The application of physical stress to microorganisms is the most widely used method to induce cell inactivation and promote food stability. To survive, microorganisms have evolved both physiological and genetic mechanisms to tolerate some extreme physical conditions. This is clearly of significance to the food industry in relation to survival of pathogens or spoilage organisms in food. In some microorganisms, the “cold shock response” has been observed in response to abrupt changes to lower temperatures. This results in the production of specific sets of proteins (cold shock proteins), the continued synthesis of proteins involved in transcription and translation, and the repression of heat shock proteins. The addition of weak acid preservatives (for example, sorbates, benzoates) also induces a specific pattern of gene expression (for example, ‘Acid Tolerance Response’), which is likely to be required for optimal adaptation of bacteria to weak acid preservatives and low pH. The primary mode of the antimicrobial action of low pH is to reduce the internal cell pH (pHi) below the normal physiological range tolerated by the cell, leading to growth inhibition. Survival mechanisms involve maintaining pH homeostasis, and this is achieved by a combination of passive and active mechanisms. Microorganisms adapt to osmotic stress by accumulating non-ionic or compatible solutes such as trehalose, glycerol, sucrose, and mannitol. These compatible solutes help balance the osmotic pressure and help preserve protein function inside the cells. By understanding and controlling such mechanisms of adaptation, it may be possible to prevent growth of key microorganisms in food products.

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Adaptation of Microorganisms to Cold Temperatures, Weak Acid Preservatives, Low pH, and Osmotic Stress: A Review

Sirloin butts from beef carcasses chilled to I or 7.2 C were vacuum packaged and stored at either 0 or S.S C for intervals of 7, 14 , 21 , 28 , or 35 days . Following storage, steaks were obtained for retail caselife studies and taste panel evaluations. Evaluation of primal cuts revea led significant differences in total desirability (p rimarily visual appearance} among storage intervals and between storage temperatures. Initial carcass temperature at fabrication influenced quality of cuts less than did storage temperature over extended storage intervals. Storage of vacuum packaged cuts at 0 C consistently enhanced shelf-life compared to storage at S.S C. Fresh meat packaging systems utilize films which vary in oxygen permeability, moisture vapor transmission rate , heat seal or shrinkage properties, tearing strength, tackiness, brittleness , clarity, and numerous other properties. Plastic films for meat have been used principally as a retail wrap, with oxygen permeability which allows optimal "bloom" yet prevents excess shrinkage . Current technology allows for almost total oxygen exclusion from packaged primal , sub-primal , or retail cuts. Vacuum packaging of fresh meats allows for maintenance of ultimate desirable color, reduces moisture loss during storage (7), inhibits aerobic microbial spoilage (2, 25), and prolongs product acceptance (24, 26, 27). According to previous work (17) , fluid loss from meat can be minimized by quick chilling of hot carcasses. Consumer studies (9, 11, 12) indicated that physical appearance of retail cuts in a display case is the most important factor determining selection. Meat cuts are selected primarily for leanness and then for appearance and freshness. Foods with relatively high microbial populations .could be stored for a considerable period without quality loss, if they were held at relatively low temperatures (19). In commercial practice, consumer 1Present address: Standard Meat Company, Ft. Worth, Texas. 'Transportation and Packaging Research Laboratory, Agricultural Marketing Research Institute, Agricultural R esearch Service, U.S. Departm ent of Agriculture, Beltsville, Maryland. cuts of fresh beef such as steaks and roasts individuall packaged in gas-permeable plastic film frequently have a shelflife of only 2 to 4 days due to development of undesirable odor and / or color (15, 22). The main cause of these undesirable changes is growth of common psychrotrophic bacteria that can produce off-odors through breakdown of meat proteins and lipids, and can indirectly favor metmyoglobin formation (brown color) by depleting the dis solved oxygen concentration at the meat surface (5, 13, 15, 22, 23). Bacterial growth reduces salable shelflife of beef because of discoloration, off-odor, and slime formation (6) . Bacterial contamination during handling and processing involved in fabrication , trimming, and packaging of retail cuts is closely associated with ultimate caselife (4) . Storage life of packaged meat could be extended (18) by reducing microorganisms , especially Pseudomonas, often a predominant organism found on fresh meat (1). Bacterial flora of vacuum-packaged lamb cuts changed from a predominance of Pseudomonas, before packaging and storage, to a predominance of Lactobacillus after 21 days of storage (31) which is similar to findings for beef (21 . 25). Vacuum packaging inhibits surface dehydration , reduces oxygen availability , and can affect bacterial action (20). Surface discoloration limited storage life of lamb cuts (16). Four parameters that influenced meat color were classified (29) as surface dehydration, temperature, oxygen requirements of the meat, and bacterial contamination. Numerous studies have shown that vacuum packaging extends storage life of meat because of the stress placed on microorganisms during storage. Also, meat cuts stored at lower temperatures exhibit longer retail caselife and lower bacterial counts. Because of the increased acceptance of the "boxed beef' distribution system , whereby carcasses are prefabricated into primal or subprimal cuts, vacuum packaged , and shipped to retail outlets and because vacuum packaging is expected to double in the future , it is imperative that , , .. QUALITY OF VACUUM PACKAGED BEEF 593 temperatures at which this vacuum-packaged product is stored be investigated . Accordingly, the objective of this study was to determine the effect of initial chill and storage temperature upon purge loss and keeping quality of vacuum-packaged beef stored for extended periods of time . 111ATERIAL AND METHODS One hundred twenty top sirloin butts (IMPS 184) were obtained from beef carcasses weighing 272 to 318 kg and grading U.S. Good and U.S. Choice, Yield Grade 2 (.30). Cuts were removed from the carcasses at 24-h postmortem for the partially chilled sirloin butts (7.2 C) and 72-h postmortem for fully chilled cuts (1 C). Cuts were vacuum packaged at 29.S inches of Hg via a chamber type, heat seal system (Conofresh B film , Conofresh 6000 chamber) and shipped by refrigerated truck to the Texas A&M Meat Laboratory (48-h post-packaging). Upon arrival the 60 cuts of each chill condition were weighed and assigned to storage coolers with temperatures of either 0 or S.S C for storage intervals of 7, 14 , 21 , 28 , or 3S days , which resulted in six cuts per chill condition/ storage temperature/ storage interval group . All cuts were weighed upon tem ination of the storage interval and again after removal of the packaging material for determination of purge loss. Upon completion of each storage treatment , a two·member experienced panel evaluated each primal cut (30 min after removal of package) for appearance of subcutaneous fat cover according to a six·point scale (6 =very fresh , 4 = normal, 1 =severe discoloration) , for muscle color using a nine-point scale (9 =very light cherry red , 6 =cherry red, 2 =very dark red) , for surface discoloration of lean on a seven -point scale (7 = no surface discoloration, 4 = 2S-SO% discoloration , 1 =total surface discoloration), for odor on a four-point scale (4 = no detectable off·odor, 1 =extreme off-odor) , and for total desirability accord ing to an eight-point scale (8 =extremely desirable, S =slightly desirable, 1 =extremely undesi rable). Microbiological assays for eval uation of the effects of chill condition, storage temperatures and storage intervals are reported separately (.3). At completion of primal cut evaluation and bacterial sampling each primal cut was trimmed of surface discoloration. Two steaks, 2.S em thick, were obtained from each primal cut. One steak from each pair was designated for retail caselife evaluation and the other for taste panel evaluation . Steaks for retail caselife evaluation studies were placed in styrofoam trays, overwrapped with PVC film , and pl~ced under reta il condtions (1 to 3 C under 82 footcandles of incandescent light) . Each retail steak was evaluated initially for muscle color and at 24-h interva ls for 4 days for surface discoloration , peripherial discoloration (five-point scale: S =no peripheral discoloration, I =extreme peripheral discoloration) and total desirability. The remaining steaks were wrapped in freezer paper and frozen at 3S C. After thawing, steaks were oven-broiled in a 177-C electric oven to an internal temperature of 7S C and evaluated by an eight-member trained sensory panel. Each steak was evaluated for tenderness, juiciness, fl avor, and overall satisfaction by use of eight-point rating scales (24). In addition, objective tenderness measurements were made using 1.3-cm diameter cores and a Warner-Bratzler shear device. Data were analyzed by use of standard analysis of variance procedures. When significant differences in main effects were identified , mean separation analyses were conducted {10). RESULTS AND DISCUSSION Appearance of fat cover, muscle color , odor, and surface discoloration of each primal cut were evaluated I individually and then assessed collectively as total desirability (Tables 1-5). At 7 and 14 days, cuts fabricated at a higher internal temperature (7 .2 C) had more desirable ratings for appearance of fat cover than cuts properly chilled (1 C) before fabrication (Table 1). After 14 days of storage there were no differences (P > .05) between treatments. The difference in the time postmortem at packaging may have influenced the fat cover appearance ratings , a variation which cannot be separated from the internal temperature treatment. At TABLE 1. Means for ratings of fat cover appearancee for primal cuts stratified according to initial intenzal temperature and storage temperature

Compendium of methods for the microbiological examination of foods

Micro-organisms require certain metallic elements for growth and function (Hughes & Poole, 1989). These include, for example, the bulk elements potassium and magnesium, and trace elements such as manganese, iron, copper, zinc and molybdenum. Essential metals are shown in Fig. 1, the Periodic Table. The need for these essential metals is, or should be, acknowledged in the composition of growth media. However, higher concentrations of these metals, and the heavy metals such as silver, cadmium, mercury, tin and lead (Fig. l), may be toxic. In any study of the biological effects of either essential or toxic metals it is imperative that full account be taken of the speciation or chemical form(s) of the metal ion in the growth medium, in the organisms’ environment, and in experimental media such as buffers; the bioavailability, and hence reactivity, of the metal is critically dependent upon its speciation (Bernhard et al., 1986; Morrison et al., 1989). Unfortunately it appears that little attention is sometimes given to these matters, and that this has led to some invalid and misleading conclusions in the microbiological literature. In this review it is intended to survey the factors that control the speciation of metal ions in media, to summarize methods for establishing the speciation of metal ions in solution and to suggest some simple guidelines for estimating the bioavailability of metal ions in media. Some commonly-used media will be assessed in the light of these comments and attention drawn to errors that have resulted from a failure to recognize the importance of metal speciation. A recent and topical example will serve to illustrate the importance of speciation effects and the ease with which they are overlooked. It has been suggested in both the scientific and popular press that levels of atmospheric carbon dioxide could be reduced by ‘sowing’ the ocean with tons of iron filings to boost growth of marine algae (Davies, 1990; Martin et al., 1990). Little mention has been made of the problems of maintaining the iron in a form which is bioavailable. Significantly, Hutner (1972) cites much earlier work, which demonstrated that the 725

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Metal speciation and microbial growth-the hard (and soft) facts

http://ssu.ac.ir/cms/fileadmin/user_upload/Mtahghighat/tfood/ARTICLES/microb-safety/A_review_of_microbial_injury_and_recovery_methods_in_food.pdf

A review of microbial injury and recovery methods in food.

The comparative survival of various fecal indicator bacteria and enteric pathogens was studied in a stable well water supply by using membrane chambers There was more variation in the 29 coliform cultures and they died more rapidly, as a group, than the 20 enterococcus cultures that were examined The comparative survival of the organisms tested follows: Aeromonas sp > the shigellae (Shigella flexneri, S sonnei, and S dysenteriae) > fecal streptococci > coliforms = some salmonellae (Salmonella enteritidis ser paratyphi A and D, S enteritidis ser typhimurium) > Streptococcus equinus > Vibrio cholerae > Salmonella typhi > Streptococcus bovis > Salmonella enteritidis ser paratyphi B S bovis had a more rapid die-off than did S equinus, but both had significantly shorter half-lives than the other streptococci The natural populations of indicator bacteria from human and elk fecal material declined similarly to the pure cultures tested, whereas the die-off of fecal streptococci exceeded the coliforms from bovine fecal material

Comparative survival of indicator bacteria and enteric pathogens in well water.

The death of bacteria may be rapid during freezing or slow during extended storage at low temperatures (Weiser and Osterud, 1945). In the latter case, especially, one may expect to find that the first stages involve nonlethal physical or metabolic injury to the bacteria and that as injury becomes progressively more extensive or critical, it eventually leads to the death of the cell. Destruction of bacteria by low temperatures has been investigated extensively but the intermediate stages of injury have received little attention. The present paper deals with the extent and nature of nonlethal injury to bacteria by storage at subfreezing temperatures.

Metabolic injury to bacteria at low temperatures.

CROXTON, W. C. 1928 Revegetation of Illinois coal stripped lands. Ecology, 9, 155-175. HEADLEE, A. J. W. 1955 Characteristics of minable coals of West Virginia. West Virginia Geological Survey, Pt. VI, The structure of bituminous coals, 13(A) 123-149. KOHNKE, H. 1950 The reclamation of coal mine spoils. Advances in Agron., 2, 317-349. PATERSON, D. D. 1939 Statistical technique in agricultural research. McGraw-Hill Book Co., Inc., New York. STOKLASA, J. AND ERNEST, A. 1905 Ueber den Ursprung, die Menge und die Bedeutung des Kohlendioxyds im Boden. Centr. Bakteriol., Parasitenk., 14(2), 723-736. TYNER, E. H. AND SMITH, R. M. 1945 The reclamation of the strip-mined coal lands of West Virginia with forage species. Soil Sci. Soc. Amer. Proc., 10, 429-436. TYNER, E. H., SMITH, R. M., AND GALPIN, S. L. 1948 Reclamation of strip-mined areas of West Virginia. J. Am. Soc. Agron., 40, 313-323. WAKSMAN, S. A. 1952 Soil microbiology. John Wiley & Sons, Inc., New York. WILSON, H. A. AND STEWART, G. 1955 Ammonification and nitrification in a strip mine spoil. Bull. 379T. West Virginia Agricultural Experiment Station, Morgantown, West Virginia. WILSON, H. A. AND STEWART, G. 1956 The number of bacteria, fungi and actinomycetes in some strip-mine spoils. Bull. 388T. West Virginia Agricultural Experiment Station, Morgantown, West Virginia.

Rapid Destruction of Bacteria in Commonly Used Diluents and Its Elimination

A selenite brilliant green medium for the isolation of Salmonella.

DAY, W. C., GOTTLIEB, S., AND PELCZAR, M. J. 1952 The biological degradatioin of lignin; The inability of Polyporous versicolor to metabolize sodium lignosulfonate. Appl. Microbiol., 1, 78-81. FOLIN, 0. 1929 Two revised copper methods for blood sugar determination. J. Biol. Chem., 82, 83-93. KAYE, S. 1949 The sterilizing action of gaseous ethylene oxide; The effect of ethylene oxide and related compounds upon bacterial aerosols. Am. J. Hyg., 50, 289-295. KAYE, S., AND PHILLIPS, C. R. 1949 The sterilizing action of gaseous ethylene oxide; Effect of moisture. Am. J. Hyg., 50, 296-306. KONETZKA, W. A., PELCZAR, M. J., AND GOTTLIEB, S. 1952 The biological degradation of lignin; The bacterial degradation of a-conidendrin. J. Bacteriol., 63, 771-778. KONETZKA, W. A. 1952 The bacterial degradation of lignin and related compounds. M. S. Thesis, University of Maryland, College Park, Maryland. KONETZKA, W. A., BURNETT, G. W., AND PELCZAR, M. J. 1955 The bacterial hydrolysis of decalcified dentin. Brit. Dental J., In press. PHILLIPS, C. R. 1949 The sterilizing action of gaseous ethylene oxide; Sterilization of contaminated objects with ethylene oxide and related compounds; Time, concentration and temperature relationships. Am. J. Hyg., 50, 280-288. PHILLIPS, C. R., AND KAYE, S. 1949 The sterilizing action of gaseous ethylene oxide; Review. Am. J. Hyg., 50, 270-279. WILSON, A. T., AND BRUNO, P. 1950 The sterilization of bacteriological media and other fluids with ethylene oxide. J. Exptl. Med., 91, 449-458.

J. L. Stokes

Papers

Metabolic injury to bacteria at low temperatures.

Rapid Destruction of Bacteria in Commonly Used Diluents and Its Elimination

A selenite brilliant green medium for the isolation of Salmonella.

A Modified Selenite Brilliant-Green Medium for the Isolation of Salmonella from Egg Products.

A rapid method for the estimation of the bacterial content of precooked frozen foods