Combined Effects of Salts and Temperature on the Thermal Destruction of Staphylococcus aureus MF-31

The combined effects of salts (NaCl and KCl) and heat treatment temperature on the thermal destruction of Staphylococcus aureus MF-31 was studied by determining the relationships between the decimal reduction time (D value), temperature and each of three parameters (water activity, osmotic pressure and water binding energy) of the heating menstruum. The results demonstrated that the resistance of S. aureus MF-31 to heat destruction increased as the degree of salt-water association increased. Salts and heat treatment temperature had an interactive effect on heat resistance; however, increasing the heat treatment temperature made the difference caused by salt concentrations become insignificant.     

热胁迫和培养温度对大肠杆菌抗热性的影响

研究大肠杆菌O157:H7 ATCC 43889经50、60 ℃和70 ℃反复多次热胁迫处理与在10、28、36 ℃和45 ℃的条件下生长培养后对其80 ℃抗热性的影响。分别对ATCC 43889进行50、60 ℃和70 ℃的热胁迫,研究在一定的热力致死温度条件下杀死某细菌数量90%所需要的时间（D值）的变化,观察ATCC 43889热胁迫前后菌落形态和个体形态的变化;将ATCC 43889置于10、28、36 ℃和45 ℃培养至稳定期,分别测定其在80 ℃的存活量,再利用Weibull模型拟合其在80 ℃的热致死曲线。结果表明,50、60 ℃和70 ℃热胁迫处理均可诱导ATCC 43889抗热性增加,经10 次热胁迫并传代培养后,其D值分别为第1次热胁迫处理后的1.88、2.38 倍和8.18 倍,D值随热处理次数的增加不断增大,说明胁迫温度越高,D值越大,其抗热性越强;经过60 ℃和70 ℃热胁迫后,ATCC 43889菌落形态和个体形态与对照组相比差异显著;在80 ℃,ATCC 43889的致死曲线表明,胁迫温度越高,其抗热性越强（P＜0.05）;在10～45 ℃培养,随培养温度的升高,ATCC 43889的抗热性显著增加（P＜0.05）。利用Weibull模型可以较好地拟合ATCC 43889经过50、60 ℃和70 ℃热胁迫处理10 次后与10、28、36 ℃和45 ℃培养后在80 ℃的抗热性曲线,随着胁迫温度和培养温度的升高,ATCC 43889的抗热性都呈增加趋势。综上,一定热处理与培养温度可胁迫诱导大肠杆菌ATCC 43889抗性热增强和形态的变化。     

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.     

The effect of oil on the heat resistance of Staphylococcus aureus

Staphylococcus aureus survived for longer periods in fish and oil than in water at temperatures up to 95°C. Heat resistance varied between strains (NCTC 10652 and 68-7895) and was affected by the age of culture prior to use.The introduction of cells in a dried state to oil increased their heat resistance. This increase was related to the growth medium and methods of heating and recovery for cells dried onto sand. An example of the highest heat resistance found was when cells taken from growth on nutrient agar, dried with sand and heated in oil at 120°C, were recovered using a filtration technique and exhibited a D value of 3 to 6 min.The increased heat resistance obtained in these experiments may be due to encapsulation of the bacteria in the oil, which protects the cells from thermal injury.     

HEAT RESISTANCE IN DIFFERENT HEATING MEDIA OF LISTERIA MONOCYTOGENES ATCC 15313 GROWN AT DIFFERENT TEMPERATURES

This paper examines the influence of growth temperature on thermal resistance of Listeria monocytogenes ATCC 15313. Regardless of the incubation temperature, the heat resistance of Listeria monocytogenes increased during incubation until the stationary phase of growth was reached. The maximum heat resistance of cells grown at 4C, 20C or 37C was attained after 14 days, 36 h and 18 h of incubation, respectively. After longer incubation times the heat resistance of cells grown at 4C and 20C did not change but that of cells grown at 37C decreased. The maximum heat resistance was usually greater at higher incubation temperatures. However, the magnitude of these differences depended on the pH and composition of heating media. By raising the incubation temperature from 4C to 37C, the D₆₂ value in pH 7 citrate-phosphate buffer increased from 0.13 to 0.34 min. However, when skimmed milk was used as menstruum this difference was not observed. Cells grown at 37C attained maximum heat resistance at pH 7 but those grown at 4C, at pH 6. The magnitude of the effect of the incubation temperature on heat resistance was constant at all heating temperatures tested (z = 6 ± 0.8C). The higher heat resistance of cells grown at higher temperatures was not due to a greater capacity of heat damage repair     

Natural deep eutectic solvents (NADES) based on citric acid and sucrose as a potential green technology: a comprehensive study of water inclusion and its effect on thermal,physical and rheological properties

In this work, NADES based on sucrose and citric acid was synthesised and evaluated. Physical properties such as density, viscosity and thermal profile were evaluated at different temperatures and water contents. Viscosity and density were inversely affected by temperature and water content. Increases in the sucrose fraction resulted in higher viscosity, but no differences in density were observed. The rheological model of Ostwald-de-Waele indicated three distinct rheological behaviours at different temperatures, water content and sucrose fractions. The Arrhenius model showed a good fit for the temperature effect on the apparent viscosity of all NADESs. The NADES decomposition temperature was approximately 393 K. The thermal characterisation revealed that all NADESs presented glass transitions at temperatures below 204.59 K, confirming their formation and stability. Based on these results, the NADES proposed appears as potential green solvent to be used in industrial processes such as extraction, separation and biochemical technology.     

THERMAL RESISTANCE OF BACILLUS STEAROTHERMOPHILUS SPORES IN DRIED PASTA AT DIFFERENT STAGES OF REHYDRATION

The thermal resistance of Bacillus stearothermophilus spores at different stages of rehydration was examined. Inoculation was achieved by rehydrating pasta samples in water containing 1‐mL spore crop (10⁶ spores/mL); the product was heat sealed in sterile pouches and processed in a laboratory retort. Thermal inactivation of spores was carried out using 10 time intervals at 121C for moisture contents, 70, 90, 105, 115, 125 and 145% dry basis. The death curves obtained consisted of two phases, an initial rapid decline (1.5 to 2.6 log reductions) followed by a slower, linear decrease of survivors from which the decimal reduction time (D value) was calculated. D₁₂₁ values were found to decrease with increasing moisture content and ranged from 4.6 to 6.5 min. The thermal resistance constant (z value) was examined to assess the effect of temperature (110–125C) on lethality. z values decreased with increasing moisture content and ranged from 10.7 to 15.6C. The results of this study demonstrate that temperature and moisture content influence the effectiveness of the intended heat process. For products that are rehydrated during heat treatment, the moisture content of the product is a critical factor. Therefore, if complete rehydration is not achieved, this may result in the heat process being less effective for the destruction of microorganisms, which may have implications in regard to food safety.     

Microbial Inactivation Kinetics in Soymilk during Continuous Flow High-Pressure Throttling

ABSTRACT:  The thermal resistance of  Clostridium sporogenes  PA 3679 ATCC 7955 was determined in soymilk (pH 7) and 0.1% peptone water (pH 7) by the capillary tube method. In the continuous flow high-pressure throttling, the temperature of soymilk increased due to instantaneous pressure release and the additional heat was supplied by a heat exchanger to achieve the set temperature. The soymilk was immediately cooled after a short preset hold time to below 40 °C. A significant increase in the heat resistance was observed in  C. sporogenes  spores when heated in soymilk in comparison to 0.1% peptone water. The  D ₁₂₁-value for spores in soymilk was approximately 3-folds higher than peptone water. The  z- value was also much higher in soymilk as compared to that in 0.1% peptone water. Continuous flow high-pressure throttling (HPT) from 207 or 276 MPa to atmospheric pressure reduced the microbial populations in inoculated soymilk up to 6 log cycles when the holding times were 10.4, 15.6, and 20.8 s and the process temperatures were 85, 121, 133, and 145 °C, respectively. The sporicidal effect increased as the operating pressure, time, and temperature were increased. More injured spores were found at 207 MPa than at 276 MPa, indicating that lower pressure caused cell injury whereas high pressure caused cell death.     

Effect of water activity on thermal resistance of Salmonella krefeld in liquid medium and on rawhide surface

The thermal resistivity of Salmonella krefeld at 60 and 65 degrees C as affected by water activity (0.99-0.60) was investigated in a liquid medium viz. Trypticase Soy Broth (TSB) and on rawhide surface. A reduction of the a(w) of the liquid medium resulted in an increase in the heat resistance of Salmonella krefeld. At a particular water activity the type of solute played a significant role with Salmonella krefeld exhibiting much higher heat sensitivity in the media with glycerol compared to the media with sucrose. The heat resistance of Salmonella krefeld inoculated onto the surface of rawhide was also investigated during hot air drying at 60 and 65 degrees C at two initial levels of a(w), 0.99 and 0.90, prior to drying as determined at 250 degrees C. The rate of water loss or a(w) decrease during drying was not significantly influenced by the initial water activity but was clearly dependent on the drying temperature with all survival curves displaying an upward concavity. Salmonella on rawhide with an initial a(w) of 0.90 exhibited higher heat tolerance than with an initial a(w) of 0.99 during a prolonged heating period with the initial a(w) significantly affecting the heat resistance particularly at higher drying temperatures.     

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.     

Phase Transitions of Amorphous Sucrose and Frozen Sucrose Solutions

Amorphous sucrose showed typical thermal transitions of amorphous materials. The temperatures of those transitions, glass transition, crystallization and melting, decreased with increasing moisture content. The glass transition temperature of dry amorphous sucrose at 57°C was decreased to -46°C as the sucrose was plasticized with excess water. That value remained constant due to maximal freeze-concentration with 72-73% sucrose in the unfrozen matrix which showed ice melting at -34°C. Concentrated solutions may become supercooled resulting in partial freeze-concentration with resultant lowering of glass transition temperature.     

Conditions Under Which Bacterial Amylases Survive Ultrahigh Temperature Sterilization

The α-amylase of the thermophile Bacillus stearothermophilus 1518 was used as a model enzyme to examine the heat resistance of a bacterial amylase at ultrahigh temperature in a starch-based pudding. When heated in the laboratory pudding at 143°C for the equivalent of 22.2 set, approximately 26% of the initial amylolytic activity was retained. The presence or absence of individual ingredient groups (starch, caseinate-nonfat dry milk, oilemulsifiers, salts, or sucrose) of the pudding had no significant effect on amylase heat resistance. A combination of any two of the starch, caseinate-nonfat dry milk, or oil-emulsifier ingredients were found to be important for thermostability. The amylase of the mesophile Xanthomonas campestris 13957–2 exhibited similar thermal inactivation characteristics.     

Coupling osmotic dehydration with heat treatment for green papaya impregnated with blackberry juice solution

Slices (1.5 mm thick) of green papaya were impregnated through osmotic dehydration with a blackberry juice–sucrose solution to produce an intermediate moisture product. The effect of processing temperature (T) and sucrose-added molality (m_sucrose) on mass transfer during the operation was assessed, using a response surface methodology (RSM). The RSM was used to model water loss, sugar and anthocyanin gain during the process. Increasing sucrose molality resulted in increasing water loss and sugar gain, but decreasing anthocyanin gain. Water transfer therefore limits anthocyanin impregnation, but not sucrose incorporation. Afterwards, the impact of heat treatment at high temperatures was analysed, using numerical simulation. The conditions of the combined process, designed to achieve an anthocyanin-rich final product, are low sucrose-added molalities (sucrose molality < 1 mol kg^–1) and high processing temperatures (T > 50 ºC) for osmotic dehydration, coupled with high-temperature, short-time (HTST) heat treatments for product stabilisation.     

Modelling effect of physical and chemical parameters on heat inactivation kinetics of hepatitis A virus in a fruit model system

While thermal destruction of pathogenic bacteria has been thoroughly studied in food industry, heat inactivation of viruses in food has been poorly investigated. Experiments were carried out to characterize the effects of controlled physical and chemical characteristics of a food matrix upon heat resistance parameters (D and z values) of hepatitis A virus (HAV), taken as model because of its reported heat resistance. Sucrose content (28-52 degrees Brix), calcium concentration (90-1700 mg kg(-1)) and pH (3.3-4.3) were selected for possible influence on thermal inactivation of HAV in strawberry mashes and thus included in an experimental design according to a Doehlert matrix. Use of this design not only allowed to detect and quantify the direct influence of sucrose concentration upon the D85 degrees C value to be higher than the one of pH, but also to reveal a sucrose concentration/pH specific interaction, while no effect of calcium concentration was evidenced. Although the model cannot be directly used to predict heat resistance in real fruit systems, because of differences observed between predicted and measured D85 degrees C values, it is useful for predicting the trends and relative changes in D values due to sucrose concentration and pH variations. Results suggested possible effects of other constituents of strawberry products on heat resistance of HAV and confirmed the importance of experimental validation of any model-derived process. Nevertheless, such a modelling approach using response surface methodology provides a rapid answer to heat resistance evaluation of a food-borne virus as a function of specific physical and chemical parameters of specific food products.     

Influence of Water Activity on Thermal Resistance of Microorganisms in Low‐Moisture Foods: A Review

A number of recent outbreaks related to pathogens in low‐moisture foods have created urgency for studies to understand the possible causes and identify potential treatments to improve low‐moisture food safety. Thermal processing holds the potential to eliminate pathogens such as Salmonella in low‐moisture foods. Water activity (a_w) has been recognized as one of the primary factors influencing the thermal resistance of pathogens in low‐moisture foods. But most of the reported studies relate thermal resistance of pathogens to a_w of low‐moisture foods at room temperature. Water activity is a thermodynamic property that varies significantly with temperature and the direction of variation is dependent on the product component. Accurate methods to determine a_w at elevated temperatures are needed in related research activities and industrial operations. Adequate design of commercial thermal treatments to control target pathogens in low‐moisture products requires knowledge on how a_w values change in different foods at elevated temperatures. This paper presents an overview of the factors influencing the thermal resistance of pathogens in low‐moisture foods. This review focuses on understanding the influence of water activity and its variation at thermal processing temperature on thermal resistance of pathogens in different low‐moisture matrices. It also discusses the research needs to relate thermal resistance of foodborne pathogens to a_w value in those foods at elevated temperatures.     

The effect of starch gelatinization and solute concentrations on T g′ of starch model system

The effect of starch gelatinization on glass transitions in a starch/water model system and how the concentrations of added solutes (sucrose and sodium chloride) affect the glass transition temperatures of the gelatinized starch solution was investigated. The starch suspension samples were heat treated in a Differential Scanning Calorimeter (DSC) under different time and temperature regimes to achieve different degrees of gelatinization. The gelatinization characteristics (onset, peak and end temperatures and enthalpy) and the glass transition values of a potato starch were determined using the DSC. The results showed that the starch concentrations had no effect on gelatinization characteristics and the T_g′ of the gelatinized potato starch but had clearly increased their ΔC_p in the T_g′ region. Annealing at a temperature slightly below the T_g′ of −5 °C, led to maximal freeze‐concentration in the total/partial gelatinized starch and a higher T_g′ value at about −3 °C was obtained. The T_g′ values of the totally gelatinized starch samples were slightly lower than those of partially gelatinized samples. The T_g′ of the gelatinized starch decreased with increasing concentrations of sucrose or sodium chloride. Sodium chloride had a stronger depressing effect on T_g′ than sucrose. © 2000 Society of Chemical Industry     

THE THERMAL STABILITY OF ASPERGILLUS ORYZAE ALPHA-AMYLASE IN PRESENCE OF SUGARS AND POLYOLS

The aim of this study was to test whether it is possible to estimate the heat stability of Aspergillus oryzae alpha‐amylase (α‐amylase) based on the amount of hydroxyl (OH) groups provided in a buffer solution. The thermal stability of the enzyme in a presence of different sugars (sucrose and trehalose) and polyols (mannitol, sorbitol, lactitol and glycerol) was investigated in the temperature range of 62–68C on a kinetic basis. It was investigated if the protective effect of additional substances was correlated to the number of hydroxyl groups (nOH) provided by each of them (per volume unit of the enzyme solution nOH). All additives showed a protective effect on the enzyme's heat stability, which was strongly dependent on the added compound concentration. Among all stabilizing compounds investigated, sucrose exhibited the largest protective effect. The decimal reduction time of α‐amylase activity increased by 33.9 times when 420 mg/mL of sucrose was added to the environment. When the same concentration of trehalose was used, the D‐value increased by 6.4 times compared to the value in the buffer system. The nOH provided in the enzyme solution could not be related to the D‐values for the enzyme thermal inactivation, meaning that the enzyme heat stability was not dependent on the nOH.     

A NEW METHOD FOR DETERMINING THE APPARENT THERMAL DIFFUSIVITY OF THERMALLY CONDUCTIVE FOOD

Two empirical parameters, j and f, have been used to determine the thermal diffusivity values of food by several researchers. Since there are considerable variations in j values, a new procedure was developed for the experimental determination of thermal diffusivity of foods without using this parametric value. A sample was filled into a cylindrical cell whose diameter and length were approximately 10 and 130 mm, respectively. The temperatures of the sample were monitored at the mid-point of the central axis and on the inside surface of the cell. These temperatures were used to estimate thermal diffusivity values together with an analytical formula for heat conduction in an infinite cylinder. The method was utilized to determine the thermal diffusivity, Biot number, and surface heat conductance values of water, 60% sucrose solution, glycerine, cherry tomato pulp, and apple pulp. There was close agreement between thermal diffusivity values determined experimentally and such values available in published literature. Mathematical procedures are presented for estimating errors in the thermophysical property values determined experimentally, where there are errors in locating temperature sensors in a sample, errors in the shape of the diffusivity cell, and errors in the temperature sensing device.