Andrzej Lenart

Osmo-convective drying of fruits and vegetables: technology and application

ABSTRACT With reference to typical food preservation processes which consist of changing the water binding to material, the technology and application of osmotic dehydration were discussed as an initial treatment before convection drying of fruit and vegetables. Particular attention was paid to the possibilities to produce more shelf-stable food while keeping the high quality of final product. The course of osmotic dehydration of plant tissue as well as its influence on convection drying and on the properties of preserved fruit and vegetables were presented.

Mass exchange during osmotic pretreatment of vegetables

Abstract Apple, pumpkin and carrot were used to study mass transfer during osmotic dehydration of plant tissue. Osmotic dehydration was carried out in 61.5% solution of sugar. Temperature of dewatering was constant at 30°C. Time of dehydration was varied from 0 to 180 min. Osmotic dehydration in sugar solution at 30°C depends on the kind of plant tissue. The most significant changes of water content, water loss and solids gain took place during the first 30 min of dewatering. During that time water content in apples was reduced by some 48%, whereas further dehydration from 60 to 180 min resulted in water content reduction by next 30%. Further dewatering of pumpkin and carrot proceeded much slower than that at the beginning of the process. Rate of water loss was 5–10 times higher than the rate of solids gain and depended on the advancement of the dewatering process.

DRYING CHARACTERISTICS OF OSMOTICALLY DEHYDRATED FRUITS COATED WITH SEMIPERMEABLE EDIBLE FILMS

This paper deals with key issues of the coating technology applied to plant materials prior to osmotic dehydration and convective drying. Coatings created with solutions of starch products and pectin were examined. It was shown that coating of foods to be dehydrated influences mass transfer in a plant tissue during osmotic pre-treatment as well as during convective drying. Each coating should be individually examined. The properties of coatings depend on their composition and the method used for the fabrication of a coating. Coating of foods with an artificial barrier on the surface may efficiently hinder the penetration of solute inside the food, not affecting much the rate of water removal. The coated plant materials had a greater water loss/solids gain ratio then the uncoated ones. Osmotic dehydration seems to be one of possible pre-treatment methods before drying. The conditions of convective drying are changed upon osmotic dehydration of plant materials. It was shown that changes in drying rate depend on the kind of coating substance. Bigger differences were observed at higher water content and these differences narrowed as water content in dried material decreased. The positive effect of coatings on the physical properties of dried fruits was found.

Freeze-drying - application in food processing and biotechnology - a review.

Freeze-Drying - Application in Food Processing and Biotechnology - A Review Freeze-drying is a method of removing water by sublimation of ice crystals from frozen material. Suitable parameters of process application allow us to obtain best quality products compared to products dried with traditional methods. Very good physical and chemical properties of food and biotechnological products make this method the best for drying exclusive products. On the domestic market there is a large selection of different types of freeze-dried products, and there is still increasing interest of consumers in these products. A high cost of the freeze-drying still limits the wide-scale application in the food industry. Equipment innovation and pretreatment of raw material can reduce the time and energy needed for this process.

KINETICS OF OSMOTIC DEHYDRATION OF APPLES WITH PECTIN COATINGS

ABSTRACT The solids gain and water loss during osmotic dehydration of coated apples were found to be dependend on concentration of pectin solution (0.5–4%) and drying time of coated apples (0–40 min). The coated apples were found to have generally a smaller the solids gain and higher or the same water loss than uncoated samples. For analysed low methyled pectin solutions and drying time the best for coating the apples before osmotic dehydration considering the highest water loss and the smallest solids gain was 2% pectin solution and 10 min drying time.

Effect of starch type on the physico-chemical properties of edible films

Food preservation is mostly related to packaging in oil-based plastics, inducing environmental problems, but this drawback could be limited by using edible/biodegradable films and coatings. Physical and chemical properties were assessed and reflect the role of the starch type (wheat, corn or potato) and thus that of the amylose/amylopectin ratio, which influences thickness, colour, moisture, wettability, thermal, surface and mechanical properties. Higher amylose content in films induces higher moisture sensitivity, and thus affects the mechanical and barrier properties. Films made from potato starch constitute a greater barrier for oxygen and water vapour though they have weaker mechanical properties than wheat and corn starch films. Starch species with higher amylose content have lower wettability properties, and better mechanical resistance, which strongly depends on the water content due to the hydrophilic nature of starch films, so they could be used for products with higher water activity, such as cheese, fruits and vegetables. It especially concerns wheat starch systems, and the contact angle indicates less hydrophilic surfaces (above 90°) than those of corn and potato starch films (below 90°). The starch origin influences optical properties and thickness: with more amylose, films are opalescent and thicker; with less, they are transparent and thinner.

Liquid and vapour water transfer through whey protein/lipid emulsion films.

BACKGROUND Edible films and coatings based on protein/lipid combinations are among the new products being developed in order to reduce the use of plastic packaging polymers for food applications. This study was conducted to determine the effect of rapeseed oil on selected physicochemical properties of cast whey protein films. RESULTS Films were cast from heated (80 degrees C for 30 min) aqueous solutions of whey protein isolate (WPI, 100 g kg(-1) of water) containing glycerol (50 g kg(-1) of WPI) as a plasticiser and different levels of added rapeseed oil (0, 1, 2, 3 and 4% w/w of WPI). Measurements of film microstructure, laser light-scattering granulometry, differential scanning calorimetry, wetting properties and water vapour permeability (WVP) were made. The emulsion structure in the film suspension changed significantly during drying, with oil creaming and coalescence occurring. Increasing oil concentration led to a 2.5-fold increase in surface hydrophobicity and decreases in WVP and denaturation temperature (T(max)). CONCLUSION Film structure and surface properties explain the moisture absorption and film swelling as a function of moisture level and time and consequently the WVP behaviour. Small amounts of rapeseed oil favourably affect the WVP of WPI films, particularly at higher humidities.

Effects of carbohydrate/protein ratio on the microstructure and the barrier and sorption properties of wheat starch-whey protein blend edible films.

BACKGROUND Starch and whey protein isolate and their mixtures were used for making edible films. Moisture sorption isotherms, water vapour permeability, sorption of aroma compounds, microstructure, water contact angle and surface properties were investigated. RESULTS With increasing protein content, the microstructure changes became more homogeneous. The water vapour permeability increases with both the humidity gradient and the starch content. For all films, the hygroscopicity increases with starch content. Surface properties change according to the starch/whey protein ratio and are mainly related to the polar component of the surface tension. Films composed of 80% starch and 20% whey proteins have more hydrophobic surfaces than the other films due to specific interactions. CONCLUSIONS The effect of carbohydrate/protein ratio significantly influences the microstructure, the surface wettability and the barrier properties of wheat starch-whey protein blend films.

Sorption Properties of Vacuum-Dried Strawberries

This work demonstrates the influence of changes in parameters of vacuum drying (temperature and pressure) on the sorption properties of dried strawberries. Fruits were dried at 50 and 70°C under pressures of 4 and 16 kPa. Vacuum drying was also conducted during the first 4 h at 70°C and then the temperature was decreased to 50°C at a pressure of 4 kPa. The other combination included increasing the pressure after the first 4 h from 4 to 16 kPa at a drying temperature of 70°C. Sorption isotherms were determined in the dried strawberries. It was shown that with increasing drying temperatures, there was a notable deterioration in the capacity for absorbing water vapor by the vacuum-dried fruit. On the other hand, the pressure at which vacuum drying proceeded did not significantly affect water vapor absorption. Changing the parameters of vacuum drying—that is, temperature in the range of 50–70°C and pressure in the range of 4–16 kPa—affected the shape and structure of the resultant dried strawberries. The combination of vacuum drying with convective drying also influenced the shape and structure of the dried fruit.

Influence of Chemical Composition and Structure on Sorption Properties of Freeze-Dried Pumpkin

The aim of this study was to investigate the influence of chemical composition and material structure on sorption properties of freeze-dried pumpkin. The chemical composition and material structure were changed by osmotic dehydration, blanching, freezing, and temperature of the freeze-drying process. Freeze-dried pumpkin obtained from nonpretreated pumpkin had the best sorption properties, whereas osmotic dehydration significantly decreased the water vapor adsorption ability of the investigated samples. Studies on the influence of different freezing methods on water vapor sorption showed that a combination method of freezing resulted in the best sorption properties. When the temperature of freeze drying was increased, the water vapor adsorption ability of the freeze-dried pumpkin also increased.