Research and Reviews: Meat

Special Circular 172-99

Dietary Intake of Vitamin E Affects the Peroxide Value of Subcutaneous Lamb Fat

C. T. Li, M. Wick¹, N.G. Marriott², and K. E. McClure
The Ohio State University Department of Animal Sciences

Abstract

Lipid oxidation is a major problem causing flavor deterioration in meat products. The objectives of this research were to analyze the effects of dietary vitamin E on the lipid oxidation of subcutaneous lamb fat employing a modified iodometric peroxide value (mPV). Lambs were fed ad libitum, an all-concentrate diet, formulated to provide 16% crude protein with 15 International Units (IU) (National Research Council recommended level; Control), 300 IU for seven days, or 300 IU for 21 days of supplemental vitamin E (per kg of diet dry matter). The mPV demonstrated significant differences in the lipid oxidation state of animals fed control, 300 IU (seven days) and 300 IU (21 days) of vitamin E (P < 0.05). In addition, mPV demonstrated significant differences (P < 0.05) in the rate of change in the lipid oxidation state during storage. mPVs demonstrated dramatic increases of the lipid oxidation state of subcutaneous lamb fat on the 11th day in all three treatments. Furthermore, the lambs fed control vitamin E had significantly higher initial PV on day one than those fed 300 IU vitamin E. These results indicate that dietary intake of vitamin E significantly affects the initial lipid oxidation state and the rate of the lipid oxidation of subcutaneous lamb fat and therefore extends the shelf-life of lamb fat.

Introduction

The evaluation of the lipid oxidation state is essential to the palatability of lipid foods. Lipid oxidation is a major problem causing flavor deterioration in meat and its by-products, which adversely affects the shelf life of animal fat. Oxidative rancidity in animal tissue starts to develop almost immediately after meat animal slaughter and carcass fabrication (Gray and Pearson, 1994). Unsaturated fatty acids become oxidized and produce undesirable organoleptic characteristics. The initial step in lipid oxidation is the generation of highly transient hydroperoxides that further degrade into malonaldehyde (MA) and other secondary compounds. Formation of hydroperoxides from unsaturated fats by oxidation has been recognized as the most important pathway for generating precursors of undesirable odors and flavors (Frankel, 1980).

Many antemortem factors potentially contribute to the lipid oxidation state of animal fat, however diet can be one of the most effective ways of inhibiting lipid oxidation. Previous research indicated that supplementing dietary vitamin E, a fat soluble antioxidant, reduces lipid oxidation and increased the shelf-life of lamb meat (Wulf et al., 1995). Current methods of lipid oxidation analyses may contribute to further lipid oxidation and generate by-products that contribute to nonspecific reactions. In addition, these analyses are generally time-consuming and involve harsh conditions, such as steam distillation or oven-heating. Such harsh conditions are postulated to contribute to further lipid oxidation (Decker et al., 1998; Shahidi, 1994). Recent research has been conducted establishing a modified iodometric peroxide value (mPV) as a practical method to monitor the lipid oxidation in lamb fat (Li et al. 1999, unpublished data). The objective of this research was to employ mPV to investigate the effects of dietary vitamin E on the lipid oxidation of subcutaneous lamb fat.

Material and Methods

Sample Preparation

The experiment was developed as a 3 (15 IU; 300 IU, 7 days; 300 IU, 21 days) x 4 (Days 1, 7, 9 and 11) factorial design (Figure 1). Twenty-four lambs were obtained from The Ohio State University’s Ohio Agricultural Research Development Center (OARDC) campus in Wooster, Ohio. They were randomly divided into three groups (eight lambs per group) and fed an all-concentrate diet, offered ad libitum, formulated to provide 16% crude protein with one of the following three treatments: 15 IU (National Research Council-recommended; Control), 300 IU (20 X NRC, 7 days) or 300 IU (20 X NRC, 21 days) of supplemental vitamin E per kg of diet dry matter. Carcass fabrication was conducted seven days post-slaughter. Subcutaneous fat from the loin area was removed, wrapped in a Styrofoam meat tray with oxygen-permeable film, and stored at 39 ± 3.6°F (4 ± 2°C) from 1 to 11 days post-slaughter. The lamb fat samples were subjected to the modified iodometric peroxide value on days 1, 7, 9, and 11 post-slaughter.

Modified Peroxide Value (mPV)

The PV method of analysis was modified from the AOAC (1990) procedure. Briefly, a 50 g sample of unrendered subcutaneous loin fat each was ground in a Waring lab blender (Waring Products Division, Dynamic Cooperation of America, New Hartford, Conn.) for 20—30 s and extracted with 30 mL ice cold (3:2 v/v) acetic acid:chloroform. The extraction was vigorously swirled to distribute the sample and reagents. After the samples were dissolved in the acetic acid:chloroform mixture, 0.5 mL of saturated potassium iodine (KI) (83.2 g solid KI /40 mL H₂O) was added and mixed vigorously for 10 s. Subsequently, 30 mL deionized water was added, and the solution mixed thoroughly. Color of the upper aqueous layer ranged from pale yellow to bright yellow, with the lower organic layer remaining white. The mixture was allowed to stand for 5 to 10 min. at room temperature then titrated with 0.01 M Na₂S₂O₃ (Sigma Chemical, Fair Lawn, N.J.) gradually with vigorous shaking. During the titration 0.5 mL of starch indicator (Starch 1% with chloroform 0.3%, Lab Chem., Inc., Pittsburgh, Pa.) was added. Color of the upper aqueous layer ranged from light purple to dark purple, and the lower organic layer remained white to gray. If the color of the lower organic layer remained yellow, the sample was vigorously swirled and allowed to stand for an additional 10 min. The end-point of titration was established when the color of the upper aqueous layer disappeared. The mPV was calculated employing the following formula:

mPV	=	(S)(N)(1000)/W
Where:
mPV	=	Modified Peroxide Value (meq oxygen/kg fat)
S	=	mL Na2S2O3
N	=	Normality of Na2S2O3 (0.01 N)
W	=	g of fat

Rate of Lipid Oxidation

The rate of lipid oxidation (m) reflects the onset of lipid oxidation on lamb fat. The rate of lipid oxidation was be calculated by following formula:

m =	PV2-PV1
	d2-d1

Where m is the rate of lipid oxidation, meq. of PV/kg/d

PV₂-PV₁ is the difference of peroxide value between two observations, meq./kg
d₂-d₁ is the time period, days

Statistical Analysis

Statistical analysis was completed by statistical software SAS 7.0 for Windows® (SAS Institute, Inc., Cary, N.C.). The significance of the differences was determined by a two-way General Linear Model (GLM).

Results and Discussion

Table 1 demonstrates that there is a significant increase of the lipid oxidation state in the subcutaneous fat obtained from lambs fed all three supplemental vitamin E treatments through day 11 (P < 0.05). In addition, the group fed 15 IU vitamin E had much higher initial peroxide value (1.83 min E (1.03 and 0.64 meq./kg), which suggests that meq./kg) on day 1 than those fed 300 IU vita high levels of dietary vitamin E supplement retarded the initiation of lipid oxidation more than the control level of vitamin E treatment. Table 1 also indicates that lipid oxidation significantly increased between day 9 and 11 in the fat from lambs fed all three vitamin E treatments (P < 0.05), which suggests high dietary vitamin E treatment reduces the rate of lipid oxidation more than low dietary vitamin E intake. The detailed information is summarized as follows.

Dietary Vitamin E

Peroxide values demonstrated that there is a significant difference in the lipid oxidation state in the fat obtained from animals fed three different levels of vitamin E (P < 0.05) (Figure 2). However, the differences in the mPV between the two 300 IU treatments were not as great as the differences between the control and 300 IU groups. In addition, animals fed 300 IU of vitamin E apparently had lower mPV than those fed 15 IU. These results suggest that a higher level of dietary vitamin E yielded lower mPVs and less oxidation of subcutaneous lamb fat.

Figure 2. Effects of three different levels of dietary vitamin E on overall peroxide values of subcutaneous lamb fat. IU: International Unit. Means followed by different letters are significantly different (P < 0.05). Total observation, n = 192.

Shelf-Life

As the data in Table 1 reflects, there is an increase in the mPV during storage in the fat obtained from lambs fed all three treatments (P < 0.05). Figure 3 demonstrates that the control group had significantly higher mPVs than the groups fed 300 IU vitamin E treatments assayed at each storage period. In previous research, Shahidi (1994) indicated that a longer period, required to reach a certain PV, increased effectiveness in inhibiting oxidation. These data suggest that either feeding higher levels of supplemental vitamin E or increasing the supplemental feeding time increases the shelf-life of animal fat through the inhibition of lipid oxidation.

Figure 3. Effects of different levels of dietary vitamin E during storage time on peroxide values of subcutaneous lamb fat. IU: International Unit. Means followed by different letters are significantly different (P < 0.05). Total observation, n=192.

Rate of Lipid Oxidation

Figure 4 indicates that mPV significantly increases between day 9 and 11 in the fat obtained from lambs fed on all three vitamin treatments (P < 0.05). These data suggest that high dietary vitamin E treatment (300 IU) reduces the rate of lipid oxidation to a greater extent than control vitamin E treatment (15 IU). The equations used to determine the rate of lipid oxidation in the fat from lambs fed the three different dietary vitamin E treatments are shown as follows:

Y₁₅ = 0.50 X₁₅ + 2.39; where Y₁₅ = Peroxide value of 15 IU group and X₁₅ = day.

(m₁₅ = 0.50; Intercept = 2.39)

Y_{300, 7} = 0.42 X_{300, 7} + 1.11; where Y_{300, 7} = Peroxide value of 300 IU, 7 days group and X_{300, 7} = day.(m_{300, 7} = 0.42; Intercept = 1.11)

Y_{300, 7} = 0.39 X_{300, 21} + 1.00; where Y_{300, 21} = Peroxide value of 300 IU, 21 days group and X_{300, 21} = day.(m_{300, 21} = 0.39; Intercept = 1.00)

Figure 4. Effects of dietary vitamin E on rate of lipid oxidation of subcutaneous lamb fat. IU: International Unit.

The rate of lipid oxidation is m_{300, 21} ³ m_{300, 7} > m₁₅, which suggests the fat in lambs fed a higher level and longer period of dietary vitamin E has a greater resistance to lipid oxidation than the fat in the control group. However, the difference between 7 days of 300 IU vitamin E treatment and 21 days of 300 IU vitamin E treatment is not great, which suggests 7 days of 300 IU vitamin E may be sufficient to retard the initiation of lipid oxidation.

Implications

Peroxides are the primary products of lipid autoxidation. Results indicate that vitamin E is effective in inhibiting lipid oxidation. Data also suggest that dietary intake of vitamin E significantly affects the initial lipid oxidation state and the rate of lipid oxidation in subcutaneous lamb fat. Despite the significance between the mPV’s at 7 days and 21 days of 300 IU, the difference is not as great as those between 15 IU and 300 IU. This data indicates that the full antioxidant effect of vitamin E can be obtained in as little as 7 days of supplemental feeding which could have a significant economic contribution for the meat animal industry in Ohio. Further evaluation is necessary to determine the optimal time and levels of feeding supplemental antioxidants on increasing the shelf life and palatability of the fat from meat animals.

Acknowledgements

Research supported by funds awarded to

M. Wick from OARDC #617216-A265.

References

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¹ For more information, contact at: The Ohio State University 230A Plumb Hall, 2027 Coffey Road, Columbus, OH 43210, (614) 292-7516, Fax (614) 292-7116; email:wick.13@osu.edu
² Department of Food Science and Technology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061