Sandra Guerrero

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.

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.

Effects of pH, temperature and glucose addition on flow behaviour of fruit purees: II. Peach, papaya and mango purées

Abstract The flow behaviour of papaya, mango and peach purees over the temperature range 10–55 °C was studied. The effect of glucose addition (soluble solids concentration in the range 12–52 °Brix) and pH (3.0 and natural) was also investigated. After eliminating time dependency, flow was adequately described by the Herschel-Bulkley model. In mango and peach purees, exponential equations were used to describe the combined effect of temperature and soluble solids content on the consistency coefficient. Temperature showed little influence over the flow index but a severe effect on the yield stress. For the three fruits, flow index and yield stress values were correlated with temperature and water activity by means of polynomial equations.

Study of the inactivation of spoilage microorganisms in apple juice by pulsed light and ultrasound.

The aim of this study was to evaluate the effect of ultrasound (US) (600 W, 20 kHz and 95.2 μm wave amplitude; 10 or 30 min at 20, 30 or 44 ± 1 °C) and pulsed light (PL) (Xenon lamp; 3 pulses/s; 0.1 m distance; 2.4 J/cm(2)-71.6 J/cm(2); initial temperature 2, 30, 44 ± 1 °C) on the inactivation of Alicyclobacillus acidoterrestris ATCC 49025 spores and Saccharomyces cerevisiae KE162 inoculated in commercial (pH: 3.5; 12.5 °Brix) and natural squeezed (pH: 3.4; 11.8 °Brix) apple juices. Inactivation depended on treatment time, temperature, microorganism and matrix. Combination of these technologies led up to 3.0 log cycles of spore reduction in commercial apple juice and 2.0 log cycles in natural juice; while for S. cerevisiae, 6.4 and 5.8 log cycles of reduction were achieved in commercial and natural apple juices, respectively. In natural apple juice, the combination of US + 60 s PL at the highest temperature build-up (56 ± 1 °C) was the most effective treatment for both strains. In commercial apple juice, US did not contribute to further inactivation of spores, but significantly reduced yeast population. Certain combinations of US + PL kept on good microbial stability under refrigerated conditions for 15 days.

Saccharomyces cerevisiae thermal inactivation kinetics combined with ultrasound

Inactivation kinetics of Saccharomyces cerevisiae during thermal treatments at moderate temperatures (45.0, 47.5, 50.0, 52.5, or 55.0 degrees C) combined with application of 20 kHz of ultrasound were evaluated. S. cerevisiae inactivation under the combined effects of heat and ultrasound followed first-order reaction kinetics, with decimal reduction times (D) that varied from 22.3 to 0.8 min. D values in treatments that combined heat and ultrasound were significantly smaller (P < 0.05) than D values obtained for thermal treatments and were more noticeable at temperatures below 50 degrees C. The dependence of the D value on temperature had a significantly (P < 0.05) greater z value for combined treatments. Yeast heat inactivation kinetics revealed decreased thermal resistance caused by ultrasound.

Optimization of a combined factors technology for preserving banana purée to minimize colour changes using the response surface methodology

A preservation process for banana puree based on combined factors technology was optimized using response surface methodology to minimize banana colour changes during storage of the product for at least 4 months. A Box-Behnken design was adopted and second-order polynomials models were computed for each month of storage to relate some process variables (sodium bisulphite concentration, storage temperature and thermal treatment time after packaging) to a colour function (Brown Index). It was seen that storage temperature had a highly significant effect on colour changes affecting the free bisulphite level and the browning kinetics. The optimum values of the three process variables for minimal colour development were determined.

Probabilistic Modeling of Saccharomyces cerevisiae Inhibition under the Effects of Water Activity, pH, and Potassium Sorbate Concentration

Probabilistic microbial modeling using logistic regression was used to predict the boundary between growth and no growth of Saccharomyces cerevisiae at selected incubation periods (50 and 350 h) in the presence of growth-controlling factors such as water activity (a(w); 0.97, 0.95, and 0.93), pH (6.0, 5.0, 4.0, and 3.0), and potassium sorbate (0, 50, 100, 200, 500, and 1,000 ppm). The proposed model predicts the probability of growth under a set of conditions and calculates critical values of a(w), pH, and potassium sorbate concentration needed to inhibit yeast growth for different probabilities. The reduction of pH increased the number of combinations of a(w) and potassium sorbate concentration with probabilities to inhibit yeast growth higher than 0.95. With a probability of growth of 0.05 and using the logistic models, the critical pH values were higher for 50 h of incubation than those required for 350 h. With lower a(w) values and increasing potassium sorbate concentration the critical pH values increased. Logistic regression is a useful tool to evaluate the effects of the combined factors on microbial growth.

Inactivation of Microorganisms

Minimal processing techniques for food preservation allow better retention of product flavor, texture, color, and nutrient content than comparable conventional treatments. A wide range of novel alternative physical factors have been intensely investigated in the last two decades. These physical factors can cause inactivation of microorganisms at ambient or sublethal temperatures (e.g., high hydrostatic pressure, pulsed electric fields, ultrasound, pulsed light, and ultraviolet light). These technologies have been reported to reduce microorganism population in foods while avoiding the deleterious effects of severe heating on quality. Among technologies, high-energy ultrasound (i.e., intensities higher than 1 W/cm2, frequencies between 18 and 100 kHz) has attracted considerable interest for food preservation applications (Mason et al., 1996; Povey and Mason, 1998).

Relationships between rheological properties, texture and structure of apple (Granny Smith var.) affected by blanching and/or osmotic dehydration

The objective of this work was to evaluate and correlate rheological properties (small-scale dynamic oscillatory and creep/recovery measurements and large-scale compression force-deformation testing), texture (sensory evaluation by trained panel) and structure (optical and transmission electronic microscopy observations) of apples osmotically dehydrated to water activity (aw) 0.97 with glucose, with or without previous blanching. All apple samples showed a solid behavior (G′ > G″) dominating the viscoelastic response, but both dynamic moduli were reduced due to processing. The instantaneous elastic compliance (J0) and the retarded compliances (J1 and J2) increased for treated tissues and the steady-state viscosity (ηN) was approximately 15% to 29% of the value of fresh apple. In general, compression parameters decreased for all treated tissues. Changes in structural features were mainly evidenced in heated samples. Partial least squares regression analysis regression models revealed that texture could be well predicted by rheological properties (compression and creep parameters). Juiciness, crispness and sensory hardness were negatively correlated to J0, J1 and J2, and ηN was negatively correlated to sensory fracturability. Some mechanical parameters (fracturability, hardness 2, area 2, modulus of deformability and cohesiveness) were positively related to sensory fracturability, crispness and sensory hardness; and juiciness was negatively correlated to hardness. Compression and creep parameters showed ability to evidence structure differences (rupture of membranes, swelling of cells and degradation of cell walls) and to explain texture of treated apples.