as well as the carrier of the sublimed moisture. Experimental results showed this to be a feasible process. Sample temperature, inlet air pressure, the freeze-dryer tube size, and the sound source, all had significant effects on the freeze-drying rates. Freeze-drying rates were 11–100% higher in the stem-jet whistle runs than in the dummy whistle runs; 7% of this improvement was attributed to the thermal effect on the air resulting from friction and adiabatic compression. The remaining increase was due to the sound pressure energy, the increase in heat and mass transfer coefficients, and the "reduced-pressure" effect in the sweeping air stream during the rarefaction cycle of the sonic vibration. The flaking action of the sound waves on near-dried tea and coffee was considered a processing advantage because of the ease in product recovery and because a new surface was continuously being exposed for sublimation.     

EFFECTS OF PROCESS VARIABLES ON RETENTION OF VOLATILES IN FREEZE-DRYING

SUMMARY— Retention of organic volatiles in freeze-dried solutions of mono-, di- end polysaccharides was studied as a function of different process conditions. Under the experimental conditions used, slow freezing and decreasing sample thickness promoted retention. The effects of platen temperature and of concentration of solution components were more complex. Effects of process variables were explained on the basis of a mechanism which postulates formation of microregions due to association of carbohydrate molecules. These microregions become impermeable to organic compounds when their water content decrease below a critical level.     

预处理对冻干黄豆芽品质的影响

主要研究了几种不同的预处理对黄豆芽冻干产品品质的影响.发现,黄豆芽可以不经漂烫处理;而经20%的麦芽糊精、饱和β-环糊精溶液浸泡1h可明显提高产品品质.     

智能湿度传感器在微波炉中的实现

热敏电阻型湿度传感器具有测湿响应速度快、滞后误差小、便于测小空间湿度的优点．把热敏电阻型湿度传感器和单片机结合起来，可根据微波炉内腔湿度的变化规律，实现微波炉加热功率和加热时间的智能控制，具有很强的实用性     

芦荟冻干及多糖提取研究

通过对库拉索芦荟、中华芦荟、木立芦荟进行真空冷冻干燥实验，得到了冻干曲线。以3种芦荟鲜叶和冻干制品及库拉索芦荟常压干燥、真空干燥制品为原料，分别进行多糖提取实验，结果表明，库拉索芦荟冻干制品多糖提取率最高。因此以库拉索芦荟冻干制品为原料，研究了用溶剂提取法提取芦荟多糖的最佳工艺条件。实验表明，水提醇沉法的最佳工艺条件为：料水比1：50，提取温度50℃，提取时间5h，提取次数1次，提取液浓缩至原体积的1／3，80％乙醇醇析。对芦荟叶片冷藏时间与多糖含量的变化做了初步研究。     

微波干燥在亚麻纺织工业的应用

介绍了微波干燥的原理和主要特点,研究了利用微波干燥亚麻纱管的工艺,探讨了微波辐射时间、微波功率对亚麻纱管脱水率的影响,考察了工业微波干燥机的干燥能力,并将实验结果与传统烘房干燥后亚麻纱的色差和断裂强度进行了比较。     

CONTINUOUS MICROWAVE STERILIZATION OF MEAT IN FLEXIBLE POUCHES

Feasibility studies originated earlier in this laboratory, on microwave energy thermal processing of foods, were continued. A means of measuring temperature distribution, as well as a photometric method for determining time/temperature integration within thermally processed foods, were developed. A knowledge of the processing parameters involved in the preservation of foods by microwave energy was obtained. Feasibility of the process was demonstrated.     

SURVIVAL OF SALMONELLA TYPHIMURIUM IN ROASTING CHICKENS COOKED IN A MICROWAVE, CONVECTION MICROWAVE, AND A CONVENTIONAL ELECTRIC OVEN

Fresh whole roasting chickens were inoculated with Salmonella typhimurium and cooked either in a microwave, convection microwave, or conventional electric oven to an internal temperature of 74°C, 77°C, 79°C, or 85°C. Cooked chickens were tested for the presence of viable Salmonella organisms using an Enzyme Linked Immunosorbent Assay (ELISA). No chicken (0/12) cooked to ± 79°C in the convection microwave or conventional oven tested positive for Salmonella. Two out of three chickens cooked to 79°C and all three chickens cooked to 85°C in the microwave tested Salmonella positive. Tetrathionate broth dilutions from all 11 microwave cooked chickens produced isolated colonies on Brilliant Green Agar. No growth was observed on Brilliant Green Agar plates inoculated with tetrathionate broth dilutions from chickens cooked to an internal temperature of 79°C or 85°C in the conventional oven or to 85°C in the convection microwave.     

TEMPERATURE PROFILES IN DOUGH HEATED WITH A SUSCEPTOR IN THE MICROWAVE OVEN

A model based on energy considerations for predicting temperature profiles in a microwaved dough placed in contact with a susceptor was developed. Using a commercial finite element software ("Ansys 5.01"), for an axisymmetric and transient heat transfer problem, the temperatures at the dough/susceptor interface and in the whole dough were calculated. The calculated temperatures at the susceptor/dough interface were superimposed on previously published curves (based on a correlation relating temperatures at the interface with heating time, dough and susceptor parameters) and compared to experimentally determined values. A very good agreement between the experimentally determined temperatures and the values predicted from this model and from the previously described correlation was obtained. In the dough, an uneven temperature distribution in the radial direction, caused mainly by evaporative cooling at the surface, was predicted. The calculated temperatures for different susceptors, heating times and various locations in the dough correlated well with experimentally determined values.     

微波消解-凯式定氮法测定牛奶中蛋白质含量

建立微波消解-凯式定氮法测定牛奶中蛋白质含量的方法,采用密闭微波消解样品,优化微波消解条件.方法的相对标准偏差RSD为1.82%,回收率在98.8%～101.7%之间.     

微波辅助提取苦参总黄酮

研究苦参中黄酮类化合物的微波提取最佳工艺。采用单因子及正交实验，考察了乙醇浓度、微波时间、微波功率及固液比对苦参黄酮类化合物提取量的影响规律。结果：苦参黄酮类化合物提取工艺的最佳参数组合为：乙醇体积分数10％，微波作用时间6min，功率为420w，固液比为1：25（g／mL）。在最佳参数组合下苦参中总黄酮的提取量可达4．83％。结论：采用微波技术提取苦参中总黄酮具有良好的应用前景。     

FREE-SPACE MICROWAVE MEASUREMENT of LOW MOISTURE CONTENT IN POWDERED FOODS

A free-space microwave transmission technique has been developed and tested for rapid inline noninvasive measurement of the moisture content of various types of food powders. the basis of this technique is the relation between the attenuation of X-band microwave radiation through a sample of the food powder to its moisture content by weight. Since food powders generally lose their utility and desirable properties, such as flowability and resistance to spoilage, at lower levels of moisture content, typically 3–7%, special techniques must be developed in order to accurately characterize the moisture content at these low levels. One such technique is to use frequency averaging to enhance the accuracy of the measurements to avoid multiple reflection effects prevalent in low-loss low-moisture attenuation measurements. This technique was implemented in the moisture content estimation. Overall accuracies in moisture content estimation are generally less than 1%, although in some cases, accuracies are in the vicinity of 5%.