Bison Meat Quality Research: An Investigation of Spray Chilling, Very Fast Chilling, and the Relationship of Grade Factors to Meat Quality

A Final Report to the Peace Country Bison Association

Jennifer A. M. Janz, PhD Candidate
Department of Agricultural, Food and Nutritional Science
University of Alberta, Edmonton, AB T6G 2P5

Dr. J. L. Aalhus 
Agriculture and Agri-Food Canada
Lacombe Research Centre

Dr. M. A. Price
Department of Agricultural, Food and Nutritional Science
University of Alberta

Dr. G. G. Greer 
Agriculture and Agri-Food Canada
Lacombe Research Centre

March 2001

Executive Summary

This report is the culmination of a set of experiments conducted at the Agriculture and Agri-Food Canada's Lacombe Research Centre (LRC), funded through the Peace Country Bison Association, with animals supplied through cooperation with Alberta bison producers. This report was written for and intended for use by producers and extension staff. All animals in care at the Lacombe Research Centre were handled in accordance with the guidelines set forth by the Canadian Council on Animal Care.

1.1. Spray Chilling

Carcass spray chilling is a cooling process that effectively controls cooler shrink loss. Since spray chilling is common practice in bison carcass processing, its influence on carcass, meat, and microbiological quality was investigated. Twelve bison bulls, with an average liveweight of 539 kg, were slaughtered at the LRC research abattoir under simulated commercial conditions. One half of each carcass was chilled under conventional cooler conditions (2°C for 24 hours) while the paired side were spray chilled (2°C, with a 60 second spray cycle 4 times per hour for 8 hours, followed by conventional chilling). Following carcass chilling, a portion of the Longissimus lumborum (striploin) was removed. A portion of each muscle section was used for meat quality analysis while the remaining portions were subject to vacuum packaged storage (0, 2, 4, or 6 weeks) and simulated retail display (0, 2, 5, 7, or 9 days). The following measurements were taken:

Carcass quality:

  • Hot side weight immediately prior to chilling

  • Cold side weight on the morning following slaughter

Meat quality:

  • Temperature and pH decline in loin muscle during chilling

  • Objective colour of lean tissue at 24 hours and 6 days postmortem

  • Sarcomere length at 24 hours postmortem

  • Drip loss following 5 days refrigerated storage

  • Shear force of striploin steaks at 7, 14, and 21 days postmortem

Microbiological quality:

  • Carcass swabs prior to and following chilling treatments

  • Growth of Anaerobes and psychrotrophs during vacuum packaged storage

  • Growth of Pseudomonads and psychrotrophs during simulated retail display

  • Retail case life based on appearance, disolouration, and odour

Spray chilling reduced cooler shrink loss and did not significantly affect other meat quality parameters. There was no influence of the chilling treatment on the bacterial counts prior to or following carcass cooling. Microbial growth during vacuum packaged storage was minimal and no spoilage occurred. During retail display, bacterial spoilage did not become a concern until the later stages of the experiment when samples had been stored for 4 or 6 weeks and on display for more than 7 days. Of some concern during retail display was the copious amount of fluid in the steak trays of samples with 0 weeks of vacuum packaged storage.As the time in storage increased, the fluid was no longer a concern because excess moisture was retained in the packaging material. An additional observation and one to be taken into consideration when merchandising fresh bison meat is the rapid discolouration of surface colour. The loss of colour quality due to the natural oxidation of muscle pigments (brown surface discolouration) was uniform across the surfaces of all steaks by 5 days of retail display.

1.2. Very Fast Chilling

Recent work with beef carcasses has indicated that very fast chilling can produce tender meat. The treatment requires the attainment of -1°C within the musculature by 5 hours postmortem by using extreme low temperature coolers.Since work with bison carcasses had not been conducted, the objective of the experiment was to determine the time/temperature chilling combination required to achieve the rapid temperature decline, and to observe the effect of such treatment on meat quality parameters. Thirty-seven bison bulls were slaughtered at the LRC research abattoir under simulated commercial conditions. Using a paired side treatment design, one half of each carcass was chilled under conventional cooler conditions (2°C for 24 hours) while the opposite side was placed into a very fast chilling treatment (2, 4, or 6 hours at -35°C, then into conventional chiller). Meat quality samples and data were collected at various times during and following carcass chilling, according to the following list:

Carcass quality:

  • Hot side weight immediately prior to chilling

  • Cold side weight on the morning following slaughter

Meat quality:

  • pH decline in loin muscle and temperature decline in loin and deep hip during chilling

  • Objective colour of lean tissue at 24 hours and 6 days postmortem

  • Sarcomere length at 24 hours postmortem

  • Drip loss following 5 days refrigerated storage

  • Moisture loss after vacuum packaged storage

  • Cooking loss prior to each shear test

  • Shear force of striploin at 6 and 13 days postmortem and following 13 days ageing with a period of frozen storage

  • Taste panel analysis of ribeye steaks after 13 days ageing and a period of frozen storage

  • Glycogen and lactate content at 0 time, 1 hour postmortem, immediately following very fast chilling, 9 and 24 hours postmortem

  • Activity of cathepsins B and L at 0 time, 24 hours, and 6 days postmortem

  • Activity of calpastatin at 0 time, 24 hours, 6 and 9 days postmortem

Very fast chilling (VFC) for 4 and 6 hours successfully lowered loin temperature into the desired range for tenderization, while 2 hours of VFC chilling was insufficient. The temperature of the deep hip region remained outside the VFC range for all treatments. VFC for 6 hours resulted in the longest sarcomere length and conventional chilling the shortest. At all times measured, a similar pattern of mean shear force values was observed. The more extreme VFC treatments produced the lowest shears while 2 hours of VFC resulted in the greatest shears and shears following conventional chilling were intermediate.Taste panelists were unable to detect a significant difference amongst chilling treatments. VFC reduced or prevented cooler shrink loss, however, drip loss and loss of moisture during vacuum packaged storage was increased. No difference in cooking loss was noted amongst chilling treatments. Until 24 hours postmortem when the lactate concentration of 4 and 6 hour VFC samples was significantly lower, chilling treatment did not result in differences in muscle glycogen and lactate concentration. There was no difference in pH amongst chilling treatments until 24 hours postmortem when the mean value from the 6 hour VFC treatment samples was significantly greater. The more rigorous chilling treatments resulted in darker meat colour at 24 hours postmortem, but this difference did not persist to 6 days postmortem. Cathepsin activity at 24 hours was greatest following the more extreme VFC treatments and increased slightly over time. Calpastatin activity was unaffected by chilling treatment and declined steadily after 24 hours postmortem.

1.3. Carcass Characteristics and Grading

Amongst bison industry participants, there has been general interest expressed in making modifications to the current Canadian bison carcass grading system.The direction of these changes has yet to be identified. From this current series of experiments, data from untreated carcass sides were compiled in order to examine the relationship between carcass and meat quality attributes and grading criteria. This information may be useful in determining where to begin with amendments to the grading system. Due to a small sample size (N=45), however, caution should be exercised when generalizing the importance of the data. In addition to liveweight and carcass grade information, hot and cold side weights, shear values, proximate analysis, and objective colour were measured.

There was an association between increased liveweight and thicker subcutaneous fat. Carcasses with thicker grade fat tended to receive A-class grades. Heavier carcasses with more subcutaneous fat cover tended to lose less weight to cooler shrink, had brighter coloured meat, and tended to receive "preferable" grades. Taste panel rating was not affected by grade or fat class.Extreme variability of shear values was observed within all grade and fat classes. Thicker fat cover seemed to be associated with more tender shears, but only a small percentage of the variability in shear force could be attributed to variability in fat thickness.

1.4. General Observations and Highlights

  • Spray chilling effectively reduced cooler shrink loss without adversely affecting other meat and microbiological parameters.

  • Drip loss from spray chilled steaks was unattractive, but excess moisture was not a problem after a period of vacuum packaged storage.

  • Retail display of aerobically packaged bison meat may be hindered by rapid discolouration of surface colour due to pigment oxidation. Vacuum packaging of retail products, thereby maintaining a purple-red colour, could be used in order to avoid the problem of pigment discolouration.

  • Chilling at -35°C for 4-6 hours was sufficient to reach very fast chilling conditions in the loin.

  • Very fast chilling reduced or prevented cooler shrink loss, however, drip loss and package loss were increased and meat colour was slightly darker at the time of grading.

  • Very fast chilling resulted in tenderization likely due to both physical restraint against contraction provided by the frozen outer surface and the increased activity of the proteolytic cathepsins.

  • Animals with greater liveweight tended to produce carcasses with thicker grade fat and tended to be assigned an A-class grade.

  • Lower cooler shrink loss and brighter colour at the time of grading, were associated with A-class grades and thicker fat cover.

  • While thicker fat and A-class grades were associated with lower mean shear values, there was extreme variability in individual shears within and amongst all grade and fat classes.

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