 |
FOOD TECHNOLOGY INTELLIGENCE INC. |
||||||||||||||
SOME ARTICLES RECENTLY PUBLISHED IN
Microbial Update InternationalApril 2006 VOL 11, IS 6 Microbial Update International (ISSN 1082-9296) is published every other month. © 2006 by Food Technology Intelligence, Inc., 215 Godwin Ave., P.O. Box 322, Midland Park, NJ 07432-0322 USA; phone 201-445-4227; fax 201-447-5904; email ftiinfo@ftipub.com. Visit our Web site at www.ftipub.com. Fee: one year, $250; two years, $400. Add $20 per year for service outside North America. TABLE OF CONTENTS (Click on title to go to story)
In this report… Consumers prefer foods that have few or no chemicals. In this light, work continues on bacteriocins. A long-range goal of some scientists is to use bacteriocins that are active against L. monocytogenes as powdered preservatives in ready-to-eat refrigerated meat products. Then see how dough conditioners and antibacterials can extend the life of flat bread. Meanwhile, with increased attention focused on the threat of bioterrorism, produce and other ready-to-eat foods may be potential vehicles for intentional contamination with disease-causing microorganisms. Read how this issue is being addressed. Produce antilisterial bacteriocins from lactic acid bacteria in dairy-based media Consumer preferences have been evolving toward foods that are minimally processed and free from chemical preservatives. As a result, there has been a great interest in developing approaches for making minimally processed products. With this in mind, several research groups have attempted to exploit microbial metabolites, such as bacteriocins from lactic acid bacteria (LAB). The long-range goal of scientists at the University of Idaho is to use bacteriocins that are active against L. monocytogenes as powdered preservatives in ready-to-eat (RTE) meat products stored at refrigeration temperatures. The researchers currently are attempting to identify antilisterial bacteriocins of interest, make comparisons among them and investigate their production in economical dairy-based media, including non-fat milk, de-mineralized whey powder, Cheddar cheese whey and Cheddar cheese whey derivatives supplemented with selected proteins. Investigators selected 118 strains of LAB and screened them for bacteriocin production using agar-well-diffusion and other techniques, as well as the following indicators: L. plantarum NCDO 995, Lb. sake ATCC 15521, and L. monocytogenes CWD 1102, CWD 1092, CWD 1157, CWD 1198 and V7 1/2a. Nisin-producing Lactococcus lactis subsp. lactis ATCC 11454 was used as a positive control. Researchers considered inhibition zones in the confluent growth of the indicators as evidence of bacteriocin production. They conducted experiments to exclude antimicrobial activity caused by organic acids and hydrogen peroxide. The proteinaceous nature of the bacteriocins was confirmed using proteases. Scientists determined that 12 LAB strains produced bacteriocins that are active against LAB indicator strains. To their knowledge, this was the first report of Lactobacillus (Weisella) virisdescens NRRL B-1951 as a bacteriocin producer. When grown in dairy-based media, nine strains exhibited inhibitory activity against L. monocytogenes indicator strains. The most successful bacteriocin producers were: Lb. bavaricus MN, Lb. curvatus L442, Lc. lactis subsp. cremoris ATCC 14365, Lb. curvatus LTH 1174 and Leuconostoc mesenteroides L124. Current research efforts involve preparing freeze-dried bacteriocins and using them in RTE meat products. Further information. Gülhan Yüksel, Department of Food Science and Toxicology, University of Idaho, Agricultural Biotechnology Laboratory, Room 205, Moscow, ID 83844; phone: 208-885-7771; fax: 208-885-9752; email: gulhan@uidaho.edu. Antimicrobial agents, dough conditioners extend shelf life, quality of flat bread More than 1.8 billion people consume flat bread on a daily basis. Arabic flat bread (AFB), unlike other breads, has a limited shelf life. You may be aware that the two major considerations of shelf life are spoilage and staling. So scientists at Kansas State University performed three related studies to determine if the shelf life and quality of AFB could be extended by adding selected preservatives or improvers. The investigators evaluated the bread using texture analysis (TA), near-infrared spectroscopy (NIRS) and sensory techniques. Their first study examined the addition of preservatives to extend the product’s shelf life. A control and three preservatives treatments—fumaric acid (0.2%), sodium propionate (0.3%) and a sodium propionate-fumaric acid mixture (PF)—were used either singly or in various combinations. The second study evaluated the addition of dough conditioners to maintain quality during storage. The conditioners examined included sodium stearoyl-2-lactylate (0.25%), monoglyceride (0.25%), hydroxy propyl methyl cellulose gum (0.75%, HPMC), high-fructose corn syrup (4.2%, HFCS) and various combinations of improvers. A third study incorporated a trained sensory panel to evaluate product attributes. The results from the first study showed that PF extended the bread’s shelf life an additional 13 days compared to the control. The second study showed that, after three days, the HFCS and an improver combination significantly improved TA and NIRS profiles compared to the control. Scientists confirmed this with the third study, in which a trained sensory panel preferred the HFCS and an improver combination over the control after product had been stored for a while. The investigators tell us that by using appropriate preservatives and improvers, the shelf life and quality of AFB may be extended. This is critical in countries in which storage and distribution chains are limited. Further information. Thomas Herald, Food Science Institute, Kansas State University, Room 220, Call Hall, Manhattan, KS 66502; phone: 785-532-1221; email: therald@ksu.edu. Chlorine, chlorine dioxide kill spores on apples With increased attention focused on the threat of bioterrorism, produce and other ready-to-eat foods may be potential vehicles for intentional contamination with disease-causing microorganisms. While chlorine, chlorine dioxide and other sanitizers are effective in killing B. anthracis and other bioterrorism agents on inert surfaces and in aqueous suspensions, the resistance of B. anthracis spores and spores of other Bacillus species to sanitizers has been given less attention by researchers. B. cereus and B. thuringiensis are genetically closely related to B. anthracis. Researchers at the University of Georgia undertook a study using B. cereus and B. thuringiensis spores as surrogates for B. anthracis. The goal was to evaluate the efficacy of chlorine and chlorine dioxide and a peroxyacetic acid-based sanitizer in killing spores of B. cereus and B. thuringiensis in suspension, on the surface of stainless steel and on apples as a ready-to eat model. Their insight will be useful when developing sanitization strategies associated with the Bacillus species. The scientists used water and 5% horse serum as carriers for spore inoculum applied to the surface of stainless steel. A 5% horse serum was used as a carrier for inoculum applied to apples. Inocula were dried on stainless steel for 5 hours and on apples for up to 24 hours before they were treated with sanitizers. At certain concentrations, the sensitivity of planktonic B. cereus and B. thuringiensis spores to chlorine and chlorine dioxide was similar. A portion of the spores surviving treatment with chlorine and, more markedly, chlorine dioxide, had less tolerance to heat. Treatment with the peroxyacetic acid-based sanitizer had little effect on the viability of spores. Planktonic spores of both species were more sensitive to chlorine and chlorine dioxide than were spores on the surface of stainless steel or apples. Chlorine was more effective than chlorine dioxide in killing spores in suspension and on stainless steel. The lethality of chlorine dioxide was markedly reduced when inoculum on stainless steel was suspended in 5% horse serum as a carrier rather than in water. This indicates that neutralization of the sanitizer occurred when it contacted the organic material. Chlorine and chlorine dioxide were equally effective in killing spores on apples. Significant reductions of 3.8 to 4.5 log CFU per apple were achieved by a treatment concentration of 100 g per mL. Treatment with the peroxyacetic acid sanitizer caused reductions of Bacillus spores on the order of 1 log CFU per apple. Further information. Michael Doyle, Center for Food Safety, University of Georgia, Griffin Campus, Melton Building, Griffin, GA 30223; phone: 770-228-7284; fax: 770-229-3216; email: mdoyle@uga.edu. PEF fluid handling system inactivates microbes in apple cider Current regulations require fresh apple cider to be heat-pasteurized. Previous research has shown that processing fresh apple cider using pulsed electric fields (PEFs) not only can extend the shelf life of the product, but it can also maintain its fresh flavor. A new pilot-scale fluid handling system with an energy recovery heat exchanger for PEF processing has been built at The Ohio State University. The system has a greater energy efficiency and is easier to operate than the previous unit. It would be suitable for processing apple cider or other juices if the FDA requirements for microbial reduction are met. The goal of Ohio State scientists was to evaluate how efficiently this system could inactivate the natural flora and surrogate to pathogens in apple ciders. Fresh apple cider was made by a commercial juice processing system. The cider was incubated at 22 C for three days, or inoculated with L. planetarium before undergoing PEF treatment. A pilot-scale pulse generator provided high-voltage pulses. In their tests, the researchers applied a PEF field strength of from 32 kV per cm to 20 kV per cm. Treatment time was 44 µs. The holding temperature depended on the PEF field strength. Holding time was 15 seconds. The flow rate was 125 L per hour, and the backpressure used was 40 psi. The PEF-treated ciders were analyzed for microbial counts by investigators. They found that the extent of microbial reductions for both the incubated and inoculated ciders depended on the PEF field strength and holding temperature. The fluid handling system was able to efficiently inactivate bacteria in apple cider. The treatment of 32 kV per cm at 70 C yielded a 5-log reduction of aerobic bacteria and a 5-log reduction of mold and yeast in the incubated cider. In addition, 5 logs of L. planetarium were inactivated in the inoculated cider. No recovery was found during one week’s storage at 4 C. Further information. Howard Zhang, Department of Food Science and Technology, The Ohio State University, 2015 Fyffe Rd., 233 Parker Food Science Building, Columbus, OH 43210; phone: 614-688-3644; email: zhang.138@osu.edu. Remove L. monocytogenes biofilms using ultrasound and ozone The removal of biofilms is an important aspect of any sanitation program. Biofilms may harbor pathogens and act as a source of post-processing contamination. One pathogen that is ubiquitous throughout the industry is L. monocytogenes. Due to their resistance to sanitizers and mechanical brushing, biofilms may best be removed by combining different techniques. The combination of power ultrasound and ozone has shown promise for eliminating biofilms of L. monocytogenes. The objective of scientists at the University of Illinois was to determine the ability of power ultrasound and ozonation, used individually and in tandem, to remove L. monocytogenes biofilms from stainless steel chips. The researchers indicate that a combination of both may be an effective treatment for removing biofilm from stainless steel food contact surfaces. In tests, the scientists inoculated stainless steel chips with L. monocytogenes. Tryptic soy broth was applied to the chips after they were rinsed with a potassium phosphate buffer (PPB, pH 7.0). Power ultrasound at 20 kHz, 100% amplitude and 120 W was applied for 30 sec or 60 sec to a biofilm chip while it was submerged in 250 ml of sterile PPB. Ozone also was cycled through the 250 ml of PPB containing the biofilm chip for 30 sec or 60 sec at concentrations of 0.25 ppm, 0.5 ppm or 1.0 ppm. Power ultrasound and ozonation were also used in tandem for testing their combined effect. Both treatments separately resulted in a significant reduction in recoverable microbial cells. Power ultrasound was the most effective treatment, yielding a 3.8 log 10 CFU per ml reduction after 60 sec. For the ozone combined with the ultrasound treatment, reductions were significantly higher than with either treatment independently. There were no recoverable cells after 60 sec of a combined treatment when an ozone concentration of 0.5 ppm was used. This treatment yielded a reduction of 7.31 log 10 CFU per ml. The research in this area is continuing. Further information. Scott Martin, Department of Food Science and Human Nutrition, University of Illinois, Urbana-Champaign, 1207 W. Gregory Dr., 486 Animal Sciences Laboratory, MC-630, Urbana, IL 61801; phone: 217-244-2877; fax: 217-244-2517; email: semartn@express.cites.uiuc.edu. High pressure inactivates V. parahaemolyticus and B. cereus High-pressure treatment, also known as high-hydrostatic-pressure processing or ultra-high-pressure processing, involves treating food at elevated pressures of 30,000 psi to 130,000 psi at a specific temperature and for a specified time. This approach inhibits microorganisms by inactivating key enzymes and by making modifications in microbial cell walls and membranes. The pressure has a marked killing effect on foodborne pathogens. Scientists in Spain evaluated the pressure-induced inactivation and sublethal injury of V. parahaemolyticus CECT 588 and B. cereus vegetative cells CECT 148 in buffered suspensions. They used three different media: nonselective, selective and thin agar layer media (TAL). Bacteria-inoculated (4 logs CFU per mL) 0.1 % buffered peptone water at pH 7.4 was treated at 100 MPa, 200 MPa and 300 MPa for both 1 minute and 5 minutes at 20 C, using a discontinuous isostatic system. Plate counts were determined on tryptic soy agar (TSA), selective media (thiosulfate-citrate-bile-sucrose [TCBS] agar for V. parahaemolyticus and B. cereus selective medium for B. cereus vegetative cells), and TAL (selective medium overlaid with TSA). The researchers used an additional enrichment step with brain-heart infusion broth at 37 C for 24 hours to confirm the complete inactivation of bacteria. The scientists added sodium chloride to the media used for V. parahaemolyticus so that the final content was 1%. They found that V. parahaemolyticus counts declined from 4 logs CFU per mL to 0.5 log CFU per mL when treated at 200 MPa for 5 minutes. The counts were nondetectable when using 300 MPa. The organism was totally inactivated when it was treated at 300 MPa for 5 minutes. B. cereus vegetative cells were reduced to 2.5 logs CFU per mL when treated at 300 MPa for 1 minute. They were completely inactivated when they were treated at 300 MPa for 5 minutes. Plate counts on TAL were generally similar to those on TSA and were greater than those on selective media, especially for V. parahaemolyticus treated at 100 MPa for 1 minute and at 200 MPa for 5 minutes. TSA counts were about 0.5 log CFU per mL higher than TCBS agar counts. Further information. J. Yuste, Departamento de Ciència Animal y de los Alimentos, Universitat Autònoma de Barcelona, CER Planta de Tecnologia dels Aliments (CeRTA, XIT), Facultat de Veterinària, Bellaterra, Barcelona 08193, Spain; phone: +34 93-581-14-46; fax: +34 93-581-14-94; email: josep.yuste@uab.es. Develop quantitative NASBA assay to detect E. coli The nucleic acid sequence-based amplification (NASBA) is a very sensitive and rapid technique for amplifying RNA and for detecting bacteria. NASBA is a specific, isothermal method of nucleic acid amplification which is highly suited for amplifying RNA. However, when using the NASBA, it is not possible to quantify the production yield of the tested target sequence. Competitive assays have been developed to quantify the target sample. These assays use internal standards that compete with the target sequence for multiplication during the amplification reaction. One current drawback of these assays entails the considerable effort that is required to construct an appropriate RNA or DNA internal standard sequence using genetic engineering. Much effort has to be put into designing a sequence that can equally compete with the target sequence for amplification in the same reaction tube. So, the development of a simple procedure that produces a competitor molecule is of great interest. Instead of cloning sequences, carrying out deletion-PCR reactions and sequencing the successful construct, Cornell University scientists were able to show that the careful design of NASBA-deletion primers can lead to a competitor molecule within only two NASBA reactions. A quantitative competitive NASBA assay that determines the initial concentration of E. coli RNA can be achieved within 3 hours with the use of an electrochemiluminesence (ECL) detection method and a simple lateral-flow assay. The ECL technique is extremely sensitive. Its dynamic range spans over several orders of magnitude. Cornell researchers integrated a lateral-flow biosensor into the internal standards onto the same assay. This made it possible to detect and quantify the target RNA in just one simple assay. Internal standards constructed by this method competed equally with the RNA of the target E. coli. Further information. AntjeBaeumner, Department of Biological and Environmental Engineering, Cornell University, 318 Riley-Robb Hall, Ithaca, NY 14853; phone: 607-255-5433; fax: 607-255-4080; email: ajb23@cornell.edu. Developing, validating processes for fermented foods We know that fermentation increases the vitamin and enzyme content of foods, aids digestion and facilitates the assimilation of nutrients. Fermented foods also colonize the intestinal tract with flora that control putrefactive bacteria, maintain proper pH balance in the colon, and increase the bulk and frequency of bowel movements. Acidified and fermented foods have enjoyed excellent safety records, with few or no cases of foodborne disease reported from the consumption of these products. However, there have been reports of disease outbreaks caused by E. coli O157:H7 and Salmonella species in juice products that have pH values of less than 4.0. These outbreaks may raise concerns about the safety of some acidified products. Acidified foods contain acid or acid food ingredients and must have a water activity of 0.85 or greater and a final equilibrium pH of 4.6 or less. These regulations were promulgated by the FDA in 1979, with the intent of preventing spore germination and any growth of C. botulinum. At that time, vegetative cells of bacteria were not considered to be a significant health risk for acidified or fermented food products. Acid foods, including fermented products, and refrigerated foods are exempt from these regulations. In 2001, the FDA began holding commercial process filings for acidified foods that were not heat-processed. Exceptions included products with an equilibrium pH value at or less than 3.3 that are acidified with acetic acid. For theses products, only a holding time of 48 hours is needed before their distribution to ensure safety. For acidified foods with a pH greater than 3.3, the minimum heat treatments needed for a 5-log reduction in the number of pathogenic bacteria are the objective of current research efforts. Research is underway in several laboratories to determine how organic acids present in fermented and acidified vegetable products can be used to destroy acid-resistant food pathogens. Scientists also are determining the molecular mechanisms by which these pathogens resist killing by acid. Results from this research may lead to specific recommendations for the safe production of acidified and fermented food products. Further information. Fred Breidt, Department of Food Science, North Carolina State University, 322 Schaub Hall, Box 7624, Raleigh, NC 27695; phone: 919-513-0186; fax: 919-515-7124; email: breidt@ncsu.edu. Understanding how bacteria communicate Bacteria talk to each other, and the implications of this capability are becoming apparent to microbiologists. This communication occurs at several levels: intraspecies (like-bacteria talking to like-bacteria); interspecies (communication among bacteria of different species and genera); and bacteria to host (bacterial communication with mammalian cells). This communication is achieved by the production of small molecules, autoinducers, which, when present at sufficient concentrations, trigger a variety of cellular responses. Thus, bacterial numbers must reach a certain level, or quorum, to generate the critical concentration of these signaling molecules. This has led to the term “quorum sensing” to describe this phenomenon. Quorum sensing—communication—controls a number of phenotypes in bacteria including biofilm formation, the synthesis of extracellular enzymes and the expression of virulence factors in many enteric bacteria. If we can understand how bacteria communicate, we may be able to develop strategies to interfere with this communication. This may lead to a reduction in the levels of biofilms found in food processing environments, an extended product shelf life (through the inhibition of the synthesis of proteases and lipases responsible for degrading foods) and a reduction in foodborne illness by preventing expression of virulence. Quorum sensing enables microbes to coordinate their behavior in response to changes in their environment. They are able to adapt to the availability of nutrients, for example, by synthesizing extracellular enzymes that scavenge nutrients or through the formation of biofilms. Quorum sensing also provides a way to compete with other microorganisms and to avoid potentially toxic compounds. Of significance is the role that quorum sensing plays in pathogenesis. It allows pathogenic bacteria to reach high populations before producing virulence factors that elicit an immune response in the host. In this way, a successful infection can be established. Different bacterial species use different molecules to communicate. In Gram negative bacteria, the main signaling molecules are acyl homoserine lactones. In Gram positive bacteria, peptides, such as nisin, are the primary means of communication. However, a single bacterial species can use more than one signal molecule, and it may respond to each molecule in a different manner. Interspecies communication in Gram negative bacteria is mediated through the production of autoinducer-2 (AI-2). The production of several virulence factors by E. coli O157:H7 are regulated by AI-2. Another autoinducer, AI-3, allows E. coli O157:H7 to talk to its host. Further information. Mansel Griffiths, Department of Food Science, University of Guelph, Canadian Research Institute for Food Safety, Room 202, 43 McGilvray St., Guelph, ON, N1G 2W1, Canada; phone: 519-824-4120; fax: 519-824-6631; email: mgriffit@uoguelph.ca. The variety of pourable salad dressings available in large containers for use in food service and home settings has increased in recent years. Many had believed that this increased the potential for postprocess contamination, such as at salad bars where portions are removed from the same container by several different people over an extended period of time. The preservation of commercially processed salad dressings depends, in part, on mild heat treatments that reduce, control or eliminate microorganisms. The presence of acetic acid, lactic and citric acids, a low pH, salt, natural antimicrobials and preservatives, such as sorbic acid or benzoic acid, create a harsh environment for pathogens. Researchers undertook a study to determine the death rates of various pathogens in shelf-stable, dairy-based, pourable inoculated commercial full-fat ranch salad dressings. They found that regardless of the initial inoculum population, the pathogens rapidly died in all salad dressings. Salmonella was undetectable in all dressings within one day, and the number of E. coli O157:H7 and L. monocytogenes bacteria were reduced to undetectable levels by enrichment between one and eight days and two and eight days, respectively. E. coli O157:H7 was not detected in four of the 10 salad dressings stored for two or more days, and in nine of the 10 dressings stored for six or more days after inoculation. In essence, shelf-stable, dairy-based, pourable ranch and blue cheese salad dressings manufactured commercially and stored at 25 C do not support the growth of Salmonella, E. coli O157:H7 or L. monocytogenes. They should not be considered as potentially hazardous foods as defined by the FDA Food Code. Contact: Michael Doyle, Center for Food Safety, University of Georgia, Griffin Campus, Melton Building, Griffin, GA 30223. Phone: 770-228-7284. Fax: 770-229-3216. Email: mdoyle@uga.edu. |
|||||||||||||||