Apparent thermal resistance of Bacillus stearothermophilus spores recovered under anaerobic conditions

 The effect of recovering Bacillus stearothermophilus spores under anaerobic conditions on their apparent thermal resistance was studied. Spores were suspended in bidistilled water as a reference medium, heated at 115, 117, 119, 121, 123 and 125°C and recovered under aerobic and anaerobic conditions. D values (decimal reduction time) obtained following recovery under anaerobic conditions were lower than those obtained under aerobic conditions. Reductions of between 31 and 48% were found for all the temperatures studied. When spores were suspended in mushroom extract and recovered under anaerobic conditions the apparent heat resistance was much lower than that obtained under aerobic conditions (D _121°C was 4.3 min and 1.7 min, under aerobic and anaerobic conditions, respectively). Heating the spores in mushroom extract and recovering the spores under anaerobic conditions produced an additive effect, decreasing the apparent heat resistance of the B. stearothermophilus spores. Received: 1 July 1997     

Kinetics of Clostridium sporogenes PA3679 Spore Destruction Using Computer-Controlled Thermoresistometer

A modified version of a computer-controlled thermoresistometer was used, with and without micropurge, to study the inactivation kinetics of Clostridium sporogenes PA 3679 spore destruction between 121–143°C in phosphate buffer (pH 7) and in mushroom extract acidified with citric acid. A shorter temperature come up time was observed with micropurge. The thermal death time (TDT) curve for spores in phosphate buffer with micropurge followed a straight line (z = 9.5°C). Without micropurge the curve could be described by two lines with z = 10.0 °C for temperatures up to 132.5 °C and z = 18.3 °C for higher temperatures. The spore heat resistance in mushroom extract was lower than in phosphate buffer. D_T. values decreased exponentially as temperature increased, but acidification did not reduce thermal resistance at high temperatures.     

Measurement of optical absorbance of Clostridium sporogenes PA 3679 spores as a method for determining their thermal inactivation

 The loss of optical density of Clostridium sporogenes PA 3679 spores after heating at temperatures of 121, 126, 130 and 135°C was studied, together with the relationship between this parameter and the recovery capacity of the heated spores. The results show that the spores suffered a greater loss of optical density when they were subjected to more severe heating. A linear relationship was observed between the loss of viability and the optical absorbance of the spores. A certain parallel was detected between the heat resistance parameters of the spores D_T (decimal reduction time) and z (thermal inactivation coefficient), and the kinetic parameters D_T ^A and z^A, which describe the reduction in absorbance. Received: 14 April 1997     

Measurement of optical absorbance of Clostridium sporogenes PA 3679 spores as a method for determining their thermal inactivation

 The loss of optical density of Clostridium sporogenes PA 3679 spores after heating at temperatures of 121, 126, 130 and 135°C was studied, together with the relationship between this parameter and the recovery capacity of the heated spores. The results show that the spores suffered a greater loss of optical density when they were subjected to more severe heating. A linear relationship was observed between the loss of viability and the optical absorbance of the spores. A certain parallel was detected between the heat resistance parameters of the spores D_T (decimal reduction time) and z (thermal inactivation coefficient), and the kinetic parameters D_T ^A and z^A, which describe the reduction in absorbance. Received: 14 April 1997     

THERMAL PROPERTIES OF CHEDDAR CHEESE: EXPERIMENTAL AND MODELING

Thermal properties (thermal conductivity, thermal diffusivity and heat capacity) of Cheddar cheese were measured as a function of cheese age and composition. The composition ranged from 30–60% moisture, 8–37% fat, and 22–36% protein (wet basis). The thermal conductivity and heat capacity ranged from 0.354–0.481 W/m °C and from 2.444–3.096 kJ/kg °C. Both thermal conductivity and heat capacity increased with moisture and protein content and decreased with fat content. The thermal diffusivity ranged from 1.07×10^?7 ? 1.53 × 10^?7 m²/s. There was no significant relationship between thermal diffusivity and moisture, fat and protein content of cheese. No statistically significant effect (at the 10% level) of age (0 to 28 wk) on thermal properties was observed. Models predicting thermal properties as a function of cheese composition were developed and their predictive ability was compared with that of empirical models available in the literature. In addition, several theoretical thermal conductivity models were evaluated for their usefulness with Cheddar cheese. Published thermal conductivity models cannot accurately predict (mean error was from 3.4 to 42%) the thermal conductivity of Cheddar cheese.     

THERMAL PROPERTIES OF SHRIMPS, FRENCH TOASTS AND BREADING

Thermal properties of fried products namely butterfly and popcorn shrimp, French toast and breading were determined for a wide range of temperature (20 to 140C), moisture content (3.3 to 87.4% w.b.) and fat content (0.005 to 0.618 kg/kg dry weight) typically encountered during deep‐fat frying. Solid density of the products ranged from 1034 to 1508 kg/m³. Solid density increased with decreasing moisture content of product. Thermal conductivity ranged from 0.03 to 0.85 W/mC. Specific heat ranged from 1.51 to 4.67 kJ/kgC. The results were in the range of published data for similar products. Temperature, moisture and fat content affected variations of thermal conductivity and specific heat. Regression equations were used to fit experimental data.     

BIOLOGICAL VALIDATION OF TOMATO PULP CONTINUOUS HEAT PROCESS

This research validated the commercial process applied to tomato pulp (pH 4.3 and 8 Brix) packed in Tetra Brik packages. Spores of Bacillus coagulans and Neosartorya fischeri were selected as targets. The heat resistance of both microorganisms, tested independently, was compared. The redesigned thermal processes were carried out in a aseptic processing and tested by indirect inoculation and retrieval with spores immobilized in alginate/tomato balls. The results showed that processes for 30 s at 115C or greater did not allow the survival of heat‐resistant molds. For bacterial spores, processes for 30 s at 109C or greater showed no survivors. Although, 30 s at 115C will control both types of spoilage spores, concern for possible C. botulinum growth attributed to metabiosis in product with varying initial populations of molds and residual oxygen content dictated, a process recommendation of 60 s at 126C for safety reasons.     

Apparent thermal resistance of Bacillus stearothermophilus spores recovered under anaerobic conditions

The effect of recovering Bacillus stearothermophilus spores under anaerobic conditions on their apparent thermal resistance was studied. Spores were suspended in bidistilled water as a reference medium, heated at 115, 117, 119, 121, 123 and 125°C and recovered under aerobic and anaerobic conditions. D values (decimal reduction time) obtained following recovery under anaerobic conditions were lower than those obtained under aerobic conditions. Reductions of between 31 and 48% were found for all the temperatures studied. When spores were suspended in mushroom extract and recovered under anaerobic conditions the apparent heat resistance was much lower than that obtained under aerobic conditions (D _121°C was 4.3 min and 1.7 min, under aerobic and anaerobic conditions, respectively). Heating the spores in mushroom extract and recovering the spores under anaerobic conditions produced an additive effect, decreasing the apparent heat resistance of the B. stearothermophilus spores.     

Effect of different rehydration temperatures on the moisture,content of phenolic compounds,antioxidant capacity and textural properties of edible Irish brown seaweed

The effect of temperature (20, 40, 60, 80 and 100 °C) on the rehydration kinetics and phytochemical constituents of dried edible Irish brown seaweed, Himanthalia elongata, were studied. The moisture content of fresh and dried seaweed was 4.07 and 0.07 g water/g dry basis, representing a 98.1% reduction in water content. All rehydration moisture curves had a clear exponential tendency, and it was observed that the rehydration time decreased when temperature was increased. Although restoration of the product to its original moisture content was achieved, rehydration resulted in losses in phytochemical content. Moisture equilibrium was achieved fastest at 100 °C (40 min) with losses of 83.2 and 93% in the total phenol and total flavonoid contents, respectively. The moisture content was fitted to empirical kinetic models; Weibull, Peleg’s, first-order and exponential association. Activation energies of 4.03, 4.28 and 3.90 kJ/mol were obtained for the parameters of Peleg’s, first-order and exponential models, respectively.     

Inactivation of Bacillus stearothermophilus spores in egg patties by pressure-assisted thermal processing

The use of pressure-assisted thermal processing (PATP) to inactivate bacterial spores in shelf-stable low-acid foods, without diminishing product quality, has received widespread industry interest. Egg patties were inoculated with Bacillus stearothermophilus spores (10⁶ spores/g) and the product was packaged in sterile pouches by heat sealing. Test samples were preheated and then PATP-treated at 105 °C at various pressures and pressure-holding times. Thermal inactivation of spores was studied at 121 °C using custom-fabricated aluminum tubes; this treatment served as a control. Application of PATP at 700 MPa and 105 °C inactivated B. stearothermophilus spores, suspended in egg matrix rapidly, (4 log reductions in 5 min) when compared to thermal treatment at 121 °C (1.5 log reduction in 15 min). Spore inactivation by PATP progressed rapidly (3 log reductions at 700 MPa and 105 °C) during pressure-hold for up to 100 s, but greater holding times (up to 5 min) had comparatively limited effect. When PATP was applied to spores in water suspension or egg patties, D values were not significantly different. While thermal inactivation of spores followed first-order kinetics, PATP inactivation exhibited nonlinear inactivation kinetics. Among the nonlinear models tested, the Weibull model best described PATP inactivation of B. stearothermophilus spores in the egg product.     

Heat Resistance of Sporolactobacillus inulinus

Sporolactobacillus inulinus was grown on 24 media varying in carbohydrate source and concentration. A heat-resistant total count (80°C–5 min) and the thermal resistance of the spores isolated from each cell crop were determined. The highest heat-resistant couunts were obtained when S. inulinus was grown on Sporolactobacillus agar with 2%α-methyl glucoside. Cells grown in Sporolactobacillus broth with 1%α-methyl glucoside yielded the most heat resistant spores. D values (decimal reduction times) and z values were determined for each spore crop. Average D values were 53.2 min at 75°C, 19.5 min at 80°C, 6.8 min at 85°C and 5.1 min at 90°C. The average z value was 13.0°C.     

Color Improvement and Metallo-chlorophyll Complexes in Continuous Flow Aseptically Processed Peas

Fresh and frozen green peas were blanched in zinc solution (50 to 500 mg/L) and thermally processed in a particle cell reactor, which simulates a continuous flow aseptic processing system. The thermal process temperatures ranged from 121 to 145 °C at holding times from 0 to 20 min. The degradation of chlorophyll and the resulting formation of Zn-pheophytin a and Zn-pyropheophytin a were monitored. Quantitative analysis of the metallo-chlorophyll complexes was performed by high-performance liquid chromatography. Complex formation increased during heat processing and was dependent on the zinc concentration absorbed within the peas during blanching. At 130 to 145 °C, the formation of Zn-pyropheophytin a increased and processing between 121 to 125 °C promoted the formation of Zn-pheophytin a. Improvements in color relative to control samples suggested that the process might be applicable to two-phase continuous aseptic processing of vegetables.     

Thermal Resistance of Bacillus coagulans in Double Concentrated Tomato Paste

Thermal resistance was determined on a strain of Bacillus coagulans in double concentrated tomato paste (a_w= 0.95 at 23°C, pH = 4.0, 30.3°Brix, 70.1% moisture and acidity 1.30 g/100g citric acid. A microsyringe method was used with an inoculum of 1.3 × 10⁴ spores/ mL. Values of D_90°c= 3.5 min and z = 9.5C° were obtained.     

The effect of pH on the thermal resistance of Clostridium sporogenes (PA 3679) in asparagus purée acidified with citric acid and glucono-delta-lactone.

The influence of type of acid, pH and temperature on heat resistance of Clostridium sporogenes (PA 3679) spores were investigated in white asparagus pureé acidified with citric acid and glucono-delta-lactone (GDL). The pH values studied were: 4.5, 4.8, 5.1 and 5.4 at temperatures of 110, 115, 118 and 121 degrees C. The addition of citric acid and GDL to reduce pH significantly diminished heat resistance of the spores. The two acids investigated differed in their effect on heat resistance at the various pH-levels. The most pronounced effect was observed at the lower heat treatment temperatures investigated. The z values ranged from 10.24 to 13.09 degrees C in the asparagus purée with acids added and with significant differences between the two acids and the various pH levels.     

Nonvolatile taste compounds in cooked Chinese Nanjing duck meat following postproduction heat treatment

Abstract: Taste‐active compounds, including free amino acids, succinic acid and 5′‐nucleotides, and chemical components including moisture, pH, protein, crude fat, and sodium chloride were evaluated in cooked and packaged Chinese Nanjing ducks following heat treatments including control, 99 °C for 40 min, 108 °C for 20 min, 92 °C microwave followed by water heating, 95 °C for 30 min, 121 °C for 25 min. Heat treatment decreased (P < 0.05) the content of Alamine and moisture and reduced the pH value in muscle, but increased (P < 0.05) the protein and 5′‐nucleotides content. The 99 °C group had a significantly lower crude fat content compared with other groups (P < 0.05). The succinic acid content in the control group was significantly higher than in the 121 °C group (P < 0.05). Groups treated at higher temperatures (108 °C, 121 °C, and microwave) had similar equivalent umami concentrations and 5′‐nucleotides, free amino acids content, as well as the derived bitter and sweet taste components compared with the groups treated at lower temperatures. It can be speculated that these differences account for the enhanced flavor of the meat in the 99 °C, 108 °C, 121 °C, and microwave groups compared with the untreated control. Therefore, heat treatment at lower temperature after packaging may prolong product shelf life without any detrimental effects on taste. The results of this study indicate that it is important to use lower temperatures in this type of food processing. However, it may be possible to modify the primary processing steps to improve the content of umami‐like taste compounds such as 5′‐nucleotides. Practical Application: Heat treatment of packaged products is an effective method for eradication of microbes, therefore increasing the shelf‐life. However, such treatment can result in major changes in the sensory perception of meat products, particularly the formation of off‐flavors. This study investigated changes in taste‐active compounds in duck meat following a number of types of heat treatment.     

Elevation of heat resistance of Clostridium sporogenes following heat shock

Heat shock of strains of spores of Clostridium sporogenes at 80 or 100°C increased their apparent resistance to heating at 121·1°C, with the exception of NCIB 10696. The presence of glucose (0·5 – 1·0%) in the sporulation medium reduced the percentage of spores which could be activated by heat. Within individual spore populations, some spores required more heat than others to induce activation. Spores of PA 3679 remained activated for 18 months at 4°C and heat shock did not alter the z value. Experiments using dimethylglutaric acid/NaOH buffer suggest that heat shock in acid conditions has maximum effect on heat resistance.     

Sporulation and Heat Resistance of Spores from a Clostridium sp. RKD

A Clostridium sp. RKD isolated from the intestine of decaying fish, showing 99% sequence identity with Clostridium tetani at a 16S rRNA level, produced a neurotoxin that was neutralized by botulinum antitoxin (A+B+E). It also showed an amplification of near‐expected size when polymerase chain reaction (PCR) was performed using group‐ and type‐specific primers for botulinum neurotoxin type B. The isolate exhibited differences with both C. tetani and Clostridium botulinum with respect to phenotypic characteristics and chemotaxo‐nomic markers. Spore production was optimized with respect to media composition and stage of growth. Time‐dependant examination of sporulation revealed 2.6% to 49.0% spores in the late stationary phase culture when grown in different broth media. A simpler method for spore production and isolation from culture grown in tryptose sulfite cycloserine (TSC)/anaerobic agar sandwich resulted in >95% sporulation, which could be purified to near homogeneity by a simple 2‐step procedure. Thermal resistance of spores revealed a biphasic inactivation at lower temperatures with D values for linear inactivation varying from 26.6, 8.0, and 4.3 min at 70 °C, 80 °C, and 90 °C, respectively. The z values of 7.86 °C and 10.47 °C were obtained in the linear and tail regions, respectively. The Weibull parameter b values at 70 °C, 80 °C, and 90 °C were 27.38, 3.55, and 0.99, respectively, with a z’ value of 13.869 °C. The shape parameter n at 70 °C, 80 °C, and 90 °C were 0.63, 0.55, 0.45, respectively. Spores produced on 2 different media (cooked meat medium [CMM] and trypticase peptone yeast‐extract glucose [TPYG] agar) exhibited differences in heat resistance. The addition of lysozyme (50 jj.g/mL) before heat treatment resulted in increased thermal resistance of spores.     

Thermophysical properties of medium grain rough rice (<Emphasis Type="Italic">LIDO cultivar</Emphasis>) at medium and low temperatures

Bulk density, specific heat capacity, bulk thermal conductivity and bulk thermal diffusivity of medium-grain rough rice (LIDO cultivar) have been studied. Specific heat was determined by DSC, thermal conductivity by the probe method and bulk thermal conductivity indirectly from bulk density, specific heat capacity and bulk thermal conductivity. All the thermal properties have been determined at different moisture contents and temperatures used during cooling and storage operations and the effect of these variables has been investigated. It has been shown that moisture content has the greatest effect on specific heat and bulk thermal conductivity. Temperature also affected these thermal properties, but to a smaller extent. Mathematical expressions have been developed to determine each of these thermophysical properties as a function of moisture content and temperature.List of Abbreviations and Symbols C_p Specific heat capacity (kJ/kg °C) - k Bulk thermal conductivity (W/m °C) - T Temperature (ºC) - W Moisture content (%, d.b.) - Bulk thermal diffusivity (m²/s) - Bulk density (kg/m³)     

Performance evaluation of aluminum test cell designed for determining the heat resistance of bacterial spores in foods

Thermal inactivation kinetic studies are necessary to determine heat resistances of spores in the development of new thermal processes for low-acid shelf-stable products. Most currently available sample holders used for solid and semi solid samples in the kinetic studies take long time to reach the target sample temperature, hence fail to provide isothermal condition. In this research, novel aluminum test cells were developed to facilitate easy loading and unloading samples in a hermetically sealed 1 ml cavity to evaluate the heat resistance of bacterial spores when heated at temperatures above 100 °C. Design of the test cell was governed by minimum come-up time. A finite element model based on the commercial software ‘FEMLAB’ was used to simulate transient heat transfer and finalize the test cell dimensions. Performance of the new test cell was evaluated against capillary and aluminum thermal death time tube methods in characterizing the heat resistance of Clostridium sporogenes PA 3679 spores in a phosphate buffer and mashed potato at 121 °C. D₁₂₁ values of PA 3679 spores in both the phosphate buffer and mashed potato using the new test cells were not significantly different (P>0.05) from those by the capillary tube method. The results indicated that the new test cell is appropriate for studying the inactivation kinetics of bacterial spores in microbial validation of conventional and novel thermal processes for low-acid shelf-stable foods.