Introduction
In Syria chicken occupies a major place in production and consumption among poultry. It has become the second most popular meat eaten and is most likely to maintain this position. Chicken meat is a good source of protein and many nutrients, and is relatively low in fat, especially if the skin is removed. Chicken meat is also characterized by versatility in menu planning, ease of preparation, consistent quality, and the availability of wide range of pre-packaged, branded, raw and ready to eat and serve products.
In many countries microwave cooking and processing have increased over the years because of its convenience and time saving. Improvements in the design of high-powered microwave ovens offer rapid and economic methods for manufacturing food products of high organoleptic properties and nutritional value [13].
In the food industry microwaves are used for pasteurization of packaged products, for sterilization, tempering of frozen foods, precooking of poultry products and snack food also. Microwave use becomes of great importance because of its extensive utilization at home for cooking, thawing and re-heating. It is used also in research due for nutrient retention in processed food [11].
Alfaia et al. [3] found the heating time, temperature, cooking method and muscle composition to be the important variables, which may influence the final desirable characteristics of meat. Although meat changes induced by cooking have been studied for many years and extensively discussed (Tornberg,[14]), only few reports have specifically dealt with the influence of different cooking conditions on the amino acid and mineral contents (Wilkinson, et al., [17]). Moreover, the nutrient composition of cooked meat available in food composition databases is quiet limited.
Happich et al. [7] reported that the quantity of nonessential amino acids varies, and that this variation is largely accounted for by the difference in glycine, hydroxylsine and hydroxyproline. Total non-essential amino acids supply nitrogen for the synthesis in the body of any of the nonessential amino acids, which may be lacking. Besides, Barr et al. [5] and Hamm [6] reported that amounts of valine, leucine, isoleucine and histidine were great in breast meat, whereas, thigh meat had more glycine, hydroxyproline, hydroxylysine, threonine and serine.
Furthermore, results indicated that area of production and related management practices appeared to influence the concentration of about half of the amino acids. Moreover, meat from males contained more hydroxyporline than that from females.
Abd El-Wahed [2] reported that the fresh breast and leg meat contained the same individual amino acids with a slight variation in their amount. Besides, Ibrahim and Shams El-Din [8] reported that amino acids content of breast chicken meat contained lysine 11,12, threonine 4,24, valine 5,22, isoleucine 5,40, histidine 4,11, arginine 5,88, aspartic acid 8,88, glutamic acid 4,92, serine 4,05, proline 4,07, glycine 4,11, alanine 5,98, tyrosine 3,77(g/100g protein).
It was reported that lysine was lost by reaction with autoxidizing fat at temperatures below 100 °C, while at high temperatures (i.e. 115–130 °C) the loss was apparently independent of the presence of fat (Lea et al.[9]). However, Macy et al. [10] found that most free amino acids increased in concentration during cooking of roasts to an internal temperature of 77 °C with the exception of threonine, serine, glutamic acids, histidine and arginine. They attributed this general increase in amino acids content to hydrolysis of protein by proteolyttic enzymes. Most free amino acids decreased during heating, when isolated from the tissue by dialysis. Besides, heat treatment (at 163 °C) was found to raise the concentration of serine, glutamic acid and valine, isoleucine, leucine, tyrosine, phenylalanine and arginine in meat (Usborn et al., [15]).
In addition, Hamm [6] analyzed broiler breast and thigh meat samples from birds grown and processed in four locations of the U.S. for their amino acid composition. On percent protein basis amounts of valine, leucine, isoleucine and histidine were significantly greater in breast and glycine, hydroxyproline, hydroxylysine, threonine and serine were greater in thigh meats. Meanwhile, Moawad, [12] reported that the total amino acids, total essential amino acids and non-essential amino acids for fresh cooked beef meat were found to be 86,34, 33,01 and 53,33g/16g N respectively.
Therefore, the aim of this research was to study the effect of different cooking methods (boiling, pressure-cooking, roasting and microwave) on amino acids composition of chicken meat (thighs and breasts).
Materials and methods
The broilers used in this study were obtained from the local Damascus markets. Their weight ranged between (1100–1400 g) and the average age 8 weeks.
Chickens were slaughtered, plucked by hand, cleaned and washed with water. The wings, neck and heads were removed by hand. The carcasses were then cut into four parts (two breast pieces and two thighs). Chicken parts (thighs and breasts) were cooked by the following methods:
1 — Boiling: Chicken parts (thighs and breasts) were cooked in sufficient amounts of boiling water to cover it at ratio 2:1 (water: chicken) for 30 minutes.
2 — Pressure-cooking: Chicken parts (thighs and breasts) were placed in a pressure cooker and cooked for 20 minutes.
3 — Roasting: Chicken parts (thighs and breasts) were roasted by using a preheated conventional electric oven to 180 °C for minutes.
4 — Cooking by microwave oven: Two thighs and two breasts were placed in a baking dish and cooked in Microwave oven on a high power level for 20 minutes, 10 minutes on one side and 10 minutes on the other.
Moisture, ash, total lipids and total protein were determined according to the methods recommended by the A.O.A.C.[4]. All determination was performed in triplicates and the mean values were reported.
Amino acids were determined at the laboratories of agriculture faculty. High performance Amino Acid Analyzer was used as described by Winder and Eggum, [16]. Acid hydrolysis was performed in sealed ampoules for the determination of all amino acids other than methionine, cysteine and tryptophan. In the oxidized hydrolysis, methionine was determined as methionine sulphone and cysteine was detected in the form of cysteine acid.
Samples were weighed (20–30 mg) in the ampoules and 5 mls. of 6 N HCl were added. The ampoules were then sealed under vacuum. Samples were then heated in an oven at 110 °C for 24 hours. The sealed ampoules were then opened and the hydrochloric acid was evaporated under vacuum. Sodium citrate buffer (pH 2,2) was used to dissolve the samples and to dilute it to the required volume. The filtrated hydrolysate was used for the amino acid analysis in High Performance Amino Acid Analyzer (a sample of 50 µL-volume was injected).
Numbers presented in the tables are the mean values of three concurrent iterations. Statistical analysis was based on the one-way analysis of variance; homogeneous groups were formed according to the Duncan test for P < 0,05. The data were statistically analysed using STATISTICA (data analysis software system).
Results and discussion
Fresh raw chicken breast and thigh meat samples were analyzed for their moisture, protein, fat, and ash. The obtained results are, hereafter shown in Table 1.
The analysis data in Table 1 indicated that the moisture content of chicken breast samples was approximately 1,0 % higher than thigh meat samples. The percentages of moisture of fresh raw breast and thigh meat were 74,15 and 73,07 %, respectively.
As expected, the percentage of fat in chicken breast meat was significantly lower than that in thigh meat (P< 0,05). Average fat contents of fresh raw chicken breast and thigh meat were 12,18 and 21,65 % (on dry weight basis), respectively.
It could be noticed that fresh raw chicken breast and thigh meat contained 3,93 and 3,72 % ash (on dry weight basis), respectively.
Table 1. Chemical composition of fresh raw chicken meat (On dry weight basis)
The amino acid composition of fresh raw chicken breast and thigh meat is shown in Table 2. From the result illustrated in the table, it could be noticed that the fresh raw chicken breast and thigh meat contained the same individual amino acids with a slight differences (P> 0,05) in their amounts. Similar results were found by Abd El- Wahed [2]. The results in the same table indicated that the fresh raw chicken breast meat contained greater amounts of serine, histidine, arginine, threonine, valine, methionine, isoleucine, leucine, phenylalanine and lysine, while fresh raw chicken thigh meat contained greater amounts of aspartic acid, glutamic acid, glycine, alanine, cysteine and tyrosine. Hamm [6] reported approximately the same results, and he found that the amino acid composition of raw broilers meat is influenced by area of production, strain, sex and age. The total non-essential and essential amino acids of fresh raw chicken breast and thigh meats were found to be 88,95 and 87,11; 50,49 and 51,37 as well as 38,49 and 35,74g/16g N, respectively. However, fresh raw chicken breast meat contained more total essential and less nonessential amino acids than fresh raw chicken thigh meat (P< 0,05) as shown in the same Table 2.
Table 2. Amino acids composition of fresh raw chicken meats (g/16g N)
The effect of cooking treatments on amino acids content of chicken meat is shown in Table 3. From the results in the table, it is clear that cooking of either fresh breast or thigh meat samples caused some decrease in all of their amino acids. The chicken breast and thigh meat samples cooked by pressure retained the highest percentages of total essential, non-essential and total amino acids, followed in a decreasing order by boiling, microwave and roasting methods (P< 0,05). However, no significant differences were found among cooking methods on amino acids content (P> 0,05) in either chicken breast or thigh meat samples. As a result of cooking of fresh breast and thigh meat samples by pressure, the total amino acids were found to decrease from 88,95 to 84,21 and from 87,11 to 81,16 g/16g N, respectively. Meanwhile, they decreased from 88,95 to 81,43 and from 87,11 to 77,43 g/16gN, in the fresh breast and thigh meat samples cooked by roasting respectively. Marked amounts of sulpher containing amino acids, i.e. leucine, tyrosine, phenylalanine and lysine (as essential amino acids) as well as serine, glycine, alanine, histidine and arginine (as non-essential amino acids) were destroyed, while as slight decrease was noticed in all the other amino acid contents under cooking of chicken breast or thigh meat samples.
Table 3. Effect of cooking methods on amino acids composition of fresh raw chicken meat
Furthermore, from the same obtained results it was clear that cooking of either fresh breast or thigh meat samples caused some decrease in all their amino acids. The reduction of amino acid content might be attributed to their loss with drippings separated during cooking as well as by the heat destruction.
Conclusion
The obtained results from this study could be summarized as follows:
— Uncooked chicken breast meat samples had higher contents of moisture, total protein, and ash but lower contents of total lipids than uncooked thigh meat.
— Fresh raw chicken breast and thigh meat contained the same individual amino acids with a slight variation in their amounts(P> 0,05)
— Fresh raw chicken breast meat contained more total essential and less non-essential amino acids than fresh raw chicken thigh meat (P< 0,05).
— The chicken breast and thigh meat samples cooked by pressure retained the highest amount of total essential, non-essential and total amino acids, if compared in a decreasing order with boiling, microwave and roasting methods, (P< 0,05) but no significant difference among these cooking methods for amino acid contents were found (P> 0,05).
This article was originally published in Theory and Practice of Meat Processing №4 | 2016. DOI 10.21323/2414-438X-2016-1-4-11-18.
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