Agnieszka Ciurzyńska

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.

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.

Osmotic dehydration of Braeburn variety apples in the production of sustainable food products

Abstract The aim of this work was to investigate the effects of osmotic dehydration conditions on the properties of osmotically pre-treated dried apples. The scope of research included analysing the most important mass exchange coefficients, i.e. water loss, solid gain, reduced water content and water activity, as well as colour changes of the obtained dried product. In the study, apples were osmotically dehydrated in one of two 60% solutions: sucrose or sucrose with an addition of chokeberry juice concentrate, for 30 and 120 min, in temperatures of 40 and 60°C. Ultrasound was also used during the first 30 min of the dehydration process. After osmotic pre-treatment, apples were subjected to innovative convective drying with the puffing effect, and to freeze-drying. Temperature and dehydration time increased the effectiveness of mass exchange during osmotic dehydration. The addition of chokeberry juice concentrate to standard sucrose solution and the use of ultrasound did not change the value of solid gain and reduced water content. Water activity of the dried apple tissue was not significantly changed after osmotic dehydration, while changes in colour were significant.

Structure influence on mechanical and acoustic properties of freeze‐dried gels obtained with the use of hydrocolloids

The influence of the structure formed by the type of hydrocolloids (low-methoxyl pectin, the mixture of xanthan gum, and locust bean gum, and mixture of xanthan gum, and guar gum) and the aeration time (3, 5, 7, and 9 min) on textural properties of freeze-dried gels were investigated. The hardest texture generating the strongest acoustic emission was obtained by freeze-dried pectin gel, characterised by the lowest porosity and the largest pore diameter. Aeration time significantly affected mechanical and acoustic properties of the pectin gel lyophilisate. No effect of gel aeration time on tested characteristics of samples with mixture of hydrocolloids was observed. Strong positive correlations between acoustic energy as well as the maximum force and work and negative ones between porosity and pore diameter indicate that greater resilience and stronger acoustic emission of freeze-dried gels was caused by the reduction of porosity and the increase in the pore size of the material. PRACTICAL APPLICATIONS The research is expected to show the phenomenon of structure formation when preparing and freeze-drying gels and explain the influence of the process parameters (time of aeration, the type of hydrocolloids) on the formation of the internal structure and physical properties of a dried product, especially mechanical and acoustic properties. This achievement will contribute to the development of the science of food and human nutrition, especially within the context of the popular research on aerated diet products. The expected result will be the ability to develop a new technology for producing food with a delicate texture, using the phenomenon of sublimation. As a result, designing changes in the structure of freeze-dried fruit gels with a delicate structure will be possible due to the choice of ingredients and aeration parameters in order to develop innovative food characterised by favorable nutritional, health and functional properties, which will be attractive for the consumers.

Effect of the aerated structure on selected properties of freeze-dried hydrocolloid gels

Abstract The ability to create diverse structures and studies on the effect of the aerated structure on selected properties with the use of freeze-dried gels may provide knowledge about the properties of dried foods. Such gels can be a basis for obtaining innovative food products. For the gel preparation, 3 types of hydrocolloids were used: low-methoxyl pectin, a mixture of xanthan gum and locust-bean gum, and a mixture of xanthan gum and guar gum. Gels were aerated for 3 and 7 min, frozen at a temperature of −45°C 2 h−1, and freeze-dried at a temperature of 30°C. For the samples obtained, structure, porosity, shrinkage, rehydration, and colour were investigated. It was shown that the type of the hydrocolloid and aeration time influence the structure of freeze-dried gels, which determines such properties of samples as porosity, shrinkage, density, rehydration, and colour. The bigger pores of low-methoxyl pectin gels undergo rehydration in the highest degree. The delicate and aerated structure of gels with the mixture of xanthan gum and locust-bean gum was damaged during freeze-drying and shrinkage exhibited the highest value. Small pores of samples with the mixture of xanthan gum and guar gum were responsible for the lower rehydration properties, but the highest porosity value contributed to the highest lightness value.

Effect of quantity of low-methoxyl pectin on physical properties of freeze-dried strawberry jellies.

Abstract The development of fruit-based products with a high proportion of fruit and good nutritional, sensory and functional properties may help to diversify market supply. These products ought to be attractive, especially to young people, easy to consume and have a reasonably long shelf life. Three recipes of freeze-dried strawberry jelly with low-methoxyl pectin (LMP) (2.0, 2.5, 3.5% LMP) with the use of strawberry pulp were obtained and physical properties were investigated to choose the sample with the best quality factors. The quantity of added low-methoxyl pectin influences the physical properties of freeze-dried strawberry jellies. The recipe with 2.5% addition of low-methoxyl pectin was chosen based on the results obtained. Despite the relatively high friability and low hardness, and higher shrinkage, it has a fast rate of rehydration. It is also characterised by high porosity and the parameters of the colour most similar to the raw material and low water content and activity.

Effect of composition changes and aeration time on the structure and rehydration of innovative freeze-dried gels

Abstract This paper seeks to explain the structural effect on selected properties of freeze-dried gels with different compositions and aeration times. Herein, sugars and citric acid were added to gels with low-methoxyl pectin, mixtures of xanthan gum and locust bean gum, as well as xanthan gum and guar gum, in order to obtain a strawberry model. Strawberry pulp was also added to hydrocolloids to develop an innovative strawberry product. All samples were aerated for 3 and 7 min, and freeze-dried. Composition changes were found to modify the structure of the examined samples and properties. Freeze-dried gels with low-methoxyl pectin seemed to reflect the best approach towards obtaining an innovative strawberry product with a tailored structure, as they had the best water absorbent capacity, and the most attractive and delicate structure. Samples with a mixture of hydrocolloids were more compact. Aeration time did not significantly affect the examined parameters, while the addition of sugar and citric acid increased them. The strawberry pulp experiment confirmed the strawberry model (with low-methoxyl pectin gels, and sugar and citric acid), which had the finest structure and the best rehydration properties.

Rehydration properties of hybrid method dried fruit enriched by natural components

Abstract The aim of the study was to determine the impact of osmotic pre-dehydration and drying of fruit on the rehydration properties of dried fruit. Herein, the effect of fruit juice, applied as a natural enriching substance was very important. In addition, the properties of dried fruits obtained through combined air-drying and subsequent microwave-vacuum drying with ‘puffing’ effect were similar to the freeze-dried fruits, but showed other rehydration properties. As raw material, frozen strawberry (Honeoye variety) and fresh apples (Idared variety) were used in the study. The apples and partially defrosted strawberries were prior dehydrated in solutions of sucrose and a mixture of sucrose with chokeberry juice concentrate at 50°C for 2 h. Next, the fruit samples were dried by one of two ways: air-drying (50°C, 5 h) and microwavevacuum drying for about 360 s; and freeze-drying (30°C, 63 Pa, 24 h). The rehydration was carried out in distilled water (20°C, 5 h). The osmotic pre-dehydration hindered fruit drying process. The impact of drying method became particularly evident while examining the kinetics of rehydration. During the rehydration of the pre-dehydrated dried fruit a slower hydration could be observed. Freeze-dried strawberries absorbed 2-3 times more water than those dried by the ‘puffing’ effect.

Sorption properties and phase transitions temperature of freeze-dried strawberry model based on hydrocolloids with a tailored structure

ABSTRACT The aim of this work was to investigate the effect of aerated structure on sorption properties and phase transitions temperature of freeze-dried hydrocolloid gels. Sugars and citric acid were added to gels with low-methoxyl pectin (LMP), the mixtures of xanthan gum and locust bean gum, xanthan gum and guar gum, to create freeze-dried strawberry model. Hydrocolloid type changed freeze-dried strawberry model properties, but aeration time was generally insignificant. LMP gels porous structure increased sorption properties and decreased the glass transition temperature. Freeze-dried strawberry models based on hydrocolloids mixture were characterized by more compact structure, which decreased the sorption ability and increased the glass transition temperature.