R. G. Cassens

Listeria monocytogenes and other Listeria spp. in meat and meat products: a review

Long known as an animal pathogen, Listeria monocytogenes has recently been recognized as a important foodborne agent in human disease. The widespread distribution of L. monocytogenes and other Listeria spp. in nature and an association with domestic livestock makes the occasional presence of these bacteria on raw meats almost unavoidable. Contamination of ready-to-eat meat products with L. monocytogenes poses a special threat to public health because of the organisms ability to grow at refrigeration temperatures and its pathogenicity within certain segments of the population. This paper reviews the prevalence of Listeria spp. in meat and meat products, analyzes the potential for survival and growth of listeriae on fresh meats and during meat processing, and addresses the effect of various meat preservation parameters on L. monocytogenes .

Fiber number and type composition in extensor digitorum longus, soleus, and diaphragm muscles with aging in Fisher 344 rats.

Histochemical (M-ATPase) fiber typing was done on extensor digitorum longus, (EDL), soleus (SOL), and diaphragm (DIA) muscles of barrier-reared Fisher 344 rats obtained at four different ages (3, 9, 28, and 30 months) from the colonies of the National Institute of Aging. In the EDL there are no differences in the percent of type I fibers among the four age groups. The percent of type IIa and IIb fibers also showed no difference between the 3 and 30 month age groups. There was no apparent trend for an increase or decrease in the percent of type IIa or IIb fibers between the four age groups. In both the SOL and DIA muscles the percent of type I fibers was greater in the aged than in the young groups. The percent of type IIa fibers was lower in the 30 month group than in the younger groups for both muscles. The percent of type IIb (DIA) and IIc (SOL) fibers did not change between groups. Total fiber number per cross section of muscle showed no change in the EDL over this age range or in the SOL after 9 months of age. These findings bring into question published results that imply that decreasing fiber number and preferential loss of type II (a and b) fibers are typical aging phenomena.

Red and White Muscle

Publisher Summary In order for the food scientist to understand the characteristics of fresh meat and to appreciate its potential for incorporation into processed meat, he must have a fundamental knowledge of muscle. The casual observation is often made that certain muscles are redder or darker than others. The level of organization, be it red and white muscle, or red and white fibers, is a situation of terminology and recognition that exists, and must be understood. Enormous differences in composition and morphology exist between red and white muscle. Red muscle has a greater concentration of the pigment myoglobin, is generally lower in soluble protein content, lower in glycogen, and higher in lipid than white muscle. Red fibers are smaller in size than white fibers, are better supplied with capillaries, and contain more mitochondria. White fibers are equipped better for glycolytic metabolism than are red fibers, which are designed for oxidative metabolism. Isolated sarcoplasmic reticulum fragments from white muscle have a greater in vitro calcium-binding ability than those from red muscle and the myosin from red muscle has a lower ATPase activity than that from white muscle. The physiological response of red and white fibers corresponds well with what has been stated about their composition and biochemical properties. Stimulation of a white fiber produces a twitch response; a twitch response is also elicited in red fibers, but it gives a slower contraction which covers a longer period of time than that from a white fiber. Histochemical techniques have produced many beautiful and useful pictures of fiber type distribution. Red fibers are high in oxidative enzymes, such as SDH, but low in glycolytic enzymes, such as phosphorylase and also low in ATPase; the opposite situation exists in white fibers. Intermediate fibers are well recognized. The fiber types differentiate clearly just before or after birth of the animal. This is demonstrated by histochemical, biochemical, and physiological techniques. Because the control of differentiation is such an important biological problem, experiments dealing with cross-innervation and the trophic influence are presented. Finally, it is pointed out that fiber type composition has been shown to be an important consideration in the conversion of pig muscle to food for human consumption. Suggested areas of research need are noted.

Animal Physiology and Meat Quality

Publisher Summary This chapter emphasizes the important role animal physiology plays generally in controlling the changes that occur in the postmortem conversion of muscle to meat, thereby affecting the meat supply for the human population. Research effort on animal stress susceptibility, ranging from very fundamental to very applied, has been reviewed. Much has been learned in recent years about the endocrinological aspects of stress susceptibility in domestic animals. The more classic approach of study of muscle biochemistry and morphology has been pursued with greater refinement. The physiology of stress–susceptible animals has been investigated and particular interest has centered on the malignant hyperthermia syndrome. The problem of porcine stress syndrome (PSS) and pale, soft, and exudative (PSE) in pigs has been recognized. Efforts are being made, particularly by the live animal producers, to identify the problem in their animals and then to minimize or eliminate it by proper management and genetic selection.The problem of animal stress susceptibility and the low–quality meat produced from such animals has been studied extensively.

Composition and safety of cured meats in the USA

The meat-consuming public is concerned about foodborne illnesses which are primarily of microbiological origin, about more long-term effects thought to be associated with dietary consumption patterns, and about the possible presence of low levels of various additives or contaminants. Even though 100% safety from foodborne illnesses can never be achieved, steady progress is being made through adoption of new technologies and improved control and prevention systems. A new generation of low-fat products gives the consumer more dietary choices, and modern cured meats have lower residual nitrite than they did twenty years ago.

Survival of Listeria monocytogenes in ground beef

Listeria monocytogenes, due to its association with animals and animal products and its proven pathogenicity, is an organism of potential importance to the meat industry. The survival of L. monocytogenes in ground beef held at 4 degrees C for 2 weeks was investigated. The ground beef was inoculated with L. monocytogenes type 1 or type 4 at a level of 5 x 10(5) to 7 x 10(6) CFU/g and then packaged in either oxygen-permeable or oxygen-impermeable bags. Packages were sampled at random at 0, 2, 3, 5, 7, 11, and 14 days post-inoculation, and assayed for L. monocytogenes counts and pH. The number of L. monocytogenes in ground beef remained constant throughout the sampling period, and survival was not affected by package permeability to oxygen. Listeriae were not isolated from the control ground beef. The pH of the meat increased slightly during storage, but was always in the range of pH 5.6 to 5.9. It appears that L. monocytogenes in ground beef can survive without any substantial increase or decrease in viable cell population during refrigerated storage for 14 days.

Characterization of bacteriocins from Enterococcus faecium with activity against Listeria monocytogenes

Laboratory cultures and environmental isolates of bacteria were screened for antagonism towards Listeria monocytogenes using an agar spot test. Seven of the 163 strains that were tested, one Streptococcus bovis, one Enterococcus casseliflavus, two E. avium and three E. faecium, consistently displayed antilisterial activity. Cell-free, pH-neutralized supernatants prepared from the three E. faecium strains (JBL1061, JBL1083 and JBL1351) exhibited strong antilisterial activity against L. monocytogenes, and were subjected to more detailed analyses. The antagonistic factors produced by these three strains were sensitive to chloroform and several proteolytic enzymes, resistant to heat (121 degrees C, 20 min), and stable over a wide pH range (3.0-10.0). Moreover, they were listericidal without causing cell lysis. These data suggest that a bacteriocin(s) is involved in the inhibition of L. monocytogenes by E. faecium JBL1061, JBL1083 and JBL1351.

Inhibition of Botulinum Toxin Formation in Bacon by Acid Development

Lactobacillus plantarum , as a producer of lactic acid, and sucrose as a fermentable carbohydrate were evaluated for use in lowering the amount of or eliminating sodium nitrite in bacon. This work was limited to effect on antibotulinal properties. Organoleptic effects were not considered. Slices of bacon were inoculated with spores of Clostridium botulinum types A and B with or without simultaneous inoculation with a culture of L. plantarum , vacuum-packaged and incubated at 27 C. Samples were taken after various periods of incubation and assayed for botulinal toxin. We found that (a) sodium nitrite alone, at 120 ppm, did not give bacon extended protection against development of botulinum toxin if a fermentable carbon source (sucrose in these instances) was not present; (b) without added lactic acid bacteria, the effectiveness of 120 ppm of sodium nitrite plus sugar was variable and depended upon growth of naturally contaminating bacteria and (c) lactic acid bacteria with an adequate amount of sucrose gave good protection against development of botulinal toxin. Upon temperature abuse, acid was produced and growth of C. botulinum was inhibited. Because the protective properties against development of botulinal toxin in the sugar-lactic acid bacteria system were not dependent on the presence of nitrite, nitrite can be lowered to the level necessary to make organoleptically acceptable products without sacrificing safety, thus less nitrosamine formation may be achieved.

Histochemical characterization of muscle differentiation

Abstract A histochemical study of fiber differentiation was conducted on the longissimus muscle of pigs varying in age from 1 day to approximately 24 weeks. The longissimus, as well as other muscles of the pig, is favorable for developmental studies because of the characteristic arrangement of the three fiber types within the fascicles. Differentiation of fiber types was not detectable in the newborn pig. Red fibers were detectable by 1 week. The differentiation of intermediate and white fibers, however, was slower, but all three fiber types were clearly distinguishable by 4-weeks of age. Throughout development there was a gradual increase in area percent composed of white fibers and a decline in area percent of both red and intermediate fibers. Myosin-ATPase and phosphorylase reactions were most sensitive for early detection of fiber type differentiation while the NADH-TR reaction showed a gradual change. The developing white fibers showed a substantial NADH-TR reaction for about 4 weeks. The mean fiber area increased more rapidly in white fibers than in either red or intermediate fibers. The unusual clump distribution of red fibers was evident at the same time that differentiation was detectable.

Effect of dietary vitamin E on pigment and lipid stability of frozen beef: A kinetic analysis.

The effect of vitamin E supplementation on pigment and lipid stability was evaluated with beef wrapped in high or low oxygen permeability films and stored in the dark or under constant illumination at -20°C. Dietary vitamin E supplementation improved pigment and lipid stability in both cases. Illumination increased metmyoglobin accumulation but did not affect lipid oxidation rate in both control and supplemented beef. A predisplay dark storage period of 30 days delayed metmyoglobin accumulation during subsequent display. Kinetic analysis showed that vitamin E supplementation stabilized the oxymyoglobin complex by enhancing the deoxymyoglobin oxygenation rate and by decreasing oxymyoglobin autoxidation rate.