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Cryogenic chilling of pork carcasses: Effects on muscle quality, bacterial populations and palatability
Meat Science 29 (19911 1-16
Cryogenic Chilling of Pork Carcasses: Effects on Muscle Quality, Bacterial Populations and Palatabilityt S. D. M. Jones, G. G. Greer, L. E. Jeremiah, A. C. Murray & W. M. Robertson Red Meat and Beef Production Section, Agriculture Canada, Lacombe Research Station, Laeombe, Alberta, Canada TOC 1S0
(Received 27 November 1989; revised version received 28 January 1990; accepted 5 February 1990)
ABSTRACT Two experiments were conducted to determine the effects o f cryogenic chilling on the carcass shrinkage, meat quality, bacterial condition and palatability o f pork. In experiment L pork sides were chilled at 1°C (n = 20), or immersed in liquid nitrogen ( L N ) for I or 3 min prior to placement in a 1°C cooler. Muscle temperature in the loin was significantly lower at 2 and 6 h post m o r t e m in treated compared to control sides, and loin muscle p H was higher ( P < 0"05) at 6 h post m o r t e m in sides immersed for 3 min in LN. Carcass side shrinkage was reduced from 29"3 g kg -1 in control sides to 20"9 and 13"5 g k g - 1 in sides dipped in L N for I and 3 min. Chilling treatment had no significant effect on the survival o f mesophilic bacteria on carcass sides, on meat colour, drip loss, protein solubility or sarcomere length, but sides dipped for I min in L N has a higher muscle shear value than control sides. In experiment 11, carcass sides from halothane posit&e ( H + ) and negative ( H - - ) pigs were conventionally chilled ( n = 49), immersed in L N for 3 min (n = 23), or electrically stimulated and chilled in L N for 3 min (n = 26). Similar results for temperature, pH, colour, protein solubility and drip loss in loin muscle were found to those in experiment L Laboratory taste panel results showed that chilling treatment had no effect on palatability. Genotype produeed meaningful differences in most palatability attributes with H + pigs t Scientifie paper no. 630, Agriculture Canada, Lacombe Researcla Station, Lacombe, Alberta, Canada T0C 1SO.
1 Meat Science 0309-1740/90/$03-50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
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S.D.M. Jones et al. having less tender, less juicy and less desirable fla~our than pork from H pigs. Laboratory studies with inoculated fresh muscle slices showed that a 3 min immersion in L N resulted in a lO-foM reduction in the aerobic spoilage pseudomonads, but effects upon other spoilage bacteria and potential human pathogens were less pronounced. It was concluded that cryogenic chilling using L N reduced carcass shrinkage during cooling, but had no consistent effects on meat quality, palatability or bacterial numbers on the carcass. In contrast, genotype had a significant effect on most pork quality and palatability attributes.
INTRODUCTION Rapid or blast-chilling has been introduced in several Western Canadian pork processing plants over the last five years, and has been in use in several European countries for a longer period of time. The general procedure has been to chill carcasses after leaving the slaughter floor at temperatures between - 20 and - 30°C for a period of 1 h followed by an equilibration period in a conventional cooler (1-4°C). Scientific studies indicate under laboratory (Dransfield & Lockyer, 1985) and plant (Barton-Gade et al., 1987; Moiler & Vestergaard, 1987) conditions that blast-chilling can induce shortening of the muscle fibres that will result in tough meat. Honikel (1986) considered that pork with a normal post-mortem glycolysis (pH 6.0-6-5 at 45 min post mortem) would not cold shorten if the temperature of the muscle did not fall below 15°C, prior to the attainment of rigor mortis. There have been relatively few studies completed in pork on the possibility of cold induced toughening using the carcass as the experimental unit rather than excised muscles. Some studies have shown that rapid chilling will result in darker pork (Borchert & Briskey, 1963; Jones et al., 1988) with less drip or purge loss (Honikel, 1986), whereas others (Swatland, 1983) have demonstrated no such effect. Research studies have also been inconclusive regarding the effects of rapid chilling on the keeping quality of meat. For example, Dann (1972) recommended the use of rapid chilling to decrease carcass contamination, whereas other studies (Greer & Dilts, 1987, 1988) have shown that neither blast-chilling (--25°C, 1 h) nor spray-chilling improved the bacterial quality or retail case life of pork. Cryogenic chilling of carcasses using liquid nitrogen represents an extreme chilling procedure where carcass heat is removed more rapidly than that obtained by blast-chilling. Although cryogenic chilling is not directly comparable to blast-chilling, it does provide information on the effects of very rapid cooling on carcass quality. The objectives of this study were to
Cryogenic chilling of carcasses
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establish if cryogenic chilling of whole pork carcasses affected muscle quality, palatability and bacterial numbers on carcasses. In addition, two separate genotypes (halothane positive and negative) were compared using different chilling regimes, and the sensitivity of certain groups of bacteria to cryogenic chilling was assessed.
EXPERIMENTAL Two experiments were conducted to assess the effects of cryogenic chilling on pork muscle quality, bacteriology and palatability.
Experiment I Twenty Lacombe pigs (halothane negative, with normal post-mortem glycolysis) were slaughtered at approximately 90 kg live weight in a research abattoir. The pigs were stunned by electric shock (400 V, 1-7 amperes) with a head to back electrode for 3--4 s. The carcasses were shackled by alternate legs to reduce the possibility of side to side variation in muscle quality which might be caused by muscle reflex activity (Swatland, 1986). Warm split sides (left and right) were weighed approximately 40 min post-slaughter, and pH and temperature (Corning pH meter, Corning Glassworks, Medfield, Mass., and Ingold combination electrode, Ingold Electronics, Andover, Mass.) were recorded in the centre of the longissimus dorsi muscle between the 12th and 13th ribs. For the first ten carcasses, the control side was placed in a cooler set at I°C with an air velocity of 0-5 m s-1 for 24 h. The treated side was completely immersed in a stainless steel tank containing liquid nitrogen (LN) for 1 min. In the second batch of ten carcasses, the same procedure was repeated except that the treated side was immersed in LN for 3 min. Following immersion, the sides were placed in a holding cooler at I°C. Temperature was recorded at 2h post mortem, and both pH and temperature were recorded at 6 h and 24 h post mortem using the same location previously described. At 6 and 24h post mortem all sides were weighed to determine shrinkage losses. The boneless muscle from the 5-13th rib was excised from both carcass sides at 24 h and used for evaluating muscle quality. A 10 mm portion was removed from the posterior end of the excised longissimus dorsi muscle to square off the muscle. A 25 mm steak was then removed from the posterior end, weighed and placed in a polythene bag for 48 h at 2°C to determine drip loss. The steak was reweighed after 48 h, after
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removing any excess moisture with a paper towel. A second 25 m m steak was removed from the posterior end of the muscle at 48 h post m o r t e m and the freshly cut surface allowed to bloom for 30 min before recording muscle colour using a Minolta C h r o m a Meter II (Minolta Camera Company, Ramsey, New Jersey). Ultimate pH was also measured on the same sample. The steak was then cooked to a final temperature between 80-85°C in a microwave oven and held overnight at 2°C. After equilibration to room temperature, two cores of 1 9 m m were removed and sheared using the Ottawa Texture Measuring System (Canners Machinery, Simcoe, Ont.) equipped with a Warner-Bratzler cell. Protein solubility was determined as described by Murray et al. (1989). Sarcomere length was determined using a phase contrast microscope and recording the length of 10 sarcomeres for each of 20 muscle fibres. A further five carcasses were used to assess the effect of LN on bacterial numbers on the carcasses pre- and post-treatment. The left sides were conventionally chilled at l°C, while the right sides were immersed in LN for 3 min, followed by conventional cooling. Samples for bacterial analyses were taken prior to and immediately following chilling treatments by aseptically excising 10cm 2 ( l m m depth) of surface tissue from the shoulder, belly and ham of each side. After sampling, the combined samples were homogenized in 0"1% peptone-water for 2 m i n using a Colwo/'th Stomacher (A. J. Seward, London, UK). Following decimal dilutions in 0.1% peptone-water, appropriate dilutions were plated by the spread plate method and mesophilic bacteria determined as previously described (Greer & Dilts, 1987).
Experiment II Twenty four halothane negative (Lacombe) pigs and 25 halothane positive pigs were slaughtered and dressed as previously outlined. Within each line, three treatments were imposed: (1) control; side chilled at I°C, (2) side immersed in LN for 3 m i n followed by the same chilling regimen as treatment 1, (3) side stimulated at 500 V, 1 ampere for 90 s (2 s on, 1 s off), followed by same LN treatment and chilling regimen as treatment 2. The muscle quality measurements were identical to those described for experiment 1. A 15"25cm section of the longissimus dorsi muscle was excised immediately posterior to the 13th rib at 24 h p o s t m o r t e m and used for the evaluation of palatability attributes. All samples were roasted to an internal temperature of 75°C in an electric convection oven preheated to 177°C. U p o n removal from the oven, all roasts were cut into 1.9 x 1-9 x 1-9 cm 3 cubes taking care to avoid large pieces of fat and connective tissue and the
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external surface. Cubes were then randomly allocated to an experienced six member laboratory panel, screened and trained according to AMSA guidelines (American Meat Science Association, 1978), by placing them in glass containers in a circulating water bath held at 75°C. All samples were evaluated within 15 min after removal from the water bath in individual, environmentally controlled booths, under 538 lux of incandescent and fluorescent white light. Distilled water and unsalted soda crackers were provided to purge the palate of residual flavours between samples. All samples were evaluated for using a 9 point structured, descriptive, category scale for initial and overall tenderness (9 = extremely tender; 1 = extremely tough), for connective tissue ( 9 = no perceptible connective tissue; 1 = abundant perceptible connective tissue), and for juiciness (9 = extremely juicy; 1 = extremely dry), and for flavour intensity (9 = extremely intense pork flavour; 1 = extremely bland pork flavour). Flavour desirability and overall palatability were assessed using a 9 point structured, hedonic, category scale (9=extremely desirable; 1 =extremely undesirable). In addition, the samples displaying sour flavours and other off-flavours were enumerated. A laboratory study was conducted to assess the survival of different bacterial strains following exposure to LN. The sources of the bacterial strains were: Pseudomonas D23 (G. G. Greer, Agriculture Canada); Pseudomonasfragi Ju 7 and Ju 14 (R. H. Dainty, A F R C Bristol); Brochothrix thermosphacta B2 (G. G. Greer, Agriculture Canada); Lactobacillus divergens M2 (A. F. Egan, CSIRO); Alteromonas putrefaciens ATCC 8071, Escherichia coli ATCCl1775, Staphylococcus aureus ATCC25923 and Salmonella typhimurium ATCC 14028 (American Type Culture Collection). All organisms, with one exception, were grown in tryptic soy broth (Difco) for 17-20 h. With Lactobacillus divergens M2, organisms were grown in MRS broth (Difco). The incubation temperature was 35°C for E. coli, S. aureus and S. typhimurium and 25°C for all other organisms. Following growth, bacteria were diluted in 0.1% peptone-water to give a bacterial density of about 1 0 7 m l - L These suspensions served as inocula for pork muscle slices. Pork muscle slices were obtained from the m. semimembranosus removed from carcasses after 24 h of post-mortem chilling at 1°C. For each bacterium examined, 15 discs of 10 c m 2 a r e a (1 mm deep) were aseptically excised from a single muscle. Muscle slices were inoculated with 0.1 ml of bacterial inoculum and the inoculum was evenly distributed using a sterile, bent, glass rod. To ensure uniform inoculation all 15 inoculated slices were mixed in a sterile Whirl-Pak bag (Fisher Scientific, Toronto, Canada). Five samples were then distributed to each of three Whirl-Pak bags. One group of five served to determine initial bacterial loads. The second was used as a control
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and was stored at 2°C for 24 h. The third group of five samples was immersed in LN for 3 min and equilibrated at 2°C for 24 h. Bacterial numbers were enumerated following homogenization of each muscle slice in 90ml of 0-1% peptone-water for 2min in a Colworth Stomacher. Specific organisms were enumerated as follows: Pseudomonas (Cephaloridine-Fucidin-Cetrimide Agar, 25°C, 48h; Mead & Adams, 1977), A. putrefaciens (Peptone Iron Agar, 20°C, 96h; Levin, 1968), B. thermosphacta (Streptomycin Sulphate-Thallous Acetate-Actidione Agar, 25°C, 48 h; Gardner, 1966), L. divergens (MRS Agar, 25°C, 72 h; Schillinger & Lucke, 1987), E. coli (Violet Red Bile Glucose Agar, 35°C, 24 h; Mossel et al., 1962), S. aureus (Baird-Parker Agar, 35°C, 48 h; ICMSF, 1978) and S. typhimurium (SS Agar, 35°C, 24 h; ICMSF, 1978). Bacterial counts were converted to common logarithms and expressed as log colony forming units c m - 2 (log C F U c m - 2). The meat quality and palatability data were analysed using the general linear model procedure with a within animal model that included treatment and error components for experiment I. The model for experiment II also included genotype, and the treatment x genotype interaction. Linear contrasts were used for the separation of means. The bacteriology data were analysed by analysis of variance and statistical significance of treatment differences were determined using the Student's 't' test.
RESULTS The pork carcasses in experiment I averaged 73.0 kg with a fat thickness of 19"5 mm measured at the 3--4th last ribs, 70 mm from the carcass mid-line. Carcass weight and fatness were similar ( P > 0-05) across treatments. In experiment II, there were no differences in chilling treatments within genetic line for carcass weight or fatness, but halothane positive pigs had heavier and fatter carcasses (79.5 vs 74.7kg and 22-2 vs 19-9mm fat) than did halothane negative pigs.
Experiment I Immersion of pork carcasses in LN caused a significant reduction of loin temperature at 2 and 6 h post mortem. The magnitude of these differences among treatments was largest at 2 h post-slaughter (Table 1). Carcass sides dipped in LN for 3 min had a deep loin temperature close to 9°C, compared to controls with a temperature of almost 26°C. Cooling rate also influenced the rate of change of pH. At 6 h post mortem, sides dipped for 3 min in LN
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TABLE 1
Temperature and pH Changes in the Longissimus dorsi Muscle Following 1 and 3 min Immersion of Warm Pork Sides in Liquid Nitrogen Time (post mortem)
Control
Liquid nitrogen immersion
SE
1 min
SE
3 min
SE
Temperature, °C: 45min 2h 6h 24 h
39"43 ~ 25"60a 8-24 * 1"94
0"15 0"63 0"25 0-08
39-96 ~'b 14"86b 4-40 b 1"84
0-25 1-32 0-24 0-03
40-1ff' 8-89c 0-36 c 1-88
0"25 0-84 0"20 0-15
pH: 45 min 6h 24 h 48 h (ultimate)
6.04 5.60a 5.50 5.49
0.05 0.04 0-02 0-02
5"93 5-74~ 5.52 5-47
0.08 0.05 0.03 0.03
6,05 5"96" 5-58 5-54
0.09 0.08 0.03 0.03
,.b.c Means in the same row with different superscripts are significantly different (P < 0.05). had a significantly higher pH than controls, whereas there were no d i f f e r e n c e s a m o n g t r e a t m e n t s a t 45 m i n o r a t 24 h p o s t m o r t e m ( T a b l e 1). Cooler shrinkage of sides was significantly reduced in the treated c o m p a r e d t o t h e c o n t r o l s i d e s ( T a b l e 2). A t 2 4 h p o s t m o r t e m , s i d e s d i p p e d f o r I o r 3 m i n i n L N h a d a s h r i n k a g e s a v i n g o f 8-4 a n d 1 5 - 8 g k g - 1 , r e s p e c t i v e l y c o m p a r e d t o c o n t r o l sides. T h e r e w e r e n o t r e a t m e n t d i f f e r e n c e s TABLE 2 Cooler Shrinkage and Pork Quality Following 1 and 3 min Immersion of Warm Sides in Liquid Nitrogen Control
Cooler shrinkage, g k g - 1: 6 h post mortem 24 h post mortem Meat colour: L* a* b* Meat drip, g k g - t Protein solubility, g k g - ~ Shear value, kg Sarcomere length,/~m
- 17"8a - 29"3a 49-5 8-2 2-4 26-2 174.7 5-17" 1.73
SE
0"56 0"83 0-67 0"32 0-30 2-8 5-9 0.27 0.04
Liquid nitrogen immersion I min
SE
3 min
SE
-8-9 b - 20-9 b
0"97 1"56
-4-I c - 13"5c
0"89 2"78
0"95 0-40 0-36 4-3 9.1 0-35 0-05
48"3 7"8 1.9 22.4 183-8 5-71a.~ 1-76
0-85 0-38 0-37 2.9 4-9 0-41 0"06
49-5 8-0 2-2 29-3 177.2 6.28 b 1.72
a.b., Means with different superscripts in the same row are significantly different (P < 0-05).
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S . D . M . Jones et al. TABLE 3 Effect o f Liquid Nitrogen Immersion on the Survival o f Mesophilic Bacteria on Pork Carcasses
Sample time
Before treatment After treatment P
log CFU cm-2 Conventional °
Liquid nitrogen
P
2.28 b 2"84 0.070
2"25 2-52 0-300
0-930 0-289
° Conventionally treated carcasses were stored in a cooler at I°C for 24h. Liquid nitrogen treated carcasses were immersed in liquid nitrogen for 3 min, followed by equilibration for 24 h at I°C. b Least squares means o f five carcasses. Standard error was 0.21.
in meat colour, protein solubility or sarcomere length (Table 2). Muscle shear value was significantly increased for sides dipped in LN for 1 min compared to control sides. However, sides dipped for 3 min in LN had similar muscle shear values to those of controls. The number ofmesophilic bacteria on pork sides before and after chilling treatments is shown in Table 3. The results show that neither conventional chilling (P > 0-05) nor LN chilling (P > 0-10) had any significant effect on the total numbers of bacteria.
Experiment II LN had a similar effect on chilling rate as previously shown for the 3 min LN treatment in experiment 1. At 2 h post mortem, LN chilled sides had loin temperatures 16-18°C lower than control sides. This difference was reduced to 7-8°C by 6 h post mortem, and by 24 h post mortem loin temperature was similar among all treatments. LN reduced the rate of pH decline in halothane negative pigs at 6 h post mortem, but there was only a trend towards a decrease in pH fall in halothane positive pigs (Table 4). Treatment had no effect on ultimate meat pH. Electrical stimulation did not increase the rate of pH decline in either halothane positive or negative pigs (Table 4). Cryogenic chilling reduced carcass shrinkage over the first 24 h of cooling (Table 5). The shrinkage saving for LN treated sides averaged 17-6 g kg-x for both halothane positive and negative pigs compared to conventionally chilled sides. LN chilling had no effect on meat colour, meat drip, protein solubility, sarcomere length or shear value compared to conventional chilling (Table 5). Genetic line on the other hand had highly significant effects on meat colour, meat drip, protein solubility and shear value.
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TABLE 4 Effects o f Electrical Stimulation a n d Liquid N i t r o g e n I m m e r s i o n o f Carcasses on T e m p e r a t u r e and p H Changes in the Longissimus dorsi Muscle o f pigs that were H a l o t h a n e Positive o r Negative
Time (post mortem)
Halothane negative Control Liquid N ES/liquid N
Temperature, °C: 45 min 2h 6h 24 h
39" 12" 27-00* 7.89 b 0-89"
39" 17° 8"34 e 1-08~ 0-61 b
39"82 b 10"69d 1.38 c 0-65 "'b
pH: 45 min 6h 24 h 48 h (ultimate)
6" 11" 5"69* 5"48" 5-52
6-05" 6-01 b 5"63 b 5-59
6"00~ 6"01 b 5"61 b 5-55
Halothane positive Control Liquid N
39-70 b 28"69 b 10.03" 0-96*
40" 12 b 12"62~ 1.75" 0"85"
5-56 b 5"54~ 5"54*'c 5.55
5"57b 5-61 "'~ 5"57°'b'~ 5-53
ES/liquid N
40-09 b 11"10~d 2-16 ~ 0"82 °
5"57 b 5"76° 5"58 ~'c 5-52
.,b.~.d.e M e a n s with different superscripts in the same row are significantly different (P < 0.05).
TABLE 5 Effects o f Electrical Stimulation and Liquid N i t r o g e n I m m e r s i o n o f Carcasses o n the Shrinkage and Quality o f Pork from H a l o t h a n e Positive (nn) and Negative (NN) Animals
Halothane negative Control Liquid N
ES/liquid N
Halothane positive Control LiquM N
ES/liquid N
C o o l e r shrinkage, gkg-l:
6h post mortem - 18-7" 24 h post mortem M e a t colour: L* a* b* M e a t drip, g k g - 1 Protein solubility, g k g -1 Shear value, kg Sarcomere length, /zm
- 26-6*
-5"33 b - 8-3 b
--3"60 b --9"2 b
49"3 ° 7-8" 2"2" 20-6"
48-1 *'c 7-9" 1"6" 23-6"
47-9 *'c 7-2 *'~ 1"4" 19-4"
183-7" 185-0 ~ 4.85* 5"03*
190"4° 5.15"
1"88
1-84
1-89
- 16"9" -4-5 b - 26"2 ° - 11"5b
-2-9 b - 7"4n
53"2 b 8-6 b 3-9b 33-2 b
53"5 b 8.3 °'b 3-7 b 36.5 ~
51-7 *'b 8.3 *'n 3-3 n 36.8 b
135.1 n 7-67 b
140-9 b 7.62 n
147-4 b 7.63 b
1-91
1-89
1-89
,.b., M e a n s with different superscripts in the same row are significantly different ( P < 0-05).
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TABLE 6
Effects of Electrical Stimulation and Liquid Nitrogen Immersion on Palatability Traits Trait
Treatment
S E Prob.
ControP Liquid Nb
Initial tenderness Overall tenderness Amount of perceptible connective tissue Juiciness Flavour desirability Flavour intensity Overall palatability Number of samples displaying sour flavours Number of samples displaying other off-flavours
Treatment
S E Prob.
ControP ES/liquid Nc
5-65 5.45
5-52 0.17 0.570 5-30 0.19 0-586
5.76 5-68
5-66 5-48
0.18 0.666 0.19 0.444
8.24 4.50 5.02 5"58 4.65
8.22 4-70 5"12 5"48 4.75
0.873 0"480 0.554 0-494 0.607
8"31 4.62 5-00 5.45 4.72
8"05 4.72 4.89 5"27 4.60
0.11 0"19 0-14 0"12 0-13
1.28
1"03 0.21 0-408
1.26
1.17
0-25 0-804
0.11
0-12 0-06 0"835
0"30
0-30
0"11 0"969
0.10 0-19 0.12 0"10 0.14
0-103 0-725 0.582 0"236 0-539
Chilled 24 h at I°C. b Dipped in liquid nitrogen for 3 min, and then chilled for 24 h at I°C. c Electrically stimulated, dipped in liquid nitrogen for 3 min, and then chilled for 24 h at I°C.
Electrical stimulation in combination with LN chilling had no significant effect in meat quality parameters. A 3min dip in LN, either alone or in combination with electrical stimulation, produced no significant effect on any of the palatability attributes measured (Table 6). However, differences were observed in most palatability attributes due to genotype (Table 7). Roasts from halothane negative pigs were consistently more tender both initially and overall,juicier, and more desirable in both flavour and overall palatability (P < 0.05). In addition, halothane positive pigs had a higher incidence of sour off-flavours than halothane negative pigs when dipped for 3 min in LN. Inoculated muscle
With one exception, the control chilling treatment (24h, 2°C) did not significantly (P > 0-05) change bacterial numbers (Table 8). In the case of P. fragi Ju 7, bacterial numbers significantly increased in the control treatment. Populations of A. putrefaciens ATCC 8071, L. divergens M2 and S. aureus ATC 25923 were not significantly (P > 0.05) altered by LN. The populations of Pseudornonas D23, P. fragi Ju 7 and Ju 14, B. thermosphacta B2, E. coli ATCC 11775, and S. typhimurium ATCC14028 were all significantly
TABLE 7 Effects o f G e n o t y p e on Palatability Traits
Trait
Liquid N c
Liquid N / E S ~
Genotype a
Initial tenderness Overall tenderness A m o u n t o f perceptible connective tissue Juiciness Flavour desirability Flavour intensity Overall palatability N u m b e r o f samples displaying sour flavours N u m b e r o f samples displaying other off-flavours
Genotype a
NN
nn
SE
Prob.
NN
nn
SE
Prob.
5-95 * 5-72 a
5.21 b 5"02b
0-17 0"19
0.003 0.013
6-05 ~ 6-01 a
5-37 b 5-15 b
0.18 0-19
0-009 0.002
8-30 4-91 ~ 5-474 5.42 5.204
8"17 4.30 b 4-67 b 5-64 4-20 b
0.10 0.19 0.12 0"10 0-14
0-360 0-031 0.000 0.013 0.000
8-31 5-03 a 5.28" 5"25 5.09"
8'05 4'32 b 4.61 b 5.47 4.23 b
0-1l 0-19 0-14 0-11 0-13
0-111 0.013 0-001 0.163 0.000
0-59 b
1-71~
0-21
0.000
1.04
1.39
0.25
0-331
0"08
0-14
0"06
0-500
0.21
0-39
0-1l
0-234
o.b M e a n s in the same experiment and row bearing different superscripts differ significantly (P < 0"05). Dipped in liquid N for 3 min and then chilled for 24 h at 1°C. Electrically stimulated, dipped in liquid N for 3 m i n a n d then chilled at 2 4 h at I°C. d Genotype: N N = h a l o t h a n e negative; n n = h a l o t h a n e positive.
TABLE 8 Effects o f Liquid N i t r o g e n on Bacterial Survival on Pork Semimembranosus Muscle
Bacteria
log CFU/cm- 2 Before treatment
Pseudomonas D23 P. fragi Ju 7 P.fragi Ju 14 A. putrefaciens A T C C 8071 B. thermosphacta B2 L. divergens M2 E. coli A T C C 11775 S. aureus A T C C 25923 S. typhimurium A T C C 14028
4"55 a 4"92 a 5"32a 3-21 a 5-23a 5-65 a 5-65" 5"52~ 5-62 ~
After treatment Control c
Nitrogen
SE
4-64 a 5"40b 5"32a 3"02a 5-43 a 5"64 a 5-62 a 5"60 a 5-57 a
3"27 b 4"00c 4"15 b 2-93 ~ 4-93 h 5"484 5"42 b 5-44a 4-83 b
0"05 0-08 0.10 0-17 0-08 0-06 0-05 0-14 0-06
a.b Least Squares M e a n s in the same row with a c o m m o n superscript were not significantly different ( P > 0-05). n = 5. c C o n t r o l samples were stored at 2°C for 2 4 h while liquid nitrogen treated samples were subjected to a 3 min dip, followed by storage at 2°C for 24 h.
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S . D . M . Jones et al.
reduced (P < 0"05) by LN (in comparison to both pre-treatment and control levels). The most dramatic decrease in bacterial numbers was observed with the three Pseudomonas spp. where populations on LN treated muscle slices were from 0.92-1-28 logs lower than the pre-treatment levels. Although treatment effects were statistically significant, LN was less effective in reducing the levels of B. thermosphacta B2 (0-30 logs), E. coli ATCC 11775 (0"23 logs) and S. typhimurium ATCC 14028 (0"79 logs) than for the three Pseudomonas spp.
DISCUSSION In both experiments paired sides were used to examine the effect ofcryogenic chilling on carcass shrinkage. Both experiments showed that rapid chilling significantly reduced carcass shrinkage compared to conventional chilling. Overall, the immersion of pork sides in LN gave a 16.1 gkg -1 saving in carcass shrinkage over the first 24 h of cooling. This would be equivalent to 1.27 kg of carcass weight on an average Canadian carcass of 79 kg. Other studies have shown that rapid chilling of pork carcasses can substantially reduce carcass shrinkage during cooling (James et al., 1983; Swatland, 1983). However, it should be noted that recent research on the application of water sprays to carcasses during cooling also gave shrinkage savings of a similar magnitude to those recorded in the present study (Jones et al., 1988). Cryogenic chilling of pork sides for 1 or 3 min in LN had a major effect on deep loin muscle temperatures. At 2 h post-slaughter, loin temperatures (8-15°C) in treated sides were within the range found for commercially operating blast-chill systems (Moiler & Vestergaard, 1987). However, it should be recognized that initial heat removal occurs in a much shorter period of time in cryogenic chilling compared to blast-chilling (3 min vs 60 min). Cryogenic chilling was also found to reduce the rate of pH decline up to 6 h post mortem. The combined effects of rapid cooling and a reduced pH fall in the early stages of cooling would be expected to improve the pH dependent aspects of meat quality (colour, drip loss) as reviewed by Honikel (1986). Experiment II provided pork carcasses with normal (halothane negative) and rapid (halothane positive) post-mortem glycolytic rates as evidenced by the measurements of initial pH. The results showed that cryogenic chilling within a genetic line appeared to have rather minor effects on all measures of meat quality in this study. There were no significant colour changes that could be attributed to rapid chilling, although there were trends towards a slightly darker pork in LN treated sides. Drip loss and protein solubility which are indicators of muscle protein quality and functionality showed no response to chilling treatment. In contrast to the
Cryogenic chilling of carcasses
13
results in the present study, other work has shown that rapid chilling resulted in pork of a slightly darker colour (James et al., 1983; Crenwelge et al., 1984a, b), whereas Swatland (1983) found that the cryogenic chilling o f hams which had been electrically stimulated had no effect on muscle colour. Early work on the cryogenic chilling of pork (Borchert & Briskey, 1963) showed that the dipping of cuts for 11 s k g - 1 and equilibration of the cut at 4°C eliminated the formation of PSE pork. The results of our study suggest that 1 or 3 min immersion of carcass sides in LN will have little effect on muscle quality. Longer periods of immersion in LN may be more effective in reducing PSE pork, but may also be impractical due to cracking of the skin (Swatland, 1983). In the case of the halothane positive pigs, the initial pH was well below 6 at the time of the LN treatment. To have any impact on improving the quality of meat in pigs with a rapid post-mortem glycolysis, the chilling process would need to be initiated as soon as possible after slaughter. Previous reports have indicated that rapid chilling of muscle m a y produce a toughening effect through muscle shortening (Locker, 1960). Moiler and Vestergaard (1987) reported that pork muscle ( p H > 6 " l ) excised from carcasses chilled for 65 min at - 2 2 to - 2 8 ° C and held in iced water had shear values that were about 50% higher along with shorter sarcomeres than muscle that was excised from carcasses subjected to the same blast-chilling treatment, but excised 30 h post-slaughter. James et al. (1983) found that carcasses chilled at -- 30°C (air speed 1 m s- ~) for 4 h produced higher shear values than meat from carcasses that were conventionally chilled at 0-4°C (air speed 0-5 m s-~), but the cause of this increase in toughness was not attributed to a shortening of sarcomere length. Another report demonstrated that freezing muscle in LN substantially reduced the solubility of intramuscular collagen (Jeremiah et aL, 1980). However, a 3 min immersion of pork carcass sides in LN, either alone or in combination with electrical stimulation, which reduced muscle temperatures to as low as 8°C within 2 h of slaughter, did not produce significant effects on sarcomere length, shear value, or any of the palatability attributes measured. Therefore it would appear that either a more severe chilling treatment is necessary to promote a toughening effect (as in the above cited studies), or that the carcasses with a pH of close to 6 were protected from cold induced toughness by the onset of the rigor process (Bendall et al., 1976). In agreement with the latter contention, Barton-Gade et al., (1987) found evidence of cold shortening in pork carcasses with a high pH 1 (initial pH) value (>6-5) and low intramuscular fat content that were chilled at - 2 5 . 5 ° C for 47 min. The fact that genotype significantly influenced most meat quality and palatability attributes is consistent with other work from our laboratory which showed significantly higher shear values for halothane positive
14
S . D . M . Jones et al.
compared to halothane negative pigs (Murray et al., 1989). While it is well established that halothane positive pigs produce meat of inferior quality (higher level of PSE) to halothane negative pigs (Barton-Gade & Olsen, 1985), there is a lack of studies in the scientific literature which have covered the palatability of pork produced from these different genotypes. The results of the present study would suggest that the meat from halothane positive pigs is of lower overall acceptability than meat from halothane negative pigs. They are also in agreement with a previous report which demonstrated that PSE pork had a more sour flavour than normal pork (Jeremiah et al., 1989), which was attributed to a more extensive glycolysis. The results reported from this study have also demonstrated that immersing a pork carcass for 3 min in LN did not significantly change the total mesophilic bacterial population contaminating the carcass surface. Bacterial densities on carcass surfaces were log C F U c m - 2 = 2 - 2 5 pretreatment and log C F U c m - 2 = 2-52 after LN treatment. Other work using air blast-chill systems (James et al., 1983; Greer & Dilts, 1987) have shown that very low temperatures during the initial stages of carcass chilling appear to have little effect on the indigenous populations of bacteria. Of all the bacterial strains tested, the aerobic spoilage pseudomonads were most susceptible to LN. The 10-fold reduction in these bacteria, attributed to LN, would be of practical significance (Kotula, 1980). It is therefore possible that LN may limit the aerobic spoilage of fresh pork. The facultatively anaerobic spoilage organisms A. putrefaciens ATCC 8071, B. thermosphacta B2, and L. divergens M2 were considerably more resistant to LN. In fact the numbers of L. divergens were not significantly (P > 0.05) changed by LN. The relatively small reduction in the numbers of A putrefaciens (0-28 logs) and B. thermosphacta (0"30 logs) would likely not have any practical consequences with respect to product shelf life. The E. coli strain ATCC 11775 was also relatively resistant to LN (0-23 log reduction). These results suggest that any inferences with respect to product safety, as reflected by faecal coliforms, m a y r e m a i n valid after LN immersion. The pathogens, Staphylococcus and Salmonella are major agents of foodborne illness attributed to meat consumption in Canada. Results of the current study have shown that S. aureus ATCC 25923 was resistant to a 3 min LN immersion. Contrarily, S. typhimurium densities were reduced by 0-79logs. It is doubtful that this reduction would reduce the risk of salmoneUosis. It should be noted that organisms inoculated onto muscle surfaces in the present study were subsequently reviewed using selective media. The stress associated with LN treatment could have injured the bacterial cells and limited their recovery in the presence of selective agents. This being the case,
Cryogenic chilling of carcasses
15
the limited bactericidal effects of LN may have been even less pronounced had non-selective culture media been used to enumerate bacteria. There are only two known previously published reports on the effects of LN on the microbiology of meat. Rey et al. (1971, 1972) examined the effects of LN freezing of primal beef cuts on the microbiology and storage life of retail steaks. In these studies, loins were shell frozen for 6 min using a LN spray. It was concluded that shell freezing with LN followed by storage at high external temperatures (25-30°C) posed no health hazard. Also of interest was their observation (Rey et aL, 1972) that LN treatment produced a longer lag phase in the growth of total aerobic bacteria in comparison to fresh loins. No comparable data were available from the current study concerning the kinetics of bacterial growth.
CONCLUSIONS Cryogenic chilling using LN was used to determine the effects of an extreme chilling rate on muscle quality, palatability and bacterial numbers on carcasses. Rapid chilling using LN resulted in a cooler shrinkage saving of 16.1gkg -1 compared to conventional chilling averaged over both experiments. Rapid chilling had no consistent effects on muscle quality, although it did result in a major reduction in muscle temperature and reduced the rate o f p H decline during the rearly stages of chilling (in the first 6 h) compared to conventional cooling. It would appear that pork with normal to low pH measured at 45 min post mortem is relatively immune to cold toughening under the conditions applied in this study. Rapid chilling had no effect on mesophilic bacteria contaminating the carcass. Laboratory studies showed that LN was effective in reducing populations of the aerobic spoilage pseudomonads. However, LN had less effect on bacteria which pose a human health hazard. Genotype was found to have a significant effect on pork palatability with pork from halothane positive pigs having lower overall acceptability than pork from halothane negative pigs.
ACKNOWLEDGEMENTS The authors wish to thank D. Brereton and his staff for the slaughter and processing of the pigs. B. Dilts, L. Gibson and P. Johnson all provided valuable technical support for this study. The technical and financial assistance of Liquid Carbonic Inc. and Farming for the Future, Alberta is gratefully acknowledged.
16
S.D.M. Jones et al.
REFERENCES American Meat Science Association (1978). Guidelines for Cooking and Sensory Evaluation of Meat. AMSA, Chicago, I11. Barton-Gade, P. & Olsen, E. V. (1985). In Evaluation and Control of Meat Quality in Pigs, Ed. P. V. Tarrant, G. Eikelenboom & G. Monin. Martinus Nijhoff; Boston, p. 117. Barton-Gade, P., Bejerholm, C. & Borup, U. (1987). In Proc. 33rdInt. Cong. Meat Sci. and Tech., Helsinki, Finland, p. 181. Bendall, J. R., Ketteridge, C. C. & George, A. R. (1976). J. Sci. FoodAgric., 27, 1123. Borchert, L. L. & Briskey, E. J. (1963). J. Food Sci., 29, 203. Crenwelge, D. D., Terrell, R. N., Dutson, T. R., Smith, G. C. & Carpenter, Z. L. (1984a). J. Food Sci., 49, 294. Crenwelge, D. D., Terrell, R. N., Dutson, T. R., Smith, G. C. & Carpenter, Z. L. (1984b). J. Anim. Sci., 59, 697. Dann, A. C. (1972). In Proc. Meat Res. Inst. Symp. No. 2., Langford, Bristol, 6.1. Dransfield, E. & Lockyer, D. K. (1985). Meat Sci., 13, 19. Gardner, G. A. (1966). J. Appl. BacterioL, 29, 455. Greer, G. G. & Dilts, B. D. (1987). Can. Inst. Food Sci. & Tech. J., 20, 94. Greer, G. G. & Dilts, B. D. (1988). Can. lnst. Food Sci. & Tech. J., 21, 295. Honikel, K. O. (1986) Recent Advances and Developments in the Refrigeration of Meat by Chilling. IIR Commission C2, Bristol, p. 45. International Commission on Microbiological Specifications for Foods (ICMSF) (1978). Microorganisms in Foods 1. Their Significance and Methods and Enumeration,, 2nd edn. Univ. of Toronto Press, Toronto, Canada. James, S. J., Gigiel, A. J. & Hudson, W. R. (1983). Meat Sci., 9, 63. Jeremiah, L. E., Murray, A. C. & Martin, A. H. (1980). Can. J. Anita. Sci., 60, 627. Jeremiah, L. E., Murray, A. C. & Gibson, L. L. (1990). Meat Sci., 27, 305. Jones, S. D. M., Tong, A. K. W. & Murray, A. C. (1988). Can. J. Anim. Sci., 67, 13. Kotula, A. W. (1980). In Proc. 26th European Meeting of Meat Res. Workers, Colorado Springs, USA, p. 66. Levin, R. E. (1968). Appl. Microbiol., 16, 1734. Locker, R. H. (1960). Food Res., 25, 304. Mead, G. C. & Adams, B. W. (1977). Br. Pouh. Sci., 18, 661. Moiler, A. J. & Vestergaard, T. (1987). Meat Sci., 19, 27. Mossel, D. A. A., Mergeriub, W. H. J. & Scholts, H. H. (1962). J. Bacteriol., 84, 381. Murray, A. C., Jones, S. D. M. & Sather, A. P. (1989). Can. J. Anim. ScL, 69, 83. Rey, C. R., Kraft, A. A. & Rust, R. E. (1971). J. Food Sci., 36, 955. Rey, C. R., Kraft, A. A. & Rust, R. E. (1972). J. Food Sci., 37, 865. Schillinger, U. & Lucke, F.-K. (1987). Fleischwirtsch., 67, 1244. Swatland, H. J. (1983). Can. Inst. Food Sci. & Tech. J., 16, 254. Swatland, H. J. (1986). Can. lnst. Food Sci. & Tech. J., 19, 170.
Cryogenic Chilling of Pork Carcasses: Effects on Muscle Quality, Bacterial Populations and Palatabilityt S. D. M. Jones, G. G. Greer, L. E. Jeremiah, A. C. Murray & W. M. Robertson Red Meat and Beef Production Section, Agriculture Canada, Lacombe Research Station, Laeombe, Alberta, Canada TOC 1S0
(Received 27 November 1989; revised version received 28 January 1990; accepted 5 February 1990)
ABSTRACT Two experiments were conducted to determine the effects o f cryogenic chilling on the carcass shrinkage, meat quality, bacterial condition and palatability o f pork. In experiment L pork sides were chilled at 1°C (n = 20), or immersed in liquid nitrogen ( L N ) for I or 3 min prior to placement in a 1°C cooler. Muscle temperature in the loin was significantly lower at 2 and 6 h post m o r t e m in treated compared to control sides, and loin muscle p H was higher ( P < 0"05) at 6 h post m o r t e m in sides immersed for 3 min in LN. Carcass side shrinkage was reduced from 29"3 g kg -1 in control sides to 20"9 and 13"5 g k g - 1 in sides dipped in L N for I and 3 min. Chilling treatment had no significant effect on the survival o f mesophilic bacteria on carcass sides, on meat colour, drip loss, protein solubility or sarcomere length, but sides dipped for I min in L N has a higher muscle shear value than control sides. In experiment 11, carcass sides from halothane posit&e ( H + ) and negative ( H - - ) pigs were conventionally chilled ( n = 49), immersed in L N for 3 min (n = 23), or electrically stimulated and chilled in L N for 3 min (n = 26). Similar results for temperature, pH, colour, protein solubility and drip loss in loin muscle were found to those in experiment L Laboratory taste panel results showed that chilling treatment had no effect on palatability. Genotype produeed meaningful differences in most palatability attributes with H + pigs t Scientifie paper no. 630, Agriculture Canada, Lacombe Researcla Station, Lacombe, Alberta, Canada T0C 1SO.
1 Meat Science 0309-1740/90/$03-50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
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S.D.M. Jones et al. having less tender, less juicy and less desirable fla~our than pork from H pigs. Laboratory studies with inoculated fresh muscle slices showed that a 3 min immersion in L N resulted in a lO-foM reduction in the aerobic spoilage pseudomonads, but effects upon other spoilage bacteria and potential human pathogens were less pronounced. It was concluded that cryogenic chilling using L N reduced carcass shrinkage during cooling, but had no consistent effects on meat quality, palatability or bacterial numbers on the carcass. In contrast, genotype had a significant effect on most pork quality and palatability attributes.
INTRODUCTION Rapid or blast-chilling has been introduced in several Western Canadian pork processing plants over the last five years, and has been in use in several European countries for a longer period of time. The general procedure has been to chill carcasses after leaving the slaughter floor at temperatures between - 20 and - 30°C for a period of 1 h followed by an equilibration period in a conventional cooler (1-4°C). Scientific studies indicate under laboratory (Dransfield & Lockyer, 1985) and plant (Barton-Gade et al., 1987; Moiler & Vestergaard, 1987) conditions that blast-chilling can induce shortening of the muscle fibres that will result in tough meat. Honikel (1986) considered that pork with a normal post-mortem glycolysis (pH 6.0-6-5 at 45 min post mortem) would not cold shorten if the temperature of the muscle did not fall below 15°C, prior to the attainment of rigor mortis. There have been relatively few studies completed in pork on the possibility of cold induced toughening using the carcass as the experimental unit rather than excised muscles. Some studies have shown that rapid chilling will result in darker pork (Borchert & Briskey, 1963; Jones et al., 1988) with less drip or purge loss (Honikel, 1986), whereas others (Swatland, 1983) have demonstrated no such effect. Research studies have also been inconclusive regarding the effects of rapid chilling on the keeping quality of meat. For example, Dann (1972) recommended the use of rapid chilling to decrease carcass contamination, whereas other studies (Greer & Dilts, 1987, 1988) have shown that neither blast-chilling (--25°C, 1 h) nor spray-chilling improved the bacterial quality or retail case life of pork. Cryogenic chilling of carcasses using liquid nitrogen represents an extreme chilling procedure where carcass heat is removed more rapidly than that obtained by blast-chilling. Although cryogenic chilling is not directly comparable to blast-chilling, it does provide information on the effects of very rapid cooling on carcass quality. The objectives of this study were to
Cryogenic chilling of carcasses
3
establish if cryogenic chilling of whole pork carcasses affected muscle quality, palatability and bacterial numbers on carcasses. In addition, two separate genotypes (halothane positive and negative) were compared using different chilling regimes, and the sensitivity of certain groups of bacteria to cryogenic chilling was assessed.
EXPERIMENTAL Two experiments were conducted to assess the effects of cryogenic chilling on pork muscle quality, bacteriology and palatability.
Experiment I Twenty Lacombe pigs (halothane negative, with normal post-mortem glycolysis) were slaughtered at approximately 90 kg live weight in a research abattoir. The pigs were stunned by electric shock (400 V, 1-7 amperes) with a head to back electrode for 3--4 s. The carcasses were shackled by alternate legs to reduce the possibility of side to side variation in muscle quality which might be caused by muscle reflex activity (Swatland, 1986). Warm split sides (left and right) were weighed approximately 40 min post-slaughter, and pH and temperature (Corning pH meter, Corning Glassworks, Medfield, Mass., and Ingold combination electrode, Ingold Electronics, Andover, Mass.) were recorded in the centre of the longissimus dorsi muscle between the 12th and 13th ribs. For the first ten carcasses, the control side was placed in a cooler set at I°C with an air velocity of 0-5 m s-1 for 24 h. The treated side was completely immersed in a stainless steel tank containing liquid nitrogen (LN) for 1 min. In the second batch of ten carcasses, the same procedure was repeated except that the treated side was immersed in LN for 3 min. Following immersion, the sides were placed in a holding cooler at I°C. Temperature was recorded at 2h post mortem, and both pH and temperature were recorded at 6 h and 24 h post mortem using the same location previously described. At 6 and 24h post mortem all sides were weighed to determine shrinkage losses. The boneless muscle from the 5-13th rib was excised from both carcass sides at 24 h and used for evaluating muscle quality. A 10 mm portion was removed from the posterior end of the excised longissimus dorsi muscle to square off the muscle. A 25 mm steak was then removed from the posterior end, weighed and placed in a polythene bag for 48 h at 2°C to determine drip loss. The steak was reweighed after 48 h, after
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s . D . M . Jones et al.
removing any excess moisture with a paper towel. A second 25 m m steak was removed from the posterior end of the muscle at 48 h post m o r t e m and the freshly cut surface allowed to bloom for 30 min before recording muscle colour using a Minolta C h r o m a Meter II (Minolta Camera Company, Ramsey, New Jersey). Ultimate pH was also measured on the same sample. The steak was then cooked to a final temperature between 80-85°C in a microwave oven and held overnight at 2°C. After equilibration to room temperature, two cores of 1 9 m m were removed and sheared using the Ottawa Texture Measuring System (Canners Machinery, Simcoe, Ont.) equipped with a Warner-Bratzler cell. Protein solubility was determined as described by Murray et al. (1989). Sarcomere length was determined using a phase contrast microscope and recording the length of 10 sarcomeres for each of 20 muscle fibres. A further five carcasses were used to assess the effect of LN on bacterial numbers on the carcasses pre- and post-treatment. The left sides were conventionally chilled at l°C, while the right sides were immersed in LN for 3 min, followed by conventional cooling. Samples for bacterial analyses were taken prior to and immediately following chilling treatments by aseptically excising 10cm 2 ( l m m depth) of surface tissue from the shoulder, belly and ham of each side. After sampling, the combined samples were homogenized in 0"1% peptone-water for 2 m i n using a Colwo/'th Stomacher (A. J. Seward, London, UK). Following decimal dilutions in 0.1% peptone-water, appropriate dilutions were plated by the spread plate method and mesophilic bacteria determined as previously described (Greer & Dilts, 1987).
Experiment II Twenty four halothane negative (Lacombe) pigs and 25 halothane positive pigs were slaughtered and dressed as previously outlined. Within each line, three treatments were imposed: (1) control; side chilled at I°C, (2) side immersed in LN for 3 m i n followed by the same chilling regimen as treatment 1, (3) side stimulated at 500 V, 1 ampere for 90 s (2 s on, 1 s off), followed by same LN treatment and chilling regimen as treatment 2. The muscle quality measurements were identical to those described for experiment 1. A 15"25cm section of the longissimus dorsi muscle was excised immediately posterior to the 13th rib at 24 h p o s t m o r t e m and used for the evaluation of palatability attributes. All samples were roasted to an internal temperature of 75°C in an electric convection oven preheated to 177°C. U p o n removal from the oven, all roasts were cut into 1.9 x 1-9 x 1-9 cm 3 cubes taking care to avoid large pieces of fat and connective tissue and the
Cryogenic chilling of carcasses
5
external surface. Cubes were then randomly allocated to an experienced six member laboratory panel, screened and trained according to AMSA guidelines (American Meat Science Association, 1978), by placing them in glass containers in a circulating water bath held at 75°C. All samples were evaluated within 15 min after removal from the water bath in individual, environmentally controlled booths, under 538 lux of incandescent and fluorescent white light. Distilled water and unsalted soda crackers were provided to purge the palate of residual flavours between samples. All samples were evaluated for using a 9 point structured, descriptive, category scale for initial and overall tenderness (9 = extremely tender; 1 = extremely tough), for connective tissue ( 9 = no perceptible connective tissue; 1 = abundant perceptible connective tissue), and for juiciness (9 = extremely juicy; 1 = extremely dry), and for flavour intensity (9 = extremely intense pork flavour; 1 = extremely bland pork flavour). Flavour desirability and overall palatability were assessed using a 9 point structured, hedonic, category scale (9=extremely desirable; 1 =extremely undesirable). In addition, the samples displaying sour flavours and other off-flavours were enumerated. A laboratory study was conducted to assess the survival of different bacterial strains following exposure to LN. The sources of the bacterial strains were: Pseudomonas D23 (G. G. Greer, Agriculture Canada); Pseudomonasfragi Ju 7 and Ju 14 (R. H. Dainty, A F R C Bristol); Brochothrix thermosphacta B2 (G. G. Greer, Agriculture Canada); Lactobacillus divergens M2 (A. F. Egan, CSIRO); Alteromonas putrefaciens ATCC 8071, Escherichia coli ATCCl1775, Staphylococcus aureus ATCC25923 and Salmonella typhimurium ATCC 14028 (American Type Culture Collection). All organisms, with one exception, were grown in tryptic soy broth (Difco) for 17-20 h. With Lactobacillus divergens M2, organisms were grown in MRS broth (Difco). The incubation temperature was 35°C for E. coli, S. aureus and S. typhimurium and 25°C for all other organisms. Following growth, bacteria were diluted in 0.1% peptone-water to give a bacterial density of about 1 0 7 m l - L These suspensions served as inocula for pork muscle slices. Pork muscle slices were obtained from the m. semimembranosus removed from carcasses after 24 h of post-mortem chilling at 1°C. For each bacterium examined, 15 discs of 10 c m 2 a r e a (1 mm deep) were aseptically excised from a single muscle. Muscle slices were inoculated with 0.1 ml of bacterial inoculum and the inoculum was evenly distributed using a sterile, bent, glass rod. To ensure uniform inoculation all 15 inoculated slices were mixed in a sterile Whirl-Pak bag (Fisher Scientific, Toronto, Canada). Five samples were then distributed to each of three Whirl-Pak bags. One group of five served to determine initial bacterial loads. The second was used as a control
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S . D . M . Jones et al.
and was stored at 2°C for 24 h. The third group of five samples was immersed in LN for 3 min and equilibrated at 2°C for 24 h. Bacterial numbers were enumerated following homogenization of each muscle slice in 90ml of 0-1% peptone-water for 2min in a Colworth Stomacher. Specific organisms were enumerated as follows: Pseudomonas (Cephaloridine-Fucidin-Cetrimide Agar, 25°C, 48h; Mead & Adams, 1977), A. putrefaciens (Peptone Iron Agar, 20°C, 96h; Levin, 1968), B. thermosphacta (Streptomycin Sulphate-Thallous Acetate-Actidione Agar, 25°C, 48 h; Gardner, 1966), L. divergens (MRS Agar, 25°C, 72 h; Schillinger & Lucke, 1987), E. coli (Violet Red Bile Glucose Agar, 35°C, 24 h; Mossel et al., 1962), S. aureus (Baird-Parker Agar, 35°C, 48 h; ICMSF, 1978) and S. typhimurium (SS Agar, 35°C, 24 h; ICMSF, 1978). Bacterial counts were converted to common logarithms and expressed as log colony forming units c m - 2 (log C F U c m - 2). The meat quality and palatability data were analysed using the general linear model procedure with a within animal model that included treatment and error components for experiment I. The model for experiment II also included genotype, and the treatment x genotype interaction. Linear contrasts were used for the separation of means. The bacteriology data were analysed by analysis of variance and statistical significance of treatment differences were determined using the Student's 't' test.
RESULTS The pork carcasses in experiment I averaged 73.0 kg with a fat thickness of 19"5 mm measured at the 3--4th last ribs, 70 mm from the carcass mid-line. Carcass weight and fatness were similar ( P > 0-05) across treatments. In experiment II, there were no differences in chilling treatments within genetic line for carcass weight or fatness, but halothane positive pigs had heavier and fatter carcasses (79.5 vs 74.7kg and 22-2 vs 19-9mm fat) than did halothane negative pigs.
Experiment I Immersion of pork carcasses in LN caused a significant reduction of loin temperature at 2 and 6 h post mortem. The magnitude of these differences among treatments was largest at 2 h post-slaughter (Table 1). Carcass sides dipped in LN for 3 min had a deep loin temperature close to 9°C, compared to controls with a temperature of almost 26°C. Cooling rate also influenced the rate of change of pH. At 6 h post mortem, sides dipped for 3 min in LN
Cryogenic chiHingofcarcasses
7
TABLE 1
Temperature and pH Changes in the Longissimus dorsi Muscle Following 1 and 3 min Immersion of Warm Pork Sides in Liquid Nitrogen Time (post mortem)
Control
Liquid nitrogen immersion
SE
1 min
SE
3 min
SE
Temperature, °C: 45min 2h 6h 24 h
39"43 ~ 25"60a 8-24 * 1"94
0"15 0"63 0"25 0-08
39-96 ~'b 14"86b 4-40 b 1"84
0-25 1-32 0-24 0-03
40-1ff' 8-89c 0-36 c 1-88
0"25 0-84 0"20 0-15
pH: 45 min 6h 24 h 48 h (ultimate)
6.04 5.60a 5.50 5.49
0.05 0.04 0-02 0-02
5"93 5-74~ 5.52 5-47
0.08 0.05 0.03 0.03
6,05 5"96" 5-58 5-54
0.09 0.08 0.03 0.03
,.b.c Means in the same row with different superscripts are significantly different (P < 0.05). had a significantly higher pH than controls, whereas there were no d i f f e r e n c e s a m o n g t r e a t m e n t s a t 45 m i n o r a t 24 h p o s t m o r t e m ( T a b l e 1). Cooler shrinkage of sides was significantly reduced in the treated c o m p a r e d t o t h e c o n t r o l s i d e s ( T a b l e 2). A t 2 4 h p o s t m o r t e m , s i d e s d i p p e d f o r I o r 3 m i n i n L N h a d a s h r i n k a g e s a v i n g o f 8-4 a n d 1 5 - 8 g k g - 1 , r e s p e c t i v e l y c o m p a r e d t o c o n t r o l sides. T h e r e w e r e n o t r e a t m e n t d i f f e r e n c e s TABLE 2 Cooler Shrinkage and Pork Quality Following 1 and 3 min Immersion of Warm Sides in Liquid Nitrogen Control
Cooler shrinkage, g k g - 1: 6 h post mortem 24 h post mortem Meat colour: L* a* b* Meat drip, g k g - t Protein solubility, g k g - ~ Shear value, kg Sarcomere length,/~m
- 17"8a - 29"3a 49-5 8-2 2-4 26-2 174.7 5-17" 1.73
SE
0"56 0"83 0-67 0"32 0-30 2-8 5-9 0.27 0.04
Liquid nitrogen immersion I min
SE
3 min
SE
-8-9 b - 20-9 b
0"97 1"56
-4-I c - 13"5c
0"89 2"78
0"95 0-40 0-36 4-3 9.1 0-35 0-05
48"3 7"8 1.9 22.4 183-8 5-71a.~ 1-76
0-85 0-38 0-37 2.9 4-9 0-41 0"06
49-5 8-0 2-2 29-3 177.2 6.28 b 1.72
a.b., Means with different superscripts in the same row are significantly different (P < 0-05).
8
S . D . M . Jones et al. TABLE 3 Effect o f Liquid Nitrogen Immersion on the Survival o f Mesophilic Bacteria on Pork Carcasses
Sample time
Before treatment After treatment P
log CFU cm-2 Conventional °
Liquid nitrogen
P
2.28 b 2"84 0.070
2"25 2-52 0-300
0-930 0-289
° Conventionally treated carcasses were stored in a cooler at I°C for 24h. Liquid nitrogen treated carcasses were immersed in liquid nitrogen for 3 min, followed by equilibration for 24 h at I°C. b Least squares means o f five carcasses. Standard error was 0.21.
in meat colour, protein solubility or sarcomere length (Table 2). Muscle shear value was significantly increased for sides dipped in LN for 1 min compared to control sides. However, sides dipped for 3 min in LN had similar muscle shear values to those of controls. The number ofmesophilic bacteria on pork sides before and after chilling treatments is shown in Table 3. The results show that neither conventional chilling (P > 0-05) nor LN chilling (P > 0-10) had any significant effect on the total numbers of bacteria.
Experiment II LN had a similar effect on chilling rate as previously shown for the 3 min LN treatment in experiment 1. At 2 h post mortem, LN chilled sides had loin temperatures 16-18°C lower than control sides. This difference was reduced to 7-8°C by 6 h post mortem, and by 24 h post mortem loin temperature was similar among all treatments. LN reduced the rate of pH decline in halothane negative pigs at 6 h post mortem, but there was only a trend towards a decrease in pH fall in halothane positive pigs (Table 4). Treatment had no effect on ultimate meat pH. Electrical stimulation did not increase the rate of pH decline in either halothane positive or negative pigs (Table 4). Cryogenic chilling reduced carcass shrinkage over the first 24 h of cooling (Table 5). The shrinkage saving for LN treated sides averaged 17-6 g kg-x for both halothane positive and negative pigs compared to conventionally chilled sides. LN chilling had no effect on meat colour, meat drip, protein solubility, sarcomere length or shear value compared to conventional chilling (Table 5). Genetic line on the other hand had highly significant effects on meat colour, meat drip, protein solubility and shear value.
Cryogenic chilling of carcasses
9
TABLE 4 Effects o f Electrical Stimulation a n d Liquid N i t r o g e n I m m e r s i o n o f Carcasses on T e m p e r a t u r e and p H Changes in the Longissimus dorsi Muscle o f pigs that were H a l o t h a n e Positive o r Negative
Time (post mortem)
Halothane negative Control Liquid N ES/liquid N
Temperature, °C: 45 min 2h 6h 24 h
39" 12" 27-00* 7.89 b 0-89"
39" 17° 8"34 e 1-08~ 0-61 b
39"82 b 10"69d 1.38 c 0-65 "'b
pH: 45 min 6h 24 h 48 h (ultimate)
6" 11" 5"69* 5"48" 5-52
6-05" 6-01 b 5"63 b 5-59
6"00~ 6"01 b 5"61 b 5-55
Halothane positive Control Liquid N
39-70 b 28"69 b 10.03" 0-96*
40" 12 b 12"62~ 1.75" 0"85"
5-56 b 5"54~ 5"54*'c 5.55
5"57b 5-61 "'~ 5"57°'b'~ 5-53
ES/liquid N
40-09 b 11"10~d 2-16 ~ 0"82 °
5"57 b 5"76° 5"58 ~'c 5-52
.,b.~.d.e M e a n s with different superscripts in the same row are significantly different (P < 0.05).
TABLE 5 Effects o f Electrical Stimulation and Liquid N i t r o g e n I m m e r s i o n o f Carcasses o n the Shrinkage and Quality o f Pork from H a l o t h a n e Positive (nn) and Negative (NN) Animals
Halothane negative Control Liquid N
ES/liquid N
Halothane positive Control LiquM N
ES/liquid N
C o o l e r shrinkage, gkg-l:
6h post mortem - 18-7" 24 h post mortem M e a t colour: L* a* b* M e a t drip, g k g - 1 Protein solubility, g k g -1 Shear value, kg Sarcomere length, /zm
- 26-6*
-5"33 b - 8-3 b
--3"60 b --9"2 b
49"3 ° 7-8" 2"2" 20-6"
48-1 *'c 7-9" 1"6" 23-6"
47-9 *'c 7-2 *'~ 1"4" 19-4"
183-7" 185-0 ~ 4.85* 5"03*
190"4° 5.15"
1"88
1-84
1-89
- 16"9" -4-5 b - 26"2 ° - 11"5b
-2-9 b - 7"4n
53"2 b 8-6 b 3-9b 33-2 b
53"5 b 8.3 °'b 3-7 b 36.5 ~
51-7 *'b 8.3 *'n 3-3 n 36.8 b
135.1 n 7-67 b
140-9 b 7.62 n
147-4 b 7.63 b
1-91
1-89
1-89
,.b., M e a n s with different superscripts in the same row are significantly different ( P < 0-05).
10
S.D.M.
Jones et al.
TABLE 6
Effects of Electrical Stimulation and Liquid Nitrogen Immersion on Palatability Traits Trait
Treatment
S E Prob.
ControP Liquid Nb
Initial tenderness Overall tenderness Amount of perceptible connective tissue Juiciness Flavour desirability Flavour intensity Overall palatability Number of samples displaying sour flavours Number of samples displaying other off-flavours
Treatment
S E Prob.
ControP ES/liquid Nc
5-65 5.45
5-52 0.17 0.570 5-30 0.19 0-586
5.76 5-68
5-66 5-48
0.18 0.666 0.19 0.444
8.24 4.50 5.02 5"58 4.65
8.22 4-70 5"12 5"48 4.75
0.873 0"480 0.554 0-494 0.607
8"31 4.62 5-00 5.45 4.72
8"05 4.72 4.89 5"27 4.60
0.11 0"19 0-14 0"12 0-13
1.28
1"03 0.21 0-408
1.26
1.17
0-25 0-804
0.11
0-12 0-06 0"835
0"30
0-30
0"11 0"969
0.10 0-19 0.12 0"10 0.14
0-103 0-725 0.582 0"236 0-539
Chilled 24 h at I°C. b Dipped in liquid nitrogen for 3 min, and then chilled for 24 h at I°C. c Electrically stimulated, dipped in liquid nitrogen for 3 min, and then chilled for 24 h at I°C.
Electrical stimulation in combination with LN chilling had no significant effect in meat quality parameters. A 3min dip in LN, either alone or in combination with electrical stimulation, produced no significant effect on any of the palatability attributes measured (Table 6). However, differences were observed in most palatability attributes due to genotype (Table 7). Roasts from halothane negative pigs were consistently more tender both initially and overall,juicier, and more desirable in both flavour and overall palatability (P < 0.05). In addition, halothane positive pigs had a higher incidence of sour off-flavours than halothane negative pigs when dipped for 3 min in LN. Inoculated muscle
With one exception, the control chilling treatment (24h, 2°C) did not significantly (P > 0-05) change bacterial numbers (Table 8). In the case of P. fragi Ju 7, bacterial numbers significantly increased in the control treatment. Populations of A. putrefaciens ATCC 8071, L. divergens M2 and S. aureus ATC 25923 were not significantly (P > 0.05) altered by LN. The populations of Pseudornonas D23, P. fragi Ju 7 and Ju 14, B. thermosphacta B2, E. coli ATCC 11775, and S. typhimurium ATCC14028 were all significantly
TABLE 7 Effects o f G e n o t y p e on Palatability Traits
Trait
Liquid N c
Liquid N / E S ~
Genotype a
Initial tenderness Overall tenderness A m o u n t o f perceptible connective tissue Juiciness Flavour desirability Flavour intensity Overall palatability N u m b e r o f samples displaying sour flavours N u m b e r o f samples displaying other off-flavours
Genotype a
NN
nn
SE
Prob.
NN
nn
SE
Prob.
5-95 * 5-72 a
5.21 b 5"02b
0-17 0"19
0.003 0.013
6-05 ~ 6-01 a
5-37 b 5-15 b
0.18 0-19
0-009 0.002
8-30 4-91 ~ 5-474 5.42 5.204
8"17 4.30 b 4-67 b 5-64 4-20 b
0.10 0.19 0.12 0"10 0-14
0-360 0-031 0.000 0.013 0.000
8-31 5-03 a 5.28" 5"25 5.09"
8'05 4'32 b 4.61 b 5.47 4.23 b
0-1l 0-19 0-14 0-11 0-13
0-111 0.013 0-001 0.163 0.000
0-59 b
1-71~
0-21
0.000
1.04
1.39
0.25
0-331
0"08
0-14
0"06
0-500
0.21
0-39
0-1l
0-234
o.b M e a n s in the same experiment and row bearing different superscripts differ significantly (P < 0"05). Dipped in liquid N for 3 min and then chilled for 24 h at 1°C. Electrically stimulated, dipped in liquid N for 3 m i n a n d then chilled at 2 4 h at I°C. d Genotype: N N = h a l o t h a n e negative; n n = h a l o t h a n e positive.
TABLE 8 Effects o f Liquid N i t r o g e n on Bacterial Survival on Pork Semimembranosus Muscle
Bacteria
log CFU/cm- 2 Before treatment
Pseudomonas D23 P. fragi Ju 7 P.fragi Ju 14 A. putrefaciens A T C C 8071 B. thermosphacta B2 L. divergens M2 E. coli A T C C 11775 S. aureus A T C C 25923 S. typhimurium A T C C 14028
4"55 a 4"92 a 5"32a 3-21 a 5-23a 5-65 a 5-65" 5"52~ 5-62 ~
After treatment Control c
Nitrogen
SE
4-64 a 5"40b 5"32a 3"02a 5-43 a 5"64 a 5-62 a 5"60 a 5-57 a
3"27 b 4"00c 4"15 b 2-93 ~ 4-93 h 5"484 5"42 b 5-44a 4-83 b
0"05 0-08 0.10 0-17 0-08 0-06 0-05 0-14 0-06
a.b Least Squares M e a n s in the same row with a c o m m o n superscript were not significantly different ( P > 0-05). n = 5. c C o n t r o l samples were stored at 2°C for 2 4 h while liquid nitrogen treated samples were subjected to a 3 min dip, followed by storage at 2°C for 24 h.
12
S . D . M . Jones et al.
reduced (P < 0"05) by LN (in comparison to both pre-treatment and control levels). The most dramatic decrease in bacterial numbers was observed with the three Pseudomonas spp. where populations on LN treated muscle slices were from 0.92-1-28 logs lower than the pre-treatment levels. Although treatment effects were statistically significant, LN was less effective in reducing the levels of B. thermosphacta B2 (0-30 logs), E. coli ATCC 11775 (0"23 logs) and S. typhimurium ATCC 14028 (0"79 logs) than for the three Pseudomonas spp.
DISCUSSION In both experiments paired sides were used to examine the effect ofcryogenic chilling on carcass shrinkage. Both experiments showed that rapid chilling significantly reduced carcass shrinkage compared to conventional chilling. Overall, the immersion of pork sides in LN gave a 16.1 gkg -1 saving in carcass shrinkage over the first 24 h of cooling. This would be equivalent to 1.27 kg of carcass weight on an average Canadian carcass of 79 kg. Other studies have shown that rapid chilling of pork carcasses can substantially reduce carcass shrinkage during cooling (James et al., 1983; Swatland, 1983). However, it should be noted that recent research on the application of water sprays to carcasses during cooling also gave shrinkage savings of a similar magnitude to those recorded in the present study (Jones et al., 1988). Cryogenic chilling of pork sides for 1 or 3 min in LN had a major effect on deep loin muscle temperatures. At 2 h post-slaughter, loin temperatures (8-15°C) in treated sides were within the range found for commercially operating blast-chill systems (Moiler & Vestergaard, 1987). However, it should be recognized that initial heat removal occurs in a much shorter period of time in cryogenic chilling compared to blast-chilling (3 min vs 60 min). Cryogenic chilling was also found to reduce the rate of pH decline up to 6 h post mortem. The combined effects of rapid cooling and a reduced pH fall in the early stages of cooling would be expected to improve the pH dependent aspects of meat quality (colour, drip loss) as reviewed by Honikel (1986). Experiment II provided pork carcasses with normal (halothane negative) and rapid (halothane positive) post-mortem glycolytic rates as evidenced by the measurements of initial pH. The results showed that cryogenic chilling within a genetic line appeared to have rather minor effects on all measures of meat quality in this study. There were no significant colour changes that could be attributed to rapid chilling, although there were trends towards a slightly darker pork in LN treated sides. Drip loss and protein solubility which are indicators of muscle protein quality and functionality showed no response to chilling treatment. In contrast to the
Cryogenic chilling of carcasses
13
results in the present study, other work has shown that rapid chilling resulted in pork of a slightly darker colour (James et al., 1983; Crenwelge et al., 1984a, b), whereas Swatland (1983) found that the cryogenic chilling o f hams which had been electrically stimulated had no effect on muscle colour. Early work on the cryogenic chilling of pork (Borchert & Briskey, 1963) showed that the dipping of cuts for 11 s k g - 1 and equilibration of the cut at 4°C eliminated the formation of PSE pork. The results of our study suggest that 1 or 3 min immersion of carcass sides in LN will have little effect on muscle quality. Longer periods of immersion in LN may be more effective in reducing PSE pork, but may also be impractical due to cracking of the skin (Swatland, 1983). In the case of the halothane positive pigs, the initial pH was well below 6 at the time of the LN treatment. To have any impact on improving the quality of meat in pigs with a rapid post-mortem glycolysis, the chilling process would need to be initiated as soon as possible after slaughter. Previous reports have indicated that rapid chilling of muscle m a y produce a toughening effect through muscle shortening (Locker, 1960). Moiler and Vestergaard (1987) reported that pork muscle ( p H > 6 " l ) excised from carcasses chilled for 65 min at - 2 2 to - 2 8 ° C and held in iced water had shear values that were about 50% higher along with shorter sarcomeres than muscle that was excised from carcasses subjected to the same blast-chilling treatment, but excised 30 h post-slaughter. James et al. (1983) found that carcasses chilled at -- 30°C (air speed 1 m s- ~) for 4 h produced higher shear values than meat from carcasses that were conventionally chilled at 0-4°C (air speed 0-5 m s-~), but the cause of this increase in toughness was not attributed to a shortening of sarcomere length. Another report demonstrated that freezing muscle in LN substantially reduced the solubility of intramuscular collagen (Jeremiah et aL, 1980). However, a 3 min immersion of pork carcass sides in LN, either alone or in combination with electrical stimulation, which reduced muscle temperatures to as low as 8°C within 2 h of slaughter, did not produce significant effects on sarcomere length, shear value, or any of the palatability attributes measured. Therefore it would appear that either a more severe chilling treatment is necessary to promote a toughening effect (as in the above cited studies), or that the carcasses with a pH of close to 6 were protected from cold induced toughness by the onset of the rigor process (Bendall et al., 1976). In agreement with the latter contention, Barton-Gade et al., (1987) found evidence of cold shortening in pork carcasses with a high pH 1 (initial pH) value (>6-5) and low intramuscular fat content that were chilled at - 2 5 . 5 ° C for 47 min. The fact that genotype significantly influenced most meat quality and palatability attributes is consistent with other work from our laboratory which showed significantly higher shear values for halothane positive
14
S . D . M . Jones et al.
compared to halothane negative pigs (Murray et al., 1989). While it is well established that halothane positive pigs produce meat of inferior quality (higher level of PSE) to halothane negative pigs (Barton-Gade & Olsen, 1985), there is a lack of studies in the scientific literature which have covered the palatability of pork produced from these different genotypes. The results of the present study would suggest that the meat from halothane positive pigs is of lower overall acceptability than meat from halothane negative pigs. They are also in agreement with a previous report which demonstrated that PSE pork had a more sour flavour than normal pork (Jeremiah et al., 1989), which was attributed to a more extensive glycolysis. The results reported from this study have also demonstrated that immersing a pork carcass for 3 min in LN did not significantly change the total mesophilic bacterial population contaminating the carcass surface. Bacterial densities on carcass surfaces were log C F U c m - 2 = 2 - 2 5 pretreatment and log C F U c m - 2 = 2-52 after LN treatment. Other work using air blast-chill systems (James et al., 1983; Greer & Dilts, 1987) have shown that very low temperatures during the initial stages of carcass chilling appear to have little effect on the indigenous populations of bacteria. Of all the bacterial strains tested, the aerobic spoilage pseudomonads were most susceptible to LN. The 10-fold reduction in these bacteria, attributed to LN, would be of practical significance (Kotula, 1980). It is therefore possible that LN may limit the aerobic spoilage of fresh pork. The facultatively anaerobic spoilage organisms A. putrefaciens ATCC 8071, B. thermosphacta B2, and L. divergens M2 were considerably more resistant to LN. In fact the numbers of L. divergens were not significantly (P > 0.05) changed by LN. The relatively small reduction in the numbers of A putrefaciens (0-28 logs) and B. thermosphacta (0"30 logs) would likely not have any practical consequences with respect to product shelf life. The E. coli strain ATCC 11775 was also relatively resistant to LN (0-23 log reduction). These results suggest that any inferences with respect to product safety, as reflected by faecal coliforms, m a y r e m a i n valid after LN immersion. The pathogens, Staphylococcus and Salmonella are major agents of foodborne illness attributed to meat consumption in Canada. Results of the current study have shown that S. aureus ATCC 25923 was resistant to a 3 min LN immersion. Contrarily, S. typhimurium densities were reduced by 0-79logs. It is doubtful that this reduction would reduce the risk of salmoneUosis. It should be noted that organisms inoculated onto muscle surfaces in the present study were subsequently reviewed using selective media. The stress associated with LN treatment could have injured the bacterial cells and limited their recovery in the presence of selective agents. This being the case,
Cryogenic chilling of carcasses
15
the limited bactericidal effects of LN may have been even less pronounced had non-selective culture media been used to enumerate bacteria. There are only two known previously published reports on the effects of LN on the microbiology of meat. Rey et al. (1971, 1972) examined the effects of LN freezing of primal beef cuts on the microbiology and storage life of retail steaks. In these studies, loins were shell frozen for 6 min using a LN spray. It was concluded that shell freezing with LN followed by storage at high external temperatures (25-30°C) posed no health hazard. Also of interest was their observation (Rey et aL, 1972) that LN treatment produced a longer lag phase in the growth of total aerobic bacteria in comparison to fresh loins. No comparable data were available from the current study concerning the kinetics of bacterial growth.
CONCLUSIONS Cryogenic chilling using LN was used to determine the effects of an extreme chilling rate on muscle quality, palatability and bacterial numbers on carcasses. Rapid chilling using LN resulted in a cooler shrinkage saving of 16.1gkg -1 compared to conventional chilling averaged over both experiments. Rapid chilling had no consistent effects on muscle quality, although it did result in a major reduction in muscle temperature and reduced the rate o f p H decline during the rearly stages of chilling (in the first 6 h) compared to conventional cooling. It would appear that pork with normal to low pH measured at 45 min post mortem is relatively immune to cold toughening under the conditions applied in this study. Rapid chilling had no effect on mesophilic bacteria contaminating the carcass. Laboratory studies showed that LN was effective in reducing populations of the aerobic spoilage pseudomonads. However, LN had less effect on bacteria which pose a human health hazard. Genotype was found to have a significant effect on pork palatability with pork from halothane positive pigs having lower overall acceptability than pork from halothane negative pigs.
ACKNOWLEDGEMENTS The authors wish to thank D. Brereton and his staff for the slaughter and processing of the pigs. B. Dilts, L. Gibson and P. Johnson all provided valuable technical support for this study. The technical and financial assistance of Liquid Carbonic Inc. and Farming for the Future, Alberta is gratefully acknowledged.
16
S.D.M. Jones et al.
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