The effect of 31 vs. 38 degree holding
temperature on meat quality.
South Dakota State University
December 23, 1999
Principal Investigator: D. M. Wulf
Graduate Students: B. J. Reuter and J. M. Bok
Microbiologist: D. R. Henning
Meat Chemist: D. M. Simon
Meat Laboratory Manager: L. E. Warborg
Objective:
Determine the differences in meat product quality and microbial growth during
refrigerated storage at 31°F vs. 38°F.
Design:
4 products (all packaged aerobically in retail wrap) – Ground beef, Beef top round
steak, Boneless and skinless chicken breast, Fully-cooked & sliced turkey breast
2 storage temperatures – 31°F and 38°F
15 storage times for panel evaluations, colorimeter readings, and digital
photographs
– 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 days
3 storage times for microbiology and TBARS – 0, 7, 14 days
Triplicate (3) samples of each treatment combination
Procedures:
All products were obtained through normal commercial retail channels. Two 10-lb
course-ground beef chubs, produced by a single processor, were obtained. The
course-ground beef was mixed, fine ground, pattied, and packaged at the SDSU
Meat Laboratory using clean equipment. A single beef top round was obtained that
was still in an intact vacuum package as shipped from the packer. This vacuum
package was opened at the SDSU Meat Laboratory and sliced into steaks.
Boneless, skinless chicken breasts were obtained as bulk packaged by the
processor (not case-ready) in a plastic bag-lined box. All products were packaged
at the SDSU Meat Laboratory using retail styrofoam trays, soaker pads, and
oxygen-permeable overwrap film. The ground beef, beef steak, and chicken breast
packages were held in a retail case for 24 hrs after packaging and prior to the start
of the test to simulate time in retail display. A single cooked turkey product was
obtained with a “Use-by” date that would not be reached until after the completion
of the entire 14-day study. Following the 24-hr retail display of the fresh products,
the cooked turkey breast was sliced and packaged, and then all products were
held at room temp (70˚F) for 90 minutes to simulate consumer shopping and
transportation time. Following this 90-minute period, all packages were randomly
assigned into either 31 or 38˚F refrigeration for the duration of the trial.
Ambient temperature and relative humidity were monitored in each of the two
refrigeration chambers for the duration of the trial with readings taken every 30
minutes. Internal product temperature was recorded on a beef steak (different
steak than those for microbiology, TBARS, or panel) once daily in each chamber.
Actual ambient temperature and relative humidity reading for the entire 15-day
study period were as follows:
Mean
31˚F chamber
38˚F chamber
SD
SD
Mean
Ambient temperature
31.08
0.77
37.45
0.64
Relative humidity
91.91
7.27
98.70
2.54
Panel Evaluation:
Sensory panel evaluations were conducted once daily to visually score the
packaged meat samples for percent discoloration and overall desirability. In
addition, the panelists were asked daily whether or not each package was
acceptable to eat (based only on visual appraisal). The panel consisted of eight
members. The panelists evaluated the packages between 6:00 and 10:00 a.m.
each day. Percent discoloration and overall desirability were scored on each
sample using the following hedonic scales:
Percent Surface Discoloration
Overall Desirability
8 = None
8 = Extremely desirable
7 = 1 to 5 %
7 = Very desirable
6 = 6 to 10 %
6 = Moderately desirable
5 = 11 to 25 %
5 = Slightly desirable
4 = 26 to 50 %
4 = Slightly undesirable
3 = 51 to 75 %
3 = Moderately undesirable
2 = 76 to 99 %
2 = Very undesirable
1 = Complete
1 = Extremely undesirable
Colorimeter readings:
Colorimeter readings were taken once daily using a Minolta CR-310 ChromaMeter with a D65 illuminant and a 50 mm-diameter measurement area.
Digital photographs:
Digital photographs were taken once daily on the most representative sample (of
three) for each product by temperature combination. Files were written to 15
diskettes, one diskette for each day. See enclosure for further explanation.
Microbial counts:
Samples were aseptically sampled, blended with diluent in a stomacher, and
spiral plated. A 1:10 and a 1:1,000 dilution were spiral plated after 5 days into the
experiment to ensure that countable plates were obtained. Mesophilic plates were
incubated at 32±1ºC for 48 ±4 hr and then counted. Psychrophilic plates were
incubated at 7±1ºC for 10 days and then counted.
TBARS:
Thiobarbituric reactive substances (TBARS) were determined by distillation as
described by Tarladgis et al. (1960) and Wang et al. (1997). TBARS is a measure
of rancidity which corresponds to the development of off-flavors in meat products.
Statistical analysis:
Sensory panel and colorimeter data were analyzed using a
multivariate repeated measures analysis. Microbial counts
were converted to log CFU per gram of sample. Microbial data
and TBARS data were analyzed using one-way ANOVA (day 7
data and day 14 data were analyzed separately).
Ground Beef Results
·
Ground beef discolored at a faster rate in 38˚F than in 31˚F.
·
Overall desirability declined at a faster rate in 38˚F than in 31˚F.
·
Ground beef discolored 3 to 4 days later in 31˚F than in 38˚F.
· Ground beef lasted 3.32 days longer (64% longer) in 31˚F (8.98 days) than in
38˚F (5.66 days) before panelists said that they would not eat it (Length of time was
determined as that point when 50% of the panelists said that they would not eat it).
· Instrument measures of color confirmed that 31˚F storage delayed the
discoloration of ground beef by approximately 3 days as compared to 38˚F storage.
· Both mesophiles and psychrotrophes grew faster in 38˚F storage than in 31˚F
storage.
· Psychrotrophes grew faster than mesophiles regardless of refrigeration
temperature. Psychrotrophes counts were nearly 10,000 fold higher than
mesophilic counts at 14 days.
· Ground beef had higher TBARS values in 38˚F storage than in 31˚F storage at
7 days and especially at 14 days. TBARS is a measure of rancidity, thus indicating
that ground beef stored at 38˚F would develop off flavors more quickly than ground
beef stored at 31˚F.
Beef Steak Results
·
Beef steak discolored at a faster rate in 38˚F than in 31˚F.
·
Overall desirability declined at a faster rate in 38˚F than in 31˚F.
·
Beef steak discolored 6 to 7 days later in 31˚F than in 38˚F.
· Beef steak lasted 6.53 days longer (154% longer) in 31˚F (10.76 days) than in
38˚F (4.23 days) before panelists said that they would not eat it (Length of time was
determined as that point when 50% of the panelists said that they would not eat it).
· Instrument measures of color confirmed that 31˚F storage delayed the
discoloration of beef steak by approximately 7 days as compared to 38˚F storage.
· There was no difference between 38˚F and 31˚F storage in mesophilic
bacterial counts at 7 days of storage (p>.05); however, mesophilic bacterial counts
were higher in 38˚F storage than in 31˚F storage at 14 days of storage.
· Psychrotrophic bacterial counts were higher in 38˚F storage than in 31˚F
storage at 7 days of storage; however, there was no difference between 38˚F and
31˚F storage in psychrotrophic bacterial counts at 14 days of storage (p>.05).
TBARS (rancidity) increased from day 0 to day 7 in beef
steak at both storage temperatures but did not change
significantly from day 7 to day 14. Storage temperature did not
affect rancidity development in beef steak (p>.05).
·
Beef Steak Comparisons by Day
31 Degrees
38 Degrees
Boneless, Skinless Chicken Breast Results
·
Chicken breast discolored at a faster rate in 38˚F than in 31˚F.
·
Overall desirability declined at a faster rate in 38˚F than in 31˚F.
·
Chicken breast discolored 2 to 2½ days later in 31˚F than in 38˚F.
·
Chicken breast lasted 2.04 days longer (23% longer) in 31˚F (10.88 days) than
in 38˚F (8.84 days) before panelists said that they would not eat it (Length of
time was determined as that point when 50% of the panelists said that they
would not eat it).
·
Instrument measures of color revealed only slight, although statistically
significant, differences between chicken breast stored at 31˚F versus chicken
breast stored at 38˚F.
·
Both mesophiles and psychrotrophes grew faster in 38˚F storage than in 31˚F
storage.
·
The difference in bacterial growth between 31˚F and 38˚F was greater for the
chicken breast than for the other three meat products.
·
TBARS (rancidity) increased from day 0 to day 7 in chicken breast at both
storage temperatures but did not change significantly from day 7 to day 14.
Storage temperature did not affect rancidity development in chicken breast (p>.
05).
Cooked Turkey Results
·
Cooked turkey discolored at a faster rate in 38˚F than in 31˚F.
·
Overall desirability declined at a faster rate in 38˚F than in 31˚F.
·
Cooked turkey lasted longer in 31˚F than in 38˚F before panelists said that they
would not eat it.
·
Instrument measures of color revealed only slight, although statistically
significant, differences between cooked turkey stored at 31˚F versus cooked
turkey stored at 38˚F.
·
There was no difference between 38˚F and 31˚F storage in either mesophilic or
psychrotrophic bacterial counts at 7 days of storage (p>.05); however, both
mesophilic and psychrotrophic bacterial counts were higher in 38˚F storage
than in 31˚F storage at 14 days of storage.
·
TBARS (rancidity) in cooked turkey was not affected by either storage time or
storage temperature (p>.05). This lack of rancidity development in turkey
breast was probably due to the presence of antioxidant ingredients (this was a
processed, restructured turkey product).
References:
Tarladgis, B. G., B. M. Watts, M. T. Younathan, and L. Dugan Jr. 1960. A distillation
method for quantitative determination of malonaldehyde in rancid foods.
J. Amer. Oil Chem. Soc. 37:44.
Wang, C., L. Zhu, and M. S. Brewer. 1997. Comparison of 2-thiobarbituric acid
reactive substances determination methods in various types of frozen,
fresh meat. J. Food Lipids 4:87.