Water Activity and Temperature Effects on Fungal Growth and Ochratoxin A Production by Ochratoxigenic Aspergillus carbonarius Isolated from Tunisian Grapes

ABSTRACT: This study investigated the effects of temperature (15 to 37 °C) and water activity (0.90 to 0.99) on the growth and production of ochratoxin A (OTA) by Aspergillus carbonarius cultured on synthetic nutrient medium (SNM) after 5 and 10 d of incubation. Total of 8 ochratoxigenic A. carbonarius, isolated from vineyards located in different regions of Tunisia, were used. Growth data were modeled by the flexible model of Baranyi and growth rates at each set of conditions were obtained. For both growth and OTA production, optimal water activity was 0.99; however, optimal temperature varied. The optimal temperature for growth was 30 °C. At 37 °C, the growth rate decreased significantly (P < 0.05). Maximum toxin production occurred at temperatures in the range of 15 to 25 °C with the optimum one depending on the isolate tested. Significant amounts of OTA were produced after only 5 d of incubation. Our results showed that A. carbonarius isolated from Tunisian grapes behave as those from European and Australian grapes, as reported in the literature, although some differences in trends for growth and OTA production were observed.     

The Use of an Extracellular Ferulic Acid Esterase from Lactobacillus acidophilus K1 for the Release of Phenolic Acids During Mashing

A purified extracellular ferulic acid esterase from Lactobacillus acidophilus K1 was successfully used during mashing for the release of free phenolic acids into sweet wort. The enzyme was produced in bioreactors and partially purified to obtain the monoenzyme preparation. Release of free ferulic and vanillic acid into the wort at 52°C (with the use of 4.09–14.60 enzyme activity units/L of the mash) and ferulic acid at 62°C (14.60 units/L) was observed. Free p‐OH‐benzoic and syringic acids were effectively released at 26°C at each enzyme concentration used. Free p‐OH‐benzoic acid was also released by the enzyme (14.60 U/L) at 52°C‐74°C. Free protocatechuic acid was effectively hydrolyzed by the enzyme preparation (8.75 U/L and 14.60 U/L) at 26°C‐52°C. Free caffeic acid (effectively released at 26°C‐62°C) originated from chlorogenic acid. No p‐coumaric acid was released due to the action of bacterial esterase during mashing. Ferulic acid esterase from L. acidophilus K1 exhibited no ability to release free phenolic acids during mashing at 62°C or at 74°C due to its low thermostability. In conclusion, L. acidophilus K1 is an attractive source for the production of ferulic acid esterase, which can be useful for the release of antioxidant phenolic acids in the early stages of mashing.     

Effects of heat treatment and storage temperature on the biogenic amine content of straw mushroom (Volvariella volvacea)

The changes in the biogenic amines tryptamine, 2-phenylethylamine, putrescine, cadaverine, histamine and tyramine of straw mushroom Volvariella volvacea (Bull, ex Fr) Sing produced by heat treatment and during storage at different temperatures were studied. About 80% of the original content of these amines was lost during cooking. Commercial canned straw mushroom contained low amounts of amines. The amine contents of straw mushroom increased during storage at 4°C, with particularly notable increases in the concentrations of 2-phenylethylamine and tyramine after 5 days' storage. However, the levels of all amines increased more markedly during storage at 25°C, and the increases in the putrescine and cadaverine concentrations were much greater than those reached at 4°C.     

Factors affecting amine production in Streptococcus cremoris

S. cremoris produced highest growth and amine production in 24 h at 30°C. The optimum pH for the synthesis of histamine, tyramine and tryptamine by S. cremoris was observed at pH 5·5. Maximum production of different amines was obtained in the absence of NaCl. As the rate of incorporation of NaCl in the basal medium was increased beyond 0·5%, the level of amine production was considerably suppressed with concomitant decrease in cell growth.     

Interrelationship Between Water Activity, Temperature and 5.5 Percent Oxygen on Growth and Enterotoxin A Secretion by Staphylococcus aureus in Precooked Bacon

The limiting water activity (a_w) for the growth of S. aureus A-100 sealed in cans at an oxygen concentration of 5.5% was 0.87 at 37°C and 0.91% at 20°C, values intermediate to those obtained previously for aerobic and anaerobic storage. Maximum populations attained at 37°C and 20°C were equal and once achieved population levels declined more slowly at 20°C than at 37°C. Enterotoxin A production was detected for population levels exceeding 10⁶ CFU/g bacon and were 100 ng/g bacon at 37°C and 16 ng/g bacon at 20°C. It is suggested that while temperature and oxygen are important considerations, a_w, being the main parameter in controlling microbial growth, should be measured directly rather than depending on indirect measurements such as the moisture-salt ratio.     

A Study of Factors Affecting Growth and Recovery of Listeria monocytogenes Scott A

The growth parameters for Listeria monocytogenes Scott A at various temperatures, pH values, water activity values, and in different carbohydrates were assessed. Growth was seen from 4°C through 45°C and at pH values from 4.7 through 9.2. Generation times were calculated for each parameter at which growth was observed. Optimum growth was defined as shortest generation time. L. monocytogenes grew optimally over the temperature range 30–37°C. The optimum pH was 7.0. Growth was demonstrated in solutions of up to 39.4% sucrose (a_w of 0.92). Carbohydrate fermentation was variable. Further studies involved examination of growth at optimal water activities (0.97) with suboptimal temperatures and pH values, and at 7.0, with suboptimal temperatures and water activities. Growth was seen when these parameters were combined.     

Effects of Frozen Storage on Survival of Staphylococcus aureus and Enterotoxin Production in Precooked Tuna Meat

This study investigated the survival of Staphylococcus aureus in precooked tuna meat for producing canned products during frozen storage (?20 ± 2 °C) as well as its growth and enterotoxin production at 35 to 37 °C after the storage. Samples (50 ± 5 g) of precooked albacore (loin, chunk, and flake) and skipjack (chunk and flake) tuna were inoculated with 5 enterotoxin‐producing strains of S. aureus at a level of approximately 3.5 log CFU/g and individually packed in a vacuum bag after 3 h incubation at 35 to 37 °C. Vacuum‐packed samples were stored in a freezer (?20 ± 2 °C) for 4 wk. The frozen samples were then thawed in 37 °C circulating water for 2 h and incubated at 35 to 37 °C for 22 h. Populations of S. aureus in all precooked tuna samples decreased slightly (<0.7 log CFU/g) after 4 wk of storage at ?20 ± 2 °C, but increased rapidly once the samples were thawed and held at 35 to 37 °C. Total S. aureus counts in albacore and skipjack samples increased by greater than 3 log CFU/g after 6 and 8 h of exposure to 35 to 37 °C, respectively. All samples became spoiled after 10 h of exposure to 35 to 37 °C, while no enterotoxin was detected in any samples. However, enterotoxins were detected in albacore loin and other samples after 12 and 24 h of incubation at 35 to 37 °C, respectively. Frozen precooked tuna meat should be used for producing canned tuna within 6 to 8 h of thawing to avoid product spoilage and potential enterotoxin production by S. aureus in contaminated precooked tuna meat.     

Improvement of the quality and abatement of the biogenic amines of grass carp muscles by fermentation using mixed cultures

BACKGROUND: To improve the quality of processed grass carp (Ctenopharyngodon idellus) products and control the accumulation of hazardous substances therein, minced grass carp slices were salted for 6 h at room temperature and then inoculated with mixed starter cultures of Lactobacillus casei, Streptococcus lactis, Saccharomyces cerevisiae Hansen and Monascus anka and fermented for 12 h at 30 °C. The changes in some characteristics and biogenic amine contents of the fermented muscles were investigated. RESULTS: During the 12 h fermentation at 30 °C, muscles inoculated with mixed starter cultures showed a rapid decrease in pH from 6.0 to 5.1 and suppression of the growth of enterobacteria and pseudomonads. The fermented muscles exhibited better colour, appearance, flavour and overall acceptability than the control (P < 0.05). The changes in non‐protein nitrogen and free amino acid contents of the fermented muscles and in their sodium dodecyl sulfate polyacrylamide gel electrophoresis profiles indicated that severe hydrolysis of muscle proteins occurred during fermentation. The accumulation of biogenic amines in the muscles was efficiently reduced by fermentation with mixed starter cultures. CONCLUSION: Fermentation with mixed starter cultures of L. casei, S. lactis, S. cerevisiae Hansen and M. anka significantly improved the characteristics of grass carp muscles and controlled the accumulation of biogenic amines. Copyright © 2009 Society of Chemical Industry     

Mesopic fermentation contributes more to the formation of important flavor compounds and increased growth of Lactobacillus casei Zhang than does high temperature during milk fermentation and storage

Probiotic fermented milk is more and more popular due to their positive health associated properties. However, fermentation temperature and other process conditions may affect the growth and metabolism of probiotic strains, thereby affecting quality of the final products. In this study, the growth behaviors and metabolomic profiles of yogurts induced by Lactobacillus casei Zhang at fermentation termination (FT) and d 10 of storage (S10d) under different fermentation temperatures at 37°C (low) and 42°C (high) were analyzed and compared using liquid chromatography-mass spectrometry (MS)- and gas chromatography-MS-based metabolomics approaches. At 37°C, the growth of L. casei Zhang at FT and S10d was significantly increased, and the potential relationship between riboflavin, starch, and sucrose metabolism and growth of L. casei Zhang may be mutually promoting. Fermentation temperature (37°C and 42°C) affected volatile and nonvolatile metabolomic profiles and pathways. The levels of acetaldehyde, 2,3-butanedione, acetoin, butyric acid, decanoic acid, hexanoic acid, and octanoic acid were significantly higher at 37°C than at 42°C at FT and S10d. This indicates that the low temperature (37°C) most likely contributes more to the formation of important flavor compounds during the fermentation process and production of short-chain fatty acids during storage.     

Release of Single-Cell Protein by Induced Cell Lysis

A temperature sensitive lysis mutant of Saccharomyces cerevisiae JD 109, which grows normally at 27°C, stops growing and partially lyses at a nonpermissive temperature (37°C). In studies in a 14L fermentor with temperature shifted (from 27°C to 37°C) at 3.0 g/L cells, the effect of pH was investigated with the result that the rate of lysis is faster at pH 8 than at pH 5.6; the protein in the cells decreases from 45 to 35%. When both pH (from 5.6 to 8.0) and temperature are shifted, the culture showed a more noticeable inhibition of growth and lysis than when only pH was changed and the temperature was maintained at 27°C.     

Assessment of Enterotoxin Production and Cross‐Contamination of Staphylococcus aureus between Food Processing Materials and Ready‐To‐Eat Cooked Fish Paste

This study evaluated Staphylococcus aureus growth and subsequent staphylococcal enterotoxin A production in tryptone soy broth and on ready‐to‐eat cooked fish paste at 12 to 37 °C, as well as cross‐contamination between stainless steel, polyethylene, and latex glove at room temperature. A model was developed using Barany and Roberts's growth model, which satisfactorily described the suitable growth of S. aureus with R²‐adj from 0.94 to 0.99. Except at 12 °C, S. aureus cells in TSB presented a lag time lower (14.64 to 1.65 h), grew faster (0.08 to 0.31 log CFU/h) and produced SEA at lower cell density levels (5.65 to 6.44 log CFU/mL) compare to those inoculated on cooked fish paste with data of 16.920 to 1.985 h, 0.02 to 0.23 log CFU/h, and 6.19 to 7.11 log CFU/g, respectively. Staphylococcal enterotoxin type A (SEA) visual immunoassay test showed that primary SEA detection varied considerably among different storage temperature degrees and media. For example, it occurred only during exponential phase at 30 and 37 °C in TSB, but in cooked fish paste it took place at late exponential phase of S. aureus growth at 20 and 25 °C. The SEA detection test was negative on presence of S. aureus on cooked fish paste stored at 12 and 15°C, although cell density reached level of 6.12 log CFU/g at 15 °C. Cross‐contamination expressed as transfer rate of S. aureus from polyethylene surface to cooked fish paste surface was slower than that observed with steel surface to cooked fish paste under same conditions. These results provide helpful information for controlling S. aureus growth, SEA production and cross‐contamination during processing of cooked fish paste.     

LACTIC ACID PRODUCTION IN SWEET WORTS

The production of D-(-)-lactic acid in sweet wort was due to growth of Lactobacillus delbrueckii whereas L-(+)-lactic acid was produced by Bacillus coagulans. Incubation of sweet worts at 45°C favoured the production of D-(-)-lactic acid whereas L-(+)-lactic acid was produced in greatest amounts at 55°C. Low concentrations of hop iso-α-acids (4–12 ppm) inhibited the growth of both B. coagulans and L. delbrueckii at 55 °C.     

Aspergillus Parasiticus Growth and Aflatoxin Production on Dehydrated Taro

A study was made of Aspergillus parasiticus growth and aflatoxin production on four taro media. The critical equilibrium relative humidity (ERH) for natural mold growth on unsterilized dehydrated taro was 88% at 20°C. However, nontoxigenic A. parasiticus NRRL 1957 did not grow at this ERH on dehydrated raw taro incubated at 20°, 30°, or 40°C. Instead, the growth of A. parasiticus NRRL 1957 on dehydrated taro was optimum at 30°C and an ERH of 96%. Aflatoxin production by toxigenic A. parasiticus NRRL 2999 was investigated on four taro media under optimal growth conditions. Only moderate quantities of aflatoxins were produced by A. parasiticus NRRL 2999 on uncooked dehydrated taro, but cooking or supplementation with peptone stimualted mycelial growth and aflatoxin production slightly. Nevertheless, growth and aflatoxin production on cooked or peptone-supplemented taro media was low.     

Bacterial Histamine Production as a Function of Temperature and Time of Incubation

Optimal temperature, lower temperature limit, extent, and rate of histamine production in a tuna fish infusion broth (TFIB) varied for the strains of Proteus morganii, Klebsiella pneumoniae, Hafnia alvei, Citrobacter freundii, and Escherichia coli studied. P. morganii and K. pneumoniae produced large quantities of histamine in a relatively short incubation period (<24 hr) at 15°C, 30°C, and 37°C; production was fastest at 37°C. H. alvei, C. freundii, and E. coli produced toxicologically significant levels of histamine (>2500 nmoles/ml) only at 30°C and 37°C on prolonged incubation (≥48 hr). At 72 hr of incubation, optimal temperature for histamine production was 37°C for E. coli and C freundii; 30°C for P. morganii strain 110SC2, K. pneumoniae, and H. alvei; and 15°C for P. morganii strain JM. The lower temperature limits for production of toxicologically significant levels of histamine in TFIB were 7°C for K. pneumoniae; 15°C for both P. morganii strains; and 30°C for H. alvei, C. freundii, and E. coli.     

Growth and Enterotoxin A Production by Staphylococcus aureus in Precooked Bacon in the Intermediate Moisture Range

Different water activities were obtained in precooked bacon by varying the frying time. Water activity (a_w) correlated best to the moisture, salt and protein content. When stored aerobically at 37°C, S. aureus A100 was capable of rapid growth in precooked bacon at a a_w of 0.84 or above, whereas at 20°C a a_w of 0.88 or higher was required. Under anaerobic storage at 37°C, growth was observed at a a_w of 0.90, and at 20°C slight growth was noted at a a_w of 0.91. The increase in the minimal a_w required for aerobic growth at the lower temperature was reflected in the differences between the isotherms obtained at 37°C and 20°C. The maximum populations achieved were higher for samples stored aerobically. Enterotoxin A (19–821 ng/g) was found in all aerobically stored samples where growth occurred. Enterotoxin A (38–109 ng/g) was also found in all anaerobically incubated samples where the population of S. aureus increased more than one logarithmic cycle.     

Staphylococcus aureus Growth and Enterotoxin Production in Mushrooms

An enterotoxin A (SEA) producing strain of Staphylococcus aureus was inoculated onto mushrooms and incubated under simulated pre-and post-canning conditions. S. aureus grew and produced SEA in mushrooms incubated at 25°C in 10% to 20% NaCl. Growth and SEA production occurred when mushrooms were stored in plastic bags at 37°C, but not at 25°C. Mushrooms heat-processed at 121.1°C supported S. aureus growth and SEA production. No SEA was recovered from mushrooms inoculated with 1 or 10 ng SEA/g and heated at 121.1°C for 4.5 min. At higher inoculated SEA levels (100 and 1,000 ng/g), SEA was detectable after 10 min of heating. Staphylococcal enterotoxin could be present in canned mushrooms as a result of pre-, but more likely post-processing contamination.     

Effect of Cycling Temperatures on Aflatoxin Production by Aspergillus parasiticus and Aspergillus flavus in Rice and Cheddar Cheese

Initiation of growth, sporulation and aflatoxin production at cycling temperatures took less time than at 15°C but more than at 18°C and 25°C. A. parasiticus produced more aflatoxins on rice under cycling temperatures than at 25°C, 18°C or 15°C, while A. flavus produced less aflatoxin under cycling temperatures. A. parasiticus produced more aflatoxins on cheese under cycling temperatures than at 18°C or 15°C, but much less than at 25°C. A. flavus produced less aflatoxins on cheese under cycling temperatures than at 18°C and 25°C. Both organisms produced trace amounts of toxins at 15°C on cheese. Preincubation at 25°C for 2 days before temperature cycling did not increase aflatoxin production on rice but increased production on cheese. The rate of aflatoxin production on cheese decreased as the temperature decreased. No growth, sporulation or aflatoxin production was observed at 5°C on either rice or cheese.     

Effects of oilseed substrates (ground nyjer and flax seeds) on the growth and Ochratoxin A production by Aspergillus carbonarius

Aspergillus carbonarius is one of the major Ochratoxin A (OTA) producing fungus. Nyjer and flax seeds are important oilseeds that are used for both human and animal consumption, but they are highly susceptible to fungal growth and mycotoxin contamination. The objectives of this study were to determine the growth and OTA production by A. carbonarius on ground nyjer and flax seeds with water activity levels ranging from 0.82 to 0.98 a_w at three incubation temperatures (20, 30, 37°C). It was found that A. carbonarius was not able to grow on the two types of oilseeds with 0.82 or 0.86 a_w. Also, the fungus was not able to grow on flax seeds with high water activity (0.98 a_w). The OTA was only detected on flax seed samples with 0.94 a_w at 20°C. On nyjer seeds, the highest concentration of OTA (271 μg/kg) was detected from samples with 0.98 a_w incubated at 20°C for 5 days, while on flax seeds the highest OTA (146 μg/kg) was found on the seed samples with 0.94 a_w incubated at 20°C for 15 days. Linear regression models also indicated that 0.98 a_w was optimal for both fungal growth and OTA production on nyjer seeds. Overall, ground nyjer seed is better than flax seed to support growth and OTA production by A. carbonarius.     

GROWTH OF STAPHYLOCOCCUS AUREUS S-6 AND YERSINIA ENTEROCOLITICA CDC A2635 IN COW, SOY AND WINGED BEAN MILKS

The two common foodborne pathogens, Staphylococcus aureus S-6 and Yersinia enterocolitica CDC A2635 were inoculated into whole cow, soy and winged bean milks and incubated at three different temperatures (10°, 25°, 37°C) to evaluate the potential for their growth. The legume milks were prepared by a hot grind method from either whole soybeans (Essex variety) or whole winged beans (Chimbu variety). In general, both foodborne pathogens grew well in all test media. However, S. aureus incubated at 10°C for 120 h, and Y. enterocolitica incubated at 37°C for 48 h were the only two conditions in which less than 10⁸ cells/mL were obtained.     

Histamine Formation in Abusively Stored Pacific Mackerel: Effect of CO2-Modified Atmosphere

Whole Pacific mackerel (Scomber japonicus) were abusively stored at 20°C in air or 80% CO₂, balance air. Samples were analyzed for amines using a modified amino acid analyzer. Following 24 hr storage, levels of histamine, tyramine, putrescine, and cadaverine increased only slightly above the low levels observed initially. During the next 24 hr, the amine content increased dramatically. Levels in the air control samples were about twice those in the modified atmosphere samples. In a separate trial, amine levels in fish stored 3 days were higher still and similar in the two atmospheres. Thus, in neither trial did CO₂-modified atmosphere storage lead to increased production of potentially toxic amines.