Summary and Implications

One hundred and twenty four-week-old broilers were randomly assigned to one of the three dietary treatments including control (none), oxidised oil (five per cent of diet) and antioxidants (500IU vitamin E and 200ppm BHT) and fed for two weeks. Blood samples were collected one day before slaughter and breast muscles were sampled immediately after slaughter. Degree of lipids and protein oxidation in blood and breast muscle and meat quality parameters were determined.

Compared to control group, broilers fed diet with oxidised oil significantly increased lipid oxidation in both blood (P<0.05). Dietary oxidised oil tended to increase carbonyl content in blood and muscle (P<0.05). Addition of antioxidants significantly decreased lipid oxidation in both blood and muscle samples and arrested protein oxidation in muscle (P<0.05). Meats from oxidised oil treatment showed higher drip loss at days 1 and 3 and lower water-holding capacity at day 1 than control group (P<0.05). No significant difference was found about drip loss and water-holding capacity between control and antioxidant treatments. The rate of pH decline in breast meat from oxidised oil treatment was significantly higher than that of control between 0 and one hour after slaughter (P<0.05).

However, dietary treatments did not show significant effects on body weight gain, feed consumption and feed efficiency of live birds, and cooking loss and colour of breast meat.

This suggested that degree of oxidation in diet increased the oxidation in blood and muscle, and the oxidative stress in live birds were related to the variations in quality parameters including pH decline, drip loss and protein and lipid oxidation of broiler breast meat.

Introduction

Lipid oxidation has been known to cause quality problems by forming off-odour and off-flavour compounds and decreasing nutritive values in meat, according to Wangang Zhang and co-authors. Dietary addition of unsaturated fatty acids may be related to increased level of lipid oxidation. However, limited research has been reported about the effects of dietary addition of oxidised oil on protein oxidation and meat quality. Protein oxidation can cause fragmentation and conformational changes of protein secondary and tertiary structures to modify their functions. Oxidation induced intermolecular bonds including disulphide, dityrosine and other intermolecular bridges can lead to protein aggregation and polymerisation to change protein proteolytic properties.

These alterations can influence the physical and chemical properties of proteins including solubility, hydrophobicity, water-holding capacity, meat tenderness, gelation functions and even the nutritional value.

In the current study, the researchers hypothesised that the addition of oxidised oil to the diet may cause oxidation including lipid and protein and thus influence meat quality in chicken breast. The objective of this research was to determine the effects of dietary oxidation condition on oxidation stress in live chicken and breast meat quality.

Materials and Methods

Protein carbonyl content was determined by derivatisation with 2,4-dinitrophenylhydrazine (DNPH) method. Lipid oxidation was determined by fluorometric thiobarbituric acid reactive substance method. Drip loss was measured under atmospheric conditions at 4°C.

Results and Discussion

Dietary treatments did not show significant effects on weight gain of broiler chickens between four and six weeks (P>0.05). Feed consumption of birds between four and six weeks was not significantly different among three treatments (P>0.05). No significant difference was found for feed efficiency (weigh gain/feed intake) during the experiment period (P>0.05). No significant differences in growth performance and feed consumption of broiler chickens were detected between control and antioxidant supplemented group (Table 1).

Dietary supplementation with five per cent oxidised oil resulted in higher levels of lipid oxidation in blood plasma than control group (P<0.05). Addition of vitamin E and BHA in the diet showed significant effects in lowering the level of lipid oxidation compared to oxidised oil treatment (P<0.05) in blood (Table 2). These results suggested that feeding broilers with oxidised oil increased the oxidative stress in vivo. The TBARS value of breast muscle from animals fed with a diet added with oxidised oil was significantly higher than those from control and vitamin E group (P<0.05). The increased levels of lipid oxidation in breast samples from oxidised group may be due to the decreased accumulation of α-tocopherol, which could have been denatured by feeding oxidised oil. The blood of birds fed with oxidised oil tended to have higher levels of carbonyl content than control (P=0.08) and antioxidant (P=0.10) groups. Higher carbonyl content was detected in breast muscles from oxidised group than control and vitamin E group (P<0.05). However, no significant difference was found for protein oxidation between control and antioxidant group in both blood and breast samples (Table 2).

The breast meat of broiler chickens fed with a diet containing oxidised oil showed significantly higher drip loss than control group after one day of storage under atmospheric conditions at 4°C (P<0.01, Table 3). The drip loss of meat on day 1 from oxidised oil group was 63 per cent and 44 per cent higher than that of control and antioxidant-supplemented group, respectively. This tendency was also detected after three days of storage. The control and antioxidant-supplemented group had significantly lower drip loss than oxidised oil group (P<0.01). The water-holding capacity, measured by water loss during high speed centrifuge, of breast muscle from oxidised oil group was lower compared to control (P<0.05) on day 1. However, no significant differences in drip loss were found between control and antioxidant-supplemented diet group after one and three days storage (P>0.05). Cooking loss was also not significantly different among three dietary treatments (P>0.05; Table 3).

The colour L* (lightness), a* (yellowness) and b* (brownness) values of breast muscle from three diets did not differ significantly in current study (P>0.05; Table 4). This result was consistent with the pH of breast muscle, which showed no significant differences among the three diet groups at 0, 1.0, 2.5 and 5.0 hours postmortem.

The postmortem pH at 0, 2.5 and 5.0 hours were not significantly different among three dietary treatments (Table 5). However, the pH of breast muscle from birds fed with a diet containing oxidised oil tended to be lower than those from control and antioxidant group (P=0.10) at one hour postmortem. The rate of pH decline between 0 and one hour post-slaughter in breast muscle from birds fed oxidised diet was faster than that from other two groups (P<0.05) but the rates of pH decline at 0 to 2.5 hours and 0 to 5.0 hours were not significantly (P>0.05). The faster rate of pH decline early postmortem (0 to one hour) in the breast muscles from oxidised diet may partly explain higher drip loss and lower water holding capacity in that group. This is due to the fact that a fast rate of pH decline or low pH plus high body temperature in early postmortem stage can lead to the denaturation of muscle proteins. The denaturation of myofibrillar proteins can result in the loss of their functionality which further decreases their water-holding capacity. The higher drip loss or lower water holding capacity in broiler chickens from oxidised group also could be due to the higher levels of protein oxidation in that group. Protein oxidation could change the structure and biochemical function of proteins by fragmentation, aggregation and polymerisation.

Addition of oxidised oil in diet lowered the specific SERCA activity measured in the calcium level 0.01 and 0.02mM at pH7 (P<0.05). However, no significant difference in non- and specific SERCA activity between the control and antioxidant supplemented groups (P<0.05) was detected (Table 6). The lower SERCA activity might be caused by the increased oxidative stress, which resulted in SERCA oxidation, in the birds fed with a diet containing oxidised oil. In addition, lower deposition of antioxidant in breast muscle of oxidised group could have decreased its ability to maintain the antioxidant system leading to increased accumulation of reactive nitrogen and oxygen species.

March 2011