RESULTS AND DISCUSSION
A shelf-life study is an objective, methodical means to determine the
amount of time a food product can reasonably be expected to remain fresh
without appreciable changes to quality/safety (Galic, 2009). For
oils/fats and the food products that contain them, shelf-life is
contingent upon lipid oxidation and hydrolysis during the time of
processing and storage (Hu and Jacobsen, 2016; Kilcast and Subramaniam,
2000). Lipid oxidation is a series of chemical reactions involving
oxygen which can be described in terms of oxidative rancidity, or
deterioration of oils/fat causing undesirable changes in color, taste,
odor, and palatability. This also threatens the quality of nutrients and
can lead to the formation of toxic compounds (Miller, 2010). Oxidation
rates in lipids are largely a function of the structure of the fatty
acid chains (Kilcast and Subramaniam, 2000; Osawa et al., 2008). Poultry
fats contain high percentages of unsaturated fatty acids (ranging
57%-75%) with linoleic acid (C18:2) accounting for as much as 20% of
the total fatty acid profile, much higher than that of beef tallow and
pork lard (Hu and Jacobsen, 2016). Unsaturated fatty acids contain
reactive double bonds between some of their carbon atoms, this results
in higher susceptibility to oxidation. Lipid oxidation is complex and
involves a multiple number of reactions (Osawa et al., 2008). Numerous
analytical methods exist for the measurement of lipid oxidation, each
with their own desired value and limitations (Hu and Jacobsen, 2016;
Gray, 2015).
The FFA value measures the concentration of free fatty acids cleaved
from the triacylglyceride molecules by hydrolytic breaking of the ester
bonds between the fatty acids and the glycerol backbone (Miller, 2010).
The percentage FFA in this study was expressed as a percentage (in
weight) of oleic acid based on titration with a standard solution of KOH
using phenolphthalein as the indicator (AOCS, 2009). The free fatty acid
concentration of the rendered chicken fat used in this study was
approximately 6.88% on initiation of the study and remained consistent
through week 5; however, on week 6, FFA increased slightly to 8.53%
(P<0.05) (Fig. 1). The hydrolysis of fat can occur due to the
enzymatic (lipase) hydrolysis before rendering, or through acid or steam
hydrolysis after rendering. The increase in the FFA over time,
especially in the LA added chicken fat could be due to the acid
hydrolysis of fat by lactic acids leading to more free fatty acid
production. This finding is also related to the finding by Tadesse et
al. (Tadesse et al., 2017) who also reported a constant increase in the
FFA over the incubation time with no regular pattern of increase. Osawa
et al. (2008) reported that FFA level in dry pet food samples increased
during the storage period which is indicative of hydrolytic rancidity.
They reported a range of 4.6 to 28.0% FFA (as oleic acid equivalents)
in the pet food samples. The FFA percentage as well as the range in our
study was smaller compared to their finding which could be dependent on
the fat source and storage time before analysis. Both the main effects
(time and treatments) were significant (P < 0.05)
whereas, the interactions were not.
Peroxide values measure the primary oxidative products of fat. In the
control fat PV was in the range of 0-0.67 meqv/100 gm fat throughout the
storage period (Fig. 2). The values were in a range of 0.56-0.67
meqv/100 gm fat for the first week before it declined. This decline
could be due to primary oxidation products breaking down to form
secondary oxidation product as the storage time increased. The PV value
of chicken fat stored for 7 days at 4°C was measured at 0.215 meqv/100
gm of fat in a study reported by Shantha and Decker (1994). This lower
PV value compared to our study is likely due to the lower storage
temperature compared to ours (40°C). The PV values in the SBS treated
fat were also in a close range of 0.11-0.39 meqv/100 gm of fat, with a
higher value (<0.05) on day 5 and week 4 of the sampling. The
reason for these elevated values is not immediately obvious. The
continuous increase in the PV value up to day 5 in control and SBS
treated fat in our study may be similar to the findings of Tadesse et
al. (2017), wherein the PV value in animal butter continuously increased
during storage period up to 72 hours when stored at 25°C and 65°C.
However, in the LA added chicken fat the PV values increased over the
storage period until 5 weeks, and then declined on 6thweek. The highest PV value of 2.53 meqv/100 gm of fat was recorded on
5th week. The increase in the PV values before it
started falling on 6th week may be the effect of the
acidulant (lactic acid) causing more lipid oxidation to form peroxides
before they start breaking down to secondary oxidation products. The
main effects and interactions of treatments and days were higher
(P < 0.05). The PV values in the LA treated and control
fat increased linearly (P < 0.05) over time.
The second stage of oxidation occurs as hydroperoxides are cleaved to
form carbonyl compounds such as aldehydes, which can be measured as
non-volatile secondary oxidation products such as AV. These secondary
compounds can impact the sensory quality of foods, producing off-tastes
and off-smells (Hu and Jacobsen, 2016). It is important to note that
initially the peroxide values increased during lipid oxidation but as
the secondary compounds formed the primary values dropped, so it is
essential to use these tests in tandem to achieve meaningful information
about what is occurring at the time of measurement. The AV values for
the control fat ranged between 2.11 to 6.68. The LA treated fat had AV
values between 1.89 on day 0 to 8.18 on week 2; whereas, the SBS treated
fat had AV values of 2.00 on day 0 to 10.28 on the week 5 (Fig. 3). For
a good quality fat or oil, the AV values should be lower than 10 (List
et al., 1974). In our study, both the acidulant treated fats, as well as
control fat had AV values at or below this threshold of 10, with the
single exception of 10.28 for SBS treated fat on week 5. The higher AV
value indicates lower oxidative stability of the fats and oils. Tadesse
et al. (2017) reported that the AV values of animal butter increased
during storage at 65°C which was similar to our findings at 40°C
storage. Christensen and Holmer (1996) reported that the increase in the
AV value of fats and oils were a function of storage time and
temperature. The time effect and the interaction of the main effects
were significant (P < 0.05). Both the acidulant treated
samples as well as the control showed a linear (P <
0.05 increase in the AV value over the time.
In the fats and oil industry, the TOTOX value has been proposed as a
mean to combine the anisidine value and the peroxide value (Shahidi et
al., 2002; O’ Keefe et al., 2010). The TOTOX value provides a broad
accounting for the history of an oil or fat, but it “does not have any
sound scientific basis because it combines variables with different
dimensions” (Shahidi et al., 2002). The TOTOX value is calculated by
adding the AV value with twice the PV value. There was a linear increase
(P<0.05) in TOTOX values over time for all treatments.
Individually, the TOTOX values of control fat after 1 week of storage
remained constant over the storage period. Whereas, the TOTOX values for
the acidulant treated chicken fats increased throughout the storage time
with a maximum TOTOX of 10.67 on week 5 for SBS treated fat and 11.36 on
week 5 for LA treated fat (Fig. 4) and differed (P<0.05) from
the control (TOTOX of 6.0).
In conclusion, this study demonstrated that the FFA of fat rose slightly
over the 6 weeks regardless of the acidulant treatments. The addition of
LA increased the PV of the chicken fat and SBS led to a slight increase
in AV at the last two weeks of the study. Taken in combination the
acidulant treatment each led to a rise in TOTOX values for the fat by
weeks 5 and 6. This would indicate that use of acidulants for pathogen
control are stable to oxidation for at least 4 weeks and that LA may
have greater impact on the primary oxidation product (PV), whereas SBS
more impact on secondary oxidation products (AV) beyond 4 weeks of
storage. There may be limitations to these findings because the
experiment was conducted in a “bulk oil” model and the results might
be amplified if applied in a thin layer to pet foods where exposure to
air would be greater. Future research should evaluate the effects on
shelf life on dry pet food kibbles coated with rendered chicken fat
treated with acidulants. In conclusion, while changes were observed over
time due to acidulant use for 6 weeks, the oxidation products measured
remained within acceptable levels for fat used in the production of the
pet foods.