Color helps meat quality--新西兰essay代写范文

新西兰essay代写范文:“Color helps meat quality”，这篇论文主要描述的是消费者在购买商品时会通过眼睛或气味来对商品进行初步的识别，以消费者购买肉为例，如果肉的颜色鲜艳对于消费者来说是比较的具有吸引力，能够吸引着消费者的购买行为，从而达到销售商品的目的。

Introduction

Among all the factors that contribute to the meat quality, the colour is the first thing that comes to consumers eyes which means the colour of meat can affect greatly on the choosing decisions. Also, the odor of meat cannot be directly evaluated through retailing, so a colour of bright cherry red can usually be attractive and indicating the good and fresh quality of meat. For some meat, for example, ground beef, although the interior is still red, the value of meat is usually considered low by consumers and even be discarded before the microbial safe problems happens. It is reported that the discolouration related meat market loss is more than 1 billion US dollars each year in United States (Smith et al. 2000).

In livestock carcass, the the myoglobin exists in sarcoplasm and are related to the colour of meat. The functional properties of myoglobin plays different role in meat and live muscles. Specifically, the myoglobin in live muscle mostly plays as an O2 carrier and release the O2 to cell mitochondria, while in the meat, myoglobin are associated with red meat colour. The present hemoglobin in meat is due to the residual blood trapping in the vein inside the muscles. Although other pigments (cytochrome and hemoglobin) can affect the colour of meat, they are less countable for the presentation of red colour. To be noticed, pigments in addition to myoglobin, they are more related to the colour in fish, poultry meat than livestock counterpart (Wittenberg and Wittenberg, 2003).          

Myoglobin

Myoglobin exists in muscle fibers. It is a kind of protein with single chain and contains iron ion. The structure of myoglobin is similar to the single subunit of hemoglobin, but it has a stronger ability to bind the O2 than hemoglobin in the bloodstream. Myoglobin contains a porphyrin ring wrapping the central heme. The proximal histidine group is attached to the iron and the distal histidine group attached to the face oppositely which is not bonded to the iron. the myoglobin structure determines that the heme group is water soluble and can also keep the heme group from environment and avoid potential oxidation. Since the conjugated double bonds in heme group in myoglobin has ability of resonance which gives the red visible light to myoglobin so that it can play as a pigment in meat muscle. The iron atom can absorb 6 electrons and in turn form 6 coordinate bonds. The sixth position of heme iron is free to bind O2 or other ligands (CO and NO). In addition, the size of ligand bound to sixth position in myoglobin is determined by the space arrangement between heme and distal histidine and this arrangement can also prevent the interaction between myoglobin and large biomolecules (Cornforth and Jayasingh, 2004).

Colour agent in fresh meet

Fresh meet which is also known as raw meat are usually stored in different ways to achieve a longer shelf life. Also for this reason, various of packaging methods have been applied. And there are four redox states exist in packaged fresh meat (Figure1): carboxymyoglobin (COMb), metmyoglobin (MetMb), , oxymyoglobin (OxyMb) and deoxymyoglobin (DeoxyMb).

Myoglobin diferent redox forms in fresh meat. (Mancini and Hunt, 2005).

Three of them ( DeoxyMb, OxyMb, and COMb ) are in bivalent state. A bright cherry red colour which is just by the critical point of human acceptance is provided by COMb and OxyMb while the red colour of these two compound cannot be distinguished by human (Cornforth & Hunt 2008). A purplish red colour is displayed by DeoxyMb. In CO in COMb and OxyMb, oxygen occupy the heme iron sixth coordinate, whereas heme iron is bound with no ligand in DeoxyMb. Saturated myoglobin combined with oxygen shows an fascinating cherry red colour in meat in the OxyMb formation. Since the myoglobin has a better adhesion ability of carbon monoxide than oxygen, the stability of bright cherry red colour is increased. The alteration of meat colour is contributed by the oxidation of DeoxyMb, OxyMb, and COMb from brown MetMb. Meanwhile, the ferric heme sixth coordinate of MetMb bounds water molecular and it is unable to bind oxygen for MetMb. The four different form of myoglobin can be soluted in low ionic strength buffer and water. And these forms are all distinctive in absorbance spectra which between 500 nm to 600 nm (Tang et al. 2004).

Table 1—Wavelengths of maximum absorption for myoglobin redox species and their millimolar extinction coefficients (ελ/mM/cm) (Tang et al. 2004).

From the chart, deoxymyoglobin has a maximum absorption at 557 nm, meanwhile, at 503nm, metmyoglobin shows a peak. To be noticed, both oxymyoglobin and carboxymyoglobin have two peaks, they are 542 nm, 582 nm and 543 nm, 581 nm respectively (Suman et al. 2006). The four absorption of redox forms have an intersection at 525 nm. The total concentration of myoglobin in solution and extraction from fresh meat is estimated at 525 nm absorption (Tang et al. 2004).

Colour agent in cooked meat

The cooked meat usually has a grey or dull brown colour, this is mainly due to the soluble myoglobin denaturation caused by cooking process and heat (King & Whyte 2006). The heme group in myoglobin is exposed because of denaturation and thus make the heme group more easily to oxidize. Also, the colour agent in meat coagulate after cooking due to the globin chain unfolding. Therefore, the pigments in cooked meat is not soluble in water solution (Cornforth & Jayasingh 2004). The dull brown colour in cooked meat is contributed by the result of denaturation of MetMb induced by heat treatment. Whereas, the formation of pink red denatured globin hemochrome which is oxidized to brown ferrihemochrome is contributed by globin in ferrous Mb denaturation. Cooking induced COMb denaturation leads to the pink red denatured globin carboxy-hemochrome ( Nam and Ahn, 2002).

Colour agent in cured meat

The cured meat has a stable pink colour which is formed by the reactions between myoglobin, nitrate and nitrites (Cassens 1997). In old days, people used sea salt which contains nitrates to preserve meat and nitrates are reduced to nitrites by bacteria which gives the rise to pink colour in cured meat. Nowadays, nitrite come to contact with water and generates nitrous acid which can reacts with myoglobin and it is oxidized to metmyoglobin . Then, the heme bound NO in metmyoglobin is formed by the conversion of nitrous acid. NO-metmyoglobin shows brown and it can be reduced to nitrosyl myoglobin which has a red colour. The cooking process can denature the nitrosyl myoglobin and converts it to nitrosyl haemochrome which shows pink ( Honikel 2008). To be noticed that the cured meat are usually stored in vacuum packaging and in lightproof films. The reason behind it is that the pinked nitrosyl haemochrome is sensitive to the light as well as oxygen and the binding NO in metmyoglobin can be dissociated when being exposed to light and O2 leading to the colour fading. Recently, since health are being concerned by more and more people, researchers are concentrating in substitutions of nitrites by plant and organic ingredient to achieve the pink colour (Sebranek et al., 2012).

Key factors that affect the meat colour

Recent research has showed that multiple factors responsible for the biochemistry characteristic and stability of meat coulor and they have applied scicentific principle to exploit strategies for minimizing the disclouration in processing.

Intrinsic factors

There are many intrinsic factors that responsible for the meat colour stability, among them the most outstanding factors are activity of mitochondrion, antioxidant, oxidation and muscle source (Mancini and Hunt, 2005). On top of that, different live animal related factors, for example, genetic, daily diet and management, can affect the mear colour. (Faustman and Cassens, 1990).

Muscle source is an important factor of all the several intrinsic factors in impacting meat colour and a lot of researches have done on beef muscle profiling (Von S eggern et al. 2005). Different muscles have specific physiological function and anatomical location which leads differences of muscle metabolism. Thus, muscle shows an unique colour biochemistry characteristic (Hunt and Hedrick, 1977). Researchers has found out that the discoloration and oxymyoglobin oxidation in beef are associated with different muscle source (McKennaet al. 2005, Seyfert et al. 2007). Beef muscles have been classified in two category: colour labile and colour stable based on the colour stability. Beef that have lower metmyoglobin reduction rate and higher O2 consumption rate are classified in colour labile group. While the beef with higher metmyoglobin reduction rate are colour stable (Reddy and Carpenter 1991). Longissimus lumborum is also an colourstable muscle which is usually served as New York Strip Streak. However, the Psoas which is usually used to make filet mignon is colour labile. Researches has found out that Longissmus limborum suffers less lipid oxidation, higher colour stability and higher metmyoglobin reducing activity than Psoas ( J oseph et al. 2012, McKenna et al. 2005, Seyfert et al. 2007). Researchers studies attributed the different type of meat discolouration to different enzymes previously (Hagler, et al. 1979). While the research in protein conducted by Joseph (2012) showed that the chaperone like 27kDa heat shock protein and antioxidant such as peroxiredoxin-2 and thioredoxin are abundant in colour stable beef muscle and thus these compounds keep myoglobin from oxidation, leading to a higher colour stability in beef muscle. As we know, the oxidation of lipid can generate ketones and aldehydes and other secondary compound that contribute to the off odors ( Pearson et al. 1977). This rancidity caused by lipid oxidation are known to us for long time, but Faustman et al (2010) reported that the lipid oxidation can also responsible for the off colouor phenomenon. The reactive compounds from lipid oxidation can cause the discolouration by accelerating myoglobin oxidation. The principle for this theory is that the highly reactive aldehydes are diffused to the sarcoplasm where the myoglobin molecules are adducted covalently with aldehydes. Although the cysteine residue groups in protein are the preferred ones for aldehydes in nucle adduction, the protein in poultry are lack of cyseine which the favourite target changes to histidines (Naveena et al., 2010). And heme are coordinated to a lower stability and exposed to oxidizing environment by the adduction of aldehydes, resulting in increase of myoglobin oxidation (Naveena et al. 2010). Meanwhile, researchers have found out the function of mitochondrial and myoglobin are associated closely (Wittenberg & Wittenberg 2007). Myoglobin are served as an O2 carrier and deliver O2 to mitochondria in animal muscle (Wittenberg and Wittenberg 2007). Even when animal are dead, the mitochondria still remain metabolizing O2 in skeletal muscle (Tang et al. 2005a). so, the myoglobin and mitochondria compete for the O2 and this becomes a important factor for formation of bright red colour. Mitochondria may affect the O2 consumption, metmyoglobin reducing activity and in turns affect the meat colour stability. In this case, the factors that may influence the activity of mitochondria can has an influence on meat colour. To be noticed, the extreme function of mitochondria can be harmful to stability of meat colour (Sammel et al. 2002). The competition of O2 in mitochondria and myoglobin can lead to the decrease of partial O2 pressure, leading to the heme bound O2 transfer from oxymyoglobin to mitochondria. This reaction promotes the deoxymyoglobin and makes the meat darker. Still, the decreased O2 partial pressure can increase the activity of mitochondria and keep the myoglobin in a deoxygenated state. While the high mitochondria activity leads to an loss of bright colour characteristic, the low mitochondria activity at a low temperature may has better colour performance at a higher temperature (Bendall & Taylor 1972).         

The mitochondria consumption of O2 provides an anaerobic environment in which promotes the metmyoglobin reduction. Researchers (Tang et al., 2005b) also reported that the mitochondria caused metmyoglobin reduction happens by the electrons transfer to metmyoglobin by cytochrome. Noticeable, some of the mitochondria substrates, for example, malate and succinate may also give rise to metmyoglobin reduction (Mohan et al. 2010).

Extrinsic factors

There are many extrinsic factors that can affect the meat colour. Antiocidant, ligand and prooxidants can all cause a change in meat olour. Ligand, usually gases, can play an important role in the colour of fresh meat. When fresh meat is exposed to air, O2 reacts with myoglobin and forms the oxymyoglobin which shows a cherry red colour. The myoglobin oxygenation occurs within 30 to 60 minutes and offers the consumer favourite cherry red colour. Normally, the oxygen combined myoglobin, also known as bloomed meat, suffers the discolouration with the formation of metmyoglobin and has a less than one week colour shelf life (McMillin 2008). The solution is improving the O2 level through modified atmosphere packaging, resulting in augment in colour shelf life (Jakobsen and Bertelsen, 2000). Nonetheless, the application carbon monoxide can boost the formation of carboxymyoglobin which is stable and increases the colour shelf life (Sorheim et al., 1999). Without of air of any ligands, for example, in the middle of cutted meat muscle where the myoglobin exist as deoxymyoglobin, it shows an purplish red colour and has less than one week colour shelf life (McMillin 2008). 

Primary structure of myoglobin and

Researches showed that the myoglobin in livestock and poultry contain 153 amino acids (Swiss Inst. B ioinform. 2012). While the functions of myoglobin are in high similarity for different meat species in meat production, the primary structure seems not alike well. For example, the myoglobin in poultry has less than 75% sequence alike with livestock (Table 1).

To be noticed, myoglobin in conventional poultry and livestock were characterized a few decades ago, but the characterization of those in different emerging meat of animals were happened recently. Thus, the myoglobin in a few closely correlative manimals share the same sequence of amino acid. For example, the myoglobin in bison, yak and beef share the same sequence of amino acid; the myoglobin primary structure are the same for white railed deer and red deer; the myoglobin of chicken and turkey have the 100% similarity in amino acid sequence (Swiss Inst. B ioinform. 2012). Moreover, the myoglobin in red meat is lighter than that in poultry for 300 to 400 Daltons (Table 2) which attributes to the fact that the small amino acids are substituted by the large ones (Figure 2).

Figure 2-1 Amino acid sequence o f red-meat and poultry m yoglobins.

Figure 2-2 Amino acid sequence o f red-meat and poultry m yoglobins.

The myoblobin primary structure determines the tertiary structure which can also influence the interactions between protein and biomolecule and meat colour are impacted ultimately. In addition, primary structure can also affect the myoglobin function in O2 carrying. A picture of how myoglobin primary structure influencing meat colour is well explained in Figure 3.

Figure 3 Mechanisms of myoglobin primary structure in?uences meat color.   

Autoxidation

Autoxidation can cause the iron state change from ferrous myoglobin to ferric metmyoglobin, resulting in brown discolouration. Researchers has showed that the different animal spices has different rate of oxidation. For instance, myoglobin of tuna  suffers faster oxidization than that of cattle meat and sperm whale. Researchers also found out that the oxidizable residues in the myoglobin primary structure can influence the rate of myoglobin autoxidation. For example, the myoglobin of sperm whale and livestock has no cystein residues which is oxidizable and highly contained in tuna myoglobin (Livingston and Brown, 1981). This unique oxidizable cystein residues present in tuna myoglobin can partly responsible for the high rate of autoxidation in myoglobin from tuna. Chow (1991) conducted a comparative experiment for the rate of autoxidation among three different species of tuna (yellow, bigeye and ?nbluefin tuna) which have the sequence similarity of more than 95%. The result is the myoglobin in bigrye tuna has the highest rate of autoxidation and that in bulenfin has the lowest rate of autoxidation. Researchers also studied the rate of autoxidation in myoglobin in red meat (red deer, beef and lamb) (Gutzke and Trout, 2002). The results showed that the myoglobin in pork which is genetically much different from other myoglobin in ruminants undergone less autoxidation than other ruminants. Dosi et al (2006) reported that in Italy, the buffalo meat came to the market and was to play an alternative role to beef, but later, consumers were discouraged due to the buffalo meat has an faster rate of dark in colour compared to traditional beef. So Dosi et al., (2006) compared the sequences of myoglobin in bufflo and that in beef. Nonetheless, the similarity in this two meat myoglobin was 98%, the negative charged residue present in buffalo meat myoglobin is responsible for the discolouration. Moreover, the myoglobin in eleohant, the glutamine replaces the distal histidine at position 64 which results in a low rate of autoxidation (Romero-Herrera et al., 1981).           

The O2 affinity

The O2 affinity of heme proteins are responsible for the different redox state of myoglobin. The size of heme pocket can influence the interactions in myoglobin and O2. Carver et al. (1992) reported that by replacing the sperm whale myoglobin position 29 with phenylalanine, the O2 affinity was increased for ten times and the rate of O2 dissociation was decreased as well (Carver et al., 1992). Another research which did the sequence compare experiment on fish myoglobins showed that the replacement of amino acid in nonhelical region is responsible for the low O2 affinity in myoglobins from bonito and mackerel (Marcinek et al., 2001).

Heme affinity

Research showed that the ligand binding ability and myoglobin redox state depend on the heme group retention ability of protein, it is usually determined by the primary structure (Yang and Philips, 1996). Grunwald and Richards (2006) reported that differences in heme affinity can be impacted by even a single amino acids replacement at a certain location. The researchers showed that the V68T which is a variant of myoglobin has higher heme affinity compared to the native myoglobin. However, the H 97 A which is a mutant performed a lower myoglobin affinity than native ones. This is due to the replacement of histidine 97 by small sized alanine residues, resulting in increased accessibility of water to heme pocket and in turn decrease the affinity of heme.      

Defected colour in fresh meat

Usually, when the fish meat turns to brown colour, it indicates that the quality is deteriorated. Despite of surface browning, some of other colour defects can also happen in meat which will lead to a less consuming desire for consumers and less competitive in market. One of the main problems in bone in cut meat is discolouration (Mancini et al. 2004). The presence of hemoglobin which is abundant in bone marrow is responsible for the red colour of bone marrow. Furthermore, the lipid is also rich in bone marrow, resulting the easily suffer the oxidation when exposed to air. And processing can damage the erythrocytes in bone marrow, leading to the hemoglobin exposed to the ambient and resulting in discolouration of bone. The application of antioxidant can restraint this kind of discolouration (Grobbel et al., 2006).        

Defected colour in cooked meat

Consumers normally believe the colour of meat is the good indicator for doneness and safety. For instance, the dull brown interior colour is usually believed as an welldone meat and pink colour are considered as a uncooked meat (King and Whyte, 2006). However, the myoglobin denaturation temperature depends on the state of redox which means that the colour of cooked meat cannot always determine the safety.   For example, the premature browning caused by myoglobin denaturation in cooked ground beef happens when heating temperature is lower than 71 centigrade at which the USDA recommended temperature for killing E. coli (Killinger et al., 2000). Researchers also reported that the ground beef in which mainly contains metmyoglobin has a less rate of premature browning than that in which the ferrous myoglogin is the mainly content (Hunt et al., 1999). To be noticed, the deoxymyoglobin is the most resisted to heat induced denaturation among the three types of myoglobins (metmyoglobin, oxymyoglobin and deoxymyoglobin). Whereas the metmyoglobin is the worst (Sepe et al., 2005). A few factors can inhibit the occurrence of premature browning by adjusting the raw meat myoglobin redox state. They are ways to storage, muscle species, packaging and antioxidant (Mancini et al., 2011).

Another colour defect is the pinkness in cooked turkey meat. Myoglobin in turkey meat has a genetical high resistance to heat compared to beef which means the meat still remain pink when is well done. This fact often misleads the consumers and thus loses the market (Holownia et al., 2003). It is also reported that the myoglobin is usually denaturated incompletely in normal cooking process and reacts with combustion product. Authors indicated that these two factors are the main reasons for post mature pinkness in turkey meat (Nam and Ahn, 2002).      

Conclusion

As the colour plays very important role for meat selling and as the change of colour is mainly due to the changes in myoglobin, it is critical to improve colour stability so that retailers can provide consumers meat with desirable colour and make it more profitable. This article mainly discussed the structure of myoglobin, four different types of myoglobin and their influence on meat colour. Also, the principles of interactions between myoglobin and various intrinsic and extrinsic factors are discussed. Finally, the defects in meat fresh and cooked meat colour is mentioned.     

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