F.J. Sala

Influence of different factors on the output power transferred into medium by ultrasound

The influence of several factors (amplitude of ultrasonic waves, external static pressure, temperature and viscosity of medium) acting, either individually or in combination, on the amount of power transferred to a liquid medium during ultrasonication (power output) was measured by calorimetry. At constant amplitude (150 microns) and pressure (200 kPa), the power output decreased as the temperature was raised. The effect of temperature could be compensated by increasing pressure. The magnitude of the increase in power output due to raising the pressure depended on the pressure range and the treatment temperature. At all temperatures and pressures studied, the power output increased exponentially when the amplitude was increased linearly. The magnitude of this power output did not depend on the temperature or pressure of treatment. At 40 degrees C the magnitude of the increase in power output due to increasing the pressure was not influenced by the amplitude of sonic waves. The power output increased as the viscosity of the medium was increased. The magnitude of this effect did not depend on the amplitude but on the static pressure.

Influence of the incubation temperature after heat treatment upon the estimated heat resistance values of spores of Bacillus subtilis

S. CONDÓN, A. PALOP, J. RASO AND F.J. SALA. 1996. The influence of the incubation temperature on the estimated heat resistance for survivors after heat treatment was investigated. The survival curves and the Dt values of spores of Bacillus subtilis heated at different temperatures in pH 7 buffer, obtained after incubating survivors at different temperatures (30, 37, 44 or 51°C), were compared. The incubation temperature influenced the profile of survival curves. Lower incubation temperatures led to bigger Dt values and longer shoulders. Dt values obtained after incubating at 30°C were higher (x3 approx.) than those obtained by incubating at 51°C. The incubation temperature did not modify z values (z= 9.1). These results show that shoulders are not only due to the activation of dormant spores but also to heat damage repair mechanisms. From the profile of survival curves at different incubation temperatures it would seem that heat damage is accumulative. Cells can repair the initial heat injury, but the accumulation of injuries would eventually make the damage irreversible.

Influence of different factors on the inactivation of Salmonella senftenberg by pulsed electric fields

The influence of growth phase, cell concentration, pH and conductivity of treatment medium on the inactivation of Salmonella senftenberg by high electric field pulses (HELP) was studied. Cells were more resistant to HELP treatments at the beginning of the logarithmic phase and at the stationary phase. Microbial inactivation was not a function of the initial cell concentration. At constant input voltage, electric field strength obtained in the treatment chamber depended on medium conductivity. At the same electric field strength, conductivity did not influence S. senftenberg inactivation. At the same conductivity, inactivation of S. senftenberg was bigger at neutral than acidic pH.

Inactivation of Salmonella Enteritidis, Salmonella Typhimurium, and Salmonella Senftenberg by ultrasonic waves under pressure.

The resistance of Salmonella Enteritidis (ATCC 13076), Salmonella Typhimurium (ATCC 13311), and Salmonella Senftenberg 775W (ATCC 43845) to ultrasonic waves under pressure treatments, at sublethal (manosonication) and lethal temperatures (manothermosonication) in citrate-phosphate buffer and in liquid whole egg was investigated. The influence of treatment parameters on the inactivation rate of manosonication was also studied. Decimal reduction times (Dt) of Salmonella Enteritidis, Salmonella Typhimurium, and Salmonella Senftenberg 775W corresponding to a heat treatment at 60 degrees C in pH 7 buffer and in liquid whole egg were 0.068, 0.12, and 1.0 min for buffer, and 0.12, 0.20, and 5.5 min for liquid whole egg. Those corresponding to a manosonication treatment (117 microns, 200 kPa, 40 degrees C) in both media were 0.73, 0.78, and 0.84 min, and 0.76, 0.84, and 1.4 min, respectively. When the amplitude of ultrasonic waves was increased linearly, the inactivation rate of manosonication increased exponentially. The inactivation rate also increased when pressure was raised. However, the magnitude of this increase was progressively smaller at higher pressures. The magnitude of the influence of the amplitude of ultrasonic waves and static pressure on the inactivation rate of manosonication was the same in the three serotypes investigated. Whereas a heat treatment at 60 degrees C only attained a 1/2-log cycle reduction in the number of Salmonella Senftenberg 775W survivors, a manothermosonication treatment (117 microns and 200 kPa) at this temperature attained a 3-log cycle reduction.

Inactivation of Salmonella enterica Serovar Enteritidis by Ultrasonic Waves under Pressure at Different Water Activities

ABSTRACT The inactivation of Salmonella enterica serovar Enteritidis by ultrasonic waves (20 kHz; 117-μm wavelength) under pressure (175 kPa) at nonlethal temperatures (manosonication [MS]) and lethal temperatures (manothermosonication [MTS]) in media of different water activities has been investigated. Heat decimal reduction time values increased 30 times when the water activity was decreased from nearly 1 to 0.96, but the MS resistance was increased only twofold. The inactivation of Salmonella serovar Enteritidis by ultrasound under pressure at low water activities was a phenomenon of the “all-or-nothing” type. A synergistic lethal effect was observed between heat and ultrasound in media with reduced water activity; the lower the water activity, the greater the synergistic effect. This work could be useful for improving sanitation and preservation treatments of foods, especially those which are sensitive to temperature and those in which components protect microorganisms to heat. It also contributes to our knowledge of microbial inactivation mechanisms by MS and MTS treatments.

Influence of pH on heat resistance of spores of Bacillus coagulans in buffer and homogenized foods.

The influence of pH of heating menstruum (McIlvaine buffer) on the heat resistance of Bacillus coagulans spores has been investigated and compared with the heat resistance in homogenized tomato and asparagus at pH 7 and 4 at a wide range of temperatures. Spores were less heat resistant in all menstrua at acid pH. The magnitude of this effect was greatest at the lowest heating temperatures tested. z values in buffer increased from 8.9 degrees C at pH 7 to 10.5 degrees C at pH 4. pH of menstrua was the main influencing factor, but media composition also influenced heat resistance: at pH 7 heat resistance was similar in all menstrua (D111 degrees C = 1.6 min) but at pH 4 the heat resistance in homogenized foods (D111 degrees C = 0.26 min in tomato and D111 degrees C = 0.28 min in asparagus) was lower than in buffer (D111 degrees C = 0.49 min). The reduced influence of the acidification of media on the heat resistance of B. coagulans at higher temperatures should be taken into account when a rise in the temperature of treatment for canned vegetables is considered to shorten duration of heat processes.

Effect of sodium chloride concentration on the heat resistance and recovery of Salmonella typhimurium

The survival of Salmonella typhimurium (ATCC 13311) heated and recovered in media with 0, 1, 2, 3, 4 or 5% (w/w) added sodium chloride was investigated. A protective effect in the heating medium and an inhibitory effect in the recovery medium were observed. The results showed an interaction between the effect on, D(58 degrees C) values, of sodium chloride concentration in both media. Lower concentration in the heating media led to a greater effect of the sodium chloride concentration in the recovery media. When the sodium chloride concentration was the same in both media, the protective effect exerted in the heating media dominated over its inhibitory effect in the recovery media.

Resistance of heat-shocked cells of Listeria monocytogenes to mano-sonication and mano-thermo-sonication.

Heat shocks did not increase the resistance of Listeria monocytogenes to an ultrasonication treatment under pressure (Mano‐Sonication; MS). While heat‐shocked cells (180 min, 45 °C) became sixfold more heat resistant than native cells (D62 = 1·8 min vs D62 = 0·24 min), the resistance of native and heat‐shocked cells to MS (200 kPa, 117 μm) was the same (DMS = 1·6 min). The inactivation rate of non‐heat‐shocked cells of L. monocytogenes by a combined heat/ultrasonication treatment under pressure (Mano‐Thermo‐Sonication; MTS) was an additive effect. On the contrary, on heat‐shocked cells, the inactivation rate of MTS was greater than that of heat added to the inactivation rate of MS (synergistic effect) in the range 62–68 °C.

Influence of sporulation temperature on the heat resistance of a strain of Bacillus licheniformis (Spanish type culture collection 4523)

The influence of the sporulation temperature on the heat resistance of a strain of Bacillus licheniformis isolated during a routine control for sterility of canned vegetables, has been studied. Heat resistance of this strain at any temperature of treatment increased with increasing sporulation temperature. Spores sporulated at 52°C were 10-fold more heat resistant than those sporulated at 30°C. The magnitude of this influence was not constant along the range of sporulation temperatures tested (30, 37, 44 and 52°C). No statistical significance ( P ≤0·05) differences were detected among z values obtained with spores sporulated at different temperatures. This increase of heat resistance at higher sporulation temperatures could account for the frequent failures of sterilization processes of canned vegetables, during hot seasons in warmer regions.

Sporulation temperature and heat resistance of Bacillus subtilis at different pH values

The influence of the temperature of sporulation on the heat resistance of Bacillus subtilis in citrate-phosphate buffer of different pH values was investigated. The effect of the pH of the heating menstruum on the heat resistance of spores was strongly influenced by the temperature of sporulation. Spores sporulated at 32°C were at pH 4 much less heat resistant (1/6) than at pH 7. This difference in heat resistance at both pH values was constant regardless of the temperature of treatment. On the contrary, in spores sporulated at 52°C the effect of acid pH on heat resistance was not constant and decreased as heating temperature increased. At 120°C heat resistance was the same at both pH values: z values for pH 7 and 4 were 8.7 and 11.6, respectively. The observed increase of the z value of B. subtilis at pH 4 when sporulated at high temperatures is an added risk that should be taken into account in hot climates, especially when designing sterilization processes at high temperatures for acid/acidified foods.