Cristina Ratti

HOT AIR AND FREEZE-DRYING OF HIGH-VALUE FOODS: A REVIEW

Abstract Drying is an ancient process used to preserve foods. Conventional drying (hot air) offers dehydrated products that can have an extended life of a year. Unfortunately, the quality of a conventionally dried product is drastically reduced from that of the original foodstuff. Freeze-drying is based on the dehydration by sublimation of a frozen product. Due to the absence of liquid water and the low temperatures required for the process, most of deterioration and microbiological reactions are stopped which gives a final product of excellent quality. The comparison of both preservation processes, hot air and freeze-drying, was done taking into account several important characteristics such as shrinkage, glass transition temperature, process–quality interaction, drying kinetics, costs and new improvements. An updated bibliographic research served to compare both drying processes. Experimental data as well as theoretical results, from several years of research in the subject, were presented and compiled in order to support conclusions.

Shrinkage during drying of foodstuffs

Abstract Experimental shrinkage data as a function of water content for particles of potatoes, apples and carrots of different geometry and under various drying conditions are presented. Volumetric shrinkage of individual particles is affected mainly by air velocity; other drying variables have negligible effect. Changes in surface area to volume ratio with water content are practically independent of drying conditions but dependent on sample geometry and the type of foodstuff. A simple model in which volumetric ratio is represented by one or two line segments is developed. The shrinkage of packed beds is also studied. Changes in bed volume during drying are almost only a function of bed-averaged water content. A simple way of predicting porosity in the bed is applied and its results are compared with experimental data.

SOLAR DRYING OF FOODS: MODELING AND NUMERICAL SIMULATION

A simulation code was developed to predict the batch drying performance of a packed bed of particles, e.g. cylinders or slices of carrot, apples, etc., subjected to time-varying air conditions. This model allows for shrinkage of the particles. The time-dependent inlet drying air conditions permit the simulation of the case of a solar dryer in which the inlet air temperature is necessarily a function of the hour of the day. All the parameters involved in the model were obtained independently from experimental solar dryer data. The results compared well with published experimental data for solar drying of diced carrot. Effects of various key parameters of this process are presented and discussed.

Effect of four drying methods on the quality of osmotically dehydrated cranberries

Abstract Partially dehydrated cranberries (osmotically dehydrated) were dried to low water contents using one of following four methods: hot air drying; microwave-assisted convective drying; freeze-drying; and vacuum drying. Quality evaluation was performed on all samples, including sensory evaluation (appearance and taste), texture, color, water activity, and rehydration ratio. Hot air drying produced dried cranberries with the best visual appearance while freeze-dried cranberries had the highest rehydration ratio. The other methods presented similar rehydration ratios. There was no significant difference in color measurements and water activity. Few differences in texture were found, except for freeze-dried cranberries, which had a lower toughness compared to the other drying methods including commercially available dried cranberries. Microwave-assisted to hot air drying rate ratios increased as the moisture content decreased.

FREEZE-DRYING CHARACTERISTICS OF STRAWBERRIES

ABSTRACT Drying kinetic, as well as color and volume variation, of whole and sliced strawberries were investigated after freeze-drying under various temperatures (30, 40, 50, 60 and 70°C). Dehydration time increased proportionally to the thickness of the product and heating plate temperature markedly reduced it. Freeze-drying caused a pronunciation in red color of strawberries. A decrease in hue angle by 22.5% (skin) and by 42.4% (pulp) was noted, with no significant effect of freeze-drying temperature up to 70°C. The strawberries had a volume reduction of 8% (whole) and 2% (sliced) due to freeze-drying although the level of shrinkage was also independent of freeze-drying temperature. However, the percentage of collapsed strawberries increased with process temperature. At heating temperatures higher than 50°C, the strawberry dry layer temperature was higher than the estimated glass transition temperature of dried fruit, increasing the risk of collapse.

Drying of Foamed Biological Materials: Opportunities and Challenges

An overview is provided on hot air drying of foamed materials in a thin layer (foam-mat drying), foam-spray drying, microwave-assisted drying of liquid foams, as well as microwave drying of frozen foams with and without dielectric solid inserts, used as complementary heat sources. In particular, the mechanisms of heat and moisture transport during the drying of foams are identified. The effects of foam characteristics (e.g., foam density and stability) and drying conditions (temperature, air velocity) on drying kinetics and product quality are examined, and the differences between the drying of non-foamed and foamed materials are discussed.

Advances in Food Dehydration

Dehydration of Foods: General Concepts, Monica Araya-Farias and Cristina Ratti Glass Transition Based Approach in Drying of Foods, Bhesh Bhandari and Benu Adhikari Application of Image Analysis in Food Drying, Maturada Jinorose, Sakamon Devahastin, Angelique Leonard, and Silvia Blacher Dehydration and Microstructure, Mohammad Shafiur Rahman Convective Drying of Foods, Maria Elena Carrin and Guillermo Hector Crapiste Advances in Spouted Bed Drying of Foods, Sandra C .S. Rocha and Osvaldir P. Taranto Osmotic Dehydration Technology in Food Processing - Application and Development, John Shi and Sophia Jun Xue Foods Freeze-Drying: Fundamentals and Tendencies, J. Ignacio Lombrana Rehydration and Reconstitution of Foods, Alejandro Marabi and I. Sam Saguy Dehydration Processes For Nutraceuticals And Functional Foods, Susan D. St. George and Stefan Cenkowski Drying of Microorganisms for Food Applications, Janusz Adamiec Dryer Modelling, Catherine Bonazzi, Bertrand Broyart, and Francis Courtois Non-Conventional Heating Sources During Drying, Valerie Orsat and G.S. Vijaya Raghavan Energy Aspects in Food Dehydration, Tadeusz Kudra Novel Food Dryers and Future Perspectives, Cristina Ratti

Foam mat drying of Alphonso mango pulp

The foam mat drying of Alphonso mango pulp using various food foaming agents, namely soy protein (0.25, 0.5, 1.0, and 1.5%) with methyl cellulose (0.5%), glycerol mono stearate (0.5, 1.0, 2.0 and 3.0%), and egg albumen (2.5, 5.0, 10 and 15%) with methyl cellulose (0.5%), was studied. Drying was carried out in a batch type thin layer dryer at four drying temperatures (60, 65, 70, and 75°C) on 1-, 2-, or 3-mm thickness foamed samples. The optimum concentrations of each foaming agent were determined to be 1% soy protein, 2% glycerol mono stearate, and 10% egg albumen. All were obtained after 25 min whipping time. The drying time was lower for foamed mango pulps as compared to non-foamed pulp at all drying temperatures. Biochemical analysis showed that the foam mat dried powder at 60°C retained a significantly higher (P < 0.05) content of biochemical compounds than at higher temperatures. The treatment of mango pulp with 10% egg albumen and 0.5% methyl cellulose and drying at 60°C (1-mm foam thickness), retained the highest nutritional quality characteristics than the other treatments.

Foam-Mat Freeze Drying of Egg White—Mathematical Modeling Part II: Freeze Drying and Modeling

Foam-mat freeze drying is one of the promising methods of drying, which utilizes advantages of both freeze drying and foam-mat drying. Egg white with its excellent foaming properties makes a suitable candidate for foam-mat freeze drying. Experiments were conducted to study foam-mat freeze drying of egg white, in an effort to determine the suitability of this method. Xanthan gum (XG) at 0.125% concentration was used as stabilizer for foaming. The results showed that the addition of xanthan gum during foaming has a positive impact in reducing the total drying time and also produces excellent quality egg white powder. The addition of stabilizer also plays an important role in improving drying. Simple models were applied for determining drying time and diffusion coefficients during freeze drying.

Drying of Garlic (Allium sativum) and Its Effect on Allicin Retention

Convective hot air-drying and freeze-drying were investigated as potential processes to preserve and concentrate allicin in garlic. Both temperature and air velocity had an important effect on hot air-drying kinetics. Sample size and temperature significantly affected the duration of freeze-drying, and thus the remaining moisture content of the garlic samples. Allicin content decreased with an increase of drying temperature in both convective hot air-drying or freeze-drying. Moderate air temperatures (40 and 50°C) allowed a better allicin retention than higher temperatures (60°C). However, retention of allicin was more important in garlic samples freeze-dried at a temperature of 20°C. The drying method did not show a significant impact on glass transition temperature values, indicating that garlic composition is a more important factor than internal structure. The predictions of the Gordon and Taylor model are in good agreement with the experimental data.