Daniel Y. C. Fung

The chemistry of lysozyme and its use as a food preservative and a pharmaceutical

The chemistry and use of lysozyme as a food preservative and a pharmaceutical are reviewed. Lysozyme inhibits the growth of deleterious organisms, thus prolonging shelf life. Chemicals used to improve the preservative effect of lysozyme and those that inhibit the enzyme are discussed, along with the stability of lysozyme in various chemical environments. Lysozyme has been used to preserve fresh fruits and vegetables, tofu bean curd, seafoods, meats and sausages, potato salad, cooked burdock with soy sauce, and varieties of semihard cheeses such as Edam, Gouda, and some Italian cheeses. Lysozyme added to infant-feeding formulas makes them more closely resemble human milk. Lysozyme has been used clinically in the treatment of periodontitis, administered in chewing gum, and implemented to prevent tooth decay. It has also been administered to patients suffering from cancer for its analgesic effect and has been used as a potentiating agent in antibiotic therapy.

Application of thin agar layer method for recovery of injured Salmonella typhimurium.

Xylose lysine decarboxylase (XLD) medium, a selective plating medium, can inhibit heat-injured Salmonella typhimurium from growing, whereas tryptic soy agar (TSA), a nonselective medium, does not. To facilitate recovery of heat-injured S. typhimurium cells while providing selectivity of isolation of S. typhimurium from other bacteria in the sample, a thin agar layer (TAL) procedure was developed by overlaying 14 ml of nonselective medium (TSA) onto prepoured and solidified XLD medium in a 8.5 cm diameter Petri dish. During the first few hours of incubating the plate, the injured S. typhimurium repaired and started to grow in the TSA. During the resuscitation of injured cells, the selective agents from XLD were diffused to the TSA top layer part. Once the selective agents diffused to the top part of the TAL, the resuscitated S. typhimurium started to produce a typical reaction (black color) and other microorganisms were inhibited by the selective agents. The recovery rate for heat-injured (55 degrees C for 15 min) S. typhimurium with the TAL method was compared with TSA, XLD, and the traditional overlay method (OV; pouring selective agar on top of resuscitated cells on TSA agar 3-4 h after incubation). No significant difference occurred among TSA, OV, and TAL (P > 0.05) for enumeration of heat-injured S. typhimurium, but they recovered significantly higher numbers than from XLD agar (P < 0.05).

Thin agar layer method for recovery of heat-injured Listeria monocytogenes.

A thin agar layer (TAL) method was developed to recover heat-injured Listeria monocytogenes. Modified Oxford medium (MOX), a selective plating medium, inhibits heat-injured L. monocytogenes from growing, whereas tryptic soy agar (TSA), a nonselective medium, does not. In order to facilitate recovery of heat-injured L. monocytogenes cells while providing selectivity of isolation of L. monocytogenes from other bacteria in the sample, a unique TAL procedure was developed by overlaying 5 ml of nonselective medium (TSA) onto prepoured and solidified MOX medium in an 8.5-cm-diameter petri dish. The injured L. monocytogenes repaired and started to grow in the TSA during the first few hours after incubation of the plate. During the resuscitation of injured cells, the selective agents from MOX diffused to the TSA top layer to inhibit other microorganisms. L. monocytogenes showed a typical reaction (black colonies) on TAL after 24 h of incubation at 37 degrees C. The recovery rate for heat-injured L. monocytogenes with the TAL method was compared with those rates associated with TSA, MOX, and the traditional overlay method (OV; pouring selective agar on top of resuscitated cells on TSA agar after 3 h incubation). Milk and 0.1% peptone water that were inoculated with L. monocytogenes (4 to 5 log CFU/ml) were heated for 15 min at 55 degrees C. L. monocytogenes was enumerated on TSA, MOX, OV, and TAL media and procedures. No significant difference occurred among TSA, OV, and TAL (P > 0.05) in terms of enumeration of heat-injured L. monocytogenes, but these media recovered significantly higher numbers than did MOX agar (P < 0.05)-in both samples. The TAL method involves only one step, whereas OV is a more cumbersome two-step procedure.

Tempeh: a mold-modified indigenous fermented food made from soybeans and/or cereal grains

A variety of indigenous fermented foods exist today; however, tempeh has been one of the most widely accepted and researched mold-modified fermented products. Tempeh is a traditional fermented food made from soaked and cooked soybeans inoculated with a mold, usually of the genus Rhizopus. After fermentation has occurred, the soybeans are bound together into a compact cake by dense cottony mycelium. An important function of the mold in the fermentation process is the synthesis of enzymes, which hydrolyze soybean constituents and contribute to the development of a desirable texture, flavor, and aroma of the product. Enzymatic hydrolysis also may decrease or eliminate antinutritional constituents; consequently, the nutritional quality of the fermented product may be improved. Current technology and new scientific advancements have enabled researchers to examine specific strains of Rhizopus and new substrates such as cereal grains. Because Kansas produces numerous cereal grains, production of a fermented tempeh-like product using wheat, sorghum (milo), oats, rye, barley, corn, and triticale is a definite possibility for generating a Kansas Value-Added Product. In this study, several different tempeh-like products were produced using various cereal grains inoculated with Rhizopus oligosporus NRRL 2549 or R. oligosporus NRRL 2710. Grains used included hard red winter wheat, triticale, yellow sorghum (milo), and red sorghum (milo). The grain source as well as the strain of R. oligosporus used influenced the products appearance, flavor, and patty integrity. Results showed that R. oligosporus NRRL 2549 produced more mycelium at a more rapid rate than did the R. oligosporus NRRL 2710 strain. The combination of red sorghum and R. oligosporus NRRL 2549 yielded a product with good patty texture, aroma, and appearance. Furthermore, the red sorghum fermented product was well suited for slicing. On the other hand, yellow sorghum inoculated with either R. oligosporus NRRL 2549 or R. oligosporus NRRL 2710 failed to produce an organoleptically suitable product. Triticale also was found to be an unacceptable substrate for the production of a tempeh-like product. Although the fermented wheat product had a desirable aroma and flavor, it lacked patty integrity and crumbled when sliced. Further research is needed to evaluate the economic significance and industrial applications of these tempeh-like products.

Salmonellae and food safety.

Salmonella is one of the most important foodborne pathogens around the world. The knowledge that very low numbers of Salmonella cells can be infectious emphasizes the need for stringent food safety measures Traditional methods for isolating and identifying Salmonella in food rely on preenrichment, selective enrichment in selective and differential media, biochemical tests, and serological confirmation. Recent advances in diagnostic technology have considerably altered testing methods for foodborne Salmonella. Many commercial assay systems and kits that use newer technologies are available to facilitate the identification of Salmonella in foods. These systems include miniaturized biochemical tests, new media formulations, automated instrumentation, DNA/RNA probes, antibody-dependent assays, and polymerase chain reaction. The technologies used for these systems are described, and the various kit formats are compared. Among the limitations of detection methods in terms of food safety are timeliness, limits of detection, and differentiation of virulent and nonvirulent isolates. Current efforts of prevention measures and strategies at different links of the food chain such as consumer education and hazard analysis and critical control point (HACCP) programs are reviewed, Global approaches to food safety are needed..

Alkaline-Fermented Foods: A Review with Emphasis on Pidan Fermentation

Alkaline-fermented foods constitute a group of less-known food products that are widely consumed in Southeast Asia and African countries. They can be made from different raw ingredients. For instance, Japanese natto, Thai thua-nao, and kinema are made from cooked soybeans, dawadawa from African locust beans, ogiri from melon seeds, ugba from African oil beans, kawal from fresh legale leaves, owoh from cotton seeds, and pidan from fresh poultry eggs. In alkaline-fermented foods, the protein of the raw materials is broken down into amino acids and peptides; ammonia is released during the fermentation, raising the pH of the final products and giving the food a strong ammoniacal smell. Most alkaline fermentations are achieved spontaneously by mixed bacteria cultures, principally dominated by Bacillus subtilis. In other cases, pure cultures can be used. For example, Japanese natto is inoculated with a pure culture of B. subtilis var natto. Pidan is a special example of alkaline fermentation. Instead of using microorganisms, pidan is made using an alkali-treated fermentation. Sodium hydroxide (NaOH) is produced from the reaction of sodium carbonate (Na2CO3), water (H2O), and calcium oxide (CaO) of pickle or coating mud. NaOH penetrates into the eggs, causing the physicochemical changes, color changes, and gelation. The appearance of pidan differs from fresh eggs in that the white becomes a semitransparent tea-brown color, and the yolk is solid or semisolid with a dark-green color. The nutritional value of pidan is slightly decreased compared with fresh eggs, but pidan has an extremely long shelf life and a pleasant, fragrant taste that is preferred by most people in Southeast Asian countries. In a small-scale laboratory study conducted by the authors, B. subtilis was not found in pidan. Four Staphylococcus spp. (S. cohnii, S. epidermidis, S. haemolyticus, and S. warneri) and two strains of Bacillus spp. (B. cereus and B. macerans) were isolated from pidan. Staphylococcus spp. did not contribute to the fermentation and were considered contaminants.

Antibacterial effect of water-soluble tea extracts on foodborne pathogens in laboratory medium and in a food model.

The microbial inhibition of foodborne pathogens was determined in brain heart infusion broth with 10% (wt/vol) water-soluble extracts of green, jasmine, black, dungglre, and oolong tea against Escherichia coli O157:H7, Salmonella enterica serovar Enteritidis, Listeria monocytogenes, and Staphylococcus aureus. The mixed culture (approximately 6.0 log CFU/ml), which was composed of the four pathogens, was inoculated into brain heart infusion broth with and without tea extracts. After incubation at 35 degrees C for 0, 1, 3, and 5 days, proper dilution of each sample was spiral plated on each selective agar. Viable cell counts were performed after incubation at 35 degrees C for 24 to 36 h. Green, jasmine, and black tea exhibited an approximately 5.0 log suppression of S. aureus compared with the control from days 1 to 5. Green and jasmine tea also suppressed the growth of L. monocytogenes by approximately 3.0 log CFU/ml on day 5. In contrast, no tea extracts inactivated E. coli O157:H7 and Salmonella Enteritidis. Based on the result in liquid medium, green and jasmine teas of 0.1% (vol/wt) were individually evaluated for their antimicrobial activity against L. monocytogenes and S. aureus in a food model (ground beef) stored at 7 degrees C for 0, 1, 3, 5, and 7 days. Viable cell counts of total bacteria, L. monocytogenes, and S. aureus in ground beef were not significantly different among green and jasmine tea and the control.

Inhibition of Listeria monocytogenes and Escherichia coli O157:H7 on Beef by Application of Organic Acids†

Lean beef surfaces were inoculated with Escherichia coli O157:H7 and Listeria monocytogenes and then sanitized with fumaric, acetic, or lactic acid alone and in combined solutions of those acids at 55°C for 5 s. The initial inoculum level was 8.62 log CFU/cm2 and 5.13 log CFU/cm2 for L. monocytogenes and E. coli O157:H7, respectively. Fumaric acid at a concentration of 1% was the most effective acid in reducing the populations of L. monocytogenes by up to 1 log unit and E. coli O157:H7 by up to 1.3 log units when compared with acetic or lactic acids. The rank order of acids tested against the growth of L. monocytogenes and E. coli O157:H7 was fumaric acid followed by lactic and acetic acids. Fumaric acid at concentrations of 1.0% and 1.5% was more effective than any of the combined solutions of acids.

The effect of microwaves on nutrient value of foods

Microwave cooking has gained considerable importance as an energy-saving, convenient, and time-saving cooking method. This article reviews the state of the art of microwave cooking and the existing publishing data on the effects of microwave cooking on nutritive values of moisture, protein, carbohydrate, lipid, minerals, and vitamins. Most reports indicated that microwave cooking resulted in higher moisture losses compared with conventional methods. Overall, the nutritional effects of microwaves on protein, lipid, and minerals appear minimal. There is no report on the effects of microwaves on carbohydrate fraction in foods. A large amount of data is available on the effects of microwaves on vitamins. It is concluded that there are only slight differences between microwave and conventional cooking on vitamin retention in foods. In conclusion, no significant nutritional differences exist between foods prepared by conventional and microwave methods. Any differences reported in the literature are minimal.

Off‐flavors in milk

This report is a review of off-flavors encountered in fluid milk. It includes sections on transmitted, microbial, lipolyzed, heated, light-activated, oxidized, and miscellaneous flavors. Finally, the flavor of ultrahigh temperature (UHT) milk is reviewed. Most of the literature cited for different off-flavors covers the period since a comprehensive review was done of milk flavors by Strobel et al. (1953). In addition to causative agents and volatile materials associated with off-flavors, methods for control of each of the off-flavors are presented. This report is designed to serve the dairy production and processing industry, students and faculty of food science curricula, dairy product evaluation students, regulatory people, and fieldmen.