Laura L. Zaika

Thermal destruction ofListeria monocytogenes in liver sausage slurry

Abstract Thermal destruction ofListeria monocytogenes was determined in a liver sausage slurry (1:1, liver sausage batter and water) using a submerged ampule technique.D-values forL. monocytogenes Scott A grown at 37°C were 8·9 min at 57·2°C, 2·4 min at 60·0°C, and 1·1 min at 62·8°C (Z=6·2°C) based on analysis of the linear portion of the survivor curves.D-values of 6·6, 1·6, and 0·4 min (Z=4·65°C) were obtained when the data were analyzed using non-linear techniques.L. monocytogenes strain V7 (D60=1.0min) was more thermosensitive than Scott A (D60=1·6min) or HO-VJ-S (D60=1·6min). When Scott A was grown at 19°C, there was a decrease in thermal resistance (D60=0·8min). These data indicate thatL. monocytogenes has a thermal resistance in liver sausage comparable to that observed in other food systems.

Review of compounds affecting the biosynthesis or bioregulation of aflatoxins

We have attempted to review the literature dealing with compounds that have been tested for their ability to inhibit growth and/or aflatoxin production by Aspergillus flavus and A. parasiticus . Although the list presented is by no means exhaustive, it serves as an indication of the type of research that has been carried out to date. A number of compounds and substances have been found that effectively inhibit fungal growth and aflatoxin production, while others have stimulatory properties. Unfortunately, most of these assessments have only dealt with fungal growth and/or aflatoxin production, and relatively few studies have attempted to identify possible mechanisms of action. Further research into the means by which other compounds influence aflatoxin synthesis is warranted and potentially could be highly beneficial to expand our understanding of mycotoxigenesis. Such studies are likely to yield knowledge that would lead to isolation of additional intermediate compounds of the pathway leading to aflatoxins, identification of key bioregulatory loci controlling aflatoxin synthesis and development of basic knowledge that would provide insights into new strategies for controlling aflatoxin formation in foods and feeds.

Model for Aerobic Growth of Shigella flexneri Under Various Conditions of Temperature, pH, Sodium Chloride and Sodium Nitrite Concentrations

A modified factorial design was used to measure the effects and interactions of temperature (10 to 37°C), pH (5.5 to 7.5), sodium chloride (0.5 to 5.0%), and sodium nitrite (0 to 1000 ppm) on the aerobic growth kinetics of Shigella flexneri in brain heart infusion broth. A total of 592 cultures were analyzed, with growth curves being generated using the Gompertz equation. A quadratic model for growth of S. flexneri in terms of temperature, pH, sodium chloride, and sodium nitrite concentrations was obtained by response surface analysis. This model provides an estimate of bacterial growth in response to any combination of the variables studied within the specified ranges. Estimates obtained with the model compared favorably with growth of S. flexneri in milk.

Effects of Some Spices on Acid Production by Starter Cultures

Ginger, red pepper, mustard, mace, cinnamon and clove were examined to determine their effects on growth of and acid production by a starter culture containing Lactobacillus plantarum and Pediococcus cerevisiae in a liquid medium. At 4, 8, and 12 g/l levels all spices except clove stimulated acid production by the starter bacteria but did not stimulate increases in bacterial population. Clove was inhibitory to the starter bacteria at and above the 4 g/l level, but low concentrations (0.5 - 2.0 g/l) stimulated acid production. High concentrations of cinnamon (8 and 12 g/l) delayed acid production, but bacterial counts were similar to those of the control.

Influence of NaCl content and cooling rate on outgrowth of Clostridium perfringens spores in cooked ham and beef

The effect of NaCl concentration and cooling rate on the ability of Clostridium perfringens to grow from spore inocula was studied with the use of a process that simulates the industrial cooking and cooling of smoked boneless ham and beef roasts. NaCl was added to ground cooked hams A and B (which were commercially obtained) to obtain levels of 2.4, 3.1, 3.6, and 4.1% (wt/wt) and 2.8, 3.3, 3.8, and 4.3% (wt/wt), respectively, and to raw ground beef to obtain levels of 0, 1, 2, 3, and 4% (wt/wt). Ham C, a specially formulated, commercially prepared product, was supplemented with NaCl to obtain levels of 2.0, 2.5, 3.0, and 3.5%. The samples were inoculated with a three-strain mixture of C. perfringens spores to obtain concentrations of ca. 3 log10 CFU/g. Portions of meat (5 g each) were spread into thin layers (1 to 2 mm) in plastic bags, vacuum packaged, and stored at -40 degrees C. Thawed samples were heated at 75 degrees C for 20 min and subsequently cooled in a programmed water bath from 54.4 to < or = 8.5 degrees C in 15, 18, or 21 h. For the enumeration of C. perfringens, samples were plated on tryptose-sulfite-cycloserine agar and incubated in an anaerobic chamber at 37 degrees C for 48 h. Population densities for cooked ham and beef increased as cooling time increased, and NaCl exerted a strong inhibitory effect on the germination and outgrowth of C. perfringens. For beef, while 3% NaCl completely arrested growth, pathogen numbers increased by > or = 3, 5, and 5 log10 CFU/g in 15, 18, and 21 h, respectively, when the NaCl level was <2%. C. perfringens did not grow during cooling for 15, 18, or 21 h in ham samples containing > or = 3.1% NaCl. Results obtained in this study suggest that a 15-h cooling time for cooked ham, which is normally formulated to contain >2% NaCl, would yield an acceptable product (with an increase of <1 log10 CFU/g in the C. perfringens count); however, for beef containing <2% NaCl, C. perfringens populations may reach levels high enough to cause illness.

Growth kinetics and cell morphology of Listeria monocytogenes Scott A as affected by temperature, NaCl, and EDTA.

Growth kinetics and morphological characteristics of Listeria monocytogenes Scott A grown under stress conditions induced by increasing levels of NaCl and EDTA were studied as a function of temperature. L. monocytogenes Scott A was inoculated into brain heart infusion broth (pH 6) at 19, 28, 37, and 42 degrees C. Test cultures contained NaCl (at concentrations of 4.5, 6.0, and 7.5%) or EDTA (at concentrations of 0.1, 0.2, and 0.3 mM); control cultures contained 0.5% NaCl. Growth curves were fitted from plate count data by the Gompertz equation, and growth kinetics parameters were derived. Stationary-phase cells were examined by scanning and transmission electron microscopy. Generation times (GTs) and lag phase duration times (LPDs) increased as additive levels were increased. The bacterium grew at all NaCl levels. At 37 and 42 degrees C, growth was slow in media containing 7.5% NaCl, and no growth occurred in media containing 0.3 mM EDTA. Temperature was a major factor in certain stress conditions that led to cell elongation and loss of flagella. Cells in control media at 28 degrees C grew as short rods (0.5 by 1.0 to 2.0 microm), while at 42 degrees C most cells were 4 to 10 times as long. Higher levels of NaCl at higher temperatures resulted in longer and thicker cells. At 28 degrees C, 0.1 mM EDTA had little effect on growth kinetics and morphology; however, 0.3 mM EDTA caused a sixfold increase in GT and LPD and loss of flagellae, with most cells being two to six times as long as normal. Cell length did not correlate with growth kinetics. The results of this study suggest that the effect of altered morphological characteristics of L. monocytogenes cells grown under stress on the virulence and subsequent survival of these cells should be investigated.

Effect of Sodium Polyphosphates on Growth of Listeria monocytogenes

Evaluation of sodium polyphosphates (SPP), multifunctional food additives, indicated that only the higher polymers sodaphos, hexaphos, and glass H (average chain length = 6, 13, and 21, respectively) significantly inhibited growth of Listeria monocytogenes . The effect of the three compounds (0-2%) on growth of L. monocytogenes was studied in brain heart infusion + 0.3% glucose medium, pH 6.0, at 28, 19, 10, and 5°C. The organism grew well under all test conditions in the absence of SPP. Hexaphos and glass H were considerably more inhibitory than sodaphos. The most pronounced effect of SPP was on lag times, which increased with increasing SPP concentration and decreasing temperature. At 10°C, addition of 0.3% hexaphos or glass H increased lag time from 22 h to 197 and 186 h, respectively, and no growth was observed after 40 d in the presence of 2.0% of these compounds. Addition of 2.0% NaCl increased the inhibitory effect of SPP but had little effect on growth in SPP-free media. Results suggest that high molecular weight SPP may be useful in controlling the growth of L. monocytogenes , particularly at low temperatures and in combination with NaCl.

Effect of Sodium Chloride, pH and Temperature on Growth of Shigella flexneri

A systematic study of the effect of sodium chloride (0.5, 2.0, 3.5, 5.0%), pH (7.5, 6.5, 5.5), and temperature (37, 28, 19, 10°C) on growth of Shigella flexneri is reported for the first time, using a factorial design. Experiments were done using Brain-Heart Infusion media inoculated to contain 1 × 103 cfu/ml and incubated on rotary shakers (150 rpm). Growth curves were plotted from the experimental data by means of the Gompertz equation, and growth rates, lag times, generation times, and maximum populations were derived for all variable combinations. Results indicated that the three variables interact to affect the growth of S. flexneri , and combinations of low temperature, low pH, and high sodium chloride contents are strongly inhibitory.

Effect of organic acids and temperature on survival of Shigella flexneri in broth at pH 4.

The survival of bacterial pathogens in acidified foods depends not only on the hydrogen ion concentration, but also on the type of acid and the storage temperature. Shigella flexneri is a foodborne pathogen that is acid tolerant. The survival of S. flexneri 5348 in brain heart infusion broth supplemented with 0.04 M acetic, citric, lactic, malic, or tartaric acid and adjusted to pH 4 with HCI or NaOH was studied. The control medium was brain heart infusion broth adjusted to pH 4 with HCI. Stationary-phase cells were inoculated into media at initial populations of 6 to 7 log10 CFU/ml and incubated at 4, 19, 28, and 37 degrees C. A two-phase linear inactivation model was applied to plate count data to derive lag times (tL) and slopes of the curves, from which D-values and time required for a 4-log10 decrease in population (T4D) were calculated. In all cases, survival increased with decreasing temperature. For each acid, tL, the D-value, and T4D increased with decreasing temperature. All acids inhibited S. flexneri to some extent but to differing degrees as follows: lactic acid, acetic acid > citric acid, malic acid, tartaric acid > HCl. The T4D values for the control medium and for media containing acetic, citric, lactic, malic, and tartaric acids were 64, 47, 50, 34, 58, and 52 h, respectively, at 37 degrees C and 2,607, 1,498, 1,905, 1,346, 1,726, and 2,134 h, respectively, at 4 degrees C. The results of this study indicate that organic acids may aid in the inactivation of Shigella. However, these data also suggest that foods stored at or below room temperature containing low levels (< 1%) of acids could cause illness if contaminated with Shigella.

Effect of sodium nitrite on growth of Shigella flexneri

A partial factorial design study of the effect of NaNO2 (0, 100, 200, 1000 ppm) in combination with NaCl (0.5, 2.5, 4.0%), pH (7.5, 6.5, 5.5), and temperature (37, 28, 19°C) on growth of Shigella flexneri is reported. Experiments were done aerobically in brain-heart infusion medium, using an inoculum of 1 × 103 CFU/ml. Growth curves were fitted from plate count data by the Gompertz equation; exponential growth rates, lag times, generation times, and maximum populations were derived for all variable combinations. In the absence of nitrite, the organism grew well under all test conditions at 37 and 28°C but did not grow at 19°C at pH 5.5 nor at pH 7.5 with 4% NaCl. Nitrite did not affect growth in media of pH 7.5 at 37 and 28°C. At pH 6.5 growth was inhibited by 1000 ppm NaNO2. The organism failed to grow at 19°C at all nitrite levels in the presence of 2.5 or 4.0% NaCl. The inhibitory effect of nitrite was much greater in media of pH 5.5 and increased with increasing salt levels. More inhibition was apparent at 28 than at 37°C. While lack of growth was used as a paradigm of the effect of nitrite on S. flexneri , nitrite also increased the lag and generation times and decreased the exponential growth rate. Results indicated that NaNO2 in combinations with low temperature, low pH, and high salt content can effectively inhibit the growth of S. flexneri .