Jiang Jiang, Xin Zhang, Alma D. True, Lirong Zhou, and Youling L. Xiong
This study investigated the efficacy of licorice extract (LE) to curtail lipid oxidation and protect sensory attributes of ground pork during refrigerated and frozen storage. Pork patties (20% fat) were formulated with 0%, 0.02%, 0.05%, and 0.1% (meat basis) LE or rosemary extract (RE) as comparison or 0.01% (fat basis) BHA with 0 or 1.5% NaCl. Raw and precooked (75 ◦ C) patties were packaged in polyvinylchloride overwrapped trays and stored at 2 ◦ C up to 7 and 14 d, respectively, or at –20 ◦ C up to 6 mo. Lipid oxidation (thiobarbituric acid-reactive substances [TBARS]) and sensory attributes of stored patty samples were evaluated, radical scavenging activity of the LE was measured, and the active phenolic compounds were identified. Cooking yield ( 0.05) between those treated with 0.01% BHA, and all had similar TBARS contents at 0.1 to 0.3 mg MDA/kg, irrespective of storage time. The presence of salt in precooked patties clearly diminished the efficacy of both herbal antioxidants and BHA as the magnitude of TBARS inhibition at each specific storage time was less pronounced when compared with that for unsalted pork. In spite of the salt-dependent change, LE when compared with RE at the same concentration level (0.02%, 0.05%, or 0.10%) remained to be superior during the 1st 3 mo of sample storage (P < 0.05), losing its advantage only after the storage was extended to 6 mo (Figure 2B). The fact that at the 0.1% dosage level, LE was able to inhibit TBARS production by more than 60% for any of the storage times suggested that LE was a more powerful antioxidant than RE under the current experimental conditions. Because radical chain reaction is a primary pathway leading to lipid oxidation in food, radical scavenging activity assays of LE in comparison with RE and BHA were performed to understand the possible molecular action involved in the demonstrated effect
12 10
TBARS (mg/kg)
Sensory panel evaluation The taste panel analysis was done under the approved protocol of Institutional Review Board (IRB Nr. 11-0167-X4B) on refrigerated cooked pork patties by an 8-member trained panel. Panelists (ages 18 to 65), who were selected faculty, staff, and graduate students who had previously participated in meat sensory evaluation, were trained in 2 training sessions using both freshly cooked and precooked (oxidized) pork. Before the evaluation, precooked meat samples were warmed to room temperature then provided to the panelists in a random fashion. They were evaluated for pork aroma, rancidity, herbal flavor, crumbliness, juiciness, and overall acceptability on a numeric scale of 1 to 7 as specified below: pork aroma = 1 (nondetectable) to 7 (extremely intense); rancidity = 1 (nondetectable) to 7 (extremely intense); herbal flavor = 1 (nondetectable) to 7 (extremely intense); crumbliness = 1 (well-bound) to 7 (extremely crumbly); juiciness = 1 (nonjuicy) to 7 (extremely juicy). As consumers otherwise, the panelists were also asked to rate the products’ overall acceptability based on the scores of 1 (nonacceptable) to 7 (extremely acceptable). Because samples stored for 7 d were considerably oxidized, they were only evaluated visually and by olfactory detection. Room temperature drinking water and unsalted crackers were provided for palate cleansing between samples.
8
a
A (0% NaCl)
a
Day 0 Day 7 Day 14
b c
c
TBARS (mg/kg)
6 Results d The efficacy of LE as an antioxidant ingredient to inhibit lipid e 4 oxidation and extend product shelf-life was tested in both raw and f precooked pork patties with or without salt. Lipid oxidation, meafg fgh 2 ghi ghi fgh sured as TBARS, was minimal in raw patties during refrigerated hij hij ij j ij storage (0.10 to 0.28 mg MDA/kg), and there was no significant ij j j j j j 0 effect of antioxidants on the TBARS content due to the low level Control BHA 0.02 0.05 0.10 0.02 0.05 0.10 of oxidation (result not shown). Cooking yield (74.2 ± 0.2% for Rosemary (%) Licorice (%) unsalted; 82.9 ± 0.3% for salted) also showed no antioxidant treat12 B (1.5% NaCl) ment effects. Therefore, subsequent analyses were focused on the a a Day 0 precooked pork patties during storage. a 10 Day 7 In nonsalted samples stored at refrigerator temperature, the Day 14 b TBARS content increased exponentially. However, the TBARS 8 production was significantly less (P < 0.05) in patties treated with c LE at all concentration levels after 14 d, indicating inhibition of d 6 lipid oxidation (Figure 1A). In comparison, the effect of RE at e 0.05% and 0.1% was significant for samples stored for 14 d. The 4 presence of salt appeared to have an unremarkable effect of promotfg f fg ing TBARS production for most cooked patties samples, and LE gh gh hi remained highly effective at increasing concentrations (Figure 1B). 2 ij ij There was a conspicuous linear reduction of TBARS (P < 0.05) jk k k k k k k k k in both salted and unsalted day-14 samples as the concentration 0 Control BHA 0.02 0.05 0.10 0.02 0.05 0.10 of LE was categorically increased from 0.02% to 0.05% and then Rosemary (%) Licorice (%) to 0.1%. A similar trend but less remarkable response was noted Sample for samples treated with LE or RE and stored for 7 d. Overall, at an equal concentration, LE was more effective than RE. Fur- Figure 1–Lipid oxidation (TBARS) in refrigerated nonsalted (A) and salted thermore, LE at as low as 0.05% was equivalent to 0.01% BHA to (B) cooked pork patties stored at 2 ◦ C. Between samples, means without a protect unsalted patties (Figure 1A) from oxidative changes within common letter differ significantly (P < 0.05).
Vol. 00, Nr. 0, 2013 r Journal of Food Science C3
C: Food Chemistry
Antioxidant activity of licorice . . .
Antioxidant activity of licorice . . .
Month 0 Month 1 Month 3 Month 6
a
ijk k
0.2
k
k
no
0.4
no mn mn no
gh lm
0.6
0.0
kl ijkl
0.8
hij
1.0
Month 0 Month 1 Month 3 Month 6
cde
efg hijk ghi hijk fgh defg
1.2
o
o
(trolox eqv, mM)
.DPPH Scavenging Activity
0.8 0.6 0.4 0.2 0.0 0 2.5
o
o
o
Control BHA 0.02 0.05 0.10 0.02 0.05 0.10 Rosemary (%) Licorice (%)
Sample
100
200
300
400
500
600
200
300
400
500
600
200
300
400
500
600
Licorice Rosemary BHA
2.0 1.5 1.0 0.5 0.0
B (1.5% NaCl)
bc ab cdef cde cd
ab
aa
1.4
0.2
k
Control BHA 0.02 0.05 0.10 0.02 0.05 0.10 Rosemary (%) Licorice (%)
1.8 1.6
k
no
0.0
k k
b
jkl
0.4
jk
0.6
ghijk
0.8
Licorice Rosemary BHA
1.0
0 2.5
100 Licorice Rosemary BHA
2.0
(trolox eqv, mM)
bcdefg
1.0
bcde fghijk cdefgh defghi ijk ghijk ghijk hijk bcdef bc bcd efghij bcdef bcdefg hijk hijk ghijk
1.2
1.2
.OH Scavenging Activity
1.4
TBARS (mg/kg)
A (0% NaCl)
aa
1.6
on precooked pork patties stored at refrigerator temperature for different days and the results are summarized in Table 1 (unsalted meat) and Table 2 (salted meat). For unsalted patties, pork aroma of the control and 0.10% RE samples decreased after 7 d but increased for 0.10% LE samples (P < 0.05), while all other samples showed no change during storage. On day 7, the aroma of the 0.10% LE treatment was found to be more intense than all other samples. The panel was able to consistently detect a low level of rancidity in patties treated with 0.05% and 0.10% LE when compared with control patties, but the rancidity score for other patty samples was variable. In terms of herbal flavor or sweetness from LE, the panel could not tell any difference between control and treated samples due to the extremely low level of LE application. For crumbliness, there was also no detectable treatment
ABTS+. Scavenging Activity (trolox eqv, mM)
1.8
TBARS (mg/kg)
C: Food Chemistry
in precooked pork. As presented in Figure 3, at the same dosage levels, LE, albeit being less reactive than BHA, was consistently more effective than RE. Interestingly, although the antiradical activity against the alcohol-soluble (• DPPH) and water-soluble (ABTS+• ) species was superior for BHA, it was LE that exhibited the strongest capability to neutralize • OH, an extremely potent, ubiquitous, and readily formed radical in food. It is well documented in the literature that polyphenols, including a variety of flavonoids, can stabilize • OH and other radicals (Leopoldini and others 2011) and that licorice is an excellent source of such phenolic compounds (Zhang and Ye 2009). To ascertain whether active phenolic chemicals were present in the LE used in the present study, thereby accounting for the radical removing effect, mass spectrometry was applied to identify specific compounds. More than 15 compounds with strong signals were detected from LE with 8 representative ones shown in Figure 4. These polyphenols are the major compounds that have been isolated from G. glabra root and reported to be antioxidative (Zhang and Ye 2009; Liao and others 2012). Two of the confirmed phenolics—glabrene and glabridin—are only found in G. glabra, not in G. uralensis and G. inflate (Hatano and others 1991). The feasibility of LE used as a novel antioxidative ingredient in processed food must be addressed in the context of its impact on the sensory properties of food to which it is applied. Therefore, trained panel evaluation in replicate sessions was conducted
1.5 1.0 0.5 0.0 0
100
Concentration (μg/mL)
Figure 2–Lipid oxidation (TBARS) in frozen nonsalted (A) and salted (B) cooked pork patties stored at –20 ◦ C. Between samples, means without a Figure 3–Radical scavenging activity of licorice extract in comparison with common letter differ significantly (P < 0.05). rosemary extract and BHA at various concentrations.
C4 Journal of Food Science r Vol. 00, Nr. 0, 2013
membrane, the confinement of prooxidative metal ions, and the tight association of heme with globin, which limits its catalytic potential as a prooxidant (Kristensen and Andersen 1997). The steric occlusion of heme within the native globin structure would also hinder heme’s reactivity. The remarkable inhibition of lipid oxidation in cooked pork patties stored either in the refrigerator or in a –20 ◦ C freezer by the presence of LE was attributed to the antioxidative effect of LE, which acted as strong radical scavenger. The observed antiradical activity of LE was consistent with literature reports, for example, that by Tohma and Gulc¸in (2010). In general, polyphenols are potent radical scavengers capable of terminating radical chain reactions (Rice-Evans 1995). Glabridin, an isoflavan present in G. glabra and identified in the present study, has previously been shown to be largely responsible for licorice’s inhibition of CuSO4 induced oxidation of low-density lipoprotein in mice (Fuhrman and other 1997). Okuda and others (1989) also reported that the antioxidant potency of glabrene was 3 times of vitamin E. Moreover, through the formation of complexes with Fe3+ (or Fe2+ ) and Cu2+ , polyphenols reduce the potency of these proxidative metal ions (Perron and Brumaghim 2009), which are abundant in meat. Therefore, their coexistence in the herbal extracts was the direct cause for inhibition of lipid oxidation in cooked pork. A well-established source of natural antioxidants, RE was used as a comparative antioxidant in the present study. As reported in a large body of literature, rosemary, similar to licorice, contains many active compounds, of which, carnosic acid, carnosol, and rosmarinic acid have been identified as the major compounds capable of inhibiting oxidative processes in food (Chen and others 1992; Frankel and others 1996). Our results showed that in cooked
effect. Yet, patties containing LE at all concentration levels were assigned higher juiciness scores than control patties. Apparently due to the detected higher quality attributes, the panel considered all LE-treated, unsalted pork patties to be most acceptable of all (Table 1). The sensory scores for salted patties (Table 2) exhibited a similar trend to those for unsalted sample. For example, there were no remarkable differences between control and antioxidant-treated patties in pork aroma and herbal flavor, and rancidity was found to be the lowest for samples containing 0.1% LE. The amount of LE used in the pork patties was extremely low (≤0.1%) and the extract was almost devoid of the main sweet-eliciting compound, that is, water-soluble glycyrrhizin, due to the ethanol extraction process employed. Hence, the characteristic licorice flavor was not detected by the taste panel. However, due to the presence of salt, which extracts myofibrillar proteins to form an adhesive gel (Shang and Xiong 2010), patties were found to be less crumbly (mean score 1.8 compared with 3.3) and more juicy (mean score 5.1 compared with 3.9) than unsalted patties. When these quality traits were considered altogether, the panel rated the 0.10% LE patties to be consistently more acceptable than control samples and the most acceptable product overall.
Discussion
As an excellent media for the production of • OH, • O2 − , and other radical species due to the abundance of heme and nonheme iron and hydroperoxide precursors (unsaturated fatty acids), cooked pork patties are highly susceptible to radical-initiated oxidation (Broncano and others 2009). Raw meat patties are relatively resistant to oxidation because of the integrity of the cell
O OH
O
O
O
O
HO
HO
OH
O
O
OH
OH
HO
Glabrene, 321
Liquiritigenin, 255
OH
Glabrol, 337
Glabridin, 323 OH
O
O O
OH
HO
O
OH
HO O
HO
O
O O
HO
HO
OH
Liquiritin apioside, 549
Isoliquiritin, 417
O
HO
OH
O
OH
O
O
O
OH OH
O
OH HO
O
OH
OH
OH
HO OH
Isoliquiritin apioside, 549
O
OH HO
O O
OH OH
OH O
O O
OH O
O
CH3 OH
OH
Licorice glycoside A, 725 Figure 4–Main phenolic compounds identified in the licorice extract. Numbers denote m/z values of the [M-H]− species from the mass spectrometry.
Vol. 00, Nr. 0, 2013 r Journal of Food Science C5
C: Food Chemistry
Antioxidant activity of licorice . . .
C6 Journal of Food Science r Vol. 00, Nr. 0, 2013 4.1 a CD
2.8 a AB –
3.8 a C 1.5 a B 1.5 a A 3.6 b A 3.8 a BC 4.8 a BC
3.9 a C 1.6 a BC 1.2 a A 3.1 a A 3.9 a AB 4.8 a AB 4.3 a CD
3.6 b C –
3.6 a C 2.1 a AB –
7 3.9 a BC 2.3 ab A 1.3 a A 3.4 a A 3.6 a C 3.9 a D
0 4.3 a BC 1.7 b AB 1.3 a A 3.2 a A 3.8 a BC 4.3 a B
3
0.02%
3.9 a D
2.9 a AB –
4.3 a ABC 2.5 a A –
7 4.4 a ABC 1.4 a B 1.4 a A 3.4 a A 3.9 a BC 5.1 a ABC
0 4.3 a ABC 1.8 a AB 1.4 a A 3.3 a A 3.7 a BC 4.7 ab AB
3
0.05%
4.3 b CD
3.6 a C –
3.8 a BC 2.1 a AB –
7
Rosemary extract (patty basis)
4.2 ab ABC 1.1 b B 1.6 a A 3.4 b A 3.8 b BC 5.3 a AB
0 4.4 a ABC 1.6 ab BC 1.4 a A 3.2 b A 4.2 a A 4.8 a AB
3
0.10%
4.8 a BC
2.7 a A –
3.6 b BC 1.9 a ABC –
7 4.5 a AB 1.2 b B 1.4 a A 3.2 a A 4.1 a AB 5.5 a A
0
4.6 a AB 1.4 ab BC 1.4 a A 3.2 a A 4.1 a AB 5.3 a A
3
0.02%
4.4 b BCD
3.5 a C –
3.8 b BC 1.8 a ABC –
7
4.6 a A 1.0 b B 1.2 a A 3.3 a A 4.1 a AB 5.7 a A
0
4.6 a AB 1.1 b C 1.4 a A 3.4 a A 3.8 a AB 5.2 a A
3
0.05%
5.1 a AB
3.5 a C –
4.3 a AB 1.6 a BC –
7
Licorice extract (patty basis)
3.9 b BC 1.3 a B 1.7 a A 3.3 a A 4.4 a A 5.4 a A
0
4.9 a A 1.1 a C 1.3 a A 3.0 a A 4.1 a AB 5.4 a A
3
0.10%
5.6 a A
3.3 a BC –
4.7 a A 1.3 a C –
7
Pork aroma = 1 (nondetectable) to 7 (extremely intense); rancidity = 1 (nondetectable) to 7 (extremely intense); herbal flavor = 1 (nondetectable) to 7 (extremely intense); crumbliness = 1 (well-bound) to 7 (extremely crumbly); juiciness = 1 (nonjuicy) to 7 (extremely juicy); overall acceptability = 1 (nonacceptable) to 7 (extremely acceptable). a-b Between days within the same treatment, means that do not share a common letter differ significantly (P < 0.05). A-D Between treatments within the same day, means that do not share a common letter differ significantly (P < 0.05).
Overall acceptability
Juiciness
Crumbliness
Herbal flavor
Rancidity
3.8 b BC 2.6 a A –
4.8 a A 2.1 a A 1.3 a A 3.5 b A 3.7 a C 4.6 a C
Pork aroma
4.4 Ab ABC 2.1 a A 1.3 a A 3.4 ab A 3.4 a C 4.1 a B
3
0
0
Day → 7
%, fat basis)
attribute 3
BHA (0.01
Sensory
Treatment
C: Food Chemistry
Table 1–Sensory evaluation of precooked pork patties (unsalted) stored at 2 ◦ C for different days.
Antioxidant activity of licorice . . .
7
4.4 a C
1.2 a A –
3.9 b A 2.0 a A –
0 4.7 a A 1.0 b B 1.3 a AB 1.6 ab AB 5.2 a A 5.9 a A
3 4.3 ab AB 1.9 a A 1.3 a B 2.1 b B 4.9 a A 5.0 b CD
7
4.5 c C
1.5 a ABC –
3.9 b A 1.8 a AB –
0 4.6 a A 1.0 b B 1.3 a AB 1.7 a AB 5.3 a A 5.8 a A
4.5 a AB 1.5 b AB 1.3 a B 1.9 a AB 4.9 a A 5.3 ab BCD
3
0.02% 7
4.8 b BC
1.8 a BC –
3.9 a A 2.2 a A –
0 4.4 a A 1.0 b B 1.5 a AB 2.1 b BC 5.1 a A 5.6 a A
4.4 a AB 1.6 ab AB 1.3 a B 1.6 ab AB 5.0 a A 5.3 ab ABC
3
0.05% 7
4.9 b ABC
1.0 a AB –
4.4 a A 1.8 a AB –
0 4.6 a A 1.1 b B 1.7 a A 2.3 b C 5.1 a A 5.9 a A
3.9 b B 1.3 ab B 2.3 a A 1.6 a A 5.1 a A 5.3 ab ABC
3
0.10% 7
5.2 b AB
1.3 a AB –
3.6 b A 1.6 a AB –
0 4.8 a A 1.0 b B 1.2 a B 1.9 a ABC 5.4 a A 5.7 a A
4.7 a A 1.7 a AB 1.4 a B 1.6 a AB 5.1 a A 4.8 b D
3
0.02% 7
4.8 b ABC
1.5 a ABC –
3.7 b A 1.8 a AB –
0 4.6 a A 1.0 b B 1.3 a B 1.6 a A 5.2 a A 5.9 a A
4.1 a AB 1.2 b B 1.3 a B 1.7 a AB 5.0 a A 5.6 a AB
3
0.05%
4.9 b ABC
1.9 a C –
4.3 a A 1.9 a AB –
7
Licorice extract (patty basis)
0 4.6 a A 1.0 a B 1.3 a AB 1.9 a ABC 5.2 a A 5.6 a A
4.5 a AB 1.1 a B 1.6 a B 1.9 a AB 5.3 a A 5.8 a A
3
0.10% 7
5.4 a A
1.7 a BC –
3.8 b A 1.2 a B –
Vol. 00, Nr. 0, 2013 r Journal of Food Science C7
C: Food Chemistry
Pork aroma = 1 (nondetectable) to 7 (extremely intense); rancidity = 1 (nondetectable) to 7 (extremely intense); herbal flavor = 1 (nondetectable) to 7 (extremely intense); crumbliness = 1 (well-bound) to 7 (extremely crumbly); juiciness = 1 (nonjuicy) to 7 (extremely juicy); overall acceptability = 1 (nonacceptable) to 7 (extremely acceptable). a-b Between days within the same treatment, means that do not share a common letter differ significantly (P < 0.05). A-D Between treatments within the same day, means that do not share a common letter differ significantly (P < 0.05).
Overall acceptability
Juiciness
Crumbliness
Herbal flavor
Rancidity
4.5 a AB 1.9 a A 1.7 a B 1.7 b AB 5.1 a A 4.9 a CD
3
0
Day →
4.8 a A 1.4 a A 1.3 a B 1.8 b ABC 5.1 a A 5.1 a B
fat basis)
Control
attribute
Pork aroma
BHA (0.01%,
Sensory
Rosemary extract (patty basis)
Treatment
Table 2–Sensory evaluation of precooked pork patties (1.5% NaCl) stored at 2 ◦ C for different days.
Antioxidant activity of licorice . . .
Antioxidant activity of licorice . . .
C: Food Chemistry
pork patties, LE was generally more effective than RE to delay lipid oxidation and prevent flavor deterioration during storage, and the difference was due to the specific antioxidant compounds present in each source. The stronger radical-scavenging power of LE in comparison with RE served to explain the more efficient suppression of TBARS production; this was most notable in salted pork patties whether stored at 2 or –20 ◦ C. The greater effectiveness of the synthetic antioxidant BHA (a nonpolar phenol derivative) than LE (a mixture of more polar phenolics due to polyols), especially during the later stage of storage of cooked meat patties (>7 d at 2 ◦ C; >1 mo at –20 ◦ C), suggested that bulk-phase neutral lipids or triacylglycerols were primary precursors of TBARS when considering the partitioning of polar and less polar antioxidants in the comminuted meat system. Namely, most oxygen radicals were probably formed in the lipid phase. However, the high efficacy of LE at the 0.1% dosage level for samples stored for a relatively short period suggested that radicals formed in the aqueous phase of cooked pork patties were also important catalysts of lipid oxidation. This hypothesis was supported by the remarkably high scavenging power of LE against • OH (a water-soluble radial) when compared to BHA. The liquefied fat from cooking was an excellent carrier and distributor of BHA, while both the juice and melted fat allowed the incorporation of phenolics. The latter has also been used to explain the remarkable antioxidant capacity of many plant polyphenol compounds applied to O/W emulsion systems (Maqsood and Banjakul 2010). The antioxidant activity of LE appears to be the result of synergistic effects of numerous active compounds. Gordon and An (1995) reported that individually, polyphenols in LE were less effective than α-tocopherol (a similar compound to BHA in functional structure) but were more effective than α-tocopherol when acting collectively. Although the results showed an apparent correlation between the inhibition of lipid oxidation (TBARS) and the suppression of rancidity in the presence of antioxidants, such a relationship was nonlinear. For example, of the 3 antioxidants tested, BHA was most effective in inhibiting lipid oxidation; however, it did not lower the rancidity score from the control samples as much as the 0.1% LE treatment. This suggested that in addition to lipid oxidation products there were other compounds that contributed to this negative sensory note. We had previously shown that treatments of crude myofibrillar proteins with propyl gallate (a BHA analog) and ascorbic acid substantially inhibited lipid oxidation but did not prevent protein oxidation, which was measured as carbonyl formation and loss of sulfhydryls (Srinivasan and others 1996; Parkington and others 2000). By binding to proteins through thiol-quinone (Jongberg and others 2011) or amine-quinone (Rohn and others 2004) adductions, phenolics present in LE might inhibit the oxidative degradation of proteins into rancidity-contributing volatiles. Vuorela and others (2005) reported that the incorporation of rapeseed and pine bark phenolics into cooked pork patties inhibited not only lipid oxidation (up to 80%) but also protein carbonyl formation (up to 64%).
Conclusions The extract of licorice for the 1st time proved to be an effective antioxidant in precooked pork patties capable of inhibiting lipid oxidation during refrigerated and frozen storage independent of salt content. Such antioxidant activity is attributed to the superior radical scavenging capacity of the extract, particularly against hydroxyl radical (• OH). As a result of suppression of oxidation of lipids and possibly also proteins, rancidity in stored meat samC8 Journal of Food Science r Vol. 00, Nr. 0, 2013
ples was less detectable. Overall, LE was more effective than RE and was almost comparable to BHA in efficacy when used at appropriate levels. Therefore, LE can be applied as a novel natural antioxidant additive for the protection of flavor and shelf-life of meat products.
Acknowledgments The study was supported, in part, by ICL Performance Products, LP. Paper nr 13-07-097 of the Kentucky Agricultural Experiment Station.
Disclaimer Dr. E. Allen Foegeding served as Scientific Editor and Dr. Gale Strasburg served as Associate Editor overseeing single-blinded review of this manuscript. It is the policy of JFS to blind Editorial Board members from the peer-review process of their own submissions, just as all authors are blinded.
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Shang Y, Xiong YL. 2010. Xanthan enhances water binding and gel formation of transglutaminase-treated porcine myofibrillar proteins. J Food Sci 75:E178–85. Sinnhuber RO, Yu TC. 1977. The 2-thiobarbituric acid reaction, an objective measure of the oxidative deterioration occurring in fats and oils. J. Japan Oil Chem 26:259–67. Srinivasan S, Xiong YL, Decker EA. 1996. Inhibition of protein and lipid oxidation in beef heart surimi-like material by antioxidants and combinations of pH, NaCl, and buffer type in the washing media. J Agric Food Chem 44:119–25. Tanahashi T, Mune T, Morita H, Tanahashi H, Isomura Y, Suwa T, Daido H, Gomez-Sancehz CE, Yasuda K. 2002. Glycyrrhizic acid suppresses type 2 11β-hydroxysteroid dehydrogenase expression in vivo. J Steroid Biochem 80:441–7. Tohma HS, Gulc¸in I. 2010. Antioxidant and radical scavenging activity of aerial parts and roots of Turkish liquorice (Glycyrrhiza glabra L.). Int J Food Prop 13:657–71. Vaya J, Belinky PA, Aviram M. 1997. Antioxidant constituents from licorice roots: isolation, structure elucidation and antioxidative capacity toward LDL oxidation. Free Rad Biol Med 23:302–13.
Vuorela S, Salminen H, M¨akel¨a M, Kivikari R, Karonen M, Heinonen M. 2005. Effect of plant phenolics on protein and lipid oxidation in cooked pork meat patties. J Agric Food Chem 53:8492–7. Zhang Q, Ye M. 2009. Chemical analysis of the Chinese herbal medicine Gan-Cao (licorice). J Chromatogr A 1216:1954–69. Zhang H, Kong B, Xiong YL, Sun X. 2009. Antimicrobial activities of spice extracts against pathogenic and spoilage bacteria in modified atmosphere packaged fresh pork and vacuum packaged ham slices stored at 4 ◦ C. Meat Sci 81:686– 92. Zhu L, Chen J, Tang X, Xiong YL. 2008. Reducing, radical scavenging, and chelation properties of in vitro digests of alcalase-treated zein hydrolysate. J Agric Food Chem 56: 2714–21.
Vol. 00, Nr. 0, 2013 r Journal of Food Science C9
C: Food Chemistry
Antioxidant activity of licorice . . .
This study investigated the efficacy of licorice extract (LE) to curtail lipid oxidation and protect sensory attributes of ground pork during refrigerated and frozen storage. Pork patties (20% fat) were formulated with 0%, 0.02%, 0.05%, and 0.1% (meat basis) LE or rosemary extract (RE) as comparison or 0.01% (fat basis) BHA with 0 or 1.5% NaCl. Raw and precooked (75 ◦ C) patties were packaged in polyvinylchloride overwrapped trays and stored at 2 ◦ C up to 7 and 14 d, respectively, or at –20 ◦ C up to 6 mo. Lipid oxidation (thiobarbituric acid-reactive substances [TBARS]) and sensory attributes of stored patty samples were evaluated, radical scavenging activity of the LE was measured, and the active phenolic compounds were identified. Cooking yield ( 0.05) between those treated with 0.01% BHA, and all had similar TBARS contents at 0.1 to 0.3 mg MDA/kg, irrespective of storage time. The presence of salt in precooked patties clearly diminished the efficacy of both herbal antioxidants and BHA as the magnitude of TBARS inhibition at each specific storage time was less pronounced when compared with that for unsalted pork. In spite of the salt-dependent change, LE when compared with RE at the same concentration level (0.02%, 0.05%, or 0.10%) remained to be superior during the 1st 3 mo of sample storage (P < 0.05), losing its advantage only after the storage was extended to 6 mo (Figure 2B). The fact that at the 0.1% dosage level, LE was able to inhibit TBARS production by more than 60% for any of the storage times suggested that LE was a more powerful antioxidant than RE under the current experimental conditions. Because radical chain reaction is a primary pathway leading to lipid oxidation in food, radical scavenging activity assays of LE in comparison with RE and BHA were performed to understand the possible molecular action involved in the demonstrated effect
12 10
TBARS (mg/kg)
Sensory panel evaluation The taste panel analysis was done under the approved protocol of Institutional Review Board (IRB Nr. 11-0167-X4B) on refrigerated cooked pork patties by an 8-member trained panel. Panelists (ages 18 to 65), who were selected faculty, staff, and graduate students who had previously participated in meat sensory evaluation, were trained in 2 training sessions using both freshly cooked and precooked (oxidized) pork. Before the evaluation, precooked meat samples were warmed to room temperature then provided to the panelists in a random fashion. They were evaluated for pork aroma, rancidity, herbal flavor, crumbliness, juiciness, and overall acceptability on a numeric scale of 1 to 7 as specified below: pork aroma = 1 (nondetectable) to 7 (extremely intense); rancidity = 1 (nondetectable) to 7 (extremely intense); herbal flavor = 1 (nondetectable) to 7 (extremely intense); crumbliness = 1 (well-bound) to 7 (extremely crumbly); juiciness = 1 (nonjuicy) to 7 (extremely juicy). As consumers otherwise, the panelists were also asked to rate the products’ overall acceptability based on the scores of 1 (nonacceptable) to 7 (extremely acceptable). Because samples stored for 7 d were considerably oxidized, they were only evaluated visually and by olfactory detection. Room temperature drinking water and unsalted crackers were provided for palate cleansing between samples.
8
a
A (0% NaCl)
a
Day 0 Day 7 Day 14
b c
c
TBARS (mg/kg)
6 Results d The efficacy of LE as an antioxidant ingredient to inhibit lipid e 4 oxidation and extend product shelf-life was tested in both raw and f precooked pork patties with or without salt. Lipid oxidation, meafg fgh 2 ghi ghi fgh sured as TBARS, was minimal in raw patties during refrigerated hij hij ij j ij storage (0.10 to 0.28 mg MDA/kg), and there was no significant ij j j j j j 0 effect of antioxidants on the TBARS content due to the low level Control BHA 0.02 0.05 0.10 0.02 0.05 0.10 of oxidation (result not shown). Cooking yield (74.2 ± 0.2% for Rosemary (%) Licorice (%) unsalted; 82.9 ± 0.3% for salted) also showed no antioxidant treat12 B (1.5% NaCl) ment effects. Therefore, subsequent analyses were focused on the a a Day 0 precooked pork patties during storage. a 10 Day 7 In nonsalted samples stored at refrigerator temperature, the Day 14 b TBARS content increased exponentially. However, the TBARS 8 production was significantly less (P < 0.05) in patties treated with c LE at all concentration levels after 14 d, indicating inhibition of d 6 lipid oxidation (Figure 1A). In comparison, the effect of RE at e 0.05% and 0.1% was significant for samples stored for 14 d. The 4 presence of salt appeared to have an unremarkable effect of promotfg f fg ing TBARS production for most cooked patties samples, and LE gh gh hi remained highly effective at increasing concentrations (Figure 1B). 2 ij ij There was a conspicuous linear reduction of TBARS (P < 0.05) jk k k k k k k k k in both salted and unsalted day-14 samples as the concentration 0 Control BHA 0.02 0.05 0.10 0.02 0.05 0.10 of LE was categorically increased from 0.02% to 0.05% and then Rosemary (%) Licorice (%) to 0.1%. A similar trend but less remarkable response was noted Sample for samples treated with LE or RE and stored for 7 d. Overall, at an equal concentration, LE was more effective than RE. Fur- Figure 1–Lipid oxidation (TBARS) in refrigerated nonsalted (A) and salted thermore, LE at as low as 0.05% was equivalent to 0.01% BHA to (B) cooked pork patties stored at 2 ◦ C. Between samples, means without a protect unsalted patties (Figure 1A) from oxidative changes within common letter differ significantly (P < 0.05).
Vol. 00, Nr. 0, 2013 r Journal of Food Science C3
C: Food Chemistry
Antioxidant activity of licorice . . .
Antioxidant activity of licorice . . .
Month 0 Month 1 Month 3 Month 6
a
ijk k
0.2
k
k
no
0.4
no mn mn no
gh lm
0.6
0.0
kl ijkl
0.8
hij
1.0
Month 0 Month 1 Month 3 Month 6
cde
efg hijk ghi hijk fgh defg
1.2
o
o
(trolox eqv, mM)
.DPPH Scavenging Activity
0.8 0.6 0.4 0.2 0.0 0 2.5
o
o
o
Control BHA 0.02 0.05 0.10 0.02 0.05 0.10 Rosemary (%) Licorice (%)
Sample
100
200
300
400
500
600
200
300
400
500
600
200
300
400
500
600
Licorice Rosemary BHA
2.0 1.5 1.0 0.5 0.0
B (1.5% NaCl)
bc ab cdef cde cd
ab
aa
1.4
0.2
k
Control BHA 0.02 0.05 0.10 0.02 0.05 0.10 Rosemary (%) Licorice (%)
1.8 1.6
k
no
0.0
k k
b
jkl
0.4
jk
0.6
ghijk
0.8
Licorice Rosemary BHA
1.0
0 2.5
100 Licorice Rosemary BHA
2.0
(trolox eqv, mM)
bcdefg
1.0
bcde fghijk cdefgh defghi ijk ghijk ghijk hijk bcdef bc bcd efghij bcdef bcdefg hijk hijk ghijk
1.2
1.2
.OH Scavenging Activity
1.4
TBARS (mg/kg)
A (0% NaCl)
aa
1.6
on precooked pork patties stored at refrigerator temperature for different days and the results are summarized in Table 1 (unsalted meat) and Table 2 (salted meat). For unsalted patties, pork aroma of the control and 0.10% RE samples decreased after 7 d but increased for 0.10% LE samples (P < 0.05), while all other samples showed no change during storage. On day 7, the aroma of the 0.10% LE treatment was found to be more intense than all other samples. The panel was able to consistently detect a low level of rancidity in patties treated with 0.05% and 0.10% LE when compared with control patties, but the rancidity score for other patty samples was variable. In terms of herbal flavor or sweetness from LE, the panel could not tell any difference between control and treated samples due to the extremely low level of LE application. For crumbliness, there was also no detectable treatment
ABTS+. Scavenging Activity (trolox eqv, mM)
1.8
TBARS (mg/kg)
C: Food Chemistry
in precooked pork. As presented in Figure 3, at the same dosage levels, LE, albeit being less reactive than BHA, was consistently more effective than RE. Interestingly, although the antiradical activity against the alcohol-soluble (• DPPH) and water-soluble (ABTS+• ) species was superior for BHA, it was LE that exhibited the strongest capability to neutralize • OH, an extremely potent, ubiquitous, and readily formed radical in food. It is well documented in the literature that polyphenols, including a variety of flavonoids, can stabilize • OH and other radicals (Leopoldini and others 2011) and that licorice is an excellent source of such phenolic compounds (Zhang and Ye 2009). To ascertain whether active phenolic chemicals were present in the LE used in the present study, thereby accounting for the radical removing effect, mass spectrometry was applied to identify specific compounds. More than 15 compounds with strong signals were detected from LE with 8 representative ones shown in Figure 4. These polyphenols are the major compounds that have been isolated from G. glabra root and reported to be antioxidative (Zhang and Ye 2009; Liao and others 2012). Two of the confirmed phenolics—glabrene and glabridin—are only found in G. glabra, not in G. uralensis and G. inflate (Hatano and others 1991). The feasibility of LE used as a novel antioxidative ingredient in processed food must be addressed in the context of its impact on the sensory properties of food to which it is applied. Therefore, trained panel evaluation in replicate sessions was conducted
1.5 1.0 0.5 0.0 0
100
Concentration (μg/mL)
Figure 2–Lipid oxidation (TBARS) in frozen nonsalted (A) and salted (B) cooked pork patties stored at –20 ◦ C. Between samples, means without a Figure 3–Radical scavenging activity of licorice extract in comparison with common letter differ significantly (P < 0.05). rosemary extract and BHA at various concentrations.
C4 Journal of Food Science r Vol. 00, Nr. 0, 2013
membrane, the confinement of prooxidative metal ions, and the tight association of heme with globin, which limits its catalytic potential as a prooxidant (Kristensen and Andersen 1997). The steric occlusion of heme within the native globin structure would also hinder heme’s reactivity. The remarkable inhibition of lipid oxidation in cooked pork patties stored either in the refrigerator or in a –20 ◦ C freezer by the presence of LE was attributed to the antioxidative effect of LE, which acted as strong radical scavenger. The observed antiradical activity of LE was consistent with literature reports, for example, that by Tohma and Gulc¸in (2010). In general, polyphenols are potent radical scavengers capable of terminating radical chain reactions (Rice-Evans 1995). Glabridin, an isoflavan present in G. glabra and identified in the present study, has previously been shown to be largely responsible for licorice’s inhibition of CuSO4 induced oxidation of low-density lipoprotein in mice (Fuhrman and other 1997). Okuda and others (1989) also reported that the antioxidant potency of glabrene was 3 times of vitamin E. Moreover, through the formation of complexes with Fe3+ (or Fe2+ ) and Cu2+ , polyphenols reduce the potency of these proxidative metal ions (Perron and Brumaghim 2009), which are abundant in meat. Therefore, their coexistence in the herbal extracts was the direct cause for inhibition of lipid oxidation in cooked pork. A well-established source of natural antioxidants, RE was used as a comparative antioxidant in the present study. As reported in a large body of literature, rosemary, similar to licorice, contains many active compounds, of which, carnosic acid, carnosol, and rosmarinic acid have been identified as the major compounds capable of inhibiting oxidative processes in food (Chen and others 1992; Frankel and others 1996). Our results showed that in cooked
effect. Yet, patties containing LE at all concentration levels were assigned higher juiciness scores than control patties. Apparently due to the detected higher quality attributes, the panel considered all LE-treated, unsalted pork patties to be most acceptable of all (Table 1). The sensory scores for salted patties (Table 2) exhibited a similar trend to those for unsalted sample. For example, there were no remarkable differences between control and antioxidant-treated patties in pork aroma and herbal flavor, and rancidity was found to be the lowest for samples containing 0.1% LE. The amount of LE used in the pork patties was extremely low (≤0.1%) and the extract was almost devoid of the main sweet-eliciting compound, that is, water-soluble glycyrrhizin, due to the ethanol extraction process employed. Hence, the characteristic licorice flavor was not detected by the taste panel. However, due to the presence of salt, which extracts myofibrillar proteins to form an adhesive gel (Shang and Xiong 2010), patties were found to be less crumbly (mean score 1.8 compared with 3.3) and more juicy (mean score 5.1 compared with 3.9) than unsalted patties. When these quality traits were considered altogether, the panel rated the 0.10% LE patties to be consistently more acceptable than control samples and the most acceptable product overall.
Discussion
As an excellent media for the production of • OH, • O2 − , and other radical species due to the abundance of heme and nonheme iron and hydroperoxide precursors (unsaturated fatty acids), cooked pork patties are highly susceptible to radical-initiated oxidation (Broncano and others 2009). Raw meat patties are relatively resistant to oxidation because of the integrity of the cell
O OH
O
O
O
O
HO
HO
OH
O
O
OH
OH
HO
Glabrene, 321
Liquiritigenin, 255
OH
Glabrol, 337
Glabridin, 323 OH
O
O O
OH
HO
O
OH
HO O
HO
O
O O
HO
HO
OH
Liquiritin apioside, 549
Isoliquiritin, 417
O
HO
OH
O
OH
O
O
O
OH OH
O
OH HO
O
OH
OH
OH
HO OH
Isoliquiritin apioside, 549
O
OH HO
O O
OH OH
OH O
O O
OH O
O
CH3 OH
OH
Licorice glycoside A, 725 Figure 4–Main phenolic compounds identified in the licorice extract. Numbers denote m/z values of the [M-H]− species from the mass spectrometry.
Vol. 00, Nr. 0, 2013 r Journal of Food Science C5
C: Food Chemistry
Antioxidant activity of licorice . . .
C6 Journal of Food Science r Vol. 00, Nr. 0, 2013 4.1 a CD
2.8 a AB –
3.8 a C 1.5 a B 1.5 a A 3.6 b A 3.8 a BC 4.8 a BC
3.9 a C 1.6 a BC 1.2 a A 3.1 a A 3.9 a AB 4.8 a AB 4.3 a CD
3.6 b C –
3.6 a C 2.1 a AB –
7 3.9 a BC 2.3 ab A 1.3 a A 3.4 a A 3.6 a C 3.9 a D
0 4.3 a BC 1.7 b AB 1.3 a A 3.2 a A 3.8 a BC 4.3 a B
3
0.02%
3.9 a D
2.9 a AB –
4.3 a ABC 2.5 a A –
7 4.4 a ABC 1.4 a B 1.4 a A 3.4 a A 3.9 a BC 5.1 a ABC
0 4.3 a ABC 1.8 a AB 1.4 a A 3.3 a A 3.7 a BC 4.7 ab AB
3
0.05%
4.3 b CD
3.6 a C –
3.8 a BC 2.1 a AB –
7
Rosemary extract (patty basis)
4.2 ab ABC 1.1 b B 1.6 a A 3.4 b A 3.8 b BC 5.3 a AB
0 4.4 a ABC 1.6 ab BC 1.4 a A 3.2 b A 4.2 a A 4.8 a AB
3
0.10%
4.8 a BC
2.7 a A –
3.6 b BC 1.9 a ABC –
7 4.5 a AB 1.2 b B 1.4 a A 3.2 a A 4.1 a AB 5.5 a A
0
4.6 a AB 1.4 ab BC 1.4 a A 3.2 a A 4.1 a AB 5.3 a A
3
0.02%
4.4 b BCD
3.5 a C –
3.8 b BC 1.8 a ABC –
7
4.6 a A 1.0 b B 1.2 a A 3.3 a A 4.1 a AB 5.7 a A
0
4.6 a AB 1.1 b C 1.4 a A 3.4 a A 3.8 a AB 5.2 a A
3
0.05%
5.1 a AB
3.5 a C –
4.3 a AB 1.6 a BC –
7
Licorice extract (patty basis)
3.9 b BC 1.3 a B 1.7 a A 3.3 a A 4.4 a A 5.4 a A
0
4.9 a A 1.1 a C 1.3 a A 3.0 a A 4.1 a AB 5.4 a A
3
0.10%
5.6 a A
3.3 a BC –
4.7 a A 1.3 a C –
7
Pork aroma = 1 (nondetectable) to 7 (extremely intense); rancidity = 1 (nondetectable) to 7 (extremely intense); herbal flavor = 1 (nondetectable) to 7 (extremely intense); crumbliness = 1 (well-bound) to 7 (extremely crumbly); juiciness = 1 (nonjuicy) to 7 (extremely juicy); overall acceptability = 1 (nonacceptable) to 7 (extremely acceptable). a-b Between days within the same treatment, means that do not share a common letter differ significantly (P < 0.05). A-D Between treatments within the same day, means that do not share a common letter differ significantly (P < 0.05).
Overall acceptability
Juiciness
Crumbliness
Herbal flavor
Rancidity
3.8 b BC 2.6 a A –
4.8 a A 2.1 a A 1.3 a A 3.5 b A 3.7 a C 4.6 a C
Pork aroma
4.4 Ab ABC 2.1 a A 1.3 a A 3.4 ab A 3.4 a C 4.1 a B
3
0
0
Day → 7
%, fat basis)
attribute 3
BHA (0.01
Sensory
Treatment
C: Food Chemistry
Table 1–Sensory evaluation of precooked pork patties (unsalted) stored at 2 ◦ C for different days.
Antioxidant activity of licorice . . .
7
4.4 a C
1.2 a A –
3.9 b A 2.0 a A –
0 4.7 a A 1.0 b B 1.3 a AB 1.6 ab AB 5.2 a A 5.9 a A
3 4.3 ab AB 1.9 a A 1.3 a B 2.1 b B 4.9 a A 5.0 b CD
7
4.5 c C
1.5 a ABC –
3.9 b A 1.8 a AB –
0 4.6 a A 1.0 b B 1.3 a AB 1.7 a AB 5.3 a A 5.8 a A
4.5 a AB 1.5 b AB 1.3 a B 1.9 a AB 4.9 a A 5.3 ab BCD
3
0.02% 7
4.8 b BC
1.8 a BC –
3.9 a A 2.2 a A –
0 4.4 a A 1.0 b B 1.5 a AB 2.1 b BC 5.1 a A 5.6 a A
4.4 a AB 1.6 ab AB 1.3 a B 1.6 ab AB 5.0 a A 5.3 ab ABC
3
0.05% 7
4.9 b ABC
1.0 a AB –
4.4 a A 1.8 a AB –
0 4.6 a A 1.1 b B 1.7 a A 2.3 b C 5.1 a A 5.9 a A
3.9 b B 1.3 ab B 2.3 a A 1.6 a A 5.1 a A 5.3 ab ABC
3
0.10% 7
5.2 b AB
1.3 a AB –
3.6 b A 1.6 a AB –
0 4.8 a A 1.0 b B 1.2 a B 1.9 a ABC 5.4 a A 5.7 a A
4.7 a A 1.7 a AB 1.4 a B 1.6 a AB 5.1 a A 4.8 b D
3
0.02% 7
4.8 b ABC
1.5 a ABC –
3.7 b A 1.8 a AB –
0 4.6 a A 1.0 b B 1.3 a B 1.6 a A 5.2 a A 5.9 a A
4.1 a AB 1.2 b B 1.3 a B 1.7 a AB 5.0 a A 5.6 a AB
3
0.05%
4.9 b ABC
1.9 a C –
4.3 a A 1.9 a AB –
7
Licorice extract (patty basis)
0 4.6 a A 1.0 a B 1.3 a AB 1.9 a ABC 5.2 a A 5.6 a A
4.5 a AB 1.1 a B 1.6 a B 1.9 a AB 5.3 a A 5.8 a A
3
0.10% 7
5.4 a A
1.7 a BC –
3.8 b A 1.2 a B –
Vol. 00, Nr. 0, 2013 r Journal of Food Science C7
C: Food Chemistry
Pork aroma = 1 (nondetectable) to 7 (extremely intense); rancidity = 1 (nondetectable) to 7 (extremely intense); herbal flavor = 1 (nondetectable) to 7 (extremely intense); crumbliness = 1 (well-bound) to 7 (extremely crumbly); juiciness = 1 (nonjuicy) to 7 (extremely juicy); overall acceptability = 1 (nonacceptable) to 7 (extremely acceptable). a-b Between days within the same treatment, means that do not share a common letter differ significantly (P < 0.05). A-D Between treatments within the same day, means that do not share a common letter differ significantly (P < 0.05).
Overall acceptability
Juiciness
Crumbliness
Herbal flavor
Rancidity
4.5 a AB 1.9 a A 1.7 a B 1.7 b AB 5.1 a A 4.9 a CD
3
0
Day →
4.8 a A 1.4 a A 1.3 a B 1.8 b ABC 5.1 a A 5.1 a B
fat basis)
Control
attribute
Pork aroma
BHA (0.01%,
Sensory
Rosemary extract (patty basis)
Treatment
Table 2–Sensory evaluation of precooked pork patties (1.5% NaCl) stored at 2 ◦ C for different days.
Antioxidant activity of licorice . . .
Antioxidant activity of licorice . . .
C: Food Chemistry
pork patties, LE was generally more effective than RE to delay lipid oxidation and prevent flavor deterioration during storage, and the difference was due to the specific antioxidant compounds present in each source. The stronger radical-scavenging power of LE in comparison with RE served to explain the more efficient suppression of TBARS production; this was most notable in salted pork patties whether stored at 2 or –20 ◦ C. The greater effectiveness of the synthetic antioxidant BHA (a nonpolar phenol derivative) than LE (a mixture of more polar phenolics due to polyols), especially during the later stage of storage of cooked meat patties (>7 d at 2 ◦ C; >1 mo at –20 ◦ C), suggested that bulk-phase neutral lipids or triacylglycerols were primary precursors of TBARS when considering the partitioning of polar and less polar antioxidants in the comminuted meat system. Namely, most oxygen radicals were probably formed in the lipid phase. However, the high efficacy of LE at the 0.1% dosage level for samples stored for a relatively short period suggested that radicals formed in the aqueous phase of cooked pork patties were also important catalysts of lipid oxidation. This hypothesis was supported by the remarkably high scavenging power of LE against • OH (a water-soluble radial) when compared to BHA. The liquefied fat from cooking was an excellent carrier and distributor of BHA, while both the juice and melted fat allowed the incorporation of phenolics. The latter has also been used to explain the remarkable antioxidant capacity of many plant polyphenol compounds applied to O/W emulsion systems (Maqsood and Banjakul 2010). The antioxidant activity of LE appears to be the result of synergistic effects of numerous active compounds. Gordon and An (1995) reported that individually, polyphenols in LE were less effective than α-tocopherol (a similar compound to BHA in functional structure) but were more effective than α-tocopherol when acting collectively. Although the results showed an apparent correlation between the inhibition of lipid oxidation (TBARS) and the suppression of rancidity in the presence of antioxidants, such a relationship was nonlinear. For example, of the 3 antioxidants tested, BHA was most effective in inhibiting lipid oxidation; however, it did not lower the rancidity score from the control samples as much as the 0.1% LE treatment. This suggested that in addition to lipid oxidation products there were other compounds that contributed to this negative sensory note. We had previously shown that treatments of crude myofibrillar proteins with propyl gallate (a BHA analog) and ascorbic acid substantially inhibited lipid oxidation but did not prevent protein oxidation, which was measured as carbonyl formation and loss of sulfhydryls (Srinivasan and others 1996; Parkington and others 2000). By binding to proteins through thiol-quinone (Jongberg and others 2011) or amine-quinone (Rohn and others 2004) adductions, phenolics present in LE might inhibit the oxidative degradation of proteins into rancidity-contributing volatiles. Vuorela and others (2005) reported that the incorporation of rapeseed and pine bark phenolics into cooked pork patties inhibited not only lipid oxidation (up to 80%) but also protein carbonyl formation (up to 64%).
Conclusions The extract of licorice for the 1st time proved to be an effective antioxidant in precooked pork patties capable of inhibiting lipid oxidation during refrigerated and frozen storage independent of salt content. Such antioxidant activity is attributed to the superior radical scavenging capacity of the extract, particularly against hydroxyl radical (• OH). As a result of suppression of oxidation of lipids and possibly also proteins, rancidity in stored meat samC8 Journal of Food Science r Vol. 00, Nr. 0, 2013
ples was less detectable. Overall, LE was more effective than RE and was almost comparable to BHA in efficacy when used at appropriate levels. Therefore, LE can be applied as a novel natural antioxidant additive for the protection of flavor and shelf-life of meat products.
Acknowledgments The study was supported, in part, by ICL Performance Products, LP. Paper nr 13-07-097 of the Kentucky Agricultural Experiment Station.
Disclaimer Dr. E. Allen Foegeding served as Scientific Editor and Dr. Gale Strasburg served as Associate Editor overseeing single-blinded review of this manuscript. It is the policy of JFS to blind Editorial Board members from the peer-review process of their own submissions, just as all authors are blinded.
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