Compendium of methods for the microbiological examination of foods

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