Stella M. Alzamora

Effect of ultrasound on the survival of Saccharomyces cerevisiae: influence of temperature, pH and amplitude

The resistance of Saccharomyces cerevisiae cells to the action of ultrasound (20 kHz, wave amplitude in the range 71–110 μm) was analyzed at 35, 45 and 55°C in Sabouraud broth at pH 3.0 and 5.6. The inactivation rate where a first-order kinetic was observed exhibited D values between 0.5 and 31 min. The resistance of the yeast decreased as ultrasonic wave amplitude increased, with the z values for this effect ranging between 128 and 323 μm. In the pH range investigated, the reduction of pH did not affect ultrasound yeast sensitivity except for experiments performed at 71.4 μm wave amplitude and 45°C. At moderate temperatures, decimal reduction time values were reduced by the simultaneous effect of ultrasound but at 55°C, no advantages were observed by adding sonication. Structural studies performed in cells sonicated at 45°C and 95.2 μm of wave amplitude indicated the treatment provoked puncturing of cell walls with leakage of content as well as damage at subcellular level. However, when ultrasound was applied at 55°C, no structural differences were appreciated between sonicated cells and only heat-treated cells.

Structural changes in apple tissue during glucose and sucrose osmotic dehydration: shrinkage, porosity, density and microscopic features

Abstract Micro- and macrostructural changes in apple slices during osmotic dehydration were studied. Samples were immersed into 25.0% (w/w) glucose or 34.6% (w/w) sucrose aqueous solutions at 30 °C until water and solid contents were almost constant (≅350 min). They were removed after selected times, and examined for thickness, volume, bulk and solid–liquid densities, porosity, water activity, water loss, solids gain and microscopic aspects. At relatively short times, significant weight and volume losses were observed for both treatments arising from the fast water loss. The solid–liquid density of glucose or sucrose treated samples increased slowly along the process. However, the bulk density increased up to a certain value and then fluctuated with increasing time, showing in some cases another pronounced increase for long times of treatment. Fruit porosity values decreased in line with the increase in bulk density values. At the end of the osmotic treatments in glucose or sucrose solutions, there was a recovery in porosity, although the values were lower than for fresh fruit. The changes in bulk density, porosity and volume of apple tissue along osmotic process were closely supported by microstructural and structural observations (using light microscopy and environmental electron scanning microscopy) and explained by considering osmosis as a multicomponent diffusion process through porous media and due to the relaxation of the viscoelastic shrunken cell walls.

Inhibitory effects of vanillin on some food spoilage yeasts in laboratory media and fruit purées

The effect of vanillin and essential oil of mint on the growth of different strains of food spoilage yeasts in laboratory media and fruit purées was studied. Growth of Saccharomyces cerevisiae, Zygosaccharomyces rouxii, Debaryomyces hansenii and Z. bailii was inhibited in culture media and apple purée containing 2000 ppm of vanillin for 40 days storage at 27 degrees C and a(w) 0.99 or 0.95. But 3000 ppm of the spice were not effective to inhibit Z. bailii growth in banana purée. Growth of yeasts was not affected by 100 ppm of essential oil of mint.

Air drying behaviour of apples as affected by blanching and glucose impregnation

The effect of blanching and/or glucose impregnation at atmospheric pressure or in vaccum on the kinetics of moisture transfer during the first falling rate period of air drying of apple was analyzed. The moisture diffusivity of water (Deff) was strongly decreased by glucose uptake during impregnation step as well as volume shrinking. Studies of the cell structure using transmission electron microscopy revealed that both pretreatments did not modify in a great extension the cell wall resistance to water flux.

Air Dehydration of Strawberries: Effects of Blanching and Osmotic Pretreatments on the Kinetics of Moisture Transport

Abstract The effect of blanching and glucose concentration before drying on the rate of moisture movement during the early stages of air dehydration of strawberries at 55 °C was studied. It was found that the effective diffusion coefficient of water in strawberries (Deff) was strongly affected by heat pretreatment, but glucose dipping after blanching caused no additional effect. Electron microscopic studies of strawberry tissues were used to explain this behavior through a modification of cellular structure, disruption of cell membranes of heat-treated fruit and subsequent cell wall alteration during the osmosis step.

Application of combined methods technology in minimally processed fruits

Abstract This paper presents the processes developed for preserving ‘high moisture fruit products’ (peach, pineapple, mango, papaya, chicozapote, banana) in seven countries in the frame of CYTED-D Program. The new technologies were based on the combination of a mild heat treatment, a slight water activity (aw) reduction, and addition of antimicrobials (‘hurdle effect’). These minimal processes are energy efficient, satisfactory to preserve fruits in situ and open new possibilities for export markets. High moisture fruit products are shelf-stable during at least 4–8 months of storage.

Kinetics of moisture transfer during air drying of blanched and/or osmotically dehydrated mango

Abstract The effect of previous blanching and/or osmotic dehydration (at atmospheric pressure or in vacuum) with glucose syrups on the kinetics of water transport during the first falling rate period of air drying of mango at 60°C was investigated. Both pre-drying treatments decreased strongly the effective moisture diffusivity (Deff) calculated with Ficks second law. For osmotically concentrated fruit at atmospheric pressure, the increase in glucose concentration of the immersion solution decreased the drying rate. Deff values were similar for mangoes predehydrated to aw 0.97 by vacuum or atmospheric osmosis, but previous blanching slightly decreased the Deff value for vacuum treated slices compared with those osmotically dehydrated to the same aw at atmospheric pressure. The air drying behaviour of blanched and/or osmotically dehydrated mangoes was ascribed to glucose uptake during the impregnation step, volume shrinking, low modification of cell wall resistance to water flux by the pretreatments and/or gelatinization of starch and denaturation of protein – carbohydrate mucilage.

Combined use of ultrasound and natural antimicrobials to inactivate Listeria monocytogenes in orange juice.

The presence of Listeria monocytogenes could seriously affect the safety of nonpasteurized fruit juices. High-intensity ultrasound combined with mild heat treatment and natural antimicrobials may be an alternative technology for fruit juice preservation. The response of L. monocytogenes in orange juice to combined treatments involving moderate temperature (45 degrees C), high-intensity ultrasound (600 W, 20 kHz, 95.2 microM wave amplitude), and the addition of different levels of vanillin (0, 1,000, 1,500, and 2,000 ppm), citral (0, 75, and 100 ppm), or both was investigated to find the most effective inactivation treatment. Nonlinear semilogarithmic survival curves were successfully fitted by the Weibull model. The presence of vanillin or citral greatly increased the bactericidal effect of thermosonication and changed the distribution of inactivation times. When both antimicrobials were added together and ultrasound was applied, narrowest frequency shapes, skewed to the right, with low ariances and death time means between 1.6 and 2.6 min, were obtained. Orange juices with 1,500 or 1,000 ppm of vanillin and 100 ppm of citral were pleasant for the consumers.

Effect of pH, temperature and glucose addition on flow behaviour of fruit purées I. Banana purée

The influence of pH (3.0 and 5.1) and addition of glucose to attain water activities (aw) ranging from 0.89 to 0.97 on the flow behaviour of banana purees was evaluated in the temperature range 10–55 °C. All purees were shear-thinning fluids with appreciable yield stress values, the flow curves essentially following the Herschel-Bulkley model. Glucose addition generally decreased the apparent viscosities and increased the temperature dependence of the flow properties. There were no patterns with respect to the effect of pH on flow characteristics. The effect of temperature (10–40 °C) and concentration (21.4–50.9 °Brix) on the consistency coefficient was represented by a single equation derived by combining an Arrhenius type relationship for the dependence on temperature and an exponential relationship for the influence of concentration.

Combination of preservation factors applied to minimal processing of foods

Innovative technologies for producing minimally processed (MP) foods that apply the concept of combination of preservation factors are addressed in this article with special emphasis on a new combined approach that has been successfully applied in several Latin American countries for MP high-moisture fruit products (HMFP). HMFP can be regarded as a different approach to the commercially available and widely accepted MP concept for fruits and vegetables (even if developed for the same purpose of obtaining freshlike high-quality products with an extended shelf life) that is better adapted to Latin American countries in terms of independence of the chill chain and the use of simple and energy-efficient technologies. The continuous refrigeration hurdle associated with MP refrigerated fruits is not included in the preservation system of HMFP because a different combination of hurdles must be overcome to enhance the shelf stability of nonrespiring vegetable tissues while preserving freshlike character. Guidelines to obtain safe and high-quality MP fruit products are proposed. Other products preserved by combined factors technology are also discussed, as well as some other classical and new preservation factors whose application could enhance the quality of HMFP.