Alfredo Palop

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.

Inactivation of Bacillus subtilis spores by combining ultrasonic waves under pressure and mild heat treatment

The inactivation of Bacillus subtilis spores by ultrasonic treatments under static pressure (Mano‐Sonication, MS) and a combined MS/heat treatment (Mano‐Thermo‐Sonication) was investigated. The sporicidal effect of MS treatments depended on static pressure, amplitude of ultrasonic waves and treatment temperature. At 70 °C, pressure increments up to 500 kPa caused progressively more inactivation. An MS treatment at 500 kPa and 117 μm of amplitude for 12 min inactivated approximately 99% of the B. subtilis spore population. Over 500 kPa, further increments in pressure did not increase the percentage of inactivation. In the range 90–150 μm, an exponential relationship was observed between the amplitude of ultrasonic waves under pressure and the number of survivors. While an MS treatment (20 kHz, 300 kPa, 70 °C, 12 min) at 90 μm inactivated 75% of the B. subtilis spore population, the same treatment at 150 μm inactivated 99·9% of this population. The MS treatments at temperatures higher than 70 °C (MTS) led to more spore inactivation. In the range 70–90 °C, the combination of heat with an MS treatment (20 kHz, 300 kPa, 117 μm, 6 min) had a synergistic effect on spore inactivation. The inactivating effect of ultrasound was due neither to titanium particles eroded from the sonication tip, nor to free radicals released during ultrasonic treatment. The MS treatments sensitized spores of B. subtilis to lysozyme.

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.

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.

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.

Heat Resistance of Alicyclobacillus acidocaldarius in Water, Various Buffers, and Orange Juice

The effect of the pH or the composition of the heating medium and of the sporulation temperature on the heat resistance of spores of a thermoacidophilic spore-forming microorganism isolated from a dairy beverage containing orange fruit concentrate was investigated. The species was identified as Alicyclobacillus acidocaldarius. The spores showed the same heat resistance in citrate-phosphate buffers of pH 4 and 7, in distilled water, and in orange juice at any of the temperatures tested (D120 degrees C = 0.1 min and z = 7 degrees C). A raise in 20 degrees C in the sporulation temperature (from 45 to 65 degrees C) increased the heat resistance eightfold (from D110 degrees C = 0.48 min when sporulated at 45 degrees C to 3.9 min when sporulated at 65 degrees C). The z-values remained constant for all sporulation temperatures. The spores of this strain of A. acidocaldarius were very heat resistant and could easily survive any heat treatment currently applied to pasteurize fruit juices.

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.

Influence of pH on heat resistance of Bacillus licheniformis in buffer and homogenised foods

The influence of pH of heating menstruum (McIlvaine buffer) on the heat resistance of Bacillus licheniformis was investigated and compared with the heat resistance in homogenised tomato and asparagus at pH 7 and 4 in a wide range of temperatures. Heat resistance was in all mestrua smaller at acid pH. At 99 degrees C and pH 4, heat resistance was 1/20 lower than at pH 7. However, the magnitude of this effect decreased as heat treatment temperatures were increased almost disappearing at 120 degrees C. z values increased from 6.85 at pH 7, to 10.75 at pH 4. At 99 degrees C the effect of pH on heat resistance was constant along the range of pHs tested. The increase of one pH unit increased D99 by 180%. At pH 7 and 4, heat resistance was the same in buffer as in tomato and asparagus homogenates at all temperatures tested. The diminishing influence of the acidification of some foods on the heat resistance of B. licheniformis sterilisation temperatures should be taken into account when a raise in temperature is considered to shorten the duration of heat processes.

Survival of heated Bacillus coagulans spores in a medium acidified with lactic or citric acid

The influence of the intensity of heat treatments on the capacity of citric or lactic acid to prevent growth of survivors of Bacillus coagulans spores after 10 days storage at 35 degrees C was studied. In most cases, the number of survivors during storage decreased. The extent of this spore inactivation depended on the intensity of previous heat treatment and the pH of the medium and the acidulant used. The inactivating effect of storage was pronounced even at pH values less acidic than those used by the canning industry. Citric acid was more effective than lactic acid on spores given only low heat treatments, but lactic was more effective against those given more severe heat treatments. The severity of heat treatment required for lactic to be more effective than citric acid increased with pH of the medium. Heat treatment also required increased pH for heated spores to grow. pH 4.6, regardless of acidulant used, was unable to prevent growth of unheated spores but a less acidic pH (pH 5.2) did prevent growth even when spores had been given only mild heat treatments (10 s at 100 degrees C).