Comparative effects of vacuum or conventional frying on the polyphenol chemistry and in vitro colon cancer stem cell inhibitory activity of purple-flesh potatoes.

Potatoes are an important food crop that undergo postharvest storage, reconditioning, and cooking. Colored-flesh varieties of potatoes are rich in phenolic acids and anthocyanins. Previous studies have suggested that purple-flesh potatoes can inhibit colon cancer cells in vitro and reduce colon carcinogenesis in vivo. Vacuum frying (VF), as an alternative to conventional frying (CF), reduces fat content and may promote polyphenol retention in potato chips. We examined the impacts of reconditioning (storing at 13°C for 3 weeks following the 90-day cold storage at 7°C) and frying method on phenolic chemistry and in vitro colon cancer stem cell (CCSC) inhibitory activity of purple-flesh potato chips. We found that reconditioned chips exhibited higher total phenolic content (TPC) than nonreconditioned chips. We found that VF chips had lower TPC than CF chips. We observed no interaction between treatments. We found that VF chips had 27% higher total monomeric anthocyanin levels than CF chips, and observed a significant interaction between treatments. We found that VF chips had higher concentrations of caffeic acid (42%-72% higher), malvidin (46%-98% higher), and pelargonidin (55%-300% higher) than CF chips. We found that reconditioning had no effect. We found that VF chips had greater in vitro CCSC inhibitory activity than CF chips. Our results suggest that VF can improve the phytochemical profile and health-related functionality of purple-flesh potato chips, but additional studies are needed to determine if these results translate to the in vivo situation. PRACTICAL APPLICATION: Our current study shows that vacuum frying of purple-flesh potato chips results in higher levels of total monomeric anthocyanins and concentrations of specific polyphenols as compared to chips produced by conventional frying. These differences correlated with better in vitro colon cancer stem cell inhibitory activity. Although additional in vivo studies are needed, our current results suggest that it may be possible for potato processors to improve the health-related functionality of purple-flesh potato chips through the use of vacuum frying.

Descriptive sensory analysis of heat-resistant milk chocolates.

Sensory attributes of six heat-resistant chocolates were compared with the standard chocolate using a trained sensory panel who were trained using the Sensory Spectrum method. The panel evaluated the chocolates using three tactile and ten oral attributes at 24, 29, and 38°C. The panel demonstrated consistent rating of the various samples. ANOVA showed that all of the 13 sensory attributes (Firmness to touch, Stickiness to fingers, Snap, Abrasiveness, Hardness with incisors, Fracturability, Cohesiveness of mass, Time to melt, Firmness with tongue, Adhesiveness to teeth, Number of particles, Oily mouthcoating, and Chocolate messiness) were significantly different across the samples. A higher degree of heat resistance was identified by the panelists for the low-fat gelatin and polyol samples at 38°C. Principal component analysis revealed two principal components; the first pricipal component described the variability due to temperature, and the second principal component described the variability brought about by the various technologies.

Impact of roasting on the flavan-3-ol composition, sensory-related chemistry, and in vitro pancreatic lipase inhibitory activity of cocoa beans.

Roasting is an important cocoa processing step, but has been reported to reduce the polyphenol content in the beans. We investigated the impact of whole-bean roasting on the polyphenol content, aroma-related chemistry, and in vitro pancreatic lipase (PL) inhibitory activity of cocoa under a range of roasting conditions. Total phenolics, (-)-epicatechin, and proanthocyanidin (PAC) dimer - pentamer content was reduced by roasting. By contrast, roasting at 150 °C or greater increased the levels of catechin and PAC hexamers and heptamers. These compounds have greater PL inhibitory potency. Consistent with these changes in PAC composition and this previous data, we found that roasting at 170 °C time-dependently increased PL inhibitory activity. Cocoa aroma-related compounds increased with roasting above 100 °C, whereas deleterious sensory-related compounds formed at more severe temperatures. Our results indicate that cocoa roasting can be optimized to increase the content of larger PACs and anti-PL activity, while maintaining a favorable aroma profile.

Release Kinetics of Nisin from Chitosan-Alginate Complex Films.

Understanding the release kinetics of antimicrobials from polymer films is important in the design of effective antimicrobial packaging films. The release kinetics of nisin (30 mg/film) from chitosan-alginate polyelectric complex films prepared using various fractions of alginate (33%, 50%, and 66%) was investigated into an aqueous release medium. Films containing higher alginate fractions showed significantly lower (P < 0.05) degree of swelling in water. Total amount of nisin released from films into an aqueous system decreased significantly (P < 0.05) with an increase in alginate concentration. The mechanism of diffusion of nisin from all films was found to be Fickian, and diffusion coefficients varied from 0.872 × 10-9 to 8.034 ×10-9 cm2 /s. Strong complexation was confirmed between chitosan and alginate polymers within the films using isothermal titration calorimetry and viscosity studies, which affects swelling of films and subsequent nisin release. Complexation was also confirmed between nisin and alginate, which limited the amount of free nisin available for diffusion from films. These low-swelling biopolymer complexes have potential to be used as antimicrobial packaging films with sustained nisin release characteristics.

Dynamic measurement of starch granule swelling during microwave heating.

The size of starch granules in dilute aqueous suspension was measured in-line during gelatinization in a microwave-heated, well-mixed system. The results were compared with those of a previous study conducted with conventional heating. For the starches used (common corn, waxy maize, and cross-linked waxy maize), no significant difference was found between microwave and conventional heating in terms of maximum diameter, temperature of maximum rate of diameter increase, or diameter vs. temperature behavior. These results suggest that there are no differences in the swelling behavior of common and modified maize starches between microwave and conventional heating.

Modeling development of inhibition zones in an agar diffusion bioassay.

A two-temperature agar diffusion bioassay is commonly used to quantify the concentration of nisin using Micrococcus luteus as the indicator microorganism. A finite element computational model based on Fick's second law of diffusion was used to predict the radius of the inhibition zone in this diffusion bioassay. The model developed was used to calculate nisin concentration profiles as a function of time and position within the agar. The minimum inhibitory concentration (MIC) of nisin against M. luteus was determined experimentally. The critical time (T c) for growth of M. luteus within the agar diffusion bioassay was experimentally determined using incubation studies with nisin. The radius of the inhibition zone was predicted from the computational model as the location where the predicted nisin concentration at T c was equal to MIC. The MIC was experimentally determined to be 0.156 µg mL(-1), and T c was determined to be 7 h. Good agreement (R (2) = 0.984) was obtained between model-predicted and experimentally determined inhibition zone radii.

Analysis of Cocoa Proanthocyanidins Using Reversed Phase High-Performance Liquid Chromatography and Electrochemical Detection: Application to Studies on the Effect of Alkaline Processing.

Flavan-3-ols and proanthocyanidins play a key role in the health beneficial effects of cocoa. Here, we developed a new reversed phased high-performance liquid chromatography-electrochemical detection (HPLC-ECD) method for the analysis of flavan-3-ols and proanthocyanidins of degree of polymerization (DP) 2-7. We used this method to examine the effect of alkalization on polyphenol composition of cocoa powder. Treatment of cocoa powder with NaOH (final pH 8.0) at 92 °C for up to 1 h increased catechin content by 40%, but reduced epicatechin and proanthocyanidins by 23-66%. Proanthocyanidin loss could be modeled using a two-phase exponential decay model (R(2) > 0.7 for epicatchin and proanthocyanidins of odd DP). Alkalization resulted in a significant color change and 20% loss of total polyphenols. The present work demonstrates the first use of HPLC-ECD for the detection of proanthocyanidins up to DP 7 and provides an initial predictive model for the effect of alkali treatment on cocoa polyphenols.

Listeria monocytogenes responds to cell density as it transitions to the long-term-survival phase.

Listeria monocytogenes was recently found to enter a long-term-survival (LTS) phase, which may help explain its persistence in natural environments and within food processing plants. The purpose of this study was to investigate the effects of initial cell density, initial pH and type of broth (fresh vs. spent) on the transition of L. monocytogenes to the LTS phase and model the change in viable population density with time. Initial cell density (~10(6)-~10(10)CFU/ml) and initial pH (5.36-6.85) both significantly affected the transition of L. monocytogenes to the LTS phase (P<0.001) with initial cell density being the main determining factor. In contrast, type of broth did not significantly affect cell density change during the transition of stationary-phase cells at high initial density to the LTS phase (P>0.05). After 30-d incubation no significant differences in cell densities were observed between either type of broth or between any of the initial cell density/pH treatment combinations (P>0.05), where the mean viable cell density was 4.3±1.1×10(8)CFU/ml. L. monocytogenes responded to viable cell density in accordance with the logistic equation during transition to the LTS phase. The Agr quorum-sensing system does not appear to play a role in the transition to the LTS phase. Further research is needed to better understand the control mechanisms utilized by L. monocytogenes as it transitions to a coccoid, resistant and stable density state in the LTS phase.

State of water in starch-water systems in the gelatinization temperature range as investigated using dielectric relaxation spectroscopy.

The dielectric response of native wheat starch-water slurries containing 5-60% starch (w/w) was measured in the frequency range of 0.2-20GHz after heating the slurries to 7 different temperatures between 25 and 90°C for 30min. Three relaxations, with relaxation time range of 4-9ps, 20-25ps and 230-620ps at 25°C, were identified from the dielectric spectra of starch slurries. The fastest relaxation process (4-9ps) was attributed to bulk water while the two slower relaxations were attributed to the confined water molecules present in the starch-water system. The amount of water exhibiting the slowest relaxation (230-620ps) was calculated to be 0.08-0.16gwater/g starch, which was close to the monolayer water associated with wheat starch. Mobility of bulk water was significantly reduced (P<0.001) upon gelatinization at low starch concentration (10% starch), but remained unaffected at higher starch concentrations. The mobility of two slower relaxing water species was not significantly influenced (P>0.19) by gelatinization at all starch concentrations.

Transcriptomic response of Listeria monocytogenes during the transition to the long-term-survival phase.

Listeria monocytogenes can change its cellular morphology from bacilli to cocci during the transition to the long-term-survival (LTS) phase. The LTS cells demonstrated increased baro- and thermotolerance compared to their vegetative counterparts. So far, the underlying mechanisms that trigger this morphological and physiological transition remain largely unknown. In this study, we compared the transcriptomic profiles of L. monocytogenes serotype 4b strain F2365 at different growth stages in tryptic soy broth with yeast extract (TSBYE) using a whole-genome DNA chip approach. We identified a total of 225 differentially expressed genes (≥4-fold; P < 0.05) during the transition to the LTS phase in TSBYE. Genes related to cell envelope structure, energy metabolism, and transport were most significantly upregulated in the LTS phase. The upregulation of compatible solute transporters may lead to the accumulation of cellular solutes, lowering intracellular water activity and thus increasing bacterial stress resistance during the transition to the LTS phase. The downregulation of genes associated with protein synthesis may indicate a status of metabolic dormancy of the LTS cells. The transcriptomic profiles of resuscitated LTS cells in fresh TSBYE resembled those of log-phase cells (r=0.94), as the LTS cells rapidly resume metabolic activities and transit back to log phase with decreased baro- and thermotolerance.

Ultraviolet-induced oxidation of ascorbic acid in a model juice system: identification of degradation products.

Degradation products of ultraviolet (UV-C, 254 nm) treated ascorbic acid (AA) are reported. Analysis by high-performance liquid chromatography-mass spectroscopy (HPLC-MS) conducted in a 0.5% malic acid model juice system (pH 3.3) demonstrated increased degradation of AA above untreated controls with concomitant increases in dehydroascorbic acid (DHA) and 2,3-diketogulonic acid (DKGA) levels. Electron spin resonance (ESR) spectroscopy studies, conducted in phosphate buffer (pH 7.0) to increase detection sensitivity, demonstrated that ascorbyl radical (AA•) formation occurs simultaneously with AA degradation. Consistent with a previous study in which UV treatments were shown to accelerate dark storage degradation, AA• radicals continued to form for up to 200 min after an initial UV treatment. Results from this study suggest that the mechanism for UV-induced degradation is the same as the general mechanism for metal-catalyzed oxidation of AA in juice.

Ascorbic acid degradation in a model apple juice system and in apple juice during ultraviolet processing and storage.

UNLABELLED: Ultraviolet radiation induced degradation of ascorbic acid in a model apple juice system and in apple juice was studied using a collimated beam batch UV reactor. In the model system, ascorbic acid degradation was more rapid at higher dose levels and the reaction accelerated with increasing exposure time. Ascorbic acid degradation significantly (P < 0.05) increased as the pH was raised from 2.4 to 5.5, although no difference was observed between 2.4 and 3.3. Increasing malic acid concentration between 0.1 and 1%, increased ascorbic acid degradation (P < 0.05) although there was no difference between 0.5 and 1.0%. Solution absorbance, varied by addition of tannic acid, decreased ascorbic acid degradation with increasing concentration due to absorption of UV radiation. Fructose at levels found in apple juice significantly increased ascorbic acid degradation while glucose and sucrose did not. Factors identified that accelerate ascorbic acid degradation may at least partially explain why ascorbic acid degradation occurred more rapidly in UV-treated apple juice than in the 0.5% malic acid model system. Ascorbic acid degradation continued after UV treatments during dark storage. Storage decreases were faster at higher initial UV dose levels and higher storage temperature. PRACTICAL APPLICATION: The present study shows the effect of UV processing on ascorbic acid, a key vitamin found in many fruit juices. Process developers and researchers can use this study as a model for designing experiments to identify factors that influence the stability of vitamin C and other bioactive compounds during UV processing.

Changes in barotolerance, thermotolerance, and cellular morphology throughout the life cycle of Listeria monocytogenes.

Changes in barotolerance, thermotolerance, and cellular morphology throughout the life cycle of Listeria monocytogenes were investigated. For part 1 of this analysis, L. monocytogenes ATCC 19115 was grown to log, stationary, death, and long-term-survival phases at 35 degrees C in tryptic soy broth with yeast extract (TSBYE). Cells were diluted in whole milk that had been subjected to ultrahigh temperatures (UHT whole milk) and then high-pressure processed (HPP) at 400 MPa for 180 s or thermally processed at 62.8 degrees C for 30 s. As cells transitioned from the log to the long-term-survival phase, the D(400 MPa) and D(62.8 degrees C) values increased 10- and 19-fold, respectively. Cells decreased in size as they transitioned from the log to the long-term-survival phase. Rod-shaped cells transitioned to cocci as they entered the late-death and long-term-survival phases. L. monocytogenes strains F5069 and Scott A showed similar results. For part 2 of the analysis, cells in long-term-survival phase were centrifuged, suspended in fresh TSBYE, and incubated at 35 degrees C. As cells transitioned from the long-term-survival phase to log and the stationary phase, they increased in size and log reductions increased following HPP or heat treatment. In part 3 of this analysis, cells in long-term-survival phase were centrifuged, suspended in UHT whole milk, and incubated at 4 degrees C. After HPP or heat treatment, similar results were observed as for part 2. We hypothesize that cells of L. monocytogenes enter a dormant, long-term-survival phase and become more barotolerant and thermotolerant due to cytoplasmic condensation when they transition from rods to cocci. Further research is needed to test this hypothesis and to determine the practical significance of these findings.

Effect of water activity on inactivation of Listeria monocytogenes and lactate dehydrogenase during high pressure processing.

The aim of this study was to investigate the effect of water activity (aw) on the inactivation of Listeria monocytogenes and lactate dehydrogenase (LDH) during high pressure processing (HPP). For microbial inactivation lyophilized cells of L. monocytogenes 19,115 were left dry or were suspended in 10 ml of 0.1% peptone water, 10 ml of glycerol, or mixtures of glycerol and peptone water. All samples of various aws were high pressure (HP) processed at ambient temperature at 600 MPa for 300 s. Following HPP, samples were serially diluted in 0.1% peptone and spread-plated on Tryptic Soy agar supplemented with Yeast Extract. For enzyme inactivation, 4.2 mg of lyophilized LDH was suspended in 2 ml of 100 mM phosphate buffer (pH 7.4), 2 ml of peptone water or glycerol, or in 2 ml mixtures of glycerol and peptone water. A lyophilized sample with no added liquid was also included. All enzyme samples were subjected to HPP as described above. After HPP, LDH was diluted to 0.28 microg/ml in 100 mM phosphate buffer (pH 7.4). LDH activity was assessed by measuring the change in concentration of beta-NADH as a function of time. Dynamic light scattering analysis (DLS) was performed to examine the size distribution, polydispersity, and hydrodynamic radius of LDH before and after HPP. No significant difference in CFU/g was observed between lyophilized cells not subjected to HPP and lyophilized cells subjected to 600 MPa for 300 s (P<0.05). However, lyophilized cells that were suspended in 100% to 60% peptone water showed a approximately 7.5-log(10) reduction when subjected to HPP. Survival of L. monocytogenes following HPP significantly increased (P<0.05) when the peptone water concentration was decreased below 60% (aw approximately 0.8). DLS results revealed that LDH suspended in buffer underwent aggregation following HPP (600 MPa, 300 s). Inactivation rate constants obtained using a first-order kinetic model indicated that untreated and HP processed lyophilized LDH had similar activities. When LDH was subject to HPP in solutions containing glycerol, enzyme activity decreased as the water content increased (r2=0.95). Lyophilization completely protected L. monocytogenes and LDH from inactivation by high pressure. Furthermore, enzyme activity and cell survival increased as water activity was decreased. We postulate low aw results in protein stabilization, which prevents protein denaturation and cell death during HPP.

Heat shock induces barotolerance in Listeria monocytogenes.

The aim of this study was to investigate the effect of heat shock on the resistance of Listeria monocytogenes to high pressure processing (HPP). L. monocytogenes ATCC 19115 was grown to stationary phase at 15 degrees C and inoculated into whole ultrahigh-temperature milk at approximately 10(7) CFU/ml. Milk samples (5 ml) were placed into plastic transfer pipettes, which were heat sealed and then heated in a water bath at 48 degrees C for 10 min. Immediately after heat shock, the milk was cooled in water (20 degrees C) for 25 min and then placed on ice. The samples were high pressure processed at ambient temperature (approximately 23 degrees C) at 400 MPa for various times up to 150 s. Following HPP, the samples were spread plated on tryptic soy agar supplemented with yeast extract. Heat shock significantly increased the D400 MPa-value of L. monocytogenes from 35 s in non-heat-shocked cells to 127 s in heat-shocked cells (P < 0.05). Addition of chloramphenicol before heat shock eliminated the protective effect of heat shock (P < 0.05). Heat shock for 5, 10, 15, or 30 min at 48 degrees C resulted in maximal barotolerance (P < 0.05); increasing the time to 60 min significantly decreased survival compared with that at 5, 10, 15, or 30 min (P < 0.05). These results indicate that prior heat shock significantly increases the barotolerance of L. monocytogenes and that de novo protein synthesis during heat shock is required for this enhanced barotolerance.

Effect of high-pressure processing on activity and structure of alkaline phosphatase and lactate dehydrogenase in buffer and milk.

Changes in the activity and structure of alkaline phosphatase (ALP) and L-lactate dehydrogenase (LDH) were investigated after high pressure processing (HPP). HPP treatments (206-620 MPa for 6 and 12 min) were applied to ALP and LDH prepared in buffer, fat-free milk, and 2% fat milk. Enzyme activities were measured using enzymatic assays, and changes in structure were investigated using far-ultraviolet circular dichroism (CD) spectroscopy and dynamic light scattetering (DLS). Kinetic data indicated that the activity of ALP was not affected after 6 min of pressure treatments (206-620 MPa), regardless of the medium in which the enzyme was prepared. Increasing the processing time to 12 min did significantly reduce the activity of ALP at 620 MPa (P < 0.001). However, even the lowest HPP treatment of 206 MPa induced a reduction in LDH activity, and the course of reduction increased with HPP treatment until complete inactivation at 482, 515, and 620 MPa. CD data demonstrated a partial change in the secondary structure of ALP at 620 MPa, whereas the structure of LDH showed gradual denaturation after exposure at 206 MPa for 6 min, leading to a random coil structure at both 515 and 620 MPa. DLS results indicated aggregation of ALP only at HPP treatment of 206 MPa and not above and enzyme precipitation as well as aggregation at 345, 415, 482, and 515 MPa. The loss of LDH activity with increasing pressure and time treatment was due to the combined effects of denaturation and aggregation.

The effects of growth temperature and growth phase on the inactivation of Listeria monocytogenes in whole milk subject to high pressure processing.

The aim of this study was to explore the effect of a wide range of growth temperatures, growth phases and plating media on the inactivation of Listeria monocytogenes by high pressure processing (HPP). In part one, L. monocytogenes was grown to mid-stationary phase at 4, 15, 25, 35 or 43 degrees C, inoculated into whole UHT milk at approximately 10(7) CFU/ml and high pressure processed at 400 MPa at room temperature (20-25 degrees C). Afterward, the HPP milk was plated on Tryptic Soy Yeast Extract Agar (TSYEA) and Modified Oxford Agar (MOX) to determine the degree of injury. For part two, cells were grown to mid-exponential, late-exponential or mid-stationary phase at 15 or 43 degrees C and processed in the same way. Time to reach a 5-log reduction was determined and data were analysed by ANOVA. The results from part one showed that both growth temperature and plating medium had a significant effect (P < 0.001) on the inactivation of stationary phase L. monocytogenes by HPP. Tukey's pairwise comparisons revealed that the effects of all temperatures, except 35 and 43 degrees C, were significantly different (P < 0.05). Cells grown at 15 degrees C were most sensitive to HPP, followed by cells grown at 4, 25 or 35 degrees C, with cells grown at 43 degrees C appearing to be the most resistant. Inactivation of cells grown at 4, 15 or 25 degrees C followed first order kinetics, whereas cells grown at 35 or 43 degrees C displayed non-linear inactivation kinetics due to tailing. In part two, both growth phase and plating medium had significant effects on the inactivation (P < or = 0.001) of L. monocytogenes by HPP. Cells grown at 15 degrees C to mid-stationary phase were the most pressure-resistant when tested on both media, and were significantly more resistant (P < 0.05) than cells grown at the same temperature to the other two phases of growth. There was no significant difference between mid- and late-exponential phase cells grown at 15 degrees C. When cells were grown at 43 degrees C, mid-exponential phase cells were significantly more sensitive (P < 0.05) than either late-exponential or mid-stationary phase cells, with no difference between late-exponential or mid-stationary phase cells. It was postulated that membrane composition, stationary phase proteins and/or stress proteins may affect pressure resistance.

Viability Loss of Salmonella Species, Staphylococcus aureus , and Listeria monocytogenes in Complex Foods Heated by Microwave Energy.

The chemical composition of five foods (UHT milk, beef broth, pudding, cream sauce, and liquid whole egg) was examined to determine factors important in achieving uniform temperatures within foods heated in a 700 W microwave oven. Proximate analyses were performed on all food systems to relate their chemical composition to temperatures and to destruction of microwave-heated Salmonella species, Listeria monocytogenes Scott A and V7 and Staphylococcus aureus ATCC 25923. Microwave heating times were chosen such that the final mixed mean temperature achieved by systems was 60°C for Salmonella spp. and L. monocytogenes , and 65°C for S. aureus . The amount of destruction of Salmonella spp. varied from 3.17 log CFU/ml in UHT milk to 0.44 log CFU/ml in beef broth. L. monocytogenes strains incurred the greatest amount of destruction in pudding (2.39 log CFU/g), while the least amount of destruction was observed in cream sauce (1.63 log CFU/ml). There were no significant differences in the amount of destruction of S. aureus heated in the five foods. The pH and aw of these foods did not affect survival of thermally stressed Salmonella , L. monocytogenes , or S. aureus cells. Of the food components examined, sodium content was the primary influence on the uniformity of temperatures achieved within foods, and, in turn, on the survival of bacteria.

Destruction of Salmonella Species Heated in Aqueous Salt Solutions by Microwave Energy.

Concentrations of 0.1, 0.5, 0.75, 1.0, and 1.25% wt/vol (0.017, 0.085, 0.13, 0.17, and 0.21 M, respectively) sodium chloride were added to 0.3 mM phosphate buffer, pH 6.8, and heated by microwave energy to study the relationship between salt concentration, temperatures achieved, and microbial destruction. Heating Salmonella spp. in saline solutions for a constant time (45 s) or to a constant final temperature (60°C) was also investigated. Fiberoptic thermometry was employed to obtain a temperature profile at specific sites within the solution. When heating for a constant time period, a minimum concentration of 0.75% wt/vol NaCl was necessary to afford Salmonella spp. significantly (P = 0.05) greater protection than the phosphate buffer control. Sodium, potassium, magnesium, and calcium chloride (1.0% wt/vol) in 0.3 mM phosphate buffer were also inoculated with a mixture of Salmonella spp. and heated by microwave energy. Of the salts examined, solutions containing NaCl consistently achieved the highest final surface temperature and largest temperature gradient from the surface to the bottom of the container. The amount of destruction of Salmonella spp. heated to a mixed mean final temperature of 60°C in buffer containing 1.0% wt/vol concentrations of NaCl, KCl, CaCl2, and MgCl2 was 56.4, 71.2, 72.8, and 88.7%, respectively. No relationship was found between the valency of the cation used and final temperatures achieved.

Survival of Salmonella Species Heated by Microwave Energy in a Liquid Menstruum Containing Food Components.

Common food constituents were examined to determine the protective influence they may exert on a mixture of Salmonella species heated by microwave energy. A model system was developed, wherein combinations of sucrose, sodium chloride, caseinate and corn oil, all at 1.0% (w/v) concentrations, were added to 0.3 mM phosphate buffer (pH 6.8), producing a total volume of 100 ml. Salmonella -inoculated solutions were heated for 47 sec in a 700 watt microwave oven. Temperatures at localized areas were monitored by fluorescent fiberoptic (fluoroptic) thermometry as the solutions heated, and by mercury thermometer after heating. The mean final temperature achieved for the various combinations of solutes was not significantly different and varied only by 4°C as measured by a mercury thermometer. However, solutions containing NaCl afforded the Salmonella spp. up to 170 times the protection of the phosphate buffer control. This protective effect occurred although the mean final temperature of NaCl-containing solutions would be as high as those solutions lacking salt. Fluoroptic thermometry temperature profiles revealed that surface temperatures were higher when NaCl was present in solution, due to decreased depth of penetration of the microwaves. This re-sulted in decreased temperatures at greater depths. Of the solutes evaluated, only NaCl appears to confer a significant protective effect.