Helmar Schubert

Microwave application in vacuum drying of fruits

Abstract Microwave vacuum drying of banana slices was investigated experimentally. This type of drying procedure is preferable to conventional drying techniques in order to avoid product degradation due to high temperatures encountered in convective drying. The drying process was examined by introducing pulsegenerated microwave power in banana samples. The material temperature was monitored. Temperature peaks in the last stages of drying indicated that drying could be favoured if temperature was maintained below a maximum level, so that the final product should not be burned by hot spots during microwave drying. This procedure produced dehydrated products of excellent quality as examined by taste, aroma, smell and rehydration tests.

Developments in the continuous mechanical production of oil-in-water macro-emulsions

Abstract Emulsification processes are determined both by droplet disruption and droplet re-coalescence. This paper explains the fundamentals of droplet disruption in laminar and turbulent flow. Emulsification results from different continuous emulsification machines are compared with respect to the energy density (specific volumetric energy input) achieved. This is the first time that such a comparison has been made for both droplet disruption itself and droplet disruption superimposed by re-coalescence.

Stabilization of emulsions by OSA starches

In continuous mechanical emulsification, droplets of a coarse premix are first deformed and disrupted. Secondly, the newly formed interface has to be stabilized. Emulsions are thermodynamically unstable, as droplets tend to coalesce. To avoid coalescence of droplets, different effects have to be taken into account. The objective of this paper is to investigate the possibility of producing finely dispersed emulsions by using OSA starches as additive. Experiments were carried out using two commercially available OSA starches that differ mainly in viscosity of their aqueous solutions. In the experiments, the dispersed phase content, starch concentration as well as pH value and the influence of dissolved salts of different ion strength have been investigated. By droplet size analysis directly after production of oil-in-water emulsions the emulsifying properties of such starches were determined. Measurements of interfacial tension prove that OSA starches are surface active substances. It was shown by emulsification experiments that the stabilization of droplets by OSA starches is very efficient. The emulsification results achieved with OSA starches have been found to be independent from the type of starch and the starch concentration above the lower limit of starch needed for stabilization. The emulsification results are independent of pH value and ion valence. Therefore it can be concluded that steric hindrance is the main stabilizing mechanism of OSA starches. As proteins stabilize emulsion droplets mainly by electrostatic repulsive forces, the emulsification result depends strongly on pH, which leads to poor emulsification at pH values near the iso-electric point. The results of the work presented here suggest that whey proteins can be substituted by OSA starches with a specific advantage at low pH values near the iso-electric point of the protein.

Influence of continuous phase viscosity on emulsification by ultrasound

Power ultrasound is one means among others of mechanically producing emulsions. In spite of numerous publications on the basic principles of this technique, there is insufficient knowledge of continuous ultrasound emulsification processes and the main parameters of practical relevance. A comparison of this system with other continuous mechanical emulsifying devices is made. The effect of continuous phase viscosity on droplet disruption due to ultrasound is the subject of a more detailed investigation. Continuous phase viscosity is varied by means of water soluble stabilizers (o/w systems) and different oils (w/o systems). At constant energy density, droplet size decreases when adding stabilizers, whereas the viscosity of the oil in w/o emulsions has no effect. Qualitative investigations of the local distribution of cavitation have shown very small penetration depths of cavitation into the liquid. This emphasizes the need for improvement of apparatus design to optimize the emulsification process.

Emulsification in High-Pressure Homogenizers

High‐pressure homogenizers are frequently employed for the homogenization of low‐viscosity emulsions containing a proportion of disperse phase which is not too high. High‐pressure homogenizers essentially consist of a high‐pressure pump and a homogenizing nozzle. The design of the homogenizing nozzle influences the flow of the emulsion in the nozzle itself and hence the results of droplet disruption. It is shown which mechanism in frequently used homogenizing nozzles is usually responsible for disruption. Experimental results reveal the effects of the dispersed phase content and the viscosity of the disperse and continuous phases in different nozzles. The results can be explained on the basis of the mechanisms of disruption. Finally, the homogenizing nozzles presented are directly compared with one another.

Preparation and analysis of oil-in-water emulsions with a narrow droplet size distribution using Shirasu-porous-glass (SPG) membranes

Abstract Shirasu-porous-glass (SPG) membranes with a mean pore size from 0.4–6.6 μm were used to produce O/W emulsions consisting of vegetable (rape seed) oil as the dispersed phase and Span 80 dissolved in demineralized water as the continuous phase. The emulsion droplets with a mean droplet size 3.5 times larger than the mean pore size and the span of the droplet size distribution between 0.26 and 0.45 were produced using 2% emulsifier at a transmembrane pressure slightly exceeding the capillary pressure. Under these conditions the dispersed phase flux through the membrane was in the range of 0.7–7 1·m−2·h−1 and only about 2% of the pores were active. However, if the transmembrane pressure was considerably higher than the capillary pressure, the dispersed phase flux strongly increased and droplets with a broad droplet size distribution were produced. The hydraulic resistance of the SPG membrane was inversely proportional to the square of the mean pore size, which is in agreement with the Hagen-Poiseuille law. The membrane porosity is independent on the pore size and ranged from 53–60%.

Combined osmotic and microwave-vacuum dehydration of apples and strawberries ☆

Abstract The combination of osmotic pre-treatment – mainly in sucrose solutions – and microwave-vacuum dehydration of strawberries and apples has been studied. Water removal during osmotic treatment is accompanied by solute uptake from the osmotic solution. The resulting changes in composition bring about better properties of the final product in terms of structure and volume. Compared to the solely microwave-vacuum dried samples, osmotic pre-treatment improved volume retention from 20% to 50% (strawberries) and from approximately 20% to 60% (apples) based on the fresh volume. SEM pictures revealed that the cellular structure is also preserved better, when osmotic pre-treatment is used. Gel formation between pectins, sucrose and in some cases, calcium ions is believed to be the main cause of structure build-up. Vitamin C retention was around 60% with the microwave procedure applied here.

Influence of process parameters on droplet size distribution in SPG membrane emulsification and stability of prepared emulsion droplets

SPG membranes were used to prepare monodispersed O/W and W/O/W emulsions over a wide range of membrane wall shear stress (0.37–40 Pa), dispersed phase content (1–20 vol.%) and transmembrane pressure. Although the most uniform droplets were prepared at the membrane wall shear stress of 30 Pa, a monodispersed O/W emulsion can be even obtained at the wall shear stress of 0.37 Pa, corresponding to laminar flow regime of continuous phase inside the membrane tube. The minimum droplet size somewhat decreased with time, probably due to gradual activation of smaller pores. There was no significant difference in the size distribution curve of pure oil droplets of O/W emulsions and W/O drops of W/O/W emulsions, if they were both prepared under the same conditions. No significant change in droplet size distribution of prepared O/W emulsions was observed during the storage time of up to 159 days.

The Microwave Processing of Foods

Part 1 Principles: Introducing microwave processing of food: Principles and technologies Dielectric properties of foods Measuring the dielectric properties of foods Microwave heating and the dielectric properties of foods Microwave processing, Nutritional and sensory quality. Part 2 Applications: Microwave technology for food processing: an overview Baking using microwave processing Drying using microwave processing Blanching using microwave processing Thawing and tempering using microwave processing Packaging for microwave foods. Part 3 Measurement and process control: Measuring the heating performance of microwave ovens Measuring temperature distributions during microwave processing Improving microwave process control Maximising uniform head distribution in microwave heating.

Dispersion of powders in liquids in a stirred vessel

The dispersion of powders in liquids can be divided into four steps: wetting, submersing, dispersing and dissolving (if soluble). While the subject of dispersing and suspending particles in liquids in mechanically agitated vessels has been investigated by several authors, the immersion step has remained largely uninvestigated. However, in many cases, the wettability of the powder is of decisive importance for the process of making a dispersion. Experiments in a baffled stirred vessel are presented and compared with results of different laboratory wetting tests to illustrate the importance of wettability for the dispersion process. Further experiments in an unbaffled vessel were conducted to investigate the influence of vortex formation on the powder immersion step.