Production of Weep in Packaged Refrigerated Beef J. R. ZARATE and N. E. ZARITZKY ABSTRACT The effect of storage conditions on the production of natural exu- date (weep) in packaged refrigerated beef was analyzed. The influ- ence of temperature (0” and 4”C), storage time, characteristics of flexible plastic film used in packaging (permeability and heat shrink- ing), and area per unit volume of meat cut were determined in semi- tendinosus and L. dorsi muscle. Heat shrinking of the film reduced release of weep, and this effect became more important when tem- perature increased; at 0°C and 4°C percentage of weep represented 68% and 51% of the values corresponding to nonshrinking condi- tion. Weight of exudate was correlated to equivalent area of the meat pieces and a linear relationship was established between per- centage of weep and equivalent area per unit volume of sample. INTRODUCTION MANY STUDIES have been made seeking procedures cap- able to reduce the formation of weep, in view of its impor- tance in the case of small meat cuts sold retail. It was demonstrated that a high final pH, slow postmortem glyco- lysis and quick cooling before the rigor mortis sets in, in- creased the water-holding capacity. A fast decrease of temperature during postmortem glycolysis, induced shor- tening and loss of water-holding capacity (Hamm, 1975). The water-holding capacity Fcreased if storage temperatures were close to 0 C, since it reduced denaturation of pro- teins. Similarly, weep production decreased if the area of the cut were minimal, and if it were cut along the direction of the fibers instead of across them (Lawrie, 1974). Howard (1956), Taylor (1972), and Penny (1974), pointed out that weep percentage in meat, varied according to the area per unit volume of meat cut. The purposes of this study were: to quantify the influ- ence of temperature and storage time, type of muscle, area per unit volume of meat cut and the effects of plastic film heat shrinking on weep production in refrigerated packaged beef, and to determine the driving forces governing the release of juices during storage time. MATERIALS & METHODS CYLINDRICAL SAMPLES of beef (5 cm in diameter, 1.5 cm thick) from semitendinosus (ST) and longissimus dorsi (LD) muscles were stored at 0°C and 4°C under different conditions of packaging to analyze the effects of storage time, temperature, type of muscle and characteristics of the film on weep production. Two kinds of films were used: low density polyethylene, 60 pm thick (water vapor permeability (WVP) = 12g of vapor/m* day, at 30°C and RH = 78%; oxygen transmission rate (OTR) = 6.500 cm3/m2 atm day at 23”C), and for vacuum packaging a composite EVA/SARAN/EVA coextruded film, 60 pm thick, (trade name Super Cryovac, WVP = 7.2 g/m* day, at 30°C and RH = 78%; OTR = 37.5 cm3/m2 atm day, at 25°C and RH = 75%). Saran is a polyvinyl and polyvinylidene chloride copolymer and EVA is ethyl- ene vinyl acetate copolymer. Oxygen transmission rate of the composite film decreases at low temperatures, following an Arrhenius relationship (Eustace, Authors Zarate and Zaritzky are affiliated with Centro de Investi- gacibn y Desarrollo en Criotecnolog;a de Alimentos IClDCAl- (CONICET-CIC-UNLP), Facultad de Ciencias Exactas, 47 y 116 La Plata (19001, Provincia de Buenos Aires, Argentina; 1981); the OTR of these films at 3.5’C was generally less than 10% of the value at 25°C. The EVA/SARAN/EVA is a heat shrinkable material; shrinking reduces the OTR approximately in proportion to the reduction in area. Heat shrinkage of the bags was achieved by immersing the packaged samplesin water at 83-85’C for 1 set; during this time the bags were reduced to 65% of the original area. Differences in weep production between heat shrinking (SH) and nonshrinking (NSH) treatments were compared. The ST and LD muscles were removed from steers classified as U2 grade A, according to the Argentine National Meat Board classi- fication (carcass weight of 240 kg) with a postmortem time of 48 hr at 4“C; initial pH values measured with INGOLD LOT 405-M4 electrode were in the range 5.6 - 5.8. Packaging of samples under vacuum conditions with EVA/ SARAN/EVA film was carried out using Minidual equipment with a single chamber and heat sealing, at a residual pressure in the equip- ment of 4.5 mm Hg. Vacuum inside the packages was measured by placing the sam- ples inside an anaerobic jar (Oxoid) which was gradually evacuated. The pressure at which the film was detached from the meat was determined by manometric reading. The averagepressure of the in- terior atmosphere in the packages was 5 cm Hg. The storage temperatures were recorded with thermocouples inserted in the samplesand connected to a Datalogger Fluke 2240B. Storage times of the samples were determined by maximum total bacterial counts of lo7 CFU/cm* starting from initial average values of lo4 CFU/cm*. These conditions lead to the following periods: 11 and 7 days in polyethylene at 0 and 4”C, respectively; 21 and 14 days in vacuum at the same temperatures (Ayres, 1960; Shaw, 1972; Sutherland et al., 1975; Newton and Gill, 1978; Newton and Rigg, 1979; Zamora and Zaritzky, 1982). In order to quantify weep production, the packaged samples were weighed initially (MO). Sample weight at different storage times (M) was also determined after drying the surfaces with filter paper without pressure to eliminate the residual liquid (% Weep = (MO-M)/Mo). In each storage condition two groups of experiments were per- formed: (a) four animals were assigned at random to each treatment, (b) sections of ST and LD muscles from each animal were submitted simultaneously to all the tested conditions in order to analyze the influence of biological variability on the average results. The influence of the ratio area per unit volume of sample (A/V) on weep percentage was determined using cylinders of ST muscle (1 - 7 cm in diameter and 0.5 - 3 cm thick) packaged in EVA/ SARAN/EVA (NSH) and kept at 4°C during 1 hr for weight loss determinations. The effect of storage time on these results was analyzed storing similar samples at 0°C during 15 days; whole semi- tendinosus muscles were included to extend the range of A/V ratios. The driving forces governing the release of weep were studied using cylindrical samples (2.3 - 5 cm in diameter and 0.7 - 1.7 cm thick) removed from the same ST muscle. The samples were sec- tioned with the principal axis parallel to muscle fibers and arranged in acrylic cells with filter paper attached to the bases to collect the natural exudate during storage time; radial flow of weep was inhibited by proper sealing the lateral area of samples and condi- tions of unidirectional flow of matter were achieved. In the statistical study of results, analysis of variance was applied to pairs of regression equations (Neter and Wasserman, 1974) to determine the significance of treatment effects. RESULTS & DISCUSSION Influence of the storage time, temperature and packaging conditions on the production of weep Volume 50 (1985)-JOURNAL OF FOOD SCIENCE-755