酵母(Saccharomyces cerevisiae)及酵母提取物对肉鸡肉质的影响

ZHANG Aiwu JU Guichun XUE Jun ZUO Luya DONG Bin

(1.Chinese Medicine Materials College of Jilin Agricultural University,Changchun 130118,China;2.China National Group of Traditional＆Herbal Medicine,Beijing 102600,China)

Saccharomyces cerevisiae(SC),one of the most commercialized yeast types,has been fed to animals for a long time[1].Previous researches had evaluated the effects of yeast products on performance and ileal morphology of poultry[2-4],yet a few researches had been focused on the effects of yeast products on broiler meat quality.

Meat quality characteristics,such as tenderness,juiciness,drip-loss,cooking loss,pH,and shelflife,are important to the consumers while purchasing the product as well as to the processors while processing the value-added meat products[5-7].Some trials show that the tenderness[8]and water holding capacity[9]of the edible meat from broiler chickens is increased when fed a chromium-enriched SC diet.

In addition,yeast products can improve the anti-oxidative status of living animals and increase the oxidative stability of meat,which are considered to bebeneficial to the consumers and the processing industry[10].The effect of yeast products on oxidative stability of chicken meat has not been extensively studied,albeit the yeast culture supplementation in diet could increase the anti-oxidative capacity of Cashmere goats[11].

Therefore,the present study was conducted to evaluate the effects of SC and yeast extracts(YE)on meat quality of broiler chickens.

1 MATERIALS AND METHODS

1.1 Birds,diets,and experimental design

One hundred and eighty one-day-old male broiler chickens(Ross 308 strain)were randomly assigned to 18 wire-floored,suspended cages in a temperaturecontrolled room.Continuous lighting was provided throughout the experimental period.Room temperature was decreased from 32℃on d 0 to 25℃on d 21 and was kept constant thereafter,and d 0 was the day when eggs were hatched out.There were 3 dietary treatments with 6 replicates each,and each replicate contained 10 birds.The 3 treatments consisted of a control diet,a 0.5%SC diet and a 0.3%YE diet to make sure that the YE content was the same between the two diets,and the yeast was cell wall-removed.Starter and finisher diets were formulated(Table 1)and used as a control diet(without the addition of SC or YE).SC and YE were added to the control diet at the expense of soybean meal,respectively.The colony-forming units(cfu)of SC were counted to be 1.3×1010cfu/g.Feed and water was provided ad libitum throughout the 35-day feeding trial.

Table 1 Composition and nutrient levels of the diets(DM basis) %

1.2 Collection of meat and skin samples

On the last day of the 35-day feeding trial,1 bird from each cage was selected randomly and euthanized by cervical dislocation.Immediately after slaughter,the entirebreast and drumstick meat(with skin)were sampled.One half of the raw breast and drumstick meat were sampled and stored at 4℃before the pHu,moisture content,cooking loss and shear force was determined.The rest of the raw meat and skin samples were stored at-20℃for the lipid oxidation assay.

1.3 Determination of p Hu

Approximately 10 g of the ground breast and drumstick meat was homogenized in 50 mL distilled water 24 h after slaughter[12].The pHu of the homogenate was measured using a portable pH meter(HI8424,Beijing Hanna Instruments Science＆Technology Co.Ltd.,Beijing,China).

1.4 Determination of moisture content

In order to determine the moisture content,5 g of breast and drumstick meat sample was dried at 105℃for 24 h[13].The weight of samples before and after dried was recorded.

The moisture content(%)=[(weight before driedweight after dried)/weight before dried]×100.

1.5 Determination of cooking loss

The cooking loss was determined according to the procedure of Sen et al.[14].Each breast and drumstick meat sample was sealed in a vacuum plastic bag and boiled in a water bath without pressure for 20 min until an internal end point temperature of 70℃was reached(which was measured using a digital probe thermometer).Each piece of the meat was cooled,removed from the bag and weighed.The weight of samples before and after cooking was recorded.

The cooking loss(%)=[(weight before cooking-weight after cooking)/weight before cooking]×100.

1.6 Determination of shear force

To determine the shear force of breast and drumstick meat,boiled meat samples were used for cooking loss assays and the raw meat samples were cut into squares(35 mm×25 mm×6 mm)[15].An application of cutting force to meat samples was performed using a TA-XT2 texture analyzer equipped with a TA-7 Warner-Bratzler blade(Stable Micro Systems Ltd.Surrey,England,UK).

1.7 Determination of 2-thiobarbituric(TBARS)value acid-reactive substances

The raw breast and drumstick meat samples that had been stored at-20℃were thawed at 4℃and homogenized.Four subsamples from each breast and drumstick sample(i.e.72 breast meat subsamples and 72 drumstick meat subsamples),weighing approximately 2.5 g,were placed into 50 mL screwcapped centrifuge tubes and then incubated at 30℃for 0,1,3,6,10,15 and 21 d.After incubation,each subsample was immediately subjected to a malondialdehyde acid(MDA)assay[16]to determine the extent of lipid oxidation.The TBARSvalues were expressed as mg MDA per kg sample.The determination of oxidative stability in skin samples was the same as breast and drumstick samples except for the homogenization.Intact skin samples were incubated from 0 to 21 d.After incubation,skin samples were immediately homogenized with 6 mL 20%trichloroacetic acid and processed as described above to determine the TBARSvalues.

1.8 Statistical analysis

All data were evaluated by One-way ANOVA using the GLM procedure of SAS 7.0.Significant differences of mean TBARS values among dietary treatments were analyzed by repeated measures and Tukey's test in GLM procedure.The level of statistical significance was preset at P＜0.05.

2 RESULTS

2.1 pHu

The p Hu of breast and drumstick meat of each treatment were shown in Table 2.There was no significant difference in the pHu of breast meat from broilers fed different sources of yeast(P＞0.05).But the pHu of drumstick meat from YE-fed broilers were significantly higher than those of the control(P＜0.05).

Table 2 Effects of SC and YE on p Hu of male broiler chickens %

2.2 Moisture content

Effects of SCand YE on moisture content of breast and drumstick meat were summarized in Table 3.Dietary SC and YE had no significant effect on moisture content of breast and drumstick meat(P＞0.05).Although there was no significant difference among the three treatments,an increasing trend can be observed in SC and YE treatments.

Table 3 Effects of SC and YE on moisture content of male broiler chickens %

2.3 Cooking loss

There was no significant difference in cooking loss between breast and drumstick meat(P＞0.05)from the three dietary treatments(Table 4).Although there was no significant difference in cooking loss of these three treatments(P＞0.05),a decreasing trend can be observed in SC and YE treatments.

Table 4 Effects of SC and YE on cooking loss of male broiler chickens %

2.4 Shear force

Effects of SC and YE on shear force in the raw and boiled breast and drumstick meat were shown in Table 5.There was no significant difference in shear force of raw breast meat among all treatments(P＞0.05).However,in raw drumstick meat samples,the shear force of SC was significantly lower than that of the control(P＜0.05),and YEwas intermediate.For boiled meat,the shear force of breast and drumstick meat of SC-fed and YE-fed birds significantly decreased compared with the control(P＜0.05).

Table 5 Effects of SC and YE on shear force of male broiler chickens g

2.5 TBARSvalues

There were no significant differences in TBARS values of breast meat(P＞0.05)among the three treatments up to d 6 of the incubation(Fig.1).On d 10,15 and 21 of the incubation,the TBARS values of SC and YE were significantly lower(P＜0.05)than that of the control.On d 21 of incubation,TBARS values were not significantly different between SC and YE(P＞0.05).Up to d 3,no significant difference was found in TBARS values of drum-stick meats(P＞0.05)among the three treatments(Fig.2).On d 6,10,15 and 21,TBARS values were lower(P＜0.05)in SC and YE than those in the control.On d 21 of incubation,TBARS values were not significantly different between SC and YE(P＞0.05).There were no significant differences for the skin TBARS values(P＞0.05)up to d 6(Fig.3).On d 6 and 10,TBARS values were significantly lower(P＜0.05)in YEthan those in the control.On d 15 and 21,TBARSvalues were significantly lower(P＜0.05)in both SC and YE than the control.

3 DISCUSSION

The pH is a direct reflection of muscle acid content and can affect the moisture content,cooking loss and shear force in meat.The pHu of drumstick meat from YE-fed broilers in this study were significant higher than those of the control.This might be related to the difference in muscle glycogen concentration[17].The pH decline depends on the muscle glycogen concentration.Daly et al.[18]also showed that a higher initial muscle glycogen concentration resulted in a faster rate of pH decline.On the other hand,Campo et al.[17]reported that this might be related to the difference in stress susceptibility in pre-slaughter handling,and high p Hu in the meat of YE-fed broilers suggested that YE-fed broilers were generally highly prone to the stress.

Fig.3 Effects of SC and YE on TBARSvalues in the skin of male broiler chickens

A low p H is associated with poor water-holding capacity(WHC),which is also associated with lower moisture content.In this study,an increasing trend of moisture content could be observed in SC and YE treatment.The results were in accordance with those obtained by Qiao et al.[19]who reported that higher pH was generally associated with higher meat moisture content.

The term,WHC,was originally used to describe the ability of muscle and muscle products to bind water[20],while cooking loss was measured to obtain an overall assessment of the water binding properties of meat.Samples with the lowest amount of cooking loss exhibited the highest amount of WHC.From a practical standpoint,cooking loss represents the most important parameter to the industry,which is looking for the raw materials with high moisture retention capacity[21].In this study,a decreasing trend of cooking loss could be observed in SC and YE treatment.It suggested that the meat of YE-fed and SC-fed broilers was prone to increasing its WHC.Madruga et al.[22]reported that differences in cooking loss were often linked to the differences in time and temperature of cooking,p Hu and different fat contents.The pHu is known to influence the structure of myofibrils and consequently the WHC and cooking loss[23].These correlations were in agreement with previous findings[21,24-25]which were reported that the poultry meat with low pH was associated with low WHC,which resulted in increased cooking loss.The data from Table 2 and Table 4 were consistent with previous studies[26-27]that higher pHu were associated with higher WHC,i.e.,lower cooking loss.

Tenderness is the sum of the mechanical strength of skeletal muscle tissue after rigor mortis during the postmortem storage[28].It is the most important textural characteristic of meat and has the greatest influence on consumer preference.Meat tenderness can be estimated by measuring the shear force,and lower shear force indicates tenderer meat.Our study suggested that the dietary SC and YE can improve the tenderness of broiler meat,although the underlying mechanism was not readily understood.There are some effective factors on the tenderness of meat.Du et al.[29]reported that it could be due to the changes in fatty acid compositions of muscle lipids.The more the saturated fatty acids content,the higher the melting point of fat,and the harder the meat.Other reason could be due to the protein content in the meat,the higher the protein content,the harder the meat after cooking[30].It suggested that SC-fed and YE-fed broilers prone to changing the compositions of muscle which need to be further studied.In addition,another reason could be due to the changes in p Hu.Silva et al.[31]reported that the tenderness depends on pH.The present results(Table 2 and Table 5)were in accordance with the results reported by Froning et al.[24]and Silva et al.[31]that the meat with higher pHu had higher tenderness.

The results(Fig.1,Fig.2,and Fig.3)provided evidence that the SC and YE could improvethe oxidative stability of broiler meat.Zhang et al.[32]also reported that chromium yeast could enhance the capacity of inhibiting the hydroxy radical(◦OH)and resisting the superoxide anion radical(O2-◦)in the serum and liver,reduce the MDA content,increase the activities of GSH-Px and T-SOD in theserum and liver of heat stressed broilers.In the present study,Fig.1,Fig.2,and Fig.3 showed that the oxidation rates were the same among all treatments at the early stage of incubation.However,the oxidation rates were reduced in SC and YE compared to the control at the later stage of incubation,indicating that the meat from chicks fed SC and YE might contain less oxidative fat(or fatty acids).At present,a clear explanation for the anti-oxidative effect of SC and YE is not available,but it may due to the different oxidative fat or fatty acids contents between SC-fed/YE-fed and the control broilers.Some researchers had reported that lipid deposition in broiler chicks[33-35]fed yeast diets is significantly lower than in chicks fed a cornsoybean meal diet.This would provethe above point.

4 CONCLUSIONS

①SC and YE can improve the meat quality.

②SC and YE can increase the meat tenderness.

③SC and YE can increase the anti-oxidative stability of meat and skin of broilers,especially the YE.

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