Food Protection Trends - March/April 2018 - 98

Pak bags were pre-filled with 205 g of de-ionized water to
facilitate sufficient coating of the sample piece during the
recovery process, and so that each rinse solution titration
sample weighed at least 100 g. Individual meat samples
were placed into the bags after set drain times had elapsed.
The bags were sealed and manually shaken for 30-40 s,
in such a way that the rinse solution completely covered
the sample. The bags were then opened, the meat samples
were removed and discarded, and the rinse solutions were
analyzed for peracetic acid and peroxide levels.
Sampling and analysis
Both types of meat samples were tested in duplicate
at each time point, and measurements were repeated on
three separate days, for a total of n = 6 samples at each
time point. For each meat sample, two separate titrations
were performed on the same rinse solution, so that for
each time point, 12 data points were obtained. Titrations
used the same suppressed peroxide iodometric method
(10), optimized for sensitivity. Briefly, to a clean 250 mL
Erlenmeyer flask with a Teflon-coated stir bar, one or two
ice cubes were added to maintain peroxide suppression. The
flask was tared on a balance, at least 100 g of the chicken
or red meat rinse sample solution was poured into the
flask from the Whirl-Pak bag, and the sample weight was
recorded. The flask was placed onto a lighted stir plate;
2-4 ml each of glacial acetic acid (Ecolab Supply Chain,
peracid grade), starch indicator (Ecolab Cat. No. 56869),
and potassium iodide (10%, Ecolab Cat. No. 56867) were
added while stirring. Reagents are added in excess, and
exact volumes are not critical, as the measurement relies
on the weight of the sample solution added to the flask
for peracetic acid or peroxide quantitation. A change in
the rinse solution color to blue indicated the presence
of peracetic acid. The solution was titrated using 0.01 N
thiosulfate standard (BDH, Cat. No. BDH7256, obtained
through VWR Scientific), delivered in 200 µL increments
(for short drain time samples, when the peracetic acid
or peroxide levels were high), or 20 µL increments using
an Eppendorf Multipette, Repeater Plus model (Cat. No.
2687194), fitted with a 1 ml pipette tip. Rapidity in the
procedure is accomplished using this titrant addition
method, allowing for distinguishable lightening of solution
color with lower titrant volume deliveries, thus preventing
overshooting of the titration endpoint. A color change
from blue to white/clear which held for at least 10 s was
considered a completed titration for peracetic acid.
Immediately following peracetic acid titration (or if no
blue color was observed after addition of the first reagents),
and on the same test solution, 2-4 mls each of 9N sulfuric
acid (prepared from 50% stock, Ecolab Supply Chain,
peracid grade) and oxygen catalyst (1.2% Molybdate
(w/v), Ricca Chemical Company, Cat. No. 5190-1) were
added to the flask and mixed for 3-5 minutes. A change

Food Protection Trends March/April

in the solution color from white or colorless and back to
dark blue indicated the presence of hydrogen peroxide.
The solution was further titrated using the same 0.01 N
thiosulfate standard, previously described, delivered in
200 µl or 20 µl increments.
Limits of detection (LOD) and quantitation (LOQ)
LODs and LOQs were determined for this titration
method via visual observation of the colored indicator
and changes imparted by titrant addition. LODs and
LOQs were calculated based on potential observable
color change, combined with the smallest titrant volume,
a defined minimum sample weight, and the concentration
of the titrant addition. For this study, a change in the
coloration upon addition of the titrant 0.01 N thiosulfate
was considered to be an indication of either species being
present (depending on place in the titration procedure),
and thus helps define the Limit of Detection. As the
smallest titrant volume delivered was 20 µl, and the sample
size of at least 100 g was maintained throughout, the LOD
determination is defined according to the equation (10)
used to measure the peracetic level:
((0.02 ml * 0.01 N * 38 * 100) / (100 g * 1000))*10000 = 0.08 ppm

where 0.02 ml is the minimum titrant volume, 0.01 N the
thiosulfate concentration, 38 is the equivalence factor for
peracetic acid (MW/2 to account for the titration reaction,
which requires 2 molar electrons), 100 g is the weight of the
sample, and 1000 converts the results, from milliequivalents
to equivalents. The 100 factor in the numerator converts the
result to a percent, and the 10,000 factor further converts
the result to parts per million. For peroxide, the factor 38 is
changed to 17, and the resulting LOD is 0.03 ppm. Limits
of Quantitation were defined as 3 × these factors (0.24 ppm
peracetic acid and 0.09 ppm hydrogen peroxide).
Data analysis
A response surface design of one factor (time) and
two responses (residual analytes-hydrogen peroxide
and peracetic acid) were used for this study. Two independent samples of meat were measured in duplicate via
titration at each time point. The duplicate readings on
each sample were averaged prior to statistical analysis.
The entire experiment was run on three separate occasions and all the data combined in a single data set. This
resulted in six samples at each time point for each type
of meat.
Data were further processed to normalize the results
based on the individual meat sample weights, such that
an amount of peracetic acid or hydrogen peroxide was
calculated and then divided by the sample weight. Error
in the sample weight measurement was considered
negligible and was not taken into account.

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