Norman N. Potter

Effects of Organic Acids on Thermal Inactivation of Bacillus stearothermophilus and Bacillus coagulans Spores in Frankfurter Emulsion Slurry

Malic, acetic, citric, lactic and hydrochloric acids were compared for their effects on thermal inactivation of Bacillus stearothermophilus and Bacillus coagulans spores in a frankfurter emulsion slurry adjusted to specific pH values. For B. stearothermophilus at 121°C and pH 5.2 no differences in thermal death rate constants attributable to the acids were noted, but at pH 4.6 a greater inactivation rate was obtained using lactic, citric or acetic acids than malic or hydrochloric acids. For B. coagulans at 110 or 105 °C and pH 4.5, there was no difference in spore inactivation noted between the five acids, but at these same temperatures and pH 4.2 a faster inactivation rate of this organism was achieved with acetic and lactic acids. Holding the inoculated meat slurry in the acidified state at 4°C for 70 h before heating decreased the thermal resistance of B. coagulans spores at pH 4.5.

Growth of Aeromonas hydrophila and Pseudomonas tragic on Mince and Surimis Made from Atlantic Pollock and Stored Under Air or Modified Atmosphere

Mince, salt-added surimi, and low-salt surimi prepared from Atlantic pollock had different protein, NaCl, and carbohydrate levels. Samples of these products were steamed, cooled, and coinoculated with Aeromonas hydrophila and Pseudomonas fragi . Jars containing modified atmosphere (MA)-stored samples were flushed so that initial headspace composition was 51% N2, 13% O2, and 36% CO2. These, and samples under air, were incubated at 5 and 13°C. Headspace composition of sample jars determined throughout storage at 5°C indicated that greater growth occurred on air-stored mince and low-salt surimi than on air-stored salt-added surimi, or on MA-stored samples. Colony counts of both species were appreciably reduced by the MA storage. In addition, the high salt level of salt-added surimi decreased growth of both species, although A. hydrophila was affected more than P. fragi . Results indicated that A. hydrophila was quite capable of competing with P. fragi on mince and low-salt surimi stored under air or MA at both 5 and 13°C.

Survival of Campylobacter jejuni at Different Temperatures in Broth, Beef, Chicken and Cod Supplemented with Sodium Chloride

Growth and survival of Campylobacter jejuni strains ATCC 33250 and ATCC 29428 with NaCl levels ranging from 0 to 3% were studied in brucella broth at -18, 6, 10 and 42°C. Both strains grew under microaerobic conditions at 42°C with 0 to 1% NaCl, but counts declined sharply with higher salt levels. At -18°C, there was a large initial decline in counts with little subsequent change during 5 d of storage and no appreciable effect of NaCl. At 6 or 10°C, counts decreased with increasing NaCl concentration over the 5-d period, but the organism survived in substantial numbers even with 3% NaCl. Survival of C. jejuni in ground chicken, beef and cod with 0, 1 and 2% added NaCl was determined at -18, 6 and 10°C over a 5-d period. At -18°C, survival of C. jejuni was similar in all three types of flesh, whether raw or previously cooked, and C. jejuni counts during frozen storage were not affected by the level of NaCl. Survival patterns of C. jejuni at 6 and 10°C in raw and cooked chicken and beef were very similar. Statistically significant decreases in counts occurred with increasing NaCl concentration, but the differences were slight. Substantial decreases in counts occurred in refrigerated raw cod, but less in the cooked fish. In the raw cod, counts decreased significantly with increasing NaCl concentration and the decline was more pronounced during storage at 10°C than at 6°C.

Microbial Growth in Surimi and Mince made from Atlantic Pollock

Surimi preparation from minced Atlantic pollock decreased protein, fat, and NaCl levels and increased moisture and carbohydrate levels of the mince. Aerobic plate counts and psychrotrophic counts of samples from three trials stored at 5 and 13°C were initially similar, but reached higher levels in surimi with time. The pH of surimi tended to decrease relative to that of mince during storage at both temperatures. Careful handling and storage are equally important for both products if micro-biological quality is to be maintained.

Survival and Growth of Aeromonas hydrophila, Vibrio parahaemolyticus, and Staphylococcus aureus on Cooked Mince and Surimis made from Atlantic Pollock

Mince, salt-added surimi, and low-salt surimi prepared from Atlantic pollock had significantly (p<0.01) different protein and NaCl levels. These three products were steamed 16 min, cooled and inoculated with A. hydrophila , V. parahaemolyticus , or S. aureus . Samples inoculated with A. hydrophila were stored at 5, 13, and 25°C; all other samples were stored at 5 and 25°C. A. hydrophila grew well on the mince and low-salt surimi but not on the salt-added surimi stored for 5 d at 5°C, 36 h at 13°C and 27 h at 25°C. Populations on the mince and low-salt surimi increased log10 3.0 CFU/g at 5 and 13°C, and log10 5.0 CFU/g at 25°C. V. parahaemotyticus counts decreased slightly on all three products during 48 h storage at 5°C. At 25°C V. parahaemolyticus counts initially decreased on all three products but by 27 h rose at least log10 2.0 MPN/g on the mince and salt-added surimi. Counts on the low-salt surimi rose <log10 1.0 MPN/g after the initial decrease. S. aureus counts did not increase on any of the products stored for 5 d at 5°C. During 27 h storage at 25°C, S. aureus counts were consistently at least log10 0.9 CFU/g higher on the surimis than on the mince, with highest counts on the low-salt-surimi. These results indicate that compositional differences, including NaCl levels, between surimis and fish flesh used in their preparation can affect pathogen growth.

Growth and Death of Selected Microorganisms in Ultrafiltered Milk

Studies were made to compare the growth and death of Staphylococcus aureus , Streptococcus faecalis and Escherichia coli in skim milk concentrated by ultrafiltration to that in unconcentrated skim milk. Skim milk was volume concentrated to 2× in laboratory-scale stirred UF cells. Behavior of the organisms was analyzed in four inoculated milk samples: 2× retentate, 1× water-diluted retentate, milk equivalent (retentate plus permeate) and unconcentrated skim milk. Growth of each organism and of total aerobes did not vary in the four milk samples at either 7 or 13°C. For S. faecalis and E. coli , D-values for samples heated to 62.7°C did not significantly differ in the four milk samples (p>0.01). The D-value of S. aureus in water-diluted retentate was slightly but significantly lower than those in the other three milk samples (p<0.01), possibly due to the lowered lactose level in this sample.

Diluents and the Enumeration of Stressed Campylobacter jejuni

Significant discrepancies in calculated counts of Campylobacter jejuni strains ATCC 33250 and 29428 were observed between undiluted samples and samples diluted in 0.1% peptone water when cultures had been incubated at 42°C under micro-aerobic conditions in brucella broth containing 2% NaCl. The influence of 0.1% peptone water, brucella broth and phosphate buffer as diluents for enumeration of C. jejuni stored at 6 and -18°C in brucella broth with 0.5 and 2% NaCl was studied. The three diluents were equally effective for the recovery of C. jejuni from refrigerated broth containing 0.5% NaCl. However, when the organism had been refrigerated in the broth with 2% NaCl or frozen in the broth with either level of salt, dilution with brucella broth produced significantly higher counts than dilution with 0.1% peptone water or phosphate buffer.

Effects of Potassium Sorbate on Normal Flora and on Staphylococcus aureus Added to Minced Cod

Minced cod and pasteurized minced cod, with and without 0.5% potassium sorbate, were subjected to abusive storage temperatures of 7 and 15°C. Staphylococcus aureus FRI 100 was inoculated into the cod before storage. Total aerobic plate counts (20 and 35°C), pH changes, S. aureus counts and the presence of thermonuclease were monitored throughout the studies. With the unpasteurized minced cod, potassium sorbate caused slightly lower aerobic plate counts (at 20 and 35°C) in the 7°C study over an 11-day storage period. Psychrotrophic organisms were inhibited to a slightly greater extent than were mesophilic organisms. Inoculated S. aureus was quickly outgrown by the normal microflora without or with sorbate. Similar results were obtained at the still more abusive temperature of 15°C over a storage period of 5 d, but the inhibitory effect of sorbate was less evident. Pasteurized minced cod, inoculated with S. aureus and stored at 15°C, showed a considerable difference in growth of S. aureus with and without sorbate. Potassium sorbate resulted in a markedly slower rate of growth of the pathogen and a substantial delay of several days in production of detectable levels of thermonuclease. This delay in nuclease production is indicative of a similar delay in enterotoxin production.

Effects of Nitrite and Sorbate on Bacterial Populations in Frankfurters and Thuringer Cervelat

Four batches of frankfurter emulsion were prepared with no additives, 0.26% potassium sorbate, 140 ppm of sodium nitrite plus 550 ppm of sodium isoascorbate, and 40 ppm sodium nitrite plus 0.26% potassium sorbate plus 550 ppm sodium isoascorbate, and processed. Five batches of thuringer cervelat emulsion were prepared with no additives, 0.26% potassium sorbate, 156 ppm of sodium nitrite, 78 ppm of sodium nitrite plus 0.26% potassium sorbate, and 78 ppm of sodium nitrite plus 156 ppm of sodium nitrate, and processed. The finished products were stored aerobically and bacterial growth patterns were monitored. At 20 C, presence of sodium nitrite and potassium sorbate, separately or together, in the frankfurters were without appreciable effect on total aerobic, total anaerobic, gram-positive, and lactobacillus-pediococcus counts, although at 7 to 9 C these additives moderately lowered bacterial counts. Bacterial counts of the thuringer cervelat were not affected by sodium nitrite, potassium sorbate or sodium nitrate at either temperature. Staphylococcus aureus and Clostridium perfringens were inoculated into all emulsions before further processing to determine if the modified cures, or possible changes in normal microflora, influenced these pathogens. S. aureus was reduced to below detectable levels after heat-processing in all systems. C. perfringens survived processing and then underwent equally slow death in all stored frankfurter emulsions, and stabilization of counts in thuringer cervelat emulsions. Results indicate that the modified cures did not appreciably alter the natural microflora of these products, nor survival of added pathogens.

Effects of Storage in Recovery Media on Sublethally Heated Polio and Echo Viruses

In vitro renaturation of heat-denatured virus particles was studied using poliovirus type 1, strain CHAT and echovirus type 6, strain DAmori. Six recovery media were chosen to simulate heat-processed foods and mildly processed or raw foods and to represent a range of compositions. Virus was suspended in four heating media and processed to various degrees of inactivation. After cooling, the stressed virus was resuspended in the recovery media and incubated at 30 C for up to 7 days. Titers were determined at intervals from 3.5 h to 7 days in African Green Monkey kidney cells. The suspending medium influenced poliovirus susceptibility to heat treatments. The acidic medium produced a higher degree of inactivation in shorter time than the nutrient and ion-rich media. Echovirus was less affected than poliovirus by medium composition. No increase in titer of either virus occurred in any of the recovery media, indicating absence of renaturation mechanisms. Instead, independent of medium composition, virus titers slowly decreased over the 7-day storage period.