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Accelerated chilling of carcasses to improve pork quality¹

Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409-2162

³ Correspondence:
Meat Laboratory, P.O. Box 42162 (phone: 806-742-2804; fax: 806-742-0169; E-mail:
mfmrraider{at}aol.com).

	Abstract

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Our objectives were to determine the optimal accelerated chill time immediately postmortem necessary to improve the quality of pork muscle and to decrease the incidence of pale, soft, and exudative pork. Carcasses from 81 market hogs were cooled either by conventional chill (CC) at 2°C or by accelerated chill (AC) at -32°C for 60, 90, 120, or 150 min, and then placed into a 2°C cooler for the remainder of the 24-h chill period. Loin muscle pH was higher (P < 0.05) for the carcasses that were accelerated chilled longer than 60 min. Although loin visual color, texture, and firmness scores increased (P < 0.05) with AC time, no improvements were noted beyond 60 min. Color, pH, texture, firmness, and CIE L*a*b* values of fresh ham muscles were not (P > 0.05) affected by AC. In addition, AC did not (P > 0.05) affect purge, drip, or thaw loss of fresh products, sensory scores of loins or processed hams (except initial juiciness; P < 0.05), water-holding capacity of processed hams, or processing characteristics of hams. Cooking loss and Warner-Bratzler shear values for hams and loins were not (P > 0.05) affected by AC. Accelerated chilling caused loins to be darker (lower L* value; P < 0.05) and to have lower (P < 0.05) b* values (less yellow) than CC loins. Accelerated chilling increased water-holding capacity in fresh hams, bound water being the greatest (P < 0.05) in the 120- and 150-min AC groups. These results demonstrate that improvements in pork loin quality can be made using freezer-accelerated chilling for carcasses.

Key Words: Chilling • Meat Quality • Pigs • Pork

	Introduction

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Pale, soft, and exudative pork occurs when muscle proteins are denatured in response to accelerated pH decline when muscle temperature is high (Borchert and Briskey, 1965). One method of preventing PSE pork is to decrease the muscle temperature during early postmortem metabolism and pH declines. Accelerated chilling can quickly reduce temperatures and has allowed for improvements in pork quality by lessening the incidence of PSE muscle (Borchert and Briskey, 1964; Milligan et al., 1998). Methods for accelerated chilling include blast or freezer chilling, hot-fat trimming, cold-water showering, and propylene glycol immersion, which have been shown to have both positive and negative effects on quality. Weakley et al. (1985) found that muscle toughened if rapidly chilled. Conversely, Long and Tarrant (1990) found that rapid chilling had no effect on tenderness. Weakley et al. (1985) also found normally chilled loins had higher L* values, as well as lighter visual color scores than those that were crust frozen or immersed in propylene glycol. Crenwelge et al. (1984) found no differences in Warner-Bratzler shear (WBS) force values or tenderness scores of blast and conventionally chilled carcasses, but found rapid chilling improved muscle color scores for hams and blast chilling improved firmness of all cuts studied. Although some conflicting results exist, the importance of decreasing muscle temperature to slow pH decline and prevent PSE pork cannot be denied. The objectives of this experiment were to improve pork quality by determining the optimal freezer chilling time immediately postmortem and its effects on pH, temperature decline, and quality attributes of fresh pork, as well as the effects of freezer chilling time on processing characteristics of hams.

	Materials and Methods

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Selection and Slaughter
Market hogs (n = 81) were produced, finished and slaughtered at a commercial facility (Premium Standard Farms, Inc., Milan, MO). Carcasses were randomly assigned to treatment groups of conventionally chilled (CC, 2°C, air movement 1.2 m³/s) or accelerated chilled (AC,-32°C, air movement of 2 m³/s) for 60, 90, 120, or 150 min (CC, n = 21; AC 60 min, n = 15; AC 90 min, n = 16; AC 120 min, n = 15; and AC 150 min, n = 15). After AC, all carcasses were chilled at 2°C for the remainder of the 24-h chilling time.

Carcass Characteristics
All carcasses were measured for midline backfat depth opposite the first and last ribs and the last lumbar vertebra. Carcass muscle scores (9 = thick+ to 1 = thin-) were subjectively determined by trained personnel from Texas Tech University using a three-point scale and further dividing it into a nine-point scale (Thick +, o, -; Medium Thick +, o, -; and Thin +, o, -; NPPC, 1991). Carcass length from the anterior edge of the first rib to the anterior edge of the aitchbone was also measured. Hot carcass weight was recorded before carcasses left the harvest floor and cold carcass weight was taken after the 24-h chilling period.

Temperature and pH
Temperature and pH were measured with puncture probes on the left side of each carcass. The first measurements were collected at 0.5 h postmortem, immediately before the carcasses exited the harvest floor, and at 1.5, 2.5, 3.5, 4.5, 5.5, and 24 h postmortem following the completion of the chill treatment. Measurements were not taken on the AC carcasses at 150 min (2.5 h) because the carcasses were still on the chain in the freezer. Temperatures were taken (Koch digital pocket thermometer 017000, Koch Supplies Inc., Kansas City, MO) in the longissimus muscle by inserting the thermometer between the transverse processes of the 5th and 6th lumbar vertebrae and at the center of the hams in the semimembranosus muscle near the femur. The pH was monitored with an Orion model 230A temperature-compensating pH meter (Orion Research, Inc., Boston, MA). A glass probe (Cole-Parmer, model 05998-20, Vernon Hills, IL) was inserted between the 6th and 7th lumbar vertebra region of the longissimus muscle and in the semimembranosus muscle.

Fabrication
Loins (IMPS No. 412B, 8th rib, boneless center cut loins; USDA, 1990) and inside hams (gracillus, semimembranosus, and adductor muscles) were collected from each carcass during fabrication about 24 h postmortem. After colorimeter and quality attribute evaluations, the products were individually vacuum packaged and chilled to 4°C.

Colorimeter Readings and Quality Attributes
Twenty minutes after ribbing, the longissimus dorsi at the 10th-rib interface of each loin was measured for Commission Internationale de l’Eclairage (CIE) L* a* b* values with a Minolta Chroma Meter (CR-200b; Minolta Corp., Osaka, Japan) equipped with an 8-mm diameter viewing area, a 0° viewing angle, and the area was illuminated with diffuse illumination from a zenon arc lamp, C illuminant. Instrumental CIE L* a* b* values for the semimembranosus of each ham was also measured 20 min after it was separated from the carcass. Hams and loins also were evaluated visually for muscle color, texture, and firmness, and loins were evaluated for marbling by a four-member team (1 = pale, pinkish-gray, coarse, very soft, and devoid to practically devoid, 5 = dark purplish-red, fine, very firm, and moderately abundant or greater) in the same location as the colorimeter measurements were taken using NPPC standards (1991).

Storage Evaluations
Vacuum-packaged loins and inside hams were packed in coolers with ice and transported to Texas Tech University. They were stored at 2°C for 14 d after harvest. Loins and hams were evaluated for purge loss by measuring percentage weight loss during storage. Loin drip loss was measured by suspending 100 ± 2-g samples overnight at 2°C (24 ± 2 h) and calculating percentage weight lost. Samples also were removed from inside hams for water-holding capacity (WHC) and drip-loss evaluations, whereas the percentage of free, bound, and immobilized water was measured using a Carver Press (model #5154-42; Fred S. Carver Inc., Summit, NJ; Ockerman, 1981). Loins were cut into 2.5-cm-thick chops, and then individually vacuum packaged and frozen (-20°C) for WBS force, sensory panel, and cooking loss evaluations. Thaw loss was measured when the chops for shearing were thawed (2°C for 24 h) by calculating percentage weight lost during thawing.

Inside hams were further processed using a 10% brine solution that included 11.34 kg of water, 1.31 kg of NaCl, 0.82 kg of dextrose, 0.15 kg of sugar, 0.15 kg of sodium phosphate, 0.13 of kg cure salt (Prague Powder), and 0.02 kg of sodium erythorbate (for 10 hams). Hams were then pumped to 30% of green weight, double bagged, and vacuum tumbled at 5 rpm for 2 h and allowed to equilibrate overnight at 2°C. At 0800 h, hams were stuffed into size 9 fibrous casings (Dewied Int. Corp., San Antonio, TX) and cooked to an internal temperature of 65°C in a Vortron processing oven (model 2500, Vortron, Beloit, WI) by the schedule in Table 1. Hams were cooled to 7°C, vacuum packaged, and frozen (-20°C).

Processing Characteristics
The cured hams were assessed for sliceability (the percentage of unbroken 0.3-cm-thick slices cut from a 5.1-cm-thick section) using a Hobart slicer (Hobart Corp. model 1612E, Troy, OH), color (5 = dark, purplish red; 1 = pale, pinkish gray), uniformity (5 = extreme two-toning; 1 = uniform), cooking loss, WBS, and WHC. Instrumental color (CIE L*a*b* values) was measured in three locations on each processed ham surface. A six-member trained sensory panel, using the same procedures given for loin chops, also evaluated cold ham samples after training on commercial hams. Hams were evaluated for initial and sustained tenderness, juiciness, flavor intensity, ham flavor, and overall mouth feel (8 = extremely juicy, extremely tender, extremely intense, extremely flavorful, and extremely ham-like; 1 = extremely dry, extremely tough, extremely bland, extremely unsavory, and extremely unham-like).

Statistical Analysis
The experiment was conducted as a completely randomized design with five treatments (Ott, 1988; Schulman, 1992). Data were analyzed using the GLM procedures of SAS (SAS Inst., Inc., Cary, NC) and least squares means were separated by the probability difference (PDIFF) option when main effects were significant (P < 0.05). Pairwise comparisons of percentages were completed according to Ott (1988) to compare two binomial portions. Analysis was completed using an alpha set at 5%.

	Results

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Carcass Characteristics
The pigs in this study had an average backfat thickness of 3.1 cm, USDA muscle score of 6.4 (slightly well muscled), carcass length of 82.1 cm, and hot carcass weight of 86.2 kg. None of these traits differed among chilling treatments (P > 0.05, data not shown).

Muscle Temperature
Muscle temperatures of loins from CC carcasses were higher (P < 0.05) than those from AC carcasses at every measuring time except at 0.5 h (before treatments were started, Figure 1). Differences (P < 0.05) in temperature at each time measured were detected between treatments at almost every measurement time except the 120- and 150-min AC treatments, which were not different at any measuring time.

View larger version (25K): [in this window] [in a new window]   Figure 1. Mean loin and ham temperature changes by chill treatments. CC = conventionally chilled at 2°C for 24 h; 60-, 90-, 120-, and 150-min freezer time for the accelerated chilled (AC) treatments. Standard errors were always <5% of the mean.

Muscle pH
Loin pH at 3.5 h was higher (P < 0.05) in carcasses that spent either 90 or 120 min in the freezer (Figure 2), but at 4.5 and 5.5 h post-treatment, the 90-, 120-, and 150-min loins produced higher (P < 0.05) pH values than those in the CC carcasses. However, differences in pH were not detected (P > 0.05) at 24 h.

View larger version (21K): [in this window] [in a new window]   Figure 2. Mean loin and ham muscle pH changes by chill treatment. CC = conventionally chilled at 2°C for 24 h; 60-, 90-, 120-, and 150-min freezer time for the accelerated chilled (AC) treatments. Standard errors were always <5% of the mean.

Quality Attributes
Accelerated chilling of carcasses for any length of time improved (P < 0.05) loin muscle color by at least 0.9 on a five-point scale compared to CC carcasses (Table 2). However, longer times in the freezer did not (P > 0.05) improve color. Loin texture and firmness were also improved (P < 0.05) by AC, except at 120 min when texture was not different (P > 0.05) from the CC treatment. For both texture and firmness, 150 min in the freezer caused higher (P < 0.05) scores than 120 min, but was not different (P > 0.05) from 60 or 90 min. Marbling was not affected (P > 0.05) by any length of time in the freezer. All AC carcasses had lower (P < 0.05) L* values (darker) for loins, but the two longer times (120 or 150 min) in the freezer decreased (P < 0.05) L* values (darker) more than 60 min. Accelerated chilling did not affect (P > 0.05) loin a* values, but lowered (P < 0.05) b* values (less yellow). Ham color, texture, firmness, and L* a* b* values were not affected (P > 0.05) by accelerated chilling treatments.

Purge Loss, Drip Loss, Thaw Loss, and Water-Holding Capacity
No differences (P > 0.05) were detected in purge or drip loss of hams or loins or in thaw of loin chops (Table 4). The WHC and percentage of moisture of loins were not affected (P > 0.05) by accelerated chilling for any length of time (Table 5). However, fresh ham moisture, bound water, and immobilized water percentages were greatest (P < 0.05), and free water percentage was least (P < 0.05), for 120- and 150-min AC carcasses. Although statistically significant differences in water binding were produced by the treatments, these differences did not translate to reductions in purge, drip, or thaw loss. Cured and cooked hams were also not affected (P > 0.05) by chilling treatment.

	Discussion

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For loin chops, the improvements in juiciness and flavor were not seen in this study as in Kerth et al. (2001) with accelerated chilling. However, similar reductions in the percentage of PSE hams and loins were observed in both studies. Finally, this study indicates that accelerated chilling does not negatively affect the sensory attributes of either chops or processed hams (except initial juiciness of cured hams); thus, increased toughness was not a problem, as reported by Long and Tarrant (1990) and Crenwelge et al. (1984). Similarly, Jones et al. (1991) reported no effect on palatability when carcasses were rapidly chilled in liquid nitrogen. Jeremiah et al. (1992) observed an actual improvement in tenderness at 60 min of blast chilling; however, additional chill time did reduce tenderness (Jeremiah et al., 1992; van der Wal et al., 1995).

Perhaps the most significant results from the use of accelerated chilling are in the color, texture, and firmness of loins and hams, and the WHC categories. Accelerated chilling for any length of time reduced the incidence of unacceptable color scores compared to conventional chilling. Very long times in the freezer (120 or 150 min) increased the percentage of moisture in the fresh hams while reducing free water and increasing bound and immobilized water. Bound water is water that is held because of charges on the proteins. Therefore, as pH approaches the isoelectric point, more repelling forces are present, resulting in less water binding by mofibrillar proteins (Hedrick et al. 1994). Generally, PSE pork loses water because of a decreased ability to hold water in the bound state. Our results differ from those of Jeremiah et al. (1992) and van der Wal et al. (1995), who reported no improvement in water properties, such as drip or cooking loss. The findings of the current study indicate that longer times (120 and 150 min) in the freezer significantly increased bound water. The improvement in the percentage of bound water is a step in combating the exudation of PSE pork. It is evident that improvements can be made in traits such as color, texture, firmness, WHC, and CIE L*a*b values, all of which should lead to increased profitability by reduction of PSE.

	Implications

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Accelerated chilling improved several important quality attributes (longissimus muscle color, texture, and firmness) and did not negatively affect sensory panel scores or processing characteristics. In most categories, an accelerated chilling time of 90 min improved scores over conventional chilling, whereas 120 and 150 min chilling treatments did not further improve scores. Therefore, the results of this study indicate that 90 min of accelerated chilling should be used by processors unless a substantial increase in profit or reduction in the incidence of pale, soft, and exudative pork can be achieved by employing accelerated chilling times longer than 90 min.

	Footnotes

 
¹ The authors thank the National Pork Producers Council of Des Moines, IA, for funding this research and the employees of Premium Standard Farms packing facility in Milan, MO, for their assistance in conducting the research.

² Current address: Dept. of Agriculture, Angelo State University, San Angelo, TX 76909.

Received for publication June 6, 2002. Accepted for publication January 6, 2003.

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