Changes in Lipid Oxidation During Cooking of Refrigerated Minced

Chapter 20

Changes in Lipid Oxidation During Cooking of Refrigerated Minced Channel Catfish Muscle

Downloaded by TUFTS UNIV on March 24, 2018 | https://pubs.acs.org Publication Date: August 5, 1992 | doi: 10.1021/bk-1992-0500.ch020

M . C . Erickson Department of Food Science and Technology, Food Safety and Quality Enhancement Laboratory, University of Georgia, Georgia Agricultural Experiment Station, Griffin, GA 30223

Refrigerated storage of minced channel catfish affected oxidative changes induced by cooking. Following refrigeration for 0, 2, 5 or 7 days, samples were analyzed before and after baking for thiobarbituric acid-reactive substances (TBA-RS), fluorescent pigment and tocopherol content. Larger increases in TBA-RS were found after cooking 2-day refrigerated samples than fresh samples, whereas smaller increases were found in 5 and 7-day samples. In contrast, the largest increases in fluorescent pigment content after cooking were found in the 5-day refrigerated samples. Loss in gamma-tocopherol upon cooking remained fairly constant (15%) whereas losses of alpha-tocopherol were greater in the 2- and 5- day refrigerated samples (40%) than in the 7-day refrigerated sample (14%). Oxidative decomposition of meat lipids during cooking is a recognized event (1-11) and is responsible for many desirable flavor characteristics. Unfortunately, due to the autocatalytic nature of lipid oxidation, cooking also accentuates undesirable flavors in products which have experienced lipid oxidation during storage in the raw state. While storage of raw product prior to cooking alters the oxidative response during cooking, both promotion and inhibition of lipid oxidation during cooking has been noted (1-3). To further explore the lipid oxidative response to cooking, lipid oxidation was monitored by a variety of methods in cooked, minced channel catfish which had previously been refrigerated for various periods of time. Materials and Methods Samples. Live channel catfish (lctalurus punctatus) obtained locally were stunned, deheaded, gutted, skinned and filleted. The fillets from each fish were pooled and minced to obtain a homogeneous sample. The minced samples were distributed into cups or tubes and heated for five minutes at 177°C. TBA-RS. Minced channel catfish muscle tissue (10.0 gm) was placed in a shallow cup and spread in a thin layer. Both cooked and uncooked meat were scraped from the cup and analyzed for thiobarbituric acid reactive substances (TBA-RS) by the

0097-6156/92/0500-0344$06.00/0 © 1992 American Chemical Society St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

20. ERICKSON

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Refrigerated Minced Channel Catfish Muscle

distillation procedure described by Rhee (72). Results were expressed in terms of mg malonaldehyde (MA)/g tissue. L i p i d Extraction. Minced channel catfish muscle tissue (1.0 gm) was placed in 16 x 125 mm screw-cap test tubes and spread in a thin layer. Lipids from both cooked and uncooked samples were extracted by chloroform:methanol (2:1) as described by Erickson (75).


Fluorescent Pigments. Fluorescent pigments were determined on 10.0 ml of a lipid extract (25 ml total) which had been washed with 2.50 ml of 0.88% KC1. Diluted samples of both the aqueous and organic layers were measured in a Turner fluorometer, Model 112, using a quinine sulfate standard (1 x 10"° M ) set equal to 100 fluorescence units (FU). Fatty Acid Esterification. Total lipid extracts were esterified in the presence of an internal standard, heptadecanoic acid, using 4% H2SO4 in methanol as described by Erickson and Selivonchick (14). In addition, both cooked and uncooked tissue samples (1.0 gm), distributed into 16 x 125 mm screw-cap test tubes, were freezedried, followed by direct esterification using 4% H2SO4 in methanol. A HewlettPackard 5790A Series gas chromatograph equipped with a glass capillary column (J & W DB-225; 30 m x 0.25 mm, 0.15 fim film) was used for separation of the esterified fatty acids. Tocopherol. Under saponification conditions described by Erickson (75), extracted tocopherols were analyzed using the reverse-phase high performance liquid chromatographic conditions designated by Vatassery and Smith (75). The chromatographic system consisted of a Micromeritics 752 Gradient Programmer, Micromeritics 750 Solvent Delivery System, and a Brinkmann 656 Electrochemical detector. Statistical Analyses. Analysis of variance and Fisher's least significant difference test were used to analyze the data for statistical differences. Results and Discussion Response to Cooking of Fresh Product. Level of TBA-RS increased by over 500% after cooking the fresh channel catfish samples (Table I). Previously, the Table I. Oxidative Response to Cooking of Fresh Minced Channel Catfish

a

TBA-RS (mg MA/g)

Extraction/ Esterification (% P U F A )

Direct Esterification (% P U F A )

Raw Fish

0.54 + 0.32

22.0 + 0.3

22.2 + 0.3

Cooked Fish

2.91 ± 0 . 8 4

21.9 + 0.2

22.1+0.2

Reported values are expressed as mean +. standard deviation based on five observations.

St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


346

LIPID OXIDATION IN FOOD

T B A number increased 406% for baked minced carp (4), 190% for cooked chicken (J) and 405% for cooked beef (6). Fatty acid composition of the tissue, TBA analytical methodology, adjustments for drip loss, as well as temperature and time of heating must all be considered to account for these variations exhibited by different products in response to cooking. Other measurements of oxidation which have similarly been noted to increase with cooking are the fluorescent pigments (5,7), total carbonyls (8) and oxysterols (9). The increase in lipid oxidation that occurs during cooking may be attributed to increases in nonheme iron content (16), disruption of muscle membranes, as well as to the increased catalytic activity encountered at higher temperatures (17). In the present study, the polyunsaturated fatty acids (PUFA) were also monitored before and after cooking to detect i f changes occurred in the levels of the oxidative substrate. Two procedures for esterification of catfish samples were employed to determine i f differences exist in their sensitivities for detecting losses of P U F A in cooked tissues. In one procedure, the lipids were extracted from tissue by chloroform:methanol (2:1) prior to esterification. In the second procedure, freezedried tissue was esterified directly to ensure that all unoxidized fatty acids were released. While direct esterification recovered 52.1 ± 3 . 1 and 50.0 +. 2.6 mg total F A M E / g in raw and cooked tissue, extraction followed by esterification recovered 48.2 ± 2.9 mg total F A M E / g in raw tissue but only 32.9 ± 2.2 mg total F A M E / g in cooked tissue. When P U F A content was expressed as a percentage of the total fatty acids, however, no preferential losses of P U F A could be detected with either procedure (Table I). The inability to detect losses of P U F A due to cooking, though, is not uncommon in muscle foods with large triglyceride contents (4,18). Similar to the situation in chicken breast (10), preferential oxidation and loss of phospholipid P U F A may have occurred in catfish, however, the large triglyceride content (43 mg triglyceride/g tissue) would have obscured these losses. Response to Cooking of Stored Product. Due to the propagative nature of lipid oxidation, the degree of oxidation incurred during cooking should theoretically be dependent on the initial oxidative rate in the raw product. Under such an assumption, an increase in the initial levels of oxidation during refrigerated or frozen storage would lead to much larger quantities of oxidative products being generated during cooking. Support for this line of reasoning was reported in studies examining the sequential treatments of frozen storage and cooking of chicken breast and leg meat (3). The results reported by Igene et al. (2), however, contradict this expectation that frozen raw meat is more susceptible to oxidation during cooking than fresh unfrozen meat. Similarly, when minced channel catfish was stored under refrigeration for varying periods of time prior to cooking, cooking did not lead to significant increases in TBA-RS for those samples stored more than 3 days, whereas the positive response to cooking of samples stored 3 days or less was statistically significant (Table II). Since a similar trend in T B A numbers has been observed for broiler tissues when stored under refrigeration for different times prior to cooking (1), the results suggest that additional factors other than the initial oxidation levels must be accounted for when estimating the potential effects of cooking. To aid in identifying the mechanism for this modification in the response to cooking, another batch of minced catfish muscle tissue was stored for 2, 5 and 7 days of refrigeration. For these samples, fluorescent pigments were also analyzed in addition to TBA-RS in the raw and cooked product (Table III). The levels of TBA-RS identified in these raw and cooked samples were greater than those found previously (Table II) which may be attributed to different lipid and/or antioxidant compositions in the catfish. Similar to the data presented in Table II, though, the data in Table III shows that cooking of 5- and 7-day samples resulted in much smaller increases in TBA-RS than cooking of 2-day fish. Increases in fluorescent

St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

20. ERICKSON

Refrigerated Minced Channel Catfish Muscle

347

Table II. Response to Cooking of Refrigerated Minced Channel Catfish as Measured by T B A - R S


a

mg M A / g tissue Cooked Fish

Days Stored

Raw Fish

1 2 3 4 5 6

0.14 + 0.06 0.40 + 0 . 1 1 0.24 + 0.06 0.46±0.16 0.41 + 0.10 0.27 + 0 . 0 6

a

a

b

b c

b

c d

c

a b

0.55 ± 0.89 ± 0.98 ± 0.60 ± 0.48 ± 0.36 +

c

0.06 0.15 0.18 0.18 0.09 0.07

1 1

Change d

e

e

c

d

d

b c

0.41 0.49 0.74 0.14 0.07 0.09

Reported values are expressed as mean +. standard deviation based on four observations. Raw and cooked values followed by the same superscript letter are not significantly different (P

Changes in Lipid Oxidation During Cooking of Refrigerated Minced

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